For a successful industrial implementation of Kesterites Cu2ZnSn(SxSe1-x)4 (CZTSSe) as semiconductor materials for photovoltaics, the long-term stability of the materials and their devices is a key parameter next to efficiency and production costs. Although much research has been conducted on CZTSSe in recent years, the processes leading to degradation are still largely unknown. Hence in this paper we analyze the main degradation processes and their causes as well as how they affect the performance of photovoltaic devices. A temporary CZTSSe-back contact interface barrier and a long-term increase of the front contact resistance and recombination mainly at the cadmium sulfide buffer layer contact of photovoltaic devices is investigated. Copper diffusion in CZTSSe is suggested to be one of the reasons for the change in resistance and recombination and thus for the deterioration of solar cell parameters during degradation or ageing.
