I did my PhD at the LSI Theoretical Spectroscopy group on ab initio calculations of the electronic properties of CuIn(S,Se)2 and other materials for photovoltaic applications. It was defended in spring 2010. It mainly focused on the relevance of these techniques depending on the physics at work. Advanced methods such as GW reveal the presence of a hidden compensating mechanism between lattice distortion and intrinsic defect concentration in CuIn(S,Se)2. Such phenomenon may explain the high efficiency obtained for photovoltaic cells based on Cu(In,Ga)S2 thin film absorbers.