Diffusion of Rb in polycrystalline Cu(In, Ga)Se-2 layers and effect of Rb on solar cell parameters of Cu(In, Ga)Se-2 thin-film solar cells

The diffusion of the heavy alkali element rubidium (Rb) in Cu(In, Ga)Se-2 (CIGS) layers was investigated over a temperature range from 148 degrees C to 311 degrees C by outdiffusion from a rubidium fluoride layer. The diffusion profiles were measured by secondary ion mass spectrometry. By using CIGS layers with different grain sizes, diffusion along grain boundaries could be distinguished from diffusion into the grain interior. Rb was found to diffuse from the CIGS surface along grain boundaries but also within the grain bulk. Based on these data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation D-V (Rb) = 3.8.10(-8) exp(-0.44 eV/k(B)T) cm(2) s(-1) and the fast diffusion along the grain boundaries by D-GB (Rb) = 5.7.10(-9) exp(-0.29 eV/k(B)T) cm(2) s(-1). Further, the effect of Na on Rb diffusion was investigated by comparing Rb diffusion into a Na-containing CIGS layer in contrast to Rb diffusion into an alkali-free CIGS layer. This comparison revealed some aspects of the ion exchange mechanism. Finally, the effect of Rb on the solar cell parameters of CIGS thin-film solar cells was investigated. Rb was found to enhance the open-circuit voltage, the fill factor, and charge carrier density in a similar manner as observed for potassium and sodium. (C) 2018 Author(s).