Impact of Na on structural properties and interdiffusion of CuInSe/sub 2/ and CuGaSe/sub 2/ thin films

We have grown CuInSe/sub 2/ (CIS) and CuGaSe/sub 2/ (CGS) thin films with the 3-stage process in the presence and absence of Na and observed a Na-induced decrease of grain size for CGS, but not for CIS. In addition, sodium was found to hinder the interdiffusion of In and Ga in Cu(In,Ga)Se/sub 2/ (ClGS) layers, which has consequences for the achievement of band-gap grading in high-efficiency absorbers. Two different mechanisms which can explain both Na effects are discussed. A post-deposition Na incorporation method was used to process solar cells with high efficiency (14.4%) at low substrate temperature (450/spl deg/C). The strong enhancement of device performance compared with Na- free cells is attributed to passivation of defects at grain boundaries. The result is promising for the development of flexible solar cells.