ZnO layers deposited by the ion layer gas reaction on Cu(In,Ga)(S,Se)2 thin film solar cell absorbers: Morphology, growth mechanism, and composition

Abstract

Cu ( In , Ga ) ( S , Se ) 2 (CIGSSe) based solar cells with a ZnO window extension layer (WEL) deposited by the ion layer gas reaction (ILGAR) reach competitive efficiencies compared to corresponding references with CdS buffer and lead to a simplified device structure. The WEL replaces not only the CdS buffer, but also the undoped part of the usually applied rf-sputtered ZnO window bilayer. Since the performance of corresponding solar cell devices depends strongly on the ILGAR process parameters (number of deposition cycles and process temperature), respective ILGAR-ZnO∕CIGSSe test structures were investigated by means of scanning electron microscopy and x-ray photoelectron spectroscopy. Thereby, the growth mechanism of ILGAR-ZnO on CIGSSe absorbers and its morphology was investigated. In addition, the surface composition was determined, showing that ILGAR-ZnO layers contain a certain amount of metastable hydroxide. Due to the systematic variation of the ILGAR process parameters it could be demonstrated that it is possible to directly tune the hydroxide content in the ILGAR-ZnO layers.