Recent Progress of Counter Electrode Catalysts in Dye-Sensitized

Article pubs.acs.org/JPCC

Recent Progress of Counter Electrode Catalysts in Dye-Sensitized Solar Cells Mingxing Wu*,† and Tingli Ma*,‡,§ †

College of Chemistry and Material Science, Key Laboratory of Inorganic Nano−materials of Hebei Province, Hebei Normal University, No. 20 Rd. East of Second Ring South, Yuhua District, Shijiazhuang City, Hebei Province, P. R. China ‡ State Key Laboratory of Fine Chemicals, College of Chemistry, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province, P. R. China § Graduate School of Life Sciences and Systems Engineering, Kyushu Institute of Technology, 2−4 Hibikino, Wakamatsu, Kitakyushu 808−0196, Japan ABSTRACT: To realize long-term developments and practical application of the dye-sensitized solar cells (DSCs) requires a robust increase of the power conversion efficiency (PCE) and a significant decrease of the production cost. Fortunately, a new record PCE value of 12.3% was achieved by using cobalt-based redox couples combined with organic dye. Evidently, dye design is the key path to improve the PCE, while developing low cost counter electrode (CE) catalysts is one of the promising paths to reduce the production cost of DSCs by replacing the expensive Pt CE. In this article, we review the recent progress of CE catalysts involving Pt, carbon materials, inorganic materials, multiple compounds, polymers, and composites. We discuss the advantages and disadvantages of each catalyst and put forward ideas for designing new CE catalysts in future research for DSCs and other application fields.

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INTRODUCTION As a third-generation solar cells, dye-sensitized solar cells (DSCs) is a potential candidate for the traditional silicon-based solar cells due to their simple assembly procedure, good plasticity, environmental friendliness, and ease of building combination.1−3 In DSCs, the dyes (sensitizers) provide a vital role for light absorption, determining the PCE of the device to a large extent. Metal complex, porphyrins, phthalocyanines, and organic dyes are the widespread sensitizers in DSCs, and the design of highly effective dyes is always the supreme task for the development of DSCs. Several reviews have summarized the advancement of the sensitizers.4−6 In addition to dyes, the semiconductor is another key component of DSCs, and a great breakthrough was brought by introduction of mesoporous TiO2 semiconductor film.7 Up till now, TiO2 is still the dominant semiconductor in DSCs in spite of the emergence of new semiconductors, such as SnO2, Nb2O5, WO3, etc.8−17 Another key part of the DSCs, electrolyte, relates with the PCE, opencircuit voltage (Voc), and short-circuit current density (Jsc) closely. Intensive studies have been made on each component of the electrolyte, such as solvents, additives, and redox couples (or hole transport materials).18−34 In the DSCs system, the counter electrode (CE) behaves as a catalyst for the redox couples regeneration, as well as an electron collector from the external circuit. The CE materials should possess two advantages of high catalytic activity and electrical conductivity. Generally, platinum (Pt) deposited on © XXXX American Chemical Society

F-doped tin oxide (FTO) conductive glass is generally used as a CE, in which Pt performs as a catalyst and FTO as an electron collector. Pt has been proved to be an excellent CE catalyst which has become a criterion in the field of CE catalyst development due to its high catalytic activity and stability.35,36 On the viewpoint of cost cutting, Pt is not the appropriate CE catalyst due to the high cost and limited reserves. To overcome this issue, several kinds of low-cost Pt-free materials have been proposed to be used as CE catalysts summarized by previous research.37,38 In a word, each component of DSCs has made great progress, and a renaissance of DSCs is underway.39 In this review, we give a fresh summary of the CE catalysts in DSCs. These catalysts contain Pt, carbon materials, polymers, inorganic materials, multiple compounds, and composites.

1. PLATINUM (PT) As a noble metal, Pt is a conventional catalyst in various fields involving the DSCs. To form a CE, Pt can be deposited on the substrate via a range of methods, such as electrodeposition, pyrolysis, sputter, chemical reduction, and vapor deposition.36,40−43 Recently, Calogero et al. prepared transparent Pt Special Issue: Michael Grätzel Festschrift Received: December 28, 2013 Revised: February 4, 2014

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dx.doi.org/10.1021/jp412713h | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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CEs on FTO glass via a bottom-up synthetic method. They used tetraoctylammonium bromide (TOAB)/toluene as the extraction agent to abstract H2PtCl6 from aqueous solution of H2PtCl6 to obtain homogeneous H2PtCl6/TOAB solution. After reduction by NaBH4, the Pt nanoparticle was prepared. Adding n-dodecylmercaptan (as a stabilizer), the Pt particle solution was deposited on FTO glass at kept at 350 °C for about 1 h to remove the organic molecules and to fix the Pt nanoparticles onto the FTO glass. Then, the Pt CE was achieved, and the DSCs gave a PCE of 3.77%, slightly lower than the DSCs using sputter Pt CE (3.90%).44 The Ma group prepared a transparent flexible Pt CE on ITO/PEN substrate using Pt ink at low temperature. The homogeneous Pt ink was prepared by reducing H2PtCl6 with NaBH4 with the help of Triton-100 as dispersant. Then, the cleaned ITO/PEN film was first dipped in an aqueous solution containing 4% conditioner at 60 °C for 5 min. This step is very important because the surface of the ITO/PEN becomes rougher after conditioner corrosion. By immersing the ITO/PEN film into the Tritoncapped Pt ink for 5 min at room temperature followed by drying at 130 °C for 30 min, the flexible Pt CE was prepared. The DSCs using this CE yielded a PCE of 5.18%.45 The Yu group synthesized FTO nanocrystals with sizes ranging from 20 to 30 nm which were used to fabricate a porous FTO film as the conductive framework (PFTO). Obviously, this kind of porous FTO can absorb more Pt particles, and the DSCs using this Pt CE showed a PCE of 6.09%.46 In this research, the substrate is still the FTO glass, and we think it is meaningful to replace the FTO glass with cheap common glass. Choi et al. reported a low-temperature method to prepare transparent Pt CE directly using a dry plasma reduction technique without using any toxic chemicals under atmospheric pressure below or at 70 °C. The DSCs using this flexible Pt CE gave a PCE of 5.05% under rear side illumination.47 Meng et al. used the press−transfer method to prepare flexible Pt CE. The DSCs using the flexible Pt CE under different pressure yielded PCE values of 7.21 (100 MPa) and 5.51% (50 MPa).48 Choi et al. prepared Pt nanoparticles (Pt NPs) with different morphologies through controlling the heating rate of thermodecomposition of Pt precursor molecules. They found that the heating rate was a sensitive parameter to determine the morphology of Pt NPs, which subsequently influenced the catalytic activity. Dense distribution of Pt NPs with uniform size was achieved on FTO glass substrate at the heating rate lower than 1.2 °C min−1, and the Pt CE with the highest activity was obtained with the heating rate of 1.2 °C min−1. The DSCs using the best Pt CE gave a PCE of 9.30%.49 The widely used Pt CE is commonly prepared with the chemical reduction method because of the simple procedure, low temperature, high catalytic activity, and low Pt loading. Figure 1 shows the top view of the FTO glass deposited with Pt particles, and the white points refer to the Pt particles without regular shape. The particle size is around 1−5 nm. The Grätzel group found that very low Pt loading (3 μg cm−2) was needed to render Pt electrodes optically transparent with the advantage of economy in the quantity of platinum used.50 The Ma group investigated the effects of Pt film thickness on the catalytic activity. For the Pt film ranging from 2 to 10 nm, no obvious mirror image was observed, while the other films, ranging from 25 to 415 nm, showed obvious mirror images, which favored the second utilization of the light. It was found that 2 nm was sufficiently thick for the Pt film to obtain good catalytic activity.51 Kim et al. used polystyrene (PS) spheres as a

Figure 1. Scanning electron microscopy (SEM) image of the Pt particle prepared by chemical reduction on the FTO surface.

template and deposited a thin layer of Pt on the PS surface to achieve a Pt sphere on the FTO glass. As shown in Figure 2, the

Figure 2. FESEM images of (a) polystyrene (PS) spheres and (b) PS spheres covered with Pt-sputtered material.52

diameter of the Pt sphere was 1 μm. This kind of Pt electrode gave a larger surface area than the sputter Pt electrode evidenced by electrochemical impedance spectroscopy (EIS) analysis. The DSCs using the Pt sphere CE yielded a PCE of 8.20%, higher than the DSCs using sputter Pt CE (7.89%). The authors pointed out that there was still more potential for these electrodes to provide even higher catalytic activity through the use of smaller PS nanospheres with diameters of 500 or 200 nm.52 Pt nanocrystals have been demonstrated to be an effective catalyst in many heterogeneous catalytic processes, and the behaviors of Pt nanoparticles have been found to be highly dependent on the exposed facets.53 As CE catalysts in DSCs, atomic arrangement on the exposed crystal facet of Pt for triiodide reduction is still inexplicable. Yang et al. applied density functional theory (DFT) to investigate the catalytic reaction processes of triiodide reduction over (100), (111), and (411) facets.54 The DFT results indicate that the activity follows the order of Pt(111) > Pt(411) > Pt(100). In addition, they further synthesized Pt nanocrystals mainly bounded by (100), (111), and (411) facets as shown in Figure 3. Then they used these Pt materials with different exposed crystal facets as CEs in DSCs, and the DSCs using Pt (111) CE showed the highest PCE value of 6.91% followed by Pt(411) and Pt(100). The photovoltaic results coincide with the predictions of the theoretical study. The photovoltaic parameters of the DSCs using Pt CEs were summarized in Table 1. Generally, Pt has been the most frequently used CE material in DSCs, and the champion DSCs with the highest PCE of 12.3% also used Pt as the CE.55 However, there remains a critical topic for the researchers to resolve, the high cost and limited reserves. Thus, the future studies on Pt CE could focus on the development of new methods and Pt-based composite B



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As early as 1996, Grätzel et al. used graphite and Cb as the CE in DSCs. The carbon CE exhibited high catalytic activity which can be attributed to the high lateral conductivity and the large surface area of the carbon CE, and the DSCs gave a PCE of 6.7%.56 In addition to Cb, Ca, Cf, Cm, CNTs, and graphene have been also introduced into DSCs as CE.57−62 Readers can obtain detailed information about the carbon CEs from a previous review.37 The Ma group did comprehensive research on nine kinds of carbon materials of Ca, Cb, Cc, Cd, Cf, CNTs, ordered mesoporous carbon (Com), discarded toner (Cp), and C60. The photovoltaic results indicated that Com and Cd showed the highest catalytic activity, and the DSCs gave a high PCE of 7.5%. The conventional carbon materials (Ca, Cb, Cc, CNTs, and Cf) showed decent catalytic activity, and the PCE values of the DSCs ranged from 6.3% to 7.0%.63 Recently, it is interesting that pen ink can be used as a carbon CE on different substrates for DSCs, and the optimized DSCs gave a PCE of 6.18%. The photovoltaic results suggested that pen ink could be a low cost alternative to Pt for DSCs through its high catalytic activity, good adhesion, and suitability for large-scale manufacturing of electronic devices via ink-jet printing technology.64 The Lee group synthesized hollow Ca nanofiber (HACNF) with core and shell structure through concentric electrospinning and thermal techniques. The core and shell diameters were approximately 200−360 nm with a total surface area of 1191 m2 g−1. The PCE value (7.21%) of the DSCs using HACNF CE can be comparable to that of Pt CE based DSCs (7.69%). The authors attributed the high catalytic activity to the high surface area and 1-D conducting pathway of HACNF.65 Very recently, Fang et al. prepared porous carbon nanoparticle CEs on the Ti substrate as CE, and the DSCs showed a PCE of 6.6%.66 Among the carbon materials, CNTs and graphene are always hotter research topics than the other carbon materials. Peng et al. fabricated slender DSCs using CNT fibers with various diameters (25−100 μm). The results showed that the CNT fiber with diameter of 60 μm had the best catalytic behavior. The DSCs using the T−/T2 electrolyte achieved a maximal PCE of 7.33%, much higher than the value of 2.06% for the DSCs using Pt CE. Interestingly, the CNT fiber based DSCs using the conventional I−/I3− electrolyte produced a PCE of 5.97%, lower than the DSCs using the T−/T2 electrolyte.67 Lin et al. prepared vertically aligned single-walled carbon nanotubes (VASWCNTs) onto FTO glass via contact transfer methods which was implemented as CE catalyst in DSCs.68,69 The VASWCNT CE showed high catalytic activity toward T−/T2 redox couples over conventional Pt CEs. Impressively, the device with VASWCNT CEs demonstrated a high fill factor (FF) of 0.68 and a PCE of 5.25%, which were significantly

Figure 3. Representative morphologies and structures of three types of Pt nanocrystals. Respectively, with single kinds of facets in TEM (a−c) and HRTEM (d−f) images: (a, d) Pt(111), (b, e) Pt(100), (c, f) Pt(411).54

CEs to reduce Pt loading. Fortunately, there are so many candidates that can be used as CE catalysts in DSCs, and we will give a summary of the Pt-free catalysts in the following section.

2. CARBON MATERIALS Carbon has been proved to be a distinguished material including activated carbon (Ca), Carbon black (Cb), conductive carbon (Cc), carbon dye (Cd), carbon fiber (Cf), mesoporous carbon (Cm), carbon nanotubes (CNTs), fullerene, graphene, etc. Carbon materials are promising candidates to replace Pt because of the merits of low cost, high catalytic activity, high electric conductivity, high thermal stability, and good corrosion resistance.

Table 1. Photovoltaic Performance of the DSCs Using Various Pt CEs CE catalysts

substrate

redox couples

dye

FF

PCE/%

ref

Pt Pt Pt Pt Pt Pt Pt sphere Pt crystal Pt

FTO glass ITO/PEN PFTO glass ITO/PET ITO/PET FTO glass FTO glass FTO glass FTO glass

I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− Co2+/Co3+

N3 N-719 N-719 N-719 N-719 N-719 N-719 N-719 Y123/YD-o-C8

0.56 0.70 0.6869 0.577 0.68 0.6721 0.7119 0.58 0.74

3.77 5.18 6.09 5.0 7.21 9.33 8.20 6.91 12.30

44 45 46 47 48 49 52 54 55

C



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Table 2. Photovoltaic Performance of the DSCs Using Various Carbon and Polymer CEs CE catalysts Ca Cb Cc Cd Cf CNTs Cm Com C60 pen ink hollow Cf CNTs fiber CNTs fiber VASWCNTs bucky paper graphene graphene graphene graphene carbon paste Com transparent carbon PEDOT PProDOT PProDOT-Et2 PPy PANI a

substrate FTO glass FTO glass FTO glass FTO glass glass FTO glass FTO glass FTO glass FTO glass stainless steel FTO glass N/A N/A FTO glass FTO glass FTO glass FTO glass FTO glass ITO glass FTO glass Carbon FTO glass ITO/PEN FTO glass FTO glass FTO glass Glass

dye

FF

PCE/%

PCE (Pt)/%a

ref

N3 N-719 N-719 N-719 N-719 N-719 N3 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N3 N3 N-719 Y-123 C106TBA N-719 N-719 N3 N-719 N3 N3 N-719 N3

0.607 0.685 0.668 0.697 0.57 0.64 0.65 0.646 0.323 0.712 0.60 0.69 0.60 0.68 0.70 0.60 0.65 0.70 0.74 0.68 0.73 0.60 0.73 0.61 0.59 0.64 0.604

3.89 9.1 6.7 7.5 2.7 7.67 6.18 7.5 2.8 6.18 7.21 7.33 5.97 5.25 4.02 4.99 5.73 9.3 9.54 6.71 8.11 6.07 8.0 7.08 7.88 7.73 6.54

4.30 n/a 7.5 7.5 4.75 7.83 6.26 7.5 7.5 6.75 7.69 2.06 n/a 3.49 4.08 5.48 6.89 8.1 9.14 7.06 8.16 6.89 6.8 7.7 7.7 8.20 6.69

57 58 63 63 59 60 61 63 63 64 65 67 67 68 70 71 62 73 74 77 79 80 83 84 84 87 88

redox couples −

−

I /I3 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− T−/T2 I−/I3− T−/T2 I−/I3− I−/I3− I−/I3− Co2+/Co3+ I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

The power conversion efficiency (PCE) of the Pt CE based DSCs that was used as the reference cell.

linearly with the graphene film’s optical absorbance, and they were 1−2 orders of magnitude larger than those for I−/I3− on the same electrode. The graphene films with optical transmission below 88% behaved better than platinized FTO for the Co2+/Co3+ redox reaction, and the DSCs with Y123 dye, Co2+/ Co3+ electrolyte, and graphene CE produced a high PCE of 9.3%. Very recently, Wang and Grätzel prepared graphene CE using layer-by-layer assembly of negatively charged graphene oxide and positively charged poly(diallyldimethylammonium chloride) and a following electrochemical reduction procedure. The DSCs using the graphene CE combined with the heteroleptic Ru complex C106TBA as sensitizer produced a high PCE of 9.54%, surpassing the DSCs using Pt CE (9.14%). As far as the authors know, 9.54% is the efficiency record of the DSCs using graphene CE. In conjunction with low volatility and solvent-free ionic liquid electrolytes, the graphene-based DSCs exhibited good durability (60 °C for 1000 h in a solar simulator, 100 mW cm−2) during the accelerated tests.74 The Hu group synthesized 3D honeycomb-like structured graphene sheets for the DSCs as CEs which gave a PCE as high as 7.8%, close to the performance of the DSCs with an expensive Pt CE.75 About the graphene CE catalysts, the readers can gather more information from a fresh review.76 In a previous review,37 we pointed out that the shortcoming of carbon CE was the poor bonding strength between the carbon film and the substrate which was a potential threat for long-term use. In the following years, to overcome this problem, we attempted to use carbon paste (like silver paste used in integrated circuit) to improve the bonding strength of the carbon CE.77 We used sticky carbon paste with high conductivity as a binder and carbon dye as the catalyst to

higher than 0.56 and 3.49% of the device using Pt CE. Misra et al. used bucky paper (BP) as flexible CE in DSCs.70 The surface morphology of BP changed dramatically after plasma treatment. The plasma treatment improved the efficiency of BP-based DSCs from 2.44% to 4.02%, which was comparable to Pt CE based DSCs (4.08%). Graphene owns the advantages of high electrical conductivity, catalytic activity, and corrosion resistance. Aksay et al. used graphene as CE in DSCs and found that the C/O ratio had a strong impact on the catalytic activity.71 When the C/O ratio was up to 13, high catalytic activity was obtained, and the DSCs showed a PCE of 5.0%, close to that of the DSCs using Pt CE (5.5%). Jeon et al. also found that oxygen functional groups decreased with annealing, whereas the DSCs using graphene CEs with the fewest number of oxygen functional groups performed the best (5.69%).72 In other words, the catalytic activity increased as the number of oxygen functional groups decreased. After combining analysis of the results of Aksay and Jeon’s works, we claim that there exists an optimum number of oxygen functional groups for graphene to achieve optimal catalytic activity. Grätzel et al. used graphene to prepare optically transparent CEs for DSCs. It was found that the graphene CE performed better for the regeneration of I−/ I3− in ionic liquid solvent than in the traditional organic solvent. The charge transfer resistance (Rct) value of the ionic solvent is smaller than the traditional solvent by a factor of 5−6.62 This means the mechanism for the regeneration of I−/I3− in the graphene surface is determined by solution-related events rather than viscosity. Further, they applied the graphene CE in Co2+/Co3+ redox couples based solar cells.73 The exchange current density (J0) for the Co2+/Co3+ redox reaction scaled D



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Figure 4. SEM images of PProDOT−Et2 films with various deposition charge capacities (×10,000): (a) 10 mC cm−2, (b) 20 mC cm−2, (c) 40 mC cm−2, (d) 80 mC cm−2, (e) 120 mC cm−2, (f) 160 mC cm−2, and (g) 200 mC cm−2. (h) Image of sputtered−Pt (100 nm) for comparison.84

used as CEs in DSCs, such as PEDOT/PSS, polypyrrole (PPy), polyaniline (PANI), etc. As early as 2003, Hayase et al. applied PEDOT/PSS CE for quasi-solid DSCs, finding that this CE performed better than Pt for an ionic liquid electrolyte (ILE).81 The EIS results showed that the Rct in the electrode/ILE interface was merely one tenth of the value for the Pt/ILE. For the organic liquid electrolyte (OLE), the result was opposite. Yanagida et al. indicated that the high viscosity and low conductivity of ILE necessitated high I2 concentration conditions; therefore, the porous PEDOT/ PSS CE was more suitable than Pt for an ILE.82 Pringle et al. prepared PEDOT film on an ITO/PEN flexible substrate using the electrodeposition technique with different deposition times (5−45 s). The results showed that very short deposition time, 5 s, was enough to achieve a highly effective and transparent PEDOT film. The DSCs using this CE and OLE electrolyte produced a high PCE of 8.0%.83 Ho et al. deposited PProDOT−Et2 film on FTO substrate by the electropolymerization method. The pore size of the PProDOT−Et2 film can be regulated by changing the charge capacities (10, 20, 40, 80, 120, 160, and 200 mC cm−2, Figure 4). When the charge capacity reached 40 mC cm−2, the PProDOT−Et2 film owned the largest active surface area, generating the best catalytic ability. The DSCs using this polymer CE yielded a PCE of 5.20%, close to that of the DSCs using Pt CEs.84 They also compared the catalytic activities of PEDOT, PProDOT, and PProDOT−Et2. The DSCs using PProDOT−Et2 and PProDOT CEs showed PCE values of 7.88% and 7.08%, much higher than that of the DSCs using PEDOT CE (3.93%). Zhang et al. used PEG and acetylene black (AB) to modify PEDOT:PSS films as CEs for DSCs. The catalytic activity for triiodide reduction was improved by adding PEG, which behaved as a conductivity promoting agent and binding agent. Catalytic activity can be further improved by adding a little amount of AB. The DSCs using a PEG/PEDOT/PSS/AB composite CE exhibited a PCE of 4.39%, slightly lower than the Pt CE based DSCs (4.50%).85 In addition to the polythiophene polymer, Xia et al. synthesized polypyrrole (PPy) films on FTO glass using vapor-phase polymerization (VPP) and electropolymerization

improve bond strength successfully, which had been confirmed by the subsequent durability test.78 Besides, the introduction of carbon paste also enhanced the conductivity of the CEs, resulting in the improvement of the photovoltaic performance of DSCs showing a PCE of 6.71%, reaching 95% of that of DSCs using sputter Pt CEs.77 Wang et al. prepared integrated carbon CE using a porous carbon sheet as a conductive substrate and Com as the catalytic layer. This kind of carbon CE showed good mechanical strength and bonding strength. The integrated carbon CE showed very low series resistance (Rs), owing to the high conductivity of the carbon sheet and low Rct originating from the large specific surface area of the Com layer. The device with this CE produced a high PCE of 8.11%, comparable to that of a Pt CE based device (8.16%).79 As pointed out previously,37 opacity is another disadvantage of carbon CEs, and development of transparent carbon materials for DSCs will become a promising research direction in the future. Recently, Zhao et al. prepared transparent carbon electrodes via an in situ carbonization method on FTO glass.80 The transparency and catalytic activity of carbon CEs were dramatically affected by the composition and concentration of the precursor. After optimization, the transparent carbon CE exhibited high catalytic activity for the regeneration of iodide redox couples, and the DSCs gave a high PCE of 5.04% under rear-side illumination, which approached 85% of the front-side illumination (6.07%). Meanwhile, the problem of poor mechanical stability for traditional carbon CEs has also been solved by this in situ carbonization method. The carbon materials hold the merits of high catalytic activity, a simple preparation process, low cost, and considerable stability, and the authors think that the carbon CE is the most competitive candidate in commercial production of low cost and effective DSCs. The photovoltaic parameters of the DSCs using carbon CEs are summarized in Table 2.

3. POLYMERS To highlight the merits of transparency and flexibility for DSCs, it is essential to develop transparent and flexible CEs. Up to date, there are several kinds of conductive polymers that can be E



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(EP) techniques.86 The PPy particle prepared by the VPP method exhibited a uniform size of 100−150 nm, while the PPy synthesized with an EP method had a larger particle size of 200−300 nm. The DSCs using the PPy CEs gave PCE values of 3.4 (VPP−PPy) and 3.2% (EP−PPy), both lower than that of the Pt CE based DSCs (4.4%). Meanwhile, Im et al. synthesized a PPy spherical nanoparticle with a uniform size of 85 nm which was subsequently used as CE catalyst for DSCs.87 The conductivity can be improved by treating the PPy with HCl. After optimization, the DSCs using PPy CE showed a high PCE of 7.73%, as compared to the DSCs using Pt CE (8.20%). Zhao and colleagues fabricated transparent polyaniline (PANI) CE via an in situ polymerization method on FTO glass used as CE for DSCs. The DSCs using this transparent PANI CE showed a PCE of 6.54% at the front-side illumination and a PCE of 4.26% at rear-side illumination.88 He et al. fabricated nanostructured PANI CE with different H2SO4 doping levels and investigated the impact of doping level on the catalytic activity in DSC systems.89 The specific surface area of the PANI CEs increased with the variation of H2SO4 concentration from 0.15 to 0.35 M which can afford more catalytic sites for the I−/ I3− redox reaction. The PCE of the DSCs using PANI CE reached 5.57%, close to that of the DSCs using Pt CE (6.00%). The photovoltaic parameters of the DSCs using polymer CEs were summarized in Table 2. Organic polymers are the most promising transparent flexible CEs to replace Pt CEs due to the advantages of transparency, high catalytic activity, easy availability, and low cost. However, there remains much work to be done in this kind of polymer CE, such as development of new polymers, a gentle method to prepare flexible polymer CE, investigation the durability of the polymer CEs, and so forth.

Except ZrC and SiC, the other carbides all showed high catalytic activity. Moreover, the authors used the carbide CEs for catalyzing the regeneration of the redox couples of T−/T2. All of the carbide CEs highlighted the advantages to the T2/T− system. The DSCs using TiC, VC, and Cr3C2 CEs gave high PCE values of 4.96, 4.06, and 4.54%. The PCE values were significantly improved to 35.5, 10.9, and 24.4% compared to the Pt CE based DSCs. To further reduce the cost of DSCs, we prepared a TiC CE on the substrate of bare glass (BG), Ti foils, and polyimide (PI), discarding the expensive TCO layer.97,98 For the bare glass and polyimide substrate, we used conductive paste (CC) to replace the TCO layer. The DSCs using TiC/Ti, TiC/CC/BG, and TiC/CC/PI CEs showed PCE values of 7.15, 5.71, and 3.90%. Obviously, the DSCs using CEs on the Ti substrate showed the best photovoltaic performance, surpassing the CEs on the expensive FTO glass. The DSCs with CEs on the BG substrate also exhibited relatively high efficiency close to FTO glass. Further, the impact of TiC film thickness on the performance of the catalytic activity was also investigated.99 The photovoltaic results showed that the DSCs fabricated with the TiC CE with an optimal film thickness of 20 μm achieved the highest PCE of 6.46%, which can match the performance of the DSCs assembled with a Pt CE. 4.2. Nitrides. Similar to the carbides, the nitrides of TiN, Mo2N, MoN, W2N, WN, Fe2N, NiN, VN, NbN, CrN, and Ta4N5 were also introduced into DSCs as CE catalysts. Gao et al. synthesized TiN nanotube arrays with the anodization of Ti foil, followed by simple nitridation. The DSCs using the TiN nanotube on the Ti sheet as CE showed a PCE of 7.73%, higher than the performance of the DSCs using Pt CE (7.45%).100 Cui et al. synthesized TiN sphere CEs by coating hierarchical micro/nano-TiO2 paste onto Ti foil followed by a nitridation reaction. Compared with particulate TiN and TiN flat CEs, the TiN sphere CE based DSCs showed higher photovoltaic performance due to the presence of the hierarchical structure. After optimization, the highest PCE reached 7.83%, 30% higher than that of Pt CE based DSCs (6.04%).101 We prepared TiN, VN, CrN, NbN, and ZrN nanoparticles using a urea-metal path and applied these nitrides as CE catalysts in DSCs, producing PCE values of 6.23 (TiN), 5.92 (VN), 5.44 (CrN), 1.20 (NbN), and 3.68% (ZrN), respectively.94 Further, we prepared flexible W2N and Mo2N CEs on a Ti sheet by the sputtering method. The DSCs using the two nitride CEs gave PCE values of 5.81 (Mo2N) and 6.38% (W2N), reaching 83 and 91% of the photovoltaic performance of the DSCs with Pt CE (7.01%). The EIS results showed that the series resistances (Rs) of W2N and Mo2N CEs were 1.8 and 1.1 Ω, due to the high conductivity of the Ti sheet and the good bonding strength between the Mo2N or W2N film and the Ti sheet.102 Gao et al. prepared surface-nitrided Ni foil to be used as CE in DSCs which produced a PCE of 5.68%, much lower than that of the Pt CE based DSCs (8.41%).103 The low catalytic activity of the surface-nitrided Ni foil can be attributed to the compact nitride film without mesoporous structure. To improve the catalytic activity, the NiN particle with a mesoporous structure was prepared, and the DSCs gave a high PCE of 8.31%, proving that large surface area was critical for high catalytic activity. Further, they also synthesized MoN, WN, and Fe2N by nitridation of the oxide (MoO2, WO3, Fe2O3) precursors in ammonia atmosphere, after which they were introduced into DSCs as CE catalysts. 104 The corresponding DSCs showed PCE values of 5.57 (MoN), 3.67 (WN), and 2.65% (Fe2N). These results confirm that the

4. METAL COMPOUNDS Transition metal carbides and nitrides (TMCs and TMNs), named as interstitial phase or interstitial compounds, possess Pt-like catalytic behavior as previous studies proved.90,91 As alternatives to the noble metals, TMCs and TMNs have been used in the fields of ammonia synthesis, hydrogenation and dehydrogenation, methanol oxidation, among others. Since 2009, some metal compounds have been applied in DSCs as CE catalysts to replace the expensive Pt CE. These compounds contain carbides, nitrides, oxides, sulfides, phosphides, and so on. The following sections will give a detailed introduction. 4.1. Carbides. Lee et al. prepared tungsten carbide (WC) with polymer-derived and microwave-assisted methods (WCPD and WC-MW) and then introduced the WC in DSCs as CE catalysts.92 The DSCs based on these CEs showed PCE values of 6.61 (WC-PD) and 7.01% (WC-MW), which were still lower than that of the DSCs using Pt CE (8.23%). The Ma group also introduced tungsten and molybdenum carbides as CE catalysts for DSCs.93 The DSCs using these commercial materials as CEs showed PCE values of 5.35 (WC) and 5.70% (Mo2C), much lower than the corresponding PCE of the Pt CE based DSCs (7.89%). This can be attributed to the large particle size and low surface area of the two carbides. Next, they synthesized nanoscaled WC and W2C using the metal−urea route, and the DSCs using nanoscaled tungsten carbides as CEs achieve high PCE values of 6.68 (W2C) and 6.23% (WC). Compared to the large-sized WC particle, the nanoscaled WC showed much improved catalytic activity.37 Besides WC and MoC, TiC, VC, NbC, Cr3C2, Ta4C3, and SiC have been subsequently introduced in iodide electrolyte based DSCs as CEs.94−96 F



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Table 3. Photovoltaic Performance of the DSCs Using Various Carbide and Nitride CEs CE catalysts

substrate

redox couples

dye

FF

PCE/%

PCE (Pt)/%

ref

WC WC W2C Mo2C TiC TiC TiC TiC VC VC Cr3C2 Cr3C2 ZrC SiC Ta4C3 TiN TiN VN CrN ZrN NbN W2N Mo2N NiN WN MN Fe2N

ITO/PEN FTO glass FTO glass FTO glass FTO glass FTO glass Bare glass Ti foil FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass Ti sheet FTO glass FTO glass FTO glass FTO glass FTO glass Ti sheet Ti sheet Ni foil FTO glass FTO glass FTO glass

I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− T−/T2 I−/I3− I−/I3− I−/I3− T−/T2 I−/I3− T−/T2 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719

0.65 0.60 0.65 0.61 0.62 0.63 0.56 0.70 0.56 0.54 0.62 0.65 0.44 0.37 0.72 0.64 0.61 0.64 0.64 0.20 0.47 0.57 0.61 0.69 0.54 0.66 0.41

7.01 6.23 6.68 5.83 6.50 4.96 5.71 7.15 4.92 4.06 5.79 4.54 3.85 3.29 7.39 7.73 6.23 5.92 5.44 1.20 3.68 5.81 6.38 8.31 3.67 5.57 2.65

8.23 7.65 7.65 7.50 7.50 3.66 7.35 7.35 7.50 3.66 7.50 3.66 7.50 7.18 7.57 7.45 7.50 7.50 7.50 7.50 7.50 7.01 7.01 8.41 6.56 6.56 6.56

92 37 37 94 94 94 97 97 94 94 94 94 94 95 96 100 94 94 94 94 94 102 102 103 104 104 104

M−Nb2O5 (monoclinic) were prepared to investigate the impact of crystal forms on the catalytic activity.108 The DSCs using niobium oxide CEs showed PCE values of 5.68 (H− Nb2O5), 4.55 (O−Nb2O5), and 5.82% (M−Nb2O5). Meanwhile, another niobium oxide, NbO2, was synthesized which showed the highest catalytic activity, resulting in a PCE of 7.88%, surpassing the DSCs using Pt CE (7.65%). Xia et al. used V2O5 as CE in solid DSCs.109 The performance of the solid DSCs reached 2.0%, close to the performance of the DSCs using Ag CE. In our previous review, we mentioned that RuO2 showed a low Rct of 20 Ω cm2,37 indicating RuO2 might be a promising CE catalyst. Recently, Wang et al. prepared RuO2 nanocrystals via hydrothermal and sintering processes, and the prepared RuO2 nanocrystals subsequently worked as CE catalyst in DSCs.110 A high PCE of 7.22% was obtained, exceeding the Pt-based DSCs (7.17%). The author indicated that the high catalytic activity was attributed to the ideal conjunction of the superior catalytic activity and high electrical conductivity, and this result also proved our conceivement.37 Generally, the photoanode mesoporous films are oxide semiconductors, such TiO2, SnO2, WO3, etc. As pointed out previously, WO3 can be used as CE catalyst. If WO3 is used as the photoanode film, the direct contact of WO3 and I3−/I− redox couples may cause the I3− to be reduced by the electrons injected in the conduction band (CB) of WO3 due to the autocatalytic activity. That is to say, a number of electrons injected in the CB will be consumed by the I3− rather than flow into the external circuit. The autocatalytic activity of WO3 can result in a large dark current density, which has been observed.111 This may be a key reason for the poor performance of the DSCs using WO3 as photoanodes.105 We found that prepared SnO2 and Nb2O5 under N2 atmosphere

nitrides are promising alternatives for Pt. The photovoltaic parameters of the DSCs using carbide and nitride CEs were summarized in Table 3. In addition to the high catalytic activity, the carbides and nitrides combine the advantages of high hardness, high melting point, and high electric and thermal conductivity. Thus, we contend that the TMCs and TMNs are promising low cost catalysts to replace the noble Pt to be used in many fields, such as fuel cells, hydrogenation of aromatic hydrocarbon, hydrogenation of unsaturated alkene, and so on, not limited to DSC systems. 4.3. Oxides. Differently from the carbides and nitrides, transition metal oxides are rarely used as catalysts to replace Pt. However, Ma et al. synthesized WO2 nanorods which hold excellent catalytic activity, and the iodide electrolyte based DSCs using WO2 CE showed a high PCE of 7.25%, close to that of the Pt CE based DSCs (7.57%).105 This is an unexpected result. For the T−/T2 redox couples, the WO2 performed better than Pt, and the DSCs gave a PCE of 4.66%.106 Meanwhile, the WO3 was also prepared, while the DSCs using WO3 CE showed a low PCE of 4.67%. Similar to WO2, WO2.72 also showed higher catalytic activity, and the DSCs produced a high PCE of 8.03%, close to the Pt CE based DSCs (8.08%).107 The fundamental reason for the high catalytic activity of tungsten oxides is still unclear and requires further study. Subsequently, TiO2, V2O3, ZrO2, Nb2O5, Cr2O3, MoO2, TaOx, and RuO2 were successively introduced into DSCs as CE catalysts. The DSCs using V2O3, Nb2O5, and TaOx showed decent PCE values of 5.40, 4.84, and 6.79%.94,96 By contrast, the TiO2, ZrO2, Cr2O3, and MoO2 gave poor catalytic activity for the triiodide reduction. For the Nb2O5, three kinds of H−Nb2O5 (hexagonal), O−Nb2O5 (orthorhombic), and G



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Table 4. Photovoltaic Performance of the DSCs Using Various Oxide and Sulfide CEs CE catalysts WO2 WO2 WO3 WO2.72 Nb2O5 NbO2 TiO2 ZrO2 V2O3 V2O5 Cr2O3 MoO2 TaOx RuO2 SnO2(Air) SnO2(N2) CoS Co9S8 Co8.4S8 NiS Ni3S2 Cu1.8S Bi2S3 Fe2S MoS2 MoS2 WS2 WS2 a

substrate

redox couples

dye

FF

PCE/%

PCE (Pt)/%

ref

FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass Al FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass ITO/PEN FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass ITO/PEN FTO glass FTO glass FTO glass FTO glass

I−/I3− T−/T2 I−/I3− I−/I3− T−/T2 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− Spiro-MeTAD I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− T−/T2 I−/I3− T−/T2

N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 Z-907 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719

0.64 0.57 0.48 0.70 0.71 0.60 0.70 0.33 0.28 0.63 0.34 0.16 0.40 0.68 0.54 0.36 0.53 0.73 0.69 0.66 0.64 0.66 0.40 n/a 0.68 0.73 0.63 0.70 0.64

7.25 4.66 4.46 8.03 5.05 4.84 7.88 0.76 2.60 5.40 2.0 1.07 2.40 6.79 7.22 1.84 6.08 6.5 7.00 6.50 6.83 7.01 3.79 3.5 7.31 7.59 4.97 7.73 5.24

7.57 3.06 7.57 8.08 2.73 7.50 7.65 7.50 7.50 7.50 2.6a 7.50 7.50 7.57 7.17 7.50 7.50 6.5 7.13 7.32 7.00 7.32 7.32 n/a 7.52 7.64 3.70 7.64 3.70

105 106 105 106 106 94 108 94 94 94 109 94 94 96 110 112 112 113 114 118 116 118 118 119 120 122 122 122 122

PCE of 2.6% was obtained from the Ag CE based solid-state DSCs.

can improve the catalytic activity significantly.112 The DSCs which were fabricated with SnO2 and Nb2O5 CEs prepared in N2 atmosphere yielded PCE values of 6.09 and 4.65%, much higher than the PCE values (1.84 and 0.97%) of the DSCs fabricated with the same SnO2 and Nb2O5 CEs prepared in air. XPS measurements showed a significant difference in the chemical states of Sn 3d data of the SnO2 CE prepared in different sintering atmosphere. This indicated that the N2− SnO2 and Air−SnO2 exhibited widely different surface electronic structures, resulting in the variation of catalytic activity. 4.4. Sulfides. Grätzel et al. prepared semitransparent CoS film on ITO/PEN which was then introduced into DSCs as a novel CE catalyst. The Tafel polarization curves showed a large slope for CoS CE, indicating a large J0 on the CoS electrode surface, comparable to Pt electrode. The DSCs using CoS CE showed a PCE of 6.5%, equal to the Pt CE based DSCs. Moreover, the long-term stability test under light soaking at 60 °C proved that CoS was stable in severe conditions.113 Tuan et al. reported the development of Co9S8 nanocrystals as costeffective CE catalyst for large-area DSCs.114 Single 2 cm2 sized DSCs using Co9S8 CEs showed an average PCE of 7.0%, slightly lower than Pt CE based DSCs. Ho et al. used a CoS nanorod array as CE catalyst in DSCs, producing a PCE of 7.67%, similar to Pt CE based DSCs (7.70%).115 Meng et al. prepared NiS CEs by periodic potential reversal (PR) and potentiostatic (PS) techniques. The DSCs using PR-NiS and PS-NiS CEs showed PCE values of 6.83 (PR-NiS) and 3.22% (PS-NiS).116 Meanwhile, the NiS nanoarray film was prepared

by a two-step low-temperature solution route, and the DSCs using this sulfide CE achieved a PCE of 7.10%, compared to the Pt CE based DSCs (7.35%).117 Further, Batabyal et al. used Co8.4S8, Ni3S2, and Cu1.8S as CEs in DSCs which produced PCE values of 6.50, 7.01, and 3.79%, respectively.118 Chen et al. compared the catalytic activity of (130) and (211) facets of Bi2S3 in DSCs as CEs.119 The DSCs with (130) and (211) faceted Bi2S3 CEs exhibited PCE values of 3.5 and 1.9%, respectively. The high performance of the (130) faceted Bi2S3 can be ascribed to the larger surface energy, the high electronic conductivity, and the highest position of CB minima, indicating the smooth electron transfer from CEs to triiodide. Recently, Chen et al. prepared semitransparent FeS2 film on ITO/PEN. After modification, the DSCs using FeS2 CE showed a PCE of 7.31%.120 Meanwhile, Du et al. used the FeS2 film and FeS2 nanorod as CEs in DSCs, giving PCE values of 4.78 and 5.88%.121 Ma et al. synthesized MoS2 and WS2 for DSCs as CEs. As a result, the MoS2 and WS2 both performed well for triiodide reduction, and the DSCs yield high PCE values of 7.59 (MoS2) and 7.73% (WS2), comparable to the performance of the Pt CE based DSCs (7.64%). In addition, the two sulfide CEs both surpass Pt in T−/T2 electrolyte based DSCs.122 The photovoltaic performance of the DSCs using oxide and sulfide CEs are summarized in Table 4. 4.5. Phosphides. There are relatively less reports on the phosphides used as CE catalysts, compared to the sulfides. Ma et al. prepared Ni5P4 and MoP used as CE catalysts in DSCs, which produced PCE values of 5.71 (Ni5P4) and 4.92% (MoP), both lower than Pt CE based DSCs using iodide electrolyte.123 H



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Table 5. Photovoltaic Performance of the DSCs Using Various Phosphide, Telluride, Selenide, and Multiple Compound CEs CE catalysts Ni5P4 Ni5P4 Ni5P12 MoP MoP Ni0.85Se NiSe2 Co0.85Se CoSe CoTe NiTe2 CZTS CZTSSe CZTSe CuInS2 NiCo2S4 CoMoS4 NiMoS4

substrate FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO FTO

glass glass glass glass glass glass glass glass glass glass glass glass glass glass glass glass glass glass

redox couples

dye

FF

PCE/%

PCE (Pt)/%

ref

I−/I3− T−/T2 I−/I3− I−/I3− T−/T2 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719

0.54 0.54 0.44 0.51 0.54 0.72 0.743 0.75 0.747 0.71 0.65 0.403 0.522 0.656 0.730 0.60 0.69 0.69

5.71 4.40 3.94 4.92 3.87 8.32 8.69 9.40 7.30 6.92 7.21 2.62 7.37 7.82 6.33 6.14 4.96 5.27

7.76 3.38 6.08 7.76 3.38 8.64 8.04 8.64 6.91 7.04 7.04 7.04 7.04 7.56 6.07 6.29 7.46 7.46

123 123 124 123 123 125 126 125 127 128 128 130 130 132 133 135 136 136

compounds (comprising three or more elements) were also introduced into DSCs as CE catalysts. The Lin group prepared copper zinc tin sulfide (CZTS) nanocrystals using a solution-based synthesis approach and got CZTSSe after Se vapor treatments.130,131 Subsequently, they used the two multiple compounds as CEs in DSCs, which achieved PCE values of 3.62 (CZTS) and 7.37% (CZTSSe). Wu et al. used CZTSe as CE catalyst in DSCs.132 They investigated the impact of the thickness of the CZTSe layer on the catalyst activity and found that 1.2 μm was the optimal thickness. The DSCs using CZTSe CE showed a PCE of 7.82%, which can match the performance of the DSCs using Pt CE. Yu et al. prepared vertically oriented CuInS2 nanosheet thin films via a facile one-step solvothermal process which was next used as CE in DSCs. The DSCs based on the optimized CuInS2 CE yielded a PCE of 6.33%, comparable to that of sputtering Pt (6.07%).133 At the same time, Wu et al. synthesized CuInS2 nanocrystals by a simple one-pot route for the DSCs as CE catalysts.134 The DSCs with CuInS2 CE gave a PCE of 6.35%, slightly lower than Pt-based DSCs (6.87%). Lin et al. synthesized NiCo2S4 via a facile solvothermal method. The NiCo2S4 CE exhibited high transmittance (>75%), and the DSCs using NiCo2S4 gave a PCE of 6.14%, slightly lower than the Pt CE based DSCs (6.29%).135 The Li group prepared porous chalcogels CoMoS4 and NiMoS4 via a facile solution reaction, and these multiple compounds displayed high catalytic activity for triiodide reduction. The DSCs using these multiple compounds as CEs gave PCE values of 4.96 (CoMoS4) and 5.27% (NiMoS4).136 The photovoltaic performance of the DSCs using phosphides, selenides, tellurides, and multiple compound CEs are summarized in Table 5. Introduction of multiple compounds as CE catalysts in DSCs just started. The authors believe that more and more kinds of multiple compounds will appear in future research, while the long-term stability of the multiple compounds should be checked in harsh conditions.

By contrast, the Ni5P4 and MoP behaved better than Pt for the T−/T2 regeneration, and the devices gave PCE values of 4.40 (Ni5P4) and 3.87% (MoP), both higher than Pt CE based DSCs (3.38%). Meanwhile, Gao et al. used Ni12P5 as CE for DSCs, yielding a PCE of 3.94%.124 4.6. Selenides. The Wang group synthesized cobalt selenide (Co0.85Se) and nickel selenide (Ni0.85Se) with a facile one-step strategy which were then used directly as CEs in DSCs.125 The DSCs using Co0.85Se resulted in a PCE of 9.40%, surpassing the PCE of 8.64% for Pt CE based DSCs under the same conditions, while the DSCs using Ni0.85Se showed a relatively low PCE of 8.32%. In other work, they also used NiSe2 as CE, and the DSCs produced a PCE of 8.69%, higher than the Pt CE based DSCs (8.04%).126 Cui et al. used CoSe as CEs for DSCs. After optimization, the DSCs gave a PCE of 7.30%, higher than the DSCs using Pt CE (6.91%).127 Recently, the Ma group prepared NbSe2 nanosheets (NSs) and nanorods (NRs), via a facile and controllable reductant-free solvothermal approach. Then, the prepared NSs and NRs were used as CEs for DSCs, which produced PCE values of 7.34 and 6.78%, close to Pt CE based DSCs (7.9%).128 4.7. Tellurides. Ma et al. synthesized metal tellurides of CoTe and NiTe2 using a composite-hydroxide-mediated (CHM) approach. The tellurides were next used as the CEs in DSCs, which behaved better for the reduction of triiodide, and the CoTe and NiTe2 CE based DSCs yielded PCE values of 6.92 and 7.21%, respectively, comparable to the DSCs using Pt CE (7.04%).129 Up till now, so many kinds of metal compounds, like carbides, nitrides, oxides, sulfides, phosphides, selenides, and tellurides, have been introduced into DSCs as CE catalysts. Although most of the metal compounds showed high catalytic activity, the synthesis procedures of metal compounds consume a large amount of energy. Developing new synthesis routes with low energy consumptions is a major issue to be resolved for the metal compound CE.

6. COMPOSITE MATERIALS Composites comprise two or more components. In the past years, preparing composite CEs for DSCs has become increasingly popular because of the high catalytic activity. In

5. MULTIPLE COMPOUNDS Besides the binary compounds (comprising two elements, like transition metal carbides, nitrides, and oxides), multiple I



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CE based DSCs (5.00%).140 The Wu group prepared MWCNTs decorated with tungsten sulfide (MWCNTs/WS2) via the hydrothermal method, which was used as a low cost platinum-free CE in DSCs, resulting in a PCE of 6.41%, comparable to 6.56% for the DSCs with Pt CE.141 In addition, MWCNTs/MoS2 also had been used as a CE in DSCs, producing a PCE of 6.45%.142 Recently, CuInS2/ZnS was synthesized as CE catalyst for DSCs which produced a PCE of 7.5%, exceeding the conventional Pt CE based DSCs.143 Wang et al. reported that they prepared a TaON/graphene oxide (GRO) nanocomposite that showed a much higher electrocatalytic activity for the regeneration of Co3+/Co2+ redox couples. Combining with an organic dye (FNE29), the DSCs using TaON/RGO CE yielded a PCE of 7.65%, comparable to the Pt CE based DSCs (7.91%).144 Moreover, TaO/MC, TaC/MC, FeC3/MC, and TiC/graphene/PEDOT:PSS composites have also been introduced into DSCs as CE catalysts, and all of the composites showed decent catalytic activity.145−147 Ouyang et al. used SWCNTs/GRO as a composite CE catalyst for iodide electrolyte based DSCs which gave a PCE of 8.37%, higher than the Pt CE based DSCs (7.79%).148 Significantly, the SWCNT/GRO-based DSCs showed a high Voc of 0.86 V, much higher than the Pt-based DSCs (0.77 V). The high Voc value stemmed from the decrease in the overpotential for the I3− reduction on the SWCNT/GRO electrode. With the electropolymerized technique, Kang et al. made a flexible CE using a PEDOT/Exfoliated graphite (EFG) composite for solid-state DSCs.149 The PEDOT/EFG CE based DSCs showed a PCE of 5.7%, higher than conventional Pt CE based DSCs (4.4%). Moreover, the PEDOT:PSS/PPy composite film on FTO substrate was prepared for CE in DSCs with a facile electrochemical polymerization method. The PCE of the DSCs using the PEDOT:PSS/PPy CE reached 7.60%, comparable to that of the DSCs using sputtered Pt CEs.150 Deposited Pt particles on vertically ordered silicon nanowires (SiNWs) can form a Pt/SiNW composite CE for monolithic DSCs. After optimization, a PCE of 8.30% was achieved from this DSCs, better than the device using Pt CEs (7.67%).152 Yang et al. prepared a composite of Pt nanoparticles and carbon nanotubes (Pt/MWCNTs) with a sulfur-assisted strategy. The DSCs using Pt/MWCNT CEs showed a PCE of 7.69%, while the corresponding PCE value of the DSCs with Pt CE was 6.31%.151 Wu et al. prepared TiC/Pt, WO2/Pt, and VN/Pt binary composite CEs for DSCs, and high PCE values of 7.63, 6.94, and 6.80% were obtained, respectively. Moreover, the TiC/Pt was used as a CE in large-scale DSC modules (Figure 6), producing a PCE of 4.94%.153 The binary composite of WO2/Pt further formed a ternary composite of TiO2/WO2/Pt, which showed a higher catalytic activity, and the DSCs using the ternary CE yielded a PCE of 7.23%.154 The photovoltaic performance of the DSCs using composite CEs is summarized in Table 6. As mentioned above, so many composites can be used as CE catalysts in DSCs, and all of the composite catalysts work better than their components. We still do not know the fundamental reasons for the higher catalytic activity of the composite, and it is necessary to figure out the role of each part of the composite in future research.

our previous review, we summarized several composites used in DSCs as CE catalysts, such as WC/OMC, MoC/OMC, TiN/ CNTs, Pt/Carbon, etc.37 In this review, we will give a fresh summary of the composite CE catalysts. Carbon material was the most common component to form composite catalysts. We synthesized tungsten dioxide imbedded in mesoporous carbon (WO2/MC) via an in situ method for DSCs as CE catalyst. The iodide electrolyte based DSCs using WO2/MC as CE showed a PCE of 7.76%, surpassing the performance of the DSCs using traditional Pt CE (7.55%). Moreover, the WO2/MC showed higher catalytic activity than Pt for the regeneration of T−/T2. The PCE values of the T−/T2 electrolyte based DSCs using WO2/MC or Pt CEs were 5.22 or 3.09%, respectively. The high catalytic activity for the composite catalyst can be attributed to the incorporation of the high electrical conductivity and catalytic activity.106 Moreover, VC/MC was also synthesized as shown in Figure 5. The DSCs using VC/MC CE yielded PCE values of 7.73 and

Figure 5. SEM image of the prepared VC/MC.94

5.15%, when using iodide and sulfide redox couple based electrolytes.94 Both WO2/MC and VC/MC showed higher catalytic activity than Pt, and this merit was magnified in iodidefree electrolyte. Han et al. synthesized a class of M/PPy/C composite catalysts (M = Co, Fe, and Ni) for the CE of DSCs.137 The DSCs with M/PPy/C CEs exhibited PCE values of 7.64 (Co/PPy/C), 7.44 (Ni/PPy/C), and 5.07% (Fe/PPy/ C), while the DSCs using bare carbon CE gave a PCE of 6.26%. The internal resistances of the devices based on Ni/PPy/C and Co/PPy/C were dramatically reduced because of the high activity of the Ni−N2 and Co−N2 sites through the entrapment of the metal atoms in the PPy matrix, and this was the major reason for the high catalytic activity of Ni/PPy/C and Co/PPy/ C. Graphene/MoS2 was employed as a CE catalyst for DSCs. After optimization, the DSCs assembled with the MoS2/ graphene CE exhibited a PCE of 6.07%, up to 95% of the level obtained by a conventional Pt CE (6.41%).138 Meanwhile, Lin et al. prepared transparent MoS2/graphene CEs in DSCs which produced a PCE of 5.81%, up to 93% of that of Pt CE based DSCs (6.24%).139 Huang et al. prepared NiS/graphene and CoS/graphene composites by directly decomposing precursors of Ni(C3H5OS2)2 and Co(C3H5OS2)2 on highly conductive graphene film substrates. The composites were employed as CEs in DSCs, which showed PCE values of 5.25 (NiS/ graphene) and 5.04% (CoS/graphene), higher than that of Pt

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SUMMARY AND OUTLOOKS Development of Pt-free CE catalysts can make DSCs more competitive among various photovoltaic devices. First, Pt-free J



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CE catalysts: (1) use of a commercial substrate to make CEs; (2) develop multiple compounds and composite catalysts; (3) explore a low-temperature method to fabricate flexible CEs; (4) apply Pt-free catalysts in large-scale DSC modules; (5) carry out a long-term stable test for Pt-free catalysts in harsh conditions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No. 21303039, 51273032), the National High Technology Research and Development Program for Advanced Materials of China (grant 2009AA03Z220), the Natural Science Foundation of Hebei Province (Grant No. B2013205171), and the Support Program for Hundred Excellent Innovation Talents from the Universities of Hebei Province, BR2-220).

Figure 6. Photographs of the large-scale DSCs using Pt and TiC/Pt CEs.153

CEs can reduce the cost of DSCs; second, it is found that Ptfree CEs are more suitable for the regeneration of iodide-free redox couples, which in turn are beneficial for achieving high efficiency. In a word, the CE catalysts developed rapidly in recent years, and the varieties of the catalysts are broadened from the expensive Pt to low cost catalysts, such as carbon materials, organic polymers, metal, carbides, nitrides, oxides, sulfides, phosphides, tellurides, selenides, multiple compounds, and composites. There are several developmental directions for

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REFERENCES

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Table 6. Photovoltaic Performance of the DSCs Using Various Composite CEs CE catalysts WO2/MC WO2/MC VC/MC VC/MC Fe/PPy/C Co/PPy/C Ni/PPy/C MoS2/Graphene CoS/Graphene NiS/Graphene WS2/MWCNTs MoS2/MWCNTs CuInS2/ZnS TaON/Graphene TaO/MC TaC/MC FeC3/C TiC/Graphene/PEDOT:PSS SWCNTs/GRO PEDOT/graphite PEDOT:PSS/PPy Pt/SiNWs Pt/MWCNTs TiC/Pt WO2/Pt TiO2/WO2/Pt VN/Pt a

substrate

redox couples

dye

FF

PCE/%

PCE (Pt)/%

ref

FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass Graphene Graphene FTO glass FTO glass FTO glass FTO glass FTO glass FTO glass TCO glass ITO/PET FTO glass graphite FTO glass Si FTO glass FTO glass FTO glass FTO glass FTO glass

I−/I3− T−/T2 I−/I3− T−/T2 I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− Co3+/ Co2+ I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3− I−/I3−

N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 FNE29 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719 N-719

0.71 0.63 0.72 0.63 0.57 0.75 0.73 0.61 0.693 0.704 0.65 0.65 0.64 0.69 0.68 0.68 0.63 n/a 0.76 0.72 0.71 0.66 0.63 0.64 0.68 0.70 0.69

7.76 5.22 7.63 5.15 5.07 7.64 7.44 6.07 5.04 5.25 6.41 6.41 7.5 7.65 8.09 7.93 6.04 4.5 8.37 5.78 7.60 8.30 7.69 7.63 6.94 7.23 6.80

7.55 3.09 7.50 3.66 6.26a 6.26a 6.26a 6.41 5.00 5.00 6.56 6.45 7.1 7.91 7.32 7.32 6.40 4.3 7.79 4.49 7.73 7.67 6.31 7.16 7.03 7.03 7.03

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The PCE value of 6.26% was obtained from a carbon CE based reference DSCs. K



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