Studies of recrystallization of CdTe thin films after CdCl2 treatment [solar cells]

CdTe thin film solar cells deposited by physical vapor deposition (PVD) and close-spaced sublimation (CSS) have been treated with CdCl₂ at 350° and 400°C. Atomic force microscopy (AFM) analysis showed that the films started recrystallizing during the 350°C CdCl₂ treatment. These results were confirmed by the presence of two lattice parameters, detected using X-ray diffraction (XRD) analysis. The PVD films treated at 400°C were completely recrystallized and grain growth was observed. The formation of Cd(S_1-xTe_x) alloy in these films was evidenced by the appearance of extra peaks close to the CdTe peaks in the diffraction patterns. No major changes were observed in the structural properties of CSS CdTe films treated at the same conditions. It was concluded that the effect of the CdCl₂ treatment in the CdTe films is to promote recrystallization and grain growth, but only if enough lattice-strain energy is available (as is the case for PVD films). Time-resolved photoluminescence (TRPL) analysis showed, for PVD and CSS films, an increase in minority-carrier lifetime with the treatment, mainly at 400°C, probably due to elimination of deep levels within the band gap