In this paper, we focus on optimizing the solar performance of a CZTSSe-based solar cell by adjusting the zinc and sulfur concentrations in the CdZnS buffer layer and the quinary absorber CZTSSe. The state-of-the-art work is to combine the ZnS and CdS binaries into CdZnS ternary used as a buffer layer in both CZTS- and CZTSSe-based solar cells. An overall study of its properties is carried out taking into account the strain present at the heterointerface, defect density, bandgap energy and the interface state density. As a result, the highest efficiency η = 14.59% was achieved with a sulfur content of 0.55 and a zinc content of 0.70 to bandgap energies of 1.25 and 3.12 eV for CZTSSe and CdZnS materials, respectively. Our simulation is validated by the reproducibility of solar cell performance under the same conditions, and an enhancement of the conversion efficiency of about Δη = 5.55% will be achieved when the CdS layer is replaced by CdZnS in the ZnO/CdS/CZTSSe/Mo/Glass solar device.