Berlin 2005 – scientific programme

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HL: Halbleiterphysik

HL 64: Photovoltaik II

HL 64.11: Talk

Wednesday, March 9, 2005, 13:15–13:30, TU P-N202

Band alignment at the CdS/Cu(In,Ga)S₂ interface in thin film solar cells — •L. Weinhardt¹, O. Fuchs¹, D. Groß¹, G. Storch¹, E. Umbach¹, N.G. Dhere², A.A. Kadam², S.S. Kulkarni², and C. Heske³ — ¹Experimentelle Physik II, Universität Würzburg — ²Florida Solar Energy Center, Cocoa, USA — ³Dept. of Chemistry, University of Nevada, Las Vegas, USA

While the Se-based part of the Cu(In,Ga)(S,Se)₂ thin film solar cell family has reached cell efficiencies above 19%, the S-based systems are today still limited to 13%. In principle, cells based on CuInS₂ should be superior to CuInSe₂ cells because of their optimal band gap of 1.4 eV (compared to 1.0 eV for CuInSe₂) and their potentially higher voltage, which reduces the series resistance in a module. However, the open circuit voltage shows a lower increase as would be expected by the larger band gap, the reason for which is speculated to be a non-ideal band alignment at the CdS/CuInS₂-interface. To clarify this issue, we have directly determined the conduction and valence band alignment at the CdS/Cu(In,Ga)S₂ interface by using photoelectron spectroscopy and inverse photoemission. We find an unfavorable conduction band offset of -0.45 (±0.15) eV, which can explain the low open circuit voltage. The results will be discussed in the context of the band alignment at the CdS/CuIn(S,Se)₂ derived in earlier measurements.