Low temperature incorporation of selenium in Cu2ZnSnS4: Diffusion and nucleation

Highlights: • Three sulfide Cu₂ZnSnS₄ samples with different average grain sizes are fabricated. • Se is incorporated in sulfide Cu₂ZnSnS₄ by selenization at 337, 360 and 409 °C. • Grain boundary diffusion of Se is observed at 337 and 360 °C. • Nucleation and recrystallization of a Cu₂ZnSn(S_x,Se_1-x)₄ phase is seen at 409 °C. • Sodium diffuses from the Cu₂ZnSnS₄/Mo interface to support recrystallization. - Abstract: Band gap grading of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells can be achieved by varying the S_r = [S]/([S] + [Se]) ratio in the absorber layer with depth. One approach is a two-step annealing process where the absorber is first sulfurized to Cu₂ZnSnS₄ (CZTS) followed by selenization to form CZTSSe. However, once nucleation of CZTSSe initiates, the rapid interchange of S and Se limits the control over the S_r ratio with depth. Here, we have studied incorporation of Se into CZTS and observed the behavior of Se below and up to the nucleation temperature of CZTSSe. Se diffusion at 337 and 360 °C is dominated by grain boundary diffusion while some increase of Se is also seen in the region from 100 to 800 nm from the surface. After selenization at 409 °C, recrystallization is observed and CZTSSe grains are formed. The recrystallization is more rapid for a smaller average grain size and is facilitated by diffusion of Na from the back contact. The grain boundary diffusion is identified with secondary ion mass spectrometry measurements by measuring the accumulation in the CZTS/Mo interface for three samples with different average grain size.