ITO Porous Film-Supported Metal Sulfide Counter Electrodes for High

Jan 23, 2013 - porous film-supported metal sulfide CEs exhibited much higher catalytic activity for polysulfide electrolyte than ITO glass-supported m...
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ITO Porous Film-Supported Metal Sulfide Counter Electrodes for High-Performance Quantum-Dot-Sensitized Solar Cells Haining Chen, Liqun Zhu, Huicong Liu, and Weiping Li* Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China ABSTRACT: In this paper, ITO porous films were prepared by the doctor-blade technique to support metal sulfide (CuS, CoS, NiS, and PbS) counter electrodes (CEs) in quantum-dotsensitized solar cells (QDSCs). The successive ionic layer adsorption and reaction (SILAR) method was used to deposit metal sulfides on the ITO porous films. Since the ITO porous films have high mechanical properties and could offer a large surface area for the large deposition of metal sulfides, the ITO porous film-supported metal sulfide CEs exhibited much higher catalytic activity for polysulfide electrolyte than ITO glass-supported metal sulfide CEs. As a result, the photoeletrochemical performance of QDSCs was greatly improved. In addition, ITO porous film-supported CuS CEs at 12 SILAR cycles exhibited the highest catalytic activity and performance among different CEs, and ITO porous film-supported CoS CEs achieved the second highest catalytic activity and performance, still far higher than the Pt CE, while both ITO porous filmsupported NiS and PbS CEs showed similar catalytic activity and performance, significantly lower than that of Pt CE. It is also suggested that many more CE materials can be easily explored and investigated by employing ITO porous films as substrates. disintegration of the film.19 To solve this problem, some deposition methods, such as electrodeposition21 and successive ionic layer adsorption and reaction (SILAR),11,25 have been applied to deposit metal sulfides on ITO or FTO glass. Though ITO and FTO are stable in polysulfide electrolyte, the adhesion of metal sulfide on ITO and FTO glass is poor, leading to low coverage, thin film thickness, and hence low catalytic activity of CEs. As discussed above, ITO or FTO glass is a promising substrate for metal sulfide CEs, but the deposition amount and the mechanical properties of metal sulfide CEs on ITO or FTO glass are needed to be increased to improve the catalytic activity of CEs and the performance of QDSCs. Herein, we have settled this issue by employing ITO porous films to support metal sulfides (CuS, CoS, NiS, and PbS), and the SILAR method was used to deposit metal sulfides on the ITO porous films. Scheme 1 illustrates the structure of the QDSCs based on ITO porous film-supported metal sulfide CEs. Since the ITO porous films prepared by the doctor-blade technique had proper mechanical properties and could offer a large surface area for the large deposition of metal sulfides, the ITO porous film-supported metal sulfide CEs exhibited much higher catalytic activity and photoeletrochemical performance in QDSCs than ITO glasssupported metal sulfide CEs.

1. INTRODUCTION In the past few years, quantum-dot-sensitized solar cells (QDSCs) as one of the third-generation solar cells have attracted more and more attention, because the fabrication of QDSCs is simple and low-cost, and quantum dots (QDs) can adjust the band gap to tailor optical absorption over a wide wavelength range and is expected to exploit multiple exciton generation to greatly improve conversion efficiency in the future.1−8 However, the reported conversion efficiencies of QDSCs (typically below 5%9−15) are still far below their theoretical value (44%3,5,11) and that of dye-sensitized solar cells (DSSCs) (11%16−18). Poor charge transfer to the oxidized redox polysulfide species (Sn2−) on counter electrodes (CEs) is considered to be a major hurdle in attaining a high fill factor and conversion efficiency in QDSCs.11,19,20 Pt has exhibited excellent catalytic activity for the reduction of I3− in DSSCs, but very poor for the reduction of Sn2− in QDSCs.5,9−11,19,21,22 The QDSCs based on Pt CE usually achieved a very low fill factor and conversion efficiency.9,11,20,23 Therefore, other effective CE materials are needed to improve the performance of QDSCs. Until now, some other effective CE materials for polysulfide electrolyte have been studied. The best result has been achieved by metal sulfides, especially CuS (or Cu2S) and CoS.11,21 However, the studies on the preparation methods of metal sulfide CEs are still obviously inadequate. The most simple preparation method is to directly immerse the metal foils of Cu, Co, or Pb into sulfide solution to obtain an interfacial layer of metal sulfides.19,24 The problem of this method is that some metal substrates may suffer from continual corrosion and mechanical instability, which would result in ultimate © XXXX American Chemical Society

Received: October 9, 2012 Revised: January 15, 2013

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with DI water, and dried with the dryer. They were then immersed into the aqueous solution of 0.5 M Na2S for 5 min, followed by rinsing with DI water and drying with the dryer.11 2.2. Characterizations. The surface morphology and elemental compositions of ZnO nanostructures were characterized by using a field emission scanning electron microscope (FESEM Apollo 300) equipped with an on-system energydispersive X-ray (EDX) analysis system (EDAX Oxford 7788). A transmission electron microscope (TEM, JEM-2100F) was used to evaluate transmission electron microscopy images of the samples. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB diffractometer with monochromatized Cu Ka radiation (k = 1.5418). Current−voltage curves of different CEs were measured in a three-electrode system with a saturated calomel electrode (SCE) and a Pt foil as reference electrode and counter electrode, respectively. The aqueous solution containing 2 M Na2S and 3 M S29 was used as electrolyte, which was also used as the electrolyte of QDSCs. A CHI 600A electrochemical analyzer was used to record current−voltage curves, and the scaning rate was set at 10 mV/s. For I−V measurement, a two-electrode photoelectrochemical cell was constructed, and the electrolyte was the aqueous solution containing 2 M Na2S and 3 M S.29 A xenon lamp (500 W) with the illumination intensity of ∼100 mW·cm−2 and the wavelength range of 380−700 nm was used as the light source. The CHI 600A electrochemical analyzer was employed to record I−V curves under illumination with an active area of 0.25 cm2. EIS experiments were conducted in a symmetrical dummy cell fabricated with two identical CEs using a CHI 600A electrochemical analyzer in the dark. The measured frequency ranged from 10 mHz to 1 MHz, and the amplitude was set at 10 mV. The spectra were fitted by the Zview software. Tafel polarization measurements were also carried out on the CHI 600A electrochemical analyzer in the symmetrical dummy cell with a scan rate of 10 mV·s−1.

Scheme 1. Structure of the QDSCs Based on ITO Porous Film-Supported Metal Sulfide CEs

2. EXPERIMENTAL DETAILS 2.1. Preparation. Preparation of ITO Porous Films. ITO nanoparticles were synthesized by adding 2 M NH4·OH into the solution of 25.5 g/L InCl3·4H2O, 3.1 g/L SnCl4·5H2O, and 3.3 g/L (NH4)2SO4 to adjust the pH to 7 at 60 °C while stirring vigorously. The indium tin hydroxide suspension was then aged at 60 °C for 2 h to get the complete ITO precipitation. The resulting ITO precipitation was washed with deionized (DI) water and then ethanol and dried at 100 °C for 3 h.26,27 For preparing ITO porous films, ITO nanoparticles were mixed with DI water and some surfactants (acetylactone (0.4 mL/1 g ITO), octylphenol ether (0.2 mL/1 g ITO), and polyethylene glycol (0.5 g/1 g ITO)) by grinding until a slurry was formed, and the doctor-blade technique was used to prepare ITO porous films on cleaned ITO glass, followed by sintering at 450 °C for 30 min in an air atmosphere.26−28 Preparation of Metal Sulfide CEs. The SILAR method was applied to deposit metal sulfides (CuS, CoS, NiS, and PbS) on ITO porous films to serve as CEs. First, ITO porous films were dipped into the aqueous solution containing the corresponding metal nitrate (0.1 M) for 1 min and then rinsed with DI water. They were then dipped into the aqueous solution containing 0.1 M Na2S for 1 min, followed by rinsing with DI water. The process was repeated 1−12 times. For comparison, these metal sulfides were also deposited on ITO glass by the same processes. For “X” SILAR cycles of CuS, the CuS CEs were named as ITO porous film-supported CuS(X) CE or ITO glass-supported CuS(X) CE. Since the SILAR cycles of CoS, NiS, and PbS were all 12, SILAR cycles would not be mentioned below. For comparison, Pt foil was used as the Pt CE. Preparation of Working Electrodes. TiO2 porous films with the thickness of about 11−12 μm were prepared by the doctorblade technique similar to ITO porous films using P25 TiO2 nanoparticles.28 For the deposition of CdS QDs, TiO2 porous films were immersed into the ethanol solution of 0.5 M Cd(NO3)2 for 5 min, rinsed with DI water, and dried with a dryer. They were then immersed into the aqueous solution of 0.5 M Na2S for 5 min, rinsed with DI water, and dried with the dryer. The process was conducted at room temperature and repeated three times. For the deposition of CdSe QDs, the TiO2/CdS electrodes were immersed into the ethanol solution of 0.5 M Cd(NO3)2 for 5 min at room temperature, followed by rinsing with ethanol and then immersing into an aqueous solution of Na2SeSO3 for 1 h at 50 °C, followed by rinsing with DI water and drying with the dryer. The process was repeated two times. Finally, TiO2/CdS/CdSe electrodes were immersed into the ethanol solution of 0.5 M Zn(NO3)2 for 5 min, rinsed

3. RESULTS AND DISCUSSION 3.1. Evaluation of ITO Porous Film. ITO porous film prepared by the doctor-blade technique was characterized by SEM, TEM, XRD, and EDS. The SEM image in Figure 1A shows that the ITO porous film exhibits a highly porous structure on the surface, which can offer a large surface area for the deposition of metal sulfide. The cross-sectional SEM image in the inset of Figure 1A indicates that the film thickness is about 5 μm and the film is highly porous along with the thickness direction. The ITO porous film has also been evaluated using TEM by scratching off the ITO porous film from ITO glass. It can easily be determined in Figure 1B that the ITO porous film is composed of nanoparticles with the grain size ranging from 12 to 18 nm. The XRD pattern of ITO porous film in Figure 1C presents many sharp diffraction peaks. It can be readily determined that all the peaks can be indexed to cubic In2O3 (JPCDS 88-2160), and the major peaks of SnO2, at 26.5° and 35.5°, and SnO, at 33.2°, are not observed in the pattern. Since the EDS pattern in the inset of Figure 1C clearly indicates the presence of Sn element in the ITO porous film and the ratio of Sn:In is calculated to be about 1:7, it can be suggested that Sn ions are finely dispersed in the lattice of In2O3. Similar results can also be found in our previous work.26,27 B

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large deposition of CuS. Besides, the ITO porous filmsupported CuS(12) CE shows a much higher catalytic activity than the Pt CE. It should be noted that higher current densities at high overpotentials would be obtained for the Pt CE than for the ITO porous film-supported CuS(12) CE, which should be due to the reduction of H+ to H2 at the Pt CE. Compared with the results of current−voltage curves for CEs in refs 11 and 12, the current density obtained by the ITO porous film-supported CuS(12) CE is slightly higher or comparable at similar overpotentials. To evaluate the photoelectrochemical performance of ITO porous film-supported CuS as the CEs of QDSCs, we employ TiO2/CdS/CdSe/ZnS as the working electrode. Under the illumination of 100 mW/cm2, I−V curves for the ITO porous film-supported CuS CEs with the different SILAR cycles of CuS were recorded and are shown in Figure 3A. For comparison, I− V curves for Pt CE and ITO glass-supported CuS CEs were also recorded and are shown in Figure 3A,B. To make a vivid evaluation, the values of Isc, Voc, ff, and η calculated from I−V curves are plotted in Figure 4. As indicated in Figure 3A, the increase in the SILAR cycles of CuS obviously improves the performance of QDSCs. The ITO porous film-supported CuS(3) CE exhibits higher performance than the Pt CE (shortcircuit photocurrent (Isc) = 6.70 mA/cm2, open-circuit potential (Voc) = 0.37 V, fill factor (ff) = 0.22, and conversion efficiency (η) = 0.56%). This result is in contrast to the results of current−voltage curves, and the reduction of H+ to H2 at the Pt CE that would not account for photocurrent may be an important reason. It can be determined in Figure 4 that all parameters (Isc, Voc, ff, and η) show similar trends as SILAR cycles increase. For bare ITO porous film, very low performance is achieved, that is, Isc = 3.66 mA/cm2, Voc = 0.35 V, ff = 0.15, and η = 0.18%, indicating the very low catalytic activity of ITO porous film for polysulfide electrolyte, which is consistent with the result of current−voltage curves. By depositing CuS on ITO porous film using the SILAR method, the performance of QDSCs is improved, but the improvement in the first two SILAR cycles is not obvious. As SILAR cycles increase to three, the performance is obviously boosted. The Isc, Voc, ff, and η increase up to 8.66 mA/cm2, 0.41 V, 0.28, and 0.98%, respectively. Further increasing SILAR cycles from 3 to 12 can still improve the performance gradually, and the values of Isc, Voc, ff, and η for the ITO porous filmsupported CuS(12) CE are 9.38 mA/cm2, 0.42 V, 0.37 and 1.47%, respectively. It can also be implied that the performance seems to be stable when SILAR cycles are more than eight. Since the catalytic property of CuS for polysulfide electrolyte is considerably higher than that of ITO, the increase in the deposition amount of CuS could improve the charge transfer at the CE/electrolyte interface to reduce internal resistances, recombination rates, and concentration gradients in the electrolyte.19,24 As a result, Isc and ff could be improved. Besides, the reduction in electron recombination rates would also shift up the electron Fermi level of TiO2 porous film, and hence, Voc could also be increased because Voc is determined by the difference between the electron Fermi level of TiO2 porous film and the redox potential of the S2−/Sn2− couple. For ITO glass-supported CuS CEs, the increase in the SILAR cycles of CuS also improves the photoelectrochemical performance, as shown in Figure 3B. However, all ITO glass-supported CuS CEs exhibit a far lower performance than the Pt CE; even SILAR cycles increase up to 12. As shown in Figure 4, all ITO glass-supported CuS CEs show a more inferior performance to

Figure 1. (A) SEM and (B) TEM images and (C) XRD pattern of ITO porous film. Insets in (A) and (C) are the cross-sectional view and EDS spectrum of ITO porous film, respectively.

3.2. ITO Porous Film-Supported CuS CEs. Current− voltage curves (Figure 2) were first recorded to study the

Figure 2. Current−voltage curves of ITO glass-supported CuS(X), ITO porous film-supported CuS(X), and Pt CEs.

electrocatalytic activity of ITO porous film-supported CuS CEs for polysulfide electrolytes. Since only the reduction of Sn2− to S2− at CEs is the useful activity for QDSCs and the reduction of H+ to H2 will happen at high overpotentials, we only focus the attention on the negative side of the current−voltage curves at low overpotentials. It can be indicated that the increase in the SILAR cycles of CuS increases the catalytic activity of CEs regardless of ITO porous films or glass serving as substrate, and even the ITO porous film CE exhibits higher catalytic activity than the ITO glass-supported CuS(12) CE, suggesting that CuS has a considerably higher catalytic activity than ITO and ITO porous film could obviously improve the catalytic activity of CuS CEs. This is attributed to the highly porous structure of ITO porous film that could offer a much larger surface area for C

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Figure 3. I−V curves obtained by (A) ITO porous film and (B) ITO glass-supported CuS CEs with the different SILAR cycles of CuS. Besides, the I−V curve obtained by Pt CE is also presented in both (A) and (B).

Figure 4. Performance comparison between ITO porous film and ITO glass-supported CuS CEs: (A) Isc, (B) Voc, (C) η, and (D) ff.

ITO porous film-supported CuS CEs regardless of SILAR cycles. Even the bare ITO porous film CE exhibits a much higher performance than the ITO glass-supported CuS(12) CE, which agrees well with the result of current−voltage curves. Therefore, it can be further reasonably suggested that the ITO porous film exhibits much more superior advantages over ITO glass to support CuS CEs, mainly due to its highly porous structure with high mechanical properties that could offer a much larger surface area for large deposition of CuS, which would greatly improve the charge-transfer process at the CEs. EIS experiments were carried out in a symmetrical dummy cell fabricated with two identical CEs. In Figure 5A,B, the highfrequency intercept on the real axis represents the series resistance (Rs). The semicircle at middle frequency regions arises from the charge-transfer resistance (Rct) and the corresponding constant phase angle element (CPE) at the CE/electrolyte interface.19,30−32 It can be easily indicated in both panels A and B in Figure 5 that the dimension of the semicircles decreases as the SILAR cycles of CuS increase. Since Rct is inversely proportional to the electrocatalytic activity

of CEs and the dimension of the semicircles in Nyquist plots represents the value of Rct,19,30−32 it can be determined that the electrocatalytic activity increases as SILAR cycles increase regardless of ITO porous films or glass serving as the substrate, which confirms well the results of current−voltage curves and I−V curves. As shown in Figure 5A, the bare ITO porous film CE exhibits lower electrocatalytic activity for polysulfide electrolyte than the Pt CE, whereas the ITO porous filmsupported CuS(3) CE exhibits higher electrocatalytic activity than the Pt CE. However, all ITO glass-supported CuS CEs exhibit similar electrocatalytic activity, significantly lower than that of the Pt CE. By fitting the EIS results using the equivalent circuit in Figure 5C, the Rct values for different CuS CEs were obtained, as shown in Figure 5D. It can be clearly indicated that the ITO porous film-supported CuS CEs exhibit much lower Rct’s than ITO glass-supported CuS CEs at the same SILAR cycles of CuS. EIS results confirm well that the ITO porous film could offer a much larger surface area for the deposition of CuS than ITO glass, which hence increases the electrocatalytic activity of CuS CEs for the reduction of Sn2− to S2−. D

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Figure 5. Nyquist plots obtained by (A) ITO porous film and (B) ITO glass-supported CuS CEs with the different SILAR cycles of CuS. Inset in (A) is the Nyquist plot obtained by ITO porous film-supported CuS(12) CE. Besides, the Nyquist plots obtained by Pt CE are also presented in both (A) and (B). (C) Equivalent circuit used for symmetrical dummy cells: Rs, serial resistance; Rct, charge-transfer resistance of CE−electrolyte interface; CPE, constant phase element of CE−electrolyte interface; and W, Warburg impedance. (D) Charge transport resistance for ITO porous film- and ITO glass-supported CuS CEs with different SILAR cycles.

Figure 6. Tafel polarization obtained by (A) ITO porous film and (B) ITO glass-supported CuS CEs with different SILAR cycles of CuS. Besides, the Tafel polarization obtained by Pt CE is also presented in both (A) and (B).

film CEs used in DSSCs.31 Increasing the SILAR cycles of CuS would improve the catalytic activity, which results in the increase in the slope and logarithmic current densities of Tafel polarization, as shown in Figure 6A. The catalytic activity of the ITO porous film-supported CuS(3) CE is much higher than that of the Pt CE. For ITO glass-supported CuS CEs, as shown in Figure 6B, increasing the SILAR cycles of CuS also increases the logarithmic current densities on anodic and cathodic branches of the Tafel polarization due to the improvement in the catalytic activity. However, the logarithmic current densities on anodic and cathodic branches for ITO glass-supported CuS CEs are all much lower than that for the Pt CE, suggesting the far lower catalytic activity of ITO glass-supported CuS CEs than that of the Pt CE. Thus, Tafel polarization results further confirm that ITO porous film-supported CuS CEs show superior catalytic activity over ITO glass-supported CuS CEs.

To further examine the interfacial charge-transfer properties of the S2−/Sn2− couple on the electrode surface, Tafel polarization measurements were carried out in the symmetrical dummy cell similar to the one used in the EIS experiments. Figure 6 shows the logarithmic current density (log J) as a function of the voltage (V) for the oxidation/reduction of the S2−/Sn2−couple. Since the slope and the logarithmic current densities on anodic and cathodic branches for the bare ITO porous film CE are all lower than those for the Pt CE, it can be suggested that the exchange current density (J0) for the bare ITO porous film CE is lower than that for the Pt CE in terms of the Tafel equation.31,33 Therefore, the bare ITO porous film CE exhibits very low catalytic activity for polysulfide electrolyte. The bias of equilibrium potential from the location of zero potential might be caused by the absorption of Sn2− on the ITO surface of ITO porous film CEs and/or by a high scan rate. A similar phenomenon has been observed on the TiO2 porous E

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Figure 7. I−V curves of QDSCs, Nyquist plots, and Tafel polarization of the symmetrical dummy cell obtained based on different CEs. CoS CEs: (A) I−V curves, (A1) Nyquist plots, and (A2) Tafel polarization. NiS CEs: (B) I−V curves, (B1) Nyquist plots, and (B2) Tafel polarization. PbS CEs: (C) I−V curves, (C1) Nyquist plots, and (C2) Tafel polarization. The I−V curves, Nyquist plots, and Tafel polarization for Pt CE are also presented. Inset in (A1) is the Nyquist plot obtained by ITO porous film-supported CoS CE. Inset in (B1) is the zoom of (B1).

the ITO porous film-supported CoS CE. The Tafel plots for the three CEs also clearly indicate that the ITO glass-supported CoS CE exhibits the lowest catalytic activity, whereas the ITO porous film-supported CoS CE presents the highest catalytic activity. Both EIS and Tafel results agreed well with the I−V results. NiS and PbS are also the effective CE materials for polysulfide electrolyte11,21,24 and have been investigated as the CEs of QDSCs.11,24 Here, ITO porous film-supported NiS and PbS CEs were also prepared and investigated. As indicated in Figure 7B−B2,C−C2, both ITO porous film-supported NiS and PbS CEs show higher performance than ITO glasssupported NiS and PbS CEs. However, all NiS and PbS CEs show far lower performance than the Pt CE. Both EIS and Tafel results indicate that both ITO glass-supported NiS and PbS CEs exhibits far lower catalytic activity than the Pt CE, while both ITO porous film-supported NiS and PbS CEs show the highest catalytic activity. According to EIS and Tafel results, the Rct and J0 for both ITO porous film-supported NiS and PbS CEs are smaller and higher, respectively, than those for the Pt CE. It was expected that the Isc for both ITO porous filmsupported NiS and PbS CEs should be larger than that for Pt CE. However, I−V results are opposite to this expectation. This

3.3. ITO Porous Film-Supported CoS, NiS, and PbS CEs. CoS has been proved to exhibit similar catalytic activity for polysulfide electrolyte to CuS and could also be easily deposited by the SILAR method.11,21 Compared with CuS, CoS shows superior stability in polysulfide electrolyte.11,21 Therefore, ITO porous film and ITO glass-supported CoS CEs are also prepared by the SILAR method, and their photoelectrochemical performance and catalytic activity were also investigated. It can be readily determined in Figure 7A that the photoelectrochemical performance for the ITO glass-supported CoS CE (Isc = 4.39 mA/cm2, Voc = 0.37 V, ff = 0.18, and η = 0.29%) is much lower than that for the Pt CE (Isc = 6.70 mA/ cm2, Voc = 0.37 V, ff = 0.22, and η = 0.56%) due to the low surface area of ITO glass for the deposition of CoS. When ITO porous film was used to support CoS, the photoelectrochemical performance is greatly improved, that is, Isc = 9.52 mA/cm2, Voc = 0.47 V, ff = 0.32, and η = 1.41%. To further reveal the catalytic activity of ITO porous film and glass-supported CoS CEs, EIS and Tafel polarization were also recorded, as shown in Figure 7A1,A2, respectively. It can be easily implied that the Rct for the ITO glass-supported CoS CE is larger than that for the Pt CE, whereas the Rct for the ITO porous film-supported CoS CE is the smallest, suggesting the superior catalytic activity of F

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Figure 8. Performance comparison between different ITO porous film-supported metal sulfide CEs and Pt CE: (A) η, (B) ff, and (C) Rct.

12 SILAR cycles were used to make a performance comparison. At this SILAR cycle, though the deposition of CoS was still larger than that of CuS, the amount of CuS is enough to achieve the highest performance. Therefore, it is thought that, under the same deposition amount, the CuS CE would exhibit higher performance than the CoS CE. Figure 8B presents the ff comparison, showing the similar order to η, that is: CuS > CoS > Pt > NiS > PbS. The value of Rct is used to make the catalytic activity comparison between different CEs. It can be easily determined in Figure 8C that the CuS CE and Pt CE exhibit the highest and lowest catalytic activity in polysulfide electrolyte, respectively, and the CoS CE exhibits the second highest catalytic activity that is very close to CuS CE. The NiS CE and PbS CE exhibit the similar and intermediate catalytic activity. It can be clearly observed that the Pt CE exhibits lower catalytic activity than the NiS CE and PbS CE, though its photoelectrochemical performance is higher, suggesting that another reaction or reactions may happen at the NiS CE and PbS CE in addition to the redox between Sn2− and S2−.

could be simply explained that another reaction or reactions may happen in addition to the redox between Sn2− and S2−, which could decrease Rct and increase J0 but could not account for increasing photocurrent and cell performance. Very recently, Tachan et al have developed a new method to prepare the highly catalytic PbS CE on Pb foil using PbSO4 as an intermediate state.24 The PbS CE exhibited higher performance than the Pt CE. Our present work has presented the completely opposite results, which needs to be further studied in detail. The difference between ITO porous film and Pb foil substrates and quantity difference of PbS should be all considered. Besides, Pt foil is used as the Pt CE here, which may have higher performance than the CE of Pt particle-coated FTO glass used in ref 24. In Hodes et al.’s work, RuS2 is proved to be another efficient CE material that is intermediate in activity between platinized Pt and the active sulfides (Cu2S and CoS).21 However, the RuS2 film prepared by electroplating is very thin, much less than the several micrometers usually plated of the other sulfide electrodes, and the measured lesser activity is due only to a lower active surface area. Therefore, it was suggested in their paper that RuS2 will show similar catalytic activity to other sulfides. Our present work has offered an effective way, combining ITO porous film and the SILAR method, to prepare the RuS2 CEs with high catalytic activity. Of course, many other efficient CEs may be easily explored and investigated by employing ITO porous films as substrates in the future. 3.4. Performance Comparison between Different CEs. Though the photoelectrochemical performance and catalytic activity for different CEs have been calculated and discussed above, they are still listed in Figure 8 together to make a vivid performance comparison between different CEs, in which all metal sulfide CEs are supported by ITO porous films with the same SILAR cycles of 12. A conversion efficiency comparison presented in Figure 8A shows that the CuS CE exhibits the highest η and the CoS CE presents a very close η. The Pt CE achieves the third highest η, followed by the NiS CE, and the PbS CE achieves the lowest η. In ref 11, the CoS CE was reported to show higher η than the CuS CE, which is opposite to our result. This can be explained as follows. During the deposition experiment of CoS and CuS by the SILAR method, the deposition amount of CoS is observed to be more than that of CuS at the same SILAR cycles. In refs 11 and 5, SILAR cycles have been applied to deposit CoS and CuS on FTO glass. Therefore, the amount of CuS on FTO glass might be insufficient, which could result in the lower performance for the CuS CE than that for the CoS CE. This was confirmed by the lower performance of ITO glass-supported CEs in our paper. In our work, the ITO porous film-supported CoS and CuS with

4. CONCLUSION We have prepared ITO porous films by the doctor-blade technique to support metal sulfide (CuS, CoS, NiS, and PbS) CEs in QDSCs. The metal sulfides were simply deposited on the ITO porous films by the SILAR method. For CuS CEs, all ITO porous film-supported CuS CEs achieved a much higher photoelectrochemical performance and catalytic activity than ITO glass-supported CuS CEs. Increasing the SILAR cycles of CuS could increase the performance and catalytic activity of CuS CEs, and the ITO porous film-supported CuS CEs exhibited higher performance and catalytic activity than the Pt CE when the SILAR cycles are more than three. The QDSCs based on ITO porous film-supported CuS(12) CEs have achieved the highest performance (9.38 mA/cm2, 0.42 V, 0.37, and 1.47%), significantly higher than that based on the Pt CE (Isc = 6.70 mA/cm2, Voc = 0.37 V, ff = 0.22, and η = 0.56%). ITO porous film-supported CoS, NiS, and PbS CEs also exhibited much higher catalytic activity and performance than ITO glass-supported CoS, NiS, and PbS CEs. The ITO porous film-supported CoS CE achieved the catalytic activity and performance (Isc = 9.52 mA/cm2, Voc = 0.47 V, ff = 0.32, and η = 1.41%) close to that of the ITO porous film-supported CuS CE, which are also far higher than that of the Pt CE. ITO porous film-supported NiS and PbS CEs showed similar catalytic activity and performance, both significantly lower than that of the Pt CE. Furthermore, many more CE materials can be easily explored and investigated by employing ITO porous films as substrates. G

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 1082317113. Fax: +86 1082317133. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding of this research by a grant from the Innovation Foundation of BUAA for Ph.D. Graduates is gratefully acknowledged.

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