Phthalocyanines for dye-sensitized solar cells

Coordination Chemistry Reviews 381 (2019) 1–64

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Phthalocyanines for dye-sensitized solar cells Maxence Urbani a,c, Maria-Eleni Ragoussi a, Mohammad Khaja Nazeeruddin b,⇑, Tomás Torres a,c,d,⇑ a

Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, EPFL, Valais Wallis, Rue de l’Industrie 17, 1950 Sion, Switzerland c IMDEA-Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain d Institute for Advanced Research in Chemical Sciences (IAdChem) Universidad Autónoma de Madrid, 28049 Madrid, Spain b

a r t i c l e

i n f o

Article history: Received 6 September 2018 Accepted 30 October 2018

Keywords: Phthalocyanine Molecular photovoltaics Dye-sensitized solar cell Solar energy conversion

a b s t r a c t Phthalocyanines (Pcs) are robust and intensely colored macrocycles (blue pigments) with high chemical, thermal and light stability, properties that are of paramount importance for realistic photovoltaic applications. In particular, Pcs have played a very important role in the development of dye-sensitized solar cells (DSSCs), as they are promising candidates for incorporation in these devices. Good efficiencies have been obtained by the use of Pcs as the light harvester, and, most importantly, a number of synthetic strategies have been developed for engineered dyes based on the Pc scaffold, due to the synthetic versatility and robustness of these macrocycles. In this review, recent advances in the use of phthalocyanines as photosensitizers for DSSC applications are presented. Ó 2018 Published by Elsevier B.V.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Phthalocyanines: generalities and nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Analogues and hybrid systems of phthalocyanines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Photovoltaic technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4. Background and history of Pcs in organic photovoltaics (OPV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5. Dye sensitized solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1. Operating principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2. Photosensitizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3. Electrolyte, co-adsorbent and additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.4. Scope and limitation of Pcs in n-type DSSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 3 3 3 4 5 6 6 7

Abbreviations: AcCN, acetonitrile; AM1.5G, standard air mass conditions for solar spectral irradiance; BET, back electron transfer; BMII, 1-butyl-3-methylimidazolium iodide; BHJ, bulk heterojunction; BTD, benzothiadiazole; CB, conduction band; CHENO or CDCA, chenodeoxycholic acid; DL, dye-loading; DMF, dimethylformamide; DMSO, dimethylsulfoxide; DMII, 1,3-dimethylimidazolium iodide; DMPII, 2,3-dimethyl-1-propylimidazolium iodide; DMPImI, dimethyl-3-n-propylimidazolium iodide; DSSC, dyesensitized solar cell; EI, electron injection; EQE, external quantum yield efficiency; ERD, energy relay dye; ETE, excitation transfer energy; F.F., fill factor; FRET, Förster resonant energy transfer; FTO, fluorine doped tin oxide; GuNCS, guanidinium thiocyanate; GSB, ground state bleach; H2Pc, free-base or metal-free phthalocyanine; HOMO, highest occupied molecular orbital; HTM, hole transporting material; Io, photon flux; Pin, incident intensity of solar light (one/full sun = 100 mW/cm2); IPCE, incident photons to photocurrent conversion efficiency; JSC, short-circuit current; LUMO, lowest unoccupied molecular orbital; MPII, 1-methyl-3-propylimidazolium iodide; NBB, 1-butylbenzimidazole; NIR, near infra red; MBII, 1-methyl-3-butylimidazolium iodide; MO, molecular orbital; MPc, metalo phthalocyanine; NMB, 1-methyl-benzimidazole; NPc, naphthalocyanine; OPV, organic photovoltaic; Pc, phthalocyanine; PCE (g), solar-to-electric power conversion efficiency; PDT, photodynamic therapy; PMII, 1-propyl-3methyl imidazolium iodide; Por, porphyrin; PSC, perovskite solar cell; PL, photo luminescence; PV, photovoltaic; Pz, porphyrazine; QD, quantum dot; SAM, self-assembled monolayer; SOMO, singly occupied molecular orbital; Spiro-OMeTAD, 2,20 ,7,70 -tetrakis(N,N0 -di-p-methoxyphenylamine)-9,90 -spirobifluorene; TAP, tetraazaporphyrin; TBP, 4tertbutylpyridine; TAS, transient absorption spectroscopy; TCSPC, time-correlated single photon counting; THAI, tetra-n-hexylammonium iodide; TCO, transparent conductive oxide; VB, valence band; VOC, open-circuit voltage. ⇑ Corresponding authors at: Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain (T. Torres). Institute of Chemical Sciences and Engineering, EPFL, Valais Wallis, 1950 Sion, Switzerland (M.K. Nazeeruddin). E-mail addresses: [email protected] (M. Urbani), [email protected] (M.-E. Ragoussi), [email protected] (M.K. Nazeeruddin), tomas. [email protected] (T. Torres). https://doi.org/10.1016/j.ccr.2018.10.007 0010-8545/Ó 2018 Published by Elsevier B.V.

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2. 3.

Symmetrically-substituted phthalocyanines (A4-type) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Unsymmetrically-substituted phthalocyanine dyes (AAAB-type; Fig. 13) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1. Background and history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Optimization of peripheral and non-peripheral substitution patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.1. Variation of the peripheral and non-peripheral substitution patterns in Pc dyes bearing a 2-succinic acid moiety as anchoring group (PCH001 analogues; Fig. 16) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.2. Variation of the peripheral and non-peripheral substitution patterns in Pc dyes bearing a 4-carboxyphenyl moiety as anchoring group (TT3 analogues; Fig. 19) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.3. Variation of the peripheral and non-peripheral substitution patterns in Pc dyes bearing a carboxyl moiety as anchoring group (TT1 analogues; Fig. 22) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.3. Modification of the anchoring groups and spacers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4. Mechanistic aspects: Scope and limitations on the performances of Pcs in DSSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.1. Effect of the metal center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.2. Effect of aggregation and adsorption geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.3. Impact of the anchoring group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5. Enhancement of light harvesting properties and panchromatic response of Pcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 5.1. Tuning optical properties and energetics of the Pc trough directly-bonded heteroatoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 5.2. Pcs functionalized with p-conjugated systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.3. p-Elongated Pcs and related analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.4. ABAC-type or ‘‘Push-Pull” Pcs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 6. Axially substituted phthalocyanine dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.1. Ru(II) Pcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.2. Si(IV) Pcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 6.3. Ti(IV) Pcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 6.4. Hf(IV) and Zr(IV)- Pcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 7. Co-sensitization and energy relay dyes (ERDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 8. Phthalocyanines for p-type dye-sensitized solar cells (p-DSSCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 9. Conclusion and outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

1. Introduction 1.1. Phthalocyanines: generalities and nomenclature Phthalocyanines (Pcs) are thermally and chemically stable, twodimensional tetrapyrrolic macroheterocycles containing 18 delocalized p-electrons responsible of their intense absorption in the red/near-infrared (NIR) region of the solar spectrum. They are distinguished for their intense Soret- and Q-band, with the second displaying high molar extinction coefficients extinction coefficients as high as 300,000 M1 cm1 and high fluorescence quantum yields, and represent, hence, ideal light-harvesting antennae [1,2]. The Q-band can be single or split depending on the symmetry of the derivative, and the bandwidth of absorption can shift by the incorporation of different substituents. Other than their strong absorption characteristics, numerous more properties stem from

the extensively conjugated aromatic chromophoric system of Pcs, which render them promising building blocks for the construction of new molecular materials at different technological fields [3–6]. These planar and highly conjugated aromatic macrocycles tend to organize spontaneously into stacks through the occurrence of p– p supramolecular interactions (J- or H- aggregates) [7–9]. In addition, they possess a rich redox chemistry that can be tuned by their metal centre and/or peripheral, non-peripheral, and axial substituents (Fig. 1). The physical and optical properties of these red/NIR pigments make them excellent candidates in many fields of applications in organic electronics [3–6], photovoltaics [10–13], catalysis [14] and photodynamic therapy (PDT) [15]. With some exceptions, the synthesis of a phthalocyanine starting from a monosubstituted phthalonitrile(s) usually affords a mixture of regioisomers due to the relative position of the R groups attached to the

Fig. 1. General structure and numbering of the positions of a phthalocyanine: peripheral (a), non-peripheral (b) and axial positions.

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isoindole units. Moreover, for DSSC applications in particular, the anchoring group is most usually located at one (or more) bposition(s) for obvious reasons related to favourable adsorption geometry whereas the three other can be either a- or b- substituted. Hence, because of the numerous regioisomers that can be obtained, the condensed structural formulae notation (type 1, Fig. 2) is most often too complex to be fully detailed. For this reason, the type II notation, though not formal, is often preferred to type I for sake of clarity, and will be used in the current review when necessary. 1.2. Analogues and hybrid systems of phthalocyanines Tetraazaporphyrin (TAP), also known as porphyrazines (Pz) [16], are contacted analogue molecules that lack the four fused benzo rings of the Pc (Fig. 3a). To the best of our knowledge, the only examples of Pz-type dyes reported in DSSC are hybrid systems (Fig. 3d) reported by us recently [17,18] achieving promising efficiency of up to 3.4% [17]. On the contrary, naphthalocyanines (Fig. 3c) are p-elongated analogues that have a naphthalo- instead of a benzo- group, fused to the macrocycle. Various hybrid systems (Fig. 3e, f) based on naphthalocyanines have been recently reported in DSSC and will be dealt in details in Chapter 5.3.

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traditional fuels [19,20]. Established and commercialized solar energy conversion technologies include silicon-based and thinfilm devices, which have reached outstanding performances [2–23]. Organic photovoltaics (OPV), including single-, double-, or bulk hetero-junction (BHJ) [24], dye-sensitized solar cells (DSSC) [25–31], and more recently perovskite solar cells (PSC) [32–52], are very promising alternatives to standard silicon photovoltaics, with the last two holding a privileged position and being recognized as one of the most promising solutions towards a largely decarbonized energy future [19]. Remarkable progress has been reported in the DSSC field and significant breakthroughs, marking the performance roadmap [53,54], have led to today’s efficiencies above 14% [55]. Undoubtedly, with the 10% threshold for commercialization having been long overcome, the entrance of these technologies into the market is now closer than ever. With regard to perovskite solar cell technology, outstanding results have been obtained in a very short timeframe, with efficiencies approaching now 22% [56–58]. The use of Pc as hole transporting material (HTM) in perovskite cells were recently reported and have opened the way to implicating these spectacular macromolecules in new challenging paths, and will be mentioned but not reviewed herein [59–61]. 1.4. Background and history of Pcs in organic photovoltaics (OPV)

1.3. Photovoltaic technologies Research on photovoltaic technologies has been on a blooming ride the last two decades, in an effort to take advantage of the enormous amount of sunlight that strikes the earth daily to tackle the current and future energy challenges: meet the planet’s constantly increasing energy needs and provide carbon-free alternatives to

Unsubstituted phthalocyanines, free base (H2Pc) or metalated (MPc), represent the most simple class of these compounds (Fig. 4). Before the arrival of the DSSC technology developed by O’Regan and Grätzel in 1991 [62], the use of phthalocyanines in organic photovoltaics (OPV) was already envisaged from as early as 1958, when magnesium phthalocyanine (MgPc) was sand-

Fig. 2. Condensed and simplified structural chemical formulae used for phthalocyanine having an a- and/or b- (mono)substitution pattern of their isoindole units. Note that both types of nomenclature will be used in this review.

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Fig. 3. General structures of a) Tetraazaporphyrin (TAP), b) Phthalocyanines (Pc), c) Naphthalocyanine (NPc), d), e), and f) some known related hybrid systems (arrows schematize the ring-expansion).

Fig. 4. Molecular structures of the first unsubstituted phthalocyanines tested as sensitizers of n-type semiconductors (TiO2, WO3, SrTiO2, ZnO, CdS, CdSe, SnO2, SnS2, etc. . .): free-base Pc (H2Pc) [64] and Metallo Pcs [65] ZnPc, MgPc [66] CoPc, FePc, CuPc [67] AlClPc, InPcCl [75], TiOPc [72].

wiched between two glass electrodes in a single-layer mode, giving rise to 0.2 V of photovoltage [63]. Back in the 80’s, Bard and coworkers also observed a photocurrent response after photosensitization of various single-crystal n-type semiconductors (TiO2, WO3, SrTiO2, ZnO, CdS, CdSe, SnO2, SnS2, etc. . .) by unsubstituted H2Pc [64] or MPcs [65] thin films. At that time, the efficiency of these cells was so low that it was not even measurable, but these examples brought in the spotlight the potential of Pcs as sensitizers in photovoltaic applications. Further investigations followed with both MgPc [66] and copper phthalocyanine (CuPc) [67], and a significant breakthrough came with Tang’s heterojunction OPV device in 1986 [68], a thin-film two-layer cell composed of CuPc as the electron-donating layer and a perylene tetracarboxylic derivative as the electron-accepting counterpart, reaching an overall efficiency of 1% under simulated AM2 illumination, a record for the time. This alluring result gave more focus by the research community on these exceptional macrocycles [69] and their capabilities for optoelectronic and molecular photovoltaic applications [70,71]. In view of the above, it is not, thus, surprising that Pcs were among the first molecules to be evaluated in firstgeneration DSSC devices [71–75] and in photocatalysis [76–78]. Nowadays, however, unsubstituted Pcs are no longer of interest in the DSSC field because of the absence of an anchoring group to allow their attachment to the semiconductor surface, not to

mention their strong tendency towards aggregation and poor solubility. Some other unsubstituted phthalocyanines were reported in DSSC more recently achieving efficiencies well-below 1%, and will be mentioned but not reviewed herein [79–83]. Indeed, due to the absence of anchoring group, no dye can be anchored (chemisorbed) to the surface, resulting in a marginal amount of adsorbed molecules through physisorption process. 1.5. Dye sensitized solar cells Dye-sensitized solar cells were introduced by O’Regan and Grätzel in 1991 and triggered a boom for further research and development [62]. Before this breakthrough, the concept of dye sensitization of wide bandgap semiconductors had already been envisaged, with various attempts being reported, but efficiencies remained consistently low, mainly due to the flat surface of the semiconductor that allowed only for a monolayer of dye to be adsorbed and there was, thus, no efficient absorption of incident light [84,85]. The novelty of this architecture that differentiated it from previous attempts was twofold: increased light harvesting, attributed to the roughness of the semiconductor surface, which allowed for a larger amount of dye to be directly adsorbed on it, and new robust dyes, stable under experimental conditions. These cells stand out for their low production cost and energy expendi-

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Fig. 5. Schematic representation of n-type (left) and p-type (right) DSSCs (VB = valence band, CB = conduction band, M(Ox/Red) = oxidized and reduced redox shuttle, S* = excited dye, S+ = oxidized dye, S = reduced dye).

ture, basically due to the fact that the standards for material purity are much lower, so processing under vacuum and high temperatures is not required. This derives from the fact that DSSCs are major carrier devices, which means that all important charge carrier processes occur at the interface, and thus, the bulk properties of the materials are of minor importance. In addition, DSSCs possess attractive technical features, such as light weight, transparency, flexibility and robustness over temperature [86–89]. All of the above render them a very promising alternative to standard silicon photovoltaics. Presently, the record efficiencies in DSSC technology stand above 14% for n-type [55], and only 2.5% for ptype [90]. 1.5.1. Operating principle A dye-sensitized solar cell consists of three main components: the organic dye, the nanocrystalline semiconductor and the redox couple in the electrolyte [19,26]. It is noteworthy that, in contrast

to other kinds of solar cells, in this case electron transport, light absorption and hole transport are each handled by different materials in the device. The photophysical sequences that occur in a cell all depend on the properties of these components, which naturally require fine-tuning. In the first seminal example of the 90 s, the active components were a trimeric ruthenium complex as the chromophore, a n-type TiO2 nanocrystalline semiconductor and an iodide/triiodide redox electrolyte [62]. Since then, several thousands of active components [91–93] and configurations [94] have been developed and evaluated. The operating principle of a ntype DSSC [95–97] consists of four main steps (Fig. 5, left): (1) Absorption of light by the sensitizer leading to the excited state of the dye (S + hm ? S*), (2) the excited dye injects an electron into the conduction band of a semiconductor (typically TiO2) leading to the oxidation of the dye (S* ? S+ + e/CBn-SC), (III) the oxidized dye is restored to the fundamental state by the mediator (M) present in the electrolyte (S+ + M(Red) ? M(Ox)), (4) the electrons in the

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Fig. 6. Molecular structures and efficiencies of some performing sensitizers in (n-type) DSSCs: Ru(II)-bipyridines complexes N3/N719 [113], C106 [116] and CYC-B11 [115]; Zn(II)-porphyrins YD2 [117], YD2-O-C8 [118], GY50 [54] and SM315 [53]; D-p-A organic dyes Y123 (in solid state-DSSC) [112] and [ADEKA-1+LEG4] [55] (in parenthesis are 3+/2 given the year of publication and the type of charge-career shuttle: I/I and [Co(phen)3]3+/2+ are redox couples used in liquid electrolytes, and Spiro-OMeTAD 3 , [Co(bpy)3] is a solid-state hole transporting material).

photoanode flow through an external circuit to reach the counter electrode where the oxidized mediator is regenerated (M(Ox) + e/CE ? M(Red)). The operating principle of a p-type DSSC is inverted [98,99], which is the reason why this type of cell is also known as ‘‘inverted DSSC” (Fig. 5, right): (1) after light absorption by the sensitizer, (2) the excited dye injects a hole in the valence band (VB) of a semiconductor (typically NiO or CuO) leading to the reduction of the dye (S* ? S + h+/VBp-SC), (3) the reduced dye is regenerated to the fundamental state by the mediator (S + M (Ox) ? M(Red)), and (4) the electrons in the photoanode flow through an external circuit to reach the counter electrode where the reduced mediator is regenerated (M(Red) ? e/CE + M(Ox)). Pcs have been scarcely considered as sensitizers for p-type DSSCs and only very few examples have reported on this topic, which will be detailed at the end of this review (Section 8). 1.5.2. Photosensitizers The design of the photosensitizer is fundamental in the race for performance optimization [19]. Major synthetic efforts have been reported over the years and a number of dyes have marked important milestones in the field, especially porphyrins [25,100–102], phthalocyanines [103–105], metal-free all-organic dyes [91,112] and Ru(II)polypyridyl complexes [93,113–116]. The driving force for the sophisticated synthetic strategies that have been developed has been the spur to meet dye requirements for optimal performance: suitable energy levels for charge injection and dye regeneration, broad absorption in the visible and NIR region, orientation and geometry adopted by the dye onto the TiO2 surface when adsorbed, and an anchoring group to attach to the semiconducting material. Ruthenium-based dyes were since the beginning a key asset in the triumph of DSSCs and the 10% PCE barrier was very early broken with the benchmark dyes N3/N719 and N749 (black dye) [113]. Notably, remarkable IPCEs of 80% across the visible part of the solar spectrum were also reported. Improvements in their molecular design have resulted to today’s most successful

analogues, that are CYC-B11 [115] (PCE of 11.5%) and C106 [116] (PCE of 11.7%) bearing conjugated thiophenes which greatly enhanced light harvesting (Fig. 6). Limitations in the further development of Ru-dyes consist in limited harvesting capability, constrained resources due to rarity of the metal and low durability. Much attention has also been directed towards porphyrin dyes, as a way to broaden the sensitizer’s absorption spectrum to higher wavelengths and circumvent ruthenium’s high cost [100]. Lately reported derivatives have achieved not only to overcome the 10% efficiency threshold, but also to yield the best performing sensitizer to date in DSSCs. Dyes YD2 [117] and YD2-oC8 [118], have set major landmarks in the evolution of the field (Fig. 6). The first breakthrough came with YD2-oC8, where an exceptional VOC of 1 V was reported, attributed to the long peripheral alkoxy chains that tackled the charge recombination processes [118]. A PCE of 11.9% was reported in conjunction with a Co-based redox electrolyte, which rose to 12.3% when co-sensitized with another organic dye. Following this impressive breakthrough for porphyrin standards performance, powerful panchromatic dyes SM315 [53] and GY50 [54], came in the spotlight, and the efficiency rose further to 13%. The performance of porphyrin-based solar cells is, however, limited by the robustness of the dyes and poor absorbance at the solar flux maximum, especially in the red and NIR regions. More recently, a record efficiency of 14.3% in n-type TiO2 DSSC was obtained by co-sensitisation of an alkoxysilyl anchor (ADEKA-1) and a carboxy-anchor (LEG4) organic dyes [55] (Fig. 6). Other organic sensitizers have also been targeted, but even though the reported conversion efficiencies have increased significantly in the last years, their usually poor absorption extinction coefficients result in efficiencies consistently remaining below the theoretical commercialization limits [106–111]. 1.5.3. Electrolyte, co-adsorbent and additives It is widely accepted that other than enhancing the lightharvesting capabilities of the organic sensitizer, another feasible

M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

Fig. 7. Typical absorption spectra of a Zn(II)-Pc (TT1 in THF solution), an axiallysubstituted Ru(II)-Pc (TT35 in THF solution) and a Ru(II)-bipyridyl complex (TT206 in DMF solution).

way to improve the performance of a cell is to enhance the open circuit potential [19,26]. In this context, aside to the growing work on delivering new optimal dyes, intense research efforts are also dedicated on developing electrolytes that will push performances to higher levels [119]. More precisely, design of electrolytes with suited redox potentials to achieve high VOC is a key strategy, since the energy mismatch between the electrolyte and the oxidized dye results in big losses of voltage during the regeneration of the dye. The most commonly used redox couple, and until recently the most effective, is I/I 3 in liquid electrolytes [120–128]. It stands out for slow recombination kinetics leading to long-lived electron lifetimes, however, it is chemically aggressive and corrosive toward the metal electrodes, it has complex multi-electron redox chemistry, and the triiodine anions partially absorb sunlight. Other iodine-free redox shuttles have been pursued to overcome these disadvantages and also to reach higher oxidation potential and lower the required driving force for efficient dye-regeneration, increasing the Voc [129,130]. Cobalt-based electrolytes have, in this context, given outstanding results, becoming the to-date most powerful alternative to iodine-based analogues. Following the first example, tris(2,20-bipyridyl)cobalt(II/III) [131], numerous other engineered Cobalt complexes came in the spotlight [132]. An appealing feature of cobalt complexes is their synthetic flexibility, allowing introduction of various ligands to modulate their electronic and redox properties. Additionally, it has been shown that the additives presents in the electrolyte influence drastically the photovoltaic parameters (VOC, JSC, F.F.) and long-term stability of the cell, as well as suppress dye aggregation [133]. Some additives can cause a shift of the semiconductor conduction band, either upward or on the contrary downward, that results in changes in the VOC and JSC. For instance, LiI produces a downward shift of the CB, which can improve significantly the JSC because the driving force for electron injection becomes larger, but concurrently decreases the VOC. Another well-known additive commonly used in liquid electrolytes is 4tertbutylpyridine (TBP), which has an opposite effect regarding the shift of the semiconductor CB [134–137]. It plays a dual role 1) it produces an upward shift of the CB, which usually greatly improves the VOC, but is most often accompanied by a decrease in the JSC (because the driving force for electron injection becomes smaller), and 2) it can decrease notably interfacial charge recombination between the oxidized species of the electrolyte and the injected TiO2 electron. In some cases, TBP have also been used directly in the dye-uptake solution to reduce p-p stacking between metallo macrocycles (e.g. Pc) through centre metal-pyridine axial coordination. Another important element in DSSC, especially for

7

aggregated compounds such as Pcs, is the incorporation of a coadsorbent in the dye uptake solution (the most commonly used being CHENO or CDCA [138,139]) which stands out for minimizing the penetration of oxidized species of the electrolyte to the metal oxide surface, reducing the dark current, and lowering dye aggregation [140]. Along with the research in liquid electrolytes, solid hole conductors have also been envisaged and studied as a means to overcome the drawbacks of liquid electrolytes, mainly corrosiveness, leakage and volatility [141]. Initial reports on all-solid-state hybrid devices (ss-DSSCs) described very poor efficiencies, but recently this value has increased tremendously, albeit not to the range of liquid electrolyte-based systems [142,143]. The highest VOC values have been obtained with the use of small-molecule hole conductors [144], such as Spiro-MeOTAD [112,145]. Further development of solid hole transporting materials is mainly hampered by the difficulty in achieving complete pore-filling in thicker films which greatly influences morphology, as well as their low lightharvesting properties and internal quantum efficiencies.

1.5.4. Scope and limitation of Pcs in n-type DSSC There are various limitations restraining the further development of the most efficient chromophores used as artificial antennae systems. Some dyes are constrained by insufficient lightharvesting properties and others by low photo- and thermalstability or poor electron injection capability. Pthalocyanines came into the picture as a means to tackle these issues. Pcs hold several advantages because of their light-absorbing capabilities in the red and near-IR spectral region, as well as their thermal and chemical stability, synthetic versatility and appealing redox properties, suitable for the electronic processes that occur in a cell. Typical absorption spectra of a Zn(II)-Pc sensitizer TT1 [17] and a Ru(II)-Pc axially substituted (TT35) [146] are displayed in Fig. 7, together with a typical heteroleptic Ru(II)-bipyridyl complex exemplified with TT206 [147]. From the point of view of light-harvesting capabilities, the huge potential of Pcs as Red/NIR sensitizers can be clearly appreciated, the 600–800 nm region being the most photon rich part of sunlight. Nevertheless, in terms of efficiency Ru(II)bipyridyl complexes have reached 10–12% [113] while those of the best Pcs have only achieved PCE of 3.52% [148] 3.65% [17] (TT1, 2007–2017), 5.5% [149] 6.1% [150] (TT40, 2012–2014,) and 6.4% (PcS20, 2014) [151]. To be efficient in a (n-type) DSSC, the LUMO level of the Pc should lay above the TiO2 CB, to ensure a satisfactory sensitization of the semiconductor, while the HOMO level should remain below the redox potential of the electrolyte’s redox couple to ensure an efficient dye regeneration. The Pc’s synthetic flexibility allows for

Fig. 8. Molecular structures of tetracarboxyl (TcPc) and tetrasulfo (TsPc) phthalocyanines [154–158].

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the systematic modification of their chemical structure, by selective functionalization at the periphery or inner cavity, for optimization of their physicochemical responses and improvement of processability. The performance of Pc-based devices has improved along the years, reaching 6%, by a rational design of the dye’s structure, but despite several major advances and the great number of molecules tailored over the years to meet the required conditions for DSSC applications, the performance remains consistently significantly lower to the state-of-the-art porphyrinand ruthenium-based analogues. As a matter of fact, there are two main factors that have limited the performance of phthalocyanines in DSSC: (i) Their strong tendency to aggregate, which degrades drastically the electron-injection efficiency of the excited Pc through a competitive non-radiative de-excitation process and delocalization of the exciton. In this context, the incorporation of a co-adsorbent, usually CHENO, in the dye-uptake solution has been a commonly used strategy to prevent the aggregation of Pcs and hence improve the performance. However, the necessary amount of CHENO per dye molecule to reach the optimal efficiency is usually huge (typically a CHENO/Pc ratio of 100:1), which constitutes a major drawback because it reduces the total number of adsorbed molecules on the surface, thus the absolute amount of injected electrons from the excited-dye to the electrode, which in fine limits the current density of the cell. (ii) Their low-lying LUMO level, which limits their electroninjection capability in the TiO2 CB. A common strategy consists to lower the TiO2-CB in order to improve the electron-efficiency (e.g. by incorporating higher contents of LiI) but presents a serious limitation: it concurrently decreases the VOC hence limiting the overall power conversion efficiency of the DSSC. In practice, the composition of the electrolyte is usually specific to Pc sensitizers in order to maintain the TiO2 CB edges low enough to enable an efficient electron-injection process. Concretely, the content

of LiI is usually larger (0.1–1.0 M) and that of TBP lower (typically 0.25 M) than in other electrolyte mixtures commonly used for others classes of dyes, such as Ru(II) complexes or porphyrins (typically 0.025–0.1 M LiI and 0.5 M TBP). Indeed, TBP and LiI have opposite effects: the former raises the TiO2 CB edges, while the latter shifts them down. On the other hand, TBP is also known for improving the F.F. Besides these ‘‘external” improvements (i.e.: co-adsorbent CHENO and electrolyte composition), others solutions were sought in molecular structure design of the Pc itself [152,153], and research divided in four main focal points:  Optimization of peripheral and non-peripheral substitution patterns  Modification of the anchoring group(s) and spacer(s)  Central metal and axial substitution effects  Improvement of light-harvesting efficiency (LHE) by expansion of the UV–Vis absorption or by cosensitization techniques 2. Symmetrically-substituted phthalocyanines (A4-type) The application of anchoring groups for the sensitization of metal oxide surfaces in DSSCs has largely been preferred over physical adsorption of the sensitizer. The most common and widely used group, in this regard, is the carboxylic acid. Tetracarboxyl ([M]TcPcs) and tetrasulfo ([M]TsPcs) Pcs belong historically to this first generation of Pc sensitizers that were tested in DSSCs (Fig. 8). In 1995, Shen et al. reported the sensitizer ZnTcPc in TiO2-DSSC, achieving an incident-photon-to-current-conversion efficiency (IPCE) of about 4% at 690 nm, which at the time was one of the highest value ever reported for Pcs in the field of photovoltaics [154]. Three years later, Deng et al. reported the tetrasulfonated gallium phthalocyanine (GaTsPc) on a titanium dioxide (TiO2) nanostructured electrode. GaTsPc molecules display a strong tendency to aggregate, and exist mainly as dimeric forms on the TiO2 films (H-aggregates; blue-shift of the absorption maximum (Q-band) at 620 nm in comparison with that of the

Fig. 9. Anchoring of Pc dyes PcBu and ZnPcBu onto a pre-treated TiO2 surface through in situ saponification, reported by Lindquist and co-workers [156] (for the sake of clarity, only one of the possible adsorption modes of the dye on TiO2 is represented (bridging-bidentate mode).

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Fig. 10. Symmetrical metalo-Pcs bearing carboxyl or sulfonyl-based anchoring groups, reported by Nazeeruddin et al. [157].

Table 1 Effect of CHENO on the photovoltaic parameters, and optimal PCE and IPCE values for DSSCs sensitized with Pcs 1–4 and 7–9, under simulated AM1.5G illumination (power 1000 W/m2).a Data derived from [157].

a b c d

Without CHENOb

With CHENO (Ratio Dye/CHENO of 1:1000)b,c

Dye

VOC (mV)

JSC (mA/cm2)

VOC (mV)

DVOC/VOCd

JSC (mA/cm2)

DJSC/JSCd

IPCE700 (%)

1 2 3 4 7 8 9

385 417 409 494 462 408 409

3.1 0.7 1.0 0.6 0.4 0.4 1.5

416 466 459 535 453 382 419

+8.1% +12% +12% +8.3% 2.0% 6.4% +2.4%

5.4 1.1 2.1 1.0 0.48 0.60 3.2

+74% +57% +110% +67% +20% +50% +113%

43 13 18 8 30 10 14

(ZnTcPc) (AlTcPc)

(ZnTsPc) (AlTsPc)

nm

gmax (%) 1.00 0.42 0.59 0.40 0.77 0.14 0.42

Composition of the electrolyte: 0.5 M LiI, 0.05 M LiI3 in propylene carbonate. Dye solutions were composed of 0.05 mM of Pc in EtOH + 3% of DMSO and 7% of TBP (v/v). 50 mM of CHENO was incorporated in the dye-uptake solution. Relative variation in comparison with the value obtained without CHENO.

monomeric species with a maximum at 675 nm) [155]. The authors proposed that the dye molecules are adsorbed on the TiO2 surface via supramolecular assemblies bound by electrostatic interaction between the negatively charged sulfonate (SO 3 ) and the positively charged TiO2 surface, and are oriented perpendicular or slightly tilted to the TiO2 surface. The photocurrent action spectrum showed a response close to zero at the maximum of absorption of the film (620 nm), and a maxima as low as 100 lA at 675 nm. The surface photovoltage followed the same trend, but with a contribution of 0.5:1 dimeric/monomeric species (100 and 200 lV, respectively). The authors concluded that only monomeric species adsorbed on the TiO2 surface can generate a photovoltaic response, while the dimeric stacked ones cannot, or at least not with the same efficiency. In contrast, when using GaTsPc in sublimed films, the photovoltage was much larger for the dimeric species with a contribution 1.2:1 with respect to the monomers, twice higher than in the films made by direct adsorption of dyes. The authors explained this difference in terms of changes in

morphology and orientation of the dimeric aggregates adsorbed on the TiO2 surface. Alternatively, among the early studies in Pc-sensitized solar cells, an attempt to use ester precursors was reported by Lindquist and co-workers, in order to meet the poor solubility associated with carboxylates [156] (Fig. 9). Following classical methods, an ester does not react spontaneously with the TiO2 surface as carboxylic acids or carboxylate salts do. For this purpose, the metal oxide surface was pre-treated with (CH3)3COLi to change the hydroxyl groups of the surface into oxygen anions and render the surface reactive toward ester functionalities, generating afterwards the corresponding carboxylate function through an in situ saponification reaction. Two butyl ester derivatives, the free-base PcBu and its zinc analogue ZnPcBu were prepared and adsorbed on TiO2 following this method. Absorption measurements of the sensitized surface revealed the presence of aggregates, and, in fact, these were accounted for the very low obtained IPCEs, which only reached 0.30% for the free base and 4.3% for the ZnPc at the Q-band.

Table 2 Effect of annealing temperature and acid treatment of TiO2 electrodes on the photovoltaic parameters of [FeII]TsPc/DSSC devices using a modified PEDOT:PSS counter electrode.a,b Data derived from [158]. TiO2 electrode Bare Bare Bare HCl-threated HNO3-threated a b c

Annealing temperature (°C)

C/108c

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm2)

300 350 450 450 450

7.08 6.97 6.23 (n/a) (n/a)

940 970 980 960 940

3.20 4.26 5.16 6.10 6.94

51 54 61 63 63

1.44 2.23 3.08 3.65 4.10

TBP (10 mM) was added into the dye-uptake solution in order to decrease dye aggregation. Composition of the electrolyte: 0.5 M KI and 0.05 M I2 in a 1:1 mixture (v/v) of AcCN and BuOH. Amount of adsorbed of dyes.

(%)

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Additionally, unsuitable energy levels for electron injection as well as fast back electron transfer were also implicated in the observed behaviour of the cell. The IPCE values were accompanied by equally unimpressive JSC, VOC and F.F. with the overall efficiency remaining consistently below 0.1%. Grätzel and co-workers reported in 1999 on the utilization of several symmetric Al(III) and Zn(II) Pcs as light harvesting materials (Fig. 10) [157]. Under optimized conditions and using CHENO as co-adsorbent and TBP as additive in the electrolyte, the device sensitized with dye 1 (ZnTcPc) achieved an overall PCE of 1% and reached a maximum IPCE value of 43% at 700 nm (Table 1), which were both significantly higher than all previously studied Pcs. Indeed, as opposed to earlier examples, where IPCEs had not exceeded 7% and PCEs were lower than 0.1%, this study was considered as a huge breakthrough, bringing about one order of magnitude increase in efficiency. This article was pioneering in many senses. First, the use of CHENO and TBP improved strikingly the photovoltaic performance of all devices (Table 1). In fact, the presence of CHENO and TBP in the dye-uptake solution during the fabrication of the devices decreased notably the aggregation of the Pcs on the TiO2 film, and hence the self-quenching of the phthalocyanine excited singlet state, which enhanced notably the electroninjection efficiency of the exited dye in the TiO2 CB. It is worth noting the high CHENO/Dye ratio of 1000:1 necessary to reach optimal performances in the final DSSC, witnessing the high degree of aggregation of these Pcs. The combined effect of CHENO and TBP considerably improved the JSC for all devices (up to +113%), which mostly contributes to the systematic increase observed in the PCEs (in some cases accompanied in greater VOC and F.F. that can also contribute to the improvement of the PCE). Regarding the molecular structure of the Pc dyes, the effect of the binding group, metal centre ion, length and nature of the spacer bridging binding moiety and Pc core, on the photovoltaic performances were also studied. Following the blooming example of Ru polypyridyl-sensitized devices, the use of a carboxyl-based binding group (dyes 1–4) was selected, as it allows an efficient attachment on the metal oxide surface and strong electronic coupling dye/TiO2. Another set of analogues endowed with sulfonic acid anchoring groups instead were also probed (dyes 7–9). These dyes were efficiently adsorbed on the TiO2 nanoparticles and displayed similar injection efficiency than their carboxyl-anchored analogues, but gave rise clearly to lower performances. In a different aspect, the metal in the central cavity altered between zinc (dyes 1, 3, 4, 7 and 9) and aluminium (dyes 2 and 8), was found to be critical toward the

Fig. 11. Symmetrically substituted metalo-Pcs reported by Gül and co-workers [159].

photovoltaic performance, with the Zn(II) derivatives performing substantially better than the Al(III) ones. Next, the influence of the spacer’s length was studied in ZnPcs 3, 4, and 9. In comparison with dyes 1 (ZnTcPc) and 7 (ZnTsPc) with no spacer, the performance drastically and systematically dropped for the analogues incorporating longer phenoxy bridges (–[OPh]– or –[OPhC2H4]) in both carboxyl- (1: 1.0% >> 3: 0.59% > 4: 0.40%) and sulfonyl- series (7: 0.77% >> 9: 0.42%). Four important conclusions were drawn from this study: 1) Zn (II) Pcs always outperformed noticeably their Al(III) analogues; 2) carboxyl-anchored dyes perform better than their sulfonyl analogues due to a stronger attachment and better electronic interaction with the TiO2 through the former; 3) a non-conjugated bridge between the Pc and the binding functional group brakes the electronic communication between the macrocycle and TiO2, resulting in a poorer electron efficiency, and hence decreases drastically the performance of these dyes in DSSC, and 4) the combined effect of CHENO and TBP reduces dye aggregation, which considerably improves the overall PCE. Following an alternative but complementary strategy, Balraju et al. focused on the optimization of the electrodes rather than the molecular design itself, to improve the efficiency of DSSC devices sensitized with tetrasulfonated iron phthalocyanine FeTsPc (Table 2) [158]. The counter electrode was made of a conductive polymer, a DMSO-treated PEDOT:PSS (carbon added) film, coated on an FTO glass to form a sandwiched type structure (PEDOT:PSS = poly(3,4-ethylenedioxythiophene) polystyrene sulfonate). This kind of electrode displays comparable efficiency and good stability in comparison with the traditional FTO/Pt commonly used in DSSC, but present the obvious advantage to be much cheaper than that made with the rare, and expensive platinum metal. Regarding the photoanode, both annealing temperature and chemical treatment modification had a large impact on recombination processes, hence on the photovoltaic performance of the DSSC. Increase of the annealing temperature had two effects: i) decreasing the surface area of the films (because of the formation of larger TiO2 nanoparticles), leading to a 12% decrease in the amount of adsorbed dye for the annealed film from 300 to 450 °C, and ii) the crystallinity of the nc-TiO2 film was improved by means of reduced grain boundary (bulk traps), which decreases recombination processes (Dye+ M TiO2(e) and/or TiO2(e)MI 3) and charge transfer resistance. Moreover, the higher degree of crystallization of the TiO2 nanoparticles was also believed to improve the electron injection efficiency of the excited dye into the metaloxide semiconductor. Thus, the slight decrease in the amount of adsorbed dye (which in principle would decrease the absolute amount of photogenerated electrons, and hence the JSC) was overcompensated by reduced charge recombination processes and charge transfer resistance, and supposedly, better electron injection, which improved significantly all the photovoltaic parameters of the devices. As a result, the overall PCE increased from 1.44% to 3.08% for the film annealed at 450 °C with respect to that at 300 °C. The acid pre-treatment also affected the photovoltaic performance of the cells. In comparison with bare TiO2 film annealed at the same temperature (450 °C), the acid-modified films displayed similar F.F. and slightly lower VOC, which was overcompensated by a significantly larger JSC that mostly accounts for their superior overall PCE (3.65% (HCl) and 4.10% (HNO3) vs. 3.08% (bare)). It is worth to notice that these efficiencies were obtained without the use of CHENO as co-adsorbent (although 10% (v/v) of TBP was added in the dye-uptake solution to decrease dye-aggregation). In a recent study (2018), Gül and co-workers reported three different metalo-Pc (with M = Zn or Co) symmetrically substituted with four peripheral anchoring group that are linked trough different hetero atoms (S-bridged or O-bridged) [159] (Fig. 11). Remark-

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M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64 Table 3 Dye-loading (DL) and photovoltaic data of the TiO2 Pc-sensitized filmsa,b,c under standard AM 1.5 sun light. Data derived from [159].

a b c

Dye

DL/108 (mol/cm2)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

4-HBa-ZnPc 4-HBa-CoPc 4-MKBa-CoPc

3.19 5.83 6.72

860 910 880

6.82 8.40 9.70

51 48 49

2.99 3.70 4.18

(%)

Dye-solutions consisted of 5 mM of Pc sensitizer in DMSO (dipping time 24 h). Composition of the electrolyte: 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in AcCN. The TiO2 films have a total thickness of 20 lm.

Fig. 12. Molecular structures of ZnPcGly and ZnPcTyr, reported by Sun, Sundström, and co-workers [160].

able performances were obtained for this kind of Pc (symmetrically substituted), with PCEs of ca. 3–4%. For the oxo-bridged Pcs, better performances were obtained when cobalt was used as metal centre (4-HBa-CoPc) instead of zinc (4-HBa-ZnPc). The electrochemical impedance studies revealed lower resistance and prolonged electron lifetime for the CoPc device, which was correlated to a better excited electron photogeneration and electron injection, and therefore explains their relative efficiency. Next, the effect of the heteroatom bridge was compared between oxo- and aza- bridged cobalt-Pcs. It was proposed that the S-bridged Pc cell displays a more efficient electron-injection and reduced recombination when compared to the O-bridged one. In addition, the presence of the sulphur atoms confers better light harvesting of the Pc, factor that should also contribute, in part, to the differences in the photovoltaic performances. These three factors were taken into account to explain the better overall efficiency of 4-MKBa-CoPc (4.18%) over 4-HBa-CoPc (3.70%) (see Table 3). In a different aspect, one of the major issues of these kinds of Pcs is their poor solubility in common organic media (dichloromethane, chloroform, alcohols, acetonitrile, etc.), which renders synthetic procedures and purification difficult. This also raises other technical difficulties during the fabrication of the DSSC device itself, because the Pc should display enough solubility in the dye-uptake solvent for an efficient adsorption. Moreover, the high-tendency to aggregate of these systems (as for Pc dyes in

general) constitutes one of the main reasons for their low efficiency in an operative DSSC. In 2002, Sun, Sundström and coworkers attempted to improve the molecular design of Pcs by incorporating amino acid groups at the peripheral positions of the Pc (Fig. 12) [160]. The performances in DSSC of ZnPcTyr incorporating tyrosine groups, was compared to those of a ZnPcGly analogue with glycine’s. It was observed that the presence of tyrosines promoted higher solubility and disaggregation of the molecules on the TiO2 surface, and the obtained Pcs proved to have the necessary driving force for electron injection, as supported by electrochemical studies and calculations. Both dyes displayed good solubility in EtOH, but ZnPcTyr also exhibited reduced dye-aggregation by the virtue of steric repulsion effect of the bulkier tyrosine group. In consequence, ZnPcTyr/DSSC displayed a 4-fold improvement of the overall PCE with respect to ZnPcGly/DSSC, with and without the use of CHENO and TBP (Table 4). This was explained in terms of faster electron recombination for the glycine derivative, as established by time-resolved transient absorption measurements.

3. Unsymmetrically-substituted phthalocyanine dyes (AAABtype; Fig. 13) 3.1. Background and history The article published by Gratzel et al. in 1999 and discussed in Section 2 (vide supra; Fig. 10 and Table 1), drew on several important findings, and was, in fact, a landmark in setting the different areas of research in the molecular design of phthalocyanines for DSSCs [157]. Other than the study of several symmetrical Pcs, this work also reported on the photovoltaic performance in DSSCs of a number of unsymmetrical Pcs with a bulky tert-butyl peripheral substitution (Fig. 14) [157]. For various symmetrical Pcs reported in this work, it was shown that the dual use of CHENO and TBP was necessary to prevent aggregation of the dye on the TiO2 surface, resulting in a systematic improvement in device performances (Table 5). Instead, in the case of the unsymmetrical derivatives, the presence of bulky tert-butyl groups afforded good solubility in organic media and prevented aggregation. The latter

Table 4 Optical parameters and photovoltaic data under simulated 1 sun illumination (power 100 mW/cm2) of the TiO2 films sensitized with ZnPcTyr and ZnPcGly.a,b,c Data derived from [160]. Dye

TBP

CHENO/Dye Ratio

Aa/Amf

IPCE at 690 nm (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

ZnPcTyr

no yesd no yesd

0:1 200:1e 0:1 200:1e

0.80 0.59 1.01 0.83

6.9 24.2 4.0 5.5

360 360 310 320

1.27 2.25 0.36 0.61

64 67 65 68

0.29 0.54 0.07 0.13

ZnPcGly a b c d e f

(%)

Devices were made with single-layer TiO2 films of 7.6 lm thickness. Composition of the electrolyte: 0.5 M LiI and 0.05 M I2, in propylene carbonate solution. For the preparation of the photoanodes, the TiO2 films were immerged in the dye solutions (0.05 mM of Pc in EtOH; containing 3% (v:v) of DMSO for ZnPcGly). Incorporating TBP (7%, v/v) in the dye-uptake solution. Incorporating 10 mM of CHENO in the dye-uptake solution. Ratio of absorbance between the aggregate band (Aa) and that of monomer (Am).

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and weak electronic coupling between the LUMO of the dye and the Ti 3d orbital caused by the poorly conjugated and flexible bridge between the binding moiety and the Pc core. Following this unprecedented development, the appearance of articles focusing on Pc design and relationship with photovoltaic performance in DSSCs was exponential over the coming years.

3.2. Optimization of peripheral and non-peripheral substitution patterns

Fig. 13. Typical structures of unsymmetrically-substituted AAAB-type Pcs used for DSSC applications.

could be appreciated by the more modest improvement in the JSC (+20 to 80%) when using CHENO for the unsymmetrical derivatives (5, 6, 10) with respect to the symmetric analogues (3, 4, 9, respectively; DJSC/JSC = +67 to 113%). Moreover, while a systematic increase in the VOC was observed for the later ones (+2.4 to 12%), it tended to decrease for the formers (9.5 to +2%). Surprisingly, dye 11 tethered with two 4-sulfophenoxy anchoring groups at the b,b’-positions of the Pc exhibited significantly lower performances compared to the similar structured dye 5 with only one at the b-position (PCE = 0.14% vs. 0.50%). As well unexpected, the unsymmetrical Pcs 5, 6, and 10 achieved a relative poor PCE (0.12–0.50%) despite reduced aggregation, and most often underperformed their symmetrical analogues 3, 4, 9 (0.40–0.59%). Notably, these efficiencies are considerably lower than that of A4-type ZnTcPc reported in the same work and under same conditions (dye 1, g = 1.0%; vide supra), for which the carboxyl groups are directly connected to the Pc macrocycle. These surprising results can be explained by the lack of directionality in the excited state,

Studies on peripheral substitution patterns of Pcs met an important boost with ZnPc sensitizers PCH001 (Nazeeruddin and co-workers [161]) and TT1 (Nazeeruddin, Torres and co-workers [148]; Fig. 15), where the reported efficiencies climbed for the first time to over 3% in 2007 (Table 6, entries 1 and 2). Both dyes employed three bulky tert-butyl groups in the periphery of the macrocycle to suppress stacking and enhance solubility. Nevertheless, their huge asset laid on the introduction of directionality and the so-called ‘‘push-pull” approach. As opposed to previously synthesized Pcs, PCH001 and TT1 achieved proper adjustment of the electron densities of the donor moieties and adequate MO levels for efficient charge transfer to take place from the LUMO of the dye and the Ti 3d orbital. In terms of anchoring groups, PCH001 was grafted with two adjacent carboxyl groups, whereas TT1 bore one carboxyl directly connected to the macrocycle. The most striking result for PCH001 was the large raise of the IPCE response reaching a maximum value of 75% at 700 nm, accompanied by a JSC of 6.5 mA/cm2, a VOC of 635 mV and a F.F. of 74%, resulting in an overall PCE of 3.05% under simulated AM1.5G one sun illumination. TT1 showed slightly improved values (IPCEmax at 680 nm = 80%, JSC = 7.60 mA/cm2, VOC = 617 mV and F.F. = 75%) to yield a PCE of 3.5% (Table 6, entry 2). This difference between the two was assigned to a more efficient electron-injection pathway into the TiO2 conduction band in the case of TT1, due to a stronger ‘‘push-pull” character. In fact, the carboxyl group being conjugated to the aromatic ring clearly boosts the directionality of the dye. An analogue dye of TT1 anchored trough a carboxyphenyl group was first reported in 2008 by Nagata (Dye 2, g = 1.16%; Table 6, entry 4) [167], followed by Nazeeruddin, Torres and collaborators in 2009 with improved efficiency (TT3, g = 2.2%; Table 6, entry 3) [162]. The same dye was also reported later by Mori and

Fig. 14. Molecular structures of unsymmetrical Pcs with a tert-butyl peripheral substitution (5, 6, 10 and 11) and symmetrical Pc analogues (3, 4 and 9), reported by Grätzel and co-workers [157].

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Table 5 Effect of CHENO on the photovoltaic parameters and optimal PCE values for DSSCs sensitized with Pcs 3–6 and 9–11, under simulated AM1.5G illumination.a Data collected from [157]. Without CHENOb

a b c d

Dye

VOC (mV)

JSC (mA/cm2)

With CHENO (Dye/CHENO ratio of 1:1000)b,c DVOC/VOCd VOC (mV) (%)

9 (bench.) 10 11 3 (bench.) 5 4 (bench.) 6

409 355 422 409 467 494 465

1.5 0.5 0.4 1.0 0.8 0.6 1.47

419 348 382 459 435 535 437

+2.4% +2.0% 9.5% +12% 6.9% +8.3% 6.0%

JSCd (mA/cm2)

DJSC/JSCd (%)

gmax

3.2 0.6 0.5 2.1 1.5 1.0 1.6

+113% +20% +25% +110% +88% +67% +81%

0.42 0.18 0.14 0.59 0.50 0.40 0.12

(%)

Composition of the electrolyte: 0.5 M LiI and 0.05 M LiI3 in propylene carbonate solution. Dye solutions were composed of 0.05 mM of Pc in EtOH + 3% of DMSO and 7% of TBP (v/v). 50 mM of CHENO was incorporated in the dye-uptake solution. Relative variation in comparison with the value obtained without CHENO.

Fig. 15. Molecular structures of PCH001 [161], TT1 [148], and TT3/2/p-PcS2 [162,167,169,170].

Table 6 Photovoltaic data under simulated one sun illumination (power 100 mW/cm2) of the TiO2 films sensitized with, PCH001, TT1, TT3, dye 2, and PcS2. Entry

1a 2b 3b 4c,d 5e 6e a b c d e f g

Dye

PCH001 TT1 TT3 2 PcS2 p-PcS2

Year

2007 2007 2009 2008 2010 2014

Dye-uptake solution (in EtOH) [Dye] (mM)

[CHENO] (mM)

CHENO/Dye Ratio

0.02–0.03 0.05 0.05–0.1 0.05 0.05 0.05

60 10 120 5 10 1

2000:1–3000:1 200:1 1:1200–1:2400 100:1 200:1 20:1

Film thickness (lm)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

[6+4]f [10+4]f [9–10+4]f 10g [7–8+5–6]f 6.5g

75 80 56 31 30 18

635 617 ± 20 610 ± 10 460 580 563

6.5 7.60 ± 0.20 4.8 ± 0.20 3.9 5.3 1.99

74.3 75 ± 2 74 ± 2 65 74 70.4

3.05 3.52 2.20 1.16 2.3 0.79

Ref

(%)

[161] [148] [162] [167] [169] [170]

Composition of the electrolyte: 0.05 M LiI, 0.05 M I2, and 0.5 M TBP in a 1:1 mixture (v/v) of valeronitrile and AcCN. Composition of the electrolyte: 0.6 M BMII, 0.025 M LiI, 0.05 GuNCS, 0.04 M I2 and 0.28 M TBP in a 15:85 mixture (v/v) of valeronitrile and AcCN. Composition of the electrolyte: 0.04 M I2, 0.4 M LiI, 0.4 M THAI and 0.3 M NMB in AcCN. Retionic acid [0.1 M], TBP 7% (v/v) and DMSO 3% (v/v) were additionally added in the EtOH dye-uptake solutions. Composition of the electrolyte: 0.6 M DMPII, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in AcCN. Double-layered TiO2 films: thickness of the transparent and scattering layer, respectively. Single-layered TiO2 film.

co-workers (PcS2 or p-PcS2) under different conditions with comparable or lower efficiencies (2.3% and 0.79%; Table 6, entries 5 and 6) [169,170]. Further molecular engineering of Pcs came from the modification of the peripheral and/or non-peripheral substituents (nature and number), which can be seen as an approach that consists in changing the tert-butyl groups in these three lead designs, that

are PCH001, TT1 and TT3. In the discussion below, we followed this (arbitrary) classification in order to focus on this first aspect of the molecular design of Pcs, independently to that of the anchoring group(s) (which will be detailed afterward in Section 3.3). These structural modifications seek two main aims (cumulative or not) to improve the performance in DSSC: 1) reduce dye aggregation by incorporation of bulkier groups (steric repulsion), and 2) induce

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M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

Fig. 16. Schematic representation of the variation of peripheral and non-peripheral substitution patterns from the PCH001 design.

Fig. 17. Molecular structure of PCH003 reported by Giribabu and collaborators [163,164].

a ‘‘pushing” effect by introducing groups with a strong electrondonor character (electron-releasing groups). 3.2.1. Variation of the peripheral and non-peripheral substitution patterns in Pc dyes bearing a 2-succinic acid moiety as anchoring group (PCH001 analogues; Fig. 16) In an attempt to further intensify the ‘‘push-pull” effect of PCH001, Giribabu et al. reported on a new analogue PCH003 decorated with six butyloxy groups at the a-peripheral positions to act as electron releasing counterparts (‘‘push”) and also to promote

higher solubility (Fig. 17) [163,164]. In terms of attaching moiety, the same double-carboxyl structure as for PCH001 was employed. Contrary to the expected, the obtained efficiency only reached 1.13%, as a consequence of a low-lying LUMO level unsuitable for efficient electron injection, which was mirrored in the low IPCE response with a maximum value of only 25% at 710 nm, in agreement with the poor JSC of 2.81 mA/cm2 (Table 7). The same authors described the synthesis of two other Pc analogues of PCH001, the dyes DMPCH-2 and DMPCH-3 (Fig. 18), distinguished for the steric hindrance induced by the 3,4-dimethoxy phenyl or 2,6-dimethoxy phenoxy substituents [165]. Consequently, both dyes display only low-aggregation tendency as supported by UV experiments, which is even more remarkable given that no co-adsorbent was used in this study. Regarding the MOs energy, deduced from electrochemical measurements and optical data, the HOMO level pointed out that the electron injection process dye* ? TiO2 is thermodynamically possible in each cases,  while the LUMOs lie below the redox potential of I 3 /I , making dye-regeneration feasible. However, despite adequate MOs energy levels, the PCEs did not exceed 1.07% at full sun illumination, with the JSC remaining consistently very low in agreement with the low IPCE response that did not exceed 20% (Table 8). 3.2.2. Variation of the peripheral and non-peripheral substitution patterns in Pc dyes bearing a 4-carboxyphenyl moiety as anchoring group (TT3 analogues; Fig. 19) One of the first reported examples of such variation came in 2008 by Imahori and co-workers, who introduced six 4-tertbutylphenyl groups at the peripheral b-positions of a phthalocyanine

Table 7 Photovoltaic data under simulated AM1.5G conditions of the TiO2 films sensitized with PCH003 and benchmark PCH001. Entry

Dye

1b 2b

PCH001 PCH003

3c

PCH001 PCH003

a b c d e

Dye-uptake solution (in EtOH) [Dye] (mM)

[CHENO] (mM)

CHENO/Dye Ratio

0.02–0.03 0.02–0.03 0.02–0.03 0.02–0.03 0.05 0.05 0.05 0.05

60 60 60 60 10 10 10 10

2000:1–3000:1 2000:1–3000:1 2000:1–3000:1 2000:1–3000:1 200:1 200:1 200:1 200:1

Ilum. (sun)

Film thickness (lm)a

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

1 1 0.5 0.1 1 1 1 1

[6+4] [8+4] [8+4] [8+4] [10+4] [10+4] [10+4] [10+4]

75 25 (n/a) (n/a) 67d 71e 16d 16e

635 525 510 466 620 ± 30d 633 ± 30e 517 ± 30d 510 ± 30e

6.5 2.81 1.54 0.30 6.20 ± 0.1d 6.25 ± 0.1e 1.94 ± 0.1d 1.33 ± 0.1e

74.3 76.4 76.8 78.5 73 ± 3d 74 ± 3e 77 ± 3d 77 ± 3e

3.05 1.13 1.14 1.10 2.80d 2.86e 0.77d 0.52e

Ref

(%)

[161] [163]

[164]

Double-layered TiO2 films: thickness of the transparent and scattering layer, respectively. Composition of the electrolyte: 0.6 M BMII, 0.05 M I2, 0.05 M LiI, and 0.5 M TBP in a 1:1 mixture (v/v) of valeronitrile/AcCN. Two different electrolytes were used in this study (see footnotes d and e). Performances obtained with electrolyte A7117: 0.6 M BMII, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in a 85:15 mixture (v/v) of AcCN and valeronitrile. Performances obtained with electrolyte M1: 0.6 M BMII, 0.025 M LiI, 0.05 M I2 and 0.275 M TBP, and 0.05 M GuNCS in a 85:15 mixture (v/v) of AcCN and valeronitrile.

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Fig. 18. Molecular structures of sterically demanding Pcs DMPCH-2 and -3 reported by Giribabu and co-workers [165].

Table 8 Photovoltaic data of DSSCs sensitized with DMPCH-2 and -3 under various light intensities illumination (AM1.5G).a,b,c,d Data derived from Ref. [165]. Dye

Illumination (Sun)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

DMPCH-2

0.1 0.5 1 0.1 0.5 1

– – (>20)e – – (n/a)

383 463 604 441 488 504

0.30 1.66 3.26 0.22 1.20 2.33

38 64 67 74 75 75

0.46 0.97 1.07 0.78 0.87 0.89

DMPCH-3

a b c d e

(%)

Double-layered TiO2 films of [(8–10)+4.5] lm thickness (transparent and scattering layer, respectively). Active area of 0.158 cm2. Dye-uptake solutions consisted of DMPCH-2 or 3 (3  106 M) in THF (soaking time >18 h). Composition of the electrolyte: 0.6 M DMII, 0.03 M I2, 0.05 M LiI, 0.05 M GuNCS and 0.25 M TPB in a 15/85 mixture (v/v) of valeronitrile and AcCN. Value roughly estimated from the IPCE graph given in the Supplemental Information of the article from [165].

Fig. 19. Schematic representation of the variation of peripheral and non-peripheral substitution patterns from the TT3 design.

macrocycle (ZnPc I-1 and H2Pc I-2; Fig. 20) to impede stacking phenomena and reduce the number of possible regioisomers (only one in these cases) [166]. The latter is expected to simplify the monolayer configuration on the metal oxide surface and thus, led to a better packing of the molecules. These dyes were also decorated with two benzoic acids for a stronger attachment to the TiO2 surface (therefore this dye is formally not an analogue of TT3 from a strict structural consideration, but will be dealt in this section). The authors’ studies revealed that the induced steric

hindrance, in this case, was very efficient in preventing aggregation, in a way that the use of CHENO was unnecessary (Table 9, entry 1). This was, however, not sufficient a factor to bring about optimized results, since the driving force for electron injection was far from satisfactory. Specifically, the H2Pc-sensitized (I-2) cell did not show any photocurrent response because the LUMO level of the Pc lies bellow the TiO2-CB, which disables electron injection into the metal oxide. The zinc analogue possessed more appropriate energy levels, but the overall PCE was limited to 0.57% (Table 9,

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Fig. 20. Phthalocyanine dyes ZnPc I-1 and H2Pc I-2 [166] (left; Imahori et al.), and ZnPcs SS-1 [168] (middle; Sastre-Santos et al.) and T-4 [167] (right; Taya et al.).

Table 9 Parameters and photovoltaic data under simulated AM1.5G sun illumination (power 100 mW/cm2) of the TiO2 films sensitized with and ZnPcs I-1, SS-1 and T-4. Entry

Dye

1a,b

I-1

2c,d 3a,e

SS-1 T-4

Dye-uptake solution [Dye] (mM)

[CHENO] (mM)

CHENO/Dye Ratio

0.05 0.05 0.1 0.05

– 2.5 – 5

0:1 50:1 0:1 100:1

IPCE(Q)f (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

4.9 (n/a) 8.1 51

540 ± 20 540 ± 30 530 500

1.47 ± 0.05 1.44 ± 0.06 2.26 4.5

71 ± 3 70 ± 3 61 70

0.57 ± 0.03 0.54 ± 0.03 0.73 1.59

Ref

(%)

[166] [168] [167]

Devices consist of single-layered TiO2 films of 10 lm thickness. Composition of the electrolyte: 0.1 M LiI, 0.05 M I2, 0.6 M DMPII, and 0.5 M TBP in MeCN solution. c Devices consist of double-layered TiO2 films of [(8–10)+(3–4)] lm thickness (transparent and scattering layer, respectively). d Composition of the electrolyte: 0.1 M LiI, 0.03 M I2, 0.5 M TBP, 0.1 M GuNCS, 1 M BMII in a 85:15 mixture (v/v) of acetonitrile and valeronitrile. e Data obtained for the device under optimized conditions with respect to the electrolyte composition (four different electrolytes were tried; the data presented in the table were obtained with an electrolyte composed of 0.04 M I2, 0.4 M LiI, 0.4 M THAI and 0.3 M NMB in AcCN solution). f Maximum IPCE value in the Q-band region. a

b

Fig. 21. Molecular structures of sterically crowded phthalocyanines dyes PcS’s bearing a 4-carboxyphenyl moiety as anchoring group, reported by Mori and collaborators [169,170,173] (note that PcS5 exists under different regioisomers; for the sake of clarity only one is depicted (abbb)).

entry 1). As well, the IPCE did not exceed 4.9% (Q-band). Once again, the result was reasoned in terms of inadequate LUMO level of the dye (low-lying) with respect to the TiO2-CB, resulting in unfavourable charge injection kinetics, and poor electronic coupling between the dye and TiO2. Along the same lines, Taya and co-workers designed a planarly enlarged ZnPc (T-4; Fig. 20, right), in an attempt to efficiently confront stacking phenomena [167]. Although aggregation was strongly reduced, it was not completely suppressed since coadsorbent (Pc/CHENO ratio of 1:100) was still necessary to reach the

optimum PCE of 1.59% (1% without CHENO). The ratio is however considerably less than for other Pcs discussed previously in this section (vide supra). The JSC and IPCE response were both strongly improved with respect to the former dye (Table 9, entry 2), witnessing a better electron injection capability of this Pc despite being anchored through only one carboxyphenyl group (instead of two for ZnPc I-1). This can be mostly reasoned in terms of destabilization of the LUMO level caused by the presence of two acceptor groups in ZnPc I-1, which renders the electron-injection process unfavourable (this point will be broached more in detail in

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M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64 Table 10 Photovoltaic data of the TiO2 films sensitized with PcS5–6, and PcS13–15, under simulated AM1.5G one sun illumination (power 100 mW/cm2). Entry

1a b

2 3b

4c a b c d e f

Dye

PcS5 PcS6 PcS6 PcS6 PcS13 PcS14 PcS15 PcS15 (Co)

Dye-uptake solution [Dye] (mM)

[CDCA] (mM)

0.05 0.05 0.05 0.05 0.05 0.05 0.05 (n/a)

10 – – – – – – (n/a)

IPCEmax (%)

Film Thickness (lm)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

52 78 57 (n/a) (<70)d (<70)d (>75)d (n/a)

[7–8+5–6]e [7–8+5–6]e 7–10f (n/a) (n/a) (n/a) (n/a) (n/a)

580 630 615 610 610 600 610 (n/a)

4.8 10.4 6.29 11.0 10.9 11.5 12.8 (n/a)

77 70 74.6 70 70 70 68 (n/a)

2.1 4.6 2.89 4.7 4.7 4.8 5.3 <1%

Ref

(%)

[169] [170] [173]

[173]

Composition of the electrolyte: 0.1 M LiI, 0.6 M DMPImI, 0.05 M I2 and 0.5 M TBP in AcCN solution. Composition of the electrolyte: 0.1 M LiI, 0.6 M DMPII, 0.05 M I2 and 0.5 M TBP in AcCN solution. The Co(III/II)tris(2,20 bipyridine) tetracyanoborate complex was used as redox shuttle in the electrolyte instead of the conventional I/I 3 couple. Values roughly estimated from the IPCE graphs given in the main article and supplemental information from Ref. [173]. Double-layer TiO2 films: thickness of transparent sensitized-film and scattering layer, respectively. Single-layer TiO2 transparent sensitized-film.

Section 4). Overall, even though significant progress was made regarding the decrease of aggregation in these two studies, the overall PCEs of 0.57% for ZnPc I-1 and 1.59% for T-4, were not encouraging. In agreement with these results, Sastre-Santos and co-workers reported recently a similar structured ZnPc (SS-1) decorated with six peripheral tert-octylphenoxy groups, acting as electron-releasing moieties (‘‘push”); (Fig. 20, middle) [168]. Similar conclusions were drawn concerning aggregation, which was again strongly reduced (thought not fully avoided) by the peripheral bulky groups. As well, the performances of this new Pc (SS1) were quite unimpressive, and especially the poor obtained photocurrent. The authors proposed that the low dye-loading adsorption must be an important issue leading to a poor JSC value. In this regard, the use of co-adsorbent CHENO was also attempted in this work, but no sensitization of the TiO2 by SS-1 occurred under these conditions (uncoloured photoanodes), which supports this hypothesis. A new breakthrough in Pc molecular design came later in 2010 by Mori and collaborators [169,170], when the 2,6diphenylphenoxy moiety was introduced as an intriguing peripheral (PcS6) or non peripheral (PcS5) substituent (Fig. 21). The strong steric hindrance of the 2,6-diphenylphenoxy groups forces them to lie perpendicular to the planar Pc macrocycle, giving rise to a three-dimensional enlargement of the molecular structure, this being the rationale behind the triumph of the newly designed dyes. In particular, analogues PcS5 and PcS6, bearing three or six of the above-mentioned moieties, were synthesized and used in nanocrystalline-based solar cells. It is noteworthy that whereas PcS6 was isolated as one single isomer, PcS5 was obtained as a mixture of regioisomers. The UV–Vis spectra of both dyes showed sharp Q bands in solution, indicating efficient suppression of aggregation, with PcS6 presenting a superior optical profile. An excellent 4.6% overall efficiency, with an IPCE of 78% at the maximum of the Q-band, was achieved with PcS6 (Table 10, entry 1). Additionally, the best performance for the PcS6sensitized device was observed without the use of CDCA, whereas the less substituted derivative, PcS5, required the coadsorbent to reach the optimal efficiency (Pc/CDCA ratio of 1:200). In a latter study (Table 10, entry 2), the performances of PcS6 were also found to be much higher without(g = 2.89%) than with- (g = 0.33%) CDCA. Notably, it was suggested that the remarkable properties of 2,6-diphenylphenoxy substituents worked favourably not only by suppressing aggregation very efficiently, but also by blocking the interactions between the Pc aromatic surface and the I 3 ions present in the electrolyte, reducing the catalysis of recombination and unwanted dark currents [122,171,172].

In 2012, Mori and collaborators went a step further to implement methyl or methoxy groups on the thriving 2,6diphenylphenoxy moiety in an attempt to modulate and/or increase the electron-releasing character. This synthetic approach led to analogues PcS13-15 (Fig. 21), distinguished for better solubility, improved photoresponse and higher overall efficiencies (Table 10, entry 3) [173]. PcS15 yielded a PCE of 5.3%, attributed to the higher JSC compared to the benchmark PcS6 under the same condition, the latter showing a slightly improved PCE of 4.7% in this new study in comparison with the 4.6% previously reported. Interestingly, PcS13 exhibited similar VOC, JSC and PCE as PcS6, which indicates that the introduction of the methoxy groups on the outer benzene ring of the diphenylphenoxy moieties did not produce the expected enhancement in performance. In turn, however, PcS14 and PcS15 bearing respectively a methyl and methoxy group at the 4-position of the diphenylphenoxy, showed slight improvements in comparison with PcS6 (unsubstituted at the 4-position). The improvement of PCE in the order PcS6 (4.7%) < PcS14 (4.8%) < PcS15 (5.3%) followed the increase of electron-donating ability of these substituents H (PcS6) < methyl (PcS14) < methoxy (PcS15), and this was accounted for the distinct performances. Mori and collaborators also attempted to use Co-based electrolytes in the photovoltaic studies of Pcs15, enticed by the success they have had in porphyrin-DSSCs. Co(III/II)tris(2,20 bipyridine) complex has emerged as an alternative redox shuttle to the conventional electrolytes based on the iodine/triodine couple, with its main advantage being the possibility to increase the VOC of a device because of a lower redox potential than I/I 3 (theoretical maximum VOC value is defined by the difference between the potential of the redox couple and the Fermi level of TiO2 [95–97]). Unfortunately, the PcS15 cell showed poor efficiency (<1%) with the cobalt–based electrolyte (Table 10, entry 4). 3.2.3. Variation of the peripheral and non-peripheral substitution patterns in Pc dyes bearing a carboxyl moiety as anchoring group (TT1 analogues; Fig. 22) Based on the excellent performances obtained with their peripherally crowed PcS15 bearing 2,6-diphenyl-4methoxyphenoxy substituents, Mori and co-workers prepared the analogue PcS17, as well as PcS18 with smaller 2,6diisopropylphenols, both new Pcs distinguished for the direct bonding of the carboxyl to the Pc core (Fig. 23 and Table 11) [174]. It was observed that direct attachment brought about better electron injection kinetics, even though PcS17 performed poorly due to lower adsorption density of the sensitizer onto the TiO2 surface. In fact, molecular modelling was also conducted in this study, and it revealed that the COOH binding sites were located deep

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Fig. 22. Schematic representation of the variation of peripheral and non-peripheral substitution patterns from the TT1 design.

Fig. 23. Molecular structures of PcS17 and PcS18 reported by Kimura et al. [174].

within the three-dimensional structure, with restricted access to the TiO2. On the other hand, the favourable electron injection properties of PcS18 together with the less sterically compromised architecture, promoted easier attachment to the metal oxide and increased packing density, giving the desired result, thus, a boost in the PCE up to 5.9%. At the same time, Nazerruddin, Torres and co-workers reported on the sterically hindered Pc TT58, substituted at the periphery with six 2,6-diphenylphenoxy groups (Fig. 24) [150]. As well, the optimal photovoltaic performances (Table 12) were obtained without CHENO, witnessing that aggregation was successfully and fully suppressed in TT58. In comparison with the tert-butyl substitution of TT1, the steric hindrance induced around the Pc core by the bulkier substitution in TT58 can be appreciated by looking at the geometry-optimized structures presented in Fig. 24. The IPCE reached impressive values with maximum superior to 90% in the Q-band region, which is so far the highest ever reported for a Pc dye. TT58 achieved an overall PCE of 6.05% and 5.57% under 0.1 and 1 sun respectively, making it one of the best performing dyes of this new generation of sterically hindered Pcs. It is noteworthy that the molecular design of TT58, bearing an equally bulky peripheral substitution as PcS17, came to opposite conclusions in terms of adsorption density, giving out a successful performance. Following their strategy previously developed for sterically demanding PcS dyes (vide supra), Mori and collaborators further prepared a new series of alkoxy-substituted analogues PcS19–21 (Fig. 25) [151]. These new analogues were endowed with alkoxy chains of different length, which sought three aims: prevent aggregation of the Pc, fill the space among dyes on the nanocrystalline semiconductor surface (by covering around the macrocycle core) and, as a strategy successfully implemented in the previous PcS13-15, enhance the electron-releasing character of these substituents. Moreover, the reason behind the variation of number and length of the alkoxy chains might rely on finding a good

compromise of these aims without precluding adsorption densities of the Pcs, an issue previously encountered for PcS17 (vide supra). The new PcS19 was decorated with twelve octyloxy chains, and PcS21 with six octyloxy chains together with a methoxy substituent in order to reinforce the ‘‘push-pull” character. PcS20, on the other hand, had twelve shorter butoxy chains around its core. PcS20 showed impressive IPCE values over 86% between 600 nm and 720 nm, and achieved an outstanding PCE of 6.4% under simulated AM1.5G full sun illumination, which stands out as the actual record efficiency to date in phthalocyanine-based DSSC (Table 13). As regards the points of concern of the successful PcS20, aggregation was not completely impeded due to the smaller size of the butoxy chains. Compared to PcS20, PcS19 displays a very similar IPCE response over the whole absorption range and quite high F. F., which indicates that both dyes performed efficiently. However, a 0.8 mA/cm2 loss in the JSC was observed for PcS19 with respect to PcS20, which mostly accounts for the lower PCE (5.9% vs. 6.4%). This loss in the JSC was mainly ascribed to the difference in dyecoverage achieved by the two dyes, three times less for the former, (with C = 2.7  1011 and 8.3  1011 mol/cm2, respectively) which reduced the absolute amount of photogenerated electrons. The molecular structure of the two dyes being the same, except for the alkoxy chains at the peripheral phenyl groups, the difference in dye-coverage can be therefore explained by the longer octyloxy than butyloxy chains. In turn, the 20 mV gain in the VOC for PcS20 with respect to PcS19 was assigned to reduced charge recombination Dye*/electrolyte due to the presence of the longer octyloxy chains protecting more efficiently the Pc core against the oxidized species (I 3 ). This was reflected in the difference of the electron lifetime between the two dyes, PcS19 having an electron lifetime about 3 times longer than PcS20. PcS21 displayed a much lower IPCE response than the two other dyes over the whole spectral window, and especially between 600 and 720 nm with values half less (43%) than PcS19 and PcS20 (86%), matching well the poorer observed photocurrent of only 9.8 mA/cm2. With comparable surface coverage, VOC and F.F. as those of PcS20, the poorer JSC and IPCE values of PcS21 witness that the later dye performed less efficiently than the two others. Based on the IPCE spectrum of PcS21 that displays less sharply split Q-band peaks, the authors assumed higher degree of aggregation than the other dyes. This might be explained by a less efficient ‘‘wrapping” of the Pc core by the octyloxy chains located at only one ortho position of the peripheral phenyl groups in PcS21, in contrast with the two sets of alkyl chains at the ortho,ortho’-positions in the case of PcS19 and PcS20. Moreover, the authors assumed that PcS21 bearing the smallest amount of electron donor groups (EDGs), showed the poorest directionality characteristics. Both factors, electronic

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Table 11 Photovoltaic data under simulated AM1.5G one sun illumination (power 100 mW/cm2) of the TiO2 films sensitized with PcS17, PcS18, and benchmark PcS15.a Data derived from Ref. [174].

a b c d

Dyeb

Dye-loading (mol/cm3)

Film thickness (lm)c

Timed (h)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

PcS15

10.5104

[7.8+4] [7.8+4] [7.8+4] [7.8+4] [7.8+4] [7.8+4] [12+5.7] [12+5.7]

24 0 24 0 48 0 48 0

610 600 586 566 622 598 613 600

12.8 12.0 11.4 10.3 12.8 11.7 13.7 12.3

68 70 69 69 71 70 70 70

5.3 5.0 4.6 4.0 5.7 4.9 5.9 5.2

4

PcS17

8.810

PcS18

13.6104

(%)

Composition of the electrolyte: 0.1 M LiI, 0.6 M DMPII, 0.05 M I2 and 0.5 M TBP in AcCN solution. Dye-uptake solutions were composed of 0.05 M of Pc in toluene. Double-layered TiO2 films: thickness of the transparent and scattering layer, respectively. Period of time after cell fabrication and before measurement.

Fig. 24. Molecular structures of TT1 and TT58 reported by Torres’s group and molecular modelling of the geometry-optimized structures at the semi-empirical level (PM6 method); only one isomer was computed for TT1 (white: hydrogen, light blue: carbon, dark blue: nitrogen, red: oxygen, and pink: zinc, atoms).

and aggregation-related, should explain the drop observed in the JSC for PcS21. In another interesting example, Giribabu and co-workers described the dye DMPCH-1 (Fig. 26) distinguished for a direct

bonding of the carboxyl group to the Pc and using the same 3,4dimethoxyl phenyl peripheral substitution as for the DMPCH-2 analogue (bearing a 2-sucicinic acid moiety as anchoring group) previously discussed in Section 3.2.1 (vide supra). Except an

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Table 12 Photovoltaic data under various light intensity (simulated AM1.5G conditions) of the TiO2 films sensitized with TT58.a,b,c,d Data derived from [150]. Dye

I0e (mW/cm2)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

TT58

9.5 50 100 9.5 50 100

– – >90 – – (n/a)

547 587 601 566 604 618

1.4 6.94 13.1 1.32 6.68 12.8

75 73 71 76 72 69

6.05 5.94 5.57 6.02 5.84 5.46

TT58+CHENOf

a b c d e f

(%)

All data reported were measured one day after cell assembling. Double-layered TiO2 films of [7+5] lm thickness (transparent and scattering layer, respectively). Dye-uptake solutions were composed of TT58 [0.1 mM] in toluene. Composition of the electrolyte: 0.6 M DMII, 0.03 M I2, 0.05 M LiI, 0.05 M GuNCS and 0.25 M TBP in a 15/85 (v/v) mixture of valeronitrile and AcCN. With I0 the intensity of incident photon flux CHENO (0.05 M) was added to the dye-uptake solution, which corresponds to a CHENO/Dye ratio of 1:2.

Fig. 25. Molecular structures of sterically crowded phthalocyanines PcS19-21 reported by Mori et al. [151].

Table 13 Adsorption densities (C) and photovoltaic data under simulated AM1.5G one sun illumination (power 100 mW/cm2) of the TiO2 films sensitized with PcS19–20.a,b Data derived from [151]. Dye PcS19 PcS20 PcS21 PcS20 a b c d e

Electrolytec,d I/I 3 I/I 3   I /I3 Co(III/II)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm2) 2.7 8.3 8.3 (n/a)

(<86)e 86 (43)e (n/a)

620 600 618 614

14.3 15.1 9.8 4.2

67 71 69 52

5.9 6.4 4.2 1.4

C/1011

(%)

Double-layered TiO2-films have a total thickness of 17.7 lm (including 5.4 lm of scattering layer). Active area of 0.25 cm2. The iodine–based electrolyte was composed of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in AcCN solution. For the cobalt-based electrolyte, Co(III/II)tris(2,20 bipyridine) tetracyanoborate complex was used as redox shuttle. Estimated values based on the information given in the article from [151].

improved F.F., the photovoltaic performances of DMPCH-1 were, however, surprisingly lower than DMPCH-2 under the same conditions (Table 14), with an overall PCE of 0.74% under one sun against 1.07% for the later (note that CHENO was not considered in this study). In a recent work, Tejerina et al. reported on the synthesis of regioisomerically pure, non-peripherally (a)-substituted ZnIIPcs

and evidenced for the first time the influence of bulky aryl groups and their orientation on the adsorption density and photovoltaic performance in DSSC [175,176]. The suggested synthetic strategy aimed at reducing the number of obtained Pc regioisomers. In fact, normally, A3B-type Pcs having only one substituent attached at each peripheral b-positions are obtained as mixtures of numerous regioisomers, as in the case of Pc-2 (eight regioisomers; Fig. 27). By

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Fig. 26. Molecular structure of DMPCH-1 reported by Giribabu and co-workers [165].

using a bulky 3,5-di-tert-butylphenyl a-substituted phthalonitrile as starting reagent, the possible regioisomers formed for the A3B-type Pc were reduced to four, by means of strong steric repulsion between two neighbouring bulky groups. The polarity of the b-carboxyl group and the relative position of the a-aryl substituents toward it, produced enough differences in polarity between the products to allow for the isolation of pure regioisomers 1a and 1b by standard column chromatography. Instead, regioisomers 1c and 1d could not be separated since they only differ in the relative position of the aryl bulky group diametrically opposite to the carboxyl group. With regard to steric compromise, as can be seen also in Fig. 27, compound 1a was the less hindered around the anchoring carboxylic group, whereas 1c and 1d where the most ones, with two rigid and bulky groups pointing towards the COOH. By measuring the absorbance of the dyes released from the dye-stained films, the adsorption density was found significantly higher for 1a than for 1b and 1-c,d (Table 15). Interestingly when conducting the same experiment with a mixture of all the regioisomers, the density was lower than the one for 1a alone, proving that each isomer adsorbs in a different quantity onto the titania surface, respective to the steric compromise around the anchoring group. The photovoltaic properties of all compounds were measured, indicating the highest Jsc, Voc and overall efficiency for the less sterically hindered, and hence more adsorbed, Pc 1a. Additionally, an IPCE of 73% at the Q-band was observed. In this study, a comparison was also conducted with b-substituted Pc-2, which is more planar than the a-analogue, and it was shown, not only that the presence of co-adsorbent (CDCA) was necessary to obtain optimal results, impeding aggregation phenomena, but also that the overall efficiency was lower than the one of the Pc 1a-based device.

Later, the same authors reported another interesting work using semi-flexible 2,6-diphenylphenoxy groups at the non-peripheral positions (a-substitution) of a Pc substituted with a carboxyl group at one b-position (Fig. 28) [177]. In contrast with the previous rigid di-tertbutyl group, it was not possible in this case, to separate the eight possible obtained regioisomers by chromatography column (SiO2), and only 3 fractions could be isolated: 1a containing the two most strained regioisomers around the carboxyl group, followed by 1b (two regioisomers) and finally 1c the four less strained ones (unsubstituted at the position 25). Similar conclusions were drawn regarding the efficiencies obtained for the different regioisomers (Table 16): 1) The highest efficiencies were obtained without CDCA in the dye solution, emphasizing the low aggregation properties of these molecules, and 2) the greater steric hindrance around the anchoring group the lower adsorption densities were obtained, and consequently, lower Jsc and efficiencies. Overall, it was reached in this work the highest efficiency ever obtained for an a-substituted Pc in DSSC (g = 4.1%), which demonstrates that this kind of Pcs can be as effective as their bsubstituted analogues. 3.3. Modification of the anchoring groups and spacers After the successive breakthrough of TT1, and more recently TT58 and PcS19-20, in which the Pc is directly connected through a carboxyl group (vide supra), much attention was drawn in the recent years around the thorough study and optimization of the binding moiety as well as the spacer that connects it to the main macrocycle core. Several reports have demonstrated that their influence toward the photovoltaic properties is crucial, since they highly control electron dynamics and kinetics. In this context, Nagata and co-workers initially prepared a series of tert-butyl Pcs, where the carboxyl binding unit was connected to the main core via varying spacers (Fig. 29) [167]. Dye 2 anchored through a carboxyphenyl directly C-bonded to the Pc core, displayed strikingly higher JSC than dyes 1 and 3 connected through an ether linkage, which mainly accounts for the higher overall PCE (Table 17). The carboxyphenyl in dye 2 should allow a better electronic coupling between the Pc and the TiO2, enhancing the electron injection efficiency and hence the JSC with respect to the poorly or nonconjugated ether-linked spacer in 1 and 3. Moreover, the rigidly and linearly oriented carboxyphenyl should imply higher directionality in dye 2, while the more flexible anchoring groups in 1 and 3, induce a more tilted and loose orientation that should impact on the geometry adopted by the dye on the surface and on the recombination processes. Alternatively, Giribabu et al. reported PCH008, tethered with a conjugated methylenemalonic acid anchoring group, for which a PCE of 2.35% and 2.22% were achieved using two different electrolytes (Fig. 30 and Table 18) [164]. Surprisingly, however, these values are lower under the same conditions than those of the

Table 14 Photovoltaic data of DSSCs sensitized with DMPCH-1 under simulated AM1.5G illumination of various light intensities (those of DMPCH-2 analogue under one sun are reminded for comparison purpose).a,b,c,d Data derived from [165]. Dyed

Illumination (Sun)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

DMPCH-1

0.1 0.5 1 1

– – (<15)e (>20)e

449 500 518 604

0.17 1.00 1.94 3.26

75 75 74 67

0.61 0.71 0.74 1.07

DMPCH-2 a b c d e

Double-layered TiO2-films of [8–10+4.5] lm thickness (transparent and scattering layer, respectively). Active area of 0.158 cm2. Dye-uptake solutions was 3  106 M of Pc in THF (soaking time >18 h). Composition of the electrolyte: 0.6 M DMII, 0.03 M I2, 0.05 M LiI, 0.05 M GuNCS and 0.25 M TPB in a 15/85 (v/v) mixture of valeronitrile and AcCN. Values roughly estimated form the IPCE graphs given in the Supplemental Information of the article from [165].

(%)

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Fig. 27. Molecular structures of ZnIIPcs 1a-d (a-substituted) and 2 (b-substituted), reported by Tejerina et al. [175,176]; red arrows schematize the steric hindrance of the 3,5di-tert-butylphenyl groups toward the COOH group.

Table 15 Adsorption density (C) and photovoltaic parameters of the TiO2-DSSCs sensitized with ZnPc 1a–d and 2 on [8+4] lm thick TiO2 films.a,b Data derived from [175]. Dye (0.1 mM)c

[CHENO] (mM)c

C/108

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm2)

1 (mixture)d

0 30 0 30 0 0 0 10 30

3.0 (n/a) 3.9 (n/a) 2.1 1.2 (n/a) (n/a) (n/a)

523 495 577 546 533 512 523 490 514

7.4 5.0 8.8 7.6 2.4 0.8 5.5 6.6 7.0

72 70 73 72 73 71 72 75 74

2.8 1.7 3.7 3.0 0.9 0.3 2.1 2.4 2.7

1a 1b 1c/d 2 (mixture)d

a b c d

(%)

Composition of the electrolyte: 0.86 M DMII, 0.2 M LiI, 0.04 M I2, 0.29 M TBP and 0.05 M GuNCS in AcCN. Double-layered TiO2 films: thickness of the active and scattering layer, respectively. In EtOH solution. Mixture of all the regioisomers.

benchmark analogue PCH001, with a non-conjugated succinic acid group (PCE = 2.80% and 2.86%). This result was astonishing since the double bond connecting the main structure with the anchoring carboxyls not only increased directionality, but also the extinction coefficient of the new dye, which is an important parameter to reach greater JSC. On the positive side, this Pc analogue exhibited great stability at high temperatures, which is an important factor for suitability in solar cell applications. The same year, the groups of Nazeeruddin, Grätzel and Torres conducted a series of well-designed studies, reporting on the preparation of ZnPcs bearing a vast number of different spacers and adsorption sites [162]. Initially, four TT1 analogues, TT2-5, were prepared, with TT2 bearing a flexible and non-conjugated alkoxy linker, and the rest bearing conjugated spacers with varying

distances (Fig. 31, left panel, and Table 19, entry 1). In terms of electron-injection efficiency, time-correlated single-photoncounting (TCSPC) measurements were carried out in order to estimate the yields, with samples showing similar absorbance values at the excitation wavelength in order to ensure similar dye coverage. In agreement with calculations studies, the estimated yields were found higher than 90% for all Pcs and with fast kinetics (173–277 ps). Among them, the benchmark TT1 displayed higher light-to-energy conversions and faster kinetics (170 ps) with respect to the other dyes. Next, laser transient absorption spectroscopy (TAS) experiments revealed that, except for TT3, the electron recombination dynamics for these Pcs were ten times slower than that of the standard champion ruthenium dye N719. In contrast, electron recombination kinetic studies in complete

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Fig. 28. Molecular structures of the mixtures of regioismomers ZnIIPcs 1a–c non-peripherally (a-)substituted with semi-flexible bulky groups, reported by Terejina et al. [177]; red arrows schematize the steric hindrance between the 2,6-diphenylphenoxy and COOH groups.

Table 16 Adsorption density (C), dipping time and photovoltaic parameters of the TiO2-DSSCs sensitized with ZnPc 1a–c on [8+4] lm thick TiO2 films.a,b Data derived from [177]. Dye/Conc. (mM)c

[CHENO]c (mM)

Dipping Time (h)

C/108

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm2)

1/0.01

– – 0.05 – – 0.05 – – 0.05 – – 0.05

6 13 6 6 13 6 6 13 6 6 13 6

(n/a) (n/a) (n/a) 2.3 (n/a) (n/a) 5.6 (n/a) (n/a) 8.8 (n/a) (n/a)

540 548 549 520 504 527 536 526 539 539 540 564

7.0 5.9 8.1 4.7 4.8 3.9 8.6 5.4 6.8 10.4 8.5 9.2

67 70 65 75 75 74 72 74 75 73 65 75

2.5 2.2 2.9 1.9 1.8 1.5 3.3 2.1 2.7 4.1 3.0 3.9

1a/0.01

1b/0.01

1c/0.01

a b c

(%)

Composition of the electrolyte: 0.6 M DMII, 0.05 M LiI, 0.03 M I2, 0.25 M TBP and 0.05 M GuNCS in AcCN solution. Double-layered TiO2 films: thickness of the active and scattering layer, respectively. Dye-uptake solutions consisted of 0.1 mM of Pc and the indicated amount of CHENO in EtOH.

functional devices revealed important differences in charge (electrons) density in each case, which were found less for TT2 and TT4 than for TT1, TT3 and TT5 devices. Accordingly, TT1, TT3 and TT5 devices gave larger photocurrent (and hence PCE) when compared to TT2 and TT4. In addition, TT4 cell showed faster recombination dynamics than TT2, which explains not only its lower photocurrent but also lower photovoltage (550 mV versus 610 mV). In terms of IPCEs, TT2 exhibited the lowest values, and this was attributed to the hampering of the electronic coupling of the Pc core with the TiO2 3d orbitals, as a result of the non-conjugated spacer. When changing to a rigid, conjugated bridge, in TT3 and TT5, efficient electronic connection was achieved, leading to a remarkable boost in photovoltaic performance, with TT5 slightly surpassing TT3 due to better light harvesting properties. High directionality in the excited state of the sensitizer was, hence, accounted for the improved performance in these cases. Interestingly, TT4, endowed with a flexible instead

of rigid conjugated spacer, was characterized by reduced directionality, reflected in the lower observed IPCE. From these results, the authors concluded that faster electron injection, and thus orbital coupling and injection directionality between this type of dye and the titanium orbitals, is requisite for a Pc sensitizer to achieve higher efficiencies. A related analogue Pc of TT4, the dye PCAZnPc-1 incorporating an additional pyrazole spacer (pyrazole-3carboxyl group) was recently reported by Yildiz et al., and similar conclusions were also drawn regarding the flexible and nonconjugated nature of the linker [178]. Nonetheless, a better, but still relatively modest, PCE of 1.74% could be achieved, which was ascribed by the authors to the stronger binding ability of the pyrazole-3-carboxylic acid motif that might help to improve electron injection capability of their system. Nazeeruddin, Palomares, Torres and collaborators went further to investigate other conjugation patterns and addition of a second anchoring site, exploring bidentate binding group (Fig. 31, middle

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Fig. 30. Molecular structures of PCH008 and the benchmark PCH001 used in this study, reported by Giribabu et al. [164] (note that PCH008 has the same molecular structure than TT15 [179] reported by the groups of Nazeeruddin and Torres, see Fig. 31).

Fig. 29. Molecular structures of Pc dyes reported by Nagata et al. [167] (note that dye 2 reported in this work, has the same molecular structure than PcS2 [169,170] reported by Mori and co-workers and TT3 [162] by Nazeeruddin, Torres and collaborators).

panel, and Table 19, entry 2) [179]. In phthalocyanines TT6, TT15 and TT16 the anchoring group consisted in acrylic-, methylenemalonic- and 2-allylidenemalonic acid, respectively (note that TT15 has the same molecular structure than PCH008 previously reported by Gribabu et al., vide supra). In turn, cyanoacetic acid was employed in TT7 and TT8, following earlier studies that suggested improvement of the ‘‘pull” character of the dye and enhancement of the photocurrent response [180–182]. UV– Vis absorption data showed little or no formation of molecular aggregates and electrochemical data, on the other hand, revealed no significant changes in the energy levels of the dyes caused by the implementation of the spacers. As concerns electron injection, TT15 proved to be the fastest, and, in fact, it was the analogue with the strongest performances, giving out a JSC of 9.15 mA/cm2 and a PCE of 3.96%, the only dye with higher performances than TT1. Unexpectedly, TT16 characterized by a p-extended conjugated spacer compared to TT15, gave, in turn, lower JSC and VOC values that were translated in a lower PCE of 2.87%. The analogue TT6, with only one- instead of two- carboxyl groups in TT15, gave a slightly lower conversion efficiency in comparison to TT1 (3.28%), with small losses in the JSC and VOC. Surprisingly, cyanoacetic acid derivatives TT7 and TT8 performed poorly, reaching voltage and photocurrent much lower than those of TT1. By looking at the broader IPCE spectrum obtained for this dye, it was suggested that this was possibly a consequence of small aggregates formation. In a later report, the same group investigated the impact of the anchoring ligand on electron injection and back electron transfer in Pcs TT1, TT6, TT7 and TT15, which will be detailed in Section 4.3

[183]. In a different study, two new Pcs (Pcs 1 and 2) decorated with rigid arylenevinylene bridges were designed to investigate the diverse conjugation pathways (Fig. 31, right panel, and Table 19, entry 3) [184]. Strong aggregation was suggested from the UV–Vis and IPCE measurements, leading to moderate PCE values of 1.87% and 2.2% respectively. In this case, also, efficiencies remained consistently below the benchmark dye TT1, where no spacer intervenes between the COOH and the Pc core. The pursuit for groups binding stronger to TiO2 and possessing higher photostability, led to the preparation of TT30 and TT32, where phosphinic acid played the role of the anchoring group (Fig. 31, right panel, and Table 19, entry 4) [185]. The TT32 analogue differed in the presence of an n-octyl moiety, aiming at suppressing the formation of molecular aggregates. Both dyes gave higher photovoltages but lower conversion efficiencies compared to the optimized values of TT1, indicating decreased electronhole recombination or an upward shift of the conduction band edge position. TT30, in particular, gave a lower and broader IPCE spectrum, which was explained by stronger aggregation compared to TT32 and TT1. This was further confirmed by the fact that the use of CHENO significantly enhanced the IPCE spectral response and brought about a 40% boost in Jsc. TT30 also showed an enhanced overall performance when using coadsorbent. With regard to photostability, both TT30 and TT32 exhibited high robustness and binding properties, meaning that after continuous light exposure, TT1 was the dye that degraded faster, while TT30 and TT32 retained most of their initial features. This result was ascribed to the structure of the anchoring group that protects the TiO2 surface and hinders desorption. In a different study, the groups of Nazeeruddin and Torres studied the long-term stability of Pc sensitizers with regard to the number of anchoring groups [186]. Phthalocyanine TT9, endowed with two carboxylic acids directly connected to the macrocycle ring, was prepared and tested in DSSC (Fig. 32 and Table 20). Using a conventional iodine-based, volatile, liquid electrolyte (A), an increase in the electron-injection efficiency from the excited

Table 17 Photovoltaic data of DSSCs sensitized with dyes 1–3 under simulated AM1.5G one sun illumination.a,b Data derived from [167]. Dye+Coadsorbentc 1+CHENO 2+CHENO 3+CHENO a b c

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

480 460 480

0.6 3.9 0.8

66 65 69

0.18 1.16 0.28

(%)

Single-layered TiO2 films of 10 lm thickness. Composition of the electrolyte: 0.04 M I2, 0.4 M LiI, 0.4 M THAI and 0.3 M NMB in AcCN solution. Dye-uptake solutions were composed of Pc [0.05 mM], CHENO [5 mM], and retionic acid [0.1 M] in EtOH solution containing 7% of TBP and 3% of DMSO; soaking time 4 h.

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M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64 Table 18 Photovoltaic data under simulated AM1.5G one sun illumination of the TiO2 films sensitized with PCH008, and benchmark PCH001.a Data derived from [164]. Dye+Coads.b

Electrolytec

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

PCH001+CHENO (ratio of 1:200) PCH008+CHENO (ratio of 1:200)

A717 M1 A717 M1

67 71 47 47

620 ± 30 633 ± 30 557 562

6.2 ± 0.1 6.25 ± 0.1 5.63 5.31

73 ± 3 74 ± 3 75 74

2.80 2.86 2.35 2.22

(%)

Double-layer TiO2 films of [10+4] lm thickness (active and scattering layer, respectively). Dye-uptake solution: Pc [0.05 mM] and CHENO [10 mM] in EtOH solution (i.e.: ratio dye/CHENO of 1:200). Two different electrolytes were used in this study: electrolyte A7117 was composed of 0.6 M BMII, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in a 85:15 mixture (v/v) of AcCN and valeronitrile; electrolyte M1was composed of 0.6 M BMII, 0.025 M LiI, 0.05 M I2 and 0.275 M TBP, and 0.05 M GuNCS in a 85:15 mixture (v/v) of AcCN and valeronitrile. a

b

c

Fig. 31. Molecular structures of tert-butyl substituted Pcs with various anchoring groups and spacers (TT dyes), reported by the groups of Nazeeruddin, Grätzel and Torres (note that TT15 [179] has the same molecular structure than PCH008 [164] reported by Girbabu et al., see Fig. 30).

Table 19 Photovoltaic data under simulated AM1.5G one sun illumination of the TiO2 films sensitized with TT dyes, and benchmark TT1. Entry 1

2

3 c

4

Dye/Conc. (mM)

[Cheno] (mM)

Film Thickness (lm)a

Electrolyteb

TT1/0.05–0.1 TT2/0.05–0.1 TT3/0.05–0.1 TT4/0.05–0.1 TT5/0.05–0.1 TT1/0.1 TT6/0.1 TT7/0.05 TT8/0.05 TT15/0.1 TT16/0.1 1/0.1 2/0.1 TT1/0.1

60 120 120 120 120 10 60 60 60 60 60 60 60 – 10 – 10 – 10 – 10 – 10 – 10

[9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [9–10+4] [6–7+5] [6–7+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5] [7.5+5]

M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 Z952 Z952 M1 M1 Z952 Z952 M1 M1 Z952 Z952

TT1/0.1 TT30/0.1 TT30/0.1 TT32/0.1 TT32/0.1 a

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

617 ± 20 550 ± 10 610 ± 10 611 ± 10 613 ± 10 611 609 587 576 600 584 587 579 519 557 540 594 539 563 525 544 544 559 582 635

7.60 ± 0.20 0.90 ± 0.20 4.80 ± 0.20 1.44 ± 0.20 6.80 ± 0.20 7.78 7.37 5.88 6.80 9.15 6.86 4.36 5.36 6.89 6.43 5.33 4.28 4.94 7.07 2.16 3.95 6.03 7.67 4.67 4.02

75 ± 2 72 ± 2 74 ± 2 75 ± 1 74 ± 1 75 74 75 69 72 72 73 71 72 75 72 80 72 75 74 77 73 76 77 80

3.52 0.4 2.20 0.67 3.10 3.56 3.28 2.55 2.64 3.96 2.87 1.87 2.20 2.57 2.75 2.09 2.10 1.90 2.97 0.83 1.63 2.39 3.24 2.10 2.06

Ref

(%) [162]

[179]

[184] [185]

Double-layered TiO2 films: thickness of the transparent and scattering layer, respectively. Composition of the electrolytes: M1 (liquid electrolyte) = 0.6 M BMII, 0.04 M I2, 0.025 M LiI, 0.05 GuNCS and 0.28 M TBP in a 15:85 mixture (v/v) of valeronitrile and AcCN; M2 (solvent-free, binary ionic liquid based electrolyte) = DMII/1-ethyl-3-methylimidazolium-iodide/1-ethyl-3-methylimidazolium tetracyanoborate/iodine/N-butylbenzoimidazole/GuNCS (molar ratio 12:12:16:1.67:3.33:0.67). c Photovoltaic parameters were measured for fresh cells when using electrolyte M1, and after light-soaking under 1 sun for 72 h at 60 °C when using electrolyte Z952. b

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Fig. 32. Molecular structures of mono- TT1 and TT22, and doubly-anchored analogues TT9 and TT23 [149,186].

Table 20 Photovoltaic performances of TT1- and TT9/DSSCs under simulated AM1.5G one sun illumination (100 mW/cm2) during successive one sun light soaking at 60 °C.a Data derived from Ref. [186]. Dye+Co-adsorbentb

Electrolytec

Time (h)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

TT1+CHENO TT9+CHENO TT1+CHENO

A A B B B B B B

_ _ 0 500 1000 0 500 1000

617 605 662 511 455 631 542 490

7.55 9.37 5.90 5.98 4.48 7.00 8.67 7.45

75.0 72.5 76.0 73.0 71.0 75.0 70.0 70.0

3.55 4.10 2.95 2.56 1.48 3.20 3.33 2.56

TT9+CHENO

(%)

a

Double-layer TiO2 films of [7+5] thickness (active and scattering layer, respectively). Dye-uptake solutions: Pc [0.1 mM] and CHENO [60 mM] in EtOH. c Composition of the electrolytes: electrolyte A (volatile) = 0.6 M MBII, 0.04 M I2, 0.05 M LiI, 0.05 M GuNCS and 0.28 M TBP in a 15/85 mixture (v/v) of valeronitrile and AcCN; Electrolyte B (non volatile) = 1.0 M PMII, 0.15 M I2, 0.1 M GuNCS and 0.5 M NBB in 3-methoxypropionitrile. b

sensitizer led to higher short-circuit photocurrent and an improved 4.1% overall efficiency, in comparison to the benchmark TT1. When using the non-volatile electrolyte (B) that did not contains LiI, the VOC was higher for both TT1- and TT9- DSSCs than that obtained with electrolyte A (0.05 M LiI). However, this increase did not compensate the significant loss in the JSC, hence leading to lower overall PCEs of 3.2% (TT9) and 2.95% (TT1). These differences were explained by the presence of LiI in the liquid electrolyte that downshifts the TiO2 band edges which has a dual effect: 1) it boosts the electron-injection efficiency thus leading to higher JSC, and 2) it concurrently decreases the VOC [187]. Stability experiments were then performed on the DSSC devices using the non-volatile electrolyte (B). The cells were submitted to successive light soaking at 60 °C and their performances were regularly measured during 1000 h. TT9 proved to be more stable under continuous exposure to light and high temperatures. Initially, the TT1/DSSC device showed higher VOC than the TT9 cell, which was ascribed to longer electron lifetime over the whole range of charge density. However, TT1/DSSC lost 207 mV of VOC and 1.40 mA/cm2 of JSC after 1000 h, resulting in a 50% drop of its initial PCE value (from 2.95% to 1.48%). In turn, the loss in the VOC for TT9/DSSC was 141 mV only and more remarkably without loss in the JSC, therefore retaining 80% of its initial PCE under the same conditions (from 3.20% to 2.56%). This exceptional robustness

was attributed to the stability of the double carboxylic acid binding site of TT9 that lead to stable current overtime. Additional insights into the function of the adsorption site came from a study involving the use of a non-flexible and conjugated spacer in combination with one or two carboxylic acids and bulky peripheral substitution [149]. In this respect, phthalocyanines TT22, TT23, TT40 and TT43, characterized by the ethynyl spacer connecting the carboxylate with the Pc core, were prepared (Figs. 32 and 33, and Table 21). The selection of the bridge was based on its very successful implementation in porphyrin dyes [117,118]. As a matter of fact, its rigidity and extended electronic delocalization, not only induces directionality but also results in a perpendicular arrangement of the dye on the TiO2 surface. In the periphery, the outstanding 2,6-diphenylphenoxy trend, introduced by Kimura et al. [169], was implemented in TT40 and TT43, whereas tertbutyl groups were used in TT22 and TT23, for comparison purposes. The best performance was reported for TT40, which gave rise to an excellent PCE of 5.5% under 1 sun irradiation (6.1% under 0.1 sun illumination (9.5 mW/cm2)). An exceptional IPCE, >85% at k = 700 nm, was also observed. In turn, TT22, endowed with the same binding site but less bulky peripheral substitution required the use of co-adsorbent CHENO to avoid aggregation phenomena, and even so, PCE did not exceed 3.26%. As expected, the non-aggregating nature of TT40 and TT43 was such

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Fig. 33. Molecular structures of mono- TT40 and doubly-anchored analogue TT43 reported by Nazeeruddin, Grätzel, Torres, and collaborators [149,150].

Table 21 Photovoltaic performance of TT-22, -23,-40 and -43/DSSCsa under different light intensities (simulated AM1.5G). Dye/Conc.b (mM)

[Cheno] (mM)

Film Thickness (lm)d

Pin (mW/cm2)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

TT22/0.1b

20

[9+5]

b

20

[6+6]

TT43/0.05b

_

[6+5]

TT40/0.05b

_

[6+5]

c

_

[7+5]

0.05

[7+5]

9.5 100 9.5 100 9.5 100 9.5 100 9.5 100 9.5 100

564 610 455 520 521 584 585 638 568 621 540 605

0.70 7.01 0.35 3.70 0.85 8.77 1.31 12.3 1.45 13.9 1.06 10.7

78 73 75 73 76 71 76 70 75 70 77 72

3.26 3.13 1.24 1.40 3.54 3.63 6.13 5.50 6.49 6.01 4.61 4.69

TT23/0.1

TT40/0.05

TT40/0.05c a b c d

Ref

(%) [149]

[150]

Composition of the electrolyte: 0.6 M DMII, 0.03 M I2, 0.05 M LiI, 0.05 M GuNCS and 0.25 M TBP in a 15/85 mixture (v/v) of valeronitrile and AcCN. In ethanol solutions. In toluene solutions. Double-layer TiO2 films: thickness of the transparent and scattering layer, respectively.

that no co-adsorbent was required. At the same time, the previously confirmed beneficial effect of the presence of two anchoring groups in the molecular sensitizer, urged the preparation of TT23 and TT43 (Figs. 32 and 33), bearing two carboxyethynyl moieties. Against expectations, a large decrease of the efficiency was observed compared to their mono-substituted analogues. The origin of this setback was ascribed to unsatisfactory alignment of the energy levels of the dyes, as determined by electrochemical characterization, leading to poor electron injection and important decrease in JSC. As well, electron lifetimes of mono-substituted analogues TT22 and TT40 were longer, a result that is consistent with similar studies on TT9. Further studies on blooming Pc TT40 optimized the device preparation conditions and gave an important boost in the overall PCE from 5.5% to 6.0% under 1 sun (6.5% under 9.5 mW/cm2, Table 21) [150]. The modified conditions for the preparation of the photoanode was the reason for this enhancement (overnight dye-sensitization and thicker [7+5] lm TiO2 films, consisting of 7 lm of transparent layer plus 5 lm of scattering layer). Based on their excellent results previously obtained with PcS15 discussed above (vide supra), Kimura et al. described a new Pc dye PcS16 functionalized with an ethynylcarboxyphenyl anchoring group (Fig. 34 and Table 22) [174], which has proven to be successful in the molecular designs and successive breakthroughs brought by porphyrin dyes YD2 [117] and YD2-o-C8 [118] in DSSC.

Fig. 34. Molecular structures of PcS15 and PcS16 reported by Mori and collaborators [174].

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Table 22 Photovoltaic data under simulated AM1.5G one sun illumination (power 100 mW/cm2) of the TiO2 films sensitized with PcS15 and PcS16.a,b Data derived from [174].

a b c d

Dyec PcS15

Dye-loading (mol/cm3) 10.5104

PcS16

12.8104

Time (h)d 24 0 24 0

VOC (mV) 610 600 610 590

JSC (mA/cm2) 12.8 12.0 11.7 9.7

g (%) 5.3 5.0 4.7 3.9

F.F. (%) 68 70 66 68

Double-layer TiO2 films of [7.8+4] lm thickness (transparent and scattering layer, respectively). Composition of the electrolyte: 0.1 M LiI, 0.6 M DMPII, 0.05 M I2 and 0.5 M TBP in AcCN solution. Dye-uptake solutions were composed of 0.05 M Pc in toluene. Period of time after cell fabrication.

Fig. 35. Molecular structures of ZnPc 1–3 and benchmark TT40 reported by the groups of Nazeeruddin, Grätzel, Torres [188].

Table 23 Photovoltaic parameters of DSSCs sensitized with ZnPcs 1–3, under 1 sun (AM1.5G; 100 mW/cm2).a,b Data derived from [188]. Dye

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

ZnPc 1 ZnPc 2 ZnPc 3

607 463 558

7.50 0.23 6.79

72 75 73

3.29 0.08 2.76

(%)

a Double-layer TiO2 films of [7+4] lm thickness (transparent and scattering layer, fabricated with 20 nm and 400 nm size particles, respectively). b Composition of the electrolyte: 0.6 M DMII, 0.05 M LiI, 0.03 M I2, 0.25 M TBP, and 0.05 M GuNCS in a mixture of AcCN and valeronitrile (85:15).

In comparison with the predecessor PcS15 connected directly through a phenylcarboxyl group, the extended conjugation in PcS16 caused by the presence of the additional ethynyl spacer, gave rise to an increased IPCE between 400 and 520 nm, but decreased in the 700 nm region, resulting in a lower PCE of 4.7% with respect to that of PsS15 (g = 5.3%). Similarly, three novel Pc dyes analogues of TT40, anchored via a carboxyphenyl or a naphthylanhydride group, were prepared by the groups of Nazeeruddin, Grätzel, and Torres (Fig. 35) [188]. ZnPc-1 and 2 stand out for the incorporation of a benzothiadiazole (BTD) bridge at the acceptor part. It should be mentioned that this binding trend was the main asset of the champion porphyrin dyes SM315 [53] and GY50 [54] that achieved a PCE of 13% under one sun illumination (AM1.5G). The introduction of a BTD bridge with a strong acceptor character aims to enhance the charge-transfer character of the dye (‘‘push-pull” effect), and extends the absorption profile of the dye by means of p-extended conjugation. ZnPc13 showed a 10 nm redshift of the Q-bands compared to TT40, as a

direct consequence of the extension of conjugation of the Pc macrocycle over the ethyne-linked acceptor moiety. In addition, ZnPc3 (without BTD bridge) displayed more pronounced splitting of these bands, which was assigned to the possible electronic coupling between the Pc core and the naphthylanhydride unit. In addition, the consistency in the non-aggregating features of 2,6diphenylphenoxy endowed Pcs remained. The obtained results from the built Pc-sensitized devices were, nonetheless, unsatisfactory, with ZnPc1 exhibiting the highest JSC, translated in a 3.29% overall efficiency (Table 23). This poor performance was explained in terms of the lower LUMO level of the three new dyes when compared to TT40, and in fact, ZnPc2’s LUMO level was too low to enable efficient electron injection, which was the reason for a marginal JSC of 0.23 mA/cm2. This study, however, revealed the beneficial effect of the benzothiadiazolyl group in light harvesting over naphthylanhydride, since comparison of the IPCE spectra of ZnPc1 and ZnPc3 showed a significant difference in the visible response below 550 nm.

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Fig. 36. Molecular structures of pyridyl- and picolinic acid substituted Zinc(II) phthalocyanines reported by the groups of Nazeeruddin, Grätzel, and Torres. Reproduced with the permission from [189]. Copyright 2017 Wiley-VCH.

Recently, Nazeeruddin, Grätzel, Torres, and collaborators reported various pyridyl- and picolinic acid- substituted Zinc(II) phthalocyanines to study the structural effects on the overall performance in comparison to the most commonly used carboxylic acid derivatives (Fig. 36 and Table 24) [189]. Pyridyl had been previously employed successfully as a binding moiety in other dyes [190–194], considering that it can form a coordination bond with TiO2. Different pyridyl substitution’s pattern (meta or para) and number of pyridyl units at the macrocycle’s peripheral position (one or two) was designed for TT52-56. Unfortunately, relatively low efficiencies (<1.7%) were obtained for these Pcs with the best one obtained for optimized TT55-DSSC device with a PCE of 1.71%. General trends on structure-relationship could be found for these pyridyl-anchored dyes: higher photovoltaic efficiencies were systematically obtained for 1) the para- over the metasubstituted Pcs, and 2) the mono- over the bis-functionalized dyes. The quite strong aggregation tendency of these tert-butyl substituted Pcs required a relatively high ratio of co-adsorbent CHENO (ratio Pc/CHENO of 1:100) to avoid such phenomena and hence reach optimal performances. This presents a serious drawback since the adsorption of the coadsorbent (CHENO), which is carboxyl-anchored to the active surface, reduces even more drastically the dye’s adsorption in the case of pyridyl-functionalized Pcs in comparison with their carboxyl analogues. Hence, the main limitation comes from their poor adsorption onto the TiO2, which

ultimately hampers the PV performances of the DSSCs, especially the JSC that is closely related to the amount of adsorbed molecules. In an attempt to tackle this issue, a Pc analogue substituted with a picolinic acid moiety was prepared and tested, which presents two advantages: on one hand, it is a strong anchoring group through the carboxylic acid function, and on the other hand, the presence of an electron-withdrawing nitrogen atom in the aromatic ring facilitates the dye’s electron injection. By taking advantages of these two designs (COOH and pyridyl), an improvement in the PV efficiency of up to 2.1% was obtained for TT59. Following a different strategy, Kimura’s group implemented bulky substituents in the periphery of pyridyl-substituted zinc phthalocyanines PcS22-24 (Fig. 37), which strongly prevent aggregation of the Pc macrocycle, hence rendered the use of coadsorbent unnecessary in this case [195]. All Pcs were decorated with bulky phenoxy substituents in the periphery and with regard to the anchoring group, PcS22 had a 4-pyridyl group directly linked to the macrocycle, whereas PcS23 and PcS24 had a longer acetylene conjugated linker between the Pc and the pyridyl. The extended conjugation was reflected in the UV–Vis spectra of the dyes, where PcS23 and PcS24 demonstrated a red-shift of the Qband. The authors also stated that due to the steric hindrance of these macrocycles, intermolecular interactions between the pyridine and the Zn central metal were avoided. Upon preparation of the dye-stained TiO2 films it was observed that the adsorption density of the Pc was a lot lower compared to previous cases of Pcs bearing a carboxyl anchoring group, which is in agreement with the recent work of Nazeeruddin, Grätzel, Torres, and collaborators [189] (vide supra) and others reports [196]. The photovoltaic characteristics of the dyes were measured and the best performing one proved to be PcS23, with a PCE of 6.1% and a maximum IPCE of 79% at the Q band (Table 25). This result was followed by PcS24 and PcS22, following the order of dye adsorption densities. An interesting observation of these studies was also that the higher IPCE was obtained with the dyes having the longer linker, which contrasts with the trend seen in carboxylic-based analogue Pcs, and this was attributed to different recombination kinetics due to the better molecular orbital hybridization of the pyridine-based dyes with the TiO2 particle. In a different strategy, Sarker et al. probed the use of catechol anchoring group for an unsymmetrical zinc phthalocyanine sensitizer, ZnPc-Cat (Fig. 38) [197]. Remarkably, it was observed a strong red-shift of the Q-band absorption from 680 nm in solution to 750 nm on TiO2, along with an appreciable absorption tail extending to 1000 nm, as a result of strong interactions between the catechol-functionalized Pc and TiO2. However, this Pc gave poor performances in DSSC (JSC = 2.53 mA/cm2, VOC = 540 mV, and F.F. = 0.68), achieving an overall conversion efficiency of 0.92% under standard AM1.5G conditions. It was suggested that the poor

Table 24 Phototovoltaic data of the DSSC devices sensitized with TT52-55, TT59 and TT60, under simulated full sun illumination (AM1.5G) and with an active area of 0.159 cm2. Data derived from [189]. Dyea

Film thicknessb (lm)

Electrolytec

VOC (mV)

JSC (mA/cm2)

F.F. (%)

Pin (mW/cm2)

g

TT52+CHENO TT53+CHENO TT55+CHENO

[6+3] [6+3] [6+3] [10+5] [6+3] [9+4] [9+4] [9+4]

A A A B A B B B

567 555 568 537 493 491 562 532

3.44 1.08 2.01 4.14 0.301 0.877 4.65 0.463

68.0 73.6 77.3 75.7 74.3 74.6 75.0 78.5

100.1 100.6 99.4 98.6 96.5 99.3 96.5 97.9

1.3 0.44 0.89 1.71 0.11 0.32 2.03 0.20

TT56+CHENO TT59+CHENO TT60+CHENO a

(%)

Dipping solutions were prepared in THF at a concentration of 0.1 mM of ZnIIPc and with 10 mM of CHENO. Double-layered TiO2 films thickness: values in brackets refers to the thickness of the transparent and scattering layer, respectively. c Electrolyte A composition: 0.025 M LiI, 0.9 M DMII, 0.28 M TBP, 0.04 M I2 and 0.05 M GuNCS in AcCN solution. Electrolyte B has the same composition than A except 0.1 M of LiI and 0.6 M of DMII. b

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Fig. 37. Molecular structures of PcS22-24 reported by Kimura’s group [195].

Table 25 Photovoltaic parameters of the DSSC devices sensitized with PcS22-24 under 1 sun illumination (AM1.5G; 100 mW/cm2).a Data derived from Ref. [195]. Dye

Film thickness (lm)b

Dye-loading (mol/cm3)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

PcS22 PcS23

12.6 + 5.0 12.6 + 5.0 4.8 + 3.4 12.6 + 5.0

1.1105 3.8105 (n/a) 0.8105

560 580 610 600

8.1 12.6 13.5 10.0

71 74 74 76

3.2 5.4 6.1 4.6

PcS24 a b

(%)

Composition of the electrolyte: 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, 0.5 M TBP in AcCN. Double-layered TiO2 films: thicknesses of transparent and scattering layers, respectively.

performances obtained with this Pc in DSSC should be explained by fast recombination kinetics between injected electrons and Pc radical cations, as usually observed for other catechol-based sensitizers, including porphyrins [198] or Ru (II)-Polypyridyl complexes [199]. 4. Mechanistic aspects: Scope and limitations on the performances of Pcs in DSSCs Since the emergence of DSSC and during the last decades, a tremendous numerous of in-depth photophysical and mechanistic studies, both theoretical and experimental, have been conducted for various type of sensitizers (in particular Ru(II) polypyridyl complexes and porphyrins) to describe quantitatively the energetics inside the operating device and shed light on the factors limiting their efficiency. However, very few ones, until recently, had been reported for phthalocyanine dyes specifically. In the following sec-

Fig. 38. Molecular structure of ZnPc-Cat bearing a catechol moiety as anchoring group, reported by Sarker et al. [197].

tion, we have attempted to collect some of the most relevant works in this connexion.

4.1. Effect of the metal center Barea et al. studied in details the energetic factors governing injection, regeneration and recombination in DSSC for two phthalocyanine sensitizers, the metallo Zn(II)Pc ZnPc-1 and its free-base analogue H2Pc-2 (Fig. 39 and Table 26) [200]. The two dyes were designed with six peripheral tert-octylphenoxy bulky groups in order to prevent dye-aggregation and, in addition,

Fig. 39. Molecular structures of ZnPc-1 and H2Pc-2 reported by Barea et al. [200].

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Table 26 Photovoltaic parameters under 1 sun (AM1.5G; 100 mW/cm2) and estimated position of the conduction band of the DSSC devicesa,b,c,d sensitized with ZnPc-1, H2Pc-2 and benchmark N719. Data derived from [200]. Dye ZnPc-1 H2Pc-2 N719 a b c d e f g

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g (%)

n at VOCe (cm3)

Ec f (eV)

Ec-EFnf,g (eV)

440 350 450

3.48 5.71 10.90

66 57 51

1.01 1.14 2.50

4.2  1017 1.5  1018 3.0  1018

4.00 4.13 4.11

0.31 0.27 0.19

Single-layered TiO2 films of 8 lm thickness. Active area of 0.25 cm2. Dye-uptake-solutions: 0.3 mM of sensitizer in CHCl3 (dipping time: 4h). Composition of the electrolyte: 0.7 M LiI and 0.05 M of I2 in 3-propiomethoxynitrile solution. n = electron density at open circuit potential. Ec = energy vs. vacuum of the conduction edge position. Ef = Fermi level of electrons in the TiO2 at open circuit conditions.

increase the electron donor ability (‘‘push” effect). The nonaggregated nature of these Pcs was first supported by comparison of their absorption spectra in solution and those adsorbed on the TiO2: no shift in the peak position nor appearance of aggregation band were observed. In addition, no improvements of the DSSC devices were obtained when CHENO was added in the dyeuptake solution. Regarding the acceptor part, an anhydride group was chosen, which not only allows an efficient attachment of the dye to the surface, but also enhances the electron-withdrawing character (‘‘pull”) that might facilitate the dye-injection into the semiconductor. To compare the performances of the two dyes in DSSC, the benchmark N719 was also studied under the same conditions (electrolyte composition, time of adsorption and thickness of the TiO2 layer). As commented by the authors, it is important to point out that the conditions in which were measured all the samples, were those optimized for the Pc sensitizers and therefore were not optimal for N719, which achieved low performances within this configuration with a PCE of 2.50% (a PCE of 7.20% was obtained under optimal conditions). Fig. 40 summarizes the energetics of the three dyes in DSSCs. It was found that the upper limit of the TiO2 conduction band (represented by the blue lines in the figure), was rather at the same energy level for H2-Pc2 and N719, whereas that of ZnPc-1 was significantly shifted upward in comparison. According to the authors, the presence of a proton excess in H2Pc-2 and N719 dyes that interacts with the TiO2 surface, could constitute a plausible explanation for their lower energy-level position of the TiO2-CB with respect to ZnPc-1. The magnitude and direction of the interfacial dipole moment of a dye (which originates from donor–acceptor within the dye molecule in the excited state) is, among many others, a factor that influence the shift of the TiO2 band edges [106,201,202], and should also contribute to the observed differences between the three dyes. ZnPc-1 has a much larger potential shift of the TiO2-CB with respect to the free base analogue H2Pc-2, but this was not fully reflected in the VOC (440 mV and 350 mV, respectively). It was evidenced that the recombination rate in ZnPc-1/DSSC was larger, which attenuates the rise of the fermi level and hence the gain in the VOC, reaching a similar value than that of N719. This metal effect is indeed known in the case of the related porphyrin dyes analogues, for which the centre metal ion, when directly exposed to the electrolyte, tends to trap more easily the I3- ions and hence increase recombination rate at the TiO2/electrolyte interface. The main issue of these Pcs sensitizers lies on their LUMO level that is very close to the TiO2 band edges. Such close alignment limits drastically the efficiency of the electron-injection because of the small driving-force for this process. In turn, the LUMO of N719 is considerably higher in energy, thus providing sufficient driving force for efficient electron-injection efficiency. An important finding of this work was that free-base and metalo Zn(II) Pcs did not interact in a same way with the TiO2. It was evidenced that the

Fig. 40. ‘‘Energy diagram of the active components in the DSC with the different  dyes. Acceptor and donor states of the I redox couple (dashed line), taking 3 /I reorganization energy of 0.5 eV; HOMO and LUMO states of the dyes after considering absorption and solvation effects (continuous lines); density of electron states below the conduction band of the TiO2, obtained from capacitance measurements (dots) and estimated shape up to the conduction band (blue line). The dotted line represents the TiO2 density of states with optimized electrolyte.” Reproduced from [200] with permission from The Royal Society of Chemistry.

presence or absence of a metal centre in the Pc, modulates significantly the position of the TiO2 energetic levels, which affects in fine the performances of the dye in DSSC. Another important finding is that the recombination rates for the Pc dyes are in the same range of order than for the benchmark N719 dye. Thus, their relatively low performances in DSSC cannot be explained by this factor. This conclusion is in agreement with the work reported by the groups of Nazeeruddin, Grätzel, and Torres, for Pc dyes TT2-5 (vide supra, see discussion on Section 3.3) [162]. Instead, it has been proposed that the small gap difference between the LUMO of the Pcs and conduction band edges of the TiO2 should be the main factor responsible of the poor injection capability of these dyes, one key-process in DSSC. Hence, the electrolyte composition needs to be modified (LiI and TBP concentration in particular) in order to lower enough the energetic levels of the TiO2, and thus to obtain some injection from the excited Pcs. As a main drawback, lowering the TiO2-CB imposes a low photovoltage, which, together with the limitation of the electron-injection process, explains the low performances of these Pcs in DSSCs. 4.2. Effect of aggregation and adsorption geometry (p-)PcS2 (tert-butyl) and PcS6 (bis(2,6-diphenylphenoxy)) previously reported by Mori and collaborators and discussed in

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Fig. 41. Molecular structures of o-, m-, p-PcS2 and PsS6 reported by Mori and collaborators [170].

Table 27 Optical data and photovoltaic parameters of the TiO2-DSSCs sensitized with PS6, o-, m-, and p-PcS2 dyes under one-sun conditions.a Data derived from [170]. Dye (Conc./mM)

[CHENO] (mM)

Film Thicknessb (lm)

A635

o-PcS2 (0.05)

0 0.1 0 1 10 0 1 10 0

10 10 10 10 10 6.5 6.5 6.5 7

0.19 0.14 0.83 0.48 0.18 1.40 0.80 0.31 0.18

m-PcS2 (0.05)

p-PcS2 (0.05)

PcS6 (0.05) a b c d

nm/A685 nm

c

IPCEmax (%)

IQEd (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

16 15 3.7 7.8 10 7.2 18 13 57

20 23 4.3 9.1 18 9.5 23 30 66

502 512 460 525 532 462 563 589 615

1.26 0.85 0.43 0.93 0.58 0.86 1.99 0.78 6.29

70.1 70.5 65.5 73.5 69.4 58.5 70.4 71.9 74.6

0.44 0.31 0.13 0.36 0.22 0.23 0.79 0.33 2.89

(%)

Composition of the electrolyte: 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M TBP in AcCN solution. All devices were fabricated with single-layered nanoporous TiO2 electrodes. Intensity ratio of the two absorption spectrum peaks at 635 nm (aggregates) and 685 nm (non-aggregated species) was used to estimate the degree of dye aggregation. IQE = Internal quantum yield efficiency; estimated values from absorbance of the films and IPCE values.

Section 3.2 (vide supra) were further investigated by in-depth photophysical studies by the same authors, in order to compare the effect of the peripheral substitution. In this new work, they also reported and studied two new dyes, namely m-PcS2 and o-PcS2, to elucidate the influence of the dye adsorption angle on the surface by changing the position of the carboxyl group over the anchoring ligand (Fig. 41 and Table 27) [170]. Overall, this work provides significant insights regarding the effect of aggregation of Pcs on the electron-transfer dynamics and recombination processes that occur in DSSC for this type of sensitizer (Fig. 42). The aggregation behaviour of these Pcs could be directly appreciated in their absorption spectra, both in solution and adsorbed on the films, by comparison of the relative intensity of the Q-band at 635 nm (aggregates) and 685 nm (non-aggregated species). The relative intensity of these two peaks constitutes a good indicator to estimate the ratio between the two species (A635nm/A685nm, Table 27). As expected, PcS6 was only little aggregated because of the high steric hindrance given by the peripheral bulky bis (2,6-diphenylphenoxy) groups. Therefore, this non-aggregated dye reached high and optimal efficiency (2.89%) in DSSC without the need of co-adsorbent CHENO. In turn, p-PcS2, with the same anchoring group but with a tert-butyl substitution, allows a tight packing of the dyes on the TiO2 surface resulting in higher dye adsorption densities (estimated 1.7-times higher than PcS6). This was however, not translated in terms of JSC (0.86 mA/cm2 vs 6.29 mA/cm2) since higher dye-loading should, in principle, led

to more photo-generated electrons and hence produce higher current density. The strong tendency of this Pc to aggregate increases considerably the recombination process between the dye radical cation and the photo-injected electrons, and thus explained the poor JSC (0.86 mA/cm2) and overall efficiency of only 0.23% achieved by this dye. CHENO was therefore necessary to reduce the aggregation of p-PcS2, which in turn reduced the dye-loading density because of the concurrent adsorption of the coadsorbent. The optimal condition was a Pc/CHENO ratio of 1:20, under which considerably improved values by more than a twofold factor for the JSC (1.99 mA/cm2) and by three for the overall efficiency (0.79%) were obtained. This PCE is however, far from that obtained with the bulky-substituted analogue PcS6 (2.89%) without CHENO. The meta analogue m-PcS2 behaves quite similarly to p-PcS2: high aggregation tendency and tight packing (i.e. high dye loading, which was in the same range (2.1 vs. 1.7 times larger than that PsS6). In that case too, CHENO was required to reach optimal performances (PCE of 0.36%). In the opposite trend of this series, o-PcS2 presents a low degree of aggregation, in the same order than bulky PcS6, attributable to unfavourable co-facial orientation of the macrocycles within the dye attached to the TiO2 surface. However, in such conformation, o-PcS2 needs much more space to accommodate on the surface thus resulting in the poorest dye-loading of this series. Consequently, this dye gave optimal performance without CHENO and with a superior JSC of 1.26 mA/cm2 to those of the ortho and meta analogues under these conditions.

M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

33

Fig. 42. A) Schematic adsorption geometry of the dyes o-, m-, p-PcS2 and PsS6 on the TiO2 surface (BET = back electron transfer; DL = dye-loading). B) Effect of CHENO on aggregation properties and dye-loading (DL) exemplified for p-PcS2. Adapted with permission from [170]. Copyright 2014 American Chemical Society.

Fig. 43. Left: Molecular structures of PZnPc (model compound) and sensitizers TPZnPc and MPZnPc; Right: schematic mode of aggregation proposed by Ramamurthy and co-workers. Reproduced from [205] with permission from the PCCP Owner Societies.

Even so, this value remains much lower with respect to that of the non-aggregated PcS6, which can be explained, in part, by the difference in dye-loading densities between the two (relative ratio of 0.64). This was reflected in their respective IPCE spectrum, with PcS6 giving systematically superior values in the whole range of wavelength than o-PcS2, and in addition showing a much broader and greater response around 635 nm. While the IPCE spectrum of o-PcS2 match well its absorption spectrum, m- and p- PcS2 both displayed an enhanced IPCE response at around 635 nm, which

was assigned, in these cases, to the extra absorption coming from the aggregated species. For practical purpose and have a more proper idea about the potential of injection capability of each dyes regardless of the LHE properties, the internal quantum yields efficiencies (IQE) were estimated at 685 nm (i.e.: non-aggregated specie) from IPCE and absorbance (a) of the films (Table 27). In the PcS2 series and without CHENO, o- PcS2 shows the highest value of 20% followed by p- (9.5%) and m- (4.3%) PcS2, which contributes for another part

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Table 28 Data of the fluorescence lifetime measurementsa of the TiO2-DSSCsb,c,d sensitized with MPZnPc and TPZnPc dyes. Data derived from [205]. Dye

sads (ns)

MPZnPc TPZnPc

0.017 0.008

A1 (%) 47 59

sfree (ns) 0.27 0.21

A2 (%) 53 41

jinj

jrec

(s1)

(s1) 9

55.1  10 106.3  109

4.3  105 4.3  105

a With sads, sfree = lifetime of ZnPc adsorbed and un-adsorbed, respectively; A1, A2 = percentage of components with the defined fluorescence lifetime; jinj,jrec = rate of electron injection- and recombination- process, respectively. b All devices were fabricated with single-layered nanoporous TiO2 films of 4 lm thickness. c Dye-uptake solutions were composed of 50 mM of Pc (solvent not reported). d The composition of the electrolyte was not reported.

to the trend observed in the JSC (1.26, 0.86 and 0.43 mA/cm2, respectively). With CHENO, these values increased in all cases but much more pronounced in the case of meta- and para- (18% and 30%, respectively, with 10 mM of CHENO) with respect to ortho- PcS2 that experienced only a small increase from 20% (no CHENO) to 23% (with only 0.1 mM of CHENO). Though these values were much better in the presence of CHENO for the meta- and para derivatives, this trend was not followed in the JSC. The main explanation given by the authors involves the diminution of the LHE efficiency in the presence of CHENO caused by the loss of extra absorption arising from the aggregated species (vide supra). It is noteworthy that similarly structured unsymmetrical porphyrins (synthetic analogues of Pcs) anchored through the same ortho, meta, and para-carboxyphenyl, were reported by D’souza and co-workers [203]. The same conclusion were drawn regarding the location of carboxyl group and the effect on the geometry adsorption, aggregation behaviour and dye coverage. However, in sharp contrast with Pc, the authors observed much faster electron recombination in the order para > meta>> ortho which support the proposed additional through-space, in addition to through-bond, recombination pathway, the latter being predominant in the case of the ortho derivative. In another independent work [204], Galoppini and co-workers reported symmetrically substituted tetracarboxyphenyl-substitued analogues para-TCCP and metaTCCP, and proposed that the main deactivation pathway for these exited dyes occur mostly via a through-space pathway for metaTCCP that lies flat on the surface, meanwhile para-TCCP orientated perpendicular to the surface injects electrons in TiO2 mostly trough-bond. In addition, the orientation of para-TCCP allows a tight packing of porphyrins (H-aggregates) leading to higher dye coverage, but at the same time to lower electron injection efficiency because of additional deactivation pathway dye*-dye. Along the same lines, another detailed photophysical study was reported the same year by Ramamurthy and co-workers, regarding the effect of the number of anchoring groups and dye-orientation toward aggregation-mode and electron injection/recombination kinetics, using the unsymmetrical (MTZnPc) and symmetrical (TPZnPc) Pcs as sensitizers (Fig. 43) [205]. PZnP was used as a model compound for spectroscopic studies in solution (obviously, no Pc-adsorption was observed for the devices made with the model compound PZnP, lacking of anchoring group). First, the steady-state absorption spectra of all Pcs derivatives in DMF solutions did not present any of the typical aggregation features characteristic of Pcs [9], which suggests neither dimerization nor aggregation in their ground state. In contrast, the appearance of a new broad absorption band with maximum at 630 nm was observed for the sensitized Pc/TiO2 films made with MPZnPc and TPZnPc, which was assigned to dimeric- or higher ordered- Haggregates on the surface. From the relative intensity of this band with respect to that of the typical Q-band centred at 677 nm (as in solution), it was concluded that this aggregation behaviour (H-type) is more pronounced for MPZnPc than TPZnPc in the sensitized TiO2-films. In addition, another additional band shifted by

5 nm to the later was observed in the case of MPZnPc/TiO2 only (not observed for TPZnPc/TiO2), which was assigned to J-type aggregates. In other words, MPZnPc shows both H- and J-type aggregation behaviour in DSSC, while TPZnPc shows only H-type and in a lesser extend. Next, the absorption of the films were also systematically greater with TPZnPc than those made with MPZnPc, which indicates that the presence of four- instead of one- carboxyl anchoring group(s), respectively, increased significantly the total amount of adsorbed molecules on the TiO2 surface. Turning to fluorescence spectra in solution (DMF), the growing number of carboxyl groups, from PZnPc (none), MPZnPc (one) to TPZnPc (four), systematically decreases the fluorescence- and increases the triplet- quantum yields in the same order. The fluorescence decay (at 688 nm) of all the three Pcs in solution was fitted to a single exponential function with a lifetime of ca. 3 ns, corresponding to the conversion from S1 to S0 state. In turn, the Pc-fluorescence was completed quenched in the MPZnPc/TiO2 and TPZnPc/TiO2 films, which was attributed to the electroninjection from the singlet-excited state of the Pc into the TiO2CB, as corroborated by time-resolved fluorescence studies using time-correlated single photon counting (TCSPC) technique. By monitoring the injection kinetics decays it was possible to estimate the rate of this process (Table 28). They found that the EI takes place almost twice faster for TPZnPc than MPZnPc, which is in agreement with their calculated DGinj values, larger for TPZnPc because of a more negative excited state potential than MPZnPc. Remarkably, the electron injection process of the excited Pc into the TiO2 semiconductor occurs quite fast in a 10–20 picosecond timescale, in agreement with other reports for ZnII-phthalocyanines sensitizer (vide infra). On the other hand, the electron recombination processes was investigated by transient absorption spectroscopy. The spectra of both devices showed a maximum at 520 nm (20 nm shifted with respect to the triplet – triplet maximum absorption in DMF solution), which was assigned to the radical cation of the Pc formed after photoinduced electron-injection of the excited dye into the TiO2. The rate constant of electron recombination was determined by monitoring the kinetic decays at 520 nm and, surprisingly, they obtained the same value of 4.3  105 s1 for both dyes, thus suggesting that recombination process should be similar for the two. Considering that TPZnPc shows almost a twice-faster electron-injection rate than MPZnPc (vide supra), the authors suggested from these findings, that a symmetrical tetra-carboxylated Pc would be more suitable for DSSCs with respect to the (unsymmetrical) monocarboxylated one. In the last few years, Tkachenko and co-workers conducted various detailed photophysical studies on several Zn(II)Pcs using ultrafast spectroscopic technics in all solid-state (ZnO- or TiO2) DSSCs, in order to unravel the primary processes of charge separation, kinetics of injection and recombination, and this way, shed light on the factors reducing the conversion efficiency of ZnPc sensitizers [206–209]. One of the lead study involved the investigation of four ZnPcs by time-resolved spectroscopy (Fig. 44 and Table 29)

M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

35

Fig. 44. Molecular structures of Zn(II)Pcs V-1–4 studied into ZnO Nanorods-ssDSSC by Virkki, Tkachenko et al. [206].

Table 29 Time constants for the electron injection (sinj), charge separation (sCS) and electron transfer (sET) of the Pc/ZnO nanorods-DSSCs. Data derived from [206]. Pc V-1 V-2 V-3 V-4d a b c

sinja

sCSb

sETc

(ps)

(ps)

(ps)

5.0 ± 0.6 5.7 ± 0.7 0.7 ± 0.2 1.4 ± 0.4

12 ± 6 11 ± 2 1.4 ± 0.3 0.5 ± 0.3

85 ± 27 125 ± 20 82 ± 5 120 ± 35

sinj is taken from the samples without the Spiro-OMeTAD layer. sCS refers to the charge transfer at the Pc/Spiro interface. sET refers to the electron injection from the Pc anion into the ZnO that is

observed in samples with Spiro-MeOTAD as the consequence of the charge separation at the Pc/Spiro interface. d Data of Pc V-4 are derived from [207].

[206]. In this work, Spiro-OMeTAD has been chosen as holetransporting material, which presents the advantages to provide a less complex design of the cell and a better thermodynamic stability with respect to liquid electrolytes traditionally used in DSSC. Together, it facilitates a direct spectroscopic evidence on the hole transfer at the Pc/Spiro-OMeTAD interface layer and on the dye regeneration kinetics. Moreover, ZnO, the second most used semiconductor after TiO2, was chosen. It presents the advantage of higher electron mobility, and more importantly, the nanorods architecture, vertically aligned, have a simpler morphology (diameter of ca. 38 ± 1 nm and length of ca. 790 ± 25 nm). In such configuration, electron microscopy could be used to visualize and monitor the different layers of the device, such as the HTM layer that fills the voids between the nanorods. Under optimized conditions (dye-uptake solvents and dipping time), spectroscopic data analysis indicated a nearly complete formation of homogenous self-assembled monolayer (SAM) for all Pcs

on the ZnO surface, with comparable dye-loading densities (estimated area per molecule of ca. 2 nm2). Moreover, these data suggest low or no Pc-aggregation for 1/ZnO, 2/ZnO, and 4/ZnO, whereas relatively higher degree of aggregation was found in the case of 3/ZnO. Transient absorption studied were then carried out for both the Pc/ZnO and HTM/Pc/ZnO device configurations. Without HTM layer, the radical cation (Pc+) was predominately observed for all Pcs, which was assigned to a fast electron injection (sinj <5ps) from the singlet-excited state of the Pc into the ZnO nanorod conduction. However, in the complete devices within the HTM layer, these studies suggest that the primarily and predominant process that occurs is the charge separation at the Pc/ HTM interface that yields the Pc anion (Pc–) and the SpiroOMeTAD cation in a few picoseconds delay time (sCS). This is followed by injection of Pc– into the ZnO nanorod in a few tens of picoseconds (referred as ‘‘electron transfer” process with time constant rate sET), leaving a long-lived state with the electron in the ZnO and the hole in the Spiro-OMeTAD layer. The time constants of the different reactions referred above (sinj, sET, and sCS) for all three samples are summarized in Table 29. The first surprising result was the similar injection rate constant values (sinj) obtained for Pc-1 and 2, which have the same bulky substitution but differ from the linker in the anchoring group. Since the sinj values seem to be independent of the length and nature of the anchoring group, the authors assumed that the electron-injection must occur via a trough-space mechanism. They concluded that 1 and 2 must have different tilt angles, providing roughly the same distances between the Pc core and the ZnO to explain the same sinj rate constants, which was supported by molecular dynamics (MD) modelling studies. In turn, modelling of Pcs- 3 and 4 did not indicate significantly tilts in comparison with 1 and 2, but more relaxed conformations that allow their macrocycle to stay closer to the ZnO surface, which is even more pronounced for 4 owing to the specific

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structure of its linker. Therefore, the slower sinj values obtained for Pcs 1 and 2 with respect to 3 and 4, was rationalized in terms of restricted approach of the macrocycle to the ZnO surface caused by the bulky peripheral groups. In the device within the HTM, the sCS values were much slower (ca.100 ps), and surprisingly roughly the same for the four Pcs, meaning that there is no significant difference in terms of electron-injection capability between them. In the later cases, the electron-transfer from the Pc into ZnO did not occur anymore from its exited state but from the radical anion (although formally from the Pc LUMO in both cases). In this sense, the authors proposed that these differences could be explained most likely in terms of electrostatic interactions that increases the potential barrier for the electron injection, in particular with the presence of the Spiro-OMeTAD cation nearby. Moreover, based on modelling studies, a time of 50–100 ps was estimated for the motion of the Pc toward the surface, which is in the same range than sET. This seems not to affect the electron injection of the excited Pc (sinj) and charge separation (sCS) process that are much faster (<15 ps). Therefore, another possible contribution could also come from a possible reorganization energy required to achieve a conformation favourable for the electron transfer from the phthalocyanine anion. Regarding the aggregation effect, some additional insights were revealed by TA measurements. A sharp band at 995 nm was observed for 2/ZnO and 4/ ZnO but not for 1/ZnO and 3/ZnO, corresponding to the Pc anion specie. In the absence of HTM, the proposed mechanism of formation involves the charge separation in the Pc aggregates (PcPc* ? Pc+Pc), process that can compete with the electron-injection from the excited Pc into ZnO (ZnO/Pc* ? ZnOr/Pc+). In principle, the Pc anion of the charge separated aggregate (Pc+Pc) could still inject an electron but it is in competition with the charge recombination (Pc+Pc ? PcPc), which is usually the most predominant deactivation pathway, thus making the electron transfer less efficient. By comparison of the relative intensities of the band at 995 nm at 30 ps of delay, some qualitative information about the inter-Pc charge separation, and thus degree of aggregation, could be estimated. Over the four devices, Spiro/ZnO/3 displayed the highest relative intensity thus being the most aggregated of the four Pcs. Next, to quantify the efficiency of charge-separations process at the Pc/HTM interface, the relative intensity at 1250 nm was monitored as an indicator of the Spiro-MeOTAD cation band. Spiro/1/ ZnO and Spiro/4/ZnO, with band ratios of 0.76 and 0.64 respectively, have the highest yield of holes in the HTM Spiro-OMeTAD layer, whereas those of Spiro/2/ZnO (ratio of 0.36) and Spiro/3/ ZnO (ratio of 0.19) displayed the lowest. For Spiro/2/ZnO, higher degree of aggregation, and eventually a different type of packing on the ZnO surface, explained the lower yield obtained with this

Fig. 46. Anthraquinone-Pc (APC) reported by Zhang et al. [218].

Pc (almost twice less) with respect to the similarly bulkysubstituted Pc-1. Despite having the strongest Pc anion feature without the Spiro-OMeTAD layer, Spiro/3/ZnO displayed the lowest long distance charge separation yield, which was directly attributed to a strong aggregation effect, Pc-3 being indeed the most aggregated on the surface of the series (vide supra). As mentioned above, there are drastic changes is in the rate of the primary charge separation reaction, before (Pc*/ZnO ? Pc+/ZnO) and after (Spiro/ Pc*/ZnO ? Spiro+/Pc/ZnO) deposition of the HTM layer, namely electron-injection (sInj) and charge separation (sCS). With the exception of Pc 4, the sCS was found almost twice longer than sinj, which should result, in principle, in two competing process in the Spiro/Pc/ZnO devices. Nerveless, the CS pathway was found predominant within this configuration, which means that the addition of the HTM layer slows down the electron injection from the Pc into the ZnO. From these results, the authors suggested that the alignment of the phthalocyanine molecules on the ZnO nanorod surface should be different with and without the Spiro-OMeTAD layer meaning that the HTM should have an effect on the Pc layer organization. The narrowing of the Pc Q-band upon addition of Spiro was interpreted as a change in arrangement and lower degree of aggregation, which was rationalized by assuming that the SpiroOMeTAD molecules partially penetrate between the Pc molecules. This could be visualized by a more upright alignment of the Pcs in the SAMs covered by the HTM. As a result, the distance of the Pc molecules from the ZnO was increased, and slower electron injection rates into the ZnO were obtained. Among the four Pcs, Pc-1 displayed the least aggregation and no looses in photon-tophotocurrent conversion nor holes transfer efficiency into the HTM were detected by time-resolved spectroscopy, making 1 the best candidate of this series of Pc-sensitizers for DSSC applications. Importantly, the rate constant of electron injection for this Pc was determined to be 5 ps, which is roughly one order of magnitude slower than that for a typical porphyrin (ZnP) on ZnO surface (<0.2 ps) [210]. This makes a sharp contrast with the values reached by the best efficient sensitizers reported in TiO2-DSSC, for which electron injection rate are much faster, lying in the femto- to picosecond time scale [187,211,212]. Nevertheless, this should, in principle, be compensated by the rather long lifetime of its radical cation (>10 ns) to target efficient solid-state DSSC. 4.3. Impact of the anchoring group

Fig. 45. Molecular structures of TT6, TT7 and TT15 with modified anchoring moieties with respect to benchmark TT1, reported by Nazeeruddin, Grätzel, Torres, and collaborators [179].

The Zn(II)-Pcs TT1, TT6, TT7 and TT15 reported by the groups of Nazeeruddin, Grätzel, Torres, and discussed in the previous Section 3.3 (vide supra) [179] are substituted with tert-butyl groups at three peripheral positions and differ only by the fourth

M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

anchoring ligand (Fig. 45). These dyes reached 3.56%, 3.38%, 2.55%, and 3.96% of efficiency in DSSC, respectively. To shed light on these differences of performances, and assess the effect of the anchoring group on the electron-injection process (EI) and recombination dynamics at the Dye/TiO2 interface, these Pcs were further investigated by in-depth photophysical studies [183].These parameters are indeed critical (especially the EI/BET ratio), since they affect the overall device efficiency. From the transient absorption spectroscopy studies, the EI process was described by a biexponential decay, composed of an ultrafast component (s1  700 fs) for all Pcs, and another slower one (s2, ps time scale) different in each cases. Since the four Pcs differ only by the anchoring ligands, and given the comparable value of s1 for all of them, the authors concluded that the ultrafast EI process (700 fs) does not occur via the anchoring ligand but instead likely via a trough-space mechanism from the Pc core to TiO2. In turn, the second slower component (ps) was found much slower in the case of TT7 (s2 = 21 ps) and TT6 (s2 = 16 ps) than the other two dyes TT15 (s2 = 5.8 ps) and TT1 (s2 = 5.7 ps), which was therefore ascribed to a through-bond process. It was concluded that the EI process becomes less efficient upon introducing a spacer between the carboxylic acid and the Pc in TT6 and TT7 in comparison with TT1 with no spacer. The introduction of a cyano group in the linker of TT7 clearly slower even more the EI process when compared to TT6 (same linker, no cyano group). The authors postulated that the cyano group acts as an electron trap, thus precluding the EI of the dye into the TiO2. Surprisingly, TT15 using the same linker than TT6 but with two carboxylic acid instead one, gave similar kinetics than TT1. This was explained by the presence of two carboxylic acid functionalities (instead of one in TT6) that compensated the negative effect of a longer bridge, by means of stronger electronic couplings with the Ti orbitals. Next, the back electron transfer (BET) was monitored by tracking the signal at 630 nm that consists mostly of the ground state bleach (GSB). The BET occurred over a broad time windows via a three-component process (rate constants s2, s3 and s4) ranging from ps to ns time scales, involving complex recombination mechanisms between injected-electrons and oxidized dyes (Zn(II) Pcs radical cations). It was clearly observed slower recombination for TT15, which, in addition to fast EI similar to that of TT1 (vide supra), account for to its superior efficiency (3.96% vs. 3.56%). In turn, the cyano group in TT7 enhanced the BET in comparison with TT1 and TT6, which accounts for its lower performance.

Fig. 47. Non-peripheral S-aryl substituted Zn(II)Pc Zn-Thio-Pc reported by Giribabu and co-workers [219].

37

5. Enhancement of light harvesting properties and panchromatic response of Pcs To improve the light-harvesting properties of a dye, a first strategy consists in incorporating chromophore moieties that complement the absorption of the Pc, acting as secondary energy collectors (antennae). Alternatively, near-IR (NIR) absorption and extended red response of a sensitizer have been envisaged as one of the most powerful strategies to get optimum advantage of the sun’s photons [213–217] considering that 50% of the solar photon flux occurs beyond 750 nm. The connexion of p-elongated systems to the Pc core or p-extended Pcs have been the most common pathways in that direction. An alternative, and often complementary, strategy for this purpose is to tune the electronic properties of the macrocycle itself by connecting peripheral or non- peripheral groups through heteroatoms (O, S, N) directly bonded to the Pc, which can affect drastically both energetics and optical properties.

5.1. Tuning optical properties and energetics of the Pc trough directlybonded heteroatoms A first interesting and representative example was reported in 2012 by Zhang et al., who reported and tested in DSSC an unsymmetrical anthraquino-Pc (APC) decorated with three peripheral tert-butoxy groups (Fig. 46) [218]. The purpose to introduce an anthraquinone moiety connected to the Pc trough an oxygen atom was to increase the pconjugation to get a panchromatic response of the Pc, while the three tert-butoxy groups act as electron releasing groups (‘push’), which in addition, enhance the solubility of the Pc in common organic solvents. Although the fabricated APC/DSSC achieved only a modest PCE of 0.71% mainly caused by aggregation issues, the photoresponse of the Pc sensitized film was extended up to 750 nm which constituted, nevertheless, the viability of such proof-of-concept that has been followed later on successfully. One year later, Giribabu and co-workers described a Pc bearing six S-aryl groups at the a-positions (i.e. non-peripheral positions) and a carboxylic acid directly connected to the Pc core (Fig. 47) [219]. The sulphur atoms directly bonded to the Pc perturbs significantly the electronic properties of the macrocycle, destabilizing the HOMO level and red-shifting the Q-band absorption toward the NIR-region with a maximum at 750 nm. Despite these better optical properties and suitable location of the HOMO-LUMO levels, the performances of this dye were surprisingly low (Table 30), achieving a maximum PCE of only 0.4%. This was assigned to dye-loading issues because of a possible steric hindrance of the a-thio-aryl substituents with the TiO2 surface. The systematic drop in performances observed upon incorporation of a co-adsorbent (CHENO), even in low ratio, also supports this hypothesis. Along the same lines, an interesting molecular design was reported by Zhang et al. for the construction of non-aggregating, low-bandgap Pcs, and involved the implementation of bisethylamino, bispropylamino or bisbutylamino groups at the periphery (Fig. 48) [220]. Also, an amino acid moiety was used as the anchoring group. These Pcs displayed broad absorption features and redshifted Q-bands with maxima at 646 nm and 690 nm in DMF solution. When adsorbed on the TiO2 films, the absorption features of these Pcs were further significantly broadened and redshifted toward the NIR-region. Poor performance was, nonetheless, observed in all cases (Table 31), with dye TPC giving rise to the highest photocurrent of 2.07 mA/cm2 and a PCE of 1.67%. The reasoning behind these unimpressive results was based on the low solubility, aggregation issues and poor electron injection capability.

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Table 30 Photovoltaic data of DSSCs sensitized with Zn-thio-Pc and variable content of coadsorbent, under simulated AM1.5G illumination.a,b,c Data derived from [219].

a b c d

[Dye]d (mM)

[CHENO]d (mM)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

0.2 0.2 0.2

0 5 20

502 480 510

1.26 0.64 0.54

64 70 51

0.40 0.22 0.14

(%)

Double-layered TiO2-films of [8+4] lm thickness (transparent and scattering layer, respectively). Active area of 0.158 cm2. Composition of the electrolyte: 0.6 M BMII, 0.1 M LiI, 0.03 M I2, 0.05 GuNCS and 0.25 M TBP in a 15/25 mixture (v/v) of valeronitrile and AcCN. Dye–uptake solutions were prepared in a 9:1 mixture of THF and EtOH, incorporating the indicated amount of co-adsorbent (CHENO).

Fig. 48. Molecular structures of alkylamino-subsituted Pcs reported by Zhang et al. [220].

5.2. Pcs functionalized with p-conjugated systems Li et al. reported on the synthesis of a series of hyperbranched phthalocyanines bearing different metals in the internal cavity, aiming to demonstrate the ability of these structures to suppress aggregation on the titania surface and be efficient in DSSC architectures (Fig. 49) [221]. The influence of the central metal into the photovoltaic characteristics was studied in detail and showed to be significant (Table 32). This kind of hyperbranched Pcs had already been successfully used in other applications, such as solar cells [222], organic light-emitting diodes [223], and high energy density and pulsed capacitors [224]. Absorption spectra of the newly synthesized Pc showed clearly the Q-band of the monomeric species, and both the Q- and Soret- bands’ position was influenced by the central metal, with a red shift being observed in the order of Co, Cu, Zn and AlCl. Non-aggregation was also mentioned by the authors by observing the sharpness of the peaks, both in the absorption and the emission spectra. Furthermore, emission spectra revealed efficient energy transfer after photoexcitation from the Soret to the Q band for the free-base, AlCl and Zn Pcs, as well as long exciton lifetimes. Additional findings deriving from the emission spectra were that the peaks were the result of the individual Pcs’ fluorescence. For the Cu and Co analogues there was no and very weak Q-band emission observed respectively, as

expected for such metallo Pcs [225]. Further insights came from femtosecond time-resolved fluorescence spectroscopy experiments, and one of the conclusions made was that the presence of central metals in these hyperbranched structures could accelerate the ultrafast energy transfer from the Soret to the Q-band and also that excitation was more localized in the Soret band. As well, for ZnPc-5, ultrafast electron injection to the TiO2 was revealed from both the Soret and Q bands. The authors also conducted electrochemical experiments that demonstrated appropriate energy levels for both electron injection and dye regeneration for all dyes. Lastly, the photovoltaic characteristics of the new hyperbranched Pc structures were reported, with dye 5 bringing about the highest overall efficiency of 1.15% under optimized device fabrication conditions. Overall, all dyes demonstrated high F.F., which was attributed by the authors to efficient dye disaggregation and suppression of interactions with the electrolyte due to the sterically bulky surrounding substituents. The generally low photovoltages were ascribed to small blocking effect, and the low photocurrents to inefficient light-harvesting capabilities. Another interesting Pc design was described by Giribabu and co-workers, who prepared Pc-Org-1 bearing three bulky and electron-donating triphenylamine groups and two carboxyl units (Fig. 50) [226]. The triphenylamine group (TPA) was selected as a means to extend the absorption of the dye, complementing that of the Pc (B-band) in the 400–450 nm region, and filling better the gap between 450–550 nm. However, Pc-Org-1-sensitized cells exhibited relatively poor performances despite better optical properties, and adequate location of their HOMO and LUMO energy levels for electron injection and dye regeneration, respectively. As expected, the IPCE spectra of Pc-Org-1 was broader, slightly redshifted, and showed a better photoresponse in the 400–600 nm range than the analogue dye PCH001 bearing a peripheral tertbutyl decoration, attributed to the additional absorption of the TPA moieties in this spectral window. However, the IPCE response was considerably lower in the 600–800 nm range (a more photonrich region) with a maximum value of only 22%, while PCH001 reached 48% (values of optimized cells; Table 33). Thus, the overall integrated IPCE value was higher for PCH001 than Pc-Org-1 matching well their respective observed photocurrent of 5.55 mA/cm2 and 3.90 mA/cm2. In addition to significantly lower

Table 31 Photovoltaic data of DSSCs sensitized with SPC, TCP, and FPC under simulated one sun illumination (AM1.5G).a,b,c,d Data derived from [220].

a b c d

Dye

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

SPC TPC FPC

480 550 560

1.53 2.07 1.89

57 59 60

1.42 1.67 1.64

Single-layered TiO2 films of 6 lm thickness. Active area of 0.74 cm2. Dye-uptake solutions consisted of 0.05 mM of Pc in organic solvent (soaking time 16 h). Composition of the electrolyte: 0.05 M I2 and 0.5 M LiI in AcCN.

(%)

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Fig. 49. Hyperbranched Pcs HBMPc-COOH 1–5 reported by Li et al. [221].

Table 32 Photovoltaic parameters of DSSCs sensitized with HBMPc-COOH 1–5, under simulated AM1.5G one sun illumination.a Data derived from [221].

a b c

Dye

Film thickness (lm)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

1 2 3 4 5 5

[10+5]b [10+5]b [10+5]b [10+5]b [10+5]b 5c

6 24 5 9 67 (n/a)

434 424 324 420 491 530

0.837 1.05 0.425 0.805 3.20 2.05

64.4 66.2 70.0 68.8 73.1 76.7

0.23 0.29 >0.1 0.23 1.15 0.83

(%)

Composition of the electrolyte: 0.6 M BMII, 0.025 M LiI, 0.04 M I2, 0.05 M GuNCS, and 0.28 M TBP in a 15:85 mixture (v/v) of valeronitrile and AcCN. Double-layered TiO2 films, thickness of transparent and scattering layer, respectively. Single-layered TiO2 film.

VOC (420 mV vs. 570 mV), the optimized Pc-Org-1/cells achieved a maximum PCE of 1.09% without CHENO, twice less than PCH001 (2.17% in the presence of CHENO). It is worth noticing that the use of co-adsorbent did not improve this result, indicating that the bulky TPA substituents efficiently suppressed aggregation. This was also reflected in the optimal soaking time, with the dyes displaying different behaviour: the PCE of Pc-Org-1 cells gradually improved the PCE with the soaking time, reaching a maximum after 16 h, while that of PCH001 was optimal at shorter time (8 h) with longer periods resulting in a decrease of the performances, a fact that was explained by overloading caused by dyeaggregation. It was also suggested that the low overall efficiency

was due to the poor electron-donating character of the triphenylamine group (one would assume that the TPA moieties should be poorly conjugated to the Pc macrocycle through a phenyl-type bridge). In a different example, following their previous work on multialkylthienyl appended porphyrins [227] and moving along the successful thienyl-based anchoring groups developed for this class of porphyrinoid [228–230], Tan and co-workers applied this strategy to Pc with the aim to shift the absorption features towards the red part of the solar spectrum. The two dyes PC-HY1 and PC-HY2 are anchored through a thienyl-acrylic acid group, and in addition, are endowed with tert-butylphenyl (PC-HY1) or tert-butylthienyl

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(Table 35). The poor results were rationalized in terms of formation of molecular aggregates. Analogously, Erten-Ela et al. prepared two 4-carboxyphenyl ZnPcs, endowed with 2-thienyl or 50 -hexyl-2,20 -bithiophene peripheral substituents (Fig. 53) [233].The introduction of hexylchains in ZnPc-2 aimed to improve solubility of the Pc, reduce aggregation and lower recombination processes, while the purpose of the two-thiophene units was to improve the panchromatic response of the Pc thought extension of the p-aromatic system of the Pc core. As expected, absorption spectrum in THF solution showed that the Q-band absorption of the bithiophenesubstituted Pc ZnPc-2 (kmax = 700 nm) was red-shifted in comparison with 3-thienyl substituted Pc ZnPc-1 (kmax = 684 nm). In addition, a decreased of the HOMO-LUMO gap was also obtained upon increasing conjugation length in ZnPc-2. Combining these favourable, the PCE obtained for ZnPc2/DSSC (1.12%) was by far superior than that of ZnPc1 (0.25%) (Table 36). The relatively poor efficiencies of these two dyes were also rationalized, as well, in terms of high-aggregation tendency of these systems. Using a different approach, the Kimura’s group incorporated thiophene units at two non-peripheral (a,a0 ) positions in XShaped Pcs aPcS1 and aPcS2 (Fig. 54) [234]. Following the success of their non-aggregated predecessors previously reported (PcS dyes, vide supra), a similar peripheral (b)-substitution pattern using diisopropylphenoxy groups was chosen for these Pcs. Remarkably, it is important to notice that the anchoring group in these new Pcs is positioned at one of the non-peripheral (a)position, which are the only known examples for Pc-based sensitizers in DSSC. The introduction of p-conjugated thiophene units at the a-positions of the Pc macrocycle affected significantly the optical and electrochemical properties, and consequently the HOMOLUMO energy levels. In particular, an important red shift of the Q-band was observed for these low-symmetrical Pcs in comparison with the corresponding symmetrical b-(diisopropylphenoxy)octasubstituted model compound. In sharp contrast with common Pcs, aPcS1 harvests photons in the green region of the solar spectrum and showed a panchromatic response in the range of 400–800 nm. In addition, the ring extension trough the naphthalene moiety in aPcS2 produced a split and slightly redshift of the Q-band (687 nm and 708 nm) with respect to aPcS1 (702 nm), as well as a smaller bandwidth. As on could expect, the adsorption densities of these two Pcs, substituted with two large substituents at the a-positions, were found lower (2–3 times less) than similar b-substituted analogues previously reported

Fig. 50. Molecular structure of Pc-Org-1 reported by Giribabu et al. [226].

(PC-HY2) groups in the periphery (Fig. 51 and Table 34) [231]. The thienyl-decorated derivative yielded, indeed, a Q-band 13 nm bathochromically shifted in comparison to PC-HY1, and also exhibited slightly improved, even though still low, adsorption densities (C) on the TiO2 surface. Cyclic voltammetry indicated satisfactory driving force for electron injection and regeneration for both Pcs, and the overall PCE was higher for PC-HY2 (1.09%) with respect to PC-HY1 (0.79%). Enhanced light absorption and extended lifetime of the excited-state due to the peripheral thiophene units were accounted for the better result. In the same context, Ince et al. described highly conjugated panchromatic ZnPc sensitizers 1 and 2, bearing triarylamineterminated bisthiophene and hexylbisthiophene units (Fig. 52) [232]. In terms of the optical properties, a panchromatic response and significant redshift of about 30 nm in the maximum of absorption were observed for the two dyes in comparison to TT1, which was attributed to the extended p-conjugation of these systems. Aggregation phenomena could be detected from the broad absorption, which extended up to 850 nm, and decreased the extinction coefficient of the Q-band. Photovoltaic measurements indicated consistently low VOC but relatively enhanced JSC, under varying conditions of electrolytes, co-adsorbent and TiO2 film thickness, with PCEs not exceeding 2.70% even in the presence of CHENO

Table 33 Parameters and photovoltaic data of DSSC devicesa,b,c,d sensitized with Pc-Org-1 under simulated AM1.5G one sun illumination (PCH001/DSSC was used as benchmark). Best PCE values are bolded and underlined for each sensitizer. Data derived from [226]. Dye

Dye/CHENO Ratio

Soaking Time (h)

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

Pc-Org-1

1:0 1:0 1:0

2 8 16

14 19 22

450 420 420

2.40 3.22 3.90

68 67 66

0.74 0.89

1:1200e 1:1200e 1:1200e 1:0 1:0 1:0 1:1200e 1:1200e

2 8 16 2 8 16 2 8

11 16 27 33 37 10 52 48

450 420 460 590 500 510 570 570

1.58 2.25 3.36 3.06 3.18 1.16 4.50 5.55

68 66 68 72 71 67 70 68

1:1200e

16

42

590

3.73

70

PCH001

a b c d e

Double-layered TiO2 films of [8+4] lm thickness (transparent and scattering layer, respectively). Active area of 0.74 cm2. Pc-Org-1 in THF or PCH001 in EtOH at a concentration of 0.05 mM in the dye-uptake solution. Composition of the electrolyte used in this study: 1.0 M LiI and 0.05 M I2 in c-butyrolactone solution. 60 mM of CHENO was incorporated in the dye-uptake solution corresponding to a Dye/CHENO ratio of 1:1200.

(%)

1.09 0.49 0.62 1.05 1.13 1.12 0.40 1.79 2.17 1.55

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Fig. 51. Molecular structures of PC-HY1 and PC-HY2 reported by Tan and coworkers [231].

(PcS18, 14  105 mol/cm3), as a result of reduced packing density on the TiO2 surface (Table 37). On the other hand, and as successfully implemented in PcS19 and PsS20 (vide supra), aggregation was significantly reduced by the presence of the bulky phenyloxy groups at the periphery. Therefore, in these cases too, only relatively low amount of CHENO (Pc/CHENO ratio of 1:10) were necessary to reach optimal performances, which in turn decreased also the adsorption densities. Unfortunately, the performances in DSSC of the new Pcs aPcS1 and aPcS1 (PCE = 5.5% and 3.8%, respectively) were found slightly less than the former b-anchored analogue PcS18 (PCE = 5.9%) despite improved panchromatic responses. In addition to lower adsorption densities, it is believed that the main reason behind this, was the poorer electronic coupling between the ZnPc core and TiO2 caused by the a-connexion of the anchoring group, thus lowering the electron-injection efficiency. This was supported by comparison of the maximum IPCE value in the Q band region, between aPcS1 (47%) and PcS18 (ca. 80%), indicative of a less efficient electron-injection capability of the former. Another interesting example was described by Terejina et al. with TT65 bearing rigid 2,6-diarylphenyl groups connected through ethynyl linkers (Fig. 55) [235]. Remarkably, these pconjugated peripheral substituents produced a remarkable redshift of the Pc Q-band up to 700 nm. Unfortunately, this Pc achieved a moderate efficiency of only 1.6% even in the presence of CHENO. The main explanation of these low performances was the higher tendency of this Pc to aggregate by mean of extension of the pconjugation of the macrocycle trough these groups, which do not prevent efficiently p–p stacking between the Pc cores. 5.3. p-Elongated Pcs and related analogues From a different molecular design perspective, naphthalocyanines (NPcs) have relevant characteristics: their extended p-electron system compared to phthalocyanines results in modified optical characteristics (intense absorption in the NIR) [236] as well as redox potentials, electrical conductivity, photoconductivity and catalytic activity. Naphthalocyanines also stand out for their

robustness. Li et al. were the first to carry out an assessment of peripherally tetra-substituted naphthalocyanines in a photoelectrochemical solar cell, via immobilization on the TiO2 surface, but they found no significant photocurrent generation [237]. Similarly, Peng and co-workers described the synthesis and DSSC assessment of two Pc/Nc hybrids mononaphthalo-triphthalo cyanine, incorporating one carboxy-naphthalene unit and decorated with either nbutoxy (nBuO-ZnPcNc) or tert-butyl (tBuZnPcNc) groups at the remaining peripheral positions (Fig. 56) [238]. UV–Vis spectroscopy indicated broad absorption of the new dyes in the red/NIR region and a 30 nm bathochromic shift compared to most common Zn(II)Pcs, which was attributed to the incorporation of the naphthalene unit. Additionally, between tBuZnPcNc and nBuO-ZnPcNc, the later showed a 5 nm redshift compared to the former, and this was ascribed to the presence of the butoxy groups acting as electron-releasing units as discussed previously in Section 5.1. As for the spectra of the sensitized TiO2 films, enlargement of the Q-band implied aggregation phenomena. The authors also performed DFT calculations on the new ZnPcNc dyes, which revealed a large driving force for directional electron injection, based on the fact that the HOMO was found to be delocalized over the entire molecule whereas the LUMO across the Pc ring and moving to the carboxyl group. Turning to the photovoltaic behaviour, the two Pcs showed similar IPCE curves, with maxima at 60.7% for the tert-butyl analogue but only 22.9% for the nbutoxy analogue at the Q-band region. This lower photoresponse was assigned to possibly more pronounced dye stacking on the semiconductor surface for nBuO-ZnPcNc. Consequently, PCE reached 3.56% for tBu-ZnPcNc whereas it did not surpass 2.20% for nBuO-ZnPcNc. Interestingly, correlating the molecular design of tBu-ZnPcNc with TT1, the addition of one benzo group seems to extend the conjugated system sufficiently to get an enhancement in the PCE (from 3.52% in TT1 to 3.56% for tBu-ZnPcNc). Prompted by these results, the authors extended their studies on tBu-ZnPcNc, to evaluate the impact of co-adsorbent (CHENO) and adsorption temperature on the DSSC performance (Table 38) [239]. In this respect, it was observed that both the concentration of co-adsorbent and temperature of sensitization have an impact on the amount of adsorbed dye. Preparation of DSSC devices at various temperatures and Dye/CHENO ratios assisted in defining the optimal sensitization conditions, which are 5 °C and 1:150, respectively, and for which the higher JSC and VOC were obtained to lead an overall PCE of 2.89%. Notably, this result was almost twice as high as the one obtained in the absence of co-adsorbent and was therefore attributed to lower dye-aggregation, more efficient electron-injection and slower charge-recombination kinetics. Regarding the influence of the co-adsorbent, higher concentration resulted in lower dye-loading because of the competitive adsorption of the two, but the aggregation was reduced, thus improving the performance of the DSSC. This was also supported by UV–Vis experiments that indicated that the H-aggregation of tBu-ZnPcNc was diminished upon higher CHENO concentrations. As well,

Table 34 Adsorption densities (C) and photovoltaic parameters of DSSC devicesa,b,c sensitized with PC-HY1 and PC-HY2, under 1 sun illumination (AM1.5 G). Data derived from [231]. Dye PC-HY1 PC-HY2 a b c

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm2) 7.52 8.16

480 510

2.31 3.00

71 71

0.79 1.09

C/109

Dye-uptake solutions: 0.5 mg/mL of Pc in DMF. Electrolyte: 0.5 M LiI, 0.05 M I2 and 0.5 M TBP in 3-methoxypropionitrile solution. Film thickness not reported.

(%)

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Fig. 52. Bisthiophene- peripherally substituted Zn(II)Pcs 1 and 2, reported by Ince et al. [232].

electrochemical impedance spectroscopy and open-circuit photovoltage decay (OCVD) curves showed that recombination between the injected electrons and the electrolyte was impeded and electron lifetime was prolonged in the presence of CHENO. On the other hand, the temperature had an impact on the adsorption kinetics of the dye to the TiO2 surface, hence on dye-loading. Under the same optimized conditions with respect to the Dye/CHENO ratio (1:150), the optimum temperature for reaching the highest PCE value was found to be 5 °C. At low temperature (15 °C) the JSC was reduced, which was explained by incomplete formation of the dye-monolayer on the surface as well as low dye-loading. At high temperature, the JSC was also found to be reduced, which was reasoned in terms of formation of multilayer or dyeaggregation due to surface coverage that exceeded the monolayer saturation limit. Continuing on the same path, the authors went further to compare the performance of tBu-ZnPcNc (renamed Zn-tri-PcNc in this new study) with a similarly designed tetraazaporphyrin analogue (Zn-tri-TAPNc), a study that gave additional insights into the influence of an enlarged p-system (Fig. 57 and Table 39) [240]. According to the UV–Vis spectra, there was a significant blue shift of the Zn-tri-TAPNc Q-band with respect to Zn-tri-PcNc, owing to the less conjugated system (lacking three fused-benzo rings), which also resulted in lower extinction coefficients and, thus, decreased light-harvesting properties. Also, electrochemical analyses revealed a more negative HOMO for the TAP analogue, which could be translated in a better dye regeneration ability. However, the LUMO of the Pc-hybrid was found to be higher in

energy to that of the TAP analogue as a result of higher degree of conjugation, leading to a more efficient electron injection into the TiO2. Additionally, OCVD curves showed a faster VOC decay trend for the Zn-tri-TAPNc-sensitized devices, indicating increased charge recombination for the less extended p-system of the TAP analogue. Overall, t-Bu-tri-ZnPcNc yielded better DSSC performances, with a PCE more than twice higher to that of Zn-triTAPNc, and the significant differences in photocurrent being ascribed to the lower number of benzo groups in the TAP analogue, affecting the degree of p-electron conjugation. In a higher extend, Kimura’s group reported recently two ringexpanded naphthalocyanine-based sensitizers NcS1 and NcS2 (Fig. 58) [241], following the molecular design based on the success of their previously reported ZnPc-based analogues (PcS). Remarkably, these new dyes display splitting Q-bands, which was ascribed to the lower symmetry of benzonaphthaloporphyrazines. Furthermore, relatively important redshifted absorption of their Q-band (Dkmax = +80–90 nm) were observed in comparison with their Pc-analogue (PcS18), thus harvesting farther in the near-IR region of the solar spectrum (kmax = 768 nm and 780 nm for NcS1 and NcS2, respectively). The size enlargement of these ring-extended macrocycles produced a decrease in the dye-loadings of both NcS1 (C = 9.4  105 mol/cm3) and NcS2 (C = 2.4  105) with respectect to the Pc analogue PcS18 (C = 13  105 mol/cm3). This drop was only moderate for NcS1 with short iPr chains, but much more important for NcS2 with longer octyloxy chains as a result of steric crowding around the macrocycle ring. This factor can correlate directly and mostly the difference observed in the JSC between the two sensitized devices (JSC = 6.2 mA/cm2 and 1.7 mA/cm2 for NcS1 and NcS2, respectively; Table 40). Notably, NcS1/DSSC displayed an IPCE response red-shifted by 50 nm compared with that of the PcS18 cell, ranging from 600 nm to 850 nm and with a maximum value of 41% at 760 nm. However, the best PCE values of the DSSCs were relatively low for these two dyes (2.4% and 0.6% for NcS1 and NcS2, respectively) in comparison with the Pc analogue PcS18 previously reported (PCE of up 5.9%). Remarkably, the use of coadsorbent (Pc/CHENO ratio of 1:50) allowed improving the PCE of NcS1/DSSC from 2.4% to 3.2%. Concurrently, however, the dye-loading of NcS1 was considerably decreased (C = 1.2  105 mol/cm3) by mean of competitive adsorption of the coadsorbent. This witnesses the superior tendency of these p-elongated systems to aggregate, contrasting with the Pc analogue system PcS18 for which aggregation was fully suppressed by the presence of the same bulky peripheral substitution and hence did not required coadsorbent. The lower dye-loading

Table 35 Photovoltaic parameters of DSSC devices sensitized with 1 and 2 under 1 sun illumination (AM1.5 G).a,b,c,d Data derived from [232]. Dyea

CHENO/Dye Ratio

Dipping Time (h)

Electrolyteb,c

Film thickness (lm)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

1

0:1 20:1 100:1 20:1 20:1 100:1 0:1 20:1 100:1 20:1 20:1 100:1

5 5 5 5 15 15 5 5 5 5 15 15

M1 M1 M1 A6986 A6986 A6986 M1 M1 M1 A6986 A6986 A6986

6 6 6 6 [6.8+5]d [6.8+5]d 6 6 6 6 [6.8+5]d [6.8+5]d

503 541 554 483 471 490 525 543 549 498 466 487

3.26 5.25 4.17 7.35 8.32 8.18 3.65 4.96 4.46 7.05 8.34 7.05

68 73 75 70 69 71 71 74 74 70 69 72

1.11 2.07 1.72 2.49 2.65 2.85 1.36 1.98 1.82 2.45 2.70 2.47

2

a b c d

Dipping solutions consisted of 0.1 M of Pc in THF. Composition of the electrolyte M1: 0.6 M (BMII), 0.025 M LiI, 0.04 M I2 0.05 M GuNCS and 0.28 M TBP in a 15:85 (v/v) mixture of valeronitrile and AcCN. Composition of the electrolyte A6986: 0.6 M (BMII), 0.1 M LiI, 0.04 M I2 and 0.05 M TBP in a 15/85 (v/v) mixture of valeronitrile and AcCN. Double-layered TiO2 films: thickness of the transparent and scattering layer, respectively.

(%)

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Fig. 53. Thienyl- and bisthienyl- peripherally substituted Zn(II)Pcs reported by Erten-Ela et al. [233].

and higher-aggregation tendency explain the limited JSC obtained for NcS1 (8.2 mA/cm2) when compared to that of PcS18 (13– 14 mA/cm2, Table 11). 5.4. ABAC-type or ‘‘Push-Pull” Pcs Inspired by the success of push-pull porphyrins in DSSC, Fazio et al. reported the first examples of ABAC-type ‘‘push-pull” Pcs (Fig. 59) [242]. Despite the elegant and successful synthetic strategy to access these unprecedented Donor–p–Acceptor chromophores, these dyes showed, however, low-to-moderate PCEs in DSSC with a maximum value of 2.4% for Pc 8 (Table 41). It was rationalized that the presence of the CF3 moieties, with a strong electron-acceptor character, pulls the LUMO electron density over the Pc core rather than the electron-acceptor/anchoring group, as predicted by DFT and TDDFT calculations. These groups were nevertheless essential from a synthetic point of view, since only the 3,6-bis-(3,5-bistrifluoromethylphenyl)phthalonitrile has proven to be effective for the preparation of the key precursor, the iodocontaining ABAB-Pc (further functionalized to the ABAC- push-

pull dyes 7–9). A positive point is that these Pcs demonstrate a lack of aggregation in solution, as supported by UV/Vis spectroscopy. This remarkable low tendency of these Pcs to aggregate made therefore the use of co-adsorbent unnecessary for the fabrication of the DSSC and, indeed, lower performances were obtained when using CHENO, as exemplified for Pc 7. The benzothiadiazole (BTD) bridged anchoring group, successfully implemented in the champion porphyrin dyes SM315 [53] and GY50 [54], surprisingly gave rise to the worst performances of the series when transposed in Pc9, even lower than those of the analogue Pc-8 having the same structure but with an carboxyethynyl group instead. Similarly observed with porphyrins, the presence of the BTD unit improved the optical properties of the porphyrinoid, especially broader absorption features, as well as larger extinction coefficients that were observe for the Pc Q-band. The dye-loading density for this Pc was significantly lower than those of the two other ones, and in particular in comparison with the analogue Pc-8 (twice less). This constitutes a first important contribution to explain the low photocurrent obtained with this sensitizer. In addition, enhanced recombination rate caused by the BTD unit is sought to be the second contributing factor. Pc-7 and Pc-8, differing only by the donor group, respectively a 4-ethynyl-N,N-dimethylaniline and bis(hexyloxy)phenylamino, also displays rather different behaviour and performances. First, the more bulky donor group in Pc-8 than in Pc-7 logically reduced the dye-loading density by >3-time for the former. However, the opposite trend was observed in the photocurrent, with Pc-7 that displays higher JSC than Pc-8, in agreement with their IPCE response. This is indicative that the former Pc is more efficient in DSSC. A first contribution should come from their difference in light-harvesting properties, with dye 8 showing indeed a redshifted and broader IPCE response, covering better the region between 475 and 550 nm, in which Pc-8 displays a significant drop. Next, as supported by DFT calculations, the degree of

Table 36 Photovoltaic parameters of DSSC devicesa,b,c sensitized with ZnPc1 and ZnPc2 under 1 sun illumination (AM1.5 G). Data derived from [233].

a b c

Dye

Film thickness (lm)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

ZnPc1 ZnPc2

[7+5]c [7+5]c

500 500

1.08 3.81

47 59

0.25 1.12

Dye-uptake solutions: 0.5 mM of Pc in a 1:1 mixture (v/v) of THF and tBuOH. Composition of the electrolyte: 0.6 M BMII, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in AcCN solution. Double-layered TiO2 films: thickness of the transparent and scattering layer, respectively.

Fig. 54. Molecular structures of X-Shaped Zinc Phthalocyanine aPcS1 and aPcS2 reported by Kimura and co-workers [234].

(%)

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Table 37 Photovoltaic parameters of DSSC devices sensitized with aPcS1-2 under 1 sun (AM1.5G; 100 mW/cm2).a,b Data derived from [234]. Dye/Conc.c (mM)

[CHENO] (mM)

Absorption density (mol/cm3)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

aPcS1/0.05

0 0.5 0 0.5

6.8105 4.1105 5.2105 2.8105

610 620 600 630

10.1 11.9 7.1 7.8

73 75 74 77

4.5 5.5 3.2 3.8

aPcS2/0.05 a b c

(%)

Single-layered TiO2 films of 6 lm. Composition of the electrolyte: 0.1 M LiI, 0.5 TBP and 0.05 M I2 in AcCN solution Dye uptake solutions were prepared in THF at a concentration of 0.05 mM of Pc and the indicated amount of coadsorbent (CHENO); dipping time: 3 h.

6.1. Ru(II) Pcs

Fig. 55. Molecular structure of TT65 reported by Terejina et al. [235].

delocalization of the LUMO and LUMO+1 orbitals onto the acceptor unit is lower for Pc 7, which is consistent with a poorer injection capability, a second factor that should contribute to the observed difference in the JSC. 6. Axially substituted phthalocyanine dyes An approach that has been widely tested as a means to reduce the aggregation behaviour of Pcs has been that of axial substitution. A vast number of Ru(II), Si(IV) and Ti(IV) metalophthalocyanines have been prepared and studied in this respect, although other less common analogues, such as Fe(II), Hf(IV) and Zr(IV) have also been described. The ability of these metals to coordinate axial substituents works in favour of molecular disaggregation on the semiconductor surface.

Besides the advantage mentioned above, Ru(II) Pcs are also distinguished for their strong absorbance in the red region of the solar spectrum. The first example of this family of Pcs used in DSSCs was reported by Nazeeruddin et al., and it was endowed with two axial 3,4-dicarboxypyridine ligands to attach to the nanocrystalline TiO2 (JM3306, Fig. 60) [243]. In this case, aggregation was not avoided, which made the use of high content of CHENO necessary (ratio Pc/ CHENO of 1:2000). In conjunction with a lithium-rich electrolyte (1 M LiI and 0.05 M LiI3 in propylene carbonate solvent), an impressive JSC close to 10 mA/cm2 could be attained for the first time with a Pc dye. Also remarkable, the IPCE spectra extended well in the near-IR region, exceeding 60% at 660 nm. Although the overall efficiency was not reported in this work, these values were strikingly high at that time for a near-IR Pc sensitizer. Some years later, Yanagisawa et al. explored the performances of Ru(II)Pcs 1 and 2 anchored through 4-carboxypyridyl group(s), the latter bearing also long pentyl chains at the peripheral positions (Fig. 60) [244]. Analogue complexes, where the COOH was replaced by a methyl group or an ester, were also prepared in order to investigate the binding properties of the dyes. As expected, no adsorption of the non-carboxylic acid Pcs was observed, in contrast to 1 and 2 that were successfully attached. The obtained IPCEs in the red region of the spectrum were 21% for the 1-sensitized cell and only 6.6% for the 2-based device. Respectively, the overall efficiencies reached 0.61% in the first case and 0.58% in the second. The better light-harvesting properties of Pc 1 together with the presence of two anchoring groups were accounted for the enhanced performance. Additionally, four novel RuPcs (6–8) with varying peripheral substitution were prepared by McDonagh and coworkers (Fig. 61 and Table 42) [245]. As well, following an acceptor-sensitizer strategy, that is maximizing the distance between the holes of the oxidized dye and the TiO2(e) injected electrons in an attempt to create a long-lived

Fig. 56. Molecular structures of hybrids nBuO- and tBu- ZnPcNc reported by Peng and co-workers [238,239].

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Table 38 Adsorption densities (C) and photovoltaic parameters of the DSSC devicesa,b,c sensitized with tBu-Zn-Pc (0.05 mM in EtOH) fabricated under different conditions (temperature of adsorption and concentration of CHENO), under 1 sun illumination (AM1.5 G). Data derived from [239]. [Cheno](mM) 0 2.5 5 7.5 7.5 7.5 7.5 10 a b c d

Dye/CHENO Ratio

Tadsd (°C)

C/108

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm2)

1:0 1:50 1:100 1:150 1:150 1:150 1:150 1:200

5 5 5 15 5 15 25 5

3.84 3.50 2.86 (n/d) 2.66 (n/d) (n/d) 2.25

440 470 470 480 480 470 460 480

6.63 7.88 8.42 7.22 9.42 7.32 7.53 7.69

67 69 67 71 64 70 66 69

1.96 2.55 2.60 2.45 2.89 2.42 2.30 2.55

(%)

Double-layered TiO2 films of [10+4] lm thickness (transparent and scattering layer, respectively). Active area of 0.25 cm2. Composition of the electrolyte: 0.5 M LiI, 0.05 M I2 and 0.1 M TBP in a 15:85 mixture (v/v) of valeronitrile and AcCN. Tads = temperature of adsorption.

Fig. 57. Molecular structures of hybrids Zn-tri-PcNc and Zn-tri-TAPNc reported by Yu et al. [240].

charge separated state, two RuPc-Rubipyridyl dyads were also reported in the same work. In these latest examples, the complementary absorption spectra of the two chromophores aimed at panchromatic absorption, looking to increase the photocurrent generation. Spectroelectrochemical experiments conducted on the dyads demonstrated that both hole hopping (A) and stepwise electron injection (B) are processes energetically feasible. The former process (A) refers to an electron transfer from the HOMO of the Pc to the SOMO of the oxidized Ru(BiPy) complex (or ‘‘hole” of the complex, located on the Ru metal centre) formed after electron injection into the TiO2 : (1) [Ru(BiPy)]* ? [Ru+(BiPy)] + e/ TiO2; (2) Pc + [Ru+(BiPy)] ? Pc+ + [Ru(BiPy)]); the later process (B) refers to an electron transfer from the LUMO of the photoexited Pc to the LUMO of the Ru(BiPy) complex (located on the bipyridyl ligand), followed by electron injection into the TiO2 CB: (1) Pc* + [Ru(BiPy)] ? Pc+ + [Ru(BiPy)]; (2) ([Ru(BiPy)] ? Ru(BiPy)] + e/TiO2). In terms of dye adsorption on the semiconductor, it was notable that the Pc dyes showed poor coverage and the dyads showed less adsorption than the respective Ru dye alone, implying that the

Fig. 58. Molecular structures of ring-expanded naphthalocyanine NcS1 and NcS2 reported by Kimura’s group [241].

inclusion of the Pc was detrimental. However, the ability for photogeneration was enhanced, even if the overall PCE was disappointing (note that the photovoltaic performances of the DSSCs in this study were recorded under low -light intensity (500 W/m2, i.e. half-sun) and with a Xe lamp that mismatches the solar flux irradiance). Alternatively, the Pc analogues bearing two carboxylic acid anchoring groups adsorbed twice as much as the monocarboxylic acid analogues and yielded significantly higher photovoltage and photocurrent, with RuPc 7 reaching the best efficiency (Table 42). Overall, the DSSC performance of the new dyes was unimpressive,

Table 39 Photovoltaic parameters of the DSSC devicesa,b,c sensitized with Zn-tri-PcNc and Zn-tri-TAPNc under 1 sun illumination (AM1.5 G). Data derived from [240].

a b c d

Dye+Coadsorbentd

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

Zn-tri-PcNc+CHENO Zn-tri-TAPNc+CHENO

480 510

9.42 3.16

64 74

2.89 1.20

Double-layered TiO2 films of [10+4] lm thickness (transparent and scattering layer, respectively). Active area of 0.25 cm2. Composition of the electrolyte: 0.5 M LiI, 0.05 M I2 and 0.1 M TBP in a 15:85 mixture (v/v) of valeronitrile and AcCN. Dye-uptake solutions consisted of Pc [0.05 mM] and CHENO [7.5 mM] in EtOH.

(%)

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Table 40 Dye-loading density (C) and photovoltaic parameters of the DSSC devicesa,b,c,d sensitized with NcS1 and NcS2 under 1 sun illumination (AM1.5 G). Data derived from Ref. [241].

NcS1+CHENOe NcS1 NcS2 a b c d e

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(mol/cm3) 1.2 9.4 2.4

545 535 512

8.2 6.2 1.7

71 73 73

3.2 2.4 0.6

C/105

Dye

(%)

Double-layered TiO2 films (thickness of transparent and scattering layer were not reported). Active area of 0.16 cm2. Composition of the electrolyte: 0.6 M DMPImI, 0.1 M LiI, 0.05 I2, and 0.5 M TBP in AcCN solution. Performance data of the best cells values over five devices. Dye-uptake solution consisted of Pc [0.05 mM] and CHENO [2.5 mM] in toluene.

Fig. 59. Molecular structures of ‘‘push-pull” Pcs 7–9 reported by Fazio et al. [242].

Table 41 Dye-loading densities (DL) and photovoltaic data of the devices

a b c

a,b,c

made with Pc 7–9 under simulated 1 sun illumination (AM1.5G). Data derived from [242].

Dye

DL (nmol/cm2)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

7 7+Cheno 8 9

87.1 (n/a) 26.1 14.1

386 339 427 411

8.16 0.991 9.88 4.46

61 63 58 60

1.92 0.21 2.43 1.09

(%)

Double-layered TiO2 films of [9+4] lm thickness (transparent and scattering layer, respectively) and with active area of 0.159 cm2. Four to five devices of equal quality were made for each dyes; the values obtained for the best cell are reported. Composition of the electrolyte: 0.5 M LiI, 0.05 M I2 and 0.5 M NaI in AcCN solution.

although the two dyads yielded improved photocurrents owing to better light-harvesting. In contrast with these examples, Yanagisawa et al. described an unsymmetrical Pc 3 anchored, this time, through 4hydroxybenzoic acid groups located at two adjacent peripheral bpositions (Fig. 62) [246]. This Pc is decorated with six pentoxy chains at the remaining peripheral positions and with two methylpyridine ligands at the axial ones. Notably, while the UV–Vis spectra indicated no formation of aggregates and the energy levels were favourable for elec-

tron injection, the 3-sensitized cell showed overall conversion efficiency of only 0.40% with a maximum IPCE value of 23% at the Q-band. This result was attributed to the slow electron injection from an energetically low-lying triplet state of the RuPcs into the TiO2 conduction band. As it can be seen from these examples, Ru(II) Pcs had never reached efficiencies of more than ca. 1% in DSSC, which are far below their Zn(II)Pcs analogues. In an attempt to understand the key factors to achieve further improvements in the efficiency of Ru(II)Pc in DSSC, Torres, Durrant and co-workers prepared and

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Fig. 60. Ru(II) phthalocyanines bearing carboxypyridyl moiety(ies) as anchoring group(s) at the axial position(s): JM3306 [243] (Nazeeruddin et al.), and Pcs 1 and 2 [244] (Yanagisawa et al.).

Fig. 61. Ru(II)-Pcs 6–9 and Ru(II) Pc-bipyridyl dyads 1a and 1b reported by McDonagh and collaborators [245].

Table 42 Photovoltaic parameters under halogen lamp illumination (500 mW/cm2) of the DSSCs sensitized with dyes 4–7, dyads 1a and 1b, and benchmark N719.a Data derived from [245].

a b c

Dye

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

6b 7b 8b 9b 9c 1ab 1bb 1bc N719b N719c

148 231 166 232 403 275 273 308 412 632

0.30 1.28 0.37 1.23 0.35 1.13 1.33 0.05 2.74 1.50

43 41 44 44 61 46 47 32 48 59

0.04 0.24 0.05 0.25 0.17 0.29 0.34 0.01 1.01 1.11

(%)

Dye-uptake solutions consisted of 0.3 mM of sensitizer in EtOH (except for N719, in DMF solution). Composition of the electrolyte: 0.7 M LiI and 0.05 M I2 in 3-methoxypropionitrile. Composition of the electrolyte: 0.7 M LiI, 0.05 M I2 and 0.5 M TBP in 3-methoxypropionitrile.

studied the Ru(II) Pc 1 by in-depth photophysical studies in both TiO2 and ZrO2 sensitized solar cells (Fig. 63, left) [247]. Unambiguously, the authors unravelled that the most efficient pathway for electron injection into TiO2 of axially substituted Ru(II)Pc is from the triplet state (T1), which is in kinetic competition with the T1

decay-to-ground state. It was evidenced for the first time that the long lifetime of this T1 state enables electron injection to occur over hundreds of nanoseconds timescale with relatively good efficiencies (APCE > 45%), which points out that an ultra-fast injection process is not a requisite for Ru(II)Pcs to be efficient in DSSC. This

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Fig. 62. Axially substituted Ru(II) phthalocyanine Pc 3 bearing two anchoring groups at adjacent peripheral b-positions reported by Yanagisawa et al. [246].

Fig. 64. Si(IV) naphthalocyanines SiNc-1 and SiNc-2, reported by Macor et al. [248].

yielded a larger photocurrent and thus higher overall PCE, and this was explained mainly by better light-harvesting properties with respect to TT35. In this second study, it was concluded as well that fast electron injection was, in fact, not a prerequisite for enhanced performance, and that the injection process depended strongly on the energy of the injecting state and the lifetime of the excited state decay pathway to the ground state. 6.2. Si(IV) Pcs

Fig. 63. Axially substituted RuPc 1 and TT35 reported by Torres, Durrant and coworkers [146].

contrasts strikingly with the ultra-fast injection process (50– 100 ps) [206] from the singlet-excited state typically observed for Zn(II) Pcs (see Section 4, vide supra). These pioneering studies were the inspiration for further developments, and specifically, Ru(II) Pc TT35 (Fig. 63, right), bearing two 4-methylpyridines as the axial ligands and three tert-butyl groups and a carboxyl anchoring group at the peripheral positions, leading to an unsymmetrical sensitizer with an efficiency of 1.01% [146]. In this study, the same authors prepared this new dye and evaluated its injection dynamics in comparison with those of the benchmark Zn(II)Pc TT1, bearing the same peripheral substitution. The two dyes had very similar structures but differed substantially in terms of their photophysical behaviour, with the Zn(II) derivative exhibiting a relatively long-lived singlet state, while the Ru (II) one, TT35, was characterized by a rapid intersystem crossing to a long-lived triplet state, as well as higher injection efficiency. This study was, thus, quite enlightening in defining the importance of the electron injection step in the photovoltaic performance of a Pc. Unexpectedly, it was noted that TT35 reached higher absorbed photon conversion efficiency (APCE) than TT1, owing to better electron-injection efficiency. Despite this, the TT1-based cell

Si(IV)Pcs have also been explored as candidates for DSSCs. In 2009 the first such analogues appeared in the literature, and they consisted of bis(succinoyl) naphthalocyanine (SiNc-2), bearing two carboxylic acid anchoring moieties, and a dichloride derivative (SiNc-1), destined to test a direct Si–O–Ti linkage mechanism (Fig. 64 and Table 43) [248]. The UV–Vis spectra of both dyes exhibited strong sharp absorption in the NIR region, with a spectral shift of the Q-bands of 100 nm (kmax = 790 nm) compared to a typical Pc, which could be significant for extending the panchromatic response. In terms of adsorption onto the wide bandgap semiconductor, notably, SiNc-1 was adsorbed in larger amounts, a fact that was attributed to the affinity of Si derivatives with the TiO2 surface and the strong Si–O–Ti bond. As well, the IPCE reached 17% at the Q-band, demonstrating the capability of axially substituted Si–naphthalocyanines for utilization in DSSCs, and the promising behaviour of the Ti–O–Si anchoring pattern. Soon after, Sastre-Santos and co-workers published the synthesis of two Pc-analogues axially substituted with two terephthalic or 40 -carboxyphenylcyanoacrylic moieties (Fig. 65) [249]. The spectroscopic properties and energy levels of both dyes were similar, indicating no real impact of the axial substituents on the optoelectronic characteristics of the molecules. Overall, enhanced performance was observed for SiPc2, endowed with the cyanoacetic acid, owing to a larger photocurrent and photovoltage than SiPc1. Specifically, a 0.77% PCE was reported for SiPc2, whereas SiPc1 did not exceed 0.53%, and this was attributed to the presence of the cyano moiety, acting as an electron-withdrawing unit and leading to better electron injection.

Table 43 Photovoltaic parameters of the DSSCs sensitized with SiNc-1 and SiNc-2 under simulated full sun illumination (AM1.5G).a,b,c Data derived from [248].

a b c

Dye

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

SiNc-1 SiNc-2

17 (@790 nm) (n/a)

470 430

0.38 0.12

60 60

0.11 0.03

Single-layer TiO2 films of 7 lm thickness. Dye-uptake solutions consisted of SiNc-Pcs in DMF (concentration not reported). Composition of the electrolyte: 0.5 M LiI and 0.05 M I2 in metoxipropionitrile solution.

(%)

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Fig. 65. Si(IV)Pcs SiPc1 and SiPc2, reported by Sastre-Santos and co-workers [249].

Alternatively, more recent examples came by Lim et al., who described a silicon naphthalocyanine (LGB1) and a silicon naphthalo/phthalocyanine (LGB3) hybrid sensitizer, both endowed with tert-butyl peripheral and trihexylsilyloxy axial substituents, looking to reduce stacking and interfacial recombination, and also improve solubility (Fig. 66) [250]. It is important to mention that the performances of LBG1 and LBG3 in DSSCs were poor when using a common iodine-based electrolyte with standard composition. Thus, the optimal performances of these DSSCs were obtained with a Li-enriched electrolyte (1.0 M LiI and 0.05 M I2) and with no other additives (e.g. TBP and GuNCS typically used in liquid electrolytes for DSSC). This strategy aims to download sufficiently the TiO2 CB to allow a better electron-injection and thus obtain higher JSC in detriment of the F.F. and on the VOC in particular. This witnesses the quite low-injection capabilities of these dyes under standard conditions. Nonetheless, an impressive JSC value of 19 mA/cm2 could be achieved with LGB3/DSSC under these opti-

mal conditions, which stands out the record of JSC obtained for a Pc sensitizer (Table 44). Accordingly, only low-to-modest VOC (460 mV) and F.F. (51%) values were achieved, but leading overall to an excellent PCE of 4.5%. In turn, LGB1/DSSC displays much lower photocurrent with respect to LGB3 (JSC = 6.56 and 19.0 mA/cm2, respectively). This is quite surprising since the ring-expended naphthalocyanine core of LGB1 confers better optical properties to this dye, and in particular, a more pronounced redshift of the Q-band. The IPCE spectrum of LGB3 shows indeed a redshifted photoresponse in comparison with LGB1, but with values consistently much lower. LGB3 incorporating only one ring-expended subunit yielded an excellent broad and flat IPCE response, with values of ca. 80% between 600 and 750 nm, unlike the unimpressive 25–30% of LGB1 in the 650–850 nm region. The reason behind this, is believed to rely on the energetics: LGB1 has a low-lying HOMO level close to that of  the I 3 /I couple, and the resulting overpotential might be too small for an efficient dye-regeneration. On a different aspect, further understanding in the connection between dye-loading and performance of SiPc DSSCs came from two studies of Burda and co-workers [251,252]. The injection rate of Pc61 (Fig. 67) was evaluated in relation to the immersion time of the TiO2 photoanodes into the dye-uptake solution during the fabrication process of the devices, considering that the interaction between the Pc and the TiO2 is largely driven by the dyecoverage and orientation. UV–Vis spectroscopic experiments indicated that longer immersion times lead to higher dye deposition on the semiconductor surface but also the appearance of aggregation phenomena. The optimal performances were obtained after 2 h of immersion, with a maximum PCE value of 0.16% (Table 45). Above 2 h of immersion, the PCE dropped, a phenomenon that was attributed to the formation of multilayers. This was also confirmed by time-resolved photoluminescence measurements on the Pc61– TiO2 films, according to which emission lifetimes were enhanced over the 2 h threshold, which meant poorer electron injection of photoexcited electrons. The authors also prepared a test cell, based

Fig. 66. Si(IV)NPcs LGB1 and LGB3 reported by Lim et al. [250].

Table 44 Photovoltaic parameters of the DSSCs sensitized with LGB1 and LGB2 under simulated full sun illumination (AM1.5G).a,b,c Data derived from [250].

a b c

Dye/Coads.b

IPCEmax (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

LGB1+CDCA (1:10) LGB3+CDCA (1:10)

25–30 80

360 460

6.56 19.0

38 51

0.9 4.5

Double-layered TiO2 films of [8+4] lm thickness (transparent and scattering layer, respectively). Dye-uptake solutions consisted of 0.1 mM of dye in ethanol with 1 mM of CDCA. Composition of the electrolyte: 1 M LiI and 0.05 M I2 in metoxipropionitrile solution.

(%)

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6.4. Hf(IV) and Zr(IV)- Pcs

Fig. 67. Si(IV) Pc-61 reported by Burda and co-workers [251,252].

on a Pc61–Al2O3 film, to corroborate if, indeed, the observed kinetics in Pc61–TiO2 were a result of electron injection. As expected, no electron injection was exhibited in the control sample, due to the large energy bandgap of Al2O3. 6.3. Ti(IV) Pcs To a lesser extent, Ti(IV)Pcs have also been evaluated in DSSC applications. Concretely, the first analogue was prepared by Torres, Durrant and co-workers, and it was endowed with peripheral tertbutyl units and an anchoring 4-carboxycatechol group at the axial position (Pc 1, Fig. 68) [253]. The axial ligand suppressed aggregation, in a way that no co-adsorbents were necessary, nevertheless, electron injection was state-selective and detrimental for the overall efficiency, which did not go beyond 0.2%. This was supported partly by the IPCE spectrum, which showed high photocurrent generation in the UV/blue region, but very low in the red region, and also by transient absorbance measurements, where the signal was strongly dependent on the excitation wavelength. The same group went further to prepare analogues 2 and 3 (Fig. 68), distinguished for the axially coordinated naphthalenediols together with the sulfonate anchoring groups [254]. This axial substitution approach was employed not only to disrupt aggregation phenomena, but also to control the distance between the Pc and the TiO2 surface and thus electron dynamics. Surprisingly, all new sensitizers, bearing one, two or no sulfonate binding groups, yielded identical photovoltage and photocurrent, which implied that the axial ligand did not have any impact on the overall result. This led to believe that the new Pcs were unstable under the conditions used to prepare the cells, in a way that displacement of the axial moieties was taking place, to give rise to the same species (O = TiPc), which was adsorbed onto the semiconductor through a di-loxotitanium-type anchoring group. The observed PCEs were low and did not surpass 0.14%. Once again, this result was rationalized in terms of poor electron injection capability.

In 2012, Drain and co-workers reported the synthesis of novel Hf(IV) and Zr(IV)-porphyrins, following studies that had shown successful attachment to oxide surfaces through these group(IV) metal ions [255]. In this context, the Pc-analogues (Pc)Zr(OAc)2 and (Pc)Hf(OAc)2 were prepared (Fig. 69) and their HOMO-LUMO energies calculated, revealing sufficient driving force for electron injection from both from the singlet and the triplet excited states, as well as dye regeneration [256]. Interestingly, the UV–Vis reflectance absorption spectra of Pc-covered TiO2 nanoparticles implied stronger binding of the newly designed dyes on the oxide surface in comparison to free-base or Zn(II)-metallated Pcs. In terms of photovoltaic performance (Table 46), the Zr-based dye produced a PCE of 1.05% and exhibited good stability to light soaking. In fact, a 50% enhancement in the photocurrent was observed after 3 h of light illumination, bringing about a higher overall performance, and this was ascribed to the reorganization of the dye molecules on the TiO2 surface. Notably, when higher concentrations of Pc in the dipping-solutions were used for the sensitisation of the TiO2 photoanodes, in an effort to increase the dye loading, efficiencies decreased due to aggregation. On the other hand, the cells prepared with the Hf(IV) analogues did not reach the same results, and the overall PCE did not exceed 0.59%. This was considered to derive from demetalation together with lower electron injection efficiency, arising from the increased intersystem crossing to the triplet state. In this article, the authors also attempted cosensitization with both the Pc (Pc)Zr(OAc)2 and porphyrin (TPP) Zr(OAc)2 dyes, but the PCE remained slightly lower to the one of the Pc dye alone, with the reflectance spectra revealing that more of the Pc analogue had adsorbed on the semiconductor surface. Similar behaviour was detected for a mixture of the Hf(IV) analogues. Overall, it has been observed that albeit the big advantage of axially substituted Pcs in suppressing aggregation of the dye on the TiO2 surface, axial attachment usually comes with the cost of slow electron injection rates, resulting in unremarkable performances. 7. Co-sensitization and energy relay dyes (ERDs) Improving the light-harvesting capabilities of the photosensitizer is the main factor that can lead to an increase in the shortcircuit current of a cell [19]. Enhanced absorbance can be obtained by synthetically reducing the energy bandgap of a dye, but even so, it is practically inconceivable to achieve a panchromatic response with only one single species. In this respect, combinations of visible- with NIR-absorbents possessing complementary optical properties, have been vastly implemented in the so-called cosensitized cells [257,258]. For this technique to be successful, it is important for both elements of the dye-cocktail to have high extinction coefficients in order to require a small surface area on

Table 45 Photovoltaic parameters of the DSSCs sensitized with Si(IV) Pc-61 upon various immersion time, measured under simulated full sun illumination (AM1.5G).a,b Data derived from [251].

a b

Immersion time (min)

Max IPCE (%)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

30 60 90 120 150 360 1440

2.93 3.55 3.91 4.51 4.46 3.32 2.05

420 420 420 410 400 410 400

0.27 0.31 0.46 0.55 0.30 0.21 0.16

69 64 62 58 61 59 54

0.09 0.10 0.15 0.16 0.10 0.07 0.04

Single-layered TiO2 films of 7 lm thickness. Dye-uptake solutions consisted of 0.06 mM of Pc 61 in ethanol.

(%)

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Fig. 68. Ti(IV)Pcs 1–3 reported by Torres, Durrant, and co-workers [253,254].

Fig. 69. Hf(IV) and Zr(IV) Pc complexes (Pc)Zr(OAc)2 and (Pc)Hf(OAc)2 reported by Drain and co-workers, and schematic binding mode to anatase TiO2 proposed by the authors [256].

the semiconductor for efficient light absorption [27]. This way, molecular co-sensitization of thin DSSC films becomes possible without reducing light harvesting at any portion of the spectrum. Along the same lines, the energy gap must be carefully engineered in order to allow, on one hand, efficient absorption of light, but prevent, on the other hand, recombination, and give access to high VOC. A sheer number of couples have been studied and some fruitful outcomes have been reported [259–263]. It is to be noted, nevertheless, that co-adsorption of dyes onto the TiO2 photoanode may result in intermolecular interactions between the dyes that can affect the device performance detrimentally, leading to values lower than each of the dyes separately [264,265]. A couple of methods have been described to avoid these interactions [257], namely, sensitization using two separate layers [266,267], or using a double dye layer [261], however, interesting as they may be as concepts they were not used to make any highly efficient devices due to their complexity. One of the first successful

co-sensitization was reported by Grätzel et al., and involved a porphyrin (YD2-oC8) and an organic dye (Y123; Fig. 6). The panchromatic response of this cocktail led to a JSC of 17.66 mA/cm2 and a PCE of 12.3% [118]. Later on, this was followed by the two organic dyes ADEKA-1 (alkoxysilyl anchored) and LEG4 (carboxyanchored), achieving an impressive JSC value of 18.27 mA/cm2 and a record PCE of 14.33% in n-type TiO2 DSSC (Fig. 6) [55]. In terms of phthalocyanines, the first example involved a symmetrically substituted tetrasulfonated gallium Pc (GaTsPc) and a tetrasulfonated zinc porphyrin (TsZnPP) with complementary absorption spectra [268]. In this case, the light-harvesting efficiency of a TiO2 semiconductor containing a layer of both dyes was higher than that of the single dye sensitized cells and the short circuit current was markedly enhanced. Notably, the maximum IPCE value of the co-sensitized cell was 9.8%, significantly higher than that of the Pc-sensitized device (1.1%). Two points that could justify this enhancement were made through absorption spectra studies: first that the Q-band of the Pc was red-shifted going from the adsorbed to the co-adsorbed thin film. This was explained by the presence of the Pc under both dimeric and monomeric form, and on the strong interaction between the two sensitizers. The ratio of the dimeric/monomeric form decreased upon increasing the amount of porphyrins on the surface. Second, the absorption spectra of the single dye sensitized surface (Pc) showed stacking of the macrocycles and it was concluded that the Pc adopts a dimeric cofacial arrangement on the TiO2 surface, which displayed no photoresponse upon illumination. The decrease of these dimers on the co-sensitized surface was the reason for the increase of the photocurrent. Additional phenomena, such as formation of Pc/Por hetero-aggregates could also be involved in this important performance improvement. A decade later, a very interesting example was described by Nazeeruddin, Torres and collaborators [148], in a cell sensitized with Pc TT1 and the organic dye JK2 (Fig. 70a), one of the most powerful organic dyes previously reported [269].

Table 46 Photovoltaic performances of the DSSCs sensitized with Zr-Pc and Hf-Pc under simulated full sun illumination (AM1.5G, 100 mW/cm2).a,b,c Data derived from [256] (main article and supplemental information). Dye/Conc.

Conditions

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

(Pc)Zr(OAc)2/0.1 mM

After After After After After

510 470 470 480 440

1.51 3.10 3.05 0.42 1.93

75 73 68 75 68

0.57 1.05 0.97 0.15 0.59

(Pc)Hf(OAc)2/0.1 mM a b c

assembly 5 days, 1 h light soaking 9 days, 3 h light soaking assembly 5 days, 1 h light soaking

Double-layer TiO2 films (transparent + scattering layer) with a total thickness of 13.5–14.5 lm (8–9 lm of active layer). Active area of 0.0707 cm2. Composition of the electrolyte: 0.6 M BMII, 0.03 M I2, 0.1 M GuNCS and 0.5 M TBP in a 85:15 mixture (v/v) of AcCN and valeronitrile.

(%)

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Fig. 70. Molecular structures of various organic dyes that have been used to co-sensitize Pcs: a) JK2 [148], b) D2 [270], c) D102 [173], d) D131 [173] and e) DH-44 [272].

TT1, a sterically hindered and highly directional Pc, has been one of the most important milestones in the history of Pcsensitized solar cells, as described earlier in Section 3. A device based solely on TT1 as the photosensitizer gave a record, for the time, power conversion efficiency of 3.52%, and was subsequently used in combination with JK2, to complement the optical window in the 400–550 nm region, where the Pc does not show any absorption. The IPCE spectrum of the couple was astounding, displaying high photoresponse from both dyes (Fig. 71), and yielded a 7.7% overall efficiency, higher than those achieved with the single-dye devices. Another fruitful, even though less efficient, combination of TT1 with an organic dye came some years later, when Palomares and co-workers described a device sensitized with a cocktail of TT1 and the organic dye D2 (Fig. 70b) [270]. The authors mentioned that even though the two dyes are not optically completely matched, leaving an important absorption gap in some parts of the solar spectrum, their selection was based on their favourable electrochemical characteristics. The maximum efficiency obtained by the co-sensitized system was 4.08%, with a JSC of 8.6 mA/cm2, a VOC of 643 mV, and a F.F. of 72.44%. The increased light harnessing capacity of the cell brought about, as expected, enhanced short circuit current in comparison to the individual dye-sensitized electrodes, however the VOC was reduced. To rationalize this result, a number of additional experiments were conducted. In particular, charge extraction and transient photovoltage experiments revealed that the position of the TiO2 conduction band and the electron lifetimes for the dye-cocktail were lower than D2 alone but higher than TT1, which was in accordance to the observed VOC of the single-dye cells, considering also the recombination of TiO2 injected-electrons with the electrolyte. Additionally, transient absorption data indicated hole-transfer from D2 to TT1, which, surprisingly and opposite to what other studies had shown [264], did

not influence detrimentally the overall efficiency. In 2012, the group of Mori used the organic dyes D102 and D131 (Fig. 70c and 70d, respectively) in combination with their successful Pc PcS15, described earlier in Section 3.2.2, p. 120) [173]. The two dyes show absorption maxima in the 425–490 nm visible region, where this Pc lacks any spectral response. Upon measurement of the J–V characteristics of the devices and the IPCE spectrum, it was revealed that the short circuit current and photovoltage increased in both cases, with the cocktail D131+PcS15 showing a more significant enhancement, and that there were no intermolecular interactions between the organic dye and the Pc, since the IPCE values at the near-IR region were not decreased by the cosensitization. It was also suggested that the sterically hindered substituents of the Pc contributed to prevent interaction between the two dyes on the co-sensitized cell, considering that the bulky moieties increased the distance between the conjugated frameworks of the two dyes. Overall performances reported were 6.2% for D131+PcS15 and 5.6% for D102+PcS15. In 2013, Jin et al. described a two-dye cocktail consisting of lutein and octacarboxylic Zn(II)-Pc (Fig. 72) [271]. Different ratios of dyes in the uptake solution were examined for the sensitization of the photoanodes, in order to correlate them with the overall performance obtained in DSSC. A Pc/lutein ratio of 4:1 was the optimum, giving out a PCE of 0.29%. With regard to the absorption spectra of the molecules, Zn(II)Pc displayed a clearly higher extinction coefficient than lutein, and also on the co-sensitized film, and the extent of Pc H-aggregates seemed to be increasingly suppressed upon addition of lutein. Interestingly, the emission spectra revealed a quenching of the Pc fluorescence, indicative of an energy transfer from the Pc to the lutein unit. The photovoltaic characteristics of six devices were measured, two for the single-dye thin films and four for different ratios of sensitizers. Both JSC and VOC increased with higher lutein concentration up to an optimal

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Fig. 71. I/V curves and IPCE spectra (insets) of A) TT1 and B) cocktail TT1/JK2, TiO2 DSSCs (active areas of 0.2 cm2). Reproduced with the permission from Ref. [148]. Copyright 2007 Wiley-VCH.

Pc/lutein ratio of 4:1, resulting also in an enhanced overall performance. This result arose from suppression of aggregation of the macrocycles as well as better electron transport kinetics and electron lifetimes, as demonstrated through electrochemical impedance spectroscopy. A more recent case was lately presented by Peng et al. [272] who used the NIR- hybrid Pc/Nc tBu-Zn-tri-PcNc synthesized by them earlier (described in Section 5.3; Fig. 56) [238], and a bithiophene-based organic dye DH-44 (Fig. 70e), characterized for its broadened and red-shifted absorption and high extinction coefficient. In the co-sensitized cell, the absorption gap of the Pc in the visible part of the spectrum was complemented by the organic dye. To study the J-V characteristics, optimized conditions for the preparation of both single dye and co-sensitized devices were developed. Upon adsorption of the two dyes on the TiO2 film, an apparent red-shift of their absorption spectra was observed, attributed mainly to their interaction with TiO2. Looking at the IPCE spectrum of the co-sensitized device, enhanced photoresponse was detected for both counterparts, apart from the Q-part region of the Pc, for which the response was lower, which the authors attributed to the reduced amount of loaded Pc dye when compared to the individual sensitized electrode. Even so, the overall performance of the cell was improved both in terms of JSC and PCE, however VOC and F.F. dropped in comparison to those of the DH-44 sensitized cell (Table 47). Surprisingly, even though the co-sensitized cell demonstrated the highest JSC, it also showed

Fig. 72. Chemical structures of Octacarboxyl Zn(II) Pc (ZnOCPC) and Lutein, used as molecular cocktail in DSSC by Jin et al. [271].

the highest dark current, which could not be rationally explained, but it was assumed that it could be related to competitive processes between the two dyes co-sensitized on the film. Further experiments involved electrochemical impedance spectroscopy, which correlated the higher electron injection efficiency with enhanced JSC of the co-sensitized device, and electron lifetime, being in between the two single-dye cells, with the VOC value. In 2015, Kimura et al. reported on the synthesis and study of three new pyridyl-substituted Pcs in DSSCs (Pc22-24; Fig. 37) and the data for the individual DSSC devices are presented in Table 25 [195] (vide supra). In addition to this study, cosensitization of the pyridyl-red dye Pc23 and the carboxylsubstituted yellow dye D123 was also tested in this work with an interesting outcome. It had been previously demonstrated that dyes having a pyridyl anchoring moiety are adsorbed on a different site of the semiconductor surface compared to carboxylic acidbased chromophores, which works in favour of co-sensitization. This way, higher adsorption densities can be observed when two dyes with different anchoring groups are used, leading to an increased photoresponse [273,274]. In this example, the piridylbased PcS23 was co-adsorbed with carboxyl-based organic dye D131 (Fig. 70c) on a TiO2 film, resulting in a panchromatic photoresponse throughout the whole visible spectrum. Advantage was taken of the site-selective adsorption of the two dyes on the semiconductor surface. The overall conversion efficiency of the cosensitized cell (PCE = 7.4%) was significantly enhanced compared to the individual dye devices, with the IPCE spectrum showing a response from both chromophores and the JSC being twice than that of the D131 device one. All of the above-mentioned examples of cocktail dyes rely on the co-sensitization of two (or more) complementary dyes, both attached on the TiO2 surface via an anchoring group (carboxyl and/or pyridyl). In a different and elegant way, Saha and coworkers developed a stepwise sensitization/supramolecular approach to obtain a panchromatic dyad sensitizer (ZnPc-PyPMI) and improve, as well, the performances with respect to the single-dye devices based on a phthalocyanine (ZnPc) and peryleneimide (PyPMI) derivatives (Fig. 73) [275]. The formation of the complex was achieved through axial coordination of the perylene’s pyridyl group with the metal centre of the Pc. Electrochemical measurements evidenced that the electron density of the Pc was enhanced because of the coordination to the PyPMI, rendering it a better electron donor. The preparation of the DSSC devices was made by a two-step sequential deposition method. First, the TiO2-photoanodes were immersed in a PyPMI solution to make sure that the dye anchors to the surface through its anhydride end. Subsequently, the PyPMI-sensitised TiO2 films were

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Table 47 Dye-loading (DL) and photovoltaic data of the devices a,b made with dyes Zn-tri-PcNc-1 and DH-44, individually and co-sensitized, on [6.7 + 5.2]c lm thick TiO2 films, under simulated one sun illumination (AM1.5G). Data derived from [272].

a b c

Dye

DL (mol/cm2)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

Zn-tri-PcNc-1 DH-44 Co-sensitization Zn-tri-PcNc-1+DH44

3.64108 7.18108 (2.78 + 7.03)108

540 610 560

5.98 12.72 17.94

74 67 66

2.38 ± 0.13 5.16 ± 0.26 6.61 ± 0.35

(%)

Five devices were made in each conditions. Composition of the electrolyte: 0.5 M LiI, 0.05 M I2 0.1 M TBP and 0.5 M NaI in a 1:1 mixture of acetonitrile–propylene carbonate. Double-layered TiO2 films of [6.7 + 5.2] lm thickness (transparent and scattering layer, respectively).

immersed in a ZnPc solution, to form the dyad through coordination with the pyridyl end of the perylene derivative. The J-V curves of the dyad-sensitized cell showed higher JSC and VOC values with respect to those of the individual dye devices (Table 48), and achieved an overall PCE of 2.3%, which is one of the highest value in DSSCs obtained with a self-assembled sensitizer. This enhancement was attributed to the two-step electron transfer producing long-lived charge-separated state and a broader spectral response, which was further confirmed through the IPCE spectra (Fig. 74). Along the lines of panchromatic engineering, energy relay dyes (ERDs) have come forward as an alternative approach, relying on the non-radiative energy transfer from a donor molecule (ERD) to an acceptor dye within a short distance range [276,277]. There are several benefits of the use of ERDs: they do not attach to the titania so they do not influence the dye loading, and therefore the overall absorption spectrum becomes broader for the same film thickness. As well, they act through a so-called Förster resonant

energy transfer (FRET) [278,279] from the ERD to the dye, which means that since the ERD does not participate in the electron injection process, its HOMO and LUMO levels do not require to be specifically engineered. Instead, they must be strongly fluorescent and their emission should overlap with the absorption spectrum of the acceptor dye for efficient FRET to occur. As well, they should be soluble in and not quenched by the electrolyte. It is worth mentioning that with ERDs having a fundamentally different function and design rules than photosensitizers, the range of dyes that can be used in DSSCs undoubtedly expands. The first attempt for a Pc-ERD combination on a DSSC came in 2009 by McGehee and co-workers, where a perylenediimine derivative, PTCDI (Fig. 75 and Table 49), played the role of the ERD and Pc TT1 the role of the acceptor dye [280].The highly photoluminescent perylene (PTCDI) derivative was dissolved in a liquid electrolyte, the chosen solvent being chloroform for solubility reasons, despite being known for showing lower internal quantum

Fig. 73. ‘‘(a) DSSCs comprised of (b) a supramolecular ZnPc-PyPMI dyad, (c) PyPMI, and (d) ZnPc dyes.” Reproduced from [275] with permission from The Royal Society of Chemistry.

Table 48 Photovoltaic data of the devicesa,b made with the ZnPc-PyPMI dyad together with those of the individual-sensitized dyes ZnPc and PyPMI, under simulated one sun illumination (AM1.5G). Data derived from [275]. Dye

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

ZnPc-PyPMIc PyPMI ZnPcd

460 390 350

10.0 3.21 0.55

50 57 63

2.20 0.72 0.15

(%)

Single-layered TiO2 films of 5 lm thickness. Composition of the electrolyte: 1.0 M LiI + 0.06 M I2 in propylene carbonate. For the dyad-sensitized cell, the TiO2 photoanode was first immerged in a PyPMI solution (0.15 mM in CH2Cl2); after washing away the unbound dyes, the PyPMIfunctionalized photoanode was immersed in a ZnPc solution (2 mM in CH2Cl2). d For the ZnPc-sensitized cell, the TiO2 photoanode was first functionalized with pyridine-4-carboxylic acid, and then immersed in a ZnPc solution. a

b

c

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Fig. 74. ‘‘(a) A plausible PET mechanism in the ZnPc  PyPMI dyad leading to electron–hole separation, which is the driving force for photocurrent generation. (b) The IPCE spectra of DSSCs composed of the ZnPc  PyPMI dyad (red), PyPMI (pink), and ZnPc (blue).” Reproduced from [275] with permission from The Royal Society of Chemistry.

Fig. 75. ‘‘PTCDI and TT1 properties. a) PTCDI absorption (blue), PTCDI emission (red) in chloroform and TT1 absorption (black) on titania nanoparticles. b), c), Chemical structures of the energy relay dye PTCDI (b), and sensitizing dye TT1 (c)”. Reprinted by permission from Macmillan Publishers Ltd. Nature Photon. (Ref. [280]), copyright 2009.

efficiency and power conversion efficiency. Subsequently, the PTCDI-containing electrolyte was then injected in the TT1-DSSC device. For comparison, TT1-DSSC devices without EDR were made under the same conditions and composition of the electrolyte (except, obviously that no EDR was introduced in that case). PTCDI is an ideal ERD, not only due to its high photoluminescence, fast fluorescence lifetime and photostability, but also because of its strong absorption coefficient and its bulky substituents that reduce intermolecular interactions and aggregation. In this example, the TT1 absorption spectrum matched the emission spectrum of PTCDI, so energy transfer could occur upon excitation of the ERD (Fig. 75). In agreement with the EQE spectrum that indicated an enhancement of the response in the 400–600 nm region, a higher JSC was obtained for the Pc-ERD device, leading to a PCE rising from 2.55% (without ERD) to 3.21% (with ERD). Remarkably, the VOC and F.F. of the cell remained unaltered. An experiment using only the

ERD as a sensitizer in a device confirmed that the produced photocurrent is minimal, meaning that energy transfer to a sensitizer needs to take place to generate a photocurrent. The same year, Grimes et al. showed that a ZnPc, namely 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (ZnPc-TTB), can also play the role of the ERD when used in combination with the well-known ruthenium polypyridine dyes N719 and black dye [281]. It must be point out that the photoanode used in this work consisted of TiO2 crystal rutile nanowires developed by the authors [282], which was essential because such close-packed architecture allows distances between donor and acceptor that are comparable to the Förster radius (Fig. 76). With the ruthenium dyes attached covalently on the titania surface and the ZnPc dissolved in the electrolyte, the authors could achieve a satisfactory distance regime for efficient FRET between the two components.

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Table 49 Photovoltaic data of the DSSC devices made with TT1+CHENO,a,b,c withd and without energy relay dye (PTCDI) under simulated one sun illumination (AM1.5G). Data derived from [280].

a b c d

PTCDI (mM)

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

0 13

562 553

6.88 8.78

65 66

2.55 3.21

(%)

Dye uptake solutions consisted of 1  105 M of sensitizer TT1 and 10 mM of CHENO as coadsorbent. Composition of the electrolyte: 0.6 M PMII, 0.01 M LiI, 0.05 M I2, 0.04 M TBP and 0.02 M GuNCS in CHCl3. Double-layered TiO2 films of [10+5] lm thickness (transparent and scattering layer, respectively). PTCDI (13 mM) was added in the electrolyte before backfilling of the DSSC.

Notably, quantitative energy transfer was observed in these devices, which was attributed to this newly designed system configuration. The choice of the Pc was based on the presence of the four tert-butyl groups to avoid aggregation phenomena, which are detrimental for FRET. As well, improvement of light harvesting in the red region of the spectrum, where the ruthenium dyes do not show any response, was targeted, and also the emission of the macrocycle was quenched by solutions of both N719 and black dye, which is required for an efficient FRET. Upon measurement of the device characteristics, a four-fold increase in the EQE at the red portion of the spectrum was observed. The authors also mentioned that the performance of the N719-device was better compared to that of the black dye. N719 dye formed a non-agglomerated monolayer, whereas in the case of the black dye the addition of a coadsorbent (deoxycholic acid) was necessary to reduce agglomeration and form such layer. Therefore, the concentration of the acceptor available for energy transfer was reduced, which hence lowered the critical distance and energy transfer efficiencies for the black dye relative to N719 despite higher spectral overlap absorption with the emission spectrum of ZnPc. Experiments to confirm the FRET phenomena as the main contributor to the enhanced performance, included Pc concentration studies with an increase bringing about enhanced quantum yields for red photons, and a blank experiment using a Ru dye without a matching absorption spectra to the Pc emission, thus not leading to an improvement in the EQE. McGehee, Grätzel and co-workers reported on the use of 4-(di cyanomethylene)-2-methyl-6-(4-dimethyl-aminostyryl)-4H-pyran (DCM; Fig. 77, left), an ERD with a broad absorption spectrum and

Fig. 76. ‘‘Concept of the FRET photovoltaic device. Depiction of the FRET-enhanced nanowire dye-sensitized solar cell”. Reprinted with permission from Ref. [281]. Copyright 2009 American Chemical Society.

high molar extinction coefficients with maximum at 460 nm, in a TT1-sensitized solar cell [283]. An average excitation transfer energy (ETE) efficiency of 96% was demonstrated for this system with limiting factors on the ERD’s EQE being, according to the authors, its absorption and moderate solubility in the electrolyte, thus the EQEERD did not overcome 40%. It was mentioned that even though DCM does not contain an anchoring group for attachment to the TiO2, it could physisorb to the semiconductor surface, increase the dye loading on the film and directly inject electrons into the TiO2. However, it is true that the excited ERDs near the surface are more likely to undergo FRET to TT1 before charge injection, so it would be complicated to determine the exact contribution of this phenomenon to the overall ETE. Using an optimized engineereddevice, including an additional scattering layer of titania nanoparticles, the efficiency of the DCM/TT1 based device was 4.51%, over 3.5% of the individual TT1-sensitized cell, with a 27% increase in the Jsc attributed to the ERD contribution. As expected, the Voc and F.F. of the device did not undergo any degradation. Another very appealing report on the use of multiple ERDs for extending the spectral response was published one year later by the same collaborators [284]. Specifically, two molecules possessing complementary absorption spectra and matching the optical window of TT1, that is DCM and Rhodamine B (RB; Fig. 77, right), were dissolved in the electrolyte of a TT1-sensitized cell giving rise to FRET from both relay dyes directly to the Pc. The choice of the ERDs was based on their physicochemical properties that perfectly matched the requirements for this role: high absorption coefficients and photoluminescence (PL) quantum efficiencies and short PL lifetimes, which are important for minimizing electrolytic quenching, and good emission overlap with the absorption spectrum of TT1. Interestingly, even though DCM and RB have similar PL lifetimes, the later experiences significantly higher quenching by the electrolyte, which is also concentration-dependent, and this results in a much lower ETE of RB in comparison to DCM (34% over 95%). Photovoltaic studies were performed on the individual TT1 cell, as well as FRET-based energy relay systems DCM/TT1, RB/ TT1 and DCM/RB/TT1 cells. The highest PCE was observed for the cell comprising both ERDs, reaching a 3.97%, in comparison to 2.94% for the Pc-sensitized cell, and 3.68% and 3.29% for the

Fig. 77. Molecular structure of DCM (left) and Rhodamine B (RB; right), used as EDRs in TT1-sensitized DSSCs by McGehee, Grätzel and collaborators [283].

M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

DCM/TT1 and RB/TT1 respectively. The short circuit current on the optimum system was increased by 44% in comparison to the cell comprising no relay dyes, and, according to the authors, this is the highest increase observed in a DSSC by the addition of an ERD. The relay dyes exhibited an EQE of 30% over the visible spectrum, but their presence decreased the EQE in the 600–710 nm region, which was due to the RB’s possible adsorption on the semiconductor via its carboxylate moiety, this way shrinking the TT1 adsorption on the surface. In this regard, in order to exclude the possibility that the increase in photocurrent arises from attachment of the RB to the titania, the acid was protected to the ester, so that no attachment could be possible. Unsurprisingly, in this case only an 8% extra EQE was observed, indicating that the main participation of RB in the cell’s activity is that of FRET. Additional experiments on these devices involved energy transfer efficiency calculations to confirm that only minor energy transfer takes place from the large-bandgap DCM to the lower-bandgap RB to compete with direct energy transfer to TT1, and the dominant lightharvesting pathway is FRET from the DCM to TT1. Following an approach initially introduced by Odobel and coworkers for porphyrins [285], Choi et al. reported on a RuPcsensitized cell, whose performances were enhanced by the addition of thiophene-based dyes JK-107 and JK-109 acting as ERD (Fig. 78 and Table 50) [286]. The ERD was linked to the main Pc sensitizer by coordination to the metal centre, as a means to decrease the distance between the two and consequently increase the FRET rate. In particular, the formation of the dyad was achieved through axial coordination of the organic dye’s pyridyl group to the Ru metal of the Pc. The two novel components were designed to have overlapping absorption spectra and achieve panchromatic coverage, namely the thiophene organic dyes exhibited strong absorption in the 474–500 nm region, where the RuPc lacks any photoresponse. However, in terms of fluorescence, JK-107 showed a better matching with the macrocycle than its JK-109 analogue, and this

57

was accounted for the difference in performances between the two systems, as FRET would be significantly more efficient for the former combination. Additionally, molecular orbital calculations confirmed the optimal energy levels of the dyes for efficient electron transfer to the RuPc. Upon measurement of the photovoltaic properties of the systems, both IPCE spectra and J-V curves of the device based on the two-dye combination with JK-107 showed an outstanding improvement in performance in comparison to the single-RuPc one, with the PCE rising by 370% in the JK-107 system. The JK-109 also brought about an enhancement but significantly lower, as expected. The authors attributed these results to the higher obtained photocurrent due to FRET phenomena as well as electron transfer from the thiophene dye to the macrocycle. With regard to the later, transient absorption spectroscopy gave further insights into the electron injection dynamics. The results indicated a fast disappearance of ground state bleaching (500 nm) and appearance of an excited state absorption at 730 nm, both assigned to JK-107, clearly attributed to an efficient electron transfer from the organic dye to the RuPc. All of the above-mentioned studies involve liquid electrolyte architectures, but besides these, research on relay dyes has also focused on solid-state configurations. In this context, an all solidstate device was described by Snaith et al. [287]. Specifically, a cell co-sensitized with indoline-based dye D102 (Fig. 70c) and Pc TT1, and with Spiro-OMeTAD as the HTM, was constructed and tested for its photovoltaic properties. Interestingly, under optimal conditions the overall efficiency rose to 4.7%, over 3.9% for the D102-cell and 1.1% for the TT1-cell. Through a number of assessments the authors observed that the contribution of the indoline dye in the device was two-fold: it exhibited FRET to the ZnPc but could also directly inject electrons to the semiconductor surface, even though this dye tends to aggregate and form multilayers on the TiO2, meaning that electron-transfer would be limited. Looking at the photovoltaic action of the co-sensitized surfaces, there was not only a big increase in the photoconversion efficiency in the visible

Fig. 78. Chemical structures of JK-107 and JK-109 reported by Choi et al. [286].

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Table 50 Photovoltaic data of the DSSC devices made with JK-107 and JK-109,a,b,c under simulated one sun illumination (AM1.5G). Data derived from [286].

a b c

Dye

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

JK-107+CHENO JK-109+CHENO

446 323

10.43 3.39

44.3 50.6

2.06 0.56

(%)

Dye uptake solutions consisted of 0.1 mM of JK-sensitizer and 0.1 mM of coadsorbent (CHENO). Composition of the electrolyte: 0.5 M LiI and 0.02 M I2 in propyl carbonate solution. Double-layered TiO2 films of [8+4] lm thickness (transparent and scattering layer, respectively).

Fig. 79. Molecular structure of Spiro-TBT developed by Driscoll and co-workers, acting as secondary absorber in solid-state excitonic solar cells [294].

region, where D102 absorbs, but also an extension into the near IR, attributed to the Pc. Additional insights came from time-resolved photoluminescence (PL) measurements, both on TiO2 and Al2O3, the second being the non-injecting reference. On both surfaces there was important quenching of the visible emission from D102 upon cosensitization with TT1. As well, the TT1’s emission at 700 nm increased 10 times in the D102/TT1 cell. Notably, the PL decays were faster on TiO2 than on Al2O3, but the D102 emission decayed faster in the co-sensitized films on TiO2, indicating that long-lived D102 excited states contributing to the emission are more effectively quenched through energy transfer to TT1 than electron transfer to TiO2. One more important finding of this work was the fact that the ERD could actually be grafted on the semiconductor, instead of simply suspended on the HTM phase as done so far, without reducing photoresponse and acting also as a coadsorbent (instead of CHENO) to reduce aggregation phenomena. Along the same lines, two reports on the use of visible light absorbing polymer P3HT poly(3 hexylthiophene) both as a secondary absorber and as the HTM were described. P3HT had already been tested previously in this dual role of sensitizer/HTM in ssDSSCs [288–291], known for its high hole mobility and broad absorption in the 400–650 nm range, but in this case a conjunction with the widely known Pc TT1 as the main light harvester was approached. In the first example by Grätzel and co-workers, two different cell structures were probed, that is a flat twodimensional bilayer or a mesoporous three-dimensional TiO2/ P3HT films, with the latter exhibiting clearly better results [292]. Nevertheless, the size of the pores played an important role in the overall performance, since the wider the size, the better the infiltration of the regio-regular P3HT polymers, but the interfacial surface area decreased and more severe recombination seemed to occur. So, 30 nm thick mesoporous TiO2 films gave the optimum results, better than the 20 nm or 60 nm ones. Absorption spectra of the system showed, as expected, panchromatic response throughout the visible and NIR range. It should also be noted that the HOMO-LUMO levels of both sensitizers are ideally aligned for electron transfer to the titania and efficient hole transfer and subsequent dye regeneration. A significant increase in the photocurrent was, indeed, observed in the two-dye system, in comparison to the lone P3HT-based one, with the polymer and the Pc working

effectively as sensitizers in different energy regions, as demonstrated by the IPCE spectra. The suggested pathways for this improvement were ascribed by the authors to several processes: among others, energy transfer from P3HT to TT1, supported by the spectral overlap of the two and electron injection from the polymer to the TiO2, directly or through a cascade energy via TT1. The overall performance of the cell consisted in a JSC of 2.86 mA/cm2, a VOC of 0.74 V and a F.F. of 0.48, corresponding to a PCE of 1.01%. In the second example, reported by Snaith, Friend and co-workers, femtosecond transient absorption spectroscopy was employed to shed light to the charge generation processes of a P3HT/TT1-based device [293]. It was observed that an efficient FRET took place from the polymer to the Pc on picoseconds time scales, while hole transfer from TT1 to P3HT occurred as of 100 ps. FRET proved to be very efficient and the photoresponse of the mixture was enhanced, however the overall performance was not optimal. In particular, the observed EQE was lower than that obtained in a standard TT1-sensitized device. The increased charge loss was attributed to a decreased electron injection rate in the TiO2 due to formation of molecular aggregates of TT1. Another architecture in the area of solid-state device was constructed and studied by Driscoll and co-workers [294], consisting of TT1 as the primary chromophore, a newly synthesized SpiroTBT (Fig. 79) acting as a the secondary absorber that possesses ideal properties for FRET to the Pc, and Spiro-OMeTAD, the most widely used HTM in solid-state DSSCs [112]. Energy transfer from the Spiro-TBT to the Pc was confirmed by emission experiments, in which upon excitation at 540 nm of solutions containing both components at various concentrations, it was observed an increase in the longer wavelength of the TT1 emission with a simultaneous quenching of the Spiro-TBT PL. As well, the higher the TT1 concentration in the solution, the fluorescence decay time of the ERD dropped, which perfectly complies with Förster’s theory. Further experiments looked into quenching of the ERD by charge transfer to the HTM. Indeed, observing at the fluorescence spectra, there was a red shift in the emission, which, combined with the long-lived species and absence in change to the ground state absorption, revealed charge transfer between SpiroTBT and Spiro-OMeTAD. However, the authors mentioned that in this system, this charge transfer could enable enhanced photocurrent due to charge generation. In fact, looking at the alignment of the components energy levels, charge generation is possible via hole transfer from Spiro-TBT to the HTM and electron injection into the TiO2, which can occur whether directly or through an energy cascade via TT1. Direct electron injection is, however, ineffective, as proved by testing a single Spiro-TBT sensitized cell. The EQE spectra of the Spiro-TBT/TT1 devices indicated a large photoresponse from the ERD in the 450–550 nm visible region and a maximum of 21.8% in ETE at optimum concentration. As well, the Jsc increased with higher Spiro-TBT concentrations, due to enhanced light harvesting, but there was a drop in the F.F., which the authors attributed to the more complex transport associated with the addition of the ERD. Overall the PCE improved from 0.91% in the single TT1-cell to 1.06% in the energy-relay system.

M. Urbani et al. / Coordination Chemistry Reviews 381 (2019) 1–64

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Fig. 80. a) IPCE spectrum of QDs PbS-DSSC. b) IPCE spectra of QDs PbS/TT1-DSSC (black bold line)); for comparison, IPCE spectrum (gray line) of a typical TT1-sensitized solidstate cell is depicted. c) Energy diagram of all components of the PbS/TT1 DSSC. Reproduced with the permission from [297] Copyright 2009 Wiley-VCH.

In a different context, but sharing much similarity with the concept of ERD previously discussed, quantum dots (QDs) [295], and more specifically QD-Pc hybrid materials, have been scarcely considered in DSSC so far, despite the complementarity in optical properties (absorption/emission) of QD and Pcs. On this line, a recent theoretical investigation envisages Pcs as one of the best candidates for the preparation on new nanocomposites with graphene quantum dot (GQD) as efficient sensitizers for such application [296]. In a pioneer work in this field, Grätzel, Nazeerudin and coworkers took advantage of the Pc TT1 to enhance the lightharvesting capabilities over the NIR region of a PbS QD-DSSC and target a panchromatic sensitizer system [297]. The PbS QDs were prepared over a mesoporous TiO2 films by a successive ionic layer adsorption and reaction (SILAR) process, and then dipped into a TT1+CHENO dye solution (0.05 M of TT1 + 10 mM of CHENO). The resulting TiO2/PbS/TT1 hybrid photoanode, in which the Pc acts as a secondary sensitizer, was exploited in a solid-state solar cell with Spiro-OMeTAD as organic hole conductor. In comparison with the PbS QD device (Fig. 80a), the IPCE response of the hybrid

PbS/TT1 system (Fig. 80b) decreased substantially in the 400– 600 nm visible range but was overcompensated by a important increase in the visible Red/NIR region (where TT1 dye absorbs strongly), which was translated to an overall improvement in the JSC. Though the overall efficiency of these systems lied close to around 1% only, this work demonstrated the synergistic effect in both the visible and NIR range by means of using of QDs and Pc in the same DSSC architecture, and therefore their potential to get a panchromatic response. More recently, the groups of Mandal, Torres and Tkachenko, persuaded deeper investigations on TT1 in ZnCdSeS QD-DSSC, and rationalized the QD-Pc interaction [298]. In this new study, two other Pcs with different linkers (TT3 and TT6) were also studied for comparison. An important finding was that, despite a thermodynamically favoured ET transfer process, only photoinduced energy transfer (FRET-type) from the QD to phthalocyanine occurs with high efficiency in these cells, as evidenced by steady-state and time-resolved spectroscopy. Another example was described by Sastre-Santos and co-workers [299] and it described the direct attachment of a Pc

Fig. 81. Schematic representation of the hybrid Pc/quantum dots systems CdS-SPhC2ZnPc and CdS-SZnPc, reported by Sastre-Santos and co-workers [299].

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tion in this architecture was twofold: it induced a cascade injection into the TiO2 electrode and it passivated the surface of the QD improving the EQE of the QD-sensitizer. It was also pointed out that Förster resonant energy transfer was ruled out owing to the low overlapping between the emission of the Pc and the absorption of the QD.

8. Phthalocyanines for p-type dye-sensitized solar cells (pDSSCs)

Fig. 82. Molecular structures of electron-accepting phthalocyanines ZnPcs 1–2 used in CuO p-type DSSCs, reported by the groups of Guldi and Torres [302–305].

to a CdS-QD that gave rise to, besides the complementary light absorption, also to an interaction between the two species leading to an electron-cascade injection. The newly designed Pcs were decorated with a thiol, protected in the form of an ethylene nitrile or thioester, in order to be able to bind to the QD, and six tertoctylphenoxy or tert-butyl groups at the remaining peripheral positions (Fig. 81). Upon deprotection, the Pcs were used to co-sensitize the SILAR prepared TiO2/CdS-sensitized electrodes. Looking at the UV–Vis spectra of the covalently bridged QD-Pc systems, it was observed a bathochromical shift of the QD band compared to that of pristidine QD alone, and moreover an intensification of the Pc Qband. Both factors evidenced that the derivative with the thiol directly linked to the macrocycle contributed to a higher adsorption of the system on the TiO2 film. Measurement of the J-V performance of the devices using a polysulfide S2/S2 electrolyte n indicated the improved characteristics of the co-sensitized system, namely the efficiency rose from 1.1% up to 1.7% in QD-(SZnPc) and 1.5% in QD-(SPhC2ZnPc). The better performance of the QD(SZnPc) system over QD-(SPhC2ZnPc) was interpreted as the result of the closer distance between the QD and the ZnPc, favouring electron transfer, as well as higher dye loading. Switching to a cobalt-based electrolyte the results were even higher, with a large increase in the JSC and VOC values, and the PCE going from 0.8% to 2.5% and 1.9% respectively. It was rationalized that the redox potential of the cobalt-based electrolyte being closer in energy to the HOMO of the ZnPc than the polysulfide one, seems to be a crucial for an efficient dye-regeneration. Overall, contribution from both components to the EQE of the device was demonstrated, in contrast to a previous Pc-QD system reported earlier by the same research group [300]. As well, it was demonstrated that the Pc did not attach directly onto the semiconductor. The Pc’s contribu-

One of the first example of the use of a Pc-based material in a ptype organic solar cell was reported by Song and co-workers in 2012 in C60-based devices [301] within the following architecture: (ITO)/(Cu-Pc)/N,N-di-[(1-naphthyl)-N,N-diphenyl]-1,1-biphenyl)-4 ,4-diamine (NPB)/fullerene (C60)/tris-(8-hydroxyquinoline) aluminium (Alq3)/aluminium (Al). In this study, the authors employed a thin layer of Cu-Pc inserted between ITO and NPB to improve the performances of the initial cell (without Pc). In such configuration, it is important to remark that the CuPc was not used as a sensitizer herein, but, instead, as an efficient hole-extraction layer and, indeed, no contribution from the CuPc was found in the spectral response of the modified device. Nonetheless, the incorporation of the Cu-Pc layer increased the overall efficiency of the new hybrid device by 2.16 times (0.54%) with respect to the one without Cu-Pc (0.25%). It was concluded that Cu-Pc has higher hole-mobility than NPB and can form a cascade energy with it, which, together, should explain such improvement. The first use of a Pc as sensitizer for p-type DSSCs was pioneered recently by the groups of Torres and Guldi [302]. For this purpose, they developed two electron-accepting phthalocyanines thanks to the incorporation of six electron-withdrawing alkyl sulfonyl groups attached at the peripheral b-positions of the macrocycle (ZnPc-1 and -2; Fig. 82). Consequently, these Pcs display adequate HOMO/LUMO energy levels to render feasible the electroninjection of these sensitizers into the VB of the semi-conductor (CuO), as well as regeneration of the reduced dyes by the electrolyte, thus making them suitable for such application. In addition, branched- instead of linear- alkyl chains were chosen in the design of these Pcs in order to limit more efficiently their aggregation on the metal oxide surface. Concerning the anchoring group, either a directly connected (ZnPc1) or ethynyl linked (ZnPc1) carboxyl group were introduced in order to modulate, in particular, the charge-injection properties of the macrocycle. Turning to the metal oxide, an important step forward in this technology came from the use of nanorod-like CuO instead of nanoparticles, the latter being traditionally used in p-type DSSC. Initially, the DSSC cells were tested in conjunction with a standard I/I 3 electrolyte (Table 51), ZnPc2/DSSC achieving higher efficiency (g = 0.103%) than ZnPc1 (g = 0.067%). The electrochemical impedance spectroscopy measurements of the Pc-cells evidenced a better charge injection and charge transport of ZnPc2 with respect to ZnPc1, which were accounted for its superior performances. Regarding

Table 51 Photovoltaic performances of CuO/p-DSSCsa sensitized with ZnPc1-2 under 1 sun illumination (simulated, AM1.5G). Data derived from [302]. Dyeb

Electrolytec

VOC (mV)

JSC (mA/cm2)

F.F. (%)

g

ZnPc1

I/I 3 CoII/CoIII   I /I3 CoII/CoIII

93 224 102 251

1.93 1.99 2.78 2.35

38 32 36 32

0.067 0.141 0.103 0.191

ZnPc2

(%)

The thickness of the films was 4 lm. Dye-uptake solutions were prepared at 0.1 mM of Pc in EtOH. c Composition of the iodine-based electrolyte: LiI/I2 (1:0.4) in a 1:1 mixture (v/v) of AcCN and 3-methoxypropionitrile; cobalt-based electrolyte was composed of CoII/CoIII (0.01:0.1) in the form of cobalt di-tert-butyl bipyridine hexafluorophosphate ([Co(dtb-bpy)3][PF6]2/3) in AcCN. a

b

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the structure-performance relationship, it was rationalized that the conjugated ethynyl bridge in ZnPc2 enhances the electronic coupling between the Pc and CuO, thus providing an optimum balance between hole-injection and charge recombination. Although the Pcs-devices shows fairly good JSC and F.F. values that are in the same range or superior than typically obtained with other p-type sensitizers, an important drawback was encountered in the exceptionally low VOC values reached by these cells (around 100 mV only). This prompted the authors to use a cobalt-based electrolyte ([Co(dtb-bpy)3][PF6]2/3 in AcCN), such redox mediators being known to overcome this issue and to allow attaining higher voltages in DSSC and, indeed, significantly higher VOC values were obtained for both Pcs-devices under these conditions (224 mV and 251 mV for ZnPc1 and ZnPc2, respectively). In this case too, ZnPc2 DSSCs showed superior performances both in the VOC and JSC values than those obtained with ZnPc1, achieving a PCE of 0.194%, a record efficiency at that time for a p-type DSSC. Afterwards, the same groups reported various works concerning further optimization and EIS characterization of these p-type CuO cells, and in particular the calcination temperature, film thickness, and electrolyte concentration [303–305]. 9. Conclusion and outlooks Phthalocyanines (Pcs) are synthetic porphyrin analogues, consisting of four isoindole subunits linked together through nitrogen atoms and forming a planar 18 p-electron system. They are distinguished for their extraordinary light-harvesting abilities in the far red- and near IR spectral region as well as their robustness and thermal stability, overcoming the drawbacks of porphyrins and establishing themselves among the benchmark dyes in photovoltaic technologies. In particular, Pcs have played a very important role in the development of dye-sensitized solar cells (DSSCs), as they are promising candidates for incorporation in these devices. A significantly large number of synthetic variations are available on the main Pc scaffold, for instance, in the anchoring group appended to the macrocycle ring, the peripheral or axial substituents and the central metal. It has been demonstrated that structural aspects can influence considerably the performance of these sensitizers in DSSCs, so the wide number of functionalisable positions that offers a large panel of possible molecular designs for phthalocyanine dyes, is of paramount importance. The performance of Pc-based devices has improved over the years, reaching efficiencies beyond 6%, thanks to rational design of the phthalocyanine structure. Acknowledgments

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