- Email: [email protected]
In–Ga–Se precursors for high-efficiency solar cells
Solmat=190=Ravi=Venkatachala=BG
Solar Energy Materials & Solar Cells 67 (2001) 217}223
Preparation of Cu(In,Ga)Se thin "lms from Cu}Se/In}Ga}Se precursors for high-e$ciency solar cells S. Nishiwaki , T. Satoh , S. Hayashi , Y. Hashimoto , S. Shimakawa , T. Negami *, T. Wada Advanced Technology Research Laboratories, Matsushita Electric Ind. Co. Ltd., 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan
Abstract Improved preparation process of a device quality Cu(In,Ga)Se (CIGS) thin "lm was proposed for production of CIGS solar cells. In}Ga}Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se #uxes to form Cu}Se/In}Ga}Se precursor "lm at substrate temperature of over 2003C. The precursor "lm was annealed in Se #ux at substrate temperature of over 5003C to obtain high-quality CIGS "lm. The solar cell with a MgF /ITO/ZnO/CdS/CIGS/Mo/glass structure showed an e$ciency of 17.5% (< "0.634 V, J "36.4 mA/cm, FF"0.756). 2001 Elsevier Science B.V. All rights reserved. Keywords: Cu(In; Ga)Se ; Thin "lms; Fabrication process; Microstructure
1. Introduction Cu(In,Ga)Se (CIGS) is one of the most promising materials for thin "lm photovol taic devices due to the suitability of its bandgap and its high absorption coe$cient for solar radiation [1]. A variety of processes for preparing device quality CIGS "lms has been proposed [2]. We have demonstrated CIGS solar cells with an e$ciency of over 17% by the `three-stagea process [3]. In the `three-stagea process, the initial stage
* Corresponding author. E-mail address: [email protected] (T. Negami). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 2 8 4 - 1
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consists of deposition of an In}Ga}Se precursor layer. In the second stage, the precursor layer is exposed to Cu and Se #uxes to form a Cu-rich CIGS. In the third stage, small amounts of In, Ga and Se are added to the Cu-rich CIGS layer in order to obtain a slightly (In,Ga)-rich CIGS "lm. Recently, we studied growth of CIGS thin "lms during the `three-stagea process [4], and demonstrate that the phase of the "lm was changed in the second stage as follows: (In,Ga) Se P[Cu(In,Ga) Se ]PCu(In,Ga) Se PCu(In,Ga)Se . In addi tion, it was proved that the CIGS crystals grew in the second stage from the (In,Ga) Se with sub-micron size grains to the Cu(In,Ga)Se with a few micrometers grains. Based on this result, we study a simple and tolerant preparation process of device-quality CIGS "lms for high-e$ciency solar cells. The process consists of the preparation stage of a CIGS precursor "lm at low substrate temperature and postannealing stage of the precursor "lm in Se #ux at high substrate temperature. Unlike the `bi-layera[5] and `three-stagea processes, this process does not include Cu-rich condition. Furthermore, unlike the previously reported two-stage method [6}10], this process is based on the formation of chalcopyrite phase at a low temperature, not at a high temperature. 2. Experimental (In,Ga)}Se layers were deposited on Mo-coated soda-lime glasses at about 25}3503C. Then the layers were exposed to Cu and Se #uxes keeping the substrate temperature. The amount of Cu, In, and Ga deposited was not changed in all preparations. The Cu}Se/In}Ga}Se precursor "lms were post-annealed in Se #ux at over 5003C. The chemical composition of the precursor and the post-annealed "lms was determined with an energy-dispersive X-ray spectrometer (EDX). The depth pro"le of the elements of Cu, In, Ga, Se, and Mo was characterized by secondary-ion mass spectroscopy (SIMS). The phases in the "lm were identi"ed by X-ray di!raction (XRD) operated at 50 kV and 200 mA using Cu K radiation. The microstructure of the "lms ? was observed by cross-sectional high-resolution transmission electron microscopy (TEM) using a TOPCON EM-002B electron microscope operated at 200 keV. CIGS solar cells with a structure of ITO/ZnO/CdS/CIGS /Mo/glass were fabricated. The CdS bu!er layers were deposited by chemical-bath deposition (CBD). The ZnO and ITO layers were deposited by rf-magnetron sputtering. The performance of solar cells was evaluated under standard AM 1.5 illumination. 3. Results and discussion 3.1. Precursor layers We prepared four kinds of precursor "lms, precursor a, b, c and d which were obtained at substrate temperatures of about 350, 200, 100 and 253C, respectively.
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Table 1 Chemical composition of precursor "lms analyzed by EDX Precusor
a b c d
Substrate temperature (3C)
Chemical composition (at%) Cu
In
Ga
Se
350 200 100 25
25 52 36 10
21 6 3 0
4 2 1 0
50 40 60 90
Fig. 1. SIMS depth pro"les of element Cu, In, Ga, Se, and Mo in the precursor "lms: (a) precursor a deposited at about 3503C, (b) precursor b deposited at about 2003C, (c) precursor c deposited at about 1003C, and (d) precursor d deposited at about 253C.
Table 1 shows chemical compositions of the precursor "lms analyzed by EDX. The data correspond to the chemical composition of the "lm near the surface, because EDX analysis is surface sensitive. Precursor a had a stoichiometric composition of Cu(In,Ga)Se . Precursor b showed Cu-rich composition. Precursor c showed Se-rich composition, and Cu and Se occupied 96% of all constituents. Precursor d was almost Se "lm. Figs. 1(a)}(d) show SIMS depth pro"le of the elements in the precursor a to d, respectively. In precursor a deposited at about 3503C, Cu, In, and Ga distribute
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Fig. 2. XRD patterns of (a) precursor a, (b) precursor b, (c) precursor c, and (d) precursor d.
through the "lm. We should point out that surface layer with high In and low Ga composition can be observed in the "gure. The details on the gradient distribution of In and Ga will be reported elsewhere. In precursor b deposited at about 2003C, the Cu}Se/In}Ga}Se structure can be seen and inter-di!usion between Cu}Se and (In,Ga)}Se layers is also observed. In precursor c deposited at about 1003C, a small amount of inter-di!usion between the surface Cu}Se and bottom (In,Ga) }Se layers is observed. In precursor d deposited at about 253C, separation of surface Cu}Se and bottom (In,Ga)}Se layers is clearly observed. These results support EDX analyses and indicate that the di!usion of the constituents through the precursor "lm proceeds over 2003C. Fig. 2(a)}(d) shows XRD patterns of precursor a, b, c and d, respectively. In the XRD patterns, selenides and Mo are detected, but Se-de"cient phase, such as Cu}In alloy, cannot be observed. The formation of only selenide during the deposition distinguishes this sequential deposition process from the other co-evaporation processes for the preparation of precursor "lms at low substrate temperature [8}10]. In the precursor a deposited at about 3503C (Fig. 2(a)), the 101 peak characterizing tetragonal structure is observed at the position of arrow, which implies the phase is chalcopyrite. In Fig. 2(a), the peak splits can be seen due to graded compositional distribution of In and Ga between the "lm surface and the bottom as shown in Fig. 1(a). The precursor b deposited at about 2003C is considered to be a metastable zinc blend-type (Cu,In,Ga)Se. In the XRD pro"le of precursor b, the peak split can be also seen at around 2h"273. It would be attributed to compositional inhomogeneity of Cu, In, and Ga. The precursor c deposited at 1003C shows Cu}Se phase and the
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precursor d deposited at about 253C indicates amorphous and small amount of Cu}Se phase. These results are consistent with the results of SIMS depth pro"le as shown in Fig. 1. The chalcopyrite-type CIGS precursor could be obtained at substrate temperatures of more than 2003C by the inter-di!usion between surface Cu}Se and bottom (In,Ga) }Se layers. 3.2. Annealed xlms The CIGS a, b and c are obtained by post-annealing of the precursor a, b, and c, respectively. It is noted that the precursor "lm deposited at about 253C was peeled o! during the post-annealing, because of a large volume expansion. EDX and XRD showed that all the post-annealed "lms had chemical compositions of Cu/(In#Ga)&0.94 and Ga/(In#Ga)&0.25, and a chalcopyrite-type Cu(In,Ga)Se phase. By the post-annealing at over 5003C, the precursor "lms adequately react and homogenize to form the CIGS phase. Figs. 3(a)}(c) show cross-sectional TEM micrographs of CIGS a, b and c "lms. The obtained "lms are dense and voids do not appear. We consider that, due to the absence of Se-de"cient phase such as Cu}In alloy in the precursor "lms, the volume expansion accompanied by selenization during the postannealing are depressed and then result in dense "lms. Fig. 3(a) shows that the grain size of CIGS a is 1}2 lm. In Figs. 3(b) and (c), it is observed that the CIGS "lms are composed of sub-micrometer size grains. It is understood that the grain size of the post-annealed CIGS "lm depends on the preparation condition of precursor "lms. We consider that the formation reaction of chalcopyrite phase from surface Cu}Se and bottom (In,Ga)}Se layers by the inter-di!usion inhibit the grain growth. The performance of the solar cells fabricated using post-annealed CIGS a, b, and c were measured under standard AM 1.5 illumination. The cell performances are listed in Table 2 and the I}< curves are shown in Fig. 4. CIGS a whose precursor was deposited at about 3503C resulted in the best performance. As listed in Table 2, the advantage in the performances of CIGS a is open-circuit voltage (< ), suggesting higher carrier density in CIGS a than CIGS b or c. We think that the defects in the CIGS grain decrease with grain growth, resulting in the good performance of CIGS solar cell.
Fig. 3. Cross-sectional TEM micrographs of the (a) CIGS a, (b) CIGS b, and (c) CIGS c whose precursor "lms were prepared at about 350, 200 and 1003C, respectively.
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Table 2 Performance of the solar cells fabricated using CIGS a, b and c Post-annealed "lms
E!. (%)
< (V)
J (mA/cm)
FF
CIGS a CIGS b CIGS c
15.5 14.2 13.3
0.661 0.614 0.615
31.9 32.6 31.8
0.736 0.712 0.679
Fig. 4. I}< curves of solar cells using CIGS a, b, and c of which precursor "lms were prepared at about 350, 200, and 1003C, respectively.
Fig. 5. I}< curve of solar cell using CIGS "lm fabricated by optimized process.
Fig. 5 shows a I}< curve of the solar cell coated with a MgF anti-re#ection layer. The CIGS "lm was fabricated by the optimized process. The cell showed an e$ciency of 17.5% (< "0.634 V, J "36.4 mA/cm, FF"0.756). The obtained values are comparable to those of solar cells fabricated using CIGS "lms deposited by the
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`three-stagea process. The present fabrication technique was not only simple and easy to control the chemical composition, but also enable to fabricate high-performance CIGS solar cells.
4. Summary We study the improved PVD process that is simple and tolerant for fabrication of device quality CIGS "lms. Cu}Se/In}Ga}Se precursor "lms were deposited on Mo-coated soda-lime glasses at substrate temperature of about 25}3503C, and then annealed in Se #ux at substrate temperature of over 5003C. The grain size of the post-annealed CIGS "lms depended on the preparation condition of the precursor "lms. The cell fabricated by this improved PVD process showed an e$ciency of 17.5% (< "0.634 V, J "36.4 mA/cm, FF"0.756). This e$ciency is comparable to our best cell. This improved PVD process is applicable to the production of CIGS solar cells and modules.
Acknowledgements This work was supported by the NEDO as a part of the New Sunshine program conducted under the sponsorship of MITI. The authors would like to express their sincere appreciation to Ms. F. Toujho and Dr. K. Tsukamoto for their help with the SIMS analyses, Mr. T. Kouzaki for his help with the TEM observations. The authors would also like to thank Drs. O. Yamazaki, M. Takao, T. Uenoyama and M. Kitagawa for their encouragement throughout this work.
References [1] J.E. Ja!e, A. Zunger, Phys. Rev. B 29 (1984) 1882. [2] H.W. Schock, Appl. Surf. Sci. 92 (1994) 606. [3] A.M. Gabor, J.R. Tuttle, D.S. Albin, M.A. Contreras, R. Nou", A.H. Hermann, Appl. Phys. Lett. 65 (1994) 198. [4] S. Nishiwaki, T. Satoh, S. Hayashi, Y. Hashimoto, T. Negami, T. Wada, J. Mater. Res. 14 (1999) 4514. [5] A. Rockett, R.M. Birkmire, J. Appl. Phys. 70 (1991) R81. [6] A.M. Gabor, J.R. Tuttle, D.S. Albin, A.L. Tennant, M.A. Contreras, R. Nou", Proceedings of the 12th NREL Photovoltaic Program Review Meeting, AIP Conference Proceedings Vol. 306, 1994, pp. 59}67. [7] J. Kessler, D. Schmid, S. Zweigart, H. Dittrich, H.W. Schock, CuInSe "lm formation from sequential depositions of In(Se):Cu:Se, 12th European Photovoltaic Solor Energy Conference, 1994, Amsterdam, pp. 648}652. [8] B. Dimmler, D. Schmid, H.W. Schock, Properties of Cu(In,Ga)Se thin "lm surfaces and their relation to device performance, Technical Digest Sixth International Photovoltaic Science Engineering Conference, 1992, New Delhi, pp. 103}108. [9] T. Nakada, R. Onishi, A. Kunioka, Sol. Energy Mater. Sol. Cells 35 (1994) 209. [10] K. Kushiya, A. Shimizu, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 34 (1995) 54.
Solar Energy Materials & Solar Cells 67 (2001) 217}223
Preparation of Cu(In,Ga)Se thin "lms from Cu}Se/In}Ga}Se precursors for high-e$ciency solar cells S. Nishiwaki , T. Satoh , S. Hayashi , Y. Hashimoto , S. Shimakawa , T. Negami *, T. Wada Advanced Technology Research Laboratories, Matsushita Electric Ind. Co. Ltd., 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan
Abstract Improved preparation process of a device quality Cu(In,Ga)Se (CIGS) thin "lm was proposed for production of CIGS solar cells. In}Ga}Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se #uxes to form Cu}Se/In}Ga}Se precursor "lm at substrate temperature of over 2003C. The precursor "lm was annealed in Se #ux at substrate temperature of over 5003C to obtain high-quality CIGS "lm. The solar cell with a MgF /ITO/ZnO/CdS/CIGS/Mo/glass structure showed an e$ciency of 17.5% (< "0.634 V, J "36.4 mA/cm, FF"0.756). 2001 Elsevier Science B.V. All rights reserved. Keywords: Cu(In; Ga)Se ; Thin "lms; Fabrication process; Microstructure
1. Introduction Cu(In,Ga)Se (CIGS) is one of the most promising materials for thin "lm photovol taic devices due to the suitability of its bandgap and its high absorption coe$cient for solar radiation [1]. A variety of processes for preparing device quality CIGS "lms has been proposed [2]. We have demonstrated CIGS solar cells with an e$ciency of over 17% by the `three-stagea process [3]. In the `three-stagea process, the initial stage
* Corresponding author. E-mail address: [email protected] (T. Negami). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 2 8 4 - 1
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consists of deposition of an In}Ga}Se precursor layer. In the second stage, the precursor layer is exposed to Cu and Se #uxes to form a Cu-rich CIGS. In the third stage, small amounts of In, Ga and Se are added to the Cu-rich CIGS layer in order to obtain a slightly (In,Ga)-rich CIGS "lm. Recently, we studied growth of CIGS thin "lms during the `three-stagea process [4], and demonstrate that the phase of the "lm was changed in the second stage as follows: (In,Ga) Se P[Cu(In,Ga) Se ]PCu(In,Ga) Se PCu(In,Ga)Se . In addi tion, it was proved that the CIGS crystals grew in the second stage from the (In,Ga) Se with sub-micron size grains to the Cu(In,Ga)Se with a few micrometers grains. Based on this result, we study a simple and tolerant preparation process of device-quality CIGS "lms for high-e$ciency solar cells. The process consists of the preparation stage of a CIGS precursor "lm at low substrate temperature and postannealing stage of the precursor "lm in Se #ux at high substrate temperature. Unlike the `bi-layera[5] and `three-stagea processes, this process does not include Cu-rich condition. Furthermore, unlike the previously reported two-stage method [6}10], this process is based on the formation of chalcopyrite phase at a low temperature, not at a high temperature. 2. Experimental (In,Ga)}Se layers were deposited on Mo-coated soda-lime glasses at about 25}3503C. Then the layers were exposed to Cu and Se #uxes keeping the substrate temperature. The amount of Cu, In, and Ga deposited was not changed in all preparations. The Cu}Se/In}Ga}Se precursor "lms were post-annealed in Se #ux at over 5003C. The chemical composition of the precursor and the post-annealed "lms was determined with an energy-dispersive X-ray spectrometer (EDX). The depth pro"le of the elements of Cu, In, Ga, Se, and Mo was characterized by secondary-ion mass spectroscopy (SIMS). The phases in the "lm were identi"ed by X-ray di!raction (XRD) operated at 50 kV and 200 mA using Cu K radiation. The microstructure of the "lms ? was observed by cross-sectional high-resolution transmission electron microscopy (TEM) using a TOPCON EM-002B electron microscope operated at 200 keV. CIGS solar cells with a structure of ITO/ZnO/CdS/CIGS /Mo/glass were fabricated. The CdS bu!er layers were deposited by chemical-bath deposition (CBD). The ZnO and ITO layers were deposited by rf-magnetron sputtering. The performance of solar cells was evaluated under standard AM 1.5 illumination. 3. Results and discussion 3.1. Precursor layers We prepared four kinds of precursor "lms, precursor a, b, c and d which were obtained at substrate temperatures of about 350, 200, 100 and 253C, respectively.
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Table 1 Chemical composition of precursor "lms analyzed by EDX Precusor
a b c d
Substrate temperature (3C)
Chemical composition (at%) Cu
In
Ga
Se
350 200 100 25
25 52 36 10
21 6 3 0
4 2 1 0
50 40 60 90
Fig. 1. SIMS depth pro"les of element Cu, In, Ga, Se, and Mo in the precursor "lms: (a) precursor a deposited at about 3503C, (b) precursor b deposited at about 2003C, (c) precursor c deposited at about 1003C, and (d) precursor d deposited at about 253C.
Table 1 shows chemical compositions of the precursor "lms analyzed by EDX. The data correspond to the chemical composition of the "lm near the surface, because EDX analysis is surface sensitive. Precursor a had a stoichiometric composition of Cu(In,Ga)Se . Precursor b showed Cu-rich composition. Precursor c showed Se-rich composition, and Cu and Se occupied 96% of all constituents. Precursor d was almost Se "lm. Figs. 1(a)}(d) show SIMS depth pro"le of the elements in the precursor a to d, respectively. In precursor a deposited at about 3503C, Cu, In, and Ga distribute
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Fig. 2. XRD patterns of (a) precursor a, (b) precursor b, (c) precursor c, and (d) precursor d.
through the "lm. We should point out that surface layer with high In and low Ga composition can be observed in the "gure. The details on the gradient distribution of In and Ga will be reported elsewhere. In precursor b deposited at about 2003C, the Cu}Se/In}Ga}Se structure can be seen and inter-di!usion between Cu}Se and (In,Ga)}Se layers is also observed. In precursor c deposited at about 1003C, a small amount of inter-di!usion between the surface Cu}Se and bottom (In,Ga) }Se layers is observed. In precursor d deposited at about 253C, separation of surface Cu}Se and bottom (In,Ga)}Se layers is clearly observed. These results support EDX analyses and indicate that the di!usion of the constituents through the precursor "lm proceeds over 2003C. Fig. 2(a)}(d) shows XRD patterns of precursor a, b, c and d, respectively. In the XRD patterns, selenides and Mo are detected, but Se-de"cient phase, such as Cu}In alloy, cannot be observed. The formation of only selenide during the deposition distinguishes this sequential deposition process from the other co-evaporation processes for the preparation of precursor "lms at low substrate temperature [8}10]. In the precursor a deposited at about 3503C (Fig. 2(a)), the 101 peak characterizing tetragonal structure is observed at the position of arrow, which implies the phase is chalcopyrite. In Fig. 2(a), the peak splits can be seen due to graded compositional distribution of In and Ga between the "lm surface and the bottom as shown in Fig. 1(a). The precursor b deposited at about 2003C is considered to be a metastable zinc blend-type (Cu,In,Ga)Se. In the XRD pro"le of precursor b, the peak split can be also seen at around 2h"273. It would be attributed to compositional inhomogeneity of Cu, In, and Ga. The precursor c deposited at 1003C shows Cu}Se phase and the
Solmat=190=Ravi=VVC
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221
precursor d deposited at about 253C indicates amorphous and small amount of Cu}Se phase. These results are consistent with the results of SIMS depth pro"le as shown in Fig. 1. The chalcopyrite-type CIGS precursor could be obtained at substrate temperatures of more than 2003C by the inter-di!usion between surface Cu}Se and bottom (In,Ga) }Se layers. 3.2. Annealed xlms The CIGS a, b and c are obtained by post-annealing of the precursor a, b, and c, respectively. It is noted that the precursor "lm deposited at about 253C was peeled o! during the post-annealing, because of a large volume expansion. EDX and XRD showed that all the post-annealed "lms had chemical compositions of Cu/(In#Ga)&0.94 and Ga/(In#Ga)&0.25, and a chalcopyrite-type Cu(In,Ga)Se phase. By the post-annealing at over 5003C, the precursor "lms adequately react and homogenize to form the CIGS phase. Figs. 3(a)}(c) show cross-sectional TEM micrographs of CIGS a, b and c "lms. The obtained "lms are dense and voids do not appear. We consider that, due to the absence of Se-de"cient phase such as Cu}In alloy in the precursor "lms, the volume expansion accompanied by selenization during the postannealing are depressed and then result in dense "lms. Fig. 3(a) shows that the grain size of CIGS a is 1}2 lm. In Figs. 3(b) and (c), it is observed that the CIGS "lms are composed of sub-micrometer size grains. It is understood that the grain size of the post-annealed CIGS "lm depends on the preparation condition of precursor "lms. We consider that the formation reaction of chalcopyrite phase from surface Cu}Se and bottom (In,Ga)}Se layers by the inter-di!usion inhibit the grain growth. The performance of the solar cells fabricated using post-annealed CIGS a, b, and c were measured under standard AM 1.5 illumination. The cell performances are listed in Table 2 and the I}< curves are shown in Fig. 4. CIGS a whose precursor was deposited at about 3503C resulted in the best performance. As listed in Table 2, the advantage in the performances of CIGS a is open-circuit voltage (< ), suggesting higher carrier density in CIGS a than CIGS b or c. We think that the defects in the CIGS grain decrease with grain growth, resulting in the good performance of CIGS solar cell.
Fig. 3. Cross-sectional TEM micrographs of the (a) CIGS a, (b) CIGS b, and (c) CIGS c whose precursor "lms were prepared at about 350, 200 and 1003C, respectively.
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Table 2 Performance of the solar cells fabricated using CIGS a, b and c Post-annealed "lms
E!. (%)
< (V)
J (mA/cm)
FF
CIGS a CIGS b CIGS c
15.5 14.2 13.3
0.661 0.614 0.615
31.9 32.6 31.8
0.736 0.712 0.679
Fig. 4. I}< curves of solar cells using CIGS a, b, and c of which precursor "lms were prepared at about 350, 200, and 1003C, respectively.
Fig. 5. I}< curve of solar cell using CIGS "lm fabricated by optimized process.
Fig. 5 shows a I}< curve of the solar cell coated with a MgF anti-re#ection layer. The CIGS "lm was fabricated by the optimized process. The cell showed an e$ciency of 17.5% (< "0.634 V, J "36.4 mA/cm, FF"0.756). The obtained values are comparable to those of solar cells fabricated using CIGS "lms deposited by the
Solmat=190=Ravi=VVC
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223
`three-stagea process. The present fabrication technique was not only simple and easy to control the chemical composition, but also enable to fabricate high-performance CIGS solar cells.
4. Summary We study the improved PVD process that is simple and tolerant for fabrication of device quality CIGS "lms. Cu}Se/In}Ga}Se precursor "lms were deposited on Mo-coated soda-lime glasses at substrate temperature of about 25}3503C, and then annealed in Se #ux at substrate temperature of over 5003C. The grain size of the post-annealed CIGS "lms depended on the preparation condition of the precursor "lms. The cell fabricated by this improved PVD process showed an e$ciency of 17.5% (< "0.634 V, J "36.4 mA/cm, FF"0.756). This e$ciency is comparable to our best cell. This improved PVD process is applicable to the production of CIGS solar cells and modules.
Acknowledgements This work was supported by the NEDO as a part of the New Sunshine program conducted under the sponsorship of MITI. The authors would like to express their sincere appreciation to Ms. F. Toujho and Dr. K. Tsukamoto for their help with the SIMS analyses, Mr. T. Kouzaki for his help with the TEM observations. The authors would also like to thank Drs. O. Yamazaki, M. Takao, T. Uenoyama and M. Kitagawa for their encouragement throughout this work.
References [1] J.E. Ja!e, A. Zunger, Phys. Rev. B 29 (1984) 1882. [2] H.W. Schock, Appl. Surf. Sci. 92 (1994) 606. [3] A.M. Gabor, J.R. Tuttle, D.S. Albin, M.A. Contreras, R. Nou", A.H. Hermann, Appl. Phys. Lett. 65 (1994) 198. [4] S. Nishiwaki, T. Satoh, S. Hayashi, Y. Hashimoto, T. Negami, T. Wada, J. Mater. Res. 14 (1999) 4514. [5] A. Rockett, R.M. Birkmire, J. Appl. Phys. 70 (1991) R81. [6] A.M. Gabor, J.R. Tuttle, D.S. Albin, A.L. Tennant, M.A. Contreras, R. Nou", Proceedings of the 12th NREL Photovoltaic Program Review Meeting, AIP Conference Proceedings Vol. 306, 1994, pp. 59}67. [7] J. Kessler, D. Schmid, S. Zweigart, H. Dittrich, H.W. Schock, CuInSe "lm formation from sequential depositions of In(Se):Cu:Se, 12th European Photovoltaic Solor Energy Conference, 1994, Amsterdam, pp. 648}652. [8] B. Dimmler, D. Schmid, H.W. Schock, Properties of Cu(In,Ga)Se thin "lm surfaces and their relation to device performance, Technical Digest Sixth International Photovoltaic Science Engineering Conference, 1992, New Delhi, pp. 103}108. [9] T. Nakada, R. Onishi, A. Kunioka, Sol. Energy Mater. Sol. Cells 35 (1994) 209. [10] K. Kushiya, A. Shimizu, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 34 (1995) 54.