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Structural, electronic, and defect properties of kesterites for high-efficiency and low-cost solar cell absorbers

Speaker: 
Wei Su-Huai
Date: 
Tue, 29/01/2013 - 1:30pm to 2:30pm
Location: 
Physics Conference Room (S13-M01-11)
Event Type: 
Seminars

Abstract:

The kesterite-structured semiconductors Cu2ZnSnS4, Cu2ZnSnSe4 and their alloys are drawing considerable attention recently as the active layers in earth-abundant, low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. I will review our recent theoretical studies on the structural, electronic and defect properties of the kesterite materials based on systematical density functional theory calculations and compare them with the better studied chalcopyrite materials CuGaSe2 and CuInSe2. Some highlights of the results observed are: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor CuZnantisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn elemental ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn+SnZn], [VCu+ZnCu] and [ZnSn+2ZnCu] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn+SnZn] clusters, especially in Cu2ZnSnS4; (v) the absolute surface energies of the kesterites and the origin of the p-type to n-type inversion at the surfaces. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions result in the highest solar cell efficiency, as well as why there is an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S composition is high.

Speaker's Biography

Su-Huai Wei received his B.S. in Physics from Fudan University in 1981 and Ph.D. from the College of William and Mary in 1985. He joined the National Renewable Energy Laboratory in 1985 and is currently a Principal Scientist and Group Manager for the Theoretical Materials Science Group. His research is focused on developing electronic structure theory of materials, especially for semiconductors and energy related materials and applications. He has published more than 340 papers in leading scientific journals, including more than 58 in Physical Review Letters with an H index of 67. He is a Fellow of the American Physical Society.

 

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