Copper-induced recrystallization and interdiffusion of CdTe/ZnTe thin films

Abstract

ZnTe is commonly employed as a buffer layer between CdTe and the metallization layer at the back contact of state-of-the-art CdTe solar cells. Here, the critical role of Cu in catalyzing recrystallization and interdiffusion between CdTe and ZnTe layers during back contact activation is presented. Several CdTe/ZnTe:Cu thin-film samples were prepared with varying levels of copper loading and annealed as a function of temperature and time. The samples were characterized by x-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray spectroscopy. The results show that stress is present in the as-deposited bilayers and that negligible interdiffusion occurs in the absence of Cu. The presence of Cu facilitates rapid interdiffusion, predominantly via Cd migration into the ZnTe phase. Zn migration into CdTe is limited to areas around defects and grain boundaries. Ternary CdxZn1-xTe interlayers are formed, and the extent of alloy formation ranges from 0.08 < x < 0.5 throughout the whole ZnTe layer. The level of Cu loading controls the composition of the CuxTe clusters observed, while their size and migration is a function of annealing conditions.