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Characterization of superstrate type CuInS2 solar cells deposited by spray pyrolysis method
Thin Solid Films 519 (2011) 7184–7188
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Characterization of superstrate type CuInS2 solar cells deposited by spray pyrolysis method Toshihiro Ryo a, Duy-Cuong Nguyen a, Motohito Nakagiri a, Noriaki Toyoda a, Hiroaki Matsuyoshi b, Seigo Ito a,⁎ a b
Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan Energy Technology Laboratories, Osaka Gas Co., Ltd., 6-19-9, Konohana-Ku, Osaka, 554-0051, Japan
a r t i c l e
i n f o
Available online 28 December 2010 Keywords: Spray CuInS2 Solar cell Superstrate TiO2 In2Se3
a b s t r a c t CuInS2 films were deposited on glass/FTO/TiO2/In2S3 air ambient air at 300 °C by spray pyrolysis, resulting in superstrate-structured solar cells. The crystallinity of the spray-deposited CuInS2 films was generally good. The CuInS2 films with a thickness of below 2 μm showed only one layer and good adhesion. On the other hand, the CuInS2 films with a thickness of more than 3 μm were formed with several layers, and were easily peeled off during deposition. The band gap value of CuInS2 samples was around 1.3 eV. The performance of the best cell obtained was Voc = 0.37 V, Jsc = 11.2 mA/cm2, FF = 0.35, and had an efficiency = 1.7%. For large size solar cells (2 × 2 cm2), the effect of In2S3 film thickness on the cell performance was significant. In order to characterize the spray-deposited CuInS2 films, the results of EPMA, XRD, XPS, and UV–vis absorption spectra have been discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, solar cell compounds such as Cu(In1-xGax)Se2 (CIGS), CuInSe2(CISe), and CuInS2(CIS) have received much attention from research groups around the world because of their potential properties, such as high efficiency, reliability, and stability [1–4]. Solar cells fabricated using conventional vacuum methods such as sputtering, atomic layer deposition (ALD), molecular beam epitaxy, (MBE) and co-evaporation possess high quality films, and consequently high conversion efficiencies [5–8]. However, such vacuum methods have some drawbacks, such as the high cost of equipment and raw materials, and the low-speed process. Meanwhile, nonvacuum methods such as printing [9,10], spraying [11–13], selenization of metal oxide with H2S or H2Se [14], and electrochemical deposition [15] realize the low cost and high-speed PV production. For this reason, non-vacuum methods have received extensive consideration from research groups concerned with low-cost solar cells. In this work, we focus on CIS absorber materials because this material are less toxic than CISe or CIGS, have a high absorption coefficient, a band gap near the optimal value for the absorption of solar radiation, and good efficiency [16]. The spray pyrolysis method is applied so as to deposit CIS films. The spray method is chosen because it is known to be a simple and low-cost method [11–13].
⁎ Corresponding author. Tel./fax: +81 79 267 4858. E-mail address: [email protected] (S. Ito). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.176
In order to characterize the spray-deposited CIS layer, we fabricated superstrate-structured solar cells: bglass/F-doped SnO2/ TiO2/In2S3/CuInS2/MoN, which is very close structure of spraydeposited CIS solar cells shown by Nanu et al. [13]. In this study, however, flat and dense TiO2 layers have been utilized (porous TiO2 layers were used in the ref. [13]) for the simplification of the system. The functions of TiO2 and In2S3 are n-type semiconductor and buffer, respectively. Without the TiO2 layer, the photovoltaic performance deteriorated drastically (the data is not shown). The buffer layer is to align the valence band and to suppress the back flow of electrons [13]. The advances of TiO2 are stable, cheap, and easy to fabricate on FTO substrate by the spray method. The superstrate structure was chosen because it is easy to be prepared by non-vacuum method and suitable for spraying CIS in air ambient. In the case of substrate structure, Mo is usually used as back electrode, which is very easy oxidized during spraying CIS in air ambient. The substrate structure is also complex for preparation cell devices because of using vacuum methods for depositing window layers such as ZnO and Al-doping ZnO. The effects of spray rate and amount of CIS precursor on the composition, microstructure, phase, optical and photovoltaic properties have been investigated in this study. 2. Experimental The CIS films were deposited on glass/FTO/TiO2/In2S3 substrate (superstrate) at 300 °C in air ambient using the spray method. Both In2S3 buffer and TiO2 compact layers were deposited by the spray method, and their thicknesses were 100 nm and 300 nm, respectively.
T. Ryo et al. / Thin Solid Films 519 (2011) 7184–7188
The deposition temperatures of In2S3 and TiO2 films were 200 °C and 450 °C, respectively. CuCl2.H2O (97.5%, Kanta Chemical Co.), InC3 (98%, Kishida Chemical Co., Ltd) and thiourea (Tokyo Chemical Industry Co., Ltd) were chosen as starting materials, which were dissolved in purified water with concentrations of 30 mM, 24 mM, and 120 mM (Cu:In:S = 1:0.8:4), respectively, for spray deposition at 300 °C. This solution is marked as CIS source. The crystalline structure and the preferred orientation of the films were characterized by X-ray diffraction (XRD; Miniflex II, Rigaku) using CuKα radiation. The thickness and microstructure of the films were viewed by scanning electron microscopy (SEM; JSM-6510, JEOL). The distribution of elements such as Cu, In, S, O, and C in the films was confirmed by an electron probe micro-analyzer (EPMA). The chemical states of the CIS films were measured by X-ray photoelectron spectroscopy (XPS). Absorption spectra were measured by an ultraviolet–visible spectroscopy (Lambda 750 UV/VIS Spectrometer, Perkin-Elmer). The size of samples for photo current–voltage measurement was 0.5 × 0.5 cm2. Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a xenon lamp (YSS-100A, Yamashita Denso, Japan). The power of the simulated light was calibrated to 100 mW/cm2 by using a reference Si photodiode equipped with an IR-cutoff filter (BS-520, Bunkoukeiki, Japan). I–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a DC voltage current source (6240A, ADCMT, Japan). The voltage step and delay time of photocurrent were 10 mV and 40 ms, respectively. 3. Results and discussion In order to analyze the effect of the spray rate on the composition qualitatively, CIS films were deposited at different spray rates. Fig. 1 shows the EPMA images of CIS films deposited at spray rates of 3.33 ml/min (fast spray) and 0.85 ml/min (slow spray). In the case of the fast spray, Cu and S exhibited a uniform distribution within the films, while In was mostly distributed at the center of the films. Notably, the oxygen and carbon concentration was observed to be rather high in the fast-sprayed films. In the case of the slow spray, the distribution of Cu, In, and S was relatively uniform. However, the concentrations of In and S may be higher than those of the fast spray; this may be due to the diffusion of In and S from the In2S3 buffer layer. Interestingly, the concentrations of carbon and oxygen atoms in these samples were relatively low in comparison with the samples deposited with the fast spray, especially carbon. The difference of oxygen and carbon concentrations for slow and fast spraying can be
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explained as follows: thiourea is used as source for sulfur in CIS and oxygen in air ambient decomposes the thiourea on the reacting surface, which takes some time to remove the carbon atoms as CO2. In the case of fast spraying, on the other hand, the time is not enough for reaction between oxygen and thiourea to liberate gases such as NO, CO2, or H2O. For this reason, the oxygen and carbon concentrations in the fast spraying are higher than slow spraying. The existence of carbon and oxygen in the films will reduce the photovoltaic properties of the solar cell, and therefore, a slow spray appears suitable for the deposition of CIS films. Hence, all CIS films in this paper were sprayed at a slow rate of 0.85 ml/min. To confirm the phase structure and orientation of CIS films, XRD measurement was performed. The XRD patterns of CIS films with different source volumes are shown in Fig. 2. The films deposited with source volumes of 20 and 30 ml show diffraction peaks at 27.86, 37.74, 46.34, 51.48, and 54.96°. The diffraction peaks at 37.74 and 51.48° are of the substrate (FTO/glass); the remaining peaks are known CuInS2 peaks, and indexed as (112), (220), and (312), respectively. The CIS diffraction peaks observed in the films are rather sharp, indicating good crystallinity. For the samples deposited with source volumes of 40–100 ml, one peak at the position of 32.46° was observed, but not for 20- and 30-ml samples. According to JDCPS and reported data, this is indexed as CIS(004)(200) [17,18]. As a result, the XRD data shows that only CIS phases exist in the films. The existence of phases in the CIS films is more carefully analyzed using XPS data below. SEM cross-section images of full CIS solar cells with a bglass/F-doped SnO2/TiO2/In2S3/CuInS2/MoN structure and various source volumes are depicted in Fig. 3. The samples deposited with CIS source volumes of below 40 mm show smooth surfaces, and one layer was observed in the absorber layer. The average thicknesses of the 20, 30, and 40 ml samples were 1.10, 1.38, and 2.30 μm, respectively. However, the samples deposited with source volumes of above 40 ml showed rough surfaces and many layers. Slots between layers in the CIS absorber layer were observed in these films. These absorber layers were also very easily peeled off from the substrate during spraying. The peeling off of these samples can be explained by the formation of several layers in the absorber layer and poor adhesion between them. The average thicknesses of the 60, 80, and 100 ml samples were 3.20, 3.95, and 3.82 μm. In other words, the CIS films deposited by the spray method were too thick and were easily peeled off. The formation of the stacked layers may be due to the interval spraying with break. For thin films, the stacked layers may not be observed, because of the low tension in the layer. However, in
Fig. 1. EPMA images of CIS films deposited on a superstrate with a fast (3.33 ml/min) and slow (0.85 ml/min) spray rate at 300 °C.
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T. Ryo et al. / Thin Solid Films 519 (2011) 7184–7188
Fig. 2. XRD patterns of CIS films with different CIS source volumes.
the case of thicker films, the stacked layers were easily formed, and be clearly observed as shown in SEM above, because the tension in the films can increase with the film thickness. As shown in the XRD and SEM data, the 30-ml sample showed a good CIS phase and good film qualities. However, in order to investigate the chemical states in these films more carefully, XPS measurement was carried out. An XPS depth profile of the 30-ml sample deposited at 300 °C in ambient air is shown in Fig. 4. The Cu peaks at 953.05 and 933.15 eV are of Cu in the CIS phase as reported [19]. In the case of indium, excluding In peaks in the CIS phase observed at binding energies of 444.7 eV (3d5/2) and 452.1 eV (3d3/2), free In peaks were found at binding energies of 443.4 eV and 450.8 eV as shown in Fig. 4. Oxygen atoms were mainly located near the outer surface; this may be due to contamination while the sample was kept in ambient air. The XPS results indicate that free In metal in the 30-ml sample can affect the photovoltaic properties of a CIS solar cell. Fig. 5 shows the absorption spectra of CIS films deposited on a substrate at 300 °C with various source volumes. The 20- and 30-ml samples show absorption cutoff at a long wavelength of 956 nm, and
Fig. 3. SEM cross-section images of CIS with different CIS source volumes.
T. Ryo et al. / Thin Solid Films 519 (2011) 7184–7188
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Fig. 4. XPS depth profiles of CIS with a 30 ml source sprayed at 300 °C on a superstrate.
this value is consistent with the absorption edge of CIS. From this value, the band gap values of the 20- and 30-ml samples were calculated, and were similar to each other, at around 1.3 eV. However, the light absorption of these samples in the long wavelength range (≥956 nm) is rather high, which may be due to the existent of defects in the absorber layer. In the case of the samples deposited with source volumes of 40–100 ml, the absorption spectra include two wavelength ranges, short and long wavelength ranges. In the short wavelength range from 400 to 900 nm, the absorption of light is due to CIS materials. In the long wavelength range of more than
900 nm, the absorption of light may be due to defects such as slots observed in the SEM cross-section images. In order to analyze the photovoltaic properties of CIS films, CIS cells with a bglass/F-doped SnO2/TiO2/In2S3/CuInS2/MoN structure and different CIS source volumes were fabricated. The cell parameters are presented in Table 1. From the results, cell parameters such as short-circuit current, open-circuit voltage, fill factor, and efficiency do not seemingly change so much with the different CIS source volumes (CIS film thickness). The efficiency was found to be in the range of around 1–1.7% in these samples. In this work, we prepared nine small cells (0.5 × 0.5 cm2) on one large substrate (5 × 5 cm2) in one deposition scheme. The samples with CIS source volumes of 60– 100 ml showed instability in the cell parameters. For instance, in the case of the 100-ml cells on one 5 × 5 cm2 substrate, several cells showed good efficiency, at around 1.7%; however, other cells showed only around 0.11% efficiency. The instability of cells deposited with a 100 ml CIS source can be explained by the instability of CIS films as
Table 1 Cell parameters of CIS solar cells with different CIS source volumes.
Fig. 5. Absorption spectra of CIS with various CIS source volumes.
CIS source volume
Voc(V)
Jsc (mA/cm2)
FF
η (%)
20 ml 30 ml 40 ml 60 ml 80 ml 100 ml
0.43 ± 0.01 0.44 ± 0.01 0.44 ± 0.00 0.44 ± 0.03 0.38 ± 0.04 0.37 ± 0.05
8.16 ± 2.32 6.64 ± 0.46 7.61 ± 2.68 5.95 ± 0.42 5.04 ± 3.01 6.16 ± 5.14
0.36 ± 0.01 0.27 ± 0.03 0.35 ± 0.03 0.28 ± 0.01 0.32 ± 0.02 0.35 ± 0.01
1.18 ± 0.23 0.82 ± 0.14 1.14 ± 0.27 0.75 ± 0.14 0.66 ± 0.45 0.92 ± 0.81
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T. Ryo et al. / Thin Solid Films 519 (2011) 7184–7188
Table 2 Cell parameters of CIS solar cells with a 30 ml CIS source volume and different In2S3 film thicknesses.
Acknowledgement
Thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
η (%)
This work was funded by the Innovative Solar Cells project (NEDO, Japan).
100 300 500
0.17 0.35 0.48
1.44 2.42 4.10
0.25 0.26 0.27
0.06 0.22 0.53
References
discussed in the SEM results above. In this study, the effect of In2S3 buffer layer thickness on the cell performance was also considered. Next, 2 × 2 cm2 cells deposited with a 30-ml CIS source volume and various In2S3 film thicknesses at 300 °C in ambient air were prepared, and the cell parameters were as shown in Table 2. The cell parameters became better with thicker In2S3 films. The efficiencies of the cells obtained in this study are not so high, which may be due to the existence of free In metal, as revealed in the XPS data, and/or the phase composition and undesirable elements such as carbon and oxygen that XRD and XPS could not detect. However, these are our preliminary results, and we will more carefully analyze this problem in the future to improve the efficiency of CIS solar cells. 4. Conclusions CIS films deposited at a fast spray rate contain a large amount of carbon and oxygen, whereas those deposited at a slow spray rate contain significantly lower amount of carbon and oxygen. CIS films deposited on a substrate with different source volumes show a CIS phase and free In metal in the films. CIS films deposited by the spray method with a thickness of more than 3 μm are rather unstable, and are easily peeled off. The best photovoltaic properties of a spraydeposited superstrate-structured CIS solar cell were obtained with 100-ml samples with cell parameters of Voc = 0.37 V, Jsc = 11.2 mA/cm2, FF = 0.35 and efficiency = 1.7%; however, the repeatability of the 100-ml sample was rather low. In order to improve the photovoltaic properties, the composition of CIS films and the effect of the buffer layer should be more thoroughly investigated.
[1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Prog. Photovoltaics Res. Appl. 16 (2008) 235. [2] K. Kushiya, Y. Tanaka, H. Hakuma, Y. Goushi, S. Kijima, T. Aramoto, Y. Fujiwara, Thin Solid Films 517 (2009) 2108. [3] M. Powalla, G. Voorwinden, D. Hariskos, P. Jackson, R. Kniese, Thin Solid Films 517 (2009) 2111. [4] B. D. Weil, S. T. Connor and Y. Cui, J. Am. Chem. Soc. in press. [5] M.M. Islam, T. Sakurai, S. Ishizuka, A. Yamada, H. Shibata, K. Sakurai, K. Matsubara, S. Niki, K. Akimoto, J. Cryst. Growth 311 (2009) 2212. [6] S. Seyrling, S. Calnan, S. Bücheler, J. Hüpkes, S. Wenger, D. Brémaud, H. Zogg, A.N. Tiwari, Thin Solid Films 517 (2009) 2411. [7] R. Cayzac, F. Boulc'h, M. Bendahan, P. Lauque, P. Knauth, Mater. Sci. Eng., B 157 (2009) 66. [8] M. Nanu, L. Reijnen, B. Meester, J. Schoonman, A. Goossens, Chem. Vap. Deposition 10 (2004) 45. [9] V.K. Kapur, A. Bansal, O.I. Asensio, P. Le, N.K. Shigeokan, DOE Sol. Program Rev. Meet. (2004) 135. [10] T. Wada, Y. Matsuo, S. Nomura, Y. Nakamura, A. Miyamura, Y. Chiba, A. Yamada, M. Konagai, Phys. Status Solidi 203 (2006) 2593. [11] C. Camus, D. Abou-Ras, N.A. Allsop, S.E. Gledhill, T. Köhler, J. Rappich, I. Lauermann, M.C. Lux-Steiner, C.-H. Fischer, Phys. Status Solidi 207 (2010) 129. [12] T.T. John, M. Mathew, C.S. Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Sol. Energy Mater. Sol. Cells 89 (2005) 27. [13] M. Nanu, J. Schoonman, A. Goossens, Nano Lett. 5 (2005) 1716. [14] V. K. Kapur, B. M. Basol, C. R. Leidholm, R. A. Roe, United State Patent, No. 6127202, date of patent: 3rd, Oct., 2000. [15] Y. Oda, M. Matsubayashi, T. Minemoto, H. Takakura, J. Crystal, Growth 311 (2009) 738. [16] D. Braunger, D. Hariskos, T. Walter, H.W. Schock, Sol. Energy Mater. Sol. Cells 40 (1996) 97. [17] Joint committee for powder diffraction standards, powder diffraction file. No.85-1575 (JDCPS International Center Diffration Data, 1997). [18] M. Krunks, O. Bijakina, T. Varema, V. Mikli, E. Mellikov, Thin Solid Films 338 (1999) 125. [19] M.C. Zouaghi, T. Ben Nasrallah, S. Marsillac, J.C. Bernede, S. Belgacem, Thin Solid Films 382 (2001) 39.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Characterization of superstrate type CuInS2 solar cells deposited by spray pyrolysis method Toshihiro Ryo a, Duy-Cuong Nguyen a, Motohito Nakagiri a, Noriaki Toyoda a, Hiroaki Matsuyoshi b, Seigo Ito a,⁎ a b
Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan Energy Technology Laboratories, Osaka Gas Co., Ltd., 6-19-9, Konohana-Ku, Osaka, 554-0051, Japan
a r t i c l e
i n f o
Available online 28 December 2010 Keywords: Spray CuInS2 Solar cell Superstrate TiO2 In2Se3
a b s t r a c t CuInS2 films were deposited on glass/FTO/TiO2/In2S3 air ambient air at 300 °C by spray pyrolysis, resulting in superstrate-structured solar cells. The crystallinity of the spray-deposited CuInS2 films was generally good. The CuInS2 films with a thickness of below 2 μm showed only one layer and good adhesion. On the other hand, the CuInS2 films with a thickness of more than 3 μm were formed with several layers, and were easily peeled off during deposition. The band gap value of CuInS2 samples was around 1.3 eV. The performance of the best cell obtained was Voc = 0.37 V, Jsc = 11.2 mA/cm2, FF = 0.35, and had an efficiency = 1.7%. For large size solar cells (2 × 2 cm2), the effect of In2S3 film thickness on the cell performance was significant. In order to characterize the spray-deposited CuInS2 films, the results of EPMA, XRD, XPS, and UV–vis absorption spectra have been discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, solar cell compounds such as Cu(In1-xGax)Se2 (CIGS), CuInSe2(CISe), and CuInS2(CIS) have received much attention from research groups around the world because of their potential properties, such as high efficiency, reliability, and stability [1–4]. Solar cells fabricated using conventional vacuum methods such as sputtering, atomic layer deposition (ALD), molecular beam epitaxy, (MBE) and co-evaporation possess high quality films, and consequently high conversion efficiencies [5–8]. However, such vacuum methods have some drawbacks, such as the high cost of equipment and raw materials, and the low-speed process. Meanwhile, nonvacuum methods such as printing [9,10], spraying [11–13], selenization of metal oxide with H2S or H2Se [14], and electrochemical deposition [15] realize the low cost and high-speed PV production. For this reason, non-vacuum methods have received extensive consideration from research groups concerned with low-cost solar cells. In this work, we focus on CIS absorber materials because this material are less toxic than CISe or CIGS, have a high absorption coefficient, a band gap near the optimal value for the absorption of solar radiation, and good efficiency [16]. The spray pyrolysis method is applied so as to deposit CIS films. The spray method is chosen because it is known to be a simple and low-cost method [11–13].
⁎ Corresponding author. Tel./fax: +81 79 267 4858. E-mail address: [email protected] (S. Ito). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.176
In order to characterize the spray-deposited CIS layer, we fabricated superstrate-structured solar cells: bglass/F-doped SnO2/ TiO2/In2S3/CuInS2/MoN, which is very close structure of spraydeposited CIS solar cells shown by Nanu et al. [13]. In this study, however, flat and dense TiO2 layers have been utilized (porous TiO2 layers were used in the ref. [13]) for the simplification of the system. The functions of TiO2 and In2S3 are n-type semiconductor and buffer, respectively. Without the TiO2 layer, the photovoltaic performance deteriorated drastically (the data is not shown). The buffer layer is to align the valence band and to suppress the back flow of electrons [13]. The advances of TiO2 are stable, cheap, and easy to fabricate on FTO substrate by the spray method. The superstrate structure was chosen because it is easy to be prepared by non-vacuum method and suitable for spraying CIS in air ambient. In the case of substrate structure, Mo is usually used as back electrode, which is very easy oxidized during spraying CIS in air ambient. The substrate structure is also complex for preparation cell devices because of using vacuum methods for depositing window layers such as ZnO and Al-doping ZnO. The effects of spray rate and amount of CIS precursor on the composition, microstructure, phase, optical and photovoltaic properties have been investigated in this study. 2. Experimental The CIS films were deposited on glass/FTO/TiO2/In2S3 substrate (superstrate) at 300 °C in air ambient using the spray method. Both In2S3 buffer and TiO2 compact layers were deposited by the spray method, and their thicknesses were 100 nm and 300 nm, respectively.
T. Ryo et al. / Thin Solid Films 519 (2011) 7184–7188
The deposition temperatures of In2S3 and TiO2 films were 200 °C and 450 °C, respectively. CuCl2.H2O (97.5%, Kanta Chemical Co.), InC3 (98%, Kishida Chemical Co., Ltd) and thiourea (Tokyo Chemical Industry Co., Ltd) were chosen as starting materials, which were dissolved in purified water with concentrations of 30 mM, 24 mM, and 120 mM (Cu:In:S = 1:0.8:4), respectively, for spray deposition at 300 °C. This solution is marked as CIS source. The crystalline structure and the preferred orientation of the films were characterized by X-ray diffraction (XRD; Miniflex II, Rigaku) using CuKα radiation. The thickness and microstructure of the films were viewed by scanning electron microscopy (SEM; JSM-6510, JEOL). The distribution of elements such as Cu, In, S, O, and C in the films was confirmed by an electron probe micro-analyzer (EPMA). The chemical states of the CIS films were measured by X-ray photoelectron spectroscopy (XPS). Absorption spectra were measured by an ultraviolet–visible spectroscopy (Lambda 750 UV/VIS Spectrometer, Perkin-Elmer). The size of samples for photo current–voltage measurement was 0.5 × 0.5 cm2. Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a xenon lamp (YSS-100A, Yamashita Denso, Japan). The power of the simulated light was calibrated to 100 mW/cm2 by using a reference Si photodiode equipped with an IR-cutoff filter (BS-520, Bunkoukeiki, Japan). I–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a DC voltage current source (6240A, ADCMT, Japan). The voltage step and delay time of photocurrent were 10 mV and 40 ms, respectively. 3. Results and discussion In order to analyze the effect of the spray rate on the composition qualitatively, CIS films were deposited at different spray rates. Fig. 1 shows the EPMA images of CIS films deposited at spray rates of 3.33 ml/min (fast spray) and 0.85 ml/min (slow spray). In the case of the fast spray, Cu and S exhibited a uniform distribution within the films, while In was mostly distributed at the center of the films. Notably, the oxygen and carbon concentration was observed to be rather high in the fast-sprayed films. In the case of the slow spray, the distribution of Cu, In, and S was relatively uniform. However, the concentrations of In and S may be higher than those of the fast spray; this may be due to the diffusion of In and S from the In2S3 buffer layer. Interestingly, the concentrations of carbon and oxygen atoms in these samples were relatively low in comparison with the samples deposited with the fast spray, especially carbon. The difference of oxygen and carbon concentrations for slow and fast spraying can be
7185
explained as follows: thiourea is used as source for sulfur in CIS and oxygen in air ambient decomposes the thiourea on the reacting surface, which takes some time to remove the carbon atoms as CO2. In the case of fast spraying, on the other hand, the time is not enough for reaction between oxygen and thiourea to liberate gases such as NO, CO2, or H2O. For this reason, the oxygen and carbon concentrations in the fast spraying are higher than slow spraying. The existence of carbon and oxygen in the films will reduce the photovoltaic properties of the solar cell, and therefore, a slow spray appears suitable for the deposition of CIS films. Hence, all CIS films in this paper were sprayed at a slow rate of 0.85 ml/min. To confirm the phase structure and orientation of CIS films, XRD measurement was performed. The XRD patterns of CIS films with different source volumes are shown in Fig. 2. The films deposited with source volumes of 20 and 30 ml show diffraction peaks at 27.86, 37.74, 46.34, 51.48, and 54.96°. The diffraction peaks at 37.74 and 51.48° are of the substrate (FTO/glass); the remaining peaks are known CuInS2 peaks, and indexed as (112), (220), and (312), respectively. The CIS diffraction peaks observed in the films are rather sharp, indicating good crystallinity. For the samples deposited with source volumes of 40–100 ml, one peak at the position of 32.46° was observed, but not for 20- and 30-ml samples. According to JDCPS and reported data, this is indexed as CIS(004)(200) [17,18]. As a result, the XRD data shows that only CIS phases exist in the films. The existence of phases in the CIS films is more carefully analyzed using XPS data below. SEM cross-section images of full CIS solar cells with a bglass/F-doped SnO2/TiO2/In2S3/CuInS2/MoN structure and various source volumes are depicted in Fig. 3. The samples deposited with CIS source volumes of below 40 mm show smooth surfaces, and one layer was observed in the absorber layer. The average thicknesses of the 20, 30, and 40 ml samples were 1.10, 1.38, and 2.30 μm, respectively. However, the samples deposited with source volumes of above 40 ml showed rough surfaces and many layers. Slots between layers in the CIS absorber layer were observed in these films. These absorber layers were also very easily peeled off from the substrate during spraying. The peeling off of these samples can be explained by the formation of several layers in the absorber layer and poor adhesion between them. The average thicknesses of the 60, 80, and 100 ml samples were 3.20, 3.95, and 3.82 μm. In other words, the CIS films deposited by the spray method were too thick and were easily peeled off. The formation of the stacked layers may be due to the interval spraying with break. For thin films, the stacked layers may not be observed, because of the low tension in the layer. However, in
Fig. 1. EPMA images of CIS films deposited on a superstrate with a fast (3.33 ml/min) and slow (0.85 ml/min) spray rate at 300 °C.
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Fig. 2. XRD patterns of CIS films with different CIS source volumes.
the case of thicker films, the stacked layers were easily formed, and be clearly observed as shown in SEM above, because the tension in the films can increase with the film thickness. As shown in the XRD and SEM data, the 30-ml sample showed a good CIS phase and good film qualities. However, in order to investigate the chemical states in these films more carefully, XPS measurement was carried out. An XPS depth profile of the 30-ml sample deposited at 300 °C in ambient air is shown in Fig. 4. The Cu peaks at 953.05 and 933.15 eV are of Cu in the CIS phase as reported [19]. In the case of indium, excluding In peaks in the CIS phase observed at binding energies of 444.7 eV (3d5/2) and 452.1 eV (3d3/2), free In peaks were found at binding energies of 443.4 eV and 450.8 eV as shown in Fig. 4. Oxygen atoms were mainly located near the outer surface; this may be due to contamination while the sample was kept in ambient air. The XPS results indicate that free In metal in the 30-ml sample can affect the photovoltaic properties of a CIS solar cell. Fig. 5 shows the absorption spectra of CIS films deposited on a substrate at 300 °C with various source volumes. The 20- and 30-ml samples show absorption cutoff at a long wavelength of 956 nm, and
Fig. 3. SEM cross-section images of CIS with different CIS source volumes.
T. Ryo et al. / Thin Solid Films 519 (2011) 7184–7188
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Fig. 4. XPS depth profiles of CIS with a 30 ml source sprayed at 300 °C on a superstrate.
this value is consistent with the absorption edge of CIS. From this value, the band gap values of the 20- and 30-ml samples were calculated, and were similar to each other, at around 1.3 eV. However, the light absorption of these samples in the long wavelength range (≥956 nm) is rather high, which may be due to the existent of defects in the absorber layer. In the case of the samples deposited with source volumes of 40–100 ml, the absorption spectra include two wavelength ranges, short and long wavelength ranges. In the short wavelength range from 400 to 900 nm, the absorption of light is due to CIS materials. In the long wavelength range of more than
900 nm, the absorption of light may be due to defects such as slots observed in the SEM cross-section images. In order to analyze the photovoltaic properties of CIS films, CIS cells with a bglass/F-doped SnO2/TiO2/In2S3/CuInS2/MoN structure and different CIS source volumes were fabricated. The cell parameters are presented in Table 1. From the results, cell parameters such as short-circuit current, open-circuit voltage, fill factor, and efficiency do not seemingly change so much with the different CIS source volumes (CIS film thickness). The efficiency was found to be in the range of around 1–1.7% in these samples. In this work, we prepared nine small cells (0.5 × 0.5 cm2) on one large substrate (5 × 5 cm2) in one deposition scheme. The samples with CIS source volumes of 60– 100 ml showed instability in the cell parameters. For instance, in the case of the 100-ml cells on one 5 × 5 cm2 substrate, several cells showed good efficiency, at around 1.7%; however, other cells showed only around 0.11% efficiency. The instability of cells deposited with a 100 ml CIS source can be explained by the instability of CIS films as
Table 1 Cell parameters of CIS solar cells with different CIS source volumes.
Fig. 5. Absorption spectra of CIS with various CIS source volumes.
CIS source volume
Voc(V)
Jsc (mA/cm2)
FF
η (%)
20 ml 30 ml 40 ml 60 ml 80 ml 100 ml
0.43 ± 0.01 0.44 ± 0.01 0.44 ± 0.00 0.44 ± 0.03 0.38 ± 0.04 0.37 ± 0.05
8.16 ± 2.32 6.64 ± 0.46 7.61 ± 2.68 5.95 ± 0.42 5.04 ± 3.01 6.16 ± 5.14
0.36 ± 0.01 0.27 ± 0.03 0.35 ± 0.03 0.28 ± 0.01 0.32 ± 0.02 0.35 ± 0.01
1.18 ± 0.23 0.82 ± 0.14 1.14 ± 0.27 0.75 ± 0.14 0.66 ± 0.45 0.92 ± 0.81
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Table 2 Cell parameters of CIS solar cells with a 30 ml CIS source volume and different In2S3 film thicknesses.
Acknowledgement
Thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
η (%)
This work was funded by the Innovative Solar Cells project (NEDO, Japan).
100 300 500
0.17 0.35 0.48
1.44 2.42 4.10
0.25 0.26 0.27
0.06 0.22 0.53
References
discussed in the SEM results above. In this study, the effect of In2S3 buffer layer thickness on the cell performance was also considered. Next, 2 × 2 cm2 cells deposited with a 30-ml CIS source volume and various In2S3 film thicknesses at 300 °C in ambient air were prepared, and the cell parameters were as shown in Table 2. The cell parameters became better with thicker In2S3 films. The efficiencies of the cells obtained in this study are not so high, which may be due to the existence of free In metal, as revealed in the XPS data, and/or the phase composition and undesirable elements such as carbon and oxygen that XRD and XPS could not detect. However, these are our preliminary results, and we will more carefully analyze this problem in the future to improve the efficiency of CIS solar cells. 4. Conclusions CIS films deposited at a fast spray rate contain a large amount of carbon and oxygen, whereas those deposited at a slow spray rate contain significantly lower amount of carbon and oxygen. CIS films deposited on a substrate with different source volumes show a CIS phase and free In metal in the films. CIS films deposited by the spray method with a thickness of more than 3 μm are rather unstable, and are easily peeled off. The best photovoltaic properties of a spraydeposited superstrate-structured CIS solar cell were obtained with 100-ml samples with cell parameters of Voc = 0.37 V, Jsc = 11.2 mA/cm2, FF = 0.35 and efficiency = 1.7%; however, the repeatability of the 100-ml sample was rather low. In order to improve the photovoltaic properties, the composition of CIS films and the effect of the buffer layer should be more thoroughly investigated.
[1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Prog. Photovoltaics Res. Appl. 16 (2008) 235. [2] K. Kushiya, Y. Tanaka, H. Hakuma, Y. Goushi, S. Kijima, T. Aramoto, Y. Fujiwara, Thin Solid Films 517 (2009) 2108. [3] M. Powalla, G. Voorwinden, D. Hariskos, P. Jackson, R. Kniese, Thin Solid Films 517 (2009) 2111. [4] B. D. Weil, S. T. Connor and Y. Cui, J. Am. Chem. Soc. in press. [5] M.M. Islam, T. Sakurai, S. Ishizuka, A. Yamada, H. Shibata, K. Sakurai, K. Matsubara, S. Niki, K. Akimoto, J. Cryst. Growth 311 (2009) 2212. [6] S. Seyrling, S. Calnan, S. Bücheler, J. Hüpkes, S. Wenger, D. Brémaud, H. Zogg, A.N. Tiwari, Thin Solid Films 517 (2009) 2411. [7] R. Cayzac, F. Boulc'h, M. Bendahan, P. Lauque, P. Knauth, Mater. Sci. Eng., B 157 (2009) 66. [8] M. Nanu, L. Reijnen, B. Meester, J. Schoonman, A. Goossens, Chem. Vap. Deposition 10 (2004) 45. [9] V.K. Kapur, A. Bansal, O.I. Asensio, P. Le, N.K. Shigeokan, DOE Sol. Program Rev. Meet. (2004) 135. [10] T. Wada, Y. Matsuo, S. Nomura, Y. Nakamura, A. Miyamura, Y. Chiba, A. Yamada, M. Konagai, Phys. Status Solidi 203 (2006) 2593. [11] C. Camus, D. Abou-Ras, N.A. Allsop, S.E. Gledhill, T. Köhler, J. Rappich, I. Lauermann, M.C. Lux-Steiner, C.-H. Fischer, Phys. Status Solidi 207 (2010) 129. [12] T.T. John, M. Mathew, C.S. Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Sol. Energy Mater. Sol. Cells 89 (2005) 27. [13] M. Nanu, J. Schoonman, A. Goossens, Nano Lett. 5 (2005) 1716. [14] V. K. Kapur, B. M. Basol, C. R. Leidholm, R. A. Roe, United State Patent, No. 6127202, date of patent: 3rd, Oct., 2000. [15] Y. Oda, M. Matsubayashi, T. Minemoto, H. Takakura, J. Crystal, Growth 311 (2009) 738. [16] D. Braunger, D. Hariskos, T. Walter, H.W. Schock, Sol. Energy Mater. Sol. Cells 40 (1996) 97. [17] Joint committee for powder diffraction standards, powder diffraction file. No.85-1575 (JDCPS International Center Diffration Data, 1997). [18] M. Krunks, O. Bijakina, T. Varema, V. Mikli, E. Mellikov, Thin Solid Films 338 (1999) 125. [19] M.C. Zouaghi, T. Ben Nasrallah, S. Marsillac, J.C. Bernede, S. Belgacem, Thin Solid Films 382 (2001) 39.