This work investigates the impact of opto-electronical buffer (b) and high resistive window layer (w) properties, i.e. band gap Eg(b,w) and electron affinity χe(b,w), on the device performance of chalcopyrite CuIn1−xGaxSe2 (CIGS) solar cells by numerical simulations with SCAPS. We established an initial device model based on an experimental device and its J–V, C–V, and EQE data at room temperature as well as its quantified depth profile for the [Ga]/([Ga]+[In]) ratio (GGI). The device features a non-uniform CIGS doping profile as well as a strongly doped CIGS surface layer. Based on our simulations that include various buffer layer materials, we argue that the most suitable buffer and window layer is Zn1−zMgzO. The potential gain in efficiency is up to 0.9% absolute which corresponds to a relative gain of 4.1%.