Decoupling light- and oxygen-induced degradation mechanisms of Sn-Pb perovskites in all perovskite tandem solar cells

Efficiencies of all-perovskite tandem solar cells are dominantly constrained by the challenges pertaining to defects and stability within tin-lead (Sn-Pb) perovskite sub-cells. On top of the well-studied oxygen oxidation, defects related to iodide and the consequent generation of I2 upon light illumination pose significant degradation risks, leading to Sn2+ -> Sn4+ oxidation. To address this, we screen phenylhydrazine cation (PEH+)-based additives of varying polarities, which strongly coordinate with Sn for reinforcing the Sn-I bond, and interacting electrostatically with negatively charged defects (VSn, VFA, ISn, and I-i). The synergistic effects suppress the photo-induced formation of I2 and the subsequent oxidation of Sn-Pb perovskites, circumventing the stability concerns of narrow bandgap (NBG) perovskite solar cells (PSCs) under operational conditions. The reducing agent 2-mercaptobenzimidazole (MBI) was further introduced into the precursor solution, which not only demonstrates strong resistance to oxygen erosion, but also reduces the deep-level defect density of the Sn-Pb perovskites. Consequently, single-junction Sn-Pb cells achieve a champion efficiency of 23.0%. The enhanced light stability allows these cells to retain 89.4% of their initial efficiency after 400 hours of continuous operation, as assessed by tracking the maximum power point (MPP). We further integrated the Sn-Pb perovskite into a two-terminal (2T) monolithic all-perovskite tandem cell and achieved a PCE of 27.9% (27.2% certified). Meanwhile, the encapsulated tandem device maintained 90.3% of its initial PCE after 300 h through MPP tracking. The work offers new ideas for tackling the stability issues related to light-triggered oxidation. This study showed PEHCl-CN can enhance the strength of Sn-I, resulting in good light stability. The subsequent doping of MBI resulted in good air stability. This enables the integrated 2T all-perovskite device to achieve an efficiency of 27.9%.