We present a copper indium gallium selenide (CIGS) solar cell with improved performance, numerically simulated using LUMERICAL FDTD and DEVICE Multiphysics simulation software and considering the charge transportation, ambient temperature, buffer layer thickness, and defect density for detailed analysis. One of the significant concerns of the existing CIGS solar cells is the presence of the toxic cadmium sulfide (CdS) buffer layer and higher buffer/absorber layer interface dominant recombination. Replacing CdS with zinc sulfide (ZnS) is a propitious way to address such issues and enhance the CIGS solar cell’s performance while achieving efficiencies similar to that of the CIGS solar cell with CdS as the buffer layer. Furthermore, the performance is improved by employing a transparent conductive oxide of fluorine-doped tin oxide (FTO) and zinc tin oxide (ZTO) window layer for better charge flow and low-current losses. However, the real solar cell operating conditions are entirely different from the predefined standard conditions. To get a more detailed analysis, we demonstrate the impact of variations of operating temperature, ZnS buffer layer thickness, defect density on the short-circuit current, open-circuit voltage, and overall performance of the solar cell. We found that the simulated efficiency of the proposed CIGS solar cell (FTO–ZTO/ZnS/CIGS/Mo) with ZnS and FTO–ZTO layer is 29.6% with a short-circuit current of 49.3  mA  /  cm², higher than that of the CIGS solar cell (Al:ZnO/CdS/CIGS/Mo) with CdS that was achieved in our previous work. Thus the study can help to develop a more promising, efficient, and cost-effective CIGS solar cell.