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

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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,”

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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.

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Thank you!