A simple non-toxic simultaneous selenization/sulfurization process for Cu(In, Ga)(S, Se)2 solar cells

Highlights: • A non-toxic one-stage selenization/sulfurization process to produce CIGSSe solar cells. • Sulfur content of the surface absorber layer is higher than inside when S/(S + Se) ratio≧0.12. • Maximum cell efficiency of 12.8% was achieved when the S/(S + Se) ratio was 0.12. • Cell efficiency is close to two-stage selenization/sulfurization process based on toxic H₂Se/H₂S gases. This paper proposes a non-toxic simultaneous selenization/sulfurization process to produce Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells without the need for the conventional two-stage process using toxic H₂Se/H₂S gases. The absorber layer is applied via sputtering with Cu-In-Ga ternary targets, whereupon non-toxic selenium and sulfur vapor are introduced simultaneously to produce chalcopyrite CIGSSe film. Experiment results demonstrate that the total sulfur content of the absorber layer increases with an increase in the S/(S + Se) ratio. When the S/(S + Se) ratio ≧ 0.12, the sulfur content of the surface absorber layer is higher than inside the sample. This contributes to an increase in the surface energy gap and Voc of the solar cell. However, an excessively high S/(S + Se) ratio can undermine the crystallinity of the CIGSSe while introducing phase segregation at the surface of the absorber layer as well as ordered vacancy compounds (OVCs) and secondary phases, which can decrease cell efficiency. A suitable proportion of sulfur in the absorber layer can inhibit excessive growth of the Mo(S,Se)₂ layer and thereby enhance cell efficiency. A small number of CuGaSe₂ grains were observed at the bottom of the CIGSSe absorber layer in specimens with various S/(S + Se) ratios. A maximum cell efficiency of 12.8% was achieved when the S/(S + Se) ratio was 0.12. This cell efficiency is close to the reference specimen fabricated using a two-stage selenization/sulfurization process based on toxic H₂Se/H₂S gases.