Diffusion of Ga and In in Sequentially Prepared Cu(In, Ga)S 2 Thin Films for Photovoltaic Applications

This contribution investigates the diffusion of In and Ga in Cu(In1-xGax)S2 thin film absorbers for photovoltaic applications. Sequentially prepared Cu-rich CuGaS2/CuInS2thin film diffusion couples have been annealed at temperatures in the range of 500°C to 600°C. Annealed samples have been examined by XRD and SNMS depth profiling. It was found that despite significant interdiffusion of Ga and In the bilayer retains its two phase structure of two layers of almost homogeneous composition, where the composition of these layers depends on the annealing temperature. Furthermore the interdiffusion process is less effective if the binary copper-sulfide phase is removed before annealing. On the basis of a tentative two dimensional diffusion model, which includes rapid diffusion along path ways of high diffusivity, the Ga-depth distribution could be modeled by numerical fits of calculated XRD-spectra to the experimental data. CuS is proposed as a possible pathway for rapide In-Ga interdiffusion.