Ultra-thin CdTe photovoltaic solar cells with an absorber thickness of 0.5 μm were produced by metal organic chemical vapour deposition onto indium tin oxide coated boroaluminosilicate glass. A wide band gap Cd1−xZnxS alloy window layer was employed to improve spectral response in the blue region of the solar spectrum. X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy were used to monitor changes in the chemical composition and microstructure of the Cd1−xZnxS/CdTe solar cell after varying the post-deposition CdCl2 activation treatment time and annealing temperature. The CdCl2 treatment leached Zn from the Cd1−xZnxS layer causing a redshift in the spectral response onset of window absorption. S diffusion occurred across the Cd1−xZnxS/CdTe interface, which was more pronounced as the CdCl2 treatment was increased. A CdTe1−ySy alloy was formed at the interface, which thickened with CdCl2 treatment time. Small concentrations of S (up to 2 at.%) were observed throughout the CdTe layer as the degree of CdCl2 treatment was increased. Greater S diffusion across the Cd1−xZnxS/CdTe interface caused the device open-circuit voltage (Voc) to increase. The higher Voc is attributed to enhanced strain relaxation and associated reduction of defects in the interface region as well as the increase in CdTe grain size.