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Novel deposition method of anti-reflective coating for spherical silicon solar cells
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 2995–3000 www.elsevier.com/locate/solmat
Novel deposition method of anti-reflective coating for spherical silicon solar cells Takashi Minemotoa,, Mikio Murozonob, Yukio Yamaguchic, Hideyuki Takakuraa, Yoshihiro Hamakawaa a
Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan b Clean Venture 21 co. c The University of Tokyo Available online 31 July 2006
Abstract The liquid-phase deposition (LPD) as a novel deposition method of anti-reflective coating (ARC) for spherical silicon solar cells has been proposed. The LPD is a growth method in aqueous solution and can deposit thin films with uniform coverage over a spherical surface. The solar cell performance of the spherical silicon solar cell with an ARC shows more than 10% increase in short-circuit current density compared to that without an ARC. The result confirms that the LPD method is useful for ARC fabrications of spherical silicon solar cells. r 2006 Elsevier B.V. All rights reserved. Keywords: Spherical si; Solar cell; Anti-reflective coating
1. Introduction Spherical silicon solar cells have gathered much attention as a promising candidate for high performance with low-cost solar cells. We proposed the new type of a spherical silicon solar cell with semi-concentration reflector system [1]. Anti-reflective coating (ARC) is one of the most important processes to enhance short-circuit current density Jsc by reducing the reflection at the surface of a solar cell. Fabrication methods of ARCs for conventional solar cells with a flat surface, e.g., physical vapor deposition, chemical vapor deposition and sol–gel method, etc., are not suitable for spherical Si solar cells because these methods Corresponding author. Tel./fax: +81 77 561 3938.
E-mail address: [email protected] (T. Minemoto). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.06.005
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T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
cannot deposit uniform ARC layers over spherical objects effectively. We have proposed the application of liquid-phase deposition (LPD) for the novel fabrication method of ARCs especially for spherical Si solar cells. The LPD is a growth method for thin films in an aqueous solution, and also known as chemical bath deposition which is utilized in the deposition of CdS buffer layers in Cu(In,Ga)Se2 solar cells [2]. The LPD method has not been utilized in ARC fabrications because the films fabricated by the LPD shows poor uniformity for large area; however, it can be ideal for spherical silicon solar cells because of the small size of silicon spheres as the diameter of 1 mm and the ability to deposit uniform thin film over any surface. In this contribution, the fabrication of ARCs on Si spheres and the solar cell performance of spherical Si solar cell with the ARC are discussed. 2. Experimental Fig. 1 shows the schematic apparatus of the LPD method in this work. The deposition process is simple; firstly a water bath is warmed up at certain temperature, and then a glass beaker (growth bath) containing a chemical solution is soaked in the water bath, at the same time substrates are immersed in the beaker and a film deposition is done for certain duration. Here, a CdS thin film is utilized as an ARC for its well-understood deposition mechanism [3–6]. The chemical solution for the CdS deposition consists of four chemicals; 0.001 M (mol/l) Cd(CH3COO)2 2H2O, 0.005 M (NH2)2CS, 0.01 M CH3COONH4 and 0.4 M NH4OH. The growth temperature and duration are 80 1C and 13 min, respectively. The films were deposited on Si spheres and Si wafers to investigate film coverage, surface morphology and reflectance. In order to demonstrate the viability of the LPD method in solar cell applications, spherical Si solar cells were fabricated as shown in Fig. 2. After our baseline process for solar cell fabrications, not fully optimized yet, including phosphorous diffusion and pn separation, etc., CdS thin films were deposited on the silicon spheres. Finally, spherical silicon solar cells were completed by sphere arrangements with reflectors and electrode connections. Here, single-crystalline silicon spheres fabricated from a Czochralski silicon wafer by cutting into dices and polishing into spheres were used for photovoltaic parts. Growth bath
Water bath
Growth solution
Si spheres Stirrer
Heater with magnetic stirrer
Fig. 1. Schematic apparatus of the LPD method.
ARTICLE IN PRESS T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
2997
3. Results and discussion Fig. 3 shows scanning electron microscope (SEM) images of Si spheres with a CdS film deposited by LPD with (a) 80 and (b) 35,000 times magnification, respectively. The surface image of the entire sphere, as shown in Fig. 3(a), indicates the edge part of the sphere is also covered by the CdS film similar to the center part, which confirms the uniform coverage of the CdS film over the spherical surface. Fig. 3(b) reveals that the CdS film consists of crystal grains with the diameters of less than 200 nm. Fig. 4 shows the transmittance, reflectance and absorbance spectrum of a CdS/glass substrate. The spectrums indicate an almost negligible absorption for a wavelength longer than 500 nm, i.e., sub-bandgap region lights. Fig. 5 shows the reflectance spectrum of a CdS/Si wafer. The reflectance spectrum of a bare Si wafer is also shown as a reference. The
Incident light
Reflector with n-electrode
Anti-reflective coating
n-Si p-Si
p-electrode Fig. 2. Cross-sectional structure view of the spherical Si solar cell with semi-concentration reflector system.
Fig. 3. SEM images of Si spheres with a CdS film deposited by the LPD method with (a) 80 and (b) 35,000 times magnification, respectively.
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T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
1.0
T, R and A
0.8 0.6 0.4
Transmittance Reflectance Absorbance
0.2 0.0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Fig. 4. Transmittance, reflectance and absorbance spectrum of a CdS/glass substrate.
1.0
Reflectance
0.8
CdS/Si wafer Si wafer
0.6 0.4 0.2 0.0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Fig. 5. Reflectance spectrum of a CdS/Si wafer.
results confirm that the reflection become minimal around the wavelength of 600 nm, corresponding to the photon density peak in the AM1.5 solar spectrum. The thickness estimated from the reflectance spectrum is around 60 nm. Fig. 6 shows the current–voltage characteristics of typical spherical silicon solar cells with and without a CdS film. The solar cell area is 0.036 cm2 and the light concentration ratio, defined as the ratio of the aperture area of the reflector to the projection area of the sphere, is 4.4. The solar cell with the CdS film shows the energy conversion efficiency of 11.0% with Jsc of 27.6 mA/cm2, while that without a CdS film shows the efficiency of 9.56% with Jsc of 23.8 mA/cm2. The 16.0% increase in Jsc gives the enhancement in the
ARTICLE IN PRESS T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
2999
30 (a)
Current density (mA/cm2)
25 (b)
20
15
10
5
(a) with
(b) without
Effi.(%)
11.0
9.56
Jsc(mA/cm2) Voc(mV)
27.6
23.8
582
577
0.694
0.696
FF 0 0.0
0.1
0.2
0.3 0.4 Voltage (V)
0.5
0.6
Fig. 6. Current–voltage characteristics of typical spherical silicon solar cells with and without a CdS film.
efficiency proportionally. The results indicate that the LPD process is applicable to spherical Si solar cells. In this study, CdS is utilized as an ARC, however it is not an ideal material for an ARC because of its absorption for short wavelength light (o500 nm), resulting a current loss. Moreover, CdS is not an environmental-friendly material. Developments of other compounds such as ZnO and ZnS by LPD methods are in optimization in our laboratory. 4. Conclusion We have proposed the LPD method as the novel deposition method of ARCs for spherical Si solar cells. In this study, CdS is utilized as the ARC. The SEM images of CdS films deposited on Si spheres confirm the uniform coverage over the spherical surface. The spherical Si solar cell with the CdS film shows the higher Jsc by 16.0% compared to that without an ARC. The increase in Jsc gives the enhancement in the efficiency proportionally. The results have demonstrated that the application of the LPD methods in the fabrication of ARCs is especially useful for spherical silicon solar cells. Acknowledgments The authors wish to thank T. Nakamura and Y. Nishibori of Clean venture 21 co., for solar cell fabrication and measurements. This work is partly supported by NEDO as Investigation for Innovative PV Technology Project. References [1] US Patent application publication No. US2002/0096206A1.
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T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
[2] D. Lincot, R. Ortega-Borges, J. Vedel, M. Ruckh, J. Kessler, K.O. Velthaus, D. Hariskos, H.W. Schock, in: Proceedings of the 11th European Photovoltaic Solar Energy Conference, pp. 870–873. [3] M.J. Furlong, M. Froment, M.C. Bernard, R. Cortes, A.N. Tiwari, M. Krejci, H. Zogg, D. Lincot, J. Cryst. Growth 193 (1998) 114. [4] M. Froment, M.C. Bernard, R. Cortes, B. Mokili, D. Lincot, J. Electrochem. Soc. 142 (1995) 2642. [5] M. Kostoglou, N. Andritsos, A.J. Karabelas, Thin Solid Films 387 (2001) 115; Y. Hashimoto, N. Kohara, N. Nishitani, T. Wada, Sol. Energy Mater. Sol. Cells 50 (1998) 71. [6] Y. Hashimoto, T. Satoh, T. Minemoto, S. Shimakawa, T. Negami, in: Proceedings of the 17th European Photovoltaic Solar Energy Conference, pp. 1225–1228.
Solar Energy Materials & Solar Cells 90 (2006) 2995–3000 www.elsevier.com/locate/solmat
Novel deposition method of anti-reflective coating for spherical silicon solar cells Takashi Minemotoa,, Mikio Murozonob, Yukio Yamaguchic, Hideyuki Takakuraa, Yoshihiro Hamakawaa a
Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan b Clean Venture 21 co. c The University of Tokyo Available online 31 July 2006
Abstract The liquid-phase deposition (LPD) as a novel deposition method of anti-reflective coating (ARC) for spherical silicon solar cells has been proposed. The LPD is a growth method in aqueous solution and can deposit thin films with uniform coverage over a spherical surface. The solar cell performance of the spherical silicon solar cell with an ARC shows more than 10% increase in short-circuit current density compared to that without an ARC. The result confirms that the LPD method is useful for ARC fabrications of spherical silicon solar cells. r 2006 Elsevier B.V. All rights reserved. Keywords: Spherical si; Solar cell; Anti-reflective coating
1. Introduction Spherical silicon solar cells have gathered much attention as a promising candidate for high performance with low-cost solar cells. We proposed the new type of a spherical silicon solar cell with semi-concentration reflector system [1]. Anti-reflective coating (ARC) is one of the most important processes to enhance short-circuit current density Jsc by reducing the reflection at the surface of a solar cell. Fabrication methods of ARCs for conventional solar cells with a flat surface, e.g., physical vapor deposition, chemical vapor deposition and sol–gel method, etc., are not suitable for spherical Si solar cells because these methods Corresponding author. Tel./fax: +81 77 561 3938.
E-mail address: [email protected] (T. Minemoto). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.06.005
ARTICLE IN PRESS 2996
T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
cannot deposit uniform ARC layers over spherical objects effectively. We have proposed the application of liquid-phase deposition (LPD) for the novel fabrication method of ARCs especially for spherical Si solar cells. The LPD is a growth method for thin films in an aqueous solution, and also known as chemical bath deposition which is utilized in the deposition of CdS buffer layers in Cu(In,Ga)Se2 solar cells [2]. The LPD method has not been utilized in ARC fabrications because the films fabricated by the LPD shows poor uniformity for large area; however, it can be ideal for spherical silicon solar cells because of the small size of silicon spheres as the diameter of 1 mm and the ability to deposit uniform thin film over any surface. In this contribution, the fabrication of ARCs on Si spheres and the solar cell performance of spherical Si solar cell with the ARC are discussed. 2. Experimental Fig. 1 shows the schematic apparatus of the LPD method in this work. The deposition process is simple; firstly a water bath is warmed up at certain temperature, and then a glass beaker (growth bath) containing a chemical solution is soaked in the water bath, at the same time substrates are immersed in the beaker and a film deposition is done for certain duration. Here, a CdS thin film is utilized as an ARC for its well-understood deposition mechanism [3–6]. The chemical solution for the CdS deposition consists of four chemicals; 0.001 M (mol/l) Cd(CH3COO)2 2H2O, 0.005 M (NH2)2CS, 0.01 M CH3COONH4 and 0.4 M NH4OH. The growth temperature and duration are 80 1C and 13 min, respectively. The films were deposited on Si spheres and Si wafers to investigate film coverage, surface morphology and reflectance. In order to demonstrate the viability of the LPD method in solar cell applications, spherical Si solar cells were fabricated as shown in Fig. 2. After our baseline process for solar cell fabrications, not fully optimized yet, including phosphorous diffusion and pn separation, etc., CdS thin films were deposited on the silicon spheres. Finally, spherical silicon solar cells were completed by sphere arrangements with reflectors and electrode connections. Here, single-crystalline silicon spheres fabricated from a Czochralski silicon wafer by cutting into dices and polishing into spheres were used for photovoltaic parts. Growth bath
Water bath
Growth solution
Si spheres Stirrer
Heater with magnetic stirrer
Fig. 1. Schematic apparatus of the LPD method.
ARTICLE IN PRESS T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
2997
3. Results and discussion Fig. 3 shows scanning electron microscope (SEM) images of Si spheres with a CdS film deposited by LPD with (a) 80 and (b) 35,000 times magnification, respectively. The surface image of the entire sphere, as shown in Fig. 3(a), indicates the edge part of the sphere is also covered by the CdS film similar to the center part, which confirms the uniform coverage of the CdS film over the spherical surface. Fig. 3(b) reveals that the CdS film consists of crystal grains with the diameters of less than 200 nm. Fig. 4 shows the transmittance, reflectance and absorbance spectrum of a CdS/glass substrate. The spectrums indicate an almost negligible absorption for a wavelength longer than 500 nm, i.e., sub-bandgap region lights. Fig. 5 shows the reflectance spectrum of a CdS/Si wafer. The reflectance spectrum of a bare Si wafer is also shown as a reference. The
Incident light
Reflector with n-electrode
Anti-reflective coating
n-Si p-Si
p-electrode Fig. 2. Cross-sectional structure view of the spherical Si solar cell with semi-concentration reflector system.
Fig. 3. SEM images of Si spheres with a CdS film deposited by the LPD method with (a) 80 and (b) 35,000 times magnification, respectively.
ARTICLE IN PRESS 2998
T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
1.0
T, R and A
0.8 0.6 0.4
Transmittance Reflectance Absorbance
0.2 0.0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Fig. 4. Transmittance, reflectance and absorbance spectrum of a CdS/glass substrate.
1.0
Reflectance
0.8
CdS/Si wafer Si wafer
0.6 0.4 0.2 0.0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Fig. 5. Reflectance spectrum of a CdS/Si wafer.
results confirm that the reflection become minimal around the wavelength of 600 nm, corresponding to the photon density peak in the AM1.5 solar spectrum. The thickness estimated from the reflectance spectrum is around 60 nm. Fig. 6 shows the current–voltage characteristics of typical spherical silicon solar cells with and without a CdS film. The solar cell area is 0.036 cm2 and the light concentration ratio, defined as the ratio of the aperture area of the reflector to the projection area of the sphere, is 4.4. The solar cell with the CdS film shows the energy conversion efficiency of 11.0% with Jsc of 27.6 mA/cm2, while that without a CdS film shows the efficiency of 9.56% with Jsc of 23.8 mA/cm2. The 16.0% increase in Jsc gives the enhancement in the
ARTICLE IN PRESS T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
2999
30 (a)
Current density (mA/cm2)
25 (b)
20
15
10
5
(a) with
(b) without
Effi.(%)
11.0
9.56
Jsc(mA/cm2) Voc(mV)
27.6
23.8
582
577
0.694
0.696
FF 0 0.0
0.1
0.2
0.3 0.4 Voltage (V)
0.5
0.6
Fig. 6. Current–voltage characteristics of typical spherical silicon solar cells with and without a CdS film.
efficiency proportionally. The results indicate that the LPD process is applicable to spherical Si solar cells. In this study, CdS is utilized as an ARC, however it is not an ideal material for an ARC because of its absorption for short wavelength light (o500 nm), resulting a current loss. Moreover, CdS is not an environmental-friendly material. Developments of other compounds such as ZnO and ZnS by LPD methods are in optimization in our laboratory. 4. Conclusion We have proposed the LPD method as the novel deposition method of ARCs for spherical Si solar cells. In this study, CdS is utilized as the ARC. The SEM images of CdS films deposited on Si spheres confirm the uniform coverage over the spherical surface. The spherical Si solar cell with the CdS film shows the higher Jsc by 16.0% compared to that without an ARC. The increase in Jsc gives the enhancement in the efficiency proportionally. The results have demonstrated that the application of the LPD methods in the fabrication of ARCs is especially useful for spherical silicon solar cells. Acknowledgments The authors wish to thank T. Nakamura and Y. Nishibori of Clean venture 21 co., for solar cell fabrication and measurements. This work is partly supported by NEDO as Investigation for Innovative PV Technology Project. References [1] US Patent application publication No. US2002/0096206A1.
ARTICLE IN PRESS 3000
T. Minemoto et al. / Solar Energy Materials & Solar Cells 90 (2006) 2995–3000
[2] D. Lincot, R. Ortega-Borges, J. Vedel, M. Ruckh, J. Kessler, K.O. Velthaus, D. Hariskos, H.W. Schock, in: Proceedings of the 11th European Photovoltaic Solar Energy Conference, pp. 870–873. [3] M.J. Furlong, M. Froment, M.C. Bernard, R. Cortes, A.N. Tiwari, M. Krejci, H. Zogg, D. Lincot, J. Cryst. Growth 193 (1998) 114. [4] M. Froment, M.C. Bernard, R. Cortes, B. Mokili, D. Lincot, J. Electrochem. Soc. 142 (1995) 2642. [5] M. Kostoglou, N. Andritsos, A.J. Karabelas, Thin Solid Films 387 (2001) 115; Y. Hashimoto, N. Kohara, N. Nishitani, T. Wada, Sol. Energy Mater. Sol. Cells 50 (1998) 71. [6] Y. Hashimoto, T. Satoh, T. Minemoto, S. Shimakawa, T. Negami, in: Proceedings of the 17th European Photovoltaic Solar Energy Conference, pp. 1225–1228.