Influence of Stress Formation on Ga – In Interdiffusion in Thin Film CuIn(1-x)Ga(x)Se2 Absorber Layers
Stefan Schaefer, Helena Stange, Jose Marquez Prieto, Manuela Klaus, Christoph Genzel, Roland Mainz
Helmholtz-Zentrum Berlin fuer Materialien und Energie GmbH, Berlin, Germany

Abstract — One of the biggest efficiency-limiting factors in the sequential production process of thin-film Cu(In,Ga)Se2 absorber layers is the formation of a steep gallium gradient towards the back of the film. We propose in this contribution that the formation of stress inside thin-film Cu(In,Ga)Se2 absorber layers due to the inter-diffusion of indium and gallium can explain the persistence of a stable gallium gradient inside those materials. To verify our statement we developed a model using the thermodynamic theory of elastically stressed crystals and performed numerical calculations of the inter-diffusion process and subsequent stress formation. It is shown that the inter-diffusion will ultimately come to a halt before total intermixing of gallium and indium occurs, if the energy released by the diffusion process does not suffice to perform the elastic work required to deform the lattice and if no additional energy is released, e.g. by mechanisms of stress reduction through plastic deformation. We compare the results from numerical experiments to data resulting from in-situ energy-dispersive X-ray diffraction measurements at the BESSY II synchrotron facility in Berlin and show that a good qualitative agreement between measured and simulated data can be achieved using the gallium mobility inside the material as adjustable parameter.