Self-Assembled Charge Bridge Path at the Sn-Pb Perovskite/C₆₀ Interface for High-Efficiency All-Perovskite Tandem Solar Cells

Narrow bandgap mixed tin-lead perovskite solar cells (PSCs) have garnered substantial research interest owing to their remarkable optoelectronic properties. However, non-radiative recombination and carrier transport losses at the interface between the perovskite layer and the charge transport layer (C₆₀) significantly reduce the overall efficiency of mixed tin-lead PSCs. To address this challenge, 9-Fluorenylmethyl carbazate (9FC) is incorporated at the interface between perovskite and C₆₀. The hydrazide group present in 9FC effectively mitigates the oxidation of Sn²⁺. Furthermore, 9FC can engage in chemical bonding with the perovskite, while the outward-facing aromatic rings create effective π–π interactions with C₆₀, thereby promoting enhanced interfacial charge transfer. The highest-performing mixed tin-lead PSCs achieve a power conversion efficiency (PCE) of 23.97%, accompanied by an impressive open-circuit voltage of 0.91 V. Additionally, these tin-lead PSCs facilitate the development of highly efficient two-terminal and four-terminal all-perovskite tandem solar cells, which demonstrate efficiencies of 27.01% and 28.07%, respectively.