ZnS0.5Se0.5 has been proposed as a novel buffer layer, alternative to CdS for solar cell application. ZnS0.5Se0.5 layers were deposited at 300 °C using close-spaced evaporation. The layers exhibited cubic structure with a sharp (111) reflection. The films showed an optical transmittance > 80% with an energy band gap of 3.01 eV and the electrical resistivity of the films was ~ 105 ?cm. CuInS2 (CIS) films synthesized by sulphurisation process of rf sputtered Cu/In precursor layers formed on Mo coated glass were used as absorber. Structural, optical and electrical studies were performed to characterize the synthesized CuInS2 films. Thin film solar cells were fabricated by depositing a thin ZnS0.5Se0.5 buffer layer onto the Glass/Mo/CuInS2 with sprayed ZnO:Ga as a window layer. The interfacial properties of the resulting heterojunction were studied using current–voltage (I–V), capacitance–voltage (C–V) and spectral response measurements and the results were discussed.