Device modelling of copper indium gallium selenide (CIGS) solar cell with tungsten di-suphide (WS2) as a window layer has been carried out in order to achieve higher conversion efficiency. Conversion efficiency for all band-gap energies of CIGS were calculated based on proposed new CIGS/WS2 structure. Numerical modelling tools were used to investigate the effects conduction band offset and interface defect state on the photovoltaic parameters of CIGS/WS2 solar cell. The model predicts the density of defect tolerance in the interface is 1 x 10(11) cm(3). Based on optimization, the highest efficiency of 26.4% has been achieved for CIGS/WS2 solar cell with Eg((CIGS)) = 1.4 eV (V-oc = 1.026 V, J(sc) = 29.57 mA/cm(2) and FF = 86.96%) which is better than that of CIGS (23.4%) solar cell. The simulation further identifies that proposed CIGS/WS2 structure is less temperature sensitive compared to conventional Si solar cell. This research paves the way for WS2 thin film as a potential window layer material for CIGS solar cells.