ZnO layers deposited by the ion layer gas reaction on Cu(In,Ga)(S,Se)2 thin film solar cell absorbers—impact of ‘damp‐heat’ conditions on the layer properties

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 bi-layer. The long-term stability of CIGSSe-based solar modules is currently under investigation. In order to pass the respective stability tests, which include exposure to ‘damp-heat’ (DH) conditions (85% relative humidity at 85(C) to accelerate possible aging effects, a good intrinsic material stability is required. In Reference1 it was revealed, that ILGAR-ZnO contains a certain amount of meta-stable hydroxide, which can be directly tuned by the ILGAR process parameters (number of process cycles and process temperature). In order to determine the ILGAR process parameters, which result in intrinsically stable WELs, ILGAR-ZnO/CIGSSe test structures were investigated by means of scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) before and after a DH-test. It was found that, induced by the DH-conditions, a continuous dehydration of the WELs together with a disintegration of the ILGAR-ZnO layers takes place. This supports an earlier suggested mechanism of a DH-induced degradation by a release of water at the most critical location in a solar cell, at the heterointerface between window and absorber. By a systematic variation of the ILGAR process parameters it was possible to reduce the hydroxide content in the ILGAR-ZnO layers resulting in intrinsically more stable samples. Copyright © 2006 John Wiley & Sons, Ltd.