In this work, the optical and electronical properties of Cu(In,Ga)Se2 (CIGSe) based solar cells are studied with respect to the absorber layer thickness in a range of 0.2 μm to 2.8 μm. Further, the influence of a micro structured back contact on the solar cell efficiency is investigated. Therefore, the optical constants, the doping density, the bandgap gradient, the sodium content, the minority carrier lifetime, and the back contact recombination velocity of the absorbers are determined. Additionally, the solar cell parameters and the quantum efficiency of the corresponding solar cells are determined. Based on optical simulations, the back contact design is optimized, which results in a strongly increased optical back contact reflectivity. Using this concept, a relative efficiency improvement of around 40 % for an absorber layer thickness of 200 nm is achieved.