- Email: [email protected]
Pyrolysis preparation of Cu2ZnSnS4 thin film and its application to counter electrode in quantum dot-sensitized solar cells
Electrochimica Acta 118 (2014) 41–44
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pyrolysis preparation of Cu2 ZnSnS4 thin film and its application to counter electrode in quantum dot-sensitized solar cells Yanru Zhang a,b , Chengwu Shi a,b,∗ , Xiaoyan Dai a,b , Feng Liu b , Xiaqin Fang b , Jun Zhu b a b
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China Key Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, China
a r t i c l e
i n f o
Article history: Received 26 July 2013 Received in revised form 25 November 2013 Accepted 26 November 2013 Available online 11 December 2013 Keywords: Cu2 ZnSnS4 thin film Pyrolysis Counter electrode Quantum dot-sensitized solar cell
a b s t r a c t The Cu2 ZnSnS4 (CZTS) thin films with uniform and porous surface feature with the pore size of 100200 nm were successfully prepared by pyrolysis procedure using the methanol solution containing copper chloride dihydrate (0.06 mol dm−3 ), zinc chloride (0.03 mol dm−3 ), stannous chloride dihydrate (0.03 mol dm−3 ), thiourea (0.48 mol dm−3 ) and deionized water (1.92 mol dm−3 ) as precursor solution at 380◦ C in air atmosphere. Electrochemical impedance spectroscopy was applied to evaluate the electrochemical catalytic activity of the pyrolysis CZTS thin films for redox couple of Sn 2- /S2− and the charge transfer resistance was 64.08 using the pyrolysis CZTS thin film obtained by repeating the procedure of dipping the FTO substrate into the precursor solution and heating at 380◦ C for 7 cycles. The assembled quantum dot-sensitized solar cells gave an open-circuit photovoltage of 0.52 V, a short-circuit photocurrent density of 12.96 mA cm−2 , and a fill factor of 0.38, corresponding to the photoelectric conversion efficiency of 2.56%. Because the pyrolysis procedure was a facile, low cost and vacuum-free process, and had the advantage of obtaining various porous microstructure thin films and allowing deposition over large areas, the pyrolysis CZTS thin films can serve as an effective counter electrode for QDSCs. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Quantum dot-sensitized solar cells (QDSCs) have emerged as a promising candidate for the development of novel solar cells because of the versatile properties of semiconductor quantum dots such as tunability of the band gap, high absorption coefficient, generation of multiple electron carriers under high energy excitation, and delivery of hot electrons[1–4]. A typical configuration of QDSCs included a quantum dot-sensitized porous TiO2 photoanode, an electrolyte containing a redox couple of Sn 2- /S2- , and a counter electrode (CE)[5]. In particular, the counter electrode with high electrochemical catalytic activity for the electrolyte with the redox couple of Sn 2- /S2− appeared to be an indispensable component of high performance QDSCs[6]. Recently, various metal sulfides such as PbS[7], Cu2 S[8], CoS[9], Cu2 S/reduced graphene oxide composite[10], Cu2 ZnSnS4 hierarchical microspheres[11], CZTSSe nanoparticle[5,12] have been reported as potential candidates for CE in QDSCs. Xu et al. [11] explored the preparation of CZTS microspheres via a solvothermal approach and the QDSCs with the CZTS hierarchical microspheres as CE gave the photoelectric conversion efficiency of 3.73%. However, the CZTS thin films using the
pyrolysis procedure have not been reported for the CE in QDSCs, though the pyrolysis procedure was a facile, low cost and vacuumfree process, and had the advantage of obtaining various porous microstructure thin films and allowing deposition over large areas. In this paper, the CZTS thin film was firstly applied for the CE in QDSCs by facile pyrolysis procedure using the methanol solution containing copper chloride dihydrate (0.06 mol dm−3 ), zinc chloride (0.03 mol dm−3 ), stannous chloride dihydrate (0.03 mol dm−3 ), thiourea (0.48 mol dm−3 ) and deionized water (1.92 mol dm−3 ) as precursor solution at 380◦ C in air atmosphere. Its chemical composition, crystal structure, surface morphology, direct band gap and the electrochemical catalytic activity for redox couple of Sn 2- /S2− were systemically investigated by energy dispersive x-ray spectroscopy, x-ray diffraction, scanning electron microscope, the ultraviolet-visible-near infrared spectroscopy and electrochemical impedance spectroscopy, respectively. The photovoltaic performance of QDSCs with the corresponding pyrolysis CZTS thin film and the platinized conducting glass was evaluated.
2. Experimental 2.1. Preparation of pyrolysis CZTS thin films
∗ Corresponding author. Tel.: +86 551 62901450, fax: +86 551 62901450. E-mail address: [email protected] (C. Shi). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.168
All chemicals were commercially available and used without further purification. The preparation procedure of the
42
Y. Zhang et al. / Electrochimica Acta 118 (2014) 41–44
precursor solution for the pyrolysis CZTS thin film was as follows: 2.9232 g (0.0384 mol) thiourea, 0.8184 g (0.0048 mol) CuCl2 ·2H2 O, 0.3270 g (0.0024 mol) ZnCl2 , 0.5414 g (0.0024 mol) SnCl2 ·2H2 O were sequentially dissolved in 80 ml methanol, then 2.7648 g (0.1536 mol) deionized water was added to the solution to promote the hydrolysis of thiourea. Fluorine-doped tin oxide (FTO) substrates were dipped into the above precursor solution, then heated at 380 ◦ C for 5 min in air atmosphere. All these processes were termed as one cycle. After the cycles of 2, 5 and 7, three kinds of pyrolysis CZTS thin films were obtained, hereafter referred to CZTS (2, 5, 7). CdS/CdSe/ZnS co-sensitized TiO2 photoanode and the platinized conducting glass counter electrode were prepared according to our previous reports[13,14]. Aqueous polysulfide electrolyte consisting of 1 mol dm−3 sodium sulfide, 0.5 mol dm−3 sulfur was used as the electrolyte in QDSCs. 2.2. Characterization Electrochemical impedance spectroscopy (EIS) was obtained by applying sinusoidal perturbations of ±5 mV over the bias 0 V at frequencies from 105 Hz to 0.1 Hz, carried out with the electrochemical workstation (CHI 660B) using the symmetrical cells based on the pyrolysis CZTS thin films to evaluate their electrochemical catalytic activity for the redox couple of Sn 2- /S2− . The chemical composition of pyrolysis CZTS thin films was analyzed by energy disperse spectroscopy (EDS, Sivion200, USA). X-ray diffractometer (D/MAX2500 V, Rigaku, Japan) was used to record x-ray diffraction (XRD) patterns. Cu-K␣ radiation (=0.154056 nm, 40 kV and 40 mA) was used to record the spectra in the 2 range of 10-80◦ with a step size of 0.026◦ s−1 . The microstructure and the surface morphology were observed using field emission scanning electron microscope (FE-SEM, HITACHI, Japan). The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra were recorded with a double beam UV-Vis-NIR (DUV-3700, Shimadzu, Japan) in the wavelength range of 400-1400 nm at the resolution of 1 nm. The photovoltaic performance of QDSCs was measured with a solar simulator (Oriel, Newport, USA, AM 1.5, 100 mW cm−2 ) and a Keithley 2420 source meter controlled by Testpoint software. The irradiation intensity was calibrated with standard crystalline silicon solar cell (Oriel, Newport, USA). The active areas of QDSCs were set at 0.25 cm2 . 3. Results and discussion 3.1. The composition, morphology and direct band gap of pyrolysis CZTS thin films Fig. 1 displayed the SEM images of pyrolysis CZTS thin films. Compared with Fig. 1 (a), (b), (c), it was found that the cycle times obviously affect the surface morphology of pyrolysis CZTS thin films. Interestingly, the pyrolysis CZTS (7) thin film showed uniform and porous surface feature with the pore size of 100-200 nm, which was beneficial to improve the electrochemical catalytic activity for the redox couple of Sn 2- /S2− . The chemical composition of pyrolysis CZTS thin films was determined to be Cu2.29 Zn1.00 Sn1.28 S4.10 by EDS analysis. Fig. 2 showed the x-ray diffraction pattern of pyrolysis CZTS (7) thin films, indicated that the strongest peaks located at 2 = 28.55◦ with weak peaks at 2 = 47.23◦ , 56.27◦ , corresponding to the spacing of (112), (220), (312) planes of CZTS and a preferred orientation along (112) plane appeared, matched well with the phases of CZTS (JCPDS NO. 26-0575). No diffraction peaks of any impurities can be detected. The direct band gap of pyrolysis CZTS (7) thin film was estimated by extrapolating the linear region of a plot of the absorbance squared
Fig. 1. SEM images of the pyrolysis CZTS thin films. a. CZTS (2); b. CZTS (5); c. CZTS (7);
Fig. 2. XRD pattern of the pyrolysis CZTS (7) thin film.
Y. Zhang et al. / Electrochimica Acta 118 (2014) 41–44
Fig. 3. (˛h)2 vs h plots of the pyrolysis CZTS (7) thin films.
43
Fig. 5. The J–V curves of the QDSCs with different counter electrodes. a. CZTS (2); b. CZTS (5); c. CZTS (7); d. Pt.
Table 2 Photovoltaic parameters of QDSCs with various counter electrodes. Counter electrode
Voc /V
Jsc /mA cm−2
FF
/%
CZTS (2) CZTS (5) CZTS (7) Pt
0.51 0.50 0.52 0.52
1.73 12.67 12.96 12.89
0.31 0.34 0.38 0.32
0.27 2.15 2.56 2.14
3.3. Photovoltaic performance of QDSCs Fig. 4. Nyquist plots of pyrolysis CZTS thin films. a. CZTS (2); b. CZTS (5); c. CZTS (7).
(˛h)2 versus energy h as shown in Fig. 3, and the value of the direct band gap was 1.44 eV, which matches well with the reported values[15]. 3.2. Electrochemical catalytic activity of pyrolysis CZTS thin films for the redox couple of Sn 2- /S2Fig. 4 showed the EIS spectra of the symmetrical cells based on pyrolysis CZTS thin films. Experimental data were represented by symbols while the solid lines correspond to the fit using the equivalent circuit Rs (Rct CPE)[14,16], and the parameters obtained by fitting the experimental spectra with the equivalent circuit were listed in Table 1. The serial resistance (Rs ) described the resistance of the FTO and solution, Rct was the charge-transfer resistance of the CZTS/electrolyte interfaces. The constant phase element (CPE) was a nonideal frequency-dependent capacitance, and its admittance was expressed by the equation, YQ = Y0 (jω)n , where n was a constant ranging from 0≤ n ≤1[16]. From Table 1, Rct decreased from 395.50 , 92.32 to 64.08 with the increase of the cycle times. It was worth noting that the pyrolysis CZTS (7) thin film showed the lowest charge-transfer resistance (Rct ), indicating the highest electrochemical catalytic activity for the redox couple of Sn 2- /S2− . And the Y0 value of pyrolysis CZTS (7) thin film was bigger than that of CZTS (2) and CZTS (5). The results of Rct and Y0 should be related to the uniform and porous surface feature with the pore size of 100-200 nm of the pyrolysis CZTS (7).
Table 1 Parameters obtained by fitting the experimental spectra with the equivalent circuit. Thin film
Rs /
Rct /
Y0 /F sn−1
n
CZTS (2) CZTS (5) CZTS (7)
31.77 33.83 33.53
395.50 92.32 64.08
854 3910 5700
0.73 0.63 0.63
Fig. 5 showed the photocurrent density-voltage (J-V) characteristics of the QDSCs, and the resultant photovoltaic parameters were summarized in Table 2. Compared with CZTS (7) and CZTS (5), the FF of the CZTS (7) was higher than that of CZTS (5), which was because the Rct 64.08 of CZTS (7) was smaller than the Rct 92.32 of CZTS (5) and the Y0 5700 F sn−1 of CZTS (7) was higher than the Y0 3910 F sn−1 of CZTS (5). There was no obvious difference of Voc and Jsc , and this was because the QDSCs of CZTS (7) and CZTS (5) used the same photoanode and polysulfide electrolyte. Therefore, the QDSCs of CZTS (7) achieved the photoelectric conversion efficiency of 2.56%. Moreover, the short circuit photocurrent density of CZTS (2) QDSCs was only 1.73 mA cm−2 , which may be due to the coverage of the CZTS (2) thin film was not complete. The QDSCs with Pt counter electrode gave a Jsc of 12.89 mA cm−2 , a Voc of 0.52 V, a FF of 0.32, and a of 2.14%, lower than that of the QDSCs with CZTS (7) counter electrode. The results revealed that pyrolysis CZTS thin film can serve as effective counter electrode in QDSCs. 4. Conclusions The pyrolysis CZTS thin films have been demonstrated to perform the efficient CEs in QDSCs, and the photoelectric conversion efficiency of 2.56% was achieved by the QDSCs with CZTS (7) counter electrode. The good performance in the efficiency can be attributed to the high electrochemical catalytic activity of pyrolysis CZTS thin films for the redox couple of Sn 2- /S2− . The results indicated that the pyrolysis CZTS thin films can serve as alternative to noble Pt. The optimization of the chemical composition of the pyrolysis CZTS thin films was ongoing. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (51072043, 51272061), National Basic Research Program of China (2011CBA00700), Anhui Province Science and Technology Plan Project of China (2010AKND0794).
44
Y. Zhang et al. / Electrochimica Acta 118 (2014) 41–44
References [1] Y.Y. Yang, Q.X. Zhang, T.Z. Wang, L.F. Zhu, X.M. Huang, Y.D. Zhang, X. Hu, D.M. Li, Y.H. Luo, Q.B. Meng, Novel tandem structure employing mesh-structured Cu2 S counter electrode for enhanced performance of quantum dot-sensitized solar cells, Electrochim. Acta 88 (2013) 44. [2] T. Bora, H.H. Kyaw, J. Dutta, Zinc oxide-zinc stannate core-shell nanorod arrays for CdS quantum dot sensitized solar cells, Electrochim. Acta 68 (2012) 44. [3] C.J. Raj, K. Prabakar, A.D. Savariraj, H.J. Kim, Surface reinforced platinum counter electrode for quantum dots sensitized solar cells, Electrochim. Acta 103 (2013) 231. [4] M.H. Yeh, C.P. Lee, C.Y. Chou, L.Y. Lin, H.Y. Wei, C.W. Chu, R. Vittal, K.C. Ho, Conducting polymer-based counter electrode for a quantum-dot-sensitized solar cell (QDSSC) with a polysulfide electrolyte, Electrochim. Acta 57 (2011) 277. [5] Y.B. Cao, Y.J. Xiao, J.Y. Jung, H.D. Um, S.W. Jee, H.M. Choi, J.H. Bang, J.H. Lee, Highly electrocatalytic Cu2 ZnSn(S1-x Sex )4 counter electrodes for quantum-dotsensitized solar cells, ACS Appl. Mater. Interfaces 5 (2013) 479. [6] S. Ruhle, M. Shalom, A. Zaban, Quantum-dot-sensitized solar cells, ChemPhysChem 11 (2010) 2290. [7] Z. Tachan, M. Shalom, I. Hod, S. Rühle, S.A. Tirosh, Zaban, PbS as a highly catalytic counter electrode for polysulfide-based quantum dot solar cells, J. Phys. Chem. C 115 (2011) 6162. [8] V.G. Pedro, X.Q. Xu, L.M. Seró, J. Bisquert, Modeling high-efficiency quantum dot sensitized solar cells, ACS NANO 4 (2010) 5783. [9] Z. Yang, C.Y. Chen, C.W. Liu, H.T. Chang, Electrocatalytic sulfur electrodes for CdS/CdSe quantum dot-sensitized solar cells, Chem. Commun 46 (2010) 5485.
[10] J.G. Radich, R. Dwyer, P.V. Kamat, Cu2 S reduced graphene oxide composite for high-efficiency quantum dot solar cells. Overcoming the redox limitations of S2- /Sn 2- at the counter electrode, J. Phys. Chem. Lett 2 (2011) 2453. [11] J. Xu, X. Yang, Q.D. Yang, T.L. Wong, C.S. Lee, Cu2 ZnSnS4 hierarchical microspheres as an effective counter electrode material for quantum dot sensitized solar cells, J. Phys. Chem. C 116 (2012) 19718. [12] X.W. Zeng, W.J. Zhang, Y. Xie, D.H. Xiong, W. Chen, X.B. Xu, M.K. Wang, Y.B. Cheng, Low-cost porous Cu2 ZnSnSe4 film remarkably superior to noble Pt as counter electrode in quantum dot-sensitized solar cell system, J. Power Sources 226 (2013) 359. [13] Y.H. Zhang, J. Zhu, X.C. Yu, J.F. Wei, L.H. Hu, S.Y. Dai, The optical and electrochemical properties of CdS/CdSe co-sensitized TiO2 solar cells prepared by successive ionic layer adsorption and reaction processes, Sol. Energy 86 (2012) 964. [14] C.W. Shi, S.Y. Dai, K.J. Wang, X. Pan, L.Y. Zeng, L.H. Hu, F.T. Kong, L. Guo, Influence of various cations on redox behavior of I- and I3 - and comparison between KI complex with 18-crown-6 and 1, 2-dimethyl-3-propylimidazolium iodide in dye-sensitized solar cells, Eletrochim. Acta 50 (2005) 2597. [15] C. Steinhagen, M.G. Panthani, V. Akhavan, B. Goodfellow, B. Koo, B.A. Korgel, Synthesis of Cu2 ZnSnS4 nanocrystals for use in low-cost photovoltaics, J. Am. Chem. Soc 131 (2009) 12554. [16] C.W. Shi, S.Y. Dai, K.J. Wang, X. Pan, L. Guo, L.Y. Zeng, L.H. Hu, F.T. Kong, Influence of 1-methyl-3-propylimidazolium iodide on I3 - /I- redox behavior and photovoltaic performance of dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 86 (2005) 527.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pyrolysis preparation of Cu2 ZnSnS4 thin film and its application to counter electrode in quantum dot-sensitized solar cells Yanru Zhang a,b , Chengwu Shi a,b,∗ , Xiaoyan Dai a,b , Feng Liu b , Xiaqin Fang b , Jun Zhu b a b
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China Key Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, China
a r t i c l e
i n f o
Article history: Received 26 July 2013 Received in revised form 25 November 2013 Accepted 26 November 2013 Available online 11 December 2013 Keywords: Cu2 ZnSnS4 thin film Pyrolysis Counter electrode Quantum dot-sensitized solar cell
a b s t r a c t The Cu2 ZnSnS4 (CZTS) thin films with uniform and porous surface feature with the pore size of 100200 nm were successfully prepared by pyrolysis procedure using the methanol solution containing copper chloride dihydrate (0.06 mol dm−3 ), zinc chloride (0.03 mol dm−3 ), stannous chloride dihydrate (0.03 mol dm−3 ), thiourea (0.48 mol dm−3 ) and deionized water (1.92 mol dm−3 ) as precursor solution at 380◦ C in air atmosphere. Electrochemical impedance spectroscopy was applied to evaluate the electrochemical catalytic activity of the pyrolysis CZTS thin films for redox couple of Sn 2- /S2− and the charge transfer resistance was 64.08 using the pyrolysis CZTS thin film obtained by repeating the procedure of dipping the FTO substrate into the precursor solution and heating at 380◦ C for 7 cycles. The assembled quantum dot-sensitized solar cells gave an open-circuit photovoltage of 0.52 V, a short-circuit photocurrent density of 12.96 mA cm−2 , and a fill factor of 0.38, corresponding to the photoelectric conversion efficiency of 2.56%. Because the pyrolysis procedure was a facile, low cost and vacuum-free process, and had the advantage of obtaining various porous microstructure thin films and allowing deposition over large areas, the pyrolysis CZTS thin films can serve as an effective counter electrode for QDSCs. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Quantum dot-sensitized solar cells (QDSCs) have emerged as a promising candidate for the development of novel solar cells because of the versatile properties of semiconductor quantum dots such as tunability of the band gap, high absorption coefficient, generation of multiple electron carriers under high energy excitation, and delivery of hot electrons[1–4]. A typical configuration of QDSCs included a quantum dot-sensitized porous TiO2 photoanode, an electrolyte containing a redox couple of Sn 2- /S2- , and a counter electrode (CE)[5]. In particular, the counter electrode with high electrochemical catalytic activity for the electrolyte with the redox couple of Sn 2- /S2− appeared to be an indispensable component of high performance QDSCs[6]. Recently, various metal sulfides such as PbS[7], Cu2 S[8], CoS[9], Cu2 S/reduced graphene oxide composite[10], Cu2 ZnSnS4 hierarchical microspheres[11], CZTSSe nanoparticle[5,12] have been reported as potential candidates for CE in QDSCs. Xu et al. [11] explored the preparation of CZTS microspheres via a solvothermal approach and the QDSCs with the CZTS hierarchical microspheres as CE gave the photoelectric conversion efficiency of 3.73%. However, the CZTS thin films using the
pyrolysis procedure have not been reported for the CE in QDSCs, though the pyrolysis procedure was a facile, low cost and vacuumfree process, and had the advantage of obtaining various porous microstructure thin films and allowing deposition over large areas. In this paper, the CZTS thin film was firstly applied for the CE in QDSCs by facile pyrolysis procedure using the methanol solution containing copper chloride dihydrate (0.06 mol dm−3 ), zinc chloride (0.03 mol dm−3 ), stannous chloride dihydrate (0.03 mol dm−3 ), thiourea (0.48 mol dm−3 ) and deionized water (1.92 mol dm−3 ) as precursor solution at 380◦ C in air atmosphere. Its chemical composition, crystal structure, surface morphology, direct band gap and the electrochemical catalytic activity for redox couple of Sn 2- /S2− were systemically investigated by energy dispersive x-ray spectroscopy, x-ray diffraction, scanning electron microscope, the ultraviolet-visible-near infrared spectroscopy and electrochemical impedance spectroscopy, respectively. The photovoltaic performance of QDSCs with the corresponding pyrolysis CZTS thin film and the platinized conducting glass was evaluated.
2. Experimental 2.1. Preparation of pyrolysis CZTS thin films
∗ Corresponding author. Tel.: +86 551 62901450, fax: +86 551 62901450. E-mail address: [email protected] (C. Shi). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.168
All chemicals were commercially available and used without further purification. The preparation procedure of the
42
Y. Zhang et al. / Electrochimica Acta 118 (2014) 41–44
precursor solution for the pyrolysis CZTS thin film was as follows: 2.9232 g (0.0384 mol) thiourea, 0.8184 g (0.0048 mol) CuCl2 ·2H2 O, 0.3270 g (0.0024 mol) ZnCl2 , 0.5414 g (0.0024 mol) SnCl2 ·2H2 O were sequentially dissolved in 80 ml methanol, then 2.7648 g (0.1536 mol) deionized water was added to the solution to promote the hydrolysis of thiourea. Fluorine-doped tin oxide (FTO) substrates were dipped into the above precursor solution, then heated at 380 ◦ C for 5 min in air atmosphere. All these processes were termed as one cycle. After the cycles of 2, 5 and 7, three kinds of pyrolysis CZTS thin films were obtained, hereafter referred to CZTS (2, 5, 7). CdS/CdSe/ZnS co-sensitized TiO2 photoanode and the platinized conducting glass counter electrode were prepared according to our previous reports[13,14]. Aqueous polysulfide electrolyte consisting of 1 mol dm−3 sodium sulfide, 0.5 mol dm−3 sulfur was used as the electrolyte in QDSCs. 2.2. Characterization Electrochemical impedance spectroscopy (EIS) was obtained by applying sinusoidal perturbations of ±5 mV over the bias 0 V at frequencies from 105 Hz to 0.1 Hz, carried out with the electrochemical workstation (CHI 660B) using the symmetrical cells based on the pyrolysis CZTS thin films to evaluate their electrochemical catalytic activity for the redox couple of Sn 2- /S2− . The chemical composition of pyrolysis CZTS thin films was analyzed by energy disperse spectroscopy (EDS, Sivion200, USA). X-ray diffractometer (D/MAX2500 V, Rigaku, Japan) was used to record x-ray diffraction (XRD) patterns. Cu-K␣ radiation (=0.154056 nm, 40 kV and 40 mA) was used to record the spectra in the 2 range of 10-80◦ with a step size of 0.026◦ s−1 . The microstructure and the surface morphology were observed using field emission scanning electron microscope (FE-SEM, HITACHI, Japan). The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra were recorded with a double beam UV-Vis-NIR (DUV-3700, Shimadzu, Japan) in the wavelength range of 400-1400 nm at the resolution of 1 nm. The photovoltaic performance of QDSCs was measured with a solar simulator (Oriel, Newport, USA, AM 1.5, 100 mW cm−2 ) and a Keithley 2420 source meter controlled by Testpoint software. The irradiation intensity was calibrated with standard crystalline silicon solar cell (Oriel, Newport, USA). The active areas of QDSCs were set at 0.25 cm2 . 3. Results and discussion 3.1. The composition, morphology and direct band gap of pyrolysis CZTS thin films Fig. 1 displayed the SEM images of pyrolysis CZTS thin films. Compared with Fig. 1 (a), (b), (c), it was found that the cycle times obviously affect the surface morphology of pyrolysis CZTS thin films. Interestingly, the pyrolysis CZTS (7) thin film showed uniform and porous surface feature with the pore size of 100-200 nm, which was beneficial to improve the electrochemical catalytic activity for the redox couple of Sn 2- /S2− . The chemical composition of pyrolysis CZTS thin films was determined to be Cu2.29 Zn1.00 Sn1.28 S4.10 by EDS analysis. Fig. 2 showed the x-ray diffraction pattern of pyrolysis CZTS (7) thin films, indicated that the strongest peaks located at 2 = 28.55◦ with weak peaks at 2 = 47.23◦ , 56.27◦ , corresponding to the spacing of (112), (220), (312) planes of CZTS and a preferred orientation along (112) plane appeared, matched well with the phases of CZTS (JCPDS NO. 26-0575). No diffraction peaks of any impurities can be detected. The direct band gap of pyrolysis CZTS (7) thin film was estimated by extrapolating the linear region of a plot of the absorbance squared
Fig. 1. SEM images of the pyrolysis CZTS thin films. a. CZTS (2); b. CZTS (5); c. CZTS (7);
Fig. 2. XRD pattern of the pyrolysis CZTS (7) thin film.
Y. Zhang et al. / Electrochimica Acta 118 (2014) 41–44
Fig. 3. (˛h)2 vs h plots of the pyrolysis CZTS (7) thin films.
43
Fig. 5. The J–V curves of the QDSCs with different counter electrodes. a. CZTS (2); b. CZTS (5); c. CZTS (7); d. Pt.
Table 2 Photovoltaic parameters of QDSCs with various counter electrodes. Counter electrode
Voc /V
Jsc /mA cm−2
FF
/%
CZTS (2) CZTS (5) CZTS (7) Pt
0.51 0.50 0.52 0.52
1.73 12.67 12.96 12.89
0.31 0.34 0.38 0.32
0.27 2.15 2.56 2.14
3.3. Photovoltaic performance of QDSCs Fig. 4. Nyquist plots of pyrolysis CZTS thin films. a. CZTS (2); b. CZTS (5); c. CZTS (7).
(˛h)2 versus energy h as shown in Fig. 3, and the value of the direct band gap was 1.44 eV, which matches well with the reported values[15]. 3.2. Electrochemical catalytic activity of pyrolysis CZTS thin films for the redox couple of Sn 2- /S2Fig. 4 showed the EIS spectra of the symmetrical cells based on pyrolysis CZTS thin films. Experimental data were represented by symbols while the solid lines correspond to the fit using the equivalent circuit Rs (Rct CPE)[14,16], and the parameters obtained by fitting the experimental spectra with the equivalent circuit were listed in Table 1. The serial resistance (Rs ) described the resistance of the FTO and solution, Rct was the charge-transfer resistance of the CZTS/electrolyte interfaces. The constant phase element (CPE) was a nonideal frequency-dependent capacitance, and its admittance was expressed by the equation, YQ = Y0 (jω)n , where n was a constant ranging from 0≤ n ≤1[16]. From Table 1, Rct decreased from 395.50 , 92.32 to 64.08 with the increase of the cycle times. It was worth noting that the pyrolysis CZTS (7) thin film showed the lowest charge-transfer resistance (Rct ), indicating the highest electrochemical catalytic activity for the redox couple of Sn 2- /S2− . And the Y0 value of pyrolysis CZTS (7) thin film was bigger than that of CZTS (2) and CZTS (5). The results of Rct and Y0 should be related to the uniform and porous surface feature with the pore size of 100-200 nm of the pyrolysis CZTS (7).
Table 1 Parameters obtained by fitting the experimental spectra with the equivalent circuit. Thin film
Rs /
Rct /
Y0 /F sn−1
n
CZTS (2) CZTS (5) CZTS (7)
31.77 33.83 33.53
395.50 92.32 64.08
854 3910 5700
0.73 0.63 0.63
Fig. 5 showed the photocurrent density-voltage (J-V) characteristics of the QDSCs, and the resultant photovoltaic parameters were summarized in Table 2. Compared with CZTS (7) and CZTS (5), the FF of the CZTS (7) was higher than that of CZTS (5), which was because the Rct 64.08 of CZTS (7) was smaller than the Rct 92.32 of CZTS (5) and the Y0 5700 F sn−1 of CZTS (7) was higher than the Y0 3910 F sn−1 of CZTS (5). There was no obvious difference of Voc and Jsc , and this was because the QDSCs of CZTS (7) and CZTS (5) used the same photoanode and polysulfide electrolyte. Therefore, the QDSCs of CZTS (7) achieved the photoelectric conversion efficiency of 2.56%. Moreover, the short circuit photocurrent density of CZTS (2) QDSCs was only 1.73 mA cm−2 , which may be due to the coverage of the CZTS (2) thin film was not complete. The QDSCs with Pt counter electrode gave a Jsc of 12.89 mA cm−2 , a Voc of 0.52 V, a FF of 0.32, and a of 2.14%, lower than that of the QDSCs with CZTS (7) counter electrode. The results revealed that pyrolysis CZTS thin film can serve as effective counter electrode in QDSCs. 4. Conclusions The pyrolysis CZTS thin films have been demonstrated to perform the efficient CEs in QDSCs, and the photoelectric conversion efficiency of 2.56% was achieved by the QDSCs with CZTS (7) counter electrode. The good performance in the efficiency can be attributed to the high electrochemical catalytic activity of pyrolysis CZTS thin films for the redox couple of Sn 2- /S2− . The results indicated that the pyrolysis CZTS thin films can serve as alternative to noble Pt. The optimization of the chemical composition of the pyrolysis CZTS thin films was ongoing. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (51072043, 51272061), National Basic Research Program of China (2011CBA00700), Anhui Province Science and Technology Plan Project of China (2010AKND0794).
44
Y. Zhang et al. / Electrochimica Acta 118 (2014) 41–44
References [1] Y.Y. Yang, Q.X. Zhang, T.Z. Wang, L.F. Zhu, X.M. Huang, Y.D. Zhang, X. Hu, D.M. Li, Y.H. Luo, Q.B. Meng, Novel tandem structure employing mesh-structured Cu2 S counter electrode for enhanced performance of quantum dot-sensitized solar cells, Electrochim. Acta 88 (2013) 44. [2] T. Bora, H.H. Kyaw, J. Dutta, Zinc oxide-zinc stannate core-shell nanorod arrays for CdS quantum dot sensitized solar cells, Electrochim. Acta 68 (2012) 44. [3] C.J. Raj, K. Prabakar, A.D. Savariraj, H.J. Kim, Surface reinforced platinum counter electrode for quantum dots sensitized solar cells, Electrochim. Acta 103 (2013) 231. [4] M.H. Yeh, C.P. Lee, C.Y. Chou, L.Y. Lin, H.Y. Wei, C.W. Chu, R. Vittal, K.C. Ho, Conducting polymer-based counter electrode for a quantum-dot-sensitized solar cell (QDSSC) with a polysulfide electrolyte, Electrochim. Acta 57 (2011) 277. [5] Y.B. Cao, Y.J. Xiao, J.Y. Jung, H.D. Um, S.W. Jee, H.M. Choi, J.H. Bang, J.H. Lee, Highly electrocatalytic Cu2 ZnSn(S1-x Sex )4 counter electrodes for quantum-dotsensitized solar cells, ACS Appl. Mater. Interfaces 5 (2013) 479. [6] S. Ruhle, M. Shalom, A. Zaban, Quantum-dot-sensitized solar cells, ChemPhysChem 11 (2010) 2290. [7] Z. Tachan, M. Shalom, I. Hod, S. Rühle, S.A. Tirosh, Zaban, PbS as a highly catalytic counter electrode for polysulfide-based quantum dot solar cells, J. Phys. Chem. C 115 (2011) 6162. [8] V.G. Pedro, X.Q. Xu, L.M. Seró, J. Bisquert, Modeling high-efficiency quantum dot sensitized solar cells, ACS NANO 4 (2010) 5783. [9] Z. Yang, C.Y. Chen, C.W. Liu, H.T. Chang, Electrocatalytic sulfur electrodes for CdS/CdSe quantum dot-sensitized solar cells, Chem. Commun 46 (2010) 5485.
[10] J.G. Radich, R. Dwyer, P.V. Kamat, Cu2 S reduced graphene oxide composite for high-efficiency quantum dot solar cells. Overcoming the redox limitations of S2- /Sn 2- at the counter electrode, J. Phys. Chem. Lett 2 (2011) 2453. [11] J. Xu, X. Yang, Q.D. Yang, T.L. Wong, C.S. Lee, Cu2 ZnSnS4 hierarchical microspheres as an effective counter electrode material for quantum dot sensitized solar cells, J. Phys. Chem. C 116 (2012) 19718. [12] X.W. Zeng, W.J. Zhang, Y. Xie, D.H. Xiong, W. Chen, X.B. Xu, M.K. Wang, Y.B. Cheng, Low-cost porous Cu2 ZnSnSe4 film remarkably superior to noble Pt as counter electrode in quantum dot-sensitized solar cell system, J. Power Sources 226 (2013) 359. [13] Y.H. Zhang, J. Zhu, X.C. Yu, J.F. Wei, L.H. Hu, S.Y. Dai, The optical and electrochemical properties of CdS/CdSe co-sensitized TiO2 solar cells prepared by successive ionic layer adsorption and reaction processes, Sol. Energy 86 (2012) 964. [14] C.W. Shi, S.Y. Dai, K.J. Wang, X. Pan, L.Y. Zeng, L.H. Hu, F.T. Kong, L. Guo, Influence of various cations on redox behavior of I- and I3 - and comparison between KI complex with 18-crown-6 and 1, 2-dimethyl-3-propylimidazolium iodide in dye-sensitized solar cells, Eletrochim. Acta 50 (2005) 2597. [15] C. Steinhagen, M.G. Panthani, V. Akhavan, B. Goodfellow, B. Koo, B.A. Korgel, Synthesis of Cu2 ZnSnS4 nanocrystals for use in low-cost photovoltaics, J. Am. Chem. Soc 131 (2009) 12554. [16] C.W. Shi, S.Y. Dai, K.J. Wang, X. Pan, L. Guo, L.Y. Zeng, L.H. Hu, F.T. Kong, Influence of 1-methyl-3-propylimidazolium iodide on I3 - /I- redox behavior and photovoltaic performance of dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 86 (2005) 527.