Impact of the structure of Mo(S,Se)₂ interfacial region in electrodeposited CuIn(S,Se)₂ solar cells

In this study, we investigate the formation of Mo(S,Se)₂ layers in the interface region between the absorber and the back contact in electrodeposited CuIn(S,Se)₂ (CISSe) solar cells, and its influence on the resulting devices. Mo(S,Se)₂ layers are formed spontaneously in the Mo/CI(S,Se) interface during the annealing of the absorber, but have a serious impact on the solar cell performance, being responsible of the conductivity at this interface. In this paper, we demonstrate that the crystal orientation of the Mo(S,Se)₂ layer is the relevant parameter that governs the electrical properties of the interface by the generation of an energy barrier. For this, we have analysed two different groups of cells, with high and low V_oc, to determine the origin of the differences. C-V analysis suggested the presence of an energy barrier at the back contact that was related to the existence of a Mo(S,Se)2 layer with c-axis oriented perpendicular to the substrate. Simulations taking into account this feature reproduced the behaviour of the low performing cells. Finally, high-resolution transmission electron microscopy observations were performed to clarify the orientation of the Mo(S,Se)₂ layers at the CISSe/Mo(S,Se)₂ interface, which is determining for the thickness of the Mo(S,Se)₂ layer.