Effect of Ga introduction during the second stage of a coevaporation process of Cu(In,Ga)Se₂ layers at low temperature on polyimide substrates

A proper control of Ga concentration profile is mandatory to achieve high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. At low temperature, deep gradients, detrimental for carriers’ diffusion, are obtained when CIGS is deposited with a standard three-stage process: an optimization of the process is needed. In this study, we show the impact of a modify three-stage process on the depth of the notch by introducing Ga flux during the second stage from 0 nm/min to 1.1 nm/min. A higher open circuit voltage compensated by a lower short current density is obtained due to higher band gap energy. The surface and the bulk of the CIGS layer was analyzed at the end of the second stage by coupling different characterization techniques: glow discharge optical emission spectroscopy, Raman and X-ray photoelectrons spectroscopy. The presence of binary compounds as well as a Ga enrichment at the end of the second stage are observed when Ga is introduced during the second stage.