The purpose of this project is to identify defects in the absorber layer of solar cells, which limit the efficiency of the cells. The research focuses on thin film solar cells, mainly with CuIn(1-x)Ga(x)Se(2) absorber. Defects in solar cells and absorber bulk materials are characterized electrically, optically and structurally. The results of this reseach help directing the optimization of solar cells of this type.