Characterization of the ZnS thin film buffer layer for Cu(In, Ga)Se2 solar cells deposited by chemical bath deposition process with different solution concentrations

ZnS buffer layer thin films for Cu(In, Ga)Se2 (CIGS) have been prepared by chemical bath deposition (CBD) technique onto the ITO coated glasses at 80 °C bath temperature, using zinc sulfate hepta-hydrate (ZnSO4·7H2O), thiourea (SC(NH2)2) and ammonia (NH4OH) as reacting chemicals. The concentrations of thiourea and ammonia were varied while the concentration of ZnSO4 was immobilized at 0.03 M. The thickness, transmittance, morphology of ZnS films were measured and the band gap was calculated according to the equation related with the absorption coefficient. Through the analysis of the parameters, we could conclude that the physical and optical properties of different ZnS thin films were influenced severely by the concentration of the two reacting chemicals and then the optimal concentration group of SC(NH2)2 and NH4OH solutions can come to 0.3 M and 1.5 M, respectively.