Schäfer, S.; Stange, H.; Márquez, J.A.; Genzel, C.; Mainz, R.: Stress Formation During In-Ga Interdiffusion in Thin-Film CuIn1-xGaxSe2 Absorber Layers Leads to Stable Ga Gradients. Physical Review Applied 14 (2020), p. 024063/1-17
10.1103/physrevapplied.14.024063

Abstract:
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)Se2 (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.