Analysis of a novel method of alkalis treatment: Effect on energy band optimization and carrier recombination at the grain boundary

Alkali metal post-deposition treatment (PDT) is a key factor to approach high efficiency in Cu(In, Ga)Se2 (CIGS) solar cells. In addition to single alkali element treatment, co-doping of light and heavy alkali elements is also applied to the preparation of CIGS absorber for higher efficiency. In this work, the details of different methods of alkali-treatments are revealed by combining several microstructural and compositional analysis methods, especially for the NaF-PDT/CsF-PDT process which introduces extra Na before Cs into the absorber. The ion exchange of Na and Cs in the grain boundaries (GB) of the absorber is analyzed in detail, which reveals alkali-treatment restrains the formation of large bandgap compounds at GB. Extensive material analysis shows that NaF/CsF-PDT can not only tailor the surface nanoscale morphology of CIGS films but also optimize the band structure at GB to reduce carrier recombination. Such modification gains the improvement of the highest power conversion efficiency (PCE) from 15.2% to 17.4% and average PCE from 14.3% to 16.5% (without antireflective coating) after NaF/CsF-PDT. According to the analysis of data, we propose a model of energy band at GB, which explains the reason why NaF/CsF-PDT shows a better effect on CIGS devices than CsF-PDT. These results could serve as a basis for the further understanding of the effects of alkali-PDT on CIGS film and help to achieve even higher efficiency.