X-ray vision of Cu(In,Ga)Se2: from the Ga/In ratio to solar-cell performance

Abstract

Abstract Cost efficiency and defect passivation are the two major challenges that thin-film solar cells have to overcome for economic competitiveness. For Cu(In,Ga)Se 2 solar cells, the first is addressed by an increase of the Ga/In ratio, which widens the bandgap favorably for tandem applications and reduces the requirement of costly, rare In. The second is addressed by heavy alkali post-deposition treatments. However, the maximum device efficiency is typically achieved with a comparably low Ga/In ratio, which is in contrast to the economic interest of a higher Ga/In ratio and makes it paramount to identify, understand and mitigate the sources of local underperformance in Ga-rich cells. In this work, we investigate a series of Cu(In,Ga)Se 2 cells with varying Ga/In concentration in the absorber, using multi-modal scanning x-ray microscopy. In particular, we analyze differences in chemical composition and electrical performance on the nanoscale, with a focus on the effect of Rb. We find that In-rich cells show, along with a greater overall performance, a more homogeneous distribution of the nanoscale performance compared to the Ga-rich cells. Our analysis on Rb suggests that this effect is due to a more effective passivation of structural defects in the absorbers, i.e. voids and grain boundaries. These results shine light on the causes of the superiority of Ga-poor/In-rich absorbers and substantiate the trend to higher defect density for Ga-rich absorbers.