Investigation of TCO/p-interface and surface morphology of a-Si:H solar cells by in-situ ellipsometry

Different concepts for optimizing the transparent conductive oxide (TCO)/p-interface in a-Si:H based solar cells have been studied in order to avoid metal layer segregation, and hence considerable reductions in short circuit current. Therefore, the authors analysed structural and chemical changes which occur at the surface of TCOs (SnO₂, ZnO, Indium Tin Oxide-ITO) in silane, hydrogen and CO₂ plasmas. They 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 bombardment at high 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 TCO-texture during deposition, which is of crucial importance for light trapping in solar cells