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16.0% Efficiency of large area (10 cm×10 cm) thin film polycrystalline silicon solar cell
Solar Energy Materials and Solar Cells 53 (1998) 23—28
16.0% Efficiency of large area (10 cm]10 cm) thin film polycrystalline silicon solar cell H. Morikawa*, Y. Nishimoto, H. Naomoto, Y. Kawama, A. Takami, S. Arimoto, T. Ishihara, K. Namba Photovoltaic Devices Technology Center, Nakatsugawa Works, Mitsubishi Electric Corporation, 4-1, Mizuhara, Itami, Hyogo 664, Japan Received 21 November 1997
Abstract High efficient large area thin film polycrystalline Si solar cell based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. Fabrication process of the via-hole etching for the separation of thin films (VEST) is newly developed. It is found that phosphorus treatment and back surface field (BSF) are quite effective for the VEST structure and the ZMR thin film polycrystalline silicon. The conversion efficiency as high as 16.0% for a practical size (10 cm]10 cm) is achieved. This is the highest for large area thin film polycrystalline Si solar cells ever reported. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Si solar cells; Thin films; Conversion efficiency
1. Introduction Theoretical calculations [1—3] show thin film polycrystalline Si solar cells are attractive candidate for high efficiency and low cost. So in the past, many investigators have reported thin film polycrystalline Si solar cells. However a conversion efficiency up to 16.6% [4] is achieved for a small size of 1 cm2 while large area cells exhibit only around 11.6% [5]. It is reasonable that there are increased difficulties with the uniform formation of a large area thin film Si layer with high crystal quality and with film stress during Si film formation on foreign substrates. In order to overcome these
* Corresponding author. E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 8 ) 0 0 0 0 3 - 8
24
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
problems, we have developed the silicon on insulator (SOI) technology using zonemelting recrystallization (ZMR). We have reported that seeding from a Si substrate during ZMR is not necessary for high-quality thin film Si with a low defect density and the dominant (1 0 0) crystallographic orientation [6—8]. On the other hand, we have also developed a fabrication method for thin film polycrystalline Si solar cells named VEST. The principal features of VEST are the separation of the thin film from the supporting substrate, which is achieved by means of etching of the underlying layer through the via-holes formed in the silicon film, and the repetitive utilization of supporting substrate [9]. In this paper we report fabrication method and structure for the high-efficiency VEST cell. In particular, we have newly introduced phosphorus treatment and low carrier concentration BSF structure for the VEST cell.
2. Experimental The VEST cell structure and its fabrication process are shown in Fig. 1. The SOI structure was formed by the ZMR process, which is a method for obtaining large grain thin film polycrystalline Si and with (1 0 0) dominated surface on an SiO layer. After 2 the ZMR process, an active layer was grown epitaxially by chemical vapor deposition (CVD). Via-holes were formed in this active layer by anisotropic etching and the SiO 2 layer was etched away by introducing the hydrofluoric acid (HF) through the via-holes. In this way, the thin film polycrystalline Si was separated from the supporting substrate, which is reused. Next, only front surface of the thin film
Fig. 1. Schematic drawing and fabrication processes of thin film polycrystalline Si solar cell.
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
25
polycrystalline Si was processed by anisotropic etching to form a randomly pyramidal structure for effective light confinement. After the heavy doped n-type emitter was formed by phosphorus diffusion at whole surface including the inner wall of the via-holes, an etch-back method [10] was adopted to only front surface emitter. Antireflecting coating (SiN) was formed by low pressure chemical vapor deposition (LPCVD) at the front surface and the inner wall of the via-holes. By using screen printing technique, the resist was printed for the patterning of back side n layer and junction separation was processed by anisotropic etching of n` layer for p electrode. On the back side emitter electrode and base electrode are processed using screenprintable Ag paste and Al-containing Ag paste, respectively. After printing they were fired at a same time. Finally low energy (1.2 keV) H` ion implantation from the back side at a beam current of 0.75 mA/cm2 is performed for electrical passivation of crystal. Solar cells with a cell size of 2]2 cm2 were used for the optimization study of the process and structure. These with practical size of 10 cm]10 cm were also fabricated.
3. Results and discussion The effectiveness of phosphorous treatment was studied by fabricating solar cells with and without etch-back process. An etching was performed in our process in order to increase the emitter sheet resistance at only front surface from 20 )/h to 30—100 )/h by varying the etching time. Fig. 2 shows a comparison of internal quantum efficiency with and without etch-back process. The surface emitter resistance for both cells were controlled to the same value of 30 )/h. The etch-backed cell shows about 0.6 mA/cm2 improvement in J . This increase is attributed to the increase for 4# long wavelength light. It has been suggested that phosphorus atoms diffuse more
Fig. 2. Quantum efficiency of 4 cm2 thin film polycrystalline Si solar cells with and without etch-back. The surface sheet resistance is 30 )/h.
26
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
rapidly in the grain boundaries of thin film polycrystalline Si to form deeper pn junction at grain boundaries, which can increase the long wavelength response [10,11]. Furthermore, in case of the sheet resistance of etch-backed surface emitter larger than about 30 )/h, the short wavelength response increases and reaches 0.8 at 400 nm, which is clearly seen in Fig. 3. For the cell in Fig. 3, the sheet resistance is about 50 )/h. For the VEST cell, simple p`-BSF layer cannot be employed because short path of the n`p` structure was formed at the back side surface. So we have employed BSF layer with relatively low carrier concentration in order not to make short path. In this study, BSF with low carrier concentration of 2.8]1017 cm~3 (0.1 ) cm) was formed by CVD prior to the active layer formation on ZMR layer as shown in Fig. 1. BSF and active layers were continuously formed by varying the flow rate of diborane gas during CVD process, so thickness of BSF and active layer can be precisely controlled. 0.1 ) cm BSF layer also acts as a active layer and so we tried simple calculation using PC-1D [12] to know the relation between cell performance and balance of thickness for BSF layer with resistivity 0.1 ) cm and active layer with resistivity 0.5 ) cm keeping the total thickness of the cell constant. In this report, we show only the tendency of the relation, because PC-1D cannot treat two-dimensional structure such as a back side pn configuration of the VEST cell. The results of calculation show that the structure of high efficiency cell needs thick BSF layer with resistivity around 0.1 ) cm. Based on this simulation, solar cells with and without BSF were formed and the results are summarized in Table 1. The improvement of 1.0 mA/cm2 in J and 4# 15 mV in » are clearly seen as a result of introducing BSF. This increase in J is 0# 4# supported by the increase of spectral response in the wavelength range from 600 to 1000 nm due to BSF. We have already reported that hydrogen passivation is very effective to improve the performance of thin film polycrystalline Si cells fabricated by ZMR based technique [13]. Therefore, hydrogen passivation was also carried out from the back side of the
Fig. 3. Quantum efficiency of 4 cm2 thin film polycrystalline Si solar cells with and without BSF.
27
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
Table 1 Comparison of electrical parameters of 4 cm2 thin film polycrystalline Si solar cells with and without BSF
Without BSF With BSF
Active layer thickness (lm)/resistivity () cm)
BSF layer thickness (lm)/resistivity () cm)
» (mV) 0#
J (mA/cm2) 4#
Fill factor
Efficiency (%)
84/0.5 37/0.5
— 47/0.1
568 583
33.62 34.62
0.724 0.720
13.83 14.53
Fig. 4. I—» characteristics of a thin film polycrystalline Si solar cell with a cell size of 10 cm]10 cm under simulated AM1.5G 100 mW/cm2.
cell for 90 min in this experiment. In this way, we have optimized the cell structure and fabrication method, and fabricated large area thin film polycrystalline Si solar cells. As shown in Fig. 4, a conversion efficiency as high as 16.0%, measured at the JQA, was achieved with a cell size of 95.8 cm2 and 77 lm thickness (» "0.5887 V, 0# I "3.407 A, FF"0.763 under AM 1.5 100 mWcm2). 4#
28
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
4. Conclusions In conclusion, we have developed a thin film polycrystalline Si solar cell and its fabrication process. In our experiments, it is demonstrated that combination of phosphorus treatment and low carrier concentration BSF are quite effective for thin film Si solar cells, and as a result the conversion efficiency as high as 16.0% has been achieved for a practical size of 10 cm]10 cm, which is the highest value ever reported in large area thin film polycrystalline Si solar cells.
Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as a part of New Sunshine Program under Ministry of International Trade and Industry.
References [1] M. Spitzer, J. Shewchun, E.S. Vera, J.J. Loferski, in: Conf. Record of the 14th IEEE Photovoltaic Specialists Conf. IEEE, San Diego, 1980, pp. 375—380. [2] M.A. Green, High Efficiency Silicon Solar Cells, Trans Tech Publications, 1987, p. 78. [3] T. Tiedje, E. Yablonovitch, G.D. Cody, B.G. Brooks, IEEE Trans. Electron Dev. ED- 31 (1984) 711. [4] J.A. Rand, J.S. Culik, C.L. Kendall, Y. Bay, A.M. Barnett, J.C. Checchi, D.H. Ford, R.B. Hall, E.L. Jackson, 26th IEEE Photovoltaic Specialists Conf., 1997, to be published. [5] D.H. Ford, A.M. Barnett, J.C. Checchi, S.R. Collins, R.B. Hall, C.L. Kendall, S.M. Lampo, J.A. Rand, in: Technical Digest of the 9th Internat. Photovoltaic Sci. Eng. Conf., Miyazaki, 1996, pp. 247. [6] S. Arimoto, H. Morikawa, M. Deguchi, Y. Kawama, Y. Matsuno, T. Ishihara, H. Kumabe, T. Murotani, Solar Energy Mater. Solar Cells 34 (1994) 257. [7] H. Naomoto, S. Hamamoto, A. Takami, S. Arimoto, T. Ishihara, H. Kumabe, T. Murotani, S. Mitsui, in: Technical Digest of the 7th Internat. Photovoltaic Sci. Eng. Conf. Nagoya, 1993, pp. 79. [8] S. Hamamoto, H. Naomoto, A. Takami, S. Arimoto, T. Ishihara, H. Kumabe, T. Murotani, S. Mitsui, in: Technical Digest of the 7th Internat. Photovoltaic Sci. Eng. Conf., Nagoya, 1993, p. 241. [9] M. Deguchi, Y. Kawama, Y. Matsuno, Y. Nishimoto, H. Morikawa, S. Arimoto, H. Kumabe, T. Murotani, in: the Conf. Record of the 24th IEEE Photovoltaic Specialists Conf., IEEE, Hawaii, 1994, p. 1287. [10] A. Rohatgi, P. Sana, J. Salami, in: Proc. 11th E.C. Photovoltaic Solar Energy Conf., Montreux, 1992, p. 159. [11] S. Narayanan, S.R. Wenham, M.A. Green, IEEE Trans. Electron Dev. ED- 37 (1990) 382. [12] P.A. Basore, D.T. Rover, A.W. Smith, in: the Conf. Record of the 20th IEEE Photovoltaic Specialists Conf., IEEE, Las Vegas, 1988, p. 389. [13] S. Arimoto, H. Morikawa, M. Deguchi, Y. Kawama, Y. Matsuno, T. Ishihara, H. Kumabe, T. Murotani, in: the Conf. Record of the 24th IEEE Photovoltaic Specialists Conf., IEEE, Hawaii, 1994, p. 1311.
16.0% Efficiency of large area (10 cm]10 cm) thin film polycrystalline silicon solar cell H. Morikawa*, Y. Nishimoto, H. Naomoto, Y. Kawama, A. Takami, S. Arimoto, T. Ishihara, K. Namba Photovoltaic Devices Technology Center, Nakatsugawa Works, Mitsubishi Electric Corporation, 4-1, Mizuhara, Itami, Hyogo 664, Japan Received 21 November 1997
Abstract High efficient large area thin film polycrystalline Si solar cell based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. Fabrication process of the via-hole etching for the separation of thin films (VEST) is newly developed. It is found that phosphorus treatment and back surface field (BSF) are quite effective for the VEST structure and the ZMR thin film polycrystalline silicon. The conversion efficiency as high as 16.0% for a practical size (10 cm]10 cm) is achieved. This is the highest for large area thin film polycrystalline Si solar cells ever reported. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Si solar cells; Thin films; Conversion efficiency
1. Introduction Theoretical calculations [1—3] show thin film polycrystalline Si solar cells are attractive candidate for high efficiency and low cost. So in the past, many investigators have reported thin film polycrystalline Si solar cells. However a conversion efficiency up to 16.6% [4] is achieved for a small size of 1 cm2 while large area cells exhibit only around 11.6% [5]. It is reasonable that there are increased difficulties with the uniform formation of a large area thin film Si layer with high crystal quality and with film stress during Si film formation on foreign substrates. In order to overcome these
* Corresponding author. E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 8 ) 0 0 0 0 3 - 8
24
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
problems, we have developed the silicon on insulator (SOI) technology using zonemelting recrystallization (ZMR). We have reported that seeding from a Si substrate during ZMR is not necessary for high-quality thin film Si with a low defect density and the dominant (1 0 0) crystallographic orientation [6—8]. On the other hand, we have also developed a fabrication method for thin film polycrystalline Si solar cells named VEST. The principal features of VEST are the separation of the thin film from the supporting substrate, which is achieved by means of etching of the underlying layer through the via-holes formed in the silicon film, and the repetitive utilization of supporting substrate [9]. In this paper we report fabrication method and structure for the high-efficiency VEST cell. In particular, we have newly introduced phosphorus treatment and low carrier concentration BSF structure for the VEST cell.
2. Experimental The VEST cell structure and its fabrication process are shown in Fig. 1. The SOI structure was formed by the ZMR process, which is a method for obtaining large grain thin film polycrystalline Si and with (1 0 0) dominated surface on an SiO layer. After 2 the ZMR process, an active layer was grown epitaxially by chemical vapor deposition (CVD). Via-holes were formed in this active layer by anisotropic etching and the SiO 2 layer was etched away by introducing the hydrofluoric acid (HF) through the via-holes. In this way, the thin film polycrystalline Si was separated from the supporting substrate, which is reused. Next, only front surface of the thin film
Fig. 1. Schematic drawing and fabrication processes of thin film polycrystalline Si solar cell.
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
25
polycrystalline Si was processed by anisotropic etching to form a randomly pyramidal structure for effective light confinement. After the heavy doped n-type emitter was formed by phosphorus diffusion at whole surface including the inner wall of the via-holes, an etch-back method [10] was adopted to only front surface emitter. Antireflecting coating (SiN) was formed by low pressure chemical vapor deposition (LPCVD) at the front surface and the inner wall of the via-holes. By using screen printing technique, the resist was printed for the patterning of back side n layer and junction separation was processed by anisotropic etching of n` layer for p electrode. On the back side emitter electrode and base electrode are processed using screenprintable Ag paste and Al-containing Ag paste, respectively. After printing they were fired at a same time. Finally low energy (1.2 keV) H` ion implantation from the back side at a beam current of 0.75 mA/cm2 is performed for electrical passivation of crystal. Solar cells with a cell size of 2]2 cm2 were used for the optimization study of the process and structure. These with practical size of 10 cm]10 cm were also fabricated.
3. Results and discussion The effectiveness of phosphorous treatment was studied by fabricating solar cells with and without etch-back process. An etching was performed in our process in order to increase the emitter sheet resistance at only front surface from 20 )/h to 30—100 )/h by varying the etching time. Fig. 2 shows a comparison of internal quantum efficiency with and without etch-back process. The surface emitter resistance for both cells were controlled to the same value of 30 )/h. The etch-backed cell shows about 0.6 mA/cm2 improvement in J . This increase is attributed to the increase for 4# long wavelength light. It has been suggested that phosphorus atoms diffuse more
Fig. 2. Quantum efficiency of 4 cm2 thin film polycrystalline Si solar cells with and without etch-back. The surface sheet resistance is 30 )/h.
26
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
rapidly in the grain boundaries of thin film polycrystalline Si to form deeper pn junction at grain boundaries, which can increase the long wavelength response [10,11]. Furthermore, in case of the sheet resistance of etch-backed surface emitter larger than about 30 )/h, the short wavelength response increases and reaches 0.8 at 400 nm, which is clearly seen in Fig. 3. For the cell in Fig. 3, the sheet resistance is about 50 )/h. For the VEST cell, simple p`-BSF layer cannot be employed because short path of the n`p` structure was formed at the back side surface. So we have employed BSF layer with relatively low carrier concentration in order not to make short path. In this study, BSF with low carrier concentration of 2.8]1017 cm~3 (0.1 ) cm) was formed by CVD prior to the active layer formation on ZMR layer as shown in Fig. 1. BSF and active layers were continuously formed by varying the flow rate of diborane gas during CVD process, so thickness of BSF and active layer can be precisely controlled. 0.1 ) cm BSF layer also acts as a active layer and so we tried simple calculation using PC-1D [12] to know the relation between cell performance and balance of thickness for BSF layer with resistivity 0.1 ) cm and active layer with resistivity 0.5 ) cm keeping the total thickness of the cell constant. In this report, we show only the tendency of the relation, because PC-1D cannot treat two-dimensional structure such as a back side pn configuration of the VEST cell. The results of calculation show that the structure of high efficiency cell needs thick BSF layer with resistivity around 0.1 ) cm. Based on this simulation, solar cells with and without BSF were formed and the results are summarized in Table 1. The improvement of 1.0 mA/cm2 in J and 4# 15 mV in » are clearly seen as a result of introducing BSF. This increase in J is 0# 4# supported by the increase of spectral response in the wavelength range from 600 to 1000 nm due to BSF. We have already reported that hydrogen passivation is very effective to improve the performance of thin film polycrystalline Si cells fabricated by ZMR based technique [13]. Therefore, hydrogen passivation was also carried out from the back side of the
Fig. 3. Quantum efficiency of 4 cm2 thin film polycrystalline Si solar cells with and without BSF.
27
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
Table 1 Comparison of electrical parameters of 4 cm2 thin film polycrystalline Si solar cells with and without BSF
Without BSF With BSF
Active layer thickness (lm)/resistivity () cm)
BSF layer thickness (lm)/resistivity () cm)
» (mV) 0#
J (mA/cm2) 4#
Fill factor
Efficiency (%)
84/0.5 37/0.5
— 47/0.1
568 583
33.62 34.62
0.724 0.720
13.83 14.53
Fig. 4. I—» characteristics of a thin film polycrystalline Si solar cell with a cell size of 10 cm]10 cm under simulated AM1.5G 100 mW/cm2.
cell for 90 min in this experiment. In this way, we have optimized the cell structure and fabrication method, and fabricated large area thin film polycrystalline Si solar cells. As shown in Fig. 4, a conversion efficiency as high as 16.0%, measured at the JQA, was achieved with a cell size of 95.8 cm2 and 77 lm thickness (» "0.5887 V, 0# I "3.407 A, FF"0.763 under AM 1.5 100 mWcm2). 4#
28
H. Morikawa et al./Solar Energy Materials and Solar Cells 53 (1998) 23—28
4. Conclusions In conclusion, we have developed a thin film polycrystalline Si solar cell and its fabrication process. In our experiments, it is demonstrated that combination of phosphorus treatment and low carrier concentration BSF are quite effective for thin film Si solar cells, and as a result the conversion efficiency as high as 16.0% has been achieved for a practical size of 10 cm]10 cm, which is the highest value ever reported in large area thin film polycrystalline Si solar cells.
Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as a part of New Sunshine Program under Ministry of International Trade and Industry.
References [1] M. Spitzer, J. Shewchun, E.S. Vera, J.J. Loferski, in: Conf. Record of the 14th IEEE Photovoltaic Specialists Conf. IEEE, San Diego, 1980, pp. 375—380. [2] M.A. Green, High Efficiency Silicon Solar Cells, Trans Tech Publications, 1987, p. 78. [3] T. Tiedje, E. Yablonovitch, G.D. Cody, B.G. Brooks, IEEE Trans. Electron Dev. ED- 31 (1984) 711. [4] J.A. Rand, J.S. Culik, C.L. Kendall, Y. Bay, A.M. Barnett, J.C. Checchi, D.H. Ford, R.B. Hall, E.L. Jackson, 26th IEEE Photovoltaic Specialists Conf., 1997, to be published. [5] D.H. Ford, A.M. Barnett, J.C. Checchi, S.R. Collins, R.B. Hall, C.L. Kendall, S.M. Lampo, J.A. Rand, in: Technical Digest of the 9th Internat. Photovoltaic Sci. Eng. Conf., Miyazaki, 1996, pp. 247. [6] S. Arimoto, H. Morikawa, M. Deguchi, Y. Kawama, Y. Matsuno, T. Ishihara, H. Kumabe, T. Murotani, Solar Energy Mater. Solar Cells 34 (1994) 257. [7] H. Naomoto, S. Hamamoto, A. Takami, S. Arimoto, T. Ishihara, H. Kumabe, T. Murotani, S. Mitsui, in: Technical Digest of the 7th Internat. Photovoltaic Sci. Eng. Conf. Nagoya, 1993, pp. 79. [8] S. Hamamoto, H. Naomoto, A. Takami, S. Arimoto, T. Ishihara, H. Kumabe, T. Murotani, S. Mitsui, in: Technical Digest of the 7th Internat. Photovoltaic Sci. Eng. Conf., Nagoya, 1993, p. 241. [9] M. Deguchi, Y. Kawama, Y. Matsuno, Y. Nishimoto, H. Morikawa, S. Arimoto, H. Kumabe, T. Murotani, in: the Conf. Record of the 24th IEEE Photovoltaic Specialists Conf., IEEE, Hawaii, 1994, p. 1287. [10] A. Rohatgi, P. Sana, J. Salami, in: Proc. 11th E.C. Photovoltaic Solar Energy Conf., Montreux, 1992, p. 159. [11] S. Narayanan, S.R. Wenham, M.A. Green, IEEE Trans. Electron Dev. ED- 37 (1990) 382. [12] P.A. Basore, D.T. Rover, A.W. Smith, in: the Conf. Record of the 20th IEEE Photovoltaic Specialists Conf., IEEE, Las Vegas, 1988, p. 389. [13] S. Arimoto, H. Morikawa, M. Deguchi, Y. Kawama, Y. Matsuno, T. Ishihara, H. Kumabe, T. Murotani, in: the Conf. Record of the 24th IEEE Photovoltaic Specialists Conf., IEEE, Hawaii, 1994, p. 1311.