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Sn alloy precursor films
Materials Chemistry and Physics 163 (2015) 24e29
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Low-cost Cu2ZnSnS4 thin films prepared from single step electrodeposited Cu/Zn/Sn alloy precursor films Ke Cheng, Jian Meng, Xiaoyun Wang, Yuqian Huang, Jingjing Liu, Ming Xue, Zuliang Du* Key Lab for Special Functional Materials of Ministry of Education, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, Henan University, Kaifeng 475004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cu/Zn/Sn alloy films were prepared by single step electrodeposition. Suppressed hydrogen evolution reaction. Homogeneous and compact Cu2ZnSnS4 absorber layer was fabricated. Photovoltaic activities were evaluated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2015 Received in revised form 9 June 2015 Accepted 14 June 2015 Available online 7 August 2015
The Cu/Zn/Sn (CZT) alloy films were prepared by a single step electrodeposition method on fluorinedoped tin oxide (FTO) substrates. In order to suppress the hydrogen evolution reaction, the pH value of the electrolytic solution was adjusted to 7.0 using diethanol amine. The homogeneous and compact Cu2ZnSnS4 (CZTS) absorber materials with better crystallinity were obtained by the subsequent sulfurization. The Raman spectra have not shown any secondary phases when the sulfurization temperature increased to 580 C. In order to evaluate their photovoltaic activities, the obtained CZTS films were introduced into the QDSSCs as counter electrodes. The highest conversion efficiency of 1.6% with a Voc of 507 mV and a Jsc of 11.2 mA/cm2 were achieved. The high performances of the electrodeposited CZTS CE indicated that they were suitable for application in environmentally-friendly thin film solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Chalcogenides Thin films Energy dispersive analysis of X-rays Electrical properties
1. Introduction Currently, CuInxGa1xSe2 (CIGS) has been regarded as one of the most promising light absorbing materials for low-cost and highquality thin film solar cells due to its suitable band gap and large optical absorption coefficient [1e3]. To date, CIGS solar cells have demonstrated the highest photoelectric conversion efficiency exceeding 20% at laboratory scale and have reached the industrial
* Corresponding author. E-mail address: [email protected] (Z. Du). http://dx.doi.org/10.1016/j.matchemphys.2015.06.026 0254-0584/© 2015 Elsevier B.V. All rights reserved.
production level [4]. However, CIGS consist of toxic and rare elements, such as selenium (Se), cadmium (Cd), indium (In), and gallium (Ga), which could limit the CIGS production and its practical applications [5,6]. Fortunately, kesterite copper zinc tin chalcogenide (CZTS) has emerged as an earth-abundant and environmentally-friendly version of CIGS where indium, gallium, and selenium are substituted by tin, zinc, and sulfur, respectively. The device architectures and growth techniques can be inherited due to its similar optical and electronic properties with CIGS [7e9]. CZTS is a quaternary kesterite semiconductor with a direct band gap (1.45 eVe1.6 eV) and a large absorption coefficient (~105 cm1)
K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
[8]. The photovoltaic properties of CZTS were firstly investigated by Ito and Nakazawa in 1988 [9]. In recent years, CZTS based solar cells have received increasing attention, and the highest efficiency of up to 8.4% and 12.7% have been achieved for CZTS and Cu2ZnSn (S, Se)4 (CZTSSe) solar cells, respectively [10,11]. CZTS thin films can be produced by various approaches, such as co-evaporation, magnetron sputtering/selenization, hydrazine-based chemical synthesis, and electrodeposition technique [11e14]. The vacuum-based techniques, such as co-evaporation and sputtering, are neither particularly low-cost nor versatile for the large-scale production due to the low growth rates, and the restrictive fabrication conditions such as vacuum and/or high temperature. Nowadays, the best CZTS based solar cell with an efficiency of 11.1% was produced from hydrazine-based slurry of CueZneSn chalcogenide (S or SeSe) [15]. However, hydrazine-based solutions have some limitations for large-scale production because of their toxicity or the complexity of the synthesis. Among them, electrodeposition technique is considered as promising alternative tool to obtain low-cost CZTS precursor films, as it offers a number of combined advantages to lower the manufacturing costs, high materials utilization, reduced machine investments, faster coating capabilities over large areas, and compatibility with roll-to-roll processes for high throughput production. There are two main approaches to fabricate CZTS precursor films: i) a stacked elemental layer Cu/Zn/Sn (CZT) is deposited in sequence and (ii) single step electrodeposition method produces an alloy layer of CueZneSn from the same electrolyte. Araki et al. fabricated the CZTS solar cells with best efficiency of 3.16% from stacked precursors containing Cu/Sn/Zn followed subsequent sulfurization to 600 C for 2 h [16]. Scragg et al. reported a solar cell efficiency of 3.2% with an electrodeposited metallic stack of Cu/Sn/ Cu/Zn and subsequent annealing of the stack in a sulfur containing atmosphere and heated to 575 C for 2 h [17]. Ennaoui et al. reported a single step electrodeposited CZT alloy with 3.4% efficiency where the alloy was annealed using a gas containing H2S at 550 C for 2 h [18]. By contrast, the single step deposition of alloy layer of CZT in a single bath is more anticipated than the separated individual baths required by the fabrication of stacked CZT precursor film. However, the standard reduction potentials of Cu, Zn and Sn differ widely. The deposition potential of Zn element is more negative than both of Cu and Sn. Therefore, the deposition potential needs be closer to the Zn reduction potential in order to obtain an ideal alloy composition. This will lead to competition between the Zn deposition and hydrogen evolution reaction (HER) and cause hydrogen embrittlement, which hampers growth of smooth photovoltaic quality CZT precursor films without the formation of undesirable pinholes [19]. This imposes certain limitations for straight forward single step electrodeposition. In the present research paper, the photovoltaic quality CZTS films are fabricated by single step electrodeposition and subsequent annealing of the CZT alloy films in a sulfur containing atmosphere. High quality CZT precursor films with flat and compact surface are obtained, which may be due to the suppressed hydrogen evolution reaction by adjusting the pH value of the electrolytic solution to 7.0 using diethanol amine. This method avoids the additional use of surfactants or organic additives as reported in earlier publications [20,21]. To the best of our knowledge, the elimination of hydrogen evolution reaction under mild condition has not been applied to the fabrication of CZTS thin films. The structures, morphologies, optical properties, and compositions of the obtained CZTS films under different deposition potentials and post-annealing temperatures are also investigated. Additionally, the photovoltaic activities are evaluated by introducing the CZTS films into quantum-dots sensitized solar cells (QDSSCs) as counter electrodes (CEs).
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2. Experimental section 2.1. Electrodeposition of CZT alloy film and the subsequent sulfurization procedure Single step electrodeposition of CZT alloy film was performed in an electrolyte containing 20 mM copper (II) sulfate pentahydrate, 170 mM zinc sulfate heptahydrate, 10 mM tin (II) chloride dehydrate, and 200 mM tri-sodium citrate dehydrate dissolved in deionized water. The final pH value was adjusted at 7.0 by addition of diethanol amine. The experimental setup consisted of a conventional three-electrode setup with platinum gauze as a counter electrode, Ag/AgCl as a reference electrode and fluorine-doped tin oxide (FTO) glass substrate as a working electrode. Before deposition, the FTO substrates (1.5 2 cm2) were cleaned ultrasonically in soap water, ethanol and 5% hydrogen peroxide water solution. Finally, the FTO substrates were thoroughly rinsed with deionized water and then dried by flowing nitrogen. The CZT alloy thin films were deposited at 1.20 and 1.30 V versus Ag/AgCl in potentiostatic mode for 20 min at room temperature without stirring. After deposition, the CZT alloy films were immediately placed in a double-temperature-zone tube furnace together with sufficient sulfur powder. The tube was heated at a rate of 10 C/min and the argon was used as carrier gas. The temperature of sulfur powder region was kept at 350 C. Meanwhile, the temperature of CZT alloy film region was kept at 560 C and 580 C for 60 min. Finally, the samples were cooled down naturally to room temperature in the furnace. In order to evaluate the photovoltaic activities, the obtained CZTS films were introduced into the QDSSCs as CEs. Ternary ZnxCd1xSe/ZnO nanowires array photoanode was fabricated by the hydrothermal growth and the subsequent ioneexchange reaction [22]. The polysulfide electrolytes were prepared by dissolving Na2S, S, and NaOH in deionized water. The solar cells were fabricated with a sandwich-type by bonding the ZnxCd1xSe/ZnO photoanode with the CZTS counter electrode. The two electrodes were separated by a spacer, and the internal space of the cell was filled with the polysulfide electrolyte. The active area was confined by clipping a mask on the photoanode with a square window of 0.2 cm2, and the cells were characterized immediately after the electrolyte filling. 2.2. Characterization techniques The surface morphologies, cross-sectional morphologies and elemental composition of the samples were measured using a JEOL 5600 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The structures of the sample were investigated by X-ray diffractometer (XRD) with X'Pert MRDPhilips diffractometer equipped with Cu ka radiation. The UVeVis HElIOSa spectrophotometer was employed to characterize the optical properties of the samples. The performances of the solar cells were evaluated under standard AM 1 global illumination using a class AAA Newport solar simulator. 3. Results and discussion For co-deposition of Cu, Zn, and Sn in the electrolyte solution, the electrodeposition potential needs closer to the Zn reduction potential. This would lead to competition between the Zn deposition and hydrogen evolution reaction under acidic aqueous solution. As we know, the HER can be suppressed effectively by adjusting the pH value of the electrolytic solution. Here, the pH value of the electrolyte solution was adjusted conveniently using diethanol amine as the alkaline complexing agent. The nitrogen
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K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
Fig. 1. Surface SEM images of CZT alloy films electrodeposited at potentials of (A) 1.2 V and (B) 1.3 V; (C) Cross-sectional SEM image of CZT alloy film electrodeposited at a potential of 1.2 V.
lone electronic of diethanol amine can react with water to form hydrogen bonds, which increases the ionization energy of the hydrogen. On the other hand, the introducing of diethanol amine can reduce the reaction of metal ions and hydrogen ions with sodium citrate, which can effectively decrease the reduction rate of copper ions. The reaction of nitrogen lone electronic of diethanol amine with water is shown below.
Fig. 1A and B presents the surface SEM images of CZT alloy films electrodeposited at potentials of 1.2 V and 1.3 V respectively. We can see that the CZT precursor films are flat and compact without the formation of undesirable pinholes. This is may be due to the suppressed hydrogen evolution reaction by using diethanol amine as the alkaline complexing agent. The CZT alloy film deposited at potential of 1.2 V consists of spherical grains of approximately 200e300 nm in diameter as shown in Fig. 1A. From the crosssectional SEM image of the alloy film obtained at deposition potential of 1.2 V, the average film thickness is found to be about 1 mm. As the deposition potential increased to 1.3 V, the grain size increased to about 400e600 nm as shown in Fig. 1B. From the morphologies analysis, we can conclude that the surface morphologies of CZT alloy films are strongly depended on the deposition potentials. Fig. 2 shows the SEM images of CZTS thin films obtained from different deposition potentials and different annealing temperatures. After the high temperature sulfurization, the CZT alloy
precursor films were converted into the CZTS absorber materials. The CZTS films are more compact than the CZT alloy films as observed from the surface SEM images. From the cross-sectional images, a substantial volume expansion to about three times (~3 mm) that of the original thickness of CZT films (~1 mm) can be observed. No big difference can be observed from the surface SEM images for all the CZTS films. However, a few of voids can be observed in the CZTS or at the CZTS/substrate interface for the CZTS films annealed at 560 C as observed from Fig. 2C. These voids are generally due to non-uniform distribution of the elements in local regions [23]. When the sulfurization temperature increased to 580 C, the crystallinity of the CZTS film was improved and the voids were vanished as shown in Fig. 2F. The absence of voids indicates the homogeneous distribution of the CZTS throughout the film. The results reveal the higher sulfurization temperature is beneficial to form homogeneous and compact CZTS films with better crystallinity. The composition ratio of each element in CZTS films deposited at potential of 1.2 V and 1.3 V was measured by EDS and was shown in Table 1. Both the CZTS films were annealed at 580 C. The strict stoichiometric ratio of Cu, Zn, Sn and S elements should be 2:1:1:4. However, it was reported that the CZTS solar cells under Cu poor and Zn rich conditions give the higher efficiencies. The Cu/
Table 1 Composition ratio of CZTS films (580 C) determined by EDS. Samples
Cu/(Zn þ Sn)
Zn/Sn
Cu (at %)
Zn (at %)
Sn (at %)
S (at %)
1.20 V 1.30 V
0.46 1.03
0.25 0.90
9.37 20.40
4.12 9.30
16.09 10.32
70.04 59.98
Fig. 2. Surface SEM images of CZTS films annealed at (A) 560 C for potential of 1.2 V; (B) 560 C for potential of 1.3 V; (D) 580 C for potential of 1.2 V; (E) 580 C for potential of 1.3 V. Cross-sectional SEM images of CZTS films annealed at (C) 560 C for potential of 1.2 V and (F) 580 C for potential of 1.2 V.
K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
Fig. 3. XRD patterns of CZTS films obtained from different deposition potentials and different sulfurization temperatures.
27
Fig. 5. Plot of (ahy)2 versus hy for the CZTS films.
(Zn þ Sn) ratio for the most reported CZTS based solar cells is between 0.75 and 1. Meanwhile, the Zn/Sn ratio is between 1 and 1.25, with deviation by as large as 25% [6]. In our case, Cu/(Zn þ Sn) and Zn/Sn ratio of the CZTS film deposited at potential of 1.2 V is 0.46 and 0.25 respectively. The lower Cu/(Zn þ Sn) and Zn/Sn ratio indicates the CZT alloy film deposited at potential of 1.2 V was under Cu and Zn poor conditions. When the deposition potential decreased to 1.3 V, the Cu/(Zn þ Sn) and Zn/Sn ratio increased to 1.03 and 0.9, which was close to the ideal stoichiometric ratio. The composition ratio of sulfur exceeding 50% indicated both the alloy CZT films were fully sulfurized. Fig. 3 shows the X-ray diffraction patterns of the CZTS films. All the CZTS films show a polycrystalline kesterite crystal structure [JCPDS #26-0575] with major reflections along (112), (200), (220) and (312) planes [24]. The obviously reduced breadth of the XRD peak along (112) plane observed for the CZTS films sulfurized at 580 C indicated that the crystallinity of the CZTS films were improved with the increasing of sulfurization temperature. Several XRD peaks with low intensity observed in Fig. 3a, b and c may be
attributed to Cu and Sn sulphides. When the sulfurization temperature increased to 580 C for the sample deposited at 1.3 V, the XRD peaks with low intensity became undistinguishable as shown in Fig. 3d. However, the wurtzite ZnS, CuS and monoclinic CTS crystal structure are very similar to the CZTS pattern, and they are difficult to be excluded using XRD technique alone. In order to further identify and exclude the ZnS, CueSneS and CuS associated secondary phases, the CZTS films were analyzed by Raman spectroscopy as shown in Fig. 4. For the Raman measurements, the 632.8 nm line of a HeeNe laser was used for excitation. The characteristic peaks of CZTS at 285 cm1, 336 cm1 and 364 cm1 can be observed clearly which indicated the formation of CZTS phase for all the samples [25]. For both the CZTS films sulfurized at 560 C, a peak at 474 cm1 can be observed, which indicates the formation of separate CuS phase [25]. When the sulfurization temperature increased to 580 C, the Raman spectra have not shown any secondary phases as observed in Fig. 4. The plot of (ahy)2 versus hy for the CZTS films is shown in Fig. 5. The band gap energy of the CZTS films can be obtained by Tuac's relation [26].
Fig. 4. Raman spectra of the CZTS films obtained from different deposition potentials and different sulfurization temperatures.
Fig. 6. Current densityevoltage (JeV) curves of the QDSSCs based on the CZTS counter electrodes.
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K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
Table 2 Photovoltaic parameters obtained from the JeV curves. Samples 1.20 1.30 1.20 1.30
V V V V
(560 (560 (580 (580
C) C) C) C)
ðahyÞ2 ¼ A hy Eg
Voc (mV)
Jsc (mA/cm2)
FF (%)
Rsh (U)
Rs (U)
Efficiency (%)
524 541 524 507
7.30 8.10 10.4 11.2
28.1 27.6 28.5 27.9
319.9 350.2 235.1 220.2
629.5 526.3 396.2 397.1
1.16 1.22 1.56 1.60
where a is the absorption coefficient and hʋ is the photon energy. The band gap energy (Eg) can be obtained by extrapolating the linear part of the (ahy)2 vs photon energy (hy) plot. The band gap energies of the CZTS films surfurized at 580 C were estimated to be 1.45 eV (1.3 V) and 1.41 eV (1.2 V). The estimated band gap energies of CZTS films at higher sulfurization temperature agreed well with the reported band gap energy values for the CZTS films [27]. However, we should note that the band gap energy decreased to about 1.3 eV for the CZTS films sulfurized at 560 C. As observed in Fig. 4, the separate CuS phase existed in the CZTS films sulfurized at lower temperature. The lower band gap of CuS (~1.21 eV) resulted in the narrow band gap energy for the CZTS films sulfurized at 560 C [28]. Recently, it was reported that CZTS can be exploited as an effective and non-platinum-based CE material in QDSSCs [29]. The CE is responsible for catalyzing the reduction of the redox shuttle in the electrolyte by electrons from external circuit and keeping the cell running [30]. In order to evaluate the photovoltaic activities of the electrodeposited CZTS films, the solar cells were fabricated with a sandwich-type by bonding the ZnxCd1xSe/ZnO photoanode with the CZTS counter electrodes. The current densityevoltage (JeV) characteristics were shown in Fig. 6. And the corresponding key photovoltaic parameters were summarized in Table 2. A conversion efficiency of 1.16% with a Voc of 524 mV, a Jsc of 7.3 mA/cm2, and an FF of 28.1% was obtained for the CZTS CEs deposited at 1.2 V (560 C). The conversion efficiency increased slightly to 1.22% when the deposition potential decreased to 1.3 V (560 C). When the sulfurization temperature of the CZTS CEs increased to 580 C, the conversion efficiencies can be greatly improved. The highest conversion efficiency of 1.6% with a Voc of 507 mV, a Jsc of 11.2 mA/cm2, and an FF of 27.9% were achieved for the CZTS CEs deposited at 1.3 V (580 C). There are no significant differences in Voc and FF for all the CZTS CEs as can be seen from Fig. 6 and Table 2. However, we should note that the Jsc increased from 8.1 mA/cm2 to 11.2 mA/ cm2 with the sulfurization temperature increased from 560 C to 580 C. The larger Jsc can be attributed to the improved crystallinity and the elimination of secondary phases under higher sulfurization temperature as discussed in Figs. 3 and 4. The high performance of the single step electrodeposited CZTS CEs indicated that they were suitable for application in environmentally-friendly thin film solar cells. 4. Conclusions CZT films were deposited on FTO substrates using single step electrodeposition. May be due to the suppressed hydrogen evolution reaction by adjusting the pH value of the electrolytic solution to 7.0 using diethanol amine, high quality CZT precursor films with flat and compact surface are obtained. By sulfurization of the electrodeposited CZT films in a sulfur containing atmosphere, CZTS films were fabricated with better crystallinity. For the CZTS film fabricated from the CZT precursor with a deposition potential of 1.3 V and higher sulfurization temperature, the Cu/(Zn þ Sn)
and Zn/Sn ratio is 1.03 and 0.9, respectively. Such stoichiometric ratios are close to the ideal ones. The Raman spectra have not shown any secondary phases when the sulfurization temperature increased to 580 C. The band gap energy of CZTS films at higher sulfurization temperature was estimated to be 1.45 eV. The highest conversion efficiency of 1.6% with a Voc of 507 mV, a Jsc of 11.2 mA/ cm2, and an FF of 27.9% were achieved for the CZTS CEs deposited at 1.3 V (580 C). These results demonstrated the introduction of diethanol amine into the electrolytic solution was an effective strategy for the deposition of high quality CZT films. Acknowledgments This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. PCS IRT1126) and the National Natural Science Foundation of China (Nos. 11274093, 61376061 and 61240053). References [1] W. Wang, S.Y. Han, S.J. Sung, D.H. Kim, C.H. Chang, Phys. Chem. Chem. Phys. 14 (2012) 11154e11159. [2] A. Duchatelet, T. Sidali, N. Loones, G. Savidand, E. Chassaing, D. Lincot, Sol. Energy Mat. Sol. Cells 119 (2013) 241e245. [3] S. Rampino, N. Armani, F. Bissoli, M. Bronzoni, D. Calestani, M. Calicchio, M. Mazzer, Appl. Phys. Lett. 101 (2012) 132107. [4] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. Res. Appl. 19 (2011) 894e897. [5] J.J. Scragg, J.T. Watjen, M. Edoff, T. Ericson, T. Kubart, C. Platzer-Bjorkman, J. Am. Chem. Soc. 134 (2012) 19330e19333. [6] S.Y. Chen, A. Walsh, X.G. Gong, S.H. Wei, Adv. Mater. 25 (2013) 1522e1539. [7] Y.F. Liu, F.Q. Huang, Y.A. Xie, H.L. Cui, W. Zhao, C.Y. Yang, N. Dai, J. Phys. Chem. C 117 (2013) 10296e10301. [8] G. Brammertz, M. Buffiere, Y. Mevel, Y. Ren, A.E. Zaghi, N. Lenaers, J. Poortmans, Appl. Phys. Lett. 102 (2013) 013902. [9] N.M. Shinde, P.R. Deshmukh, S.V. Patil, C.D. Lokhande, Mater. Res. Bull. 48 (2013) 1760e1766. [10] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S.J. Chey, S. Guha, Prog. Photovolt. Res. Appl. 21 (2013) 72e76. [11] J. Kim, H. Hiroi, T.K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M. Hopstaken, B. Shin, Y.S. Lee, Adv. Mater. 26 (2014) 7427e7431. [12] U. Chalapathi, S. Uthanna, V.S. Raja, Sol. Energy Mater. Sol. Cells 132 (2015) 476e484. [13] S.M. Pawar, A.I. Inamdar, K.V. Gurav, S.W. Shin, J. Gwak, Y. Jo, J. Yun, H. Pak, S. Kwon, H. Kim, J.H. Kim, H. Im, Curr. Appl. Phys. 15 (2015) 59e63. [14] J.H. Tao, J. He, K.Z. Zhang, J.F. Liu, Y.C. Dong, L. Sun, P.X. Yang, J.H. Chu, Mater. Lett. 135 (2014) 8e10. [15] T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D.B. Mitzi, Adv. Energy Mater. 3 (2013) 34e38. [16] H. Araki, Y. Kubo, K. Jimbo, W.S. Maw, H. Katagiri, M. Yamazaki, K. Oishi, A. Takeuch, Phys. Status Solidi C 6 (2009) 1266e1269. [17] J.J. Scragg, D. Berg, P.J. Dale, J. Electroanal. Chem. 646 (2010) 52e56. [18] A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I. Kotschau, Thin Solid Films 517 (2009) 2511e2514. [19] K.V. Gurav, J.H. Yun, S.M. Pawar, S.W. Shin, M.P. Suryawanshi, Y.K. Kim, J.H. Kim, Mater. Lett. 108 (2013) 316e319. [20] L. Guo, Y. Zhu, O. Gunawan, T. Gokmen, V.R. Deline, S. Ahmed, L.T. Romankiw, H. Deligianni, Prog. Photovolt. Res. Appl. 22 (2014) 58e68. [21] S.G. Lee, J. Kim, H.S. Woo, Y. Jo, A.I. Inamdar, S.M. Pawar, H.S. Kim, W. Jung, H.S. Im, Curr. Appl. Phys. 14 (2014) 254e258. [22] J. Xu, C.Y. Luan, Y.B. Tang, X. Chen, J.A. Zapien, W.J. Zhang, H.L. Kwong, Z.M. Meng, S.T. Lee, C.S. Lee, ACS Nano 4 (2010) 6064. [23] S.M. Pawar, A.I. Inamdar, K.V. Gurav, S.W. Shin, J. Kim, H. Im, J.H. Kim, Vacuum 104 (2014) 57e60. [24] S.G. Lee, J. Kim, H.S. Woo, Y. Jo, A.I. Inamdar, S.M. Pawar, H.S. Im, Curr. Appl. Phys. 14 (2014) 254e258. [25] J. Lehner, M. Ganchev, M. Loorits, N. Revathi, T. Raadik, J. Raudoja,
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Low-cost Cu2ZnSnS4 thin films prepared from single step electrodeposited Cu/Zn/Sn alloy precursor films Ke Cheng, Jian Meng, Xiaoyun Wang, Yuqian Huang, Jingjing Liu, Ming Xue, Zuliang Du* Key Lab for Special Functional Materials of Ministry of Education, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, Henan University, Kaifeng 475004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cu/Zn/Sn alloy films were prepared by single step electrodeposition. Suppressed hydrogen evolution reaction. Homogeneous and compact Cu2ZnSnS4 absorber layer was fabricated. Photovoltaic activities were evaluated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2015 Received in revised form 9 June 2015 Accepted 14 June 2015 Available online 7 August 2015
The Cu/Zn/Sn (CZT) alloy films were prepared by a single step electrodeposition method on fluorinedoped tin oxide (FTO) substrates. In order to suppress the hydrogen evolution reaction, the pH value of the electrolytic solution was adjusted to 7.0 using diethanol amine. The homogeneous and compact Cu2ZnSnS4 (CZTS) absorber materials with better crystallinity were obtained by the subsequent sulfurization. The Raman spectra have not shown any secondary phases when the sulfurization temperature increased to 580 C. In order to evaluate their photovoltaic activities, the obtained CZTS films were introduced into the QDSSCs as counter electrodes. The highest conversion efficiency of 1.6% with a Voc of 507 mV and a Jsc of 11.2 mA/cm2 were achieved. The high performances of the electrodeposited CZTS CE indicated that they were suitable for application in environmentally-friendly thin film solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Chalcogenides Thin films Energy dispersive analysis of X-rays Electrical properties
1. Introduction Currently, CuInxGa1xSe2 (CIGS) has been regarded as one of the most promising light absorbing materials for low-cost and highquality thin film solar cells due to its suitable band gap and large optical absorption coefficient [1e3]. To date, CIGS solar cells have demonstrated the highest photoelectric conversion efficiency exceeding 20% at laboratory scale and have reached the industrial
* Corresponding author. E-mail address: [email protected] (Z. Du). http://dx.doi.org/10.1016/j.matchemphys.2015.06.026 0254-0584/© 2015 Elsevier B.V. All rights reserved.
production level [4]. However, CIGS consist of toxic and rare elements, such as selenium (Se), cadmium (Cd), indium (In), and gallium (Ga), which could limit the CIGS production and its practical applications [5,6]. Fortunately, kesterite copper zinc tin chalcogenide (CZTS) has emerged as an earth-abundant and environmentally-friendly version of CIGS where indium, gallium, and selenium are substituted by tin, zinc, and sulfur, respectively. The device architectures and growth techniques can be inherited due to its similar optical and electronic properties with CIGS [7e9]. CZTS is a quaternary kesterite semiconductor with a direct band gap (1.45 eVe1.6 eV) and a large absorption coefficient (~105 cm1)
K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
[8]. The photovoltaic properties of CZTS were firstly investigated by Ito and Nakazawa in 1988 [9]. In recent years, CZTS based solar cells have received increasing attention, and the highest efficiency of up to 8.4% and 12.7% have been achieved for CZTS and Cu2ZnSn (S, Se)4 (CZTSSe) solar cells, respectively [10,11]. CZTS thin films can be produced by various approaches, such as co-evaporation, magnetron sputtering/selenization, hydrazine-based chemical synthesis, and electrodeposition technique [11e14]. The vacuum-based techniques, such as co-evaporation and sputtering, are neither particularly low-cost nor versatile for the large-scale production due to the low growth rates, and the restrictive fabrication conditions such as vacuum and/or high temperature. Nowadays, the best CZTS based solar cell with an efficiency of 11.1% was produced from hydrazine-based slurry of CueZneSn chalcogenide (S or SeSe) [15]. However, hydrazine-based solutions have some limitations for large-scale production because of their toxicity or the complexity of the synthesis. Among them, electrodeposition technique is considered as promising alternative tool to obtain low-cost CZTS precursor films, as it offers a number of combined advantages to lower the manufacturing costs, high materials utilization, reduced machine investments, faster coating capabilities over large areas, and compatibility with roll-to-roll processes for high throughput production. There are two main approaches to fabricate CZTS precursor films: i) a stacked elemental layer Cu/Zn/Sn (CZT) is deposited in sequence and (ii) single step electrodeposition method produces an alloy layer of CueZneSn from the same electrolyte. Araki et al. fabricated the CZTS solar cells with best efficiency of 3.16% from stacked precursors containing Cu/Sn/Zn followed subsequent sulfurization to 600 C for 2 h [16]. Scragg et al. reported a solar cell efficiency of 3.2% with an electrodeposited metallic stack of Cu/Sn/ Cu/Zn and subsequent annealing of the stack in a sulfur containing atmosphere and heated to 575 C for 2 h [17]. Ennaoui et al. reported a single step electrodeposited CZT alloy with 3.4% efficiency where the alloy was annealed using a gas containing H2S at 550 C for 2 h [18]. By contrast, the single step deposition of alloy layer of CZT in a single bath is more anticipated than the separated individual baths required by the fabrication of stacked CZT precursor film. However, the standard reduction potentials of Cu, Zn and Sn differ widely. The deposition potential of Zn element is more negative than both of Cu and Sn. Therefore, the deposition potential needs be closer to the Zn reduction potential in order to obtain an ideal alloy composition. This will lead to competition between the Zn deposition and hydrogen evolution reaction (HER) and cause hydrogen embrittlement, which hampers growth of smooth photovoltaic quality CZT precursor films without the formation of undesirable pinholes [19]. This imposes certain limitations for straight forward single step electrodeposition. In the present research paper, the photovoltaic quality CZTS films are fabricated by single step electrodeposition and subsequent annealing of the CZT alloy films in a sulfur containing atmosphere. High quality CZT precursor films with flat and compact surface are obtained, which may be due to the suppressed hydrogen evolution reaction by adjusting the pH value of the electrolytic solution to 7.0 using diethanol amine. This method avoids the additional use of surfactants or organic additives as reported in earlier publications [20,21]. To the best of our knowledge, the elimination of hydrogen evolution reaction under mild condition has not been applied to the fabrication of CZTS thin films. The structures, morphologies, optical properties, and compositions of the obtained CZTS films under different deposition potentials and post-annealing temperatures are also investigated. Additionally, the photovoltaic activities are evaluated by introducing the CZTS films into quantum-dots sensitized solar cells (QDSSCs) as counter electrodes (CEs).
25
2. Experimental section 2.1. Electrodeposition of CZT alloy film and the subsequent sulfurization procedure Single step electrodeposition of CZT alloy film was performed in an electrolyte containing 20 mM copper (II) sulfate pentahydrate, 170 mM zinc sulfate heptahydrate, 10 mM tin (II) chloride dehydrate, and 200 mM tri-sodium citrate dehydrate dissolved in deionized water. The final pH value was adjusted at 7.0 by addition of diethanol amine. The experimental setup consisted of a conventional three-electrode setup with platinum gauze as a counter electrode, Ag/AgCl as a reference electrode and fluorine-doped tin oxide (FTO) glass substrate as a working electrode. Before deposition, the FTO substrates (1.5 2 cm2) were cleaned ultrasonically in soap water, ethanol and 5% hydrogen peroxide water solution. Finally, the FTO substrates were thoroughly rinsed with deionized water and then dried by flowing nitrogen. The CZT alloy thin films were deposited at 1.20 and 1.30 V versus Ag/AgCl in potentiostatic mode for 20 min at room temperature without stirring. After deposition, the CZT alloy films were immediately placed in a double-temperature-zone tube furnace together with sufficient sulfur powder. The tube was heated at a rate of 10 C/min and the argon was used as carrier gas. The temperature of sulfur powder region was kept at 350 C. Meanwhile, the temperature of CZT alloy film region was kept at 560 C and 580 C for 60 min. Finally, the samples were cooled down naturally to room temperature in the furnace. In order to evaluate the photovoltaic activities, the obtained CZTS films were introduced into the QDSSCs as CEs. Ternary ZnxCd1xSe/ZnO nanowires array photoanode was fabricated by the hydrothermal growth and the subsequent ioneexchange reaction [22]. The polysulfide electrolytes were prepared by dissolving Na2S, S, and NaOH in deionized water. The solar cells were fabricated with a sandwich-type by bonding the ZnxCd1xSe/ZnO photoanode with the CZTS counter electrode. The two electrodes were separated by a spacer, and the internal space of the cell was filled with the polysulfide electrolyte. The active area was confined by clipping a mask on the photoanode with a square window of 0.2 cm2, and the cells were characterized immediately after the electrolyte filling. 2.2. Characterization techniques The surface morphologies, cross-sectional morphologies and elemental composition of the samples were measured using a JEOL 5600 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The structures of the sample were investigated by X-ray diffractometer (XRD) with X'Pert MRDPhilips diffractometer equipped with Cu ka radiation. The UVeVis HElIOSa spectrophotometer was employed to characterize the optical properties of the samples. The performances of the solar cells were evaluated under standard AM 1 global illumination using a class AAA Newport solar simulator. 3. Results and discussion For co-deposition of Cu, Zn, and Sn in the electrolyte solution, the electrodeposition potential needs closer to the Zn reduction potential. This would lead to competition between the Zn deposition and hydrogen evolution reaction under acidic aqueous solution. As we know, the HER can be suppressed effectively by adjusting the pH value of the electrolytic solution. Here, the pH value of the electrolyte solution was adjusted conveniently using diethanol amine as the alkaline complexing agent. The nitrogen
26
K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
Fig. 1. Surface SEM images of CZT alloy films electrodeposited at potentials of (A) 1.2 V and (B) 1.3 V; (C) Cross-sectional SEM image of CZT alloy film electrodeposited at a potential of 1.2 V.
lone electronic of diethanol amine can react with water to form hydrogen bonds, which increases the ionization energy of the hydrogen. On the other hand, the introducing of diethanol amine can reduce the reaction of metal ions and hydrogen ions with sodium citrate, which can effectively decrease the reduction rate of copper ions. The reaction of nitrogen lone electronic of diethanol amine with water is shown below.
Fig. 1A and B presents the surface SEM images of CZT alloy films electrodeposited at potentials of 1.2 V and 1.3 V respectively. We can see that the CZT precursor films are flat and compact without the formation of undesirable pinholes. This is may be due to the suppressed hydrogen evolution reaction by using diethanol amine as the alkaline complexing agent. The CZT alloy film deposited at potential of 1.2 V consists of spherical grains of approximately 200e300 nm in diameter as shown in Fig. 1A. From the crosssectional SEM image of the alloy film obtained at deposition potential of 1.2 V, the average film thickness is found to be about 1 mm. As the deposition potential increased to 1.3 V, the grain size increased to about 400e600 nm as shown in Fig. 1B. From the morphologies analysis, we can conclude that the surface morphologies of CZT alloy films are strongly depended on the deposition potentials. Fig. 2 shows the SEM images of CZTS thin films obtained from different deposition potentials and different annealing temperatures. After the high temperature sulfurization, the CZT alloy
precursor films were converted into the CZTS absorber materials. The CZTS films are more compact than the CZT alloy films as observed from the surface SEM images. From the cross-sectional images, a substantial volume expansion to about three times (~3 mm) that of the original thickness of CZT films (~1 mm) can be observed. No big difference can be observed from the surface SEM images for all the CZTS films. However, a few of voids can be observed in the CZTS or at the CZTS/substrate interface for the CZTS films annealed at 560 C as observed from Fig. 2C. These voids are generally due to non-uniform distribution of the elements in local regions [23]. When the sulfurization temperature increased to 580 C, the crystallinity of the CZTS film was improved and the voids were vanished as shown in Fig. 2F. The absence of voids indicates the homogeneous distribution of the CZTS throughout the film. The results reveal the higher sulfurization temperature is beneficial to form homogeneous and compact CZTS films with better crystallinity. The composition ratio of each element in CZTS films deposited at potential of 1.2 V and 1.3 V was measured by EDS and was shown in Table 1. Both the CZTS films were annealed at 580 C. The strict stoichiometric ratio of Cu, Zn, Sn and S elements should be 2:1:1:4. However, it was reported that the CZTS solar cells under Cu poor and Zn rich conditions give the higher efficiencies. The Cu/
Table 1 Composition ratio of CZTS films (580 C) determined by EDS. Samples
Cu/(Zn þ Sn)
Zn/Sn
Cu (at %)
Zn (at %)
Sn (at %)
S (at %)
1.20 V 1.30 V
0.46 1.03
0.25 0.90
9.37 20.40
4.12 9.30
16.09 10.32
70.04 59.98
Fig. 2. Surface SEM images of CZTS films annealed at (A) 560 C for potential of 1.2 V; (B) 560 C for potential of 1.3 V; (D) 580 C for potential of 1.2 V; (E) 580 C for potential of 1.3 V. Cross-sectional SEM images of CZTS films annealed at (C) 560 C for potential of 1.2 V and (F) 580 C for potential of 1.2 V.
K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
Fig. 3. XRD patterns of CZTS films obtained from different deposition potentials and different sulfurization temperatures.
27
Fig. 5. Plot of (ahy)2 versus hy for the CZTS films.
(Zn þ Sn) ratio for the most reported CZTS based solar cells is between 0.75 and 1. Meanwhile, the Zn/Sn ratio is between 1 and 1.25, with deviation by as large as 25% [6]. In our case, Cu/(Zn þ Sn) and Zn/Sn ratio of the CZTS film deposited at potential of 1.2 V is 0.46 and 0.25 respectively. The lower Cu/(Zn þ Sn) and Zn/Sn ratio indicates the CZT alloy film deposited at potential of 1.2 V was under Cu and Zn poor conditions. When the deposition potential decreased to 1.3 V, the Cu/(Zn þ Sn) and Zn/Sn ratio increased to 1.03 and 0.9, which was close to the ideal stoichiometric ratio. The composition ratio of sulfur exceeding 50% indicated both the alloy CZT films were fully sulfurized. Fig. 3 shows the X-ray diffraction patterns of the CZTS films. All the CZTS films show a polycrystalline kesterite crystal structure [JCPDS #26-0575] with major reflections along (112), (200), (220) and (312) planes [24]. The obviously reduced breadth of the XRD peak along (112) plane observed for the CZTS films sulfurized at 580 C indicated that the crystallinity of the CZTS films were improved with the increasing of sulfurization temperature. Several XRD peaks with low intensity observed in Fig. 3a, b and c may be
attributed to Cu and Sn sulphides. When the sulfurization temperature increased to 580 C for the sample deposited at 1.3 V, the XRD peaks with low intensity became undistinguishable as shown in Fig. 3d. However, the wurtzite ZnS, CuS and monoclinic CTS crystal structure are very similar to the CZTS pattern, and they are difficult to be excluded using XRD technique alone. In order to further identify and exclude the ZnS, CueSneS and CuS associated secondary phases, the CZTS films were analyzed by Raman spectroscopy as shown in Fig. 4. For the Raman measurements, the 632.8 nm line of a HeeNe laser was used for excitation. The characteristic peaks of CZTS at 285 cm1, 336 cm1 and 364 cm1 can be observed clearly which indicated the formation of CZTS phase for all the samples [25]. For both the CZTS films sulfurized at 560 C, a peak at 474 cm1 can be observed, which indicates the formation of separate CuS phase [25]. When the sulfurization temperature increased to 580 C, the Raman spectra have not shown any secondary phases as observed in Fig. 4. The plot of (ahy)2 versus hy for the CZTS films is shown in Fig. 5. The band gap energy of the CZTS films can be obtained by Tuac's relation [26].
Fig. 4. Raman spectra of the CZTS films obtained from different deposition potentials and different sulfurization temperatures.
Fig. 6. Current densityevoltage (JeV) curves of the QDSSCs based on the CZTS counter electrodes.
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K. Cheng et al. / Materials Chemistry and Physics 163 (2015) 24e29
Table 2 Photovoltaic parameters obtained from the JeV curves. Samples 1.20 1.30 1.20 1.30
V V V V
(560 (560 (580 (580
C) C) C) C)
ðahyÞ2 ¼ A hy Eg
Voc (mV)
Jsc (mA/cm2)
FF (%)
Rsh (U)
Rs (U)
Efficiency (%)
524 541 524 507
7.30 8.10 10.4 11.2
28.1 27.6 28.5 27.9
319.9 350.2 235.1 220.2
629.5 526.3 396.2 397.1
1.16 1.22 1.56 1.60
where a is the absorption coefficient and hʋ is the photon energy. The band gap energy (Eg) can be obtained by extrapolating the linear part of the (ahy)2 vs photon energy (hy) plot. The band gap energies of the CZTS films surfurized at 580 C were estimated to be 1.45 eV (1.3 V) and 1.41 eV (1.2 V). The estimated band gap energies of CZTS films at higher sulfurization temperature agreed well with the reported band gap energy values for the CZTS films [27]. However, we should note that the band gap energy decreased to about 1.3 eV for the CZTS films sulfurized at 560 C. As observed in Fig. 4, the separate CuS phase existed in the CZTS films sulfurized at lower temperature. The lower band gap of CuS (~1.21 eV) resulted in the narrow band gap energy for the CZTS films sulfurized at 560 C [28]. Recently, it was reported that CZTS can be exploited as an effective and non-platinum-based CE material in QDSSCs [29]. The CE is responsible for catalyzing the reduction of the redox shuttle in the electrolyte by electrons from external circuit and keeping the cell running [30]. In order to evaluate the photovoltaic activities of the electrodeposited CZTS films, the solar cells were fabricated with a sandwich-type by bonding the ZnxCd1xSe/ZnO photoanode with the CZTS counter electrodes. The current densityevoltage (JeV) characteristics were shown in Fig. 6. And the corresponding key photovoltaic parameters were summarized in Table 2. A conversion efficiency of 1.16% with a Voc of 524 mV, a Jsc of 7.3 mA/cm2, and an FF of 28.1% was obtained for the CZTS CEs deposited at 1.2 V (560 C). The conversion efficiency increased slightly to 1.22% when the deposition potential decreased to 1.3 V (560 C). When the sulfurization temperature of the CZTS CEs increased to 580 C, the conversion efficiencies can be greatly improved. The highest conversion efficiency of 1.6% with a Voc of 507 mV, a Jsc of 11.2 mA/cm2, and an FF of 27.9% were achieved for the CZTS CEs deposited at 1.3 V (580 C). There are no significant differences in Voc and FF for all the CZTS CEs as can be seen from Fig. 6 and Table 2. However, we should note that the Jsc increased from 8.1 mA/cm2 to 11.2 mA/ cm2 with the sulfurization temperature increased from 560 C to 580 C. The larger Jsc can be attributed to the improved crystallinity and the elimination of secondary phases under higher sulfurization temperature as discussed in Figs. 3 and 4. The high performance of the single step electrodeposited CZTS CEs indicated that they were suitable for application in environmentally-friendly thin film solar cells. 4. Conclusions CZT films were deposited on FTO substrates using single step electrodeposition. May be due to the suppressed hydrogen evolution reaction by adjusting the pH value of the electrolytic solution to 7.0 using diethanol amine, high quality CZT precursor films with flat and compact surface are obtained. By sulfurization of the electrodeposited CZT films in a sulfur containing atmosphere, CZTS films were fabricated with better crystallinity. For the CZTS film fabricated from the CZT precursor with a deposition potential of 1.3 V and higher sulfurization temperature, the Cu/(Zn þ Sn)
and Zn/Sn ratio is 1.03 and 0.9, respectively. Such stoichiometric ratios are close to the ideal ones. The Raman spectra have not shown any secondary phases when the sulfurization temperature increased to 580 C. The band gap energy of CZTS films at higher sulfurization temperature was estimated to be 1.45 eV. The highest conversion efficiency of 1.6% with a Voc of 507 mV, a Jsc of 11.2 mA/ cm2, and an FF of 27.9% were achieved for the CZTS CEs deposited at 1.3 V (580 C). These results demonstrated the introduction of diethanol amine into the electrolytic solution was an effective strategy for the deposition of high quality CZT films. Acknowledgments This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. PCS IRT1126) and the National Natural Science Foundation of China (Nos. 11274093, 61376061 and 61240053). References [1] W. Wang, S.Y. Han, S.J. Sung, D.H. Kim, C.H. Chang, Phys. Chem. Chem. Phys. 14 (2012) 11154e11159. [2] A. Duchatelet, T. Sidali, N. Loones, G. Savidand, E. Chassaing, D. Lincot, Sol. Energy Mat. Sol. Cells 119 (2013) 241e245. [3] S. Rampino, N. Armani, F. Bissoli, M. Bronzoni, D. Calestani, M. Calicchio, M. Mazzer, Appl. Phys. Lett. 101 (2012) 132107. [4] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. Res. Appl. 19 (2011) 894e897. [5] J.J. Scragg, J.T. Watjen, M. Edoff, T. Ericson, T. Kubart, C. Platzer-Bjorkman, J. Am. Chem. Soc. 134 (2012) 19330e19333. [6] S.Y. Chen, A. Walsh, X.G. Gong, S.H. Wei, Adv. Mater. 25 (2013) 1522e1539. [7] Y.F. Liu, F.Q. Huang, Y.A. Xie, H.L. Cui, W. Zhao, C.Y. Yang, N. Dai, J. Phys. Chem. C 117 (2013) 10296e10301. [8] G. Brammertz, M. Buffiere, Y. Mevel, Y. Ren, A.E. Zaghi, N. Lenaers, J. Poortmans, Appl. Phys. Lett. 102 (2013) 013902. [9] N.M. Shinde, P.R. Deshmukh, S.V. Patil, C.D. Lokhande, Mater. Res. Bull. 48 (2013) 1760e1766. [10] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S.J. Chey, S. Guha, Prog. Photovolt. Res. Appl. 21 (2013) 72e76. [11] J. Kim, H. Hiroi, T.K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M. Hopstaken, B. Shin, Y.S. Lee, Adv. Mater. 26 (2014) 7427e7431. [12] U. Chalapathi, S. Uthanna, V.S. Raja, Sol. Energy Mater. Sol. Cells 132 (2015) 476e484. [13] S.M. Pawar, A.I. Inamdar, K.V. Gurav, S.W. Shin, J. Gwak, Y. Jo, J. Yun, H. Pak, S. Kwon, H. Kim, J.H. Kim, H. Im, Curr. Appl. Phys. 15 (2015) 59e63. [14] J.H. Tao, J. He, K.Z. Zhang, J.F. Liu, Y.C. Dong, L. Sun, P.X. Yang, J.H. Chu, Mater. Lett. 135 (2014) 8e10. [15] T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D.B. Mitzi, Adv. Energy Mater. 3 (2013) 34e38. [16] H. Araki, Y. Kubo, K. Jimbo, W.S. Maw, H. Katagiri, M. Yamazaki, K. Oishi, A. Takeuch, Phys. Status Solidi C 6 (2009) 1266e1269. [17] J.J. Scragg, D. Berg, P.J. Dale, J. Electroanal. Chem. 646 (2010) 52e56. [18] A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I. Kotschau, Thin Solid Films 517 (2009) 2511e2514. [19] K.V. Gurav, J.H. Yun, S.M. Pawar, S.W. Shin, M.P. Suryawanshi, Y.K. Kim, J.H. Kim, Mater. Lett. 108 (2013) 316e319. [20] L. Guo, Y. Zhu, O. Gunawan, T. Gokmen, V.R. Deline, S. Ahmed, L.T. Romankiw, H. Deligianni, Prog. Photovolt. Res. Appl. 22 (2014) 58e68. [21] S.G. Lee, J. Kim, H.S. Woo, Y. Jo, A.I. Inamdar, S.M. Pawar, H.S. Kim, W. Jung, H.S. Im, Curr. Appl. Phys. 14 (2014) 254e258. [22] J. Xu, C.Y. Luan, Y.B. Tang, X. Chen, J.A. Zapien, W.J. Zhang, H.L. Kwong, Z.M. Meng, S.T. Lee, C.S. Lee, ACS Nano 4 (2010) 6064. [23] S.M. Pawar, A.I. Inamdar, K.V. Gurav, S.W. Shin, J. Kim, H. Im, J.H. Kim, Vacuum 104 (2014) 57e60. [24] S.G. Lee, J. Kim, H.S. Woo, Y. Jo, A.I. Inamdar, S.M. Pawar, H.S. Im, Curr. Appl. Phys. 14 (2014) 254e258. [25] J. Lehner, M. Ganchev, M. Loorits, N. Revathi, T. Raadik, J. Raudoja,
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