Processing of CdTe/CdS solar cells requires annealing of CdS and CdTr/CdS in different ambients. It ha?, been proven that the application of a CdCl2 treatment (or its variant) is important for high efficiency solar cells. This treatment influences the structural and interface properties of the layers. We have grown CdS layers either by a chemical bath deposition (CBD) or a high vacuum evaporation (HVE) on different transparent conducting oxides (TCO): tin oxide doped with fluorine (FTO) and indium tin oxide (ITO) coated glass substrates. The CdTe layers have been grown by a HVE method. Effects of the CdCl2 treatment on the recrystallization of CdTe: and CdS have been studied with X-ray diffraction and scanning electron microscopy. An increase in the grain size of CdTe from about 0.5 to 3-7 mu m, along with the loss of the preferred (111) growth orientation has been observed. The strain and recrystallization of CdTe, and intermixing of the CdTe and CdS layers strongly depend on the deposition and annealing temperatures. An optimum treatment and a minimum thickness of CBD-CdS is required for high efficiency solar cells. CdS layers and the method of their deposition also have a strong influence on the microstructure of CdTe and photovoltaic properties. Solar cells with efficiency of 11.2 and 2.5% are obtained with FIVE and CBD grown CdS window layers.