Two-dimensional layered transition metal dichalcogenide exhibit important characteristics such as suitable bandgap, high absorption coefficient, and favourable electron transport properties for their uses in nano-electronic such as ultrathin solar cells. We adopted a hybrid simulation approach, where density functional calculations are performed for optoelectronic properties of semiconductor materials and macroscopic device simulation is carried out to evaluate photovoltaic response. We investigated electronic and optical properties of bulk WS2, WSe2 and noticed very high absorption coefficient, making them suitable absorber materials for solar cell. Further, the electronic and optical properties of an ultrathin WSSe Janus layer are investigated using density functional theory and noticed low reflectance and high bandgap, supporting its usefulness as a buffer layer for W(S/Se)2 absorbers. The computed density functional results for W(S/Se)2 and Janus WSSe are used to simulate the photovoltaic response of WSSe/W(S/Se)2 solar cell using macroscopic device simulation. The photovoltaic performance of a single junction solar cell is optimized for W(S/Se)2 absorbers and WSSe Janus buffer materials. The effect of absorber layer thickness, carrier concentration, and contact work function is evaluated to understand the solar cell performance. We noticed that interface recombination speed between absorber and buffer layer and minority carrier lifetime are affecting the solar cell performance. The maximum efficiency of about ~17.73% and 18.87% is noticed for optimized WSSe/WS2 and WSSe/WSe2 solar cell. The present study will provide a new approach to design, develop, and optimize a solar cell and evaluate the impact of different materials parameters on solar cell performance. © 2019 Elsevier B.V.