Influence of Substrate Temperature during InxSy Sputtering on Cu(In,Ga)Se2/Buffer Interface Properties and Solar Cell Performance

Abstract

Indium sulfide (InxSy)—besides CdS and Zn(O,S)—is already used as a buffer layer in chalcopyrite-type thin-film solar cells and modules. We discuss the influence of the substrate temperature during very fast magnetron sputtering of InxSy buffer layers on the interface formation and the performance of Cu(In,Ga)Se2 solar cells. The substrate temperature was increased from room temperature up to 240 °C, and the highest power conversion efficiencies were obtained at a temperature plateau around 200 °C, with the best values around 15.3%. Industrially relevant in-line co-evaporated polycrystalline Cu(In,Ga)Se2 absorber layers were used, which yield solar cell efficiencies of up to 17.1% in combination with a solution-grown CdS buffer. The chemical composition of the InxSy buffer as well as of the Cu(In,Ga)Se2/InxSy interface was analyzed by time-of-flight secondary ion mass spectrometry. Changes from homogenous and stoichiometric In2S3 layers deposited at RT to inhomogenous and more sulfur-rich and indium-deficient compositions for higher temperatures were observed. This finding is accompanied with a pronounced copper depletion at the Cu(In,Ga)Se2 absorber surface, and a sodium accumulation in the InxSy buffer and at the absorber/buffer interface. These last two features seem to be the origin for achieving the highest conversion efficiencies at substrate temperatures around 200 °C.