Interfacial Donor‐Acceptor Complex Engineering Boosts Hot Hole Extraction in Tin Halide Perovskite Indoor Photovoltaics

Tin‐based perovskites have long hot‐carrier lifetimes, holding great potential for developing high‐efficiency lead‐free indoor photovoltaics (IPVs). However, the severe hot carriers dissipation remains a major energy loss channel in tin‐based perovskite indoor photovoltaic devices. Here, we synthesized a novel self‐assembled monolayer (SAM) to extract the hot‐carrier energy in tin‐based perovskite, ((2,7‐dibromo‐9H‐fluorene‐9,9‐diyl)bis(propane‐3,1‐diyl))bis(phosphonic acid) (2Br‐PAFa), with a fluorene core and dual phosphonic acid anchoring groups. The fluorene‐contained SAM characterized with disrupted‐conjugation structure possesses a deeper highest occupied molecular orbital (HOMO) level (−5.97 eV), which facilitates the formation of donor‐acceptor (D‐A) complexes with perovskite. The complex enhances the spatial separation of charge carriers and suppress interfacial recombination, thus accelerate the capture of excess energy in the hot carrier. Femtosecond transient absorption spectroscopy confirmed that the 2Br‐PAFa/perovskite system demonstrates an ultrafast hot hole extraction time within femtosecond timescale and reduced cooling time of 41 ps. These advancements result in a champion power conversion efficiency (PCE) of 21.08% under 1000 lux illumination, ranking among the highest for lead‐free perovskite IPVs.