Interfacial Defect Passivation by Molecular Dipole‐Moment Engineering in Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) is critically dependent on the quality of interfaces and grain boundaries, which often lead to nonradiative recombination losses, degradation, and inefficient charge transport. Here, we introduce an asymmetric bulky cation with a dipole moment (2.48 Debye), 4‐methoxyphenethylammonium bromide (MPB), at the perovskite/hole transport layer (HTL) interface in planar (n‐i‐p) PSCs. The MPB provides dual functionality, not only as a defect passivation agent for undercoordinated Pb 2+ but also improving charge extraction at the perovskite/HTL interface. The MPB‐passivated PSCs exhibit enhanced crystallinity and an increase in hole‐carrier diffusion length (from 540 to 770 nm). We quantitatively attributed the built‐in potential increase (from 1.09 ± 0.01 to 1.12 ± 0.01 V) to a larger charge‐carrier driving force in MPB‐treated devices, as determined by Mott–Schottky analysis. The optimized PSCs achieve a higher power‐conversion efficiency (PCE) of 22.7% compared to the control device (PCE of 21.5%). Consequently, the MPB‐treated device retained 80% of its initial PCE, compared with 60% for the control, under standard illumination, ~35% relative humidity (RH), and a temperature of ~45 °C after 2000 s. This work provides valuable insights into defect passivation through dipole‐assisted molecular interface engineering, which enhances charge extraction and overall device performance, offering a pathway for high‐efficiency PSCs.