In this work, we investigate the performance of CIGS-based thin-film solar cells employing the SCAPS-1D simulation package. The paper is mainly devoted to the development of the ZrS_xSe_2−x (where 0 ≤ x ≤ 2) transition metal dichalcogenide (TMDC) as a Cd-free, nontoxic, and abundant buffer layer, the first of its kind. In the first step, we have evaluated the impact of the p-MoSe₂ interfacial layer between the GIGS absorber and Mo back contact. The J–V characteristic showed a higher slope, revealing that the p-MoSe₂ layer at the CIGS/Mo interfaces beneficially on the CIGS/Mo hetero-contact, mediating the quasi-ohmic contact rather than the Schottky type. For the optimized solar cell using the ZrS_xSe_2−x as a buffer layer, the photovoltaic parameters, such as the short-circuit current density, open-circuit voltage, Fill Factor, and efficiency, were investigated versus the thickness, carrier concentration, and bandgap values. The results reveal an optimum efficiency of ~ 25.5% at a bandgap of 1.3 eV, corresponding to ZrS_0.8Se_1.2 (i.e., x = 0.8) and 180 nm thicknesses, at a high carrier concentration of 1 × 10¹⁸ cm⁻³. Furthermore, the solar cell performance is assessed with the increment of the operating temperature from 275 to 475 K. The observed decrease in the V_oc is ascribed to the rise in the reverse saturation current associated with the higher temperatures. The study concludes an excellent potential for fabricating high-performance CIGS thin solar cells using a Cd-free nontoxic buffer layer.