In this paper, we have simulated a copper indium gallium selenide (CIGS) thin-film solar cell using a physically based two-dimensional device simulator SILVACO Atlas. The simulation of electrical characteristics and quantum efficiency was under AM1.5 illumination and a temperature of 300 K. In this work, we changed the band gap of CuInxGa1 -xSe to optimize the efficiency of the solar cell. We obtained it by varying the absorber layer thickness with different mole fractions x that affects the efficiency of the solar cell. The simulation result shows that the maximum efficiency of 16.62 % was achieved with a band gap of 1.67 eV and a thickness of 3 µm, a short-circuit current density of 29.293 mA/cm 2 , an open-circuit voltage of 1.29 V, and a fill factor of 87.79 %. The obtained results show that the proposed design can be considered as a potential candidate for high performance photovoltaic applications.