One of the major challenges in increasing the efficiency of the CZTSSe solar cells is to control the lattice defects formation and secondary phases in the absorber layer of the cells. Moreover, by controlling and decreasing lattice defects and thus improving the efficiency, larger-scale applications would be financially acceptable. In this paper, several CZTSSe thin-film solar cells have been prepared and one device with champion efficiency of 10.33% has been chosen for the modeling. Afterward, numerical simulations based on the Finite Element Method (FEM) and Finite Difference (FD) were conducted on these cells. The effect of defects density and defect types, which are located at different energy levels in the absorber layer bandgap, was evaluated. The results indicated that by decreasing the defects which are located close to the electron Fermi level and the middle of the band gap (E-g/2), the maximum efficiency of 18.47% for these solar cells can be achieved.