In this paper, based on numerical simulation, the SCAPS-1D software was used to simultaneously study the influence of the surface defect layer (SDL) and the molybdenum diselenide layer (MoSe2) on the performance of the CIGS solar cell. These two defects layers are respectively formed at the front interface (CdS/CIGS) and the back interface (CIGS/ Mo) by atomic inter-diffusion. Simultaneous analysis of SDL and MoSe2 layers thickness revealed that optimum performance is achieved for 10 nm thickness of SDL layer and 35 nm thickness of MoSe2. A study of the gap energy revealed that optimum performance is obtained for 1.25 eV energy gap SDL layer and 1.35 eV energy gap of MoSe2 layer. Next, a performance study of the ultra-thin CIGS solar cell with optimized SDL and MoSe2 layers was carried out. It was found that the presence of the SDL and MoSe2 layers optimized improves the electrical performance of the ultra-thin CIGS solar cell.