In this paper, we present a simulation study of Cu(In,Ga)Se 2 (CIGS) based solar cell using 2DSILVACO Atlas simulator under AM1.5G illumination spectrum and standard temperature of 300K. The effects of CIGS absorber layer parameters such as CIGS absorber thickness, the doping and the defects concentrations are studied numerically to improve the performance of CIGS cell. The obtained efficiency of the conventional cell is 23.39% with optimal CIGS absorber layer thickness of about 5 μm. In order to improve the efficiency of the solar cells, a back surface field (BSF) based on hydrogenated microcrystalline silicon μc-Si:H(p + ) layer has been inserted between the back contact (Mo) and the CIGS absorber layer. The obtained results show that the efficiency of CIGS solar cells is increased by the insertion of μc-Si:H(p + ) BSF layer. The proposed cell was found to give an efficiency of 23.42% using only an optimal CIGS thickness of about 1.5 μm and 10 nm for μc-Si:H(p + ) BSF layer.