ZnS was chemically deposited as a buffer layer alternative to CdS, for use as a Cd-free buffer layer in Cu(In1-xGax)Se2 (CIGS) solar cells. The deposition of a thin film of ZnS was carried out by chemical bath deposition, following which the structural and optical properties of the ZnS layer were studied. For the experiments, zinc sulfate hepta-hydrate (ZnSO4·7H2O), thiourea (SC(NH2)2), and ammonia (NH4OH) were used as the reacting agents. The mole concentrations of ZnSO4 and SC(NH2)2 were fixed at 0.03 M and 0.8 M, respectively, while that of ammonia, which acts as a complexing agent, was varied from 0.3 M to 3.5 M. By varying the mole concentration of ammonia, optimal values for parameters like optical transmission, deposition rate, and surface morphology were determined. For the fixed mole concentrations of 0.03 M ZnSO4·7H2O and 0.8 M SC(NH2)2, it was established that 3.0 M of ammonia could provide optimal values of the deposition rate (5.5 nm/min), average optical transmittance (81%), and energy band gap (3.81 eV), rendering the chemically deposited ZnS suitable for use as a Cd-free buffer layer in CIGS solar cells.
