Influence of substrate temperature and activation treatment on CdTe solar cells

For complete layer stack deposition of CdTe solar cell the coating technology on pilot scale has been established. The pilot line covers all relevant process steps such as front contact sputtering, deposition of the semiconductor layers, chemical activation, etching and back contact deposition. Substrates sizes of 10 cm x 10 cm can be processed, currently. In our present work the growth of CdTe thin films by close spaced sublimation (CSS) was systematically investigated. CdTe and CdS thin films were deposited onto FTO (fluorine-doped tin oxide) substrates with varying substrate temperature (300…500°C) and deposition rates (2,5 ?m/min and 5 ?m/ min). The microstructure of the films were examined before and after activation treatment with a high-resolution field emission scanning electron microscope (FE-SEM) on ion beam prepared cross sections. It was observed that the microstructure of CdTe layers strongly depends on substrate temperature, whereas grain growth of CdS layers is influenced mainly by the subsequent activation treatment. From this investigation it is possible to define two temperature regions in which compact CdTe layers with low porosity can be deposited. In contrast, little influence of the chamber pressure on the grain size could be observed in the range examined