The application of transition metal complexes in hole-transporting layers for perovskite solar cells: Recent progress and future perspectives

Coordination Chemistry Reviews 406 (2020) 213143

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Review

The application of transition metal complexes in hole-transporting layers for perovskite solar cells: Recent progress and future perspectives Ze Yu a,⇑, Anders Hagfeldt b, Licheng Sun a,c,⇑ a State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Center on Molecular Devices, Institute of Energy Science and Technology, Dalian University of Technology (DUT), Dalian 116024, China b Laboratory of Photomolecular Science, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland c Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm 10044, Sweden

a r t i c l e

i n f o

Article history: Received 10 October 2019 Accepted 2 December 2019 Dedicated to the retirement of Prof. Mei Wang. Keywords: Transition metal complex Perovskite solar cells Hole-transporting materials P-type dopant Phthalocyanine Porphyrin

a b s t r a c t In the past few years, the photovoltaic community has witnessed a fast growth of inorganic-organic hybrid perovskite solar cells (PSCs) due to their exceptional power conversion efficiencies with a certified record value exceeding 25%, which rivals the established commercial solar cell technologies. In the stateof-the-art PSCs, a hole-transporting layer (HTL) typically consisting of an active hole-transporting material (HTM) and some additives or dopants, takes the responsibility of transporting the photo-generated holes from the perovskite to the selective contact electrode, and is a key component for achieving high-performance and stable PSCs. In this review article, recent advances in the development of transition metal complexes in HTLs for PSCs including HTMs and p-type dopants are presented. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 p-Type dopants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Cobalt complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Other metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Hole-transporting materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Phthalocyanines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1.1. Copper phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1.2. Zinc phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1.3. Other metal phthalocyanines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3. Other transition metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Concluding remarks and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

⇑ Corresponding authors at: State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Center on Molecular Devices, Institute of Energy Science and Technology, Dalian University of Technology (DUT), Dalian 116024, China. E-mail addresses: [email protected] (Z. Yu), [email protected] (L. Sun). https://doi.org/10.1016/j.ccr.2019.213143 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

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Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

1. Introduction Recently, inorganic–organic hybrid perovskite solar cells (PSCs) have attracted considerable research attention because of their ease of fabrication, low-production cost, and excellent power conversion efficiencies (PCEs) [1–5]. Metal halide perovskites exhibit several superior optoelectronic properties, such as large absorption coefficient, a broad absorption range, high charge carrier mobility, and long charge carrier diffusion length. The certified record PCE of PSCs has quickly reached over 25%, which rivals other established commercial solar cells, such as silicon, cadimium telluride (CdTe), and copper indium gallium selenide (CIGS) [6,7]. Perovskites typically adopt a cubic structure with a general formula of ABX3, where A and B stand for the cations and X represents an anion as shown in Fig. 1 [8–10]. The most frequently used perovskites in solar cells are lead or tin halide ones, where the A sites are organic cations CH3NH+3 (MA), NH@CHNH+3 (FA) and/or inorganic Cs or Rb cations, and the X sites are the halides Cl
, Br
or I
. The device structures of PSCs typically include mesoscopic and planar (n-i-p), and inverted planar (p-i-n) configurations (Fig. 2). The mesoscopic n-i-p structure of PSCs is analogous to the solidstate dye-sensitized solar cells (ss-DSSCs) as illustrated in Fig. 2a. The perovskite is penetrated into a porous titania layer, followed by the deposition of a hole-transporting layer (HTL) and a metal electrode [1,2]. Mesoscopic PSCs typically require hightemperature sintering process, which restricts their potential use on plastic substrates. This led to the emergence of the simplified planar configurations of PSCs (planar n-i-p or p-i-n, Fig. 2b and c), in which the perovskite is simply sandwiched between a HTL and a compact electron-transporting layer (ETL), such as SnO2, C60, and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), depending on the deposition sequence of the charge-selective layers. Irrespective of the device configurations used, PSCs generally work as follows: after photo-excitation, the perovskite absorber transfers the photo-generated charges to the respective chargeselective layers (electrons to the ETL and holes to the HTL). These

photo-generated charges are then collected by the front and back contact electrodes to generate the photocurrent. A HTL is an essential component for achieving high PCE in PSCs. It takes the responsibility of transporting the photo-generated holes from the perovskite to the selective contact [11,12]. The HTL is generally composed of a hole-transporting material (HTM) and some additives or dopants. A HTM ideally should possess the following requirements [13]: 1) The highest occupied molecular orbital (HOMO) energy level of a HTM must be matching (less negative vs. vacuum) the valence band (VB) edge level of the perovskite. 2) The hole mobility of the HTM is required to be sufficient enough to effectively facilitate hole transportation/collection from the perovskite absorber. 3) The HTM is chemically and thermally robust. 4) Moreover, the HTM should be easily attainable correlated with a low-production cost in view of future large-scale application. Small molecule 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenyla mine)-9,90 -spirobifluorene (spiro-OMeTAD, Fig. 3a) has been the most widely used HTM in PSCs so far. Over 23% PCE has been obtained by using spiro-OMeTAD [14]. However, the hole mobility of such a HTM is quite low because of its amorphous nature. Several cobalt(III) complexes (i.e. FK209) were initially developed as ptype dopants (p-dopants) for spiro-OMeTAD in ss-DSSCs with satisfactory overall efficiency and high reproducibility. Chemical pdoping has proved to be an effective approach to enhance the conductivity and thus the solar cell performance. Various p-dopants have been developed for spiro-OMeTAD in ss-DSSCs and PSCs, including transition metal complexes (TMCs), ionic liquids, and organic small molecules, such as tetracyanoquinodimethane (TCNQ) derivatives, etc., [15–21]. In addition, spiro-OMeTAD suffers from tedious synthetic steps, resulting in a rather highproduction cost. Therefore, considerable research activities in this field have been dedicated to the exploration of alternative lowcost and efficient HTMs in PSCs, including small molecules, inorganic hole conductors, and conducting polymers, etc., [13,22–32]. Another interesting class of alternative HTMs is based on TMCs, such as metal phthalocyanines (Pcs) and porphyrins (Pors). They possess several unique properties, such as excellent stability, relatively high hole mobility, and high hydrophobicity [33]. The optical and electronic properties of these macrocycles can be easily modified by adjusting the metal center and the substituents. These merits make TMCs promising efficient and stable HTMs that can be explored in PSCs. This review article is specifically focused on the recent development of TMCs as active HTMs and p-dopants in HTLs for PSCs. 2. p-Type dopants

Fig. 1. Crystal structure of perovskite with a general chemical formula of ABX3.

The basic principle of the p-type doping (p-doping) process in an organic semiconductor is illustrated in Fig. 3b. A dopant is

Fig. 2. Typical device architectures of PSCs.

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

3

Fig. 3. a) Molecular structure of spiro-OMeTAD. b) Schematic illustration of p-doping mechanism in organic semiconductors (e.g. HTM).

added to a host (e.g. HTM), which accepts an electron from the latter, leaving the host partially oxidized. To make this charge transfer process efficient, the electron affinity (EA) of the dopant has to exceed the ionization potential (IP) of the host [34]. Thus, the hole concentration in the host is increased, leading to a higher hole conductivity. As discussed above, spiro-OMeTAD is subject to low hole mobility/conductivity in its pristine form. Therefore, a variety of pdopants have been explored for spiro-OMeTAD to enhance its hole conductivity. In this review, we specifically focused on the TMCbased p-dopants developed for spiro-OMeTAD as most of the studies were based on this system. If the readers are interested in other p-doping systems for HTLs in PSCs please refer to the fruitful literature [17,21,35–43]. 2.1. Cobalt complexes Cobalt(III) complex FK209 (Fig. 4) has been the most frequently used TMC-based p-dopant for spiro-OMeTAD in PSCs [3,44–50]. It was originally developed in ss-DSSCs [51]. Unlike lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which oxidizes spiroOMeTAD by oxygen typically by exposure to air, FK209 directly facilitates the electron transfer from the latter due to the large potential difference (~300 mV) between the redox potential of FK209 and the first oxidation potential of spiro-OMeTAD [51]. Chemical p-doping using Co(III) complexes thus provides an effective route to control the oxidation extent of spiro-OMeTAD, which is beneficial for the reproducibility of the solar cell efficiency. Seok and coworkers systematically studied the effect of FK209 in mesoscopic MAPbI3-based PSCs in comparison to LiTFSI and 4tert-butylpyridine (TBP) [44]. Note that TBP is another commonly used additive in the HTL, which has been reported to improve the film quality of the HTL and avoid the aggregation of the lithium salt in the HTL [52,53]. The authors found that additional FK209 reduced the charge recombination and lowered the Fermi level of the HTM, thus resulting in an improved PCE of 10.4%. Xi and Zhang et al. further confirmed this synergistic effect of FK209 and LiTFSI in planar PSCs, achieving a much higher PCE of 17.8% (Table 1) in comparison with the FK209-doped device (13.5%) and the LiTFSIdoped device (15.0%) [46]. Very recently, a PCE of 24.0% (certified 23.5%) has been yielded based on spiro-OMeTAD in conjunction with LiTFSI/TBP/FK209, by introducing methylammonium chloride in FAPbI3-based PSCs [49]. Although LiTFSI has been ubiquitously used to dope spiroOMeTAD, the hygroscopic nature of the lithium salt is detrimental to the device long-term stability. It has also been reported that the lithium ion can diffuse through the perovskite layer and may therefore affect the cell operation [54]. Grätzel and coworkers introduced Zn-TFSI2 to replace LiTFSI in the presence of FK209

and TBP [50]. The use of the zinc salt as a p-dopant for spiroOMeTAD significantly improved the hole mobility by one order of magnitude. PSCs with Zn-TFSI2 showed a higher open-circuit voltage (Voc) and fill factor (FF), resulting in a stabilized PCE of 22.0%. More importantly, the devices containing Zn-TFSI2 demonstrated superior photo-, humidity-, and thermal stability in comparison to those containing LiTFSI. The much better stability of Zn-TFSI2-based devices is related to a strong interaction between Zn-TFSI2 and spiro-OMeTAD, improved morphological stability, as well as enhanced moisture resistivity. Koh et al. synthesized MY11 with a deep redox potential of 1.27 V vs. normal hydrogen electrode (NHE) and applied it as a p-dopant in comparison with a previously reported FK102 [55,56]. In contrast to FK102, the pyrazolyl moiety in MY11 is connected to a pyrimidine moiety rather than a pyridine moiety. The introduction of the pyrimidine brings on a similar effect as incorporating an electron-withdrawing group into the ligand, which positively tunes the redox potential of the dopant. Under the same doping concentration, MY11 presented a higher spiro-to-spiro+ conversion ratio in contrast to FK102, because of the larger potential discrepancy between the redox potential of MY11 and the HOMO energy level of spiro-OMeTAD. Consequently, an enhanced conductivity of spiro-OMeTAD was obtained by MY11, which led to a slightly higher PCE of 11.9%. A series of bipyridine-based cobalt (III) complexes D1-D4 were adopted as p-dopants for spiroOMeTAD [57]. A co-solvent system containing dichloroethane and acetylacetone was specifically developed to dissolve both the dopants and spiro-OMeTAD. PSCs based on D1 exhibited the highest PCE of 14.9% amongst the cobalt dopants studied. Komatsuzaki and coworkers developed two types of cobalt(III) complexes Co1Co4 based on bidentate pyrazolyl-pyridine and tridendate pyrazolyl-bipyridine ligands, and investigated their optical and electrochemical properties as well as photovoltaic performance in mesoscopic MAPbI3-based PSCs [58]. Hydrophobic alkyl groups were incorporated into these ligands to improve the solubility of the complexes. The devices with Co2 showed the optimal efficiency of 14.8%, which surpassed that of FK209 (13.7%) under identical conditions. Very recently, Bach and Simonov et al. systematically studied another commercially available cobalt(III) p-dopants FK269 in comparison with FK209 [59]. In addition to the function of pdoping, FK269 was also found to have an impact on the morphology of the perovskite layer and interfacial charge recombination strongly correlated with its deep-lying HOMO energy level (
5.71 eV) with respect to that of FK209 (
5.45 eV). Mesoscopic PSCs (1 cm2 size) based on FK269 and a triple-cation perovskite Cs0.05FA0.79MA0.16PbI2.49Br0.51 presented a stabilized PCE of 19.0%.

4

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

Fig. 4. Molecular structures of TMC-based p-dopants developed for spiro-OMeTAD in PSCs.

2.2. Other metal complexes The research of TMC-based p-dopant in PSCs has been almost exclusively dominated by cobalt complexes. Only few studies were carried out in terms of other metal complexes. Snaith and coworkers employed Mo-(tfd-CO2Me)3 and Mo-(tfd-COCF3)3 as p-dopants for spiro-OMeTAD in place of LiTFSI [60]. As compared to pristine spiro-OMeTAD, both these two molybdenum complexes enhanced the conductivity by almost 4 orders of magnitude, which outperformed that using LiTFSI/FK209/TBP. The best devices afforded stabilized PCEs of 16.7% and 15.7% for Mo-(tfd-COCF3)3 and Mo-(tfdCO2Me)3, respectively (Table 1). Moreover, non-encapsulated PSCs based on these two dopants showed improved thermal stability at ambient conditions for 500 h, with 70% remaining of its initial

efficiency while the devices using lithium salts rapidly dropped to 5.1% from 17.1%. Badia et al. reported an iridium complex, namely IrCp*Cl(PyPyz) [TFSI], as a p-dopant for spiro-OMeTAD in mesoscopic PSCs [61]. The HOMO energy level of IrCp*Cl(PyPyz)[TFSI] is located at
6.31 eV, which is more negative relative to that of spiroOMeTAD (
5.11 eV), making it a suitable p-dopant for the latter. The best device based on such a dopant showed a 17% enhancement in PCE, amounting to 10.8% comparable to that obtained by FK209. Hagfeldt and coworkers developed a hemicage-structured iron(III) complex Fe(ttb) and applied it in planar PSCs [62]. The redox potential of this iron complex was 1.29 V vs. NHE, which showed a larger driving force for oxidation of spiro-OMeTAD relative to FK209 (1.09 V vs. NHE). The devices containing 5 mol% Fe

5

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143 Table 1 Summary of representative photovoltaic performance of PSCs using spiro-OMeTAD as HTMs with different TMC-based p-dopants. Dopant

Redox potentiala (eV)

Device Configurationb

Cell structure

Additives

Jsc (mA cm
2)

Voc (V)

FF

PCEc (%)

Ref.

FK209


5.46

M

FTO/c-TiO2/m-TiO2/FAPbI3/HTM/Aud

25.9

1.13

0.82

24.0

[49]

FK209


5.46

P

FTO/TiO2/MAPbI3–xClx/HTM/Ag

21.7

1.11

0.74

17.8

[46]

MY11


5.77

M

FTO/c-TiO2/m-TiO2/MAPbI3/HTM/Au

16.8

1.00

0.71

11.9

[55]

FK269


5.71

M

FTO/c-TiO2/m-TiO2/Cs0.05FA0.79MA0.16PbI2.49Br0.51/ HTM/Au FTO/SnO2/PCBM/FA0.85MA0.15Pb(I0.85Br0.15)3/HTM/ Au FTO/SnO2/PCBM/FA0.85MA0.15Pb(I0.85Br0.15)3/HTM/ Au FTO/SnOx/(RbCsMAFA)Pb(I0.83Br0.17)3/HTM/Au

LiTFSI/ TBP LiTFSI/ TBP LiTFSI/ TBP LiTFSI/ TBP —

23.1

1.14

0.74

19.4

[59]

22.0

1.06

0.74

17.2

[60]

—

22.1

1.07

0.76

17.8

[60]

LiTFSI/ TBP LiTFSI/ TBP LiTFSI/ TBP LiTFSI/ TBP LiTFSI/ TBP

21.7

1.20

0.74

19.2

[62]

24.0

1.09

0.71

18.5

[19]


5.22

e

P

Mo-(tfd-COCF3)3


5.49

e

P

Fe(ttb)


5.69f

P

Cu(bpcm)2


5.34

M

Mo-(tfd-CO2Me)3

a b c d e f g

JQ–1


5.17

M

JQ–2


5.27

M

JQ–3


5.44

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbI3)0.15/HTM/ Au FTO/c-TiO2/m-TiO2/(Rb0.05Cs0.05FA0.8MA0.1)Pb (I0.85Br0.15)3/HTM/Au FTO/c-TiO2/m-TiO2/(Rb0.05Cs0.05FA0.8MA0.1)Pb (I0.85Br0.15)3/HTM/Au FTO/c-TiO2/m-TiO2/(Rb0.05Cs0.05FA0.8MA0.1)Pb (I0.85Br0.15)3/HTM/Au

g

22.8

1.12

0.75

19.3

[63]

21.8

1.04

0.68

15.9g

[63]

21.9

1.10

0.73

18.0g

[63]

From electrochemistry unless stated otherwise. M and P stand for mesoscopic and planar cell configurations, respectively. The photovoltaic parameters are based on the best-performing devices. Doped with 40 mol% MACl in FAPbI3. Conversion from Fc/Fc+ by equation: E =
(5.1 + Eox). Conversion from NHE by equation: E =
(4.4 + Eox). Average PCE value.

(ttb) afforded the best PCE of 19.5%, which was on par with the reference devices based on FK209 (19.3%). Kloo and coworkers reported two copper(II) complexes, coded as Cu(bpm)2 and Cu(bpcm)2, as p-dopants in mesoscopic PSCs [19]. A chloride group was introduced on the ligands of Cu(bpcm)2 to increase its oxidation potential with an aim to increase the driving force for the oxidation process. Cu(bpcm)2-doped spiroOMeTAD (2 mol%) indeed showed a much improved conductivity of 9.36  10
5 S cm
1 relative to that of the pristine film (9.12  10
7 S cm
1). In combination with LiTFSI and TBP, the best devices based on Cu(bpcm)2 delivered a PCE of 18.5%, which rivaled that of FK209 (18.7%). Cheng et al. systematically studied three copper(II) complexes JQ1-JQ3 as p-dopants in PSCs [63]. The oxidative reactivity of the dopants was found to play a role in governing the conductivity, and thus the photovoltaic performance. By varying the doping level, the best PCE of 19.3% was achieved for the devices using JQ1.

Fig. 5. General molecular structure of MPcs.

functional groups (i.e. hydrophobic, conjugated systems), etc. As a consequence, a number of MPcs and their derivatives have been explored as HTMs in PSCs.

3. Hole-transporting materials 3.1. Phthalocyanines

3.1.1. Copper phthalocyanines

Metal phthalocyanines (MPcs) are 18-p electron aromatic macrocyclic compounds and their general structure is shown in Fig. 5. MPcs possess several excellent properties, such as facile synthesis correlated with low-production costs and robustness (chemically, thermally, and photo). These planar and highly conjugated aromatic macrocycles are prone to aggregate through p-p stacking interactions (J- or H-aggregates) [64]. Specific molecular packing of Pcs has a significant influence on the crystallinity of the Pc films, which regulates the hole transport property and thus the solar cell performance [65]. Another advantageous feature of MPcs is the synthetic flexibility of these macrocycles by tuning the metal center and substituents including different positions (peripheral (b-) or non-peripheral (a-)), different sizes (small or bulky), and varied

(a) Vacuum-processed copper phthalocyanines Lianos and coworkers first introduced CuPc into mesoscopic PSCs, in which CuPc was thermally evaporated onto the perovskite MAPbClxI3
x due to the low solubility of CuPc in common solvents [66]. A low PCE of 5.0% was obtained most likely because of the poor film quality of the perovskite fabricated. Based on the CuPc and a low-cost carbon cathode, Yang et al. further boosted the PCE of mesoscopic PSCs to 16.1% by using a sequential method to produce high-quality perovskite MAPbI3 films (Table 2) [67]. Thermally evaporated CuPc was also employed as a HTL in planar PSCs and an optimal PCE of 15.4% was achieved by tuning the CuPc thickness [68]. Liao and coworkers conducted a series of studies regarding the modification of the ETL for planar CuPc-based PSCs

6

Table 2 Summary of representative photovoltaic performance of PSCs based on MPcs as HTMs. HOMOa (eV)

lhb

Device Configurationc

Cell structured

Dopants

Jsc (mA cm
2)

Voc (V)

FF

PCEe (%)

Ref.

(cm2 V
1 s
1)

CuPc

–5.20

—

M

FTO/c-TiO2/m-TiO2/MAPbI3/CuPc/Carbon

—

20.8

1.05

0.74

[67]

CuPc CuPc CuMe2Pc


5.20
5.28
5.10

— — 4.79  10
2

P IP P

FTO/Ni:TiO2/perovskite/CuPc/Carbon ITO/VOx/CuPc/MAPbI3/C60/BCP/Ag FTO/SnO2/PCBM/MAPbI3/CuMe2Pc/Au

— — —

22.4 22.2 21.3

1.07 1.02 1.09

0.73 0.75 0.68

CuPc-DMP


5.46

9.80  10
5

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPc-DMP/Au

LiTFSI/TBP

23.2

1.04

0.71

t-Bu-CuPc t-Bu-CuPc


5.20
5.20

— —

M M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/t-Bu-CuPc/Au FTO/c-TiO2/m-TiO2/PCBM:PMMA/Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45/t-Bu-CuPc/ Au

LiTFSI/TBP LiTFSI/TBP

22.6 23.6

1.07 1.15

0.78 0.74

16.1 (15.0) 17.5 16.9 15.7 (16.4) 17.1 (16.7) 18.8 20.1

CuPc-OTPAtBu


5.22

—

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPc-OTPAtBu/Carbon

F4TCNQ

21.9

1.01

0.68

HTM

[71] [73] [75] [76] [77] [78]

( OctPhO)8CuPc


5.35

—

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/( OctPhO)8CuPc/Au

LiTFSI/TBP

19.0

0.87

0.51

(tOctPhO)8ZnPc


5.26

—

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/(tOctPhO)8CuPc/Au

LiTFSI/TBP

17.5

0.89

0.47

ZnPc-p-ZnPc


4.90

—

M

FTO/c-TiO2/m-TiO2/(Cs0.05FA0.85MA0.1)Pb(I0.9Br0.1)3/ZnPc-p-ZnPc/Au

LiTFSI/TBP

23.4

0.95

0.68

BI25


4.96

—

M

FTO/c-TiO2/m-TiO2/MAPbI3/BI25/Au

LiTFSI/TBP/ FK209

16.7

1.01

0.68

(19.7) 15.0 (15.3) 16.1 20.2 (17.6) 16.6 (16.1) 14.0 (15.8) 15.9 17.8 (17.5) 12.5 (18.4) 13.7 (18.4) 16.7 (20.2) 19.7 (20.2) 14.3 (18.8) 17.6 (18.8) 8.3 (15.2) 7.3 (15.2) 15.2 (17.9) 11.8

BL07


5.39

—

M

FTO/c-TiO2/m-TiO2/MAPbI3/BL07/Au

LiTFSI/TBP/ FK209

10.7

1.00

0.60

(16.8) 6.7

[94]

BL08


5.18

—

M

FTO/c-TiO2/m-TiO2/MAPbI3/BL08/Au

LiTFSI/TBP/ FK209

17.4

1.03

0.61

(16.8) 11.4

[94]

HT-ZnPc


5.19

—

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/HT-ZnPc/Au

LiTFSI/TBP/ FK209

21.0

1.06

0.79

(16.8) 17.5

[95]

TS-CuPc TS-CuPc


5.30
5.30

2.30  10 2.30  10
4

IP P

ITO/TS-CuPc/MAPbI3
xClx/PCBM/Bphen/Ag FTO/TiO2/MAPbI3/spiro-OMeTAD/TS-CuPc/MoO3/Ag

F4TCNQ F4TCNQ

21.7 24.3

0.96 1.12

0.77 0.74

N-CuMe2Pc


5.06 (IPS)

1.23  10
3 (FET)

P

FTO/SnO2/MAPbI3/N-CuMe2Pc:P3HT/Au

—

22.4

1.01

0.73

CuPc-TIPS


5.42

—

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPc-TIPS/Carbon

—

21.4

1.01

0.65

CuBuPc CuPrPc


5.20
4.92 (IPS)

— 2.16  10
3

M P

FTO/c-TiO2/m-TiO2:RGO/MAPbI3–xClx/GO/CuBuPc/Au FTO/SnO2/MAPbI3/CuPrPc/Au

— —

21.0 23.2

1.07 1.01

0.71 0.76

CuPcNO2-OMFPh


4.98

5.02  10
5

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPcNO2-OMFPh/Au

—

20.0

1.02

0.60

CuPcNO2-OBFPh


4.96

8.21  10
5

OMe-DPA-CuPc


4.99 (IPS)

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPcNO2-OBFPh/Au

—

20.6

1.04

0.64


3

P

FTO/SnO2/Cs0.05(MA0.13FA0.87)0.95Pb(I0.87Br0.13)3/OMe-DPA-CuPc/Au

—

22.4

1.05

0.71


3

1.55  10

OMe-TPA-CuPc


5.14 (IPS)

6.39  10

P

FTO/SnO2/Cs0.05(MA0.13FA0.87)0.95Pb(I0.87Br0.13)3/OMe-TPA-CuPc/Au

—

23.4

1.10

0.77

CuPc-Bu


5.12

1.23  10
4

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPc-Bu/Au

—

21.0

1.03

0.66

CuPc-OBu


5.11

4.30  10
4

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuPc-OBu/Au

—

22.8

1.06

0.73

t

t

(19.1)

[79] [80] [80] [82] [33] [83] [85] [86] [86] [87] [87] [88] [88] [92] [92] [93] [94]

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143


4

[103] (10.8) 16.3 0.73

e

d

c

b

a

From electrochemistry unless stated otherwise. IPS: from ionization energy measurement. Cal: form calculation. From space-charge-limited current (SCLC) method unless stated otherwise. FET: from field-effect transistor. M, P, and IP stand for mesoscopic, planar and inverted planar cell configurations, respectively. BCP, bathocuproine. PCBM, [6,6]-phenyl-C61-butyric acid methyl ester. PMMA, poly(methyl methacrylate). RGO, reduced graphene oxide. GO, graphene oxide. The photovoltaic parameters are based on the best-performing devices. The corresponding PCE values for spiro-OMeTAD are given in the parentheses.

1.06 21.1 — P 3.42  10
2
5.11 PdMe2Pc

FTO/SnO2/PCBM/MAPbI3/PdMe2Pc/Au

11.8 M —
5.14 (Cal) CoPcNO2-OPh

FTO/c-TiO2/m-TiO2/MAPbI3/CoPcNO2-OPh/Au

—

1.04

0.64

[102]

[101] 0.73 23.1 1.90  10
4
5.06 NiPc-(OBu)8


5

M

8.63  10
4.98 (IPS) H6Bu-ZnPc

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/NiPc-(OBu)8/V2O5/Au

—

1.08

[98] 0.46 21.3

6.85  10
4.96 (IPS) Me6Bu-ZnPc

P


4

FTO/SnO2/Cs0.05(MA0.13FA0.87)0.95Pb(I0.87Br0.13)3/H6Bu-ZnPc/Au

—

1.06

[98] 0.69 23.1

4.78  10
4
4.90 (IPS) ZnBu4Pc

P

7.59  10
4
4.91 (IPS) RE-ZnBu4Pc

FTO/SnO2/Cs0.05(MA0.13FA0.87)0.95Pb(I0.87Br0.13)3/Me6Bu-ZnPc/Au

—

1.09

[97] 0.61 20.7 P

1.14  10
4
4.95 ZnPcNO2-OBFPh

FTO/SnO2/MAPbI3/ZnBu4Pc/Au

—

0.95

[97] 0.63 21.8 P

—
5.15 HBT-ZnPc

FTO/SnO2/MAPbI3/RE-ZnBu4Pc/Au

—

0.94

[86]

(19.1) 15.7 (18.4) 12.9 (17.3) 12.1 (17.3) 17.4 (19.3) 10.3 (19.3) 18.3 (19.4) 8.2 0.68 21.0 M

(cm2 V
1 s
1)

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/ZnPcNO2-OBFPh/Au

—

1.10

15.5 1.10 20.2 LiTFSI/TBP/ FK209 M

lhb

HOMOa (eV) HTM

Table 2 (continued)

Device Configurationc

Cell structured

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/HBT-ZnPc/Au

Jsc (mA cm
2) Dopants

Voc (V)

FF

0.69

PCEe (%)

Ref.

[95]

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

7

[69–71]. A PCE of 17.5% was obtained in combination with a high-crystallinity Ni-doped rutile TiO2 ETL and a carbon counter electrode [71]. The authors also incorporated CuPc into inverted planar PSCs, which delivered the PCEs of 15.4% and 12.8% for rigid and flexible substrates, respectively [72]. Dong and Wu et al. employed a bilayer HTL comprising of VOx and CuPc in inverted planar PSCs [73]. The best devices based on this bilayer HTL showed a PCE of 16.9%, which surpassed that of the devices using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS, 14.3%) under the same conditions. Wang et al. synthesized two tetra-alkyl (tetra-methyl and tetraethyl)-substituted CuPcs as HTLs in planar PSCs, showing PCEs of 11.9% and 11.7% for CuMePc and CuEtPc (Fig. 6), respectively [74]. Xu and Fang et al. reported an octamethyl-substituted CuPc (CuMe2Pc), and compared its performance relative to pristine CuPc in planar MAPbI3-based PSCs [75]. It was disclosed that CuMe2Pc showed a face-on molecular orientation on top of the perovskite layer, leading to higher hole mobility and a more homogeneous morphology. As a consequence, the devices containing CuMe2Pc presented a 25% higher PCE (15.7%) than those with CuPc (12.6%). (b) Solution-processed copper phthalocyanines It is noticeable that pristine CuPc and its derivatives with shorter alkyl chains (methyl or ethyl) have to be thermally evaporated under high vacuum because of their low solubility in common organic solvents. A number of CuPc derivatives with other substituents (e.g. longer alkyl chains, aryl, and heteroaryl groups) have also been developed in PSCs with an aim to fabricate the CuPc thin films by solution-processing techniques such as spincoating. Yu and Sun et al. first introduced a-tetra-2,4-dimethyl-3pentoxy substituted CuPc (CuPc-DMP) as a solution-processable HTM in mesoscopic PSCs [76]. The incorporation of bulky branched alkoxy chains increased the solubility of the compound, which made solution-processing possible. The best devices based on CuPc-DMP combined with the perovskite absorber (FAPbI3)0.85(MAPbBr3)0.15 afforded an overall efficiency of 17.1% (Table 2), which was comparable to that obtained by spiro-OMeTAD (16.7%). Seok, Seo, and coworkers investigated the thermal stability of PSCs based on a commercially available CuPc derivative, t-Bu-CuPc, in comparison with the state-ofthe-art HTMs spiro-OMeTAD and poly(triarylamine) (PTAA) [77]. The CuPc-based devices achieved a PCE of 18.8% and excellent long-term thermal stability relative to spiro-OMeTAD and PTAA. The superior thermal stability observed in CuPc-based devices was most likely attributed to the strong interfacial properties between t-Bu-CuPc and the perovskite. By using t-Bu-CuPc and quadruple-cation perovskite Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45, a PCE of over 20% was further obtained in a following study [78]. The authors also confirmed that the devices containing t-Bu-CuPc presented outstanding thermal stability, remaining over 90% of their original performance for 2000 h at 85 °C and good light stability at 25 °C. More elaborated substituents other than simple alkyl or alkoxy chains were also incorporated into the CuPc ring. Yu and Sun et al. developed another a-tetrakis substituted CuPc derivative (CuPc-OTPAtBu) functionalized with 4-(bis(4-tertbutyl)phenyl)amino)phenoxy groups [79]. The best devices doped with 6% (w/w) F4TCNQ exhibited a PCE of 15.0%, in combination with a carbon cathode. Wang, Liao, and coworkers used F4TCNQ to dope a commercial CuPc derivative TS-CuPc [80]. Planar PSCs based on those HTMs exhibited a PCE of 16.1% in a p-i-n configuration and 20.2% in an n-i-p configuration, respectively. In another report, the same group employed a PEDOT:PSS and TS-

8

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

Fig. 6. Molecular structures of representative CuPcs developed as HTMs in PSCs.

CuPc hybrid HTL in inverted planar PSCs [81]. The hybrid HTL not only facilitated the hole transport, but also improved the per-

ovskite crystallization. As such, an optimal PCE of 17.3% was obtained by the 50 wt% composition. Pan and Xu et al. combined

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

non-peripheral octamethyl-substituted CuPc (N-CuMe2Pc) nanowire and poly(3-hexylthiophene) (P3HT) as a HTM in MAPbI3based PSCs [82]. The devices employing the hybrid HTL presented a better performance (16.6%) with respect to pure P3HT (11.9%) and N-CuMe2Pc (8.3%). The systems discussed above are either doped with dopants like LiTFSI and F4TCNQ or mixed with another HTM. The displacement of dopants or additives is of significance for not only simplifying the fabrication process, but more importantly for enhancing the device stability. In this regard, numerous dopant-free CuPcs have been attempted as HTMs in PSCs. Yu and Sun et al. developed btetrakis substituted CuPc with triisopropylsilylethynyl substituents (CuPc-TIPS) [33]. In combination with a double-cation perovskite (FAPbI3)0.85(MAPbBr3)0.15 and a carbon cathode, the best devices using pristine CuPc-TIPS exhibited a PCE of 14.0% (Table 2). Mohammadi and Lianos et al. introduced tetra-n-butyl-substituted CuPc (CuBuPc) as a dopant-free HTM and a graphene oxide buffer layer in mesoscopic PSCs, which delivered a 15.9% PCE [83]. Peng and coworkers revisited t-Bu-CuPc as a dopant-free HTM together with a low-temperature processed rutile TiO2 array and a triplecation (Cs/FA/MA) perovskite absorber Cs0.05(FA0.83MA0.17)0.95Pb (I0.9Br0.1)3 [84]. By incorporating an Al2O3 buffer layer between the perovskite and t-Bu-CuPc, an efficiency of 14.8% was obtained. Xu and coworkers employed b-tetra-propyl-substituted CuPc (CuPrPc) as a dopant-free HTM in planar PSCs, showing an optimal efficiency of 17.8% [85]. The devices containing CuPrPc showed good long-term stability with over 94% remaining of their initial performance after 800 h under ambient condition at a relative humidity (RH) of 75%, which was much better than the spiroOMeTAD-based ones. Guo and Zhang et al. synthesized asymmetrically substituted CuPcs containing (4-methyl formate) phenoxy or (4-butyl formate) phenoxy as the peripheral groups (CuPcNO2OMFPh and CuPcNO2-OBFPh) [86]. The best devices incorporating dopant-free CuPcNO2-OMFPh and CuPcNO2-OBFPh gave PCEs of 12.5% and 13.7%, respectively. Very recently, Xu, Muccini, and coworkers reported two arylamine-substituted CuPcs (OMe-DPACuPc and OMe-TPA-CuPc) as dopant-free HTMs in planar PSCs based on a mixed-ion perovskite Cs0.05(MA0.13FA0.87)0.95Pb (I0.87Br0.13)3 [87]. A higher PCE of 19.7% was yielded by the devices employing OMe-TPA-CuPc, due to a reduced charge recombination and improved hole transport characteristic. Yu and Sun et al. developed two CuPc derivatives (CuPc-Bu and CuPc-OBu) by adjusting the non-peripheral moieties in the phthalocyanine rings [88]. It was revealed that a small structural difference in non-peripheral substituents exerted a significant impact on the molecular orientation. Better crystal packing and relative crystallinity was observed for the CuPc-OBu films, which resulted in an enhanced hole transport property. Thus, devices using dopant-free CuPc-OBu achieved a better efficiency of 17.6% with respect to that obtained by CuPc-Bu (14.3%). Moreover, the un-encapsulated devices based on CuPc-OBu also displayed excellent ambient stability at a high RH of 85%. The outstanding humidity stability of CuPc-OBu-based devices should be attributed to the hydrophobic nature of the CuPc material (water contact angle, 106.5°), which can protect the perovskite film. 3.1.2. Zinc phthalocyanines Song and coworkers introduced thermally evaporated pristine ZnPc as an interfacial layer between the perovskite MAPbI3 and spiro-OMeTAD [89]. The involvement of the ZnPc modification layer enhanced both the solar cell performance and ambient stability of PSCs. The authors further boosted the PCE of planar devices based on this ZnPc/spiro-OMeTAD bi-HTL to 17.8% by incorporating a photon downshifting layer SrAl2O4:Eu2+, Dy3+, which improved the light harvesting and suppressed charge recombination [90].

9

Solution-processed ZnPcs have also been explored as HTMs in PSCs. A tetra-triphenylamine-substituted ZnPc (TPA-ZnPc, Fig. 7) was synthesized, and the PSC devices incorporating TPA-ZnPc showed a PCE of 13.7% by using an Al2O3 interfacial layer between the HTL and the perovskite MAPbIxCl3
x [91]. Santos and Ahmad et al. reported b-octakis-substituted ZnPc with 4-tertoctylphenoxy functional groups coded as (tOctPhO)8ZnPc, and compared its optical and electrochemical properties as well as photovoltaic performance with its CuPc counterpart ((tOctPhO)8CuPc) [92]. Devices using (tOctPhO)8ZnPc showed a lightly lower PCE of 7.3% (Table 2). Hagfeldt and coworkers very recently designed and synthesized a ZnPc dimer, ZnPc-p-ZnPc, in which two zinc tri-tert-butyl phthalocyanines are linked by a benzene ring in the para position [93]. From the PL measurements, it was found that the ZnPc dimer had a better hole transfer ability than its monomeric counterpart. A higher PCE of 15.2% was obtained by using the ZnPc dimer. Torres and Nazeeruddin et al. carried out a series of work by molecularly engineering the periphery of ZnPc rings with octa- and tetra-substituents [94–96]. Among these HTMs, tetra-5-hexyl-thiophene-substituted ZnPc (HT-ZnPc) recorded the highest PCE of 17.5% in mesoscopic (FAPbI3)0.85(MAPbBr3)0.15-based PSCs in conjunction with the dopants LiTFSI/ TBP/FK209 [95]. The majority of solution-processed ZnPc-based HTMs in PSCs still utilized additives and dopants such as LiTFSI. Only few examples of dopant-free ZnPcs have been reported so far. Guo and Zhang et al. employed asymmetrically substituted ZnPc (ZnPcNO2-OBFPh) as a dopant-free HTM in mesoscopic PSCs, which gave a higher efficiency (15.7%) as compared to its copper counterparts due largely to the more efficient hole extraction and transport of the former [86]. Xu and coworkers synthesized isomer-pure 2,9,16,24-tetran-butyl-ZnPc (RE-ZnBu4Pc), and applied it as a dopant-free HTM in planar MAPbI3-based PSCs [97]. A higher average PCE of 11.5 ± 0.7% was obtained in comparison with the isomer mixture counterpart (ZnBu4Pc), owing to the higher crystallinity and better film quality of the isomer-pure one. Very recently, the same group developed two asymmetrically substituted ZnPcs with A3B-type structures (Me6Bu-ZnPc and H6Bu-ZnPc) [98]. Planar PSCs using dopant-free Me6Bu-ZnPc in combination with a triple-cation perovskite Cs0.05(MA0.13FA0.87)0.95Pb(I0.87Br0.13)3 afforded an optimal PCE of 17.4%, which was substantially higher than that obtained by H6Bu-ZnPc (10.3%). The improved performance achieved by Me6Bu-ZnPc was the result of more homogenous film morphology and a better hole extraction/mobility property. Furthermore, the long-term stability of un-encapsulated devices containing Me6Bu-ZnPc and H6Bu-ZnPc were also investigated at a RH of 75% in comparison with doped spiro-OMeTAD. Me6Bu-ZnPc-based devices exhibited the best stability, remaining over 90% of their original PCE after 1400 h, which was largely ascribed to the dense and smooth film morphology of this ZnPc compound on the perovskite layer and its strongest protecting effect against moisture. 3.1.3. Other metal phthalocyanines Liu and coworkers adopted thermally evaporated NiPc in inverted planar PSCs, and an efficiency of 14.3% was obtained [99]. Two solution-processed tetrakis-substituted MPcs based on Ni and Fe (NiPc-Cou and FePc-Cou, Fig. 7) were synthesized, and their performances were evaluated in mesoscopic PSCs [100]. The devices using NiPc-Cou displayed a slightly higher PCE of 10.2% because of the higher Voc and FF obtained. Sun and coworkers introduced a-octabutoxy-substituted NiPc (NiPc-(OBu)8) as a dopant-free HTM in mesoscopic PSCs, in combination with a V2O5 buffer layer between the Au electrode and NiPc [101]. The devices based on this integrated HTL yielded an average PCE of 17.6% (Table 2), which rivaled those obtained by doped spiroOMeTAD. Guo and Zhang et al. reported an asymmetrically substi-

10

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

Fig. 7. Molecular structures of representative ZnPcs and other MPcs developed as HTMs in PSCs.

tuted CoPc (CoPcNO2-OPh), and adopted it as a dopant-free HTM in MAPbI3-based PSCs with a modest PCE of 8.2% [102]. MPcs based on other metals including Pd, Ti, and Pb have also been attempted as HTMs in PSCs by thermal evaporation [103–105]. Among them, planar devices based on octamethyl-substituted PdPc (PdMe2Pc) displayed the best PCE of 16.3% [103].

3.2. Porphyrins Metal porphyrins (MPors) are 18-p electron aromatic macrocyclic compounds consisting of four pyrrole units and four bridging carbon atoms in a planar conformation (Fig. 8) [106]. MPors have been intensively used as photosensitizers and donor materials in DSSCs and organic solar cells because of their outstanding lightharvesting property, and excellent chemical and thermal stability [107–113]. Similar to MPcs, an appealing aspect of MPors is that their properties can be modified through the structural

Fig. 8. General molecular structure of MPors.

modification of the periphery (four meso- and eight b-positions) and the change of the metal center [114]. Their highly electronrich and strong p-p molecular stacking characteristics make MPors promising HTM candidates. With this regard, a variety of MPor derivatives have been developed as HTMs in PSCs. Li et al. introduced a symmetrical meso-substituted ZnPor derivative (ZnPc-SAc, Fig. 9) as a modified HTL in inverted PSCs between PEDOT:PSS and the perovskite MAPbI3 [115]. The

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

11

Fig. 9. Molecular structures of representative MPors developed as HTMs in PSCs.

porphyrin functionalized with -SCOCH3 groups can promote the crystallization of the perovskite film, resulting in large crystal sizes and low density of traps. Consequently, the Voc and FF of the devices containing ZnPc-SAC were dramatically enhanced, leading to a PCE of 14.1%, which was 24% higher than the reference (Table 3). Chen and coworkers prepared meso-triphenylamine

(TPA) substituted ZnPor (ZnP) and CuPor (CuP), and applied them as HTMs in mesoscopic MAPbI3-based PSCs [114]. It was disclosed that the metal centers had an impact the solar cell performances. Devices employing ZnP yielded a higher PCE of 17.8% relative to the CuP counterparts (15.4%), largely due to better solubility of the former linked to a smoother surface morphology.

HOMOa (eV)

lh

Device Configurationb

Cell structurec

Dopantsd

Jsc (mA cm–2)

Voc (V)

FF

PCEe (%)

Ref.

(cm2 V
1 s
1)

ZnPor-SAc CuP


5.1
5.37

— 2.89  10
4

IP M

ITO/PEDOT:PSS/ZnPor-SAc/MAPbI3/PCBM/C60/BCP/Al FTO/c-TiO2/m-TiO2/MAPbI3/CuP/Au

— LiTFSI/TBP

21.9 20.5

0.93 1.07

0.69 0.66

[115] [114]

ZnP


5.29

3.06  10
4

M

FTO/c-TiO2/m-TiO2/MAPbI3/ZnP/Au

LiTFSI/TBP

21.8

1.10

0.71

PZn-TPA-O


5.13 (UPS)

3.51  10
4

P

ITO/ZnO/MAPbI3/PZn-TPA-O/Au

LiTFSI/TBP

19.8

1.03

0.61


5.20 (UPS)

3.38  10


4

P

ITO/ZnO/MAPbI3/PZn-TPA/Au

LiTFSI/TBP

19.4

1.04

0.60


4

P

ITO/ZnO/MAPbI3/PZn-DPPA-O/Au

LiTFSI/TBP

20.3

1.04

0.64

14.1 15.4 (18.6) 17.8 (18.6) 12.4 (14.6) 12.0 (14.6) 13.5 (14.6) 14.1 (14.6) 18.9 (19.2) 17.7 (19.2) 17.8 (18.6) 16.6 (18.0) 10.6 (18.0) 19.4 (18.6) 17.8 (18.6) 12.6 (16.6) 11.5 (16.6) 9.0 (16.6) 19.0 (17.1) 20.5 (20.0) 12.0 (12.3) 10.8 (12.3) 11.1 (17.3) 6.0 (17.3) 7.1 (17.3) 18.8 (17.6) 2.2 (5.2)

HTM

PZn-TPA


5.23 (UPS)

3.85  10

PZn-DPPA


5.36 (UPS)

4.10  10
4

P

ITO/ZnO/MAPbI3/PZn-DPPA/Au

LiTFSI/TBP

20.8

1.05

0.65

PZn-2FTPA


5.40 (UPS)

3.91  10
4

P

ITO/ZnO/MAPbI3/PZn-2FTPA/Au

LiTFSI/TBP

22.2

1.13

0.75

PZn-3FTPA


5.29 (UPS)

3.54  10


4

P

ITO/ZnO/MAPbI3/PZn-3FTPA/Au

LiTFSI/TBP

21.9

1.11

0.73

ZnPy


5.35 (Cal)

—

M

FTO/c-TiO2/m-TiO2/MAPbI3/ZnPy/Au

—

22.3

1.09

0.73

Y2


5.25 (UPS)

2.04  10
4

M

FTO/c-TiO2/m-TiO2/MAPbI3/Y2/Au

LiTFSI/TBP

22.8

0.99

0.73


5.10 (UPS)


5

M

FTO/c-TiO2/m-TiO2/MAPbI3/Y2D2/Au

LiTFSI/TBP

17.8

1.01

0.59


4

M

FTO/c-TiO2/m-TiO2/Cs0.05[(FA0.83MA0.17)PbI0.83Br0.17]0.95/WT3/Au

LiTFSI/TBP/FK209

22.6

1.10

0.79

a

c d e

1.53  10

WT3


5.20 (UPS)

4.20  10

YR3


5.10 (UPS)

9.30  10
5

M

FTO/c-TiO2/m-TiO2/Cs0.05[(FA0.83MA0.17)PbI0.83Br0.17]0.95/YR3/Au

LiTFSI/TBP/FK209

23.0

1.04

0.75

SGT-061


5.03

2.60  10
5

M

FTO/c-TiO2/m-TiO2/MAPbIxCl3
x/SGT-061/Au

LiTFSI/TBP/FK209

17.2

1.07

0.68

SGT-062


5.27

1.28  10


5

M

FTO/c-TiO2/m-TiO2/MAPbIxCl3
x/SGT-062/Au

LiTFSI/TBP/FK209

17.3

1.05

0.63

SGT-063


5.05

8.01  10
6

M

FTO/c-TiO2/m-TiO2/MAPbIxCl3
x/SGT-063/Au

LiTFSI/TBP/FK209

14.5

1.01

0.61

DPPZnP-TSEH


5.20

3.20  10
5

P

ITO/SnO2:C60/MAPbI3/DPPZnP-TSEH:PCBM/MoO3/Ag

Pyridine/DIO

23.2

1.04

0.78

Co(II)P/Co(III)P


5.30/
5.28

—

M

FTO/c-TiO2/m-TiO2-SM/(CsFAMA)Pb(BrI)3/CO(II)P:Co(III)P/Au

—

23.6

1.13

0.77


4

M

FTO/c-TiO2/m-TiO2/MAPbI3/HA1/Ag

—

17.3

0.91

0.76

HA1


5.22

6.49  10

HA2


5.34

8.38  10
4

M

FTO/c-TiO2/m-TiO2/MAPbI3/HA2/Ag

—

15.1

0.92

0.78

Y1-Ni


5.43

1.99  10
5

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/Y1-Ni/Au

LiTFSI/TBP/TeCA

16.6

1.06

0.63

Y2-Cu


5.38

2.04  10


5

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/Y2-Cu/Au

LiTFSI/TBP/TeCA

11.0

1.00

0.55

Y3-Zn


5.38

1.88  10
5

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/Y3-Zn/Au

LiTFSI/TBP/TeCA

12.6

1.00

0.56

CuH


5.38

9.76  10
4

M

FTO/c-TiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/CuH:spiro-OMeTAD/Au

—

22.6

1.11

0.75

[Fe(bpyPY4)](OTf)2.5)


5.46 (UPS)

—

P

FTO/TiO2/FAPbBr3/[Fe(bpyPY4)](OTf)2.5)/Au

—

6.0

0.86

0.42

From electrochemistry unless stated otherwise. UPS: from ultraviolet photoelectron spectroscopy. Cal: form calculation. M, P, and IP stand for mesoscopic, planar and inverted planar cell configurations, respectively. PEDOT:PSS, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid). DIO, 1,8-diiodooctane. TeCA, 1,1,2,2-tetrachloroethane. The photovoltaic parameters are based on the best-performing devices. The corresponding PCE values for spiro-OMeTAD are given in the parentheses.

[114] [116] [116] [116] [116] [117] [117] [118] [119] [119] [120] [120] [121] [121] [121] [124] [125] [126] [126] [127] [127] [127] [128] [129]

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

PZn-DPPA-O

Y2A2

b

12

Table 3 Summary of representative photovoltaic performance of PSCs based on MPors and other TMCs as HTMs.

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

Yoon, Jang, and Jung et al. synthesized a series of meso-tetrakisarylamine-substituted ZnPors as HTMs in PSCs [116,117]. The incorporation of the diphenyl-2-pyridylamine (DPPA) moiety down-shifted the HOMO energy levels relative to TPA, resulting in a better energy alignment to the perovskite MAPbI3 layer [116]. On the other hand, the involvement of DPPA also enhanced the hole mobility due to the face-on orientaction. As a result, higher efficiencies were achieved by DPPA-based devices (14.1% for PZn-DPPA and 13.5% for PZn-DPPA-O) with respect to TPAbased ones (12.0% for PZn-TPA and 12.4% for ZnP). In another report, the same collaborators further molecularly engineered tetera-TPA-substituted ZnPors by incorporating fluorine atoms in phenyl rings that are connected to the Por core [117]. The fluorinated ZnPors (PZn–2FTPA and PZn–3FTPA) displayed better properties in terms of deeper HOMO energy levels and better hole transport property with respect to the non-fluorinated one. It was also found that the position of the fluorine substituents also had an impact on the HOMO energy levels and the molecular packing of ZnPors. PZn–2FTPA-based devices showed the best overall efficiency of 18.9%, whereas those using PZn–3FTPA and PZn–TPA yielded PCEs of 17.7% and 16.4%, respectively. Lv et al. reported an acylhydrazone-based ZnPor derivative (ZnPy) as a HTM in mesoscopic PSCs [118]. Up to 17.8% PCE was recorded by ZnPy without any p-dopants, because the acylhydrazone and pyridine units of ZnPy are expected to passivate under-coordinated Pb atoms in the perovskite MAPbI3. Chen and Yeh et al. did a pioneering work by applying mesosubstituted A2B2-type ZnPors (Y2 and Y2A2) as HTMs in PSCs [119]. These ZnPor materials were designed to be structurally simple and symmetric so that the synthesis of the final compounds involved only simple steps from commercially available chemicals and reagents. Both of the two ZnPor derivatives consisted of a meso-5,15-bis(ethynylaniline)porphyrin backbone bearing bilateral alkyl chains, which were found to improve the solubility and influence the film quality of the the Pors. A higher efficiency of 16.6% was achieved by Y2 while devices based on Y2A2 showed a PCE of only 10.6% (Table 3), probably due to the longer alkyl chains in the latter compound that increased the intermolecular distance and impeded the hole transport. On the basis of Y2, the same group further designed two ZnPor dimers, namely WT3 and RT3, with disubstitution of electron-donating ethynylaniline moieties attached to two lateral meso-positions of porphyrin cores [120]. Both WT3 (4.2  10
4 cm2 V
1 s
1) and YR3 (9.3  10
5 cm2 V
1 s
1) displayed higher hole mobility as compared to Y2 (3.2  10
5 cm2 V
1 s
1), which should be attributed to the better intermolecular interaction of the dimers. In combination with a triple-cation perovskite Cs0.05[(FA0.83MA0.17)PbI0.83Br0.17]0.95 and dopants (LiTFSI/TBP/FK209), mesoscopic PSCs with WT3 afforded a PCE of 19.4%, which surpassed those containing YR3 (17.8%) and spiro-OMeTAD (18.6%). The ambient stability (RH, 40 ± 10%) of non-encapsulated devices containing WT3 were also examined as compared to spiro-OMeTAD. The WT3-based devices presented excellent stability, retaining over 90% of the original performance after 800 h, whereas the PCE of spiro-OMeTAD-based ones significantly dropped to 80% of their initial value within 300 h. The enhanced humidity stability of porphyrin-based devices originated from the hydrophobic property of WT3 (water contact angle of 103°) connected to more effective protection of the perovskite layer against the moisture in air. Furthermore, the WT3-based devices also exhibited superior thermal, and light-soaking stability in comparison to those based on spiro-OMeTAD. Kim and coworkers developed three meso-substituted A2B2type ZnPors (SGT-061, SGT-062, and SGT-063) by varying different electron-donating triarylamine groups [121]. Higher hole mobility was observed for SGT-061 with respect to SGT-062 and SGT-063, which could be attributed to the strong p-p stacking in the former

13

with less bulky units. A relatively higher PCE of 12.6% was accordingly obtained by SGT-061 in mesoscopic PSCs. Natural chlorophyll compounds have also been tested as HTMs in PSCs with moderate PCEs typically in the range of 8–11% [122,123]. An intriguing example of the application of ZnPors in PSCs was demonstrated by incorporating Por-based organic photovoltaics (OPVs) into PSCs, in which a ZnPor derivative DPPZnP-TSEH was blended with PCBM atop of the perovskite layer [124]. DPPZnPTSEH was designed by linking a porphyrin ring with an alkyl sulfide substituted thienyl group as side-chains and connecting with two diketopyrrolopyrrole units via ethynylene bridges. The absorption spectrum of this Por derivative showed a broad absorption range extending to near-infrared range. Its HOMO energy level (
5.2 eV) matched well the VB of the perovskite MAPbI3, thus effectively facilitating the hole extraction. As a consequence, the OPV/perovskite hybrid devices afforded a maximum PCE of 19.0% (Table 3). Cao et al. developed two Co(II) and Co(III)-based ZnPors, namely Co(II)P and Co(III)P, and applied the mixture of these two compounds as a HTM in mesoscopic PSCs [125]. They first investigated a natural plant sunscreen, sinapoyl malate (SM), as an interfacial modification layer on the surface of ETL, which enhanced the UV stability and improved the interfacial contact between the ETL and the perovskite. On the basis of the SM-decorated TiO2 ETL and a triple cation perovskite absorber (CsFAMA)Pb(BrI)3, the devices employing Co(II) and Co(III)-based ZnPors without any dopants achieved a PCE of 20.5% with a stabilized power output of 19.7%. Furthermore, the devices incorporating the mixed HTMs also showed excellent thermal stability, with almost no losses at 85 °C for 1000 h. On the contrary, the devices containing spiroOMeTAD remained less than 20% of their initial efficiency, and a severe morphological change was observed for such devices. 3.3. Other transition metal complexes The reports on other types of TMC-based HTMs in PSCs have been rather limited up till now. Sun and coworkers adopted two Ag-based TMCs (HA1 and HA2, Fig. 10) as dopant-free HTMs in MAPbI3-based PSCs [126]. These HTMs were easily synthesized with high yields. The HA1-based PSCs provided an overall efficiency of 12.0%, which was comparable to that obtained by doped spiro-OMeTAD (12.3%) under identical conditions (Table 3). Hua and Kloo et al. reported three TPA-based metal complexes by varying the metal ions (Y1-Ni, Y2-Cu, and Y3-Zn) [127]. The impacts of the metal centers on the energy levels, hole transport property, and photovoltaic performance in PSCs have been systematically studied. Similar HOMO energy levels were observed for these materials. However, an increase in band gaps was found from nickel to copper to zinc. The hole mobility and conductivity of these materials were low in the range of 10
5 cm2 V
1 s
1 and 10
7 S cm
1, respectively, due largely to the limited conjugated moieties in the systems. The best device performance was observed for Y1-Ni with a PCE of 11.1%, largely due to the better coverage of Y1-Ni on the perovskite. The same collaborators further developed a hybrid HTL in mesoscopic PSCs by using a copper complex (CuH) and spiro-OMeTAD [128]. The incorporation of the CuH into the spiro-OMeTAD dramatically enhanced the hole mobility and impeded the aggregation. Consequently, the best devices obtained by the hybrid HTL showed a PCE of 18.8%, which outperformed the corresponding values for devices based on CuH (15.8%) and spiro-OMeTAD (14.5%). Simonov and Bach et al. introduced a blend of iron(II/III) complexes bearing a hexadentate polypyridyl ligand ([FeII(bpyPY4)] (OTf)2 and [FeIII(bpyPY4)](OTf)3) as a HTM in PSCs [129]. The conductivity of the blend HTM was determined to be 0.24 mS m
1, which was almost 5 times higher as compared to spiro-OMeTAD.

14

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

Fig. 10. Molecular structures of other TMC-based HTMs developed in PSCs.

The HOMO energy level of [Fe(bpyPY4)](OTf)2.5 was measured to be in the range of
5.46 eV and
5.67 eV. It implied that the new iron-complex-based HTM had a mismatched energy level alignment with MAPbI3. The authors employed FAPbBr3 instead with a more negative VB. Planar PSCs were fabricated by spincoating the iron complexes on top of the perovskite layer using nitromethane as a solvent due to the poor solubility of the complexes in nonpolar solvents. Due to the high conductivity, no pdopants were used for [Fe(bpyPY4)](OTf)2.5. A modest of PCE of 2.2% was obtained by the iron-based HTM originating from the lower FF and Voc relative to spiro-OMeTAD, most likely linked to more pronounced charge recombination losses in the [Fe (bpyPY4)](OTf)2.5-based devices. 4. Concluding remarks and future outlook The HTL plays an essential role in determining the performance of PSCs in terms of both the overall efficiency and the long-term stability. As such, searching for a low-cost, efficient, and stable HTL has been an important research endeavor in view of largescale application of PSCs in the future. In this review, we comprehensively summarized the recent advancement of using TMCs in HTLs for PSCs including p-dopants and HTMs. With respect to p-dopants, numerous TMCs including cobalt and other metal complexes have been attempted. TMC-based pdopants are ease of synthesis, and their chemical and physical properties can be facilely tailored through the modification of the ligands. The incorporation of p-dopants in HTLs, on the one hand, enhances the hole conductivity and thus the overall photovoltaic performance; on the other hand, it also well controls the reproducibility of the solar cell devices. The most efficient PSCs reported thus far still relied on the dopants. However, we cannot overlook the negative impact, which the dopants bring about on the durability of the devices. Unfortunately, the majority of the studies still coupled TMC-based p-dopants with LiTFSI. LiTFSI is by no means a good option for the stability of PSCs in terms of its hydroscopic nature and the lithium ion-migration issue. The replacement of the lithium salts with more stable alternatives, for example ZnTFSI2 as demonstrated in Ref. [50], offers the possibility to achieve high PCE while simultaneously maintaining satisfactory long-term stability. The intrinsic stability of the TMCs and their reduced species is also of great importance since both the chemically reduced and unreacted dopants are remained in the HTL. It is notable that the research of the newly developed TMC-based p-dopants has been dominated by the spiro-OMeTAD systems. The combination of TMC-based p-dopants with other low-cost and efficient small molecule HTMs, in particular the thermally more stable ones, is another direction for continued investigation. A fundamental

understanding of the dopant-induced degradation mechanism is also required for further development of new TMC-based pdopants in the future. Considerable research effort has been devoted to the development of TMC-based HTMs in PSCs mainly consisting of MPcs and MPors. The synthesis of these macrocycles is relatively simple, and their optical and electronic properties can be easily tailored by tuning the substituents and the metal center. Besides, MPcs and MPors possess high hydrophobicity, and excellent thermal and photochemical stability. Encouragingly, several studies have proven that the systems incorporating MPcs and MPors have shown better moisture and/or thermal stability than spiroOMeTAD and PTAA. All these advantageous features render these metallomacrocycles promising HTM candidates that need to be further explored in PSCs. Unfortunately, the majority of the MPcs and MPors systems still used additives typically LiTFSI, i.e. the same scenario as for spiro-OMeTAD, which counteracted the merit of high stability of these macrocycles. The vast experience of stable dopants established for spiro-OMeTAD and other small moleculebased HTMs can be adopted into the MPc and MPor systems. Alternatively, we have seen some progress of dopant-free MPcs and MPors HTMs, although their efficiencies (19.7% for MPcs and 20.5 for MPors) are still lagging behind the small molecule and polymer counterparts. An in-depth understanding of the structure–property relationship is required to regulate the molecular arrangement (strong p-p interactions), and to further enhance the hole mobility and thus the overall efficiency. The bandgaps of MPcs and MPors are typically narrow correlated with lower-lying LUMO energy levels, which may not be able to effectively prevent the back electron transfer from the perovskite to the counter electrode. A solution to solve this problem is to introduce an interfacial layer between the perovskite/HTM and/or the HTM/metal electrode interfaces such as the large bandgap 2D perovskites or metal oxides. The presence of the buffer layer is expected to circumvent the charge recombination losses and to improve the overall performance. On the other hand, the barrier layer is also beneficial for enhancing the device stability in terms of the protection of moisture and metal migration at elevated temperatures. The integration of MPcs and MPors based HTMs with thermally more stable MAfree perovskites offers another opportunity to further strengthen the device durability.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Z. Yu et al. / Coordination Chemistry Reviews 406 (2020) 213143

Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (21975038, 21606039, and 51661135021), the Swiss National Science Foundation (IZLCZ2_170177), the Fundamental Research Funds for the Central Universities (DUT17JC39), the Swedish Foundation for Strategic Research (SSF), the Swedish Energy Agency, as well as the Knut and Alice Wallenberg Foundation.

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