Toward highly efficient CdS/CdSe quantum dot-sensitized solar cells incorporating a fullerene hybrid-nanostructure counter electrode on transparent conductive substrates

Abstract

At present, the performance of quantum dot-sensitized solar cells (QDSCs) is still much lower than that of conventional dye-sensitized solar cells. Besides efforts on the development of photoanodes, a new generation of materials for counter electrodes is also urgently needed to optimize the structure of QDSCs and improve their power conversion efficiency (PCE). Here, fullerene (C₆₀) films on transparent conductive substrates have been prepared through a cost-effective doctor blade process, which were subsequently applied as counter electrodes in QDSCs. Compared to conventional counter electrodes composed of Pt or activated carbon (AC) electrodes, the C₆₀-based counter electrodes exhibit better electrocatalytic activity, lower charge-transfer resistance and higher exchange current density. Based on these merits, the QDSC with C₆₀-based counter electrode and a polysulfide electrolyte shows an improved PCE of 4.18% under simulated 100 mW cm⁻² AM 1.5 illumination. In contrast, the PCEs of the QDSC using Pt or activated carbon as counter electrode under the same conditions are only 1.17% and 3.48%, respectively. Stability test reveals that the C₆₀-based counter electrode has good stability at room temperature.

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1. Introduction

Quantum dot-sensitized solar cells (QDSCs) represent a type of next-generation solar cells and have attracted much attention because of their low cost, easy fabrication and high theoretical power conversion efficiency (PCE).^1–6 The unique characteristics of quantum dots (QDs) over traditional dyes, such as Ru–polypyridine complexes and organic dyes, are their strong photo-response in the visible region, tunable band gaps simply by controlling their sizes and components. Moreover, the utilization of multiple exciton generation of QDs is expected to obtain much higher PCEs.^7–10 However, up to now, the performance of reported QDSCs is still much lower than conventional dye-sensitized solar cells (DSCs).^11–13 Thus, the development of new generation electrode materials is urgently needed to further improve the PCE of QDSCs.

The counter electrode is an important component in QDSCs, which collects electrons from the external circuit and reduces the redox shuttles in the electrolyte. In DSCs, Pt is commonly used as counter electrode. However, it is proven to be unsuitable toward polysulfide (S²⁻/S₂²⁻), which is a popular electrolyte in QDSCs,¹⁴ due to strong interaction between Pt and sulfide ions that remarkably influences the conductivity and catalytic activity of the electrode.^15–17 Alternatively, various carbon materials with good corrosion inertness toward polysulfide redox couple and larger specific surface area with porous structure, have been applied in QDSCs.^18–21 Meng et al. introduced activated carbon into QDSC as counter electrode and demonstrated that activated carbon exhibits better catalytic property against Pt toward polysulfide electrolyte.²² Fan et al. explored CdSe QDSC based on hierarchical nanostructured spherical carbon electrode with hollow core/mesoporous shell, showing 3.90% efficiency.²³ Poly(3,4-ethylenedioxythiophene) electrode (PEDOT) was prepared by electropolymerization used in CdS QDSC with a PCE of 1.56%.²⁴ Although these pioneering works exhibited considerable improvement in the photovoltaic performance of QDSCs, some issues, i.e. large internal resistance, relatively poor stability and complicated fabrication process, are still unsolved. Therefore, further investigation of counter electrodes with high catalytic activity and satisfactory stability is desirable.

Recently, fullerene (C₆₀) and its derivatives, rising stars in the carbon family, have been employed in photoanodes of QDs solar cells due to their excellent electron accepting ability and suitable conduction band level position.^25–27 However, few works have been reported to investigate DSCs or QDSCs with fullerene counter electrodes systematically.^28,29 Kuramoto et al. reported that C₆₀ and the derivatives was prepared on transparent conductive substrate by spin-coating method and applied in DSCs. The efficiency is far unsatisfactory.²⁸ Wu et al. employed Pt/C₆₀ counter electrode in QDSCs with a PCE of 2%, in which authors focused on the fabrication process of photoanode with less counter-electrode characterization.²⁹ Herein, we prepared C₆₀ films on transparent conductive substrates via a cost-effective doctor blade process, which were then utilized as counter electrodes in QDSCs. Further research demonstrates that the C₆₀-based counter electrode has hybrid nanostructure with relatively high surface area and large inner pores, which facilitates electrolyte infiltration and electron transportation. Compared to the counter electrodes based on Pt and activated carbon, the C₆₀-based counter electrode exhibits better electrocatalytic activity, lower charge-transfer resistance, and higher exchange current density, which finally leads to an improved PCE of 4.18% with a polysulfide electrolyte under simulated 100 mW cm⁻² AM 1.5 illumination. In addition, stability test reveals that our cell can be stable for more than 14 days at room temperature.

2. Experimental section

2.1 Chemicals and materials

Sodium nitrilotriacetate (N(CH₂COONa)₃, AR) and selenium powders were purchased from Aldrich. NH₄Cl, Na₂SO₃, CdCl₂, thiourea, ammonia, sulfur, titanium isopropoxide were obtained from Sinopharm Chemical Reagent Co. Ltd. Ethyl cellulose (EC), terpineol, P25, 200 nm-sized anatase TiO₂ nanoparticles were from Wuhan Jingge Solar Co. Ltd. Fullerene C₆₀ and activated carbon were supplied by Suzhou Dade Carbon Nanotechnology Co. Ltd. and Cabot Corporation, respectively. Other chemicals including Na₂S, CdSO₄ and Zn(CH₃COO)₂ were from local suppliers with the highest purities. All the chemicals were used without further purification. Solutions for electrolyte and QDs deposition were prepared using high-purity water obtained from a water purification system (Ulupure Instrument Co. Ltd). The electrode substrate was fluorine-doped tin oxide conducting glass (FTO, thickness: 2.2 mm, Nippon, sheet resistance 14 Ω per square).

2.2 Preparation of TiO₂ film

Two kinds of TiO₂ pastes used for doctor blade were prepared by using P25 and 200 nm TiO₂ particles, respectively.³⁰ A 7 μm-thick transparent TiO₂ layer was first printed on FTO glass using P25 paste, which was further coated by a 5 μm-thick scattering layer with 200 nm TiO₂ paste (see Fig. S1a†). After leveled for 15 min, the samples were heated at 80 °C for 30 min and then sintered at 450 °C for 30 min. Finally, the samples were further treated in 40 mM TiCl₄ solution, rinsed with water, dried under air flow, and heated to 500 °C in air for 30 min.

2.3 Deposition of QDs onto TiO₂ films

Chemical bath deposition (CBD) was employed to successively assemble CdS and CdSe onto TiO₂ films.³¹ The deposition was performed at 10 °C. CdS was in situ grown for about 50 min in an aqueous solution containing 20 mM CdCl₂, 66 mM NH₄Cl, 140 mM thiourea and 230 mM ammonia. After thoroughly washed with deionized water, CdSe was deposited by mixing an aqueous solution of 26 mM CdSO₄, 40 mM N(CH₂COONa)₃ and 26 mM Na₂SeSO₃ for 4.5 h. Finally, CdS/CdSe co-sensitized photoanodes were passivated with ZnS by alternately dipping into 100 mM Zn(CH₃COO)₂ and Na₂S aqueous solution. The passivation was repeated twice and the duration for each dipping is 1 min.

2.4 Formation of counter electrodes

C₆₀ paste was obtained by dispersing C₆₀ powders in terpineol with EC and titanium isopropoxide as binders.³² After ball milling at 450 rpm for 5 h, the mixture was transferred onto FTO glass with doctor blade method, followed by sintering at 400 °C for 20 min. Activated carbon (AC) electrode was fabricated by the same procedures. The average thickness of the two kinds of carbon films was about 10 μm, as shown in Fig. S1.† Pt electrode was prepared by thermal decomposition of H₂PtCl₆ (30 mM in isopropanol) on FTO glass at 385 °C for 30 min.³³

2.5 Characterizations

Scanning electron microscopy (SEM) observations were carried out on a JSM-6700F. N₂ adsorption–desorption was performed on ASAP 2020 HD88. Typical Brunauer–Emmett–Teller (BET) analysis was used to determine the average pore size of as-prepared films. The absorption and desorption data were analyzed with the BJH (Barrett, Joyner and Hanlenda) method for incremental pore volume distribution. The transmittance spectra of TiO₂ films with different CdSe deposition time were recorded by a UV-vis spectrophotometer (Shanghai Puyuan Instrument Co. Ltd.). Photocurrent density–photovoltage (J–V) measurements were carried out using a solar cell with an active area of 0.20 cm². The polysulfide electrolyte was sandwiched between the QDs-coated TiO₂ film and the counter electrode, which was separated by using a silicone spacer (thickness: 25 μm). The electrolyte contained 1 M Na₂S and 1 M S with a mixture solution of methanol and water (3/7, v/v). The cells were measured by a source meter (Keithley 2400) under an illumination of a solar simulator (SS150A, Zolix Instrument Co. Ltd.) under AM 1.5 illumination of 100 mW cm⁻². The incident-photon-to-current conversion efficiency (IPCE) was measured by DC method using an IPCE system (Solar Cell Scan 100, Zolix Instrument Co. Ltd.) without bias illumination.

Cyclic voltammogram (CV) was carried out by using a Pt wire as auxiliary electrode, a SCE electrode as reference electrode, and the FTO-supported C₆₀ film, AC or Pt as working electrode with a total exposure area of 1 cm² in a mixture of methanol and water (3/7, v/v) containing 1 M Na₂S₂. Scan rates from 10 to 40 mV s⁻¹ were used and the data were acquired with a CHI 600E electrochemical workstation. Electrochemical impedance spectra (EIS) and Tafel curve measurements were conducted in a symmetrical cell fabricated with two identical counter electrodes using a CHI 600E electrochemical analyzer in dark. The measured frequency for EIS ranged from 100 mHz to 100 kHz and the amplitude was set to 10 mV. The results were fitted by Zview software.

3. Results and discussion

The morphology of a typical C₆₀ film is shown in Fig. 1. As can be seen, the film has hybrid nanostructure of granular precipitates composing continuous layers. The grain size is in the range of 20 to 100 nm, and mesopores are uniformly distributed in the layers. The granular precipitate comes from hydrolysis of titanium isopropoxide, which improves the interconnections between C₆₀ layers as well as between C₆₀ film and FTO. The mesopores are probably associated with EC sintering. Such hybrid porous structure with high surface area is supposed to favor electron transportation.

Fig. 1 SEM image of a typical C₆₀ film (a), (b) and (c) are amplification parts of the selected areas in image a.

The mesoporous nature of the C₆₀ film was further confirmed by N₂ adsorption–desorption. For comparison, result of a typical activated carbon film is also given. As seen from Fig. 2a, C₆₀ film represents type IV isotherm, and its mesoporous nature can be judged by the hysteresis loop in the medium pressure range between 0.5 and 0.8.³⁴ The high pressure part of the hysteresis loop (0.9 < P/P₀ < 1.0) is supposed to originate from textual larger pores formed between C₆₀ layers. In comparison with AC film, C₆₀ film has a larger average pore size and a higher volume of pores. This leads to a smaller surface area of C₆₀ film (146 m² g⁻¹) compared to that of activated carbon film (234 m² g⁻¹). Despite the smaller surface area, the C₆₀ film exhibits a better electrochemical response in S²⁻/S₂²⁻ system using a Pt wire as auxiliary electrode and a SCE electrode as reference electrode, respectively. As seen from Fig. 3, one pair of redox peaks (2S²⁻ ↔ S₂²⁻ + 2e⁻) has been clearly detected for both electrodes based on C₆₀ and activated carbon. It is known that the catalytic ability of counter electrodes toward S₂²⁻ reduction in QDSCs can be evaluated by the intensity of the reduction peak.³⁵ From Fig. 3 it is obviously seen that the peak intensity of C₆₀-based electrode is higher than that of activated carbon-based one, indicating a higher electrochemical activity of C₆₀-based electrode. In contrast, under the same condition the response of Pt electrode is too weak to be recognized.

Fig. 2 N₂ adsorption–desorption isotherm (a) and pore size distribution (b) of C₆₀ and AC films.

Fig. 3 Cyclic voltammograms of different electrodes at a scan rate of 10 mV s⁻¹ in a three-electrode system with SCE as reference and methanol–water (3/7, v/v) containing 1 M Na₂S₂ as supporting electrolyte. Inset is magnified curve of Pt electrode.

Fig. 4 presents a set of CV curves of C₆₀-based electrode versus the scan rate. It is clearly seen that the intensity of the reduction peak increases and the position of the reduction peak shifts toward lower voltage with increasing scan rate from 10 to 40 mV s⁻¹. A linear relationship between the peak intensity and the square root of scan rate is found (inset of Fig. 4), indicating the redox reaction on the C₆₀-based electrode is diffusion-controlled.³⁶ The diffusion process is probably assigned to the transport of S²⁻ out of the C₆₀ film after the S₂²⁻ reduction. The relatively larger pore size and higher volume of pores towards C₆₀ electrode is benefit for the wetting of the electrode surface and the charge transportation between the electrode and the electrolyte.

Fig. 4 Cyclic voltammograms of the C₆₀-based electrode under various scan rates. Inset shows the linear relationship of the peak intensity and the square root of the scan rate.

In QDSCs, CBD is facile and straightforward and is thus one of the most popular methods to fabricate QD-sensitized photoanode. In order to find the optimized photoanode which is most suitable to match the C₆₀ counter electrode, we adjusted the CdSe deposition time from 3.5 to 6.5 h. From the UV-visible transmittance of the CdS/CdSe-coated TiO₂ electrodes at different CdSe deposition time (Fig. S2†), it is clearly seen that with increasing deposition time the transmittance edge gradually shifts to longer wavelengths, indicating that the size of QDs increases at prolonged deposition time. After assembling the photoanodes with C₆₀ counter electrodes to form QDSCs, on the best photovoltaic performance was obtained at a CdSe deposition time of 4.5 h (Table S1†). The same trend was noticed for the QDSCs with activated carbon counter electrodes. Therefore, photoanodes obtained at 4.5 h CdSe deposition time are selected for further investigation.

Fig. 5a shows the IPCE results of the QDSCs with different counter electrodes. The IPCE values increase successively for QDSCs with Pt-, AC- and C₆₀-based counter electrodes, and the values at 550 nm are 68%, 76% and 83%, respectively. Fig. 5b gives corresponding J–V curves of the three QDSCs and the derived photovoltaic parameters are summarized in Table 1, in which the trend of the results towards the three counter electrodes is repeatable and the same as those in Table S1.† The cell with C₆₀-based counter electrode exhibits the highest photovoltaic performance a short-circuit photocurrent (J_SC) of 12.6 mA cm⁻², an open-circuit photovoltage (V_OC) of 546 mV and a fill factor (FF) of 0.60. This yields a PCE (η) of 4.18%, which is better than that of the cell with activated carbon-based counter electrode (η = 3.48%) and comparable to those of reported highly efficient QDSCs using metal sulfide as counter electrodes.^37,38 Meanwhile, the performance of the cell with Pt-based counter electrode is much worse (Table 1), which is consistent with the CV and IPCE measurements.

Fig. 5 IPCE spectra (a) and J–V characteristics (b) of CdS/CdSe co-sensitized QDSCs with different counter electrodes. CdSe deposition time is 4.5 h.

Table 1 Photovoltaic performance of QDSCs with different counter electrodes

Counter electrode	JSC (mA cm^−2)	VOC (mV)	FF	η (%)
C60	12.6	546	0.60	4.18
AC	10.9	542	0.57	3.48
Pt	9.93	496	0.38	1.89

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It is known that the performance of solar cell, especially the FF, is significantly influenced by its internal resistance.^39,40 Thus, EIS measurement in a symmetrical cell was carried out to evaluate the electrochemical properties of different counter electrodes on the cell performance. The schematic structure of the symmetric thin-layer cell is shown in Fig. 6a. Fig. 6b presents typical Nyquist plots of the cells with different electrodes. Experimental curves are illustrated by symbols while the solid lines correspond to the fitted curves obtained with Zview software by using the equivalent circuit shown in the above inset of Fig. 6b. Two semicircles are obviously observed. The small one at high frequency represents the charge transfer resistance (R_CT1) and interfacial capacitance (C) at the solid–solid interfaces, while the large one reflects the charge-transfer resistance (R_CT2) and constant phase element (CPE) at the counter electrode/electrolyte interface in the mid/low frequency range.⁴¹ Similar to the reported EIS results, no obvious response can be found toward the diffusion impedance of the redox species in the electrolyte, which is assigned to short circuit in the QDSC system.^42,43 In the high frequency (over 10⁵ Hz) where the phase is zero, the ohmic series resistance (R_S) of the FTO layer, the electrode layer and the electrolyte can be determined. The fitting data using the equivalent circuit were shown in Table S2.†

Fig. 6 (a) Schematic structure of the symmetric thin-layer sandwich type cell. (b) Nyquist plots of the cells measured at zero bias potential based on different counter electrodes. Inset above shows equivalent circuit for fitting EIS, and inset below represents the abscissa expanded in the high frequency ranges. The scattered points are experimental data and the solid lines are the fitting curves. R_S: series resistance; R_CT1 and C: charge-transfer resistance and capacitance at the solid/solid interface in the high frequency ranges; R_CT2 and CPE: charge transfer resistance and constant phase element at the electrode/electrolyte interface in the low frequency ranges. (c) Tafel curves of the cells with different counter electrodes.

R_CT1 values for the three electrodes are all as low as about 2 Ω, indicating the well interconnection between the FTO glass and the films. The R_CT2 values are 84.0, 106, 338 Ω for C₆₀-, AC- and Pt-based electrodes, respectively. C₆₀-based electrode shows smaller R_CT2 value than that of AC-based one, which is mainly because the intrinsic excellent electron conductivity toward C₆₀ and the hybrid nanostructure of C₆₀ film which facilitates electron transport and reduction of charge-transfer resistance.^25–27 Meanwhile, the large R_CT2 value of Pt-based electrode demonstrates its poor catalytic activity in polysulfide electrolyte, resulting in a low FF and η in corresponding QDSC. The R_S data also reveal that the conductivity of the Pt film is distinctly influenced by the strong interaction between sulfides (S²⁻, S₂²⁻ ions) and the surface of the Pt-based electrode. Our results again demonstrate that carbon materials are superior to Pt because of its good corrosion inertness toward polysulfide electrolyte.¹⁹

To further understand the electrocatalytic activity of the different counter electrodes, Tafel curve analysis was carried out in the same symmetric cells, as seen in Fig. 6c. The C₆₀-based electrode shows higher exchange current density (J₀ = RT/nFR_CT) toward the polysulfide redox species reduction than AC- and Pt-based electrodes.^22,44 The C₆₀-based electrode has the best electrocatalytic activity characterized by the highest J₀ and the lowest R_CT, in well accordance with the CV and EIS results.

Besides PCE, another important aspect of QDSCs is their long-term stability. When a symmetric cell with C₆₀-based counter electrode and CdS/CdSe coated photoanode was stored at room temperature in the dark, no obvious decrease of both η and FF could be observed over 14 days (Fig. 7), revealing the good stability of the cell. This is crucial when put these cells in practical applications. Upon further optimizing the nanostructure of the C₆₀ film in the counter electrode as well as the QDs-sensitized TiO₂ photoanode, the performance of the QDSCs could be further improved. Efforts towards this direction is currently underway in our lab and will be reported in near future.

Fig. 7 Normalized PCE and FF of QDSCs fabricated with C₆₀ counter electrode versus conservation time.

4. Conclusions

In summary, electrodes based on fullerene C₆₀ have been successfully prepared through a simple doctor blade process. The C₆₀-based electrodes have relatively high surface area, large inner pores and hybrid nanostructure, which facilitate electrolyte infiltration and electron transportation. Compared to Pt- and traditional activated carbon-based electrodes, C₆₀ electrodes exhibit better electrocatalytic activity, lower charge-transfer resistance, and higher exchange current density. When assembled into QDSCs together with QDs-sensitized photoanodes and polysulfide electrolyte, the cells show good performance with η up to 4.18%, outperforming the identical cells with Pt- (1.17%) and activated carbon-based electrodes (3.48%). Preliminary stability test shows that the cell can be stable for more than 14 days at room temperature in the dark, indicating that the C₆₀-based electrodes are stable with polysulfide electrolyte. Our research may pave the way for optimizing the performance of QDSCs by development of nanocarbon-based counter electrodes.

Acknowledgements

This work was financially supported by the Hundred Talents Program of Chinese Academy of Sciences (Y20245YBR1) and National Natural Science Foundation of China (no. 21402215, no. 61474124).

References

1. J.-H. Bang and P. V. Kamat, ACS Nano, 2009, 3, 1467–1476.
2. Y.-L. Lee and Y.-S. Lo, Adv. Funct. Mater., 2009, 19, 604–609.
3. S. Giménez, I. Mora-Seró, L. Macor, N. Guijarro, T. Lana-Villarreal, R. Gómez, L. J. Diguna, Q. Shen, T. Toyoda and J. Bisquert, Nanotechnology, 2009, 20, 295204.
4. Q. X. Zhang, X. Z. Guo, X. M. Huang, S. Q. Huang, D. M. Li, Y. H. Luo, Q. Shen, T. Toyoda and Q. B. Meng, Phys. Chem. Chem. Phys., 2011, 13, 4659–4667.
5. Z. S. Yang, C. Y. Chen, C. W. Liu and H. T. Chang, Chem. Commun., 2010, 46, 5485–5487.
6. Z. Tachan, M. Shalom, I. Hod, S. Rühle, S. Tirosh and A. Zaban, J. Phys. Chem. C, 2011, 115, 6162–6166.
7. C. Shen, L. D. Sun, Z. Y. Koh and Q. Wang, J. Mater. Chem. A, 2014, 2, 2807–2813.
8. K. Zhao, H. J. Yu, H. Zhang and X. H. Zhong, J. Phys. Chem. C, 2014, 118, 5683–5690.
9. H. N. Chen, L. Q. Zhu, H. C. Liu and W. P. Li, J. Phys. Chem. C, 2013, 117, 3739–3746.
10. L. L. Li, P. N. Zhu, S. J. Peng, M. Srinivasan, Q. Y. Yan, A. S. Nair, B. Liu and S. Samakrishna, J. Phys. Chem. C, 2014, 118, 16526–16535.
11. I. Mora-Seró, S. Giménez, F. Fabregat-Santiago, R. Gómez, Q. Shen, T. Toyoda and J. Bisquert, Acc. Chem. Res., 2009, 42, 1848–1857.
12. C. Ratanatawanate, C. Xiong and K. J. Balkus Jr, ACS Nano, 2008, 2, 1682–1688.
13. H. Chen, L. Zhu, H. Liu and W. Li, J. Power Sources, 2014, 245, 406–410.
14. X. W. Zeng, W. J. Zhang, Y. Xie, D. H. Xiong, W. Chen, X. B. Xu, M. K. Wang and Y. B. Cheng, J. Power Sources, 2013, 226, 359–362.
15. G. Zhu, L. K. Pan, H. C. Sun, X. J. Liu, T. Lv, T. Lu, J. Yang and Z. Sun, ChemPhysChem, 2012, 13, 769–773.
16. X. L. Zhang, X. M. Huang, Y. Y. Yang, S. Wang, Y. Gong, Y. H. Luo, D. M. Li and Q. B. Meng, ACS Appl. Mater. Interfaces, 2013, 5, 5954–5960.
17. S. Peng, L. Tian, J. Liang, S. G. Mhaisalkar and S. Ramakrishna, ACS Appl. Mater. Interfaces, 2012, 4, 397–404.
18. P. K. Santra and P. V. Kamat, J. Am. Chem. Soc., 2012, 134, 2508–2511.
19. G. Hodes, J. Manassen and D. Cahen, J. Electrochem. Soc., 1980, 127, 544–549.
20. D. Cahen, G. Dagan, Y. Mirovsky, G. Hodes, W. Giriat and M. Lubke, J. Electrochem. Soc., 1985, 132, 1062–1070.
21. Y. Mirovsky, R. Tenne, D. Cahen, G. Sawatzky and M. Polak, J. Electrochem. Soc., 1985, 132, 1070–1076.
22. Q. Zhang, Y. Zhang, S. Huang, X. Huang, Y. Luo, Q. Meng and D. Li, Electrochem. Commun., 2010, 12, 327–330.
23. S. Q. Fan, B. Fang, J. H. Kim, J. J. Kim, J. S. Yu and J. Ko, Appl. Phys. Lett., 2010, 96, 063501.
24. T. Shu and Z. L. Ku, J. Alloys Compd., 2014, 586, 257–260.
25. J. H. Bang and P. V. Kamat, ACS Nano, 2011, 5, 9421–9427.
26. S. Sarkar, B. Rajbanshi and P. Sarkar, J. Appl. Phys., 2014, 116, 114303.
27. P. Brown and P. V. Kamat, J. Am. Chem. Soc., 2008, 130, 8890–8891.
28. N. Kuramoto, T. Hino and Y. Ogawa, Carbon, 2006, 44, 880–887.
29. G. T. Yue, J. H. Wu, Y. M. Xiao, J. M. Lin, M. L. Huang, L. Q. Fan and Z. Lan, Sci. China: Chem., 2013, 56, 93–100.
30. S. Ito, P. Chen, P. Comte, M. K. Nazeeruddin, P. Liska, P. Péchy and M. Grätzel, Progress in Photovoltaics: Research and Applications, 2007, 15, 603–612.
31. O. Niitsoo, S. K. Sarkar, C. Pejoux, S. Rühle, D. Cahen and G. Hodes, J. Photochem. Photobiol., A, 2006, 181, 306–313.
32. Z. Huang, X. Liu, K. Li, D. Li, Y. Luo, H. Li, W. Song, L. Chen and Q. Meng, Electrochem. Commun., 2007, 9, 596–598.
33. G. Q. Wang, R. F. Lin, Y. Lin, X. P. Li, X. W. Zhou and X. R. Xiao, Electrochim. Acta, 2005, 50, 5546–5552.
34. M. Kruk and M. Jaroniec, Chem. Mater., 2001, 13, 3169–3183.
35. M. D. Ye, C. Chen, N. Zhang, X. R. Wen, W. X. Guo and C. J. Lin, Adv. Energy Mater., 2014, 4, 1301564.
36. H. Sun, Y. Luo, Y. Zhang, D. Li, Z. Yu, K. Li and Q. Meng, J. Phys. Chem. C, 2010, 114, 11673–11679.
37. H. J. Kim, S. W. Kim, C. Gopi, S. K. Kim, S. S. Rao and M. S. Jeong, J. Power Sources, 2014, 268, 163–170.
38. F. F. Wang, H. Dong, J. L. Pan, J. J. Li, Q. Li and D. S. Xu, J. Phys. Chem. C, 2014, 118, 19589–19598.
39. N. Papageorgiou, Coord. Chem. Rev., 2004, 248, 1421–1446.
40. A. Hauch and A. Georg, Electrochim. Acta, 2001, 46, 3457–3466.
41. Y. Yang, L. Zhu, H. Sun, X. Huang, Y. Luo, D. Li and Q. Meng, ACS Appl. Mater. Interfaces, 2012, 4, 6162–6168.
42. V. González-Pedro, X. Q. Xu, I. Mora-Seró and J. Bisquert, ACS Nano, 2010, 4, 5783–5790.
43. Q. Zhang, G. Chen, Y. Yang, X. Shen, Y. Zhang, C. Li, R. Yu, Y. Luo, D. Li and Q. Meng, Phys. Chem. Chem. Phys., 2012, 14, 6479–6486.
44. J. Dong, S. Jia, J. Chen, B. Li, J. Zheng, J. Zhao, Z. Wang and Z. Zhu, J. Mater. Chem., 2012, 22, 9745–9750.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of TiO₂, C₆₀ and activated carbon films on FTO, transmittance of photoanodes, photovoltaic performances of the QDSCs with varying CdSe deposition time, EIS fitting data and BET analysis. See DOI: 10.1039/c5ra02091c

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