Synergistic Engineering of Molecular Asymmetry and Short Linkers Enables High-Performance Hole-Selective Contacts in Perovskite Solar Cells.

Self-assembled monolayers (SAMs) have emerged as highly promising hole-selective contacts for inverted perovskite solar cells (IPSCs) due to their tunable energy levels and molecular dipoles. However, deriving generalizable rules that explicitly connect halogenation symmetry and linker length to interfacial properties and device performance remains difficult. Herein, we systematically decouple these two variables by synthesizing a series of benzo[c]carbazole-phosphonate-based SAMs with orthogonal variations in bromination pattern, comparing asymmetric monobromination with symmetric dibromination, and in alkyl linker length, comparing short with long. We show that asymmetric monobromination is the dominant factor in enhancing the molecular dipole and optimizing energy-level alignment. It breaks the in-plane molecular symmetry, generating a substantial lateral dipole that drives optimized antiparallel packing. Furthermore, a short alkyl linker plays a critical role in improving interfacial quality and charge transport kinetics by fostering denser molecular packing, stronger substrate anchoring, and reduced interfacial resistance. When combined, these two design strategies produce a powerful synergistic effect. Consequently, IPSCs incorporating the optimized asymmetric, short-linker SAM (1Br2PADCB) achieve a champion power conversion efficiency (PCE) of 26.24% with a high fill factor (FF) of 86.36%, and retain over 80% of their initial efficiency after 800 h of continuous maximum power point tracking. This work establishes a concise and broadly applicable design rule: asymmetric halogenation with short linkers, providing a clear blueprint for engineering high-performance buried interfaces in perovskite photovoltaics and beyond.