Structural and Chemical Studies on CdTe/CdS Thin Film Solar Cells with Analytical Transmission Electron Microscopy.

CdTe/CdS thin £lm solar cells have been grown by closed space sublimation and high vacuum evaporation at different temperatures. After the growth, a standard CdCl2 treatment was applied for 30 min at 430◦C. The microstructure of the cells was studied by cross-sectional transmission electron microscopy. A recrystallization after the CdCl2 treatment was observed not only for CdTe but also for CdS. For all cells, diffusion of S into CdTe and Te in CdS were studied. The presence and distribution of Cl at the interface and at grain boundaries was also investigated. The lattice parameter as an indicator for a compositional change in the CdTe absorber layer after the CdCl2 treatment was determined from convergent beam electron diffraction. The results were cross-checked with X-ray diffraction and compared with results obtained for a Cd50S5Te45 polycrystal. The chemical composition at the interface of the cells was studied directly with energy dispersive X-ray spectroscopy mapping.The diffusion of S is found to depend on the extent of CdCl2 treatment. While a diffusion of Te at the interface was not observed, segregation of Cl and Te at the CdTe/CdS interface was clearly revealed. Based on these results, a formation of a Te-Cl compound is proposed to explain the catalytic role of Cl for the CdTe/CdS recrystallization