In this paper, the performance of CIS Based solar cells was investigated, using a simulation program named SCAPS-1D Software (Solar Cells Capacitance Simulator). CIS cell structure is based on Cu(In, Ga), (Se,S)2; which is a semiconductor compound as an absorber layer; un-doped Zinc Oxyd (i) ZnO as a window layer, and Sulfide Cadmium CdS as a Buffer layer, with an efficiency of ƞ=15%. We studied the influence of different layers’ thickness and their defect densities, working temperature and absorber carrier density on the CIS based solar cells. The photovoltaic parameters have been calculated, and we have obtained the optimal values of every constraint cited above. As a result, we obtained a new high efficiency value of 19.71%, under air mass (AM) 1.5 and 100 mW/cm2 illumination and the area of this device was 0.15 cm2 .