Germanium monoselenide (GeSe) has been considered to be a promising photovoltaic absorber material due to its excellent photoelectric properties such as suitable band gap, high absorption coefficient and high carrier mobility. In this paper, the thin film solar cells with the proposed structure of metal grid/AZO/i-ZnO/CdS/GeSe/Mo/glass were simulated. The solar cells output performance parameters were investigated and evaluated in response to changes in materials properties of functional layers (such as thickness, carrier concentration and bulk defect density). After optimizing the thickness and doping concentration of CdS buffer layer and GeSe absorber layer, respectively, the solar cells show a conversion efficiency of 27.59%. Finally, the effect of bulk defect density in the absorber layer on the device performance was simulated. These results show that GeSe based thin film solar cells have the potential to become a high efficiency photovoltaic device.