Characterization of polycrystalline Cu(In,Ga)Se-2 thin films produced by rapid thermal processing

In this study a simple, tolerant and reproducible process has been developed to produce device quality Cu(ln,Ga)Se-2 thin films. These films were prepared by the rapid thermal processing (RTP) of thermally evaporated Cu-In-Ga-Se metallic precursors. All precursors used in this study were prepared by a new growth process in which all metals (Cu, In and Ga) were evaporated from one single crucible in a Se atmosphere. In this process, developed at Stuttgart University, the stoichiometry of the precursor films was fixed by the amount of material in the crucible and the substrate temperature was kept constant at 200 degrees C. Various rapid thermal processes were considered to optimize the material properties (adhesion, surface morphologies and uniformity) of the compound films. In the case of relatively slow rapid thermal processes (RTP) which involved various ramping steps, films with poor structural properties (inhomogeneous film morphologies and presence of secondary phases) were obtained. A significant improvement in material properties was obtained in the case of rapid heating (in 10s from 200 degrees C to 550 degrees C) of samples. However, these films were still characterized by the presence of secondary phases. Optimum material properties (homogeneous and dense films) were obtained when the heating profile followed a root function in the critical temperature range around 300 degrees C. Preliminary solar cell devices were fabricated with conversion efficiencies above 6% (total area).