CdTe/CdS thin film solar cells have been grown by high-vacuum evaporation (HVE) and close-space sublimation. To understand the role of Cl on the microstructure and composition of the CdTe/CdS layers and interfaces, the cells were annealed for different durations after different amounts of CdCl2 had been deposited. Transmission electron microscopy shows a loss of orientational relationship between CdTe and US after annealing under the influence of Cl. The interdiffusion of S and Te across the interface is measured quantitatively and segregation of Cl, Te and O at the CdTe/CdS interface is detected by energy-dispersive X-ray mapping in the electron microscope. The results show a strong correlation with the diffusion of Cl along the CdTe grain boundaries, which is directly proved for the first time. It is suggested that recrystallization of CdTe grains starts from the CdTe surface and proceeds towards the interface. Cells grown by HVE show all the features expected from the predominance of recrystallization. (C) 2003 Elsevier Science B.V. All rights reserved.