In this study, CIGS ultrathin films of 52 nm, 89 nm, 183 nm and 244 nm thicknesses were grown on a n-Si substrate using PLD technique, then Ag/CIGS/Si/Al hetero-junction solar cells were formed by producing front finger and back contact. While the thickness of CIGS ultrathin film is increased, their grain sizes are also increased, crystal structures are developed and more light is absorbed within the thin film. The straight characteristics of hetero-junctions have been improved while the thicknesses of CIGS ultrathin film is decreased according to J-V characteristics of the hetero-junctions in the darkness. In addition, depending on CIGS ultrathin film thickness, the photovoltaic behavior of CIGS/Si hetero junction solar cells has been studied and interpreted in detail in this article. It can be concluded that CIGS/Si hetero-junction solar cell device produced based on CIGS ultrathin film of 183 nm thickness shows the highest short circuit current density and power conversion efficiency values among other thicknesses, with respect to J-V curve under the illumination (AM 1.5 solar radiation in 80 mW/cm2). Using SCAPS 1-D program, we have been able to successfully simulate CIGS/Si hetero-junction of 183 nm thickness.