Towards an optimized gallium gradient for Cu(In,Ga)Se2 thin film via an improved constant low-temperature deposition process

Abstract The gallium gradient of the CIGS film plays a key role for device performance of solar cells. However, a three-stage co-evaporation deposition process at low substrate temperature often produces a pronounced steeper gallium (Ga) gradient, which is detrimental to the transport of carriers and leads to lower conversion efficiency. In this work, we propose a modified low-temperature deposition process by introducing an overlapping process between the 1st stage and the 2nd stage. This modified low-temperature process mitigates the diffusion of In and Ga and facilitates the formation of ideal back gradient. Meanwhile, we have demonstrated the phase transition process of the back gradient during the overlapping process by characterization techniques. With the design of new reaction models during the overlapping process, growth mechanisms of the CIGS film in the modified low-temperature process could be visually displayed. This modified low-temperature process has improved the device efficiency of CIGS solar cells from 14.1% to 16.7%.