In this work, a path to overcome the highest current efficiency on SnS thin-film solar cells by the Se incorporation is presented. We carried out a theoretical study of the effect of different Se/(S + Se) compositional ratios (CRs) (from 0.0 to 1.0) on the solar cell performance. In this sense, an improvement on power conversion efficiency (PCE) by decreasing the energy band gap (theoretical Se incorporation) from 1.35 to 1.08 eV was observed. All electrical output parameters (open-circuit voltage, short-circuit current density, fill factor and PCE) were increased by an augment of the CR from 0.25 to 0.75. A PCE of 10.23% was obtained for a CR of 0.75. Furthermore, a thickness optimization of the absorber was carried out, where the greatest PCE of 11.78% was obtained at 800 nm. On the other hand, a simulation at different work functions in back contact and different bulk defect density on the absorber were performed in order to achieve higher efficiencies.