Electrochemistry and dye-sensitized solar cells

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Review Article Electrochemistry and dye-sensitized solar cells Ladislav Kavan∗

Dye-sensitized solar cells are reviewed from the viewpoints of (i) electrochemical preparation of cell components and (ii) electrochemical properties and characterization. Previous reviews in the field are updated, quoting more than half of references from the last two years. The specific features of this technology outline new perspectives for the conversion of solar light to electricity and fuel, along with pivotal challenges and open questions for future efforts. Address J. Heyrovsky Institute of Physical Chemistry, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic ∗

Corresponding author: Kavan, Ladislav ([email protected])

Current Opinion in Electrochemistry 2017, XX:XX–XX This review comes from a themed issue on Solar Cells GRAETZEL 2017 Edited by Michael Graetzel For a complete overview see the Issue and the Editorial Available online XX XXXX 2017 http://dx.doi.org/10.1016/j.coelec.2017.03.008 2451-9103/© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

The ‘incident photon to current conversion efficiency’ (IPCE) of a dye monolayer (extinction coefficient ε and surface coverage Г) is: IPCE = ηin j · ηcol l · (1 − 10−RF ε )

(1)

ηinj is the electron injection efficiency, ηcoll is the electron collection efficiency and RF is the roughness factor. Model calculations for Ru(II)-bipyridine dyes (ε ≈ 107 cm2 /mol, Г ≈ 10−10 mol/cm2 , ηinj ·ηcoll ≈ 1) give an IPCE of 0.23% or 90% for RF of 1 or 1000, respectively. The experimental value for a single-crystal anatase was 0.11% [16] and > 90% for optimized mesoporous electrodes [2] with RF ≈ 1000. The other parameter controlling light harvesting (Г) is often ignored, though it can significantly influence recombination at the Ru(II)-sensitized titania [17•• ] (for review see [18]). Electrochemical synthesis of mesoporous titania nanotubes [19• ] consists in anodic oxidation of Ti (Equation 2a) and selective dissolution of the produced TiO2 to TiF6 2− (Equation 2b): Ti + 2H2 O → TiO2 + 4H+ + 4e−

(2a)

TiO2 + 6F− + 4H+ → TiF6 2− + H2 O

(2b)

••

The discovery of a dye-sensitized solar cell (DSC) [1 ], also called the Grätzel cell, triggered significant scientific output during the last two decades [2•• ], including the advent of perovskite solar cells [3• ] and perovskite/DSC hybrid cells [4• ]. The energy conversion efficiency of DSCs under 1 sun is now exceeding 14% [5•• ]. As DSCs are inherently electrochemical devices, pertinent methods for their investigation are electrochemical in nature. In addition, certain electrode materials can be fabricated electrochemically. These works are briefly reviewed below.

2. Photoelectrode: properties and electrochemical fabrication The photoelectrode in a DSC serves to harvest light, and then to collect/transport electrons (Figure 1). It is usually a photoanode, with n-TiO2 (anatase) being the generic material [6,7• ]. Rutile is less often encountered in DSCs, but an impressive 9.6%-efficient device was reported recently [8• ]. A somewhat exotic p-TiO2 was demonstrated by several groups [9• ], yet its applicability as a holeacceptor (photocathode) is still unclear. A standard photoCurrent Opinion in Electrochemistry 2017, 2:88–96

cathode material is p-NiO, though efficiencies of p-DSCs are lower (2.5%) [10•• ]. Amongst other p-semiconductors, p-CuO [11], p-Cu(I) oxides [12] and B-doped diamond [13,14• ,15] were tested with moderate success so far.

The efficiencies of nanotube-based DSCs can sometimes outperform those with ordinary nanoparticles [2,20], but the champion devices [2,5] employ other morphologies. ZnO is the second most popular photoanode material for n-DSCs, yet the efficiencies are lower [2,6]. Electrochemical growth of ZnO is triggered by reduction of nitrate (Equation 3a) or peroxide (Equation 3b) causing local pH increase (Equations 3a-c): NO3 − + H2 O + 2e− → NO2 − + 2OH−

(3a)

H2 O2 + +2e− → 2OH−

(3b)

Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2 O

(3c)

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Electrochemistry and dye-sensitized solar cells Kavan

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Figure 1

Scheme of n-type dye sensitized solar cell (n-DSC; left) and p-type dye sensitized solar cell (p-DSC; right). In both cases the sensitizer (S) is photo-excited to S∗ . Subsequently an electron from S∗ is injected either into the conduction band of photoanode (n-DSC) or into the mediator, M (p-DSC). The accessible open-circuit voltage, VOC is also schematically shown.

Modification of this synthesis by using ionic liquids provided DSCs with enhanced open-circuit voltage (VOC ) and efficiency [21].

I3 − + 2e− → 3I− E0 = 0.35 V  3+  2+ Co(bpy )3 + e− → Co(bpy )3

Compact TiO2 layer is electrodeposited by oxidative hydrolysis of TiCl3 [16,22]:

 3+  2+ Co(bpy−pz )2 +e− → Co(bpy−pz )2 E0 = 0.86 V (5c)

Ti3+ + H2 O → TiOH2+ + H+

Cu(dmp )2 2+ + e− → Cu(dmp )2 +

TiOH

2+

−

+ H2 O - e → TiO2 + 3H

(4a) +

(4b)

O’Regan and Grätzel disclosed in their seminal paper [1] that these layers improved the performance of photoanode. However, simple treatment with TiCl4 has a similar effect [2]. Improvement was also reported upon electrochemical Al3+ -insertion [2]. Electrodeposition (Equations 4a, 4b) allows controlled growth on nanotextured templates, providing nanotubes, mosaic nanoarrays, gyroid networks or inverse opal [16]. Compactlayer/nanorod composites [22] and blocking layers [22,23] are also deposited electrochemically. Amongst various syntheses of blocking layers (electrodeposition [22,23], sol–gel [24], ALD [23] or spray pyrolysis [23]) the latter one is the most popular.

3. Counterelectrode: properties and electrochemical fabrication The counterelectrode acts for the reduction of the oxidized state of redox mediator (such as I3 − , Co3+ or Cu2+ ): www.sciencedirect.com

Cu(dmby )2

2+

−

+ e → Cu(dmby )2

(5a) E0 = 0.56 V (5b)

E0 = 0.93 V +

E0 = 0.97 V

(5d) (5e)

(bpy = 2,2 -bipyridine; bpy-pz = 6-(1H-pyrazol-1yl)-2,2 -bipyridine; dmp = bis(2,9-dimethyl-1,10phenanthroline); dmby = 6,6 -dimethyl-2,2 -bipyridine, E0 is the standard redox potential). Mediators beyond I3 − /I− were scarcely used prior to 2010 due to their poor performance with Ru-bipyridine dyes. However, the development of novel donor-π -acceptor dyes gave the first breakthrough [2], and the recent finding of 9.4%-efficient DSC using Co-mediator and Ru-bipyridine dye with high Г (cf. Equation 1) was the second breakthrough [17]. The charge-transfer (Equations 5a-e) must be catalysed by e.g. platinum, PtOx or Pt-alloys, the latter exhibit enhanced activity and stability in I-containing electrolytes [25• ]. F-doped SnO2 (FTO) is commonly activated by thermal decomposition of H2 PtCl6 . Platinum is alternatively deposited electrochemically [26• ]. This is useful for substrates which cannot be calcined, e.g. carbonnanotubes/PEDOT composites [27] or polymer-metal Current Opinion in Electrochemistry 2017, 2:88–96

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Figure 2

Example of dye-sensitized solar employing FTO-free counterelectrode. The latter is electrochemically platinized tungsten wire interwoven with PEN (polyethylene naphthalate) fibers. This material provides higher electrical conductivity, better optical transparency and assumed cheaper price compared to FTO. Left charts: SEM images. Right top: scheme of the cell architecture. Right bottom: photocurrent/voltage characteristics at 0.1 sun illumination (reference device is a DSC with the standard Pt@FTO counterelectrode). Adapted with permission from ACS Appl. Mater. Interfaces 2, 22,343, 2014. Copyright (2014) American Chemical Society.

wire grids (Figure 2) [28] (for review of fiber/wire DSCs see [29]). A flexible DSC using Ti-grids for both photoanode/cathode was demonstrated recently [30• ]. Graphene and related 2D-materials find numerous applications in photovoltaics [31]. Graphene-based catalysts are popular for Co-mediators [32,33], while even low-loading (optically transparent) films show excellent catalytic activity outperforming that of Pt [33]. Highly active graphene electrodes can be prepared at temperature <200°C and on substrates other than FTO [34]. The nature of electrocatalytic sites on graphene/carbons is a subject of conflicting debate [33,35• ]. Lattice defects, rather than oxygen-surface groups, are assumed to be responsible for Co-electrocatalysis [36•• ]. Similarly active are carbon nanohorns [37] and heteroatom-modified graphenes, i.e. edge-halogenated [38], B-doped [39] and Sb-doped [40• ]. Electrochemically grown MoS2 /graphene [41] and electro-reduced graphene oxide [42] are active for I-mediators, too. Electro-deposited cobalt sulfide is another promising catalyst for Co-mediators outperforming Pt [43]. Electrochemically grown PEDOT is a standard Current Opinion in Electrochemistry 2017, 2:88–96

catalyst for Cu-mediators [44• ,45]. It was employed in all champion devices of this kind, including the ‘Zombie Cells’ [44,46•• -48]; sometimes platinum was also used [48,49• ]. Other electrochemically grown polymers for counterelectrodes are PProDOT, polyaniline, polypyrrole, polythiophene [50,51], polypyrrole/graphene [52] and PEDOT/graphene [53].

4. Hole-transporting media; electrolyte solutions The advent of solid-state DSC (ss-DSC) was promoted by molecular hole-conductor, spiro-OMeTAD, which is still used in state-of-art devices [54]. The practical benefits of avoiding liquid electrolytes are limited by lower efficiencies of ss-DSCs. Problems of spiro-OMeTAD are low hole-conductivity, difficult infiltration into mesoporous photoanodes and high cost. Alternative hole-conductors are photoelectrochemically polymerized PEDOT (7.1%DSC) [55] or spiro-[fluorene-9,9 -xanthene] (7.3%-DSC) [56• ]. The largest efficiency was obtained for ‘Zombie Cells’, made simply by solvent evaporation from certain Cu-mediated DSCs (8.2% vs. 5.6% for a control device with spiro-OMeTAD) [46]. Another similar system (5.7% www.sciencedirect.com

Electrochemistry and dye-sensitized solar cells Kavan

efficiency) was based on solid Co-bipyridine complexes [57• ]. The chemical synthesis of Cu(dmby)2 2+ and Cu(tmby)2 2+ mediators yielded impure products [44] but a preparative electrolysis readily provided clean substances [58]. Except for works prior to 1988 [2] electrolyte solutions in DSCs were aprotic, benefiting from the upshift of TiO2 conduction-band, stability of dye-anchoring, broad selection of electrolyte additives etc. However, practical devices suffered from volatility, flammability and toxicity of organic solvents [59•• ]. These problems are minimized using solvent-free eutectic ionic liquids or composite electrolytes, yet the efficiencies are smaller [2,59,60• ]. One of reasons is the viscosity-controlled mass transport, nonetheless the electrolytes with I-mediators also show a non-Stokesian (Grotthus-type) transport [59]. Analogously, some Cu-mediators may also transport charge by fast electron self-exchange (Dahms–Ruff mechanism) [58]. Another strategy of avoiding organic solvents is the ‘aqueous-DSC’ [61•• ], sometimes called ‘thirdgeneration technology’ [62• ]. Most aqueous DSCs use an I-mediator, but a Co-mediated DSC (5.7% efficiency) was also presented [63]. A system close to aqueous nDSC is the photoelectrolytic cell in which the oxidized dye is not regenerated by a mediator (cf. Figure 1), but by a catalytic oxidation of water to O2 [64,65•• ]. A reverse process (dye-sensitized H2 production) occurs on p-NiO photocathodes [65,66• ]. Eventually, tandem cells were also demonstrated [67,68]. A DSC interfaced to photo-electrolyser provided 7.1%-efficient H2 evolution without external bias [69• ]. These devices are promising for ‘solar fuel’ generation [70,71•• ]. Target efficiencies >10% (assumed for commercial prototypes [69]) are still a challenge, but water electrolysis through two perovskite cells (12.3% efficiency) meets this benchmark [72• ]. The zero-bias photoelectrochemical water splitting is conceptually similar to the dye-sensitized photocatalysis on particles [71,73•• ].

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the number of conduction band states. Enhancing of VOC is realized by (i) an upshift of ECB or (ii) a downshift of Wredox (increase of E0 , see Equations 5a-e). ECB upshifts (≈0.1 eV) are achieved by engineering of the TiO2 structure [74] or by electrolyte additives [2]. However, the Wredox is theoretically limited only by the redox potential of the dye’s ground state (≈1 VSHE ; with some overpotential for dye regeneration) [2]. This driving-force can be as small as 0.1 V in Cu-mediated DSCs [44]. The position of ECB in titania has been a subject of conflicting debate. Photoelectron spectroscopy (PES) and most theoretical simulations support that ECB of anatase is lower than that of rutile [75,76], while electrochemical studies show the opposite [16]. The CB edge is described by the flatband potential (EFB ) measured by Mott–Schottky plots from electrochemical impedance. This approach is correct for dense layers or large crystals, but mesoporous network of low-doped nanocrystals cannot be treated in terms of space charges and band bending [77]. The relation between ECB and EFB is [78• ]: EF B =

E0SHE ζnb ECB − − (EH − EHf b ) + e e e

(7)

E0 SHE is the energy of the SHE electrode w.r.t. vacuum level (E0 SHE ≈ −4.44 eV), EH is potential drop at the Helmholtz layer, EH fb describes the contribution of specifically adsorbed ions, if any and ξ nb is the offset of ECB w.r.t. Fermi level. The controversy between electrochemical and PES studies is addressable by considering the effect of the electrolyte solution at the interface [79,80• ]. PES indicates that the CB edge of (001)-anatase is upshifted by 0.1 eV referenced to (101)-anatase in agreement with the electrochemical studies and DFT calculations [81] but there are again some works claiming the opposite (for discussion see [82]). Interestingly, the (010) facet on anatase was particularly efficient for electron injection [83• ].

5. Electrochemical studies of DSC 5.1. Photoanode

5.2. Electron collecting layer

In n-DSCs, titania photoanodes control the lightharvesting (Section 2) and the open circuit potential (VOC ). The latter stems from a difference between the quasi Fermi level in the illuminated TiO2 and the energy level (Wredox ) equivalent to the redox potential of mediator (Figure 1):

A dense layer of transparent semiconductor serves as an electron selective contact and simultaneously as a buffer against recombination at the photoanode [2]. This blocking layer is less important in liquid-junction DSCs, but it is pivotal for ss-DSCs [54] and for perovskite solar cells [3]. The quality of blocking is evaluated electrochemically by model redox probes, such as Fe(CN)6 3 −/4− , methylviologen, Ru(bpy)3 3+/2+ or spiroOMeTAD [23,84]. While methylviologen is electrochemically active on TiO2 in neutral-acidic media, the other redox probes react selectively on a native substrate (FTO), and thus monitor the pinhole-defects in the blocking layer [23,84] and/or the position of conduction band [84].

VOC

  n ph Wred ox ECB kT − = + ln e e NCB e

(6)

ECB is the energy of the conduction band edge at the surface, k is the Boltzmann constant, nph is the number of photogenerated electrons in a semiconductor, NCB is www.sciencedirect.com

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ALD- or electrochemically grown titania layers are pinhole-free in their pristine state, but crack upon calcination at ≈450°C which is needed for subsequent deposition of a mesoporous oxide [23]. Interestingly, some porous blocking layers still work well with perovskites, which naturally prevent shunting between FTO and spiro-OMeTAD [85]. Titania is a standard material for blocking layer, but enhanced blocking and perfect thermal stability of SnO2 layers was reported recently [84]. 5.3. Counterelectrode

Electrochemical activity of the counterelectrode and ionic transport in the electrolyte solution are evaluated on thinlayer symmetrical dummy cells [33,86• ]. The maximal (diffusion-controlled) current, jL is: 2nF cD jL = δ

(8)

(n is the number of electron, F is Faraday constant, c is the concentration of transport-limiting species, D is its diffusion coefficient and δ is the cell spacing). The mass transport is important particularly for the Comediators [87]. The electrochemical impedance spectra (EIS) of dummy cells (Figure 3) provide the charge transfer resistance RCT , the constant phase element CPE, the serial resistance Rs and the Warburg impedance ZW [33]. The latter is modeled by a finite-length element Ws with the parameters ‘Ws -R’ = RW and ‘Ws -T’ = TW . This offers an alternative route to diffusion coefficient (cf. Equations 8,9):  RW ZW = √ tanh iTW ω; iTW ω

TW =

δ2 D

(9)

For highly porous electrodes, like carbons, the Nernst diffusion impedance in pores of the electrode material needs to be considered too [86]. A good counterelectrode should exhibit the exchangecurrent density, j0 comparable to the short-circuit photocurrent at 1 sun (≈ 20 mA/cm2 ). It is expressed as: j0 =

RT 1−α α = F k0 (cox · cred ) nF RCT

(10)

as DC-techniques. Other methods use periodic perturbation of either the applied voltage (EIS) or the light intensity (intensity modulated photocurrent spectroscopy, IMPS or photovoltage spectroscopy, IMVS). Still other methods use single perturbation, VOC -decay or transient photovoltage-rise [2,88]. The periodic modulation of light provides a time-constant of photovoltage (at open circuit) or photocurrent (at short circuit) which describe electron transport and recombination. These techniques are available in the hard/software of most manufacturers of electrochemical instruments. The complex-plane IMPS/IMVS spectrum culminates at frequency: 1 τrec 1 = τt r

IMV S =

(11a)

IMPS

(11b)

(τ rec and τ tr are the effective electron recombination and transport time constants, respectively). They define the charge-collection efficiency (cf. Equation 1): ηcol l = 1 −

τt r τrec

(11c)

EIS gives the most complex information [89•• ], extending the data from IMPS/IMVS [2,88]. Some parameters measured on DSCs are essentially equivalent to those measured on dummy cells (chapter 5.3 and Figure 3): series resistance (Rs ), charge transfer resistance of the counter electrode (RCT ) and diffusion resistance of the electrolyte (RW ). The other impedance variables are specific for the mesoporous photoanode: chemical capacitance (Cμ ) transport resistance (Rtr = τ tr /Cμ ) and recombination resistance (Rrec = τ rec /Cμ ). These can be extracted from a model of diffusion-recombination transmission line [89]. Chemical capacitance (Cμ ) accounts for electron accumulation in delocalized or trapped states of a photoanode (porosity p thickness L). It is related to the density of states (DOS): DOS = 6.24 · 1018

Cμ L(1 − p)

(12)

R is the gas constant, T is temperature, k0 is the rate constant, cox and cred are the concentrations of oxidized and reduced mediators, respectively and α is the chargetransfer coefficient. Equation 10 provides the ‘ideal’ RCT = 1.3 ·cm2 , but values of ca. 2–10 ·cm2 are still tolerable in certain DSCs [33].

Most EIS studies deal with n-DSCs, but there are few works about p-type cells (NiO, CuO) [11]. Scanning electrochemical microscopy [35] is less-frequently used for DSCs, though it gives valuable information about the photoanode/electrolyte interface [90].

5.4. Electrochemical tests of full devices

6. Conclusion

Standard tests of DSCs, such as photocurrent/voltage and IPCE/wavelength spectra (Section 2) are classified

Electrochemistry provides important inputs for the fabrication and investigation of DSCs. Recent highlights

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Figure 3

Electrochemical impedance spectra in propionitrile electrolyte solution containing Co(bpy)3 3+/2+ redox mediator. Top chart: symmetrical dummy cell with two Pt@FTO electrodes, applied bias 0 V. Bottom chart: complete DSC with Y123-sensitized TiO2 photoanode and Pt@FTO cathode measured in dark; the applied bias equal to VOC (transport resistance is negligible at these conditions). RCT was found to be 2.5 or 2.6  cm2 from dummy cell (top) or from DSC (bottom), respectively. Both images highlight different frequency domains providing specific information about the studied systems. Adapted with permission from Electrochim.Acta 195, 34, 2016. Copyright (2016) Elsevier.

include electrocatalysts for counterelectrodes (graphene, PEDOT, inorganic catalysts), high-voltage redox mediators (Cu(II/I)-redox couples), new electrochemical materials for p-DSCs, hole-conductors for ss-DSCs and the ‘comeback’ of aqueous electrolyte solutions together with the DSC-driven water splitting for solar fuel generation. DSC is an ‘old sister’ of perovskite solar cells, which is the rising star in photovoltaics. Consequently, electrochemical material-science and methods developed for DSCs are often transferable to perovskite cells (e.g. optimization of the electron-collecting layer). Electrochemical methods (EIS, IMPV, IMVS in addition to standard electrochemical techniques) are indispensable for characterization of electrode materials, electrolyte www.sciencedirect.com

solutions, hole-transporting media and full devices. Sometimes, electrochemical studies raise debatable results (e.g. concerning the electronic structure of titania and electrocatalytic sites on graphene).

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •

Paper of special interest Paper of outstanding interest.

••

Acknowledgment This work was supported by the Grant Agency of the Czech Republic (contract No. 13-07724S). Last but not least, I am highly grateful to Prof. Michael Grätzel for the fascinating three decades of our collaboration. Current Opinion in Electrochemistry 2017, 2:88–96

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References 1.

O’Regan B, Grätzel M: A low-cost high efficiency solar cell •• based on dye-sensitized titanium dioxide. Nature 1991, 353:737–740. Discovery of DSC; demonstration of the unprecedented efficiency of 7%. 2.

Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H: •• Dye-sensitized solar cells. Chem Rev 2010, 110:6595–6663. Comprehensive and well-balanced review of all aspects of DSCs. 3.

Li X, Bi D, Yi C, Decoppet JD, Luo J, Zakeeruddin SM, Hagfeldt A, Grätzel M: A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells. Science 2016, 353:58–62. Demonstration of perovskite solar cell with efficiency over 20%. •

4.

Kinoshita T, Nonomura K, Jeon NJ, Giordano F, Abate A, Uchida S, Kubo T, Seok SI, Nazeeruddin MK, Hagfeldt A, Grätzel M, Segawa H: Spectral splitting photovoltaics using perovskite and wideband dye-sensitized solar cells. Nat Commun, vol 6 2015. The first demonstration of DSC/perovskite hybridized device (21.5 % efficiency). •

5.

Kakiage K, Aoyama Y, Yano T, Oya K, Fujisawa JI, Hanaya M: Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes. Chem Commun 2015, 51:15894–15897. Demonstration of the champion efficiency in DSC (14.3%). ••

6.

Concina I, Vomiero A: Metal oxide semiconductors for dye- and quantum-dot-sensitized solar cells. Small 2015, 11:1744–1774.

7.

Bai Y, Mora-Sero I, De Angelis F, Bisquert J, Wang P: Titanium dioxide nanomaterials for photovoltaic applications. Chem Rev 2014, 114:10095–10130. Comprehensive review about titania photoanodes for DSCs. •

Li H, Yu Q, Huang Y, Yu C, Li R, Wang J, Guo F, Jiao S, Gao S, Zhang Y, Zhang X, Wang P, Zhao L: Ultralong rutile TiO2 nanowire arrays for highly efficient dye-sensitized solar cells. ACS Appl. Mat. Interfaces 2016, 8:13384–13391. Demonstration of a top-active photoanode based on rutile (9.6% efficiency). 8. •

9.

Anitha VC, Banerjee AN, Joo SW: Recent developments in TiO2 as n- and p-type transparent semiconductors: synthesis, modification, properties, and energy-related applications. J Mater Sci 2015, 50:7495–7536. Stimulating discussion about possible p-doping in TiO2 . •

10. Perera IR, Daeneke T, Makuta S, Yu Z, Tachibana Y, Mishra A, •• Bäuerle P, Ohlin CA, Bach U, Spiccia L: Application of the tris(acetylacetonato)iron(III)/(II) redox couple in p-type dye-sensitized solar cells. Angew Chem Int Ed 2015, 54:3758–3762. The highest efficiency reported so far for p-DSC (2.51 %).

nanocrystalline diamond through non-covalent surface modification. Phys Chem Chem Phys 2015, 17:1165–1172. 16. Kavan L: Electrochemistry of titanium dioxide: some aspects and highlights. Chem Rec 2012, 12:131–142. 17. Aghazada S, Gao P, Yella A, Marotta G, Moehl T, Teuscher J, •• Moser JE, De Angelis F, Grätzel M, Nazeeruddin MK: Ligand engineering for the efficient dye-sensitized solar cells with ruthenium sensitizers and cobalt electrolytes. Inorg Chem 2016, 55:6653–6659. The first demonstration of highly efficient DSC using Co-mediator and Ru-based dye. 18. Pashaei B, Shahroosvand H, Graetzel M, Nazeeruddin MK: Influence of ancillary ligands in dye-sensitized solar cells. Chem Rev 2016, 116:9485–9564. 19. Roy P, Berger S, Schmuki P: TiO2 nanotubes: synthesis and • applications. Angew Chem Int Ed 2011, 50:2904–2939. Comprehensive review about electrochemically grown titania nanotubes. 20. Roy P, Kim D, Lee K, Spiecker E, Schmuki P: TiO2 nanotubes and their application in dye-sensitized solar cells. Nanoscale 2010, 2:45–59. 21. Azaceta E, Idigoras J, Echeberria J, Zukal A, Kavan L, Miguel O, Grande HJ, Anta JA, Tena-Zaera R: ZnO-ionic liquid hybrid films: electrochemical synthesis and application in dye-sensitized solar cells. J Mater Chem A 2013, 1:10173–10183. 22. Wu MS, Tsai CH, Wei TC: Electrochemical formation of transparent conductive TiO2 film. Chem Commun 2011, 47:2871–2873. 23. Kavan L, Tetreault N, Moehl T, Grätzel M: Electrochemical characterization of TiO2 blocking layers for dye sensitized solar cells. J Phys Chem C 2014, 118:16408–16418. 24. Kavan L, Zukalova M, Vik O, Havlicek D: Sol–gel titanium dioxide blocking layers for dye-sensitized solar cells: electrochemical characterization. ChemPhysChem 2014, 15:1056–1061. 25. Tang Q, Zhang H, Meng Y, He B, Yu L: Dissolution engineering of • platinum alloy counter electrodes in dye-sensitized solar cells. Angew Chem Int Ed 2015, 54:11448–11452. Revelation of novel highly-active and stable catalyst for counterelectrodes contacting I-mediator. 26. Wei YH, Tsai MC, Ma CCM, Wu HC, Tseng FG, Tsai CH, Hsieh CK: • Enhanced electrochemical catalytic efficiencies of electrochemically deposited platinum nanocubes as a counter electrode for dye-sensitized solar cells. Nanoscale Res Lett 2015, 10:1–8. Disclosing of an improved method for electrodeposition of Pt catalyst for counterelectrodes.

11. Langmar O, Ganivet CR, De La Torre G, Torres T, Costa RD, Guldi DM: Optimizing CuO p-type dye-sensitized solar cells by using a comprehensive electrochemical impedance spectroscopic study. Nanoscale 2016, 8:17963–17975.

27. Wang HY, Wang FM, Wang YY, Wan CC, Hwang BJ, Santhanam R, Rick J: Electrochemical formation of Pt nanoparticles on multiwalled carbon nanotubes: useful for fabricating electrodes for use in dye-sensitized solar cells. J Phys Chem C 2011, 115:8439–8446.

12. Sullivan I, Zoellner B, Maggard PA: Copper(I)-based p-type oxides for photoelectrochemical and photovoltaic solar energy conversion. Chem Mater 2016, 28:5999–6016.

28. Kavan L, Liska P, Zakeeruddin SM, Grätzel M: Optically transparent FTO-free cathode for dye-sensitized solar cells. ACS Appl Mater Interfaces 2014, 6:22343–22350.

13. Krysova H, Barton J, Petrak V, Jurok R, Kuchar M, Cigler P, Kavan L: Efficiency and stability of spectral sensitization of boron-doped-diamond electrodes through covalent anchoring of a donor-acceptor organic chromophore (P1). Phys Chem Chem Phys 2016, 18:16444–16450.

29. Peng M, Zou D: Flexible fiber/wire-shaped solar cells in progress: properties, materials, and designs. J Mater Chem A 2015, 3:20435–20458.

14. Krysova H, Kavan L, Vlckova-Zivcova Z, Yeap WS, Verstappen P, • Maes W, Haenen K, Gao F, Nebel CE: Dye-sensitization of boron-doped diamond foam: champion photoelectrochemical performance of diamond electrodes under solar light illumination. RSC Adv 2015, 5:81069–81077. The best performing dye-sensitized B-doped diamond electrode under solar light. 15. Krysova H, Vlckova-Zivcova Z, Barton J, Petrak V, Nesladek M, Cigler M, Kavan L: Visible-light sensitization of boron-doped

Current Opinion in Electrochemistry 2017, 2:88–96

30. Gerosa M, Sacco A, Scalia A, Bella F, Chiodoni A, Quaglio M, • Tresso E, Bianco S: Toward totally flexible dye-sensitized solar cells based on titanium grids and polymeric electrolyte. IEEE J Photovolt 2016, 6:498–505. New design of flexible DSCs using metal grids for current collectors of both electrodes. 31. Bonaccorso F, Colombo L, Yu G, Stoller M, Tozzini V, Ferrari AC, Ruoff RS, Pellegrini V: Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science, vol 347 2015 1246501.

www.sciencedirect.com

Electrochemistry and dye-sensitized solar cells Kavan

95

32. Yun S, Liu Y, Zhang T, Ahmad S: Recent advances in alternative counter electrode materials for co-mediated dye-sensitized solar cells. Nanoscale 2015, 7:11877–11893.

cells with molecular copper phenanthroline as solid hole conductor. Energy Environ Sci 2015, 8:2634–2637. Discovery of ‘Zombie Cells’ with outstanding efficiency (8.2%).

33. Kavan L, Yum J-H, Grätzel M: Graphene based cathodes for liquid-junction dye sensitized solar cells: electrocatalytic and mass transport effects. Electrochim Acta 2014, 128:349–359.

47. Freitag M, Giordano F, Yang W, Pazoki M, Hao Y, Zietz B, Grätzel M, Hagfeldt A, Boschloo G: Copper phenanthroline as a fast and high-performance redox mediator for dye-sensitized solar cells. J Phys Chem C 2016, 120:9595–9603.

34. Kavan L, Liska P, Zakeeruddin SM, Grätzel M: Low-temperature fabrication of highly-efficient, optically-transparent (FTO-free) graphene cathode for co-mediated dye-sensitized solar cells with acetonitrile-free electrolyte solution. Electrochim Acta 2016, 195:34–42.

48. Magni M, Giannuzzi R, Colombo A, Cipolla MP, Dragonetti C, Caramori S, Carli S, Grisorio R, Suranna GP, Bignozzi CA, Roberto D, Manca M: Tetracoordinated bis-phenanthroline copper-complex couple as efficient redox mediators for dye solar cells. Inorg Chem 2016, 55:5245–5253.

35. Lai SCS, Patel AN, McKelvey K, Unwin PR: Definitive evidence • for fast electron transfer at pristine basal plane graphite. Angew Chem Int Ed Eng 2012, 51:5405–5408. An unexpected finding about the catalytic activity of basal plane graphite.

49. Hoffeditz WL, Katz MJ, Deria P, Cutsail GE, Pellin MJ, Farha OK, • Hupp JT: One electron changes everything. A multispecies copper redox shuttle for dye-sensitized solar cells. J Phys Chem C 2016, 120:3731–3740. Disclosing of the variations in coordination sphere of Cu(II/I)-complexes relevant to their application as redox mediators.

36. Roy-Mayhew JD, Pope MA, Punckt C, Aksay IA: Intrinsic catalytic •• activity of graphene defects for the CoII/III (bpy)3 dye-sensitized solar cell redox mediator. ACS Appl Mater Interfaces 2016, 8:9134–9141. Identification of electrocatalytic sites on graphene. 37. Carli S, Casarin L, Syrgiannis Z, Boaretto R, Benazzi E, Caramori S, Prato M, Bignozzi CA: Single walled carbon nanohorns as catalytic counter electrodes for Co(III)/(II) electron mediators in dye sensitized cells. ACS Appl Mat. Interfaces 2016, 8:14604–14612. 38. Jeon IY, Kim HM, Choi IT, Lim K, Ko J, Kim JC, Choi HJ, Ju MJ, Lee JJ, Kim HK, Baek JB: High-performance dye-sensitized solar cells using edge-halogenated graphene nanoplatelets as counter electrodes. Nano Energy 2015, 13:336–345. 39. Jung SM, Choi IT, Lim K, Ko J, Kim JC, Lee JJ, Ju MJ, Kim HK, Baek JB: B-doped graphene as an electrochemically superior metal-free cathode material as compared to Pt over a Co(II)/Co(III) electrolyte for dye-sensitized solar cell. Chem Mater 2014, 26:3586–3591. 40. Kim HM, Jeon IY, Choi IT, Kang SH, Shin SH, Jeong HY, Ju MJ, • Baek JB, Kim HK: Edge-selectively antimony-doped graphene nanoplatelets as an outstanding counter electrode with an unusual electrochemical stability for dye-sensitized solar cells employing cobalt electrolytes. J Mater Chem A 2016, 4:9029–9037. Simple mechano-chemical fabrication of a counterelectrode catalyst outperforming Pt in activity and stability. 41. Li S, Min H, Xu F, Tong L, Chen J, Zhu C, Sun L: All electrochemical fabrication of MoS2 /graphene counter electrodes for efficient dye-sensitized solar cells. RSC Adv 2016, 6:34546–34552. 42. Xu X, Huang D, Cao K, Wang M, Zakeeruddin MS, Grätzel M: Electrochemically reduced graphene oxide multilayers as efficient counter electrode for dye sensitized solar cells. Sci Rep 2013, 3(1489):1–7. 43. Swami SK, Chaturvedi N, Kumar A, Kapoor R, Dutta V, Frey J, Moehl T, Grätzel M, Mathew S, Nazeeruddin MK: Investigation of electrodeposited cobalt sulphide counter electrodes and their application in next-generation dye sensitized solar cells. J Power Sources 2014, 275:80–89. 44. Saygili Y, Soderberg M, Pellet N, Giordano F, Cao Y, • Munoz-Garcia AB, Zakeeruddin SM, Vlachopoulos N, Pavone M, Boschloo G, Kavan L, Moser JE, Grätzel M, Hagfeldt A, Freitag M: Copper bipyridyl redox mediators for dye-sensitized solar cells with high photovoltage. J Am Chem Soc 2016, 138:15087–15096. Demonstration of a system with the lowest driving force for dye regeneration (0.1 eV). 45. Magni M, Biagini P, Colombo A, Dragonetti C, Roberto D, Valore A: Versatile copper complexes as a convenient springboard for both dyes and redox mediators in dye sensitized solar cells. Coord Chem Rev 2016, 322:69–93. 46. Freitag M, Daniel Q, Pazoki M, Sveinbjornsson K, Zhang J, Sun L, •• Hagfeldt A, Boschloo G: High-efficiency dye-sensitized solar

www.sciencedirect.com

50. Saranya K, Rameez M, Subramania A: Developments in conducting polymer based counter electrodes for dye-sensitized solar cells. Eur Polym J 2015, 66:207–227. 51. Ahmad S, Guillen E, Kavan L, Grätzel M, Nazeeruddin MK: Metal free sensitizer and catalyst for dye sensitized solar cells. Energy Environ Sci 2013, 6:3439–3466. 52. Liu W, Fang Y, Xu P, Lin Y, Yin X, Tang G, He M: Two-step electrochemical synthesis of polypyrrole/reduced graphene oxide composites as efficient Pt-free counter electrode for plastic dye-sensitized solar cells. ACS Appl Mat Interfaces 2014, 6:16249–16256. 53. Belekoukia M, Ramasamy MS, Yang S, Feng X, Paterakis G, Dracopoulos V, Galiotis C, Lianos P: Electrochemically exfoliated graphene/PEDOT composite films as efficient Pt-free counter electrode for dye-sensitized solar cells. Electrochim Acta 2016, 194:110–115. 54. Zhang X, Xu Y, Giordano F, Schreier M, Pellet N, Hu Y, Yi C, Robertson N, Hua J, Zakeeruddin SM, Tian H, Grätzel M: Molecular engineering of potent sensitizers for very efficient light harvesting in thin-film solid-state dye-sensitized solar cells. J Am Chem Soc 2016, 138:10742–10745. 55. Zhang J, Vlachopoulos N, Hao Y, Holcombe TW, Boschloo G, Johansson EMJ, Grätzel M, Hagfeldt A: Efficient blue-colored solid-state dye-sensitized solar cells: enhanced charge collection by using an in situ photoelectrochemically generated conducting polymer hole conductor. Nano Energy 2016, 17:1441–1445. 56. Xu B, Bi D, Hua Y, Liu P, Cheng M, Grätzel M, Kloo L, Hagfeldt A, • Sun L: A low-cost spiro[fluorene-9,9-xanthene]-based hole transport material for highly efficient solid-state dye-sensitized solar cells and perovskite solar cells. Energy Environ Sci 2016, 9:873–877. Demonstration of facile synthesis a hole-conductor as an alternative to spiro-OMeTAD. 57. Kashif MK, Milhuisen RA, Nippe M, Hellerstedt J, Zee DZ, • Duffy NW, Halstead B, De Angelis F, Fantacci S, Fuhrer MS, Chang CJ, Cheng YB, Long JR, Spiccia L, Bach U: Cobalt polypyridyl complexes as transparent solution-processable solid-state charge transport materials. Adv Energy Mater 2016, 6:1600874. Discovery of a promising solid hole-conductor based on Co-complexes. 58. Kavan L, Saygili Y, Freitag M, Zakeeruddin SM, Hagfeldt A, Grätzel M: Electrochemical properties of Cu(II/I)-based redox mediators for dye-sensitized solar cells. Electrochim. Acta 2017, 227:194–202. 59. Wu J, Lan Z, Lin J, Huang M, Huang Y, Fan L, Luo G: Electrolytes •• in dye-sensitized solar cells. Chem Rev 2015, 115:2136–2173. Comprehensive review about electrolytes in DSCs. 60. Lau GPS, Decoppet JD, Moehl T, Zakeeruddin SM, Grätzel M, • Dyson PJ: Robust high-performance dye-sensitized solar cells based on ionic liquid-sulfolane composite electrolytes. Sci Rep 2015, 5:18158.

Current Opinion in Electrochemistry 2017, 2:88–96

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Solar Cells GRAETZEL 2017

Demonstration of a solvent-free ionic-liquid electrolyte with high performance. 61. Bella F, Gerbaldi C, Barolo C, Grätzel M: Aqueous dye-sensitized •• solar cells. Chem Soc Rev 2015, 44:3431–3473. Stimulating review about DSCs using aqueous electrolyte solutions. 62. Bella F, Galliano S, Falco M, Viscardi G, Barolo C, Grätzel M, • Gerbaldi C: Unveiling iodine-based electrolytes chemistry in aqueous dye-sensitized solar cells. Chem Sci 2016, 7:4880–4890. Optimization of aqueous DSCs with I-mediators corroborating their stability against aging.

75. Mi Y, Weng Y: Band alignment and controllable electron migration between rutile and anatase TiO2 . Sci Rep 2015, 5:11482:1–10. 76. Nosaka Y, Nosaka AY: Reconsideration of intrinsic band alignments within anatase and rutile TiO2 . J Phys Chem Lett 2016, 7:431–434. 77. Berger T, Monllor-Setoca D, Jankulovska M, Lana-Villarreal T, Gomez R: The electrochemistry of nanostructured titanium dioxide electrodes. ChemPhysChem 2012, 13:2824–2875.

63. Ellis H, Jiang R, Ye S, Hagfeldt A, Boschloo G: Development of high efficiency 100% aqueous cobalt electrolyte dye-sensitised solar cells. Phys Chem Chem Phys 2016, 18:8419–8427.

78. Bisquert J, Cendula P, Bertoluzzi L, Gimenez S: Energy diagram • of semiconductor/electrolyte junctions. J Phys Chem Lett 2014, 5:205–207. Useful survey of terms and relations describing the semiconductor/electrolyte interface.

64. Lindquist RJ, Phelan BT, Reynal A, Margulies EA, Shoer LE, Durrant JR, Wasielewski MR: Strongly oxidizing perylene-3,4-dicarboximides for use in water oxidation photoelectrochemical cells. J Mater Chem A 2016, 4:2880–2893.

79. Kullgren J, Aradi B, Frauenheim T, Kavan L, Deak P: Resolving the controversy about the band alignment between rutile and anatase: the role of OH− /H+ adsorption. J Phys Chem C 2015, 119:21952–21958.

65. Yu Z, Li F, Sun L: Recent advances in dye-sensitized •• photoelectrochemical cells for solar hydrogen production based on molecular components. Energy Environ Sci 2015, 8:760–775. Stimulating review about water splitting in systems inspired by DSC technology.

80. Deak P, Kullgren J, Aradi B, Frauenheim T, Kavan L: Water • splitting and the band edge positions of TiO2 . Electrochim Acta 2016, 199:27–34. Rationalization of the electronic band structure of TiO2 in the presence of water.

66. Li L, Duan L, Wen F, Li C, Wang M, Hagfeldt A, Sun L: Visible light • driven hydrogen production from a photo-active cathode based on a molecular catalyst and organic dye-sensitized p-type nanostructured NiO. Chem Commun 2012, 48:988–990. Pioneering study about water splitting to H2 at the sensitized p-NiO. 67. Sherman BD, Bergkamp JJ, Brown CL, Moore AL, Gust D, Moore TA: A tandem dye-sensitized photoelectrochemical cell for light driven hydrogen production. Energy Environ Sci 2016, 9:1812–1817. 68. Li F, Fan K, Xu B, Gabrielsson E, Daniel Q, Li L, Sun L: Organic dye-sensitized tandem photoelectrochemical cell for light driven total water splitting. J Am Chem Soc 2015, 137:9153–9159. 69. Shi X, Jeong H, Oh SJ, Ma M, Zhang K, Kwon J, Choi IT, Choi IY, • Kim HK, Kim JK, Park JH: Unassisted photoelectrochemical water splitting exceeding 7% solar-to-hydrogen conversion efficiency using photon recycling. Nat Commun 2016, 7:11943. Novel design and optimization of a DSC-based device for efficient water splitting. 70. Armaroli N, Balzani V: Solar electricity and solar fuels: status and perspectives in the context of the energy transition. Chem Eur J 2016, 22:32–57. 71. Zhang X, Peng T, Song S: Recent advances in dye-sensitized •• semiconductor systems for photocatalytic hydrogen production. J Mater Chem A 2016, 4:2365–2402. Inspiring review about water splitting in suspension of dye-sensitized semiconductors. 72. Luo J, Im JH, Mayer MT, Schreier M, Nazeeruddin MK, Park NG, • Tilley SD, Fan HJ, Graetzel M: Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014, 345:1593–1596. Revelation of a highly efficient water-splitting device employing perovskite solar cells. 73. Willkomm J, Orchard KL, Reynal A, Pastor E, Durrant JR, •• Reisner E: Dye-sensitised semiconductors modified with molecular catalysts for light-driven H2 production. Chem Soc Rev 2016, 45:9–23. Tutorial review about dye-sensitized splitting of water at semiconductor nanoparticles. 74. Laskova B, Zukalova M, Kavan L, Chou A, Liska P, Wei Z, Bin L, Kubat P, Ghadiri E, Moser JE, Grätzel M: Voltage enhancement in dye-sensitized solar cell using (001)-oriented anatase TiO2 nanosheets. J Solid State Electrochem 2012, 16:2993–3001.

Current Opinion in Electrochemistry 2017, 2:88–96

81. De Angelis F, Vitillaro G, Kavan L, Nazeeruddin MK, Grätzel M: Modeling ruthenium dye sensitized TiO2 surfaces exposing the (001) or (101) faces: a first principles investigation. J Phys Chem C 2012, 116:18124–18131. 82. Laskova B, Moehl T, Kavan L, Zukalova M, Liu X, Yella A, Comte P, Zukal A, Nazeeruddin MK, Grätzel M: Electron kinetics in dye sensitized solar cells employing anatase with (101) and (001) facets. Electrochim Acta 2015, 160:296–305. 83. Li C, Koenigsmann C, Ding W, Rudshteyn B, Yang KR, Regan KP, • Konezny SJ, Batista VS, Brudvig GW, Schmuttenmaer CA, Kim JH: Facet-dependent photoelectrochemical performance of TiO2 nanostructures: an experimental and computational study. J Am Chem Soc. 2015, 137:1520–1529. Unexpected finding about the high injection efficiency at the (010) facet of anatase. 84. Kavan L, Steier L, Grätzel M: Ultrathin buffer layers of SnO2 by atomic layer deposition: perfect blocking function and thermal stability. J Phys Chem C 2017, 121:342–350. 85. Moehl T, Im JH, Lee YH, Domanski K, Giordano F, Zakeeruddin SM, Dar MI, Heiniger LP, Nazeeruddin MK, Park NG, Graetzel M: Strong photocurrent amplification in perovskite solar cells with a porous TiO2 blocking layer under reverse bias. J Phys Chem Lett 2014, 5:3931–3936. 86. Roy-Mayhew JD, Aksay IA: Graphene materials and their use in • dye-sensitized solar cells. Chem Rev 2014, 114:6323–6348. Thorough review about the application of graphene in DSCs. 87. Heiniger LP, Giordano F, Moehl T, Graetzel M: Mesoporous TiO2 beads offer improved mass transport for cobalt-based redox couples leading to high efficiency dye-sensitized solar cells. Adv Energy. Mater 2014, 4:1400168. 88. Zheng D, Ye M, Wen X, Zhang N, Lin C: Electrochemical methods for the characterization and interfacial study of dye-sensitized solar cell. Sci Bull 2015, 60:850–863. 89. Bisquert J: Theory of the impedance of electron diffusion and •• recombination in a thin layer. J Phys Chem B 2002, 106:325–333. Pioneering work about the applications of electrochemical impedance spectroscopy in DSC. 90. Martin CJ, Bozic-Weber B, Constable EC, Glatzel T, Housecroft CE, Wright IA: Using scanning electrochemical microscopy to examine copper(I) sensitizers for dye-sensitized solar cells. J Phys Chem C 2014, 118:16912–16918.

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