Na distribution in CIGS solar cells grown with modified three-stage processes

Highest efficiency CIGS solar cells are generally grown with a three-stage co-evaporation process where the absorber layer is in a copper-rich regime for a period of time at the end of the second stage. We investigated the influence of changing the maximum [Cu]/[In+Ga] ratio at the end of stage 2 on the distribution of sodium throughout the absorber layer when sodium is supplied by diffusion from the soda-lime glass substrate. Secondary ion mass spectrometry (SIMS) was used for depth profiling of the Na content, the surface concentration of Na was determined by wavelength dispersive X-ray analysis (WDX) from top view scanning electron micrographs. Raman investigation of the phase composition of the surface and SIMS compositional depth profiles of the investigated absorber layers suggested the possibility of the formation of a Na-rich compound on the absorber layer surface for CIGS grown with low Cu excess while absorbers grown with high excess showed a more evenly distributed Na depth profile. New WDX results further support these claims as a surface [Na]/[Cu+Na] ratio of up to 0.2 for layers grown with low Cu excess was measured while the Na surface values of absorbers grown with high Cu excess are below the detection limit.