Simulation study of the impact of interface roughness and void inclusions on Cu(In,Ga)(Se,S)2 solar cells

Typical features of sequentially processed Cu(In1−xGax)(Sey−1Sy)2 (CIGSSe) thin film solar cells such as roughness at particular interfaces and free absorber volume (voids) might influence the device characteristics significantly. We investigated the influence of both, roughness and voids, on the solar cell performance parameters as well as on the device capacitance by means of two-dimensional (2D) device simulations. Increased roughness was found to enhance light absorption and carrier collection whereas the increased interface recombination at the textured heterojunction is of minor relevance compared to the performance gain. However, the enlarged interface area must be taken into account when considering measurements of the device capacitance. Void inclusions in the rear section of the absorber support infrared absorption because of internal reflection. This leads to slight improvement of the device performance unless the back contact coverage does not come below 30%. Additionally, voids appear to be less critical in terms of surface recombination at the void/CIGSSe interface given that increased gallium concentration is present.