techniques for achieving >20% conversion efficiency si-based solar cells qingkai qian department...
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Techniques for achieving >20%conversion efficiency Si-based
solar cells
Qingkai QIAN
Department of Electronic and Computer EngineeringThe Hong Kong University of Science and Technology
December 3, 2014
Outline
I-V curve of solar cellTheoretical limitKeys to improve the efficiency
Maximize light absorptionMinimize recombinationReduce resistance
To transcend the classical limit
Outline
I-V curve of solar cellTheoretical limitKeys to improve the efficiency
Maximize light absorptionMinimize recombinationReduce resistance
To transcend the classical limit
p-n junction: soul of solar cell
Built in electric field separates the photon-generated electron-hole pair.
I-V curve of solar cell
IL : light generated current
Outline
I-V curve of solar cellTheoretical limitKeys to improve the efficiency
Maximize light absorptionMinimize recombinationReduce resistance
To transcend the classical limit
Response to different wavelength
Blue light absorbed at the front surface,Red light absorbed at the back surface,Different absorption site, different efficiency.
Artificial sun standard
International Electrotechnical Commission (IEC)IEC 60904–3: 2008 AM1.5 standard 1000 W/m2 at 25 ℃
¿1
cos (48 °)=1.5
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.
Outline
I-V curve of solar cellTheoretical limitKeys to improve the efficiency
Maximize light absorptionMinimize recombinationReduce resistance
To transcend the classical limit
Techniques to improve the efficiency
Maximize light absorptionAntireflection layerSurface texturingReduce front metal contact
Minimize recombinationFront and back passivationHeavily doped metal contactHetero-junction with amorphous silicon
Reduce resistance
Light trap: antireflection layer
Minimize reflection for a wavelength of 0.6 µm
Double layer anti-reflection coating (DLARC) ZnS+MgF2 Further reduce reflection too expensive
Bare silicon has a high surface reflection of over 30%.
Light trap: surface texturing• “Roughening" reduces reflection by bouncing back onto the surface.• Anisotropic etching of (100) in KOH
random pyramid texture
random inverted-pyramid textureRear reflector:total internal reflection
Light trap: reduce metal shadowing
EWT: Emitter wrap through solar cell
MWT: Metal wrap through solar cellPossible to move to backside?
Tandem package advantage
Reduce recombination
N-type silicon has a higher surface quality, near p-n junction SiO2 surface passivation Heavily contact region doping
Reduce recombination
• HIT: hetero-junction with intrinsic thin layer• a-Si /Si/ a-Si junction as passivation
Reduce series resistance
Doping of Base (1 Ω·cm)Doping Level of Emitter(100 Ω/ )☐
• Resistance of bulk silicon• Trade off with carrier diffusion length, recombination.
Reduce series resistance
• Resistance of busbar metal• Trade-off with light shadowing.
Effect of these techniques
Records of efficiency
2014 Panasonic HIT® Solar Cell: heterojunction with intrinsic thin layer Highest efficiency of silicon solar cell: 25.6%
Outline
I-V curve of solar cellTheoretical limitKeys to improve the efficiency
Maximize light absorptionMinimize recombinationReduce resistance
To transcend the classical limit
To transcend the classical limit
Tandem solar cell One photontwo e-h pairs two photonone e-h pair
Make the full use of sunlight
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. 2014.• [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. 2011.
• [9] F. Kiefer, C. Ulzhofer, T. Brendemuhl, N.-P. Harder, R. Brendel, V. Mertens, S. Bordihn, C. Peters, and J. W. Muller, “High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells,” IEEE J. Photovolt., vol. 1, no. 1, pp. 49–53, Jul. 2011.
• [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.
Thank you!
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