This study focuses on optimizing the influence of a zinc selenide (ZnSe) buffer layer on the performance of a CIGS thin-film solar cell, with the aim of proposing an alternative to CdS, a commonly used material that contains toxic cadmium. The choice of ZnSe is motivated by its advantageous optoelectronic properties, including a direct optical band gap of about 2.7 eV, high transparency in the visible and near-infrared ranges, an absorption coefficient on the order of 104 cm⁻1, and an electron affinity close to 4.1 eV. In addition, ZnSe can crystallize in zinc-blende or wurtzite phases, with possible structural transitions that may promote a type-II band alignment suitable for the ZnSe/CIGS interface. The main objective of this work is to analyze the effect of the optical band gap and the electron affinity of ZnSe on the key photovoltaic parameters of the cell, namely the short-circuit current density, the open-circuit voltage, the fill factor, and the conversion efficiency. The study is carried out through numerical simulation by solving the fundamental carrier transport equations. The results show that the optical band gap of ZnSe has a limited impact on the overall performance, with an optimal efficiency of about 23.16% for values between 2.3 and 2.6 eV. In contrast, the electron affinity appears to be a critical parameter: a range between 4.0 and 4.6 eV promotes optimal band alignment and enables good photovoltaic performance, with efficiencies ranging from 19.38% to 24.93%. These findings confirm the potential of ZnSe as an alternative cadmium-free buffer layer to CdS, opening promising prospects for the development of more environmentally and health-friendly CIGS solar cells.