Solvent-Actuated Self-Assembly of Amphiphilic Hole-Transporting Polymer Enables Bottom-Surface Passivation of Perovskite Film for Efficient Photovoltaics

Bottom-surface defect passivation of perovskite film, lagging far behind easily conducted bulk and top-surface passivations in perovskite solar cells (PSCs), remains rather challenging because most passivation molecules/groups can be eroded by polar solvents used for the subsequent perovskite deposition. In this work, an effective bottom-surface passivation is enabled for enhanced performance of inverted PSCs by covalently attaching a passivation group (hydroxyl) to a hole transporting polymer. A short linker (methylene) between the hydroxyl and the conjugated backbone bearing hydrophobic long alkyl chains is adopted to improve the resistance of the resultant amphiphilic polymer to polar solvents. A solvent evaporation-induced self-assembly of the amphiphilic hole transporting polymer is developed to enrich hydroxyl groups on the film surface, passivating defects of the upper perovskite layer via interactions with undercoordinated Pb²⁺ and I^– sites. Inverted PSCs based on this hole transporting film are superior in efficiency (20.12%), reproducibility, large-area fabrication, and stability to its classical poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) counterpart. This work demonstrates that rational introduction of passivation groups into the hole transporting layer combined with self-assembly-modulated component distributions is useful to realize bottom-surface passivation of the perovskite layer for improved photovoltaic performance.