Tin monosulfide (SnS) is a promising candidate for thin film solar cell technology. Despite its excellent properties, the overall efficiency of SnS based solar cell remains limited. The main aim of this work is to propose a highly efficient and environment friendly SnS based solar cell. We have developed an heterostructure defined as P+ SnS/SnS/ZnS2/ITO. The impact of the different parameters and physical properties of the absorber layer and the back surface layer such as thickness, doping density and gap energy were investigated using Scaps-1D software. In order to improve the efficiency of our structure, computational analysis based on DFT was performed to substitute Germanium (Ge) into SnS structure with different percentages 25%, 50% and 75% aiming to identify the specific band gap which yields the highest possible efficiency in our solar cell. The finding results demonstrate that Sn0.5Ge0.5S with a band gap of 1.123 eV yields a high efficiency of 30.80%, a Voc of 0.88V, a Jsc of 42.86 mA/cm2, and FF of 80.94%.