Structural Design of Bisphosphonic Acid Self‐Assembled Molecule for Stable and Reproducible Perovskite Solar Cells

ABSTRACT Self‐assembled molecules (SAMs) have recently emerged as promising hole transport monolayers in inverted perovskite solar cells (PSCs), benefiting from their tailorable energy level alignment, molecular‐level thickness, and excellent surface passivation capability. However, the low substrate binding strength, spontaneous self‐aggregation behavior, and limited structural flexibility of conventional monophosphonic acid SAMs severely compromise device reproducibility and stability. Herein, a new bisphosphonic‐acid‐based SAM (Phenyl‐terminated carbazole bisphosphonic acid, denoted as LS‐6), featuring two phosphonic acid anchoring groups connected to a π‐conjugated backbone through flexible C─O linkages, was designed and synthesized. This molecular configuration effectively suppresses intermolecular aggregation, enabling highly uniform self‐assembly on fluorine‐doped tin oxide (FTO) substrates with increased monolayer coverage and mitigated ligand desorption during solvent treatment. The LS‐6 layer improves the wettability of the perovskite precursor solution and regulates perovskite crystallization, resulting in improved crystallinity with reduced lattice strain and enhanced film uniformity. Consequently, LS‐6‐based PSCs achieve an impressive power conversion efficiency (PCE) of 26.08% and retain over 90% of their initial performance after 4600 h of storage in a nitrogen atmosphere. Importantly, the reinforced bifacial contact at the buried interface endows the devices with robust tolerance to mechanical bending fatigue, highlighting significant potential for the development of stabilized PSCs.