This paper presents a numerical characterization of copper-indium-gallium-diselenide thin film solar cells using one dimensional simulation program (SCAPS-1D). We have performed an optimization of the performances of the standard Mo/Cu(In,Ga)Se2/CdS/ZnO solar cells using current-voltage and quantum efficiency methods. With a CuIn0.7Ga0.3Se2 absorber, we have investigated the buffer layer thickness, temperature, series and shunt resistances effects on the open-circuit voltage, short-circuit current density, fill factor, conversion efficiency and quantum efficiency. The simulated results show good performances when the thickness of the buffer layer is in the range of 10-40 nm due to the reduction of absorption in the short wavelenghts (380-500 nm). High performances of the model is obtained when the series and shunt resistances is in the range of 0.1-1 Ω·cm² and 1,000 Ω·cm², respectively. Under these conditions, the cell can theoretically operate under an ambiant temperature of 370 K without any loss of its performances.