Selenization of KF Premixed CuInGa Precursor and Its Impact on Solar Cell Performance

Abstract

K-doped Cu(In,Ga)Se2 absorber thin films were prepared by a two-step process (sputtering/selenization) using a metal precursor of a KF-premixed In/CuGa/Mo/glass structure and Se vapor. KF was deposited on top of In, between In and CuGa, and below CuGa by thermal evaporation. The properties were compared with those of a precursor without KF inserted. As an effect of KF insertion, a large electrical change was observed, i.e., the disappearance of deep-level defects, and the grain size and Ga composition distribution of the absorber layer thin film were changed slightly. Nevertheless, there was no benefit in terms of the solar cell performance compared to a precursor without KF inserted. The K inserted in the precursor occupied a position competitively with Na diffused from the substrate, resulting in an undesirable carrier density profile in the surface region of the absorber layer thin film.