S–Te Interdiffusion within Grains and Grain Boundaries in CdTe Solar Cells

At the CdTe/CdS interface, a significant Te-S interdiffusion has been found a few nanometers into the CdTe grain interiors with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy. This interdiffusion happens on both as-grown and CdCl2-treated CdTe. S substitution at Te sites has been directly resolved in CdTe with STEM Z-contrast images, which further confirms the S diffusion into CdTe grain interiors. Moreover, when a sufficient amount of S substitutes for Te, a structural transformation from zinc-blende to wurtzite has been observed. In the CdCl2-treated CdTe, Cl segregation has also been found at the interface. STEM electron-beam-induced current shows that the p-n junction occurs a few namometers into the CdTe grains, which is consistent with the S diffusion range we observe. The shift of the p-n junction suggests a buried homojunction which would help reduce nonradiative recombination at the junction. Meanwhile, long-range S diffusion in CdTe grain boundaries (GBs) has been detected, as has Te and Cl diffusion in CdS GBs.