Study of Gallium Front Grading at Low Deposition Temperature on Polyimide Substrates and Impacts on the Solar Cell Properties

Cu(In, Ga)Se-2 (CIGS) solar cells have achieved the highest efficiencies among thin-film solar technologies. At low temperatures (<450 degrees C) compatible with polyimide, lower diffusion rates of copper, indium, and gallium are observed. This implies a strong composition gradient when a standard three-stage process is used. In this paper, we investigate the impact of Ga content at the interface between the CIGS and the buffer layer where we manage to create a steep front Ga grading. This gradient has a strong impact on the device performances leading to a significant increase of V-oc and FF with high Ga content at the interface. We show that the insertion of a Ga-rich layer favors the formation of ordered vacancy compound phases, and the absence of Ga during the last stage of the process impacts the overall element diffusion. Optimized Ga gradient leads to 17.8% efficiency solar cells without antireflecting coating or KF postdeposition treatment.