A Novel and Effective Strategy for Reducing the Surface Roughness of CIGS Bottom Cells by SAS Annealing in Perovskite/CIGS Tandem Solar Cells

The high surface roughness of CIGS bottom cells is a critical challenge to improve the photoelectric conversion efficiency of perovskite/CIGS tandem solar cells. In this work, a sulfurization after selenization (SAS) annealing process was introduced to reduce the surface roughness of CIGSSe absorbers in CIGS bottom cells. By independently optimizing the H₂Se and H₂S concentrations during the SAS annealing, the diffusion and distribution of In, Ga, S, and Se were exactly controlled. The individual and synergistic effects of selenization and sulfurization on the elemental diffusion in CIGSSe absorbers, the mechanisms of elemental distribution on the surface roughness of CIGSSe absorbers, and their effects on the performance of CIGSSe solar cells were systematically investigated. The results show that the H₂Se concentration during the SAS annealing determines the crystallization and phase formation of CIGSSe absorbers. And the H₂S concentration during the SAS annealing affects the diffusion and distribution of In, Ga, S, and Se along the depth direction of CIGSSe absorbers. By optimizing the SAS annealing process, a CIGSSe absorber with excellent crystallinity, low surface roughness, free of secondary phases and fine-grained layers, and a U-shaped bandgap can be obtained. Finally, the surface roughness of CIGSSe absorbers decreases by 58.2%, with the <i>R</i>_q value reduces to 89 nm. The photoelectric conversion efficiency of CIGSSe solar cell based on this absorber increases to 17.0%. This strategy significantly reduces the surface roughness of CIGS bottom cells, providing a critical foundation for high-quality perovskite growth and enabling future efficiency improvements in perovskite/CIGS tandem solar cells.