Study of Cu(In,Ga)Se₂ Thin Film Growth at Low Temperature on Polyimide Substrate in a Multistage Coevaporation Process for Photovoltaic Applications

A proper control of Ga composition gradient is mandatory to achieve high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. Steeper gradients are usually obtained when CIGS is deposited at low temperatures (<450 °C) on polymer substrates such as polyimide with a three-stage process. As a steep gallium gradient is considered detrimental to carrier transport and reduces solar cells efficiency, a modification of the deposition process has to be implemented. In this study, we analyze the CIGS growth and properties by coupling XRD, Raman spectroscopy, and GD-OES measurements at different stages of the deposition process for a classic three-stage process and a modified process so-called multistage process. To do so, several samples were stopped at different moments of the second stage of a coevaporation process. A better crystallized and Ga-richer material is obtained with the modified deposition process. This raises the advantages of low temperature deposition processes of CIGS as compared to classical high temperature ones.