The S-Te inter-diffusion in CdTe/CdS heterojunction has been studied by X-ray diffraction, Atomic force microscope and Spectroscopic ellipsometry. For the first time, the phase segregation of CdTeySx compound that is confirmed by X-ray measurement has been taken into account in the analysis of ellipsometry, which promotes the analytical reliability. Benefit from the dielectric constants obtained from the ellipsometric data, the external quantum efficiency of the solar cell has been fitted without any paradoxical assumption used before. Results show that high temperature process such as Close space sublimation leads to significant S-Te inter-diffusion and shallows the junction obviously, but this does not happen in magnetron sputtered device. Tellurium diffusion into CdS can reduce the blue response of solar cell, meanwhile sulfur diffusion into CdTe lowers the band gap of CdTe which increase the quantum efficiency at long wavelength. Finally, the conversion efficiency of solar cell can be improved by suppressing the current leakage near the edge of cell. The fill factor is 54.8% and the efficiency reaches 9.28%. (C) 2016 Elsevier Ltd. All rights reserved.