In the past years, record efficiency of copper-indium-gallium-diselenide (CIGS) based solar cells has improved reaching 22.6%. This result shows that CIGS absorbent is idealistic for thin-film solar cells. The most attractive feature in CIGS is the tunable bandgap of the absorber layer that varies from 1.06 eV to 1.7 eV depending on the gallium fraction. This feature leads to best match the solar spectrum. In the present work, the influence of the bandgap of the absorber is investigated using solar cell capacitance simulator (SCAPS). An optimum bandgap of 1.39 eV results in a maximum efficiency of 24.288%. In order to get a Cd-free CIGS based thin film solar cell the CdS buffer layer is replaced by In2S3. The results show that In₂S₃ is a proper alternative that does not degrade th ³ e cell performance.