Growth of a-Si:H on transparent conductive oxides for solar cell applications

Abstract

Different concepts for optimizing the transparent conductive oxide (TCO)/p-interface in hydrogenated amorphous silicon (a-Si:H) based solar cells have been studied in order to avoid the segregation of metal layers, and hence considerable reductions in short circuit current. We analysed structural and chemical changes which occur at the surface of transparent conductive oxides — TCOs (SnO 2, ZnO, and Indium Tin Oxide — ITO) in silane, hydrogen and C0 2 plasmas. We also used a-SiO:H instead of a-SiC:H in the p-doped layer. In-situ ellipsometry and SIMS/XPS depth profiling show that room temperature as well as fast deposition easily overcome all detrimental effects. TCO deterioration by ion and radical bombardement at high deposition rates is more than compensated if the surface is protected by a rapidly growing a-Si:H film. Using ZnO as a TCO, or as a 20 nm buffer layer only, completely suppresses metal formation. In-situ ellipsometry in conjunction with atomic force microscopy reveals significant changes in surface morphology, namely filling of the TCO-texture during deposition, which is of crucial importance for light trapping in solar cells.