CdS/CdSe quantum dots co-sensitized solar cells with ...

Journal of Photochemistry and Photobiology A: Chemistry 271 (2013) 56–64

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CdS/CdSe quantum dots co-sensitized solar cells with Cu2 S counter electrode prepared by SILAR, spray pyrolysis and Zn–Cu alloy methods Hosein Salaramoli a,∗ , Elham Maleki b , Zahra Shariatinia b,∗ , Maryam Ranjbar a a b

Department of Chemical Industries, Iranian Research Organization for Science and Technology (IROST), P.O. Box 33535-111, Tehran, Iran Department of Chemistry, Amirkabir University of Technology (Polytechnic), P.O. Box 15875-4413, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 26 May 2013 Received in revised form 31 July 2013 Accepted 8 August 2013 Available online 18 August 2013 Keywords: Quantum dot Solar cell HR-TEM SEM SILAR Spray pyrolysis

a b s t r a c t Herein, CdS/CdSe co-sensitized TiO2 photoanodes for QDSSCs were prepared by successive ionic layer adsorption and reaction (SILAR), spray pyrolysis and zinc–copper alloy processes. The HR-TEM, SEM, EDS, XRD, UV–vis and I–V curve analyses were performed to investigate the surface and structural properties of the prepared electrodes and the efficiencies of the fabricated QDSSs. Employing different methods for preparation of Cu2 S counter electrode affected the performance of QDSSCs under one illumination of sun (100 mW/cm2 ) so that various conversion efficiencies () of 3.18, 0.341 and 0.266% were measured in alloy, SILAR and spray pyrolysis methods, respectively. Therefore, among these methods, the zinc–copper alloy process with higher efficiency is preferred that gives fill factor (ff) and short circuit density (JSC ) values of 0.44 and 11.69 mA/cm2 . The HR-TEM images showed that CdS and CdSe QDs are in close contact with TiO2 nanoparticles and the sizes of CdS and CdSe QDs are about 5 and 6 nm, respectively. The energydispersive X-ray spectroscopy (EDS) measurement confirmed that CdS and CdSe QDs are successfully deposited on the surface of the TiO2 film. The band gaps estimated from Tauc plots using UV–vis spectra vary from 3.1 eV (without CdS and CdSe, bare TiO2 ) to 2.38 eV (TiO2 /CdS (3)/CdSe). The SEM images of Cu2 S counter electrodes prepared by zinc–copper alloy indicated nanosheets with high porosity that is much suitable for injection of electrolyte while in two other approaches (SILAR and spray pyrolysis), large (∼50–70 nm) and small (∼10–17 nm) nanoparticles were observed without high porosity. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Since the original model cell reported by O’Regan and Grätzel in 1991 [1], dye-sensitized solar cells (DSSCs) have attracted a lot of academic and commercial interests due to their characteristics of low cost, surrounding friendly, and high energy conversion efficiency [2]. Pursuing high efficiency is always a core task for DSSC systems, and one key factor determining the efficiency of DSSCs is the light-harvesting property of dyes anchoring on the surface of nanocrystalline semiconductors. From the light-harvesting point of view, semiconductor quantum dots (QDs) which absorb light in visible region can also be utilized as alternatives to traditional organic dyes [3]. Quantum dot-sensitized nanocrystalline TiO2 solar cells (QDSSCs) are promising third generation photovoltaic devices. Compared with conventional photosensitizers widely employed in dye-sensitized solar cells such as ruthenium (Ru) dyes [4], QDs are easier to prepare and are of lower cost. Additionally, QDs present higher extinction coefficients than the

∗ Corresponding authors. Tel.: +98 2164542766; fax: +98 2164542762. E-mail addresses: [email protected] (H. Salaramoli), [email protected], [email protected] (Z. Shariatinia). 1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2013.08.006

conventional dyes [5], and their band gaps and, consequently, absorption spectra can be conveniently tuned by controlling the nanoparticle size [6], making them extraordinarily attractive for photovoltaic applications [7]. Moreover, the quantum confinement effect of QDs makes it possible to generate multiple electron-hole pairs per photon through impact ionization effect [8–10]. Among the semiconductor QDs, CdS and CdSe has been paid much attention in QDs-sensitized solar cells because of their high potential in light harvesting under visible region along with particle size tuning properties [11–15]. When using CdS and CdSe QDs as sensitizers in QDSSCs, standard I− /I3 − redox couple employed for DSSCs is not chemically compatible whit QDs leading to fast degradation of nanocrystals. Consequently, different redox systems must be incorporated as the polysulfide [16,17] or Co-based systems [18]. When polysulfide electrolytes are used in QDSSCs, platinum counter electrodes are not suitable ones, since sulfur containing S2− compound absorbs to Pt surfaces, and thus a more suitable material should be employed (Au, Cu2 S, CoS, etc.) [19]. In the present study, three different approaches (including SILAR, spray pyrolysis and zinc–copper alloy processes) were employed for fabricating Cu2 S counter electrode. The prepared Cu2 S counter electrodes allowed the fabrication of CdS/CdSe QDSSCs having different values of 3.18, 0.341 and 0.266%.

H. Salaramoli et al. / Journal of Photochemistry and Photobiology A: Chemistry 271 (2013) 56–64

2. Experimental 2.1. Materials The commercially available copper nitrate, cadmium acetate, cadmium sulfate, nitrilotriacetic acid trisodium salt, selenium powder, zinc acetate, thioacetamide, diethylene glycol, sodium hydroxide, nitric acid, sodium sulfite, terpineol, sulfuric acid, acetic acid, titanium isopropoxide, P-25 TiO2 powder, ethyl cellulose, titanium tetrachloride, were obtained from Merck, Sigma, Aldrich and Alfa Aesar companies. 2.2. Preparation of TiO2 film Two types of TiO2 pastes yielding nanocrystalline-TiO2 (20 nm) and microcrystalline-TiO2 (400 nm) particles were prepared according to the literature method [18]. Fluorine doped tin oxide (FTO) glass having sheet resistance of 10 sq−1 was used to make both the working and counter electrodes. To make working electrode, the FTO glass used as current collector was first cleaned in a detergent solution using an ultrasonic bath for 15 min, and then rinsed with deionized water, next in ethanol using ultrasonic bath for 15 min, also in 0.1 M HCl solution in ultrasonic bath for 15 min and at the end in acetone in ultrasonic bath. The FTO glass plates were immersed into a 40 mM aqueous TiCl4 solution at 70 ◦ C for 30 min and washed with water and ethanol. The working photoelectrode of the QDSSC was made by pasting 20 nm nanoporous TiO2 paste on the 5 mm × 5 mm (0.25 cm2 active areas) FTO glass plates with doctor Blid method and dried for 6 min at 125 ◦ C. After drying the 20 nm TiO2 paste films at 125 ◦ C, one layer of TiO2 paste (with nanoporous sizes of 400 nm) was also deposited by doctor Blid, resulting in a light-scattering TiO2 film containing 400 nm sized anatase particles of 4–5 ␮m thickness. The electrodes coated with the TiO2 pastes were gradually heated under airflow at 325 ◦ C for 5 min, at 375 ◦ C for 5 min, and at 450 ◦ C for 15 min, and finally at 500 ◦ C for 30 min [19]. 2.3. Preparation of photoelectrodes Herein, the CdS and CdSe QDs were deposited directly on TiO2 . For CdS sensitization, Cd2+ precursor is 0.5 M cadmium acetate in ethanol and S2− precursor is 0.5 M Na2 S in methanol, in a successive ionic layer adsorption and reaction (SILAR) process. A single SILAR was consisted of 15 min dip-coating of the TiO2 photoanode into each of the above solutions. After every bath, the photoanode was thoroughly rinsed by immersion in the corresponding solvent about 2 min to remove the chemical residuals from the surface and then was dried in nitrogen. Typically, three cycles of CdS nanoparticles were pre-assembled on photoanode through SILAR technique and subsequently a thin-layer of CdSe nanoparticles was grown by CBD. For CdSe quantum dots sensitization, chemical bath solution was prepared by mixing CdSO4 solution as Cd2+ precursor and Na2 SeSO3 solution as S2− precursor and the CdSe QDs were directly grown on the CdS surface by CBD. Na2 SeSO3 solution was obtained by dissolving 0.31 g Se, 1.26 g Na2 SO3 and 4.0 g NaOH in 50 mL deionized water, then refluxing with heating the reagents at 80 ◦ C in a two necked flask under N2 flow and stirring for 7 h. The resulting solution was filtered and stored at 10 ◦ C in dark at least for two days prior to its use. For CdSe QDs sensitization, chemical bath solution was prepared by mixing 1.02 g of the CdSO4 in 50 mL deionization water, 1.65 g of nitriloacetic acid in 50 mL deionization water and 25 mL of the Na2 SeSO3 solution. The resultant TiO2 electrodes were immersed in the above chemical bath solution at 10 ◦ C for 12 h [20]. Finally, the film was rinsed with deionized water and dried under nitrogen. To prevent the charge recombination from TiO2 to electrolyte, a layer of ZnS was adsorbed on the QDs adsorbed TiO2 film.

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For covering the ZnS layer, the electrode was dipped alternately into a 0.1 M Zn(CH3 COO)2 aqueous solution and a 0.1 M Na2 S aqueous solution. The optimal dipping time was 1 min for both solutions and rinsing with deionized water between dips was done to obtain TiO2 /CdS (3)/CdSe/ZnS electrodes [12]. 2.4. Preparation of Cu2 S counter electrode via SILAR method A SILAR method was applied to deposit Cu2 S on FTO glass substrates. The FTO substrates were cleaned in water and ethanol using an ultrasonic bath for 10 min and dried under a nitrogen flow. In a representative cycle of SILAR process, the substrate was successively immersed into 0.5 M aqueous Cu(NO3 )2 for 90 s, rinsed with ethanol, dried with an air gun, immersed into 0.5 M aqueous Na2 S solution for another 90 s, rinsed with ethanol, and then dried with an air gun. This process was repeated up to five times [21]. The prepared electrode was dried at 60 ◦ C for 30 min. 2.5. Preparation of Cu2 S counter electrode via spray pyrolysis method The spray pyrolysis method was utilized to spray Cu2 S nanoparticles on FTO glass substrates. For using this method, synthesis of Cu2 S nanoparticles must be done. For the preparation of Cu2 S nanoparticles, copper acetate hydrate Cu(CH3 COO)2 ·H2 O (0.4 g) was mixed with 70 mL of diethylene glycol (DEG) solvent under sonication for several minutes to form a homogeneous solution and the solution was stirred and heated to 180 ◦ C. When temperature was set at 180 ◦ C, 30 mL fresh solution of C2 H5 NS (0.075 g) in DEG was slowly added and the reactor was maintained at 180 ◦ C for 3 h. The color of the solution changed rapidly from clear to a black suspension. After the system was cooled to room temperature naturally, the black precipitate at the bottom of the flask was washed with absolute ethanol and collected via centrifugation for three times and then dried in vacuum at 60 ◦ C for 6 h. 0.067 g of the synthesized Cu2 S nanoparticles were dispersed in 5 mL ethanol under sonication for 2 min. A black suspension of Cu2 S nanoparticles was injected into the tank of spray pyrolysis and then was sprayed on the FTO substrate. The temperature of the plate was set at 60 ◦ C. After spraying the black suspension on the FTO substrate, the electrode was let stay at 60 ◦ C for 30 min. After that time, the counter electrode is ready to apply in solar cell. 2.6. Preparation Cu2 S counter electrode via zinc and copper alloy For this method a piece of zinc and copper alloy covered with gum acting as a spacer is needed. To prepare this counter electrode, a drop of dense sulfuric acid was poured on the alloy and it was kept constant for 1 h. Then, it can be seen that some black particles are generated at the place the sulfuric acid was poured. These black particles are Cu2 S nanoparticles. This counter electrode is ready to apply in solar cell. 2.7. Preparation of electrolyte The polysulfide redox electrolyte used in all cases was prepared following the procedure described in Ref. [22]. Briefly, a solution of 1 M Na2 S, 1 M S and 0.1 M NaOH in deionized water was refluxed at 100 ◦ C for 45 min under N2 atmosphere to produce the required electrolyte. 2.8. Assembly of CdS/CdSe QDSSCs Here two kinds of counter electrodes are prepared, for both of them the substrate is FTO. The sensitized photoanode was

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Fig. 1. HR-TEM images of TiO2 /CdS (3)/CdSe (1)/ZnS using SILAR/CBD/SILAR processes, respectively.

sandwiched between counter electrodes. Both electrodes were separated by 25 ␮m thick surlyn foil gasket and the structure was sealed by hot press. Polysulfide redox electrolyte was put on the hole in the back of the counter electrode. The electrolyte was introduced into the cell via vacuum backfilling. The cell was placed in a small vacuum chamber to remove inside air. Finally, the hole was sealed using a piece of spacer (25 ␮m thick surlyn foil gasket) and a hot glass cover (0.1 mm thickness). For other counter electrode which uses zinc and copper alloy, a drop of electrolyte was poured on counter electrode and photoanode was put on it and they were sealed with two clips.

2.9. Measurements Absorption spectra of the photoanode were recorded by a Shimadzu UV-3103 UV–vis spectrophotometer. SEM images were acquired using a Philips XL30 instrument. HR-TEM images were taken by a TEM apparatus. Current–voltage (I–V) curves were obtained from a potentiostat galvanostat (Plamsems) equipment. The cell was illuminated using a solar simulator at AM1.5 G.

3. Results and discussion In this work, different Cu2 S counter electrodes were fabricated using three various methods including SILAR, spray pyrolysis and zinc–copper alloy approaches. The three counter electrodes were employed in CdS/CdSe QDSSCs that yielded relatively appreciable values of 3.18, 0.341 and 0.266%. It is notable that in QDSSCs with CdS or CdSe QDs as sensitizer, the iodide/triiodide redox electrolyte causes rapid photocorrosion of the QD, thus polysulfide redox couple was used as electrolyte. Since the conductivity of Pt surface decreases in the presence of S2− ion, metal sulfides like Cu2 S can be suitable choices for redox reaction of polysulfide. The HR-TEM, SEM, EDS, XRD, UV–vis and I–V analyses were performed to investigate the surface and structural properties of the prepared electrodes and the efficiencies of the fabricated QDSSs.

3.1. HR-TEM study of CdS and CdSe QDs on the TiO2 surface In order to get information on the size of QDs and their arrangement on TiO2 surface, the TEM experiment was carried out. Fig. 1 shows the HR-TEM images of CdS (3)/CdSe (1)/ZnS (1) co-sensitized TiO2 film. It can be seen that CdS and CdSe QDs are in close contact with TiO2 nanoparticles. The sizes of CdS and CdSe QDs were measured about 5 nm and 6 nm, respectively. The composition of the electrode was identified by energy-dispersive X-ray spectroscopy (EDS) measurement, as displayed in Fig. 2. Quantitative analysis of the EDS spectrum gives O:Ti atomic ratio of 2:1, indicating that high-grade TiO2 particles are formed. The atomic ratios of (Cd + Zn)/S and Cd/(S + Se) are nearly 1:1 indicating that the expected CdS, CdSe and ZnS QDs are deposited on the electrode. The result confirms that CdS and CdSe QDs are successfully assembled on the surface of the TiO2 film via the CBD and SILAR processes. 3.2. Optical properties of CdS/CdSe co-sensitized TiO2 film Fig. 3 shows the UV–vis absorption spectra of pure TiO2 , TiO2 /CdS, TiO2 /CdSe, and TiO2 /CdS/CdSe electrodes. The plates used for UV–vis study are the same as working electrode but without the layer of TiO2 paste with 400 nm nanoporous size. This plate was applied instead of quartz cell in UV–vis spectrophotometer. Fig. 3 indicates the UV–vis spectra after each cycle of SILAR. Here, the CdS/CdSe co-sensitized TiO2 photoanodes are prepared by SILAR in the form of TiO2 /CdS (3)/CdSe (1), where 3 and 1 are the number of deposition cycles of CdS and CdSe, respectively. After CdSe QDs are deposited, the color of the electrode changes from yellow to orange. Compared with the absorption spectra of the pure TiO2 film, there are obvious absorption peaks near 400 and 550 nm for the TiO2 /CdS and TiO2 /CdSe electrodes. The sizes of QDs assembled on TiO2 film can be estimated from the UV–vis spectra. According to the empirical fitting functions [R(QD) = 0.1/(0.1338–0.0002345e )], which R is the diameter of QDs and e is the absorption edge given in the literature [4], the mean diameters of CdS and CdSe QDs on the TiO2 /CdS (3)/CdSe photoanode were measured to be both 5 nm and 6 nm respectively.


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Fig. 2. EDS spectrum of TiO2 /CdS/CdSe/ZnS.

The evaluated sizes of CdS and CdSe are in well consistent with the sizes measured from the HR-TEM images. Deposition of CdSe on TiO2 /CdS electrode results in the increase of absorbance. The effects of deposition cycles of CdS and CdSe can be clearly seen on the energy band gap values of CdS/CdSe co-sensitized TiO2 films. By employing a Tauc analysis of (hv˛)2 versus hv (where h = 4.136 × 10−15 eV s, = c/ and c = 3 × 108 m/s and the UV–vis spectra were used to obtain value as wavelength and ˛ as absorbance) [23] plotted in Fig. 4, the band gap values of the CdS/CdSe co-sensitized TiO2 films corresponding to each SILAR cycle were extracted [24,25]. To achieve the band gap in the Tauc plots, a baseline is first drawn at low energies. Second, a line tangent to the slope in the linear region of the absorption onset is drawn. The intersection of the two lines corresponds to the best estimate for the energy of the band gap [26]. According to Fig. 4, the estimated band gap of bare TiO2 = 3.08 eV, TiO2 /CdS (1) = 2.71 eV, TiO2 /CdS (2) = 2.54 eV, TiO2 /CdS (3) = 2.45 eV, and TiO2 /CdS (3)/CdSe = 2.40 eV. Herein, the calculated band gaps vary

Fig. 3. UV–vis absorption spectra of bare TiO2 film and the TiO2 films sensitized by CdS/CdSe QDs.

from 3.1 eV (without CdS and CdSe, bare TiO2 ) to 2.38 eV (TiO2 /CdS (3)/CdSe), which are higher than the values reported for CdS and CdSe in bulk (2.25 eV and 1.7 eV [27,28], respectively), indicating that the sizes of CdS and CdSe on TiO2 films are still within the scale of QDs, since by decreasing the band gap the size of nanoparticles will increase [29]. 3.3. SEM study of Cu2 S counter electrode Fig. 5 displays scanning electron microscopy (SEM) images of Cu2 S counter electrode prepared by zinc and copper alloy, SILAR and spray pyrolysis methods. Fig. 5a illustrates a typical morphology of Cu2 S nanostructure composed of nanosheets with high porosity which is much suitable for injection of electrolyte and will boost the performance of the counter electrode. It should be mentioned that the nanosheets have been firmly attached to the substrate, thus they do not soluble in electrolyte and it is expected that this counter electrode will show a good performance. Fig. 5b demonstrates SEM image of Cu2 S nanoparticles which have been prepared by SILAR method. As it is seen, this nanostructure is composed of large nanoparticles (about 50–70 nm) which are covered with very small nanoparticles. This structure has a good porosity, therefore can help to injection of electrolyte into nanostructure. But this method cannot cover all areas of FTO substrate (it is shown in Fig. 5b) and it can be as a week point for this counter electrode because empty place causes defect in transfusion of electron. Moreover, because high temperature was not used for drying this counter electrode, hence the nanoparticles are soluble in electrolyte and it is expected that this counter electrode could not illustrate a good performance. Fig. 5c and d exhibits morphology of Cu2 S nanoparticles deposited on FTO substrate by spray pyrolysis approach. As it is indicated, this method yields small nanoparticles (about 10–17 nm) but the porosity is not as better as the counter electrode prepared by alloy and it has negative effect on injection of electrolyte and as a result on the performance of

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Fig. 4. Tauc plots of TiO2 at different number of deposition TiO2 (a), CdS (1) (b), CdS (2) (c), CdS (3) (d), CdSe (e).

the counter electrode. In addition, this electrode was not dried at high temperature and the nanoparticles did not attached on FTO substrate firmly, thus they will solve easily in electrolyte. The compositions of counter electrodes were identified by energy-dispersive X-ray spectroscopy (EDS) measurement, as shown in Figs. 6 and 7. Quantitative analysis of the EDS spectrum gives a Cu:S atomic ratio of about 2:1, indicating that high-grade Cu2 S particles are formed, although there is O peak in Fig. 6 indicating tiny impurity in the sample. The Zn peak in Fig. 7 reveals the presence of alloy under the nanosheet. The XRD pattern for Cu2 S nanoparticles is shown in Fig. 8. It can be see that there are seven peaks corresponding to {1 1 1}, {2 0 0}, {2 2 0}, {3 1 1}, {4 0 0},

{3 3 1} and {4 2 0} crystal planes, which matches to the cubic phase ˚ of Cu2 S named digenite (JCPDS card no. 84-1770, a = 5.6286 A). 3.4. Photovoltaic performance of CdS/CdSe/ZnS QDSSCs In order to understand the effects of the methods for preparation of counter electrode, their photovoltaic performances with polysulfide electrolyte were investigated. All the samples were coated with ZnS to inhibit the recombination at the TiO2 photoanode/polysulfide electrolyte interface. Fig. 9 presents the photocurrent density–voltage characteristics of the QDSSCs with different counter electrodes (active area of TiO2 film 0.25 cm2 ) at


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Fig. 5. SEM micrographs of Cu2 S counter electrode prepared by (a) zinc and copper alloy, (b) SILAR, (c, d) spray pyrolysis.

AM 1.5 (100 mW/cm2 ), and the related parameters of these QDSSCs are listed in Table 1. This figure shows that the power conversion efficiencies of QDSSCs are affected with the approach used for preparation of the counter electrodes, because photoanode in all the solar cells are the same. Moreover, in all counter electrodes the Cu2 S nanoparticles are just different in size and morphology with each other that are related to the method applied for their preparation. In Table 1, the TiO2 /CdS/CdSe/ZnS device with alloy counter electrode yields an open-circuit voltage (VOC ) of 0.6 V, a shortcircuit current density (JSC ) of 11.69 mA/cm2 , fill factor (ff) of

Table 1 Photovoltaic performance parameters of QDSSCs based on different counter electrodes. Sample

(%)

ff

JSC (mA/cm2 )

VOC (V)

Cu2 S (metal) Cu2 S (spray) Cu2 S (SILAR)

3.18 0.266 0.341

0.44 0.37 0.30

11.69 0.90 3.51

0.6 0.6 0.4

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Fig. 6. EDS spectra of Cu2 S counter electrode prepared by SILAR method.

0.44 and an energy conversion efficiency () of 3.18%. When using counter electrode prepared with spray pyrolysis method, no changes in VOC (0.6 V) but slightly changes in ff (0.37) value were obtained. Remarkably, the JSC decreases from 11.69 to 0.9 mA/cm2 results in a substantial reduction of (from 3.18 to 0.266%). Applying the SILAR method for preparation of counter electrode improves

the JSC from 0.9 to 3.51 mA/cm2 but decreases the VOC (from 0.6 to 0.4 V) and ff (from 0.37 to 0.3) that may be due to low coverage of nanoparticles obtained in this method on the FTO substrate. Owing to a remarkable increase in the JSC (from 0.9 to 3.51 mA/cm2 ), a relatively higher (0.341%) is obtained for QDSSC with this counter electrode.

Fig. 7. EDS spectra Cu2 S counter electrode prepared by zinc and copper alloy.


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electrodes and the efficiencies of the fabricated QDSSs. A shortcircuit current density of 11.69 mA/cm2 and a conversion efficiency of 3.18% under one sun illumination has been achieved using zinc and copper alloy as a counter electrode, which is very much greater than those obtained by spray pyrolysis and SILAR methods (0.90 and 3.51 mA/cm2 , respectively). As mentioned, Cu2 S nanoparticles were used in all counter electrodes in this work, thus it can be said that different processes for preparation of counter electrodes have their impact on the efficiencies of QDSSCs. Thus, it may be expected that by changing some parameters such as temperature in spray pyrolysis, or number of SILAR cycles and temperature in SILAR method, the efficiencies of these QDSSCs may approach the efficiency of QDSSCs with zinc and copper alloy counter electrode. Acknowledgements

Fig. 8. XRD pattern Cu2 S nanoparticles.

The financial support of this work by the Iranian Research Organization for Science and Technology (IROST) and Research Office of Amirkabir University of Technology is gratefully acknowledged. References

Fig. 9. The photocurrent–voltage (I–V) curves of the QDSSCs with different counter electrode under one sun illumination. (a) Zinc and copper alloy, (b) SILAR, (c) spray pyrolysis.

4. Conclusion The Cu2 S counter electrodes were prepared by three different methods (zinc and copper alloy, SILAR and spray pyrolysis). The HR-TEM, SEM, EDS, XRD, UV–vis and I–V analyses were performed to investigate the surface and structural properties of the prepared

[1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] Y.-L. Lee, C.-H. Chang, Efficient polysulfide electrolyte for CdS quantum dotsensitized solar cells, J. Power Sources 185 (2008) 584–588. [3] Y. Li, A. Pang, X. Zheng, M. Wei, CdS quantum-dot-sensitized Zn2 SnO4 solar cell, Electrochim. Acta 56 (2011) 4902–4906. [4] W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Experimental determination of the extinction coefficients of CdTe, CdSe, and CdS nanocrystals, Chem. Mater. 15 (2003) 2854–2860. [5] A.L. Rogach, Semiconductor Nanocrystal Quantum Dots, Springer, Wien, New York, 2008. [6] P.V. Kamat, Quantum dot solar cells. Semiconductor nanocrystals as light harvesters, J. Phys. Chem. C 112 (2008) 18737–18753. [7] A.J. Nozik, Physica E 14 (2002) 115, National Renewable Energy Laboratory, Center for Basic Sciences, 1617 Cole Boulevard, Golden, CO 80401, USA. [8] I. Mora-Seró, S. Giménez, F. Fabregat-Santiago, R. Gómez, Q. Shen, T. Toyoda, J. Bisquert, Recombination in quantum dot sensitized solar cells, Acc. Chem. Res. 42 (2009) 1848–1857. [9] X. Xu, S. Giménez, I. Mora-Seró, A. Abate, J. Bisquert, G. Xu, Influence of cysteine adsorption on the performance of CdSe quantum dots sensitized solar cells, Mater. Chem. Phys. 124 (2010) 709–712. [10] Y.L. Lee, B.M. Huang, H.T. Chien, Highly efficient CdSe-sensitized TiO2 photoelectrode for quantum-dot-sensitized solar cell applications, Chem. Mater. 20 (2008) 6903–6905. [11] O. Niitsoo, S.K. Sarkar, C. Pejoux, S. Ruhle, D. Cahena, G. Hodes, Chemical bath deposited CdS/CdSe-sensitized porous TiO2 solar cells, J. Photochem. Photobiol. A: Chem. 181 (2006) 306–313. [12] S.Q. Fan, D. Kim, J.J. Kim, D.W. Jung, S.O. Kang, J. Ko, Highly efficient CdSe quantum-dot-sensitized TiO2 photoelectrodes for solar cell applications, Electrochem. Commun. 11 (2009) 1337–1339. [13] Q. Shen, J. Kobayashi, L.J. Diguna, T. Toyoda, Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells, J. Appl. Phys. 103 (2008) 084304. [14] G. Hodes, J. Manassen, D. Cahen, Electrocatalytic electrodes for the polysulfide redox system, J. Electrochem. Soc. 127 (1980) 544–549. [15] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, Quantum dot solar cells. Harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films, J. Am. Chem. Soc. 128 (2006) 2385–2393. [16] H.J. Lee, M. Wang, P. Chen, D.R. Gamelin, S.M. Zakeeruddin, M. Grätzel, M.K. Nazeeruddin, Efficient CdSe quantum dot-sensitized solar cells prepared by an improved successive ionic layer adsorption and reaction process, Nano Lett. 9 (2009) 4221–4227. [17] S. Gimenez, I. Mora-Sero, L. Macor, N. Guijarro, T. Lana-Villareal, R. Gomez, L.J. Diguna, Q. Shen, T. Toyoda, J. Bisquert, Improving the performance of colloidal quantum-dot-sensitized solar cells, Nanotechnology 20 (2009) 295204. [18] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M.K. Nazeeruddin, M. Grätzel, Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%, Thin Solid Films 516 (2008) 4613–4619. [19] R. Vogel, K. Pohl, H. Weller, Sensitization of highly porous, polycrystalline TiO2 electrodes by quantum sized CdS, Chem. Phys. Lett. 174 (1990) 241–246. [20] P. Sudhagar, T. Song, D.H. Lee, I. Mora-Sero, J. Bisquret, M. Laudenslager, W.M. Sigmund, W.I. Park, U. Paik, Y.S. Kang, High open circuit voltage quantum dot sensitized solar cells manufactured with ZnO nanowire arrays and Si/ZnO branched hierarchical structures, J. Phys. Chem. Lett. 2 (2011) 1984–1990. [21] Z. Yang, C.-Y. Chen, C.-W. Liu, C.-L. Li, H.-T. Chang, Quantum dots-sensitized solar cells featuring CuS/CoS electrodes provide 4.1% efficiency, Adv. Energy Mater. 1 (2011) 259–264.

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[22] P. Zhano, H. Zhang, H. Zhou, B. Yi, Nickel foam and carbon felt applications for sodium polysulfide/bromine redox flow battery electrodes, Electrochim. Acta 51 (2005) 1091–1098. [23] J. Tauc, A. Menth, D.L. Wood, Optical and magnetic investigations of localized states in semiconducting glasses, Phys. Rev. Lett. 25 (1970) 749–752. [24] J. Hiie, T. Dedova, V. Valdna, K. Muska, Comparative study of nano-structured CdS thin films prepared by CBD and spray pyrolysis: annealing effect, Thin Solid Films 511–512 (2006) 443–447. [25] L.K. Pan, C.Q. Sun, Coordination imperfection enhanced electron–phonon interaction, J. Appl. Phys. 95 (2004) 3819–3821.

[26] P. Ardalan, T.P. Brennan, H.B.R. Lee, J.R. Bakke, I.K. Ding, M.D. McGehee, S.F. Bent, Effects of self-assembled monolayers on solid-state CdS quantum dot sensitized solar cells, ACS Nano 5 (2011) 1495–1504. [27] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [28] G. Hodes, Comparison of dye- and semiconductor-sensitized porous nanocrystalline liquid junction solar cells, J. Phys. Chem. C 112 (2008) 17778–17787. [29] Z.F. Liu, Y.J. Li, Z.G. Zhao, Y. Cui, K. Hara, M. Miyauchi, Block copolymer templated nanoporous TiO2 for quantum-dot-sensitized solar cells, J. Mater. Chem. 20 (2010) 492–497.