This work investigates the impact of opto-electronical buffer (b) and high resistive window layer (w) properties, i.e. band gap E-g (b, w) and electron affinity x(e) (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%.