The main objective of this research work is to improve the efficiency of conventional baseline structured CIGS solar cells by adding a Back surface field (PbS) layer between the CIGS absorber and the Mo back contact. The possible effect of the p-PbS layer on the photovoltaic parameters of the CIGS thin-film solar cell has been investigated by SCAPS-1D. The simulation of J-V characteristics showed that conversion efficiency of the conventional solar cell (Al/ITO/Al-ZnO/i-ZnO/CIGS/Mo/Substrate) is 22.67% (3 mu m CIGS layer) and an improved conversion efficiency of 24.22% has been gained after adding the BSF (PbS) layer. The PbS layer between CIGS and Mo back contact provides an additional hole tunneling action that form a quasi ohmic contact from a schottkey type near the back region and this quasi ohmic contact improves the efficiency of carrier collection as well as BSF action reduces the back surface recombination. Besides thickness, energy band gap, electron affinity of the PbS layer were varied and optimized. Again, operating temperature on the overall performance of the proposed solar cell was also investigated. These findings are very promising for future performance and cost effective solar cell.