Stress Formation During In-Ga Interdiffusion in Thin-Film CuIn1-xGaxSe2 Absorber Layers Leads to Stable Ga Gradients

To optimize the opto-electronic properties of compound semiconductors, a detailed understanding and control of compositional gradients forming during their synthesis is crucial. A common fabrication process for Cu(In, Ga)Se-2 (CIGS) thin-film solar cells uses annealing at high temperatures, which-contrary to what could be expected from simple Fickian diffusion-results in the formation of steep and stable Ga gradients, deviating from the optimal Ga profile for high-efficiency CIGS absorbers. Here, we show that the formation of elastic stresses inside the material during the interdiffusion can have a profound effect on the final Ga distribution, resulting in persistent Ga gradients inside CIGS absorber layers. A comparison of numerical diffusion and stress-formation calculations with real-time synchroton-based energy-dispersive x-ray diffraction data acquired in-situ during selenization of CIGS thin films demonstrates that the model can reproduce the stagnation of In-Ga interdiffusion. We discuss that a detailed understanding of the interplay between stress and diffusion processes in thin films may open alternative fabrication strategies for generating desired stable compositional gradients to improve the opto-electronic properties of compound semiconductors, such as chalcopyrite, kesterite, and perovskite solar cell absorbers for solar cells.Y