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Techniques for achieving >20% conversion efficiency Si-based solar cells
Qingkai QIAN Department of Electronic and Computer Engineering The Hong Kong University of Science and Technology December 3, 2014
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Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency Maximize light absorption Minimize recombination Reduce resistance To transcend the classical limit
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Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency Maximize light absorption Minimize recombination Reduce resistance To transcend the classical limit
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p-n junction: soul of solar cell
Built in electric field separates the photon-generated electron-hole pair.
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I-V curve of solar cell IL : light generated current
Light generated current basically equals to absorbed photon number
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Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency Maximize light absorption Minimize recombination Reduce resistance To transcend the classical limit
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Response to different wavelength
Blue light absorbed at the front surface, Red light absorbed at the back surface, Different absorption site, different efficiency.
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Artificial sun standard
International Electrotechnical Commission (IEC) IEC 60904–3: 2008 AM1.5 standard 1000 W/m2 at 25 ℃ = 1 cos(48°) =1.5
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Theoretical output limit
Under AM1.5 solar standard Maximum ISC 46mA/cm2. Every photon >1.12eV corresponds to an e-h pair Maximum VOC Depend on dark current Thermal equilibrium limit 0.85V Auger recombination limit 0.72V Shockley-Queisser efficiency limit 29.8% Efficiency mainly limited by voltage loss. For example, 3eV photon produces 0.7V voltage.
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Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency Maximize light absorption Minimize recombination Reduce resistance To transcend the classical limit
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Techniques to improve the efficiency
Maximize light absorption Antireflection layer Surface texturing Reduce front metal contact Minimize recombination Front and back passivation Heavily doped metal contact Hetero-junction with amorphous silicon Reduce resistance
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Light trap: antireflection layer
Bare silicon has a high surface reflection of over 30%. This wavelength is chosen since it is close to the peak power of the solar spectrum. Minimize reflection for a wavelength of 0.6 µm Double layer anti-reflection coating (DLARC) ZnS+MgF2 Further reduce reflection too expensive
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Light trap: surface texturing
“Roughening" reduces reflection by bouncing back onto the surface. Anisotropic etching of (100) in KOH random pyramid texture Rear reflector: total internal reflection random inverted-pyramid texture
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Light trap: reduce metal shadowing
Possible to move to backside? EWT: Emitter wrap through solar cell MWT: Metal wrap through solar cell Tandem package advantage
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Reduce recombination N-type silicon has a higher surface quality, near p-n junction SiO2 surface passivation Heavily contact region doping Recombination will cause Current Losses Voltage Losses Lower efficiency Recombination Within a diffusion length of the junction, electron-hole pair will split, otherwise it will recombine. Unpassivated surface as a localized high recombination site Metal contact region
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Reduce recombination HIT: hetero-junction with intrinsic thin layer
a-Si /Si/ a-Si junction as passivation
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Reduce series resistance
Resistance of bulk silicon Trade off with carrier diffusion length, recombination. higher levels of doping result in damage to the crystal to the extent that carriers recombine before reaching the junction. Doping of Base (1 Ω·cm) Doping Level of Emitter(100 Ω/☐)
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Reduce series resistance
Resistance of busbar metal Trade-off with light shadowing. The resistivity of silicon is too high. Lower resistivity metal grid is placed on the surface to conduct away the current.
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Effect of these techniques
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Records of efficiency 2014 Panasonic HIT® Solar Cell: heterojunction with intrinsic thin layer Highest efficiency of silicon solar cell: 25.6%
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Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency Maximize light absorption Minimize recombination Reduce resistance To transcend the classical limit
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To transcend the classical limit
Make the full use of sunlight Tandem solar cell One photontwo e-h pairs two photonone e-h pair
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Reference [1] D. M. Chapin, C. S. Fuller, and G. L. Pearson, “A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power,” J. Appl. Phys., vol. 25, no. 5, p. 676, 1954. [2] M. A. Green, A. W. Blakers, J. Shi, E. M. Keller, and S. R. Wenham, “19.1% efficient silicon solar cell,” Appl. Phys. Lett., vol. 44, no. 12, p. 1163, 1984. [3] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (version 44): Solar cell efficiency tables,” Prog. Photovolt. Res. Appl., vol. 22, no. 7, pp. 701–710, Jul [4] M. A. Green, “Limits on the Open-circuit Voltage and Efficiency of Silicon Solar Cells Imposed by Intrinsic Auger Processes.pdf,” IEEE Trans. Electron Devices, vol. 31, no. 5, pp. 671–678, 1984. [5] W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys., vol. 32, no. 3, p. 510, 1961. [6] T. Tiedje, E. Yablonovitch, G. D. Cody, and B. Brooks, “Limiting Efficiency of Silicon Solar Cells.pdf,” IEEE Trans. Electron Devices, vol. 31, no. 5, pp. 711–716, 1984. [7] P. Campbell and M. A. Green, “Light trapping properties of pyramidally textured surfaces,” J. Appl. Phys., vol. 62, no. 1, p. 243, 1987. [8] E. Lohmuller, B. Thaidigsmann, M. Pospischil, U. Jager, S. Mack, J. Specht, J. Nekarda, M. Retzlaff, A. Krieg, F. Clement, A. Wolf, D. Biro, and R. Preu, “20% Efficient Passivated Large-Area Metal Wrap Through Solar Cells on Boron-Doped Cz Silicon,” IEEE Electron Device Lett., vol. 32, no. 12, pp. 1719–1721, Dec [9] F. Kiefer, C. Ulzhöfer, T. Brendemühl, N.-P. Harder, R. Brendel, V. Mertens, S. Bordihn, C. Peters, and J. W. Müller, “High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells,” IEEE J. Photovolt., vol. 1, no. 1, pp. 49–53, Jul [10] A. Wang, J. Zhao, and M. A. Green, “24% efficient silicon solar cells,” Appl. Phys. Lett., vol. 57, no. 6, p. 602, 1990. [11] M. Tanaka, M. Taguchi, T. Matsuyama, T. Sawada, S. Tsuda, S. Nakano, H. Hanafusa, and Y. Kuwano, “Development of New a-Si/c-Si Heterojunction Solar cells: ACJ-HIT (Artificially Consructed Juncion-Heterojunction with Intrinsic Thin-Layer),” Jpn J Appl Phys, vol. 31, pp. 3518–3522, 1992.
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