Effects of Back Contact Instability on Cu2ZnSnS4 Devices and Processes

Cu2ZnSnS4 (CZTS) is a promising material for thin film solar cells based on sustainable resources. This paper explores some consequences of the chemical instability between CZTS and the standard Mo “back contact” layer used in the solar cell. Chemical passivation of the back contact interface using titanium nitride (TiN) diffusion barriers, combined with variations in the CZTS annealing process, enables us to isolate the effects of back contact chemistry on the electrical properties of the CZTS layer that result from the synthesis, as determined by measurements on completed solar cells. It is found that instability in the back contact is responsible for large current losses in the finished solar cell, which can be distinguished from other losses that arise from instabilities in the surface of the CZTS layer during annealing. The TiN-passivated back contact is an effective barrier to sulfur atoms and therefore prevents reactions between CZTS and Mo. However, it also results in a high series resistance and thus a reduced fill factor in the solar cell. The need for high chalcogen pressure during CZTS annealing can be linked to suppression of the back contact reactions and could potentially be avoided if better inert back contacts were to be developed.