The aim of this work is to analyze the influence of the interfacial MoSe$_2$ layer on the performance of a /n-ZnO/i-ZnO/n-Zn(O,S)/p-CIGS/p⁺-MoSe₂/Mo/SLG solar cell. In this investigation, the numerical simulation software AFORS-HET is used to calculate the electrical characteristics of the cell with and without this MoSe$_2$ layer. Different reported experimental works have highlighted the presence of a thin-film MoSe$_2$ layer at the CIGS/Mo contact interface. Under a tunneling effect, this MoSe$_2$ layer transforms the Schottky CIGS/Mo contact nature into a quasi-ohmic one. Owing to a heavily p-doping, the MoSe$_2$ thin layer allows better transport of majority carrier, tunneling them from CIGS to Mo. Moreover, the bandgap of MoSe$_2$ is wider than that of the CIGS absorbing layer, such that an electric field is generated close to the back surface. The presence of this electric field reduces carrier recombination at the interface. Under these conditions, we examined the performance of the cell with and without MoSe$_2$ layer. When the thickness of the CIGS absorber is in the range from 3.5 μm down to 1.5 μm, the efficiency of the cell with a MoSe$_2$ interfacial layer remains almost constant, about 24.6%, while that of the MoSe$_2$-free solar cell decreases from 24.6% to 23.4%. Besides, a Schottky barrier height larger than 0.45 eV severely affects the fill factor and open circuit voltage of the solar cell with MoSe2 interface layer compared to the MoSe$_2$-free solar cell.