Transition metal dichalcogenides based materials exhibit promising optoelectronic properties for solar cell application. We considered the heterostructure single junction solar cell with bulk W(S/Se)2 as an absorber material and ultrathin Janus WSeTe layer as a buffer layer. The optoelectronic properties of WSeTe monolayer are calculated using density functional theory calculations. The high absorption coefficient of WS2/WSe2 makes them suitable absorber materials. The performance of layered single junction heterostructure is simulated using SCAPS-1D simulator. The device performance is evaluated to understand the effect of an absorber layer thickness, carrier concentration, defect density, work function, buffer layer carrier concentration, and interface defect density. After optimization of all the possible parameters, the maximum efficiencies noted are 18.87% and 18.1% for AZO/WSeTe/WS2 and AZO/WSeTe/WSe2 solar cells, respectively. © 2021 American Chemical Society.