Ellipsometric investigation of S-Te inter-diffusion and its effect on quantum efficiency of CdS/CdTe thin films solar cell

The S-Te inter-diffusion at CdTe/CdS heterojunction within ITO/CdS /CdTe multilayer is studied by Spectroscopic ellipsometry. Analysis shows that consumption of CdS and S-Te inter-diffusion principally occurs during high temperature CdTe deposition rather than post CdCl2 annealing. High temperature CdTe deposition results in the 183 nm intermixed layer CdSxTe1−x, while the low temperature one exhibits only 83 nm, and the post annealing only increases the intermixed layer further by 69 nm at most. Moreover, the external quantum efficiency of the solar cell fabricated with the structure of glass/ITO/CdS/CdTe/CuxTe/Cu has been simulated considering the “bowing” effect on the band gap of intermixed alloy. Results indicate that the S-Te inter-diffusion can change the shape of EQE both in red and blue absorption edges obviously. It is found that the depth of depletion region affects the carrier collection in short wavelength region, whereas its width plays a primary role in long wavelength region. The best device in this study has the maximum photocurrent and the largest conversion efficiency with the value of 22.003 mA/cm2 and 7.8%, respectively.