Thin films of CdTe and CdTe/CdS and SnO2 used for heterojunction solar cells were deposited on glass substrates. The effects resulting from the processing with thermal heat and CdCl2 treatments are investigated. The optical properties are determined by photoluminescence (PL) and transmission spectra. The compositional changes within the CdTe film structures are studied by 2 MeV He-4(+) beam using the Rutherford backscattering (RES) technique. The optical and the RES data are then correlated to the evolution of high-efficiency solar cells.