Efficiency Improvement of Narrow Band Gap Cu(In,Ga)(S,Se) 2 Solar Cell with Alkali Treatment via Aqueous Spray Pyrolysis Deposition.

The chalcopyrite Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cell with a low band gap is a promising candidate for use as the bottom cell in high-efficiency tandem solar cells. In this study, we investigated narrow band gap CIGSSe solar cells, both with and without alkali treatment. The CIGSSe absorbers were fabricated using aqueous spray pyrolysis in an air environment, with the precursor solution prepared by dissolving constituent metal salts. We found that the power conversion efficiency (PCE) of the fabricated solar cell was significantly enhanced when rubidium postdeposition treatment (PDT) was applied to the CIGSSe absorber. The Rb-PDT facilitates defect passivation and a downshift of the valence band maximum of the CIGSSe absorber, thereby improving the power conversion efficiency and all device parameters. Due to these beneficial effects, a PCE of ∼15% was obtained with an energy band gap of less than 1.1 eV, making it suitable for use as the bottom cell in a highly efficient tandem solar cell.