Abstract Cd 1− x Zn x S films suitable for application in photovoltaic devices have been produced by annealing CdS films in vaporous zinc chloride and characterized by optical absorption, X-ray diffraction, energy dispersive X-ray spectroscopy and scanning electron microscopy. The incorporation of Zn in CdS was found to be dependent on the annealing temperature and duration, reaching a concentration as high as 50% ( x=0.5) and producing a blue shift of 0.3 eV in bandgap energy. CdTe solar cells fabricated with Cd 1− x Zn x S showed substantially higher short-circuit current than the cells with CdS as the window layer. The optimal concentration of Zn was found to be around 5%. High efficiencies were obtained in SnO 2/Cd 1− x Zn x S/CdTe cells.