In-situ study of CIGS dielectric function as a function of copper content

High efficiency CuIn_1-xGa_xSe₂ (CIGS) thin film solar cells are produced by two-stage or three-stage co-evaporation processes. Both of these methods involve a Cu-rich to Cu-poor transition. One way to control the Cu-rich to Cu-poor transition is the so-called end point detection (EPD), which monitors the change of emissivity of the film as it changes from Cu-rich to Cu-poor. This method, however, depends on the thermal response of the substrate and the location of the corresponding thermocouple, which can lead to delays in observing the phase transitions. Presented here is a novel, nondestructive way to detect the presence of such phase changes from Cu-rich to Cu-poor or vice-versa. In this study we have relied on the high sensitivity of real time spectroscopic ellipsometry (RTSE) to simultaneously measure multiple structural and electronic properties of the absorber layer by extracting dielectric functions throughout the deposition.