Anodic degradation of ZnO on soda-lime glass

We introduce a novel degradation effect of the transparent conductive oxide zinc oxide (ZnO). The effect is observed for ZnO layers deposited on commercially available soda-lime glass. When an electric field is applied, the potential induced anodic degradation causes a massive sheet resistance increase of the ZnO. We show that the increase in resistance strongly affects the power output of a small non-encapsulated thin-film photovoltaic module, in which ZnO was used as the front electrode. For the module, an amorphous silicon top and a microcrystalline silicon bottom cell were used in the tandem configuration. We found that the evolution of gaseous oxygen at the anode (ZnO) might be responsible for the increased resistance of the ZnO. A model that describes our observation is presented and solutions to suppress the effect are proposed.