This study investigates the effect of molecular design in establishing structure–property–performance relationships that govern interfacial passivation at the perovskite/hole-selective layer (HSL) interface. Interfacial defects at the interface of the perovskite and charge-selective layer junction remains one of the primary bottlenecks preventing perovskite solar cells from reaching their intrinsic efficiency limits and long-term stability [1]. While molecular passivation has emerged as a powerful strategy to suppress interfacial defects and charge recombination, the fundamental relationship between molecular functionality, interfacial energetics, and charge extraction remains poorly understood.

Using structurally analogous molecules that differ only in their terminal functional groups (ammonium, carboxylic acid, and bifunctional ammonium–carboxylic acid), we isolate the role of chemical functionality in affecting the interfacial behavior. Elemental, structural and optoelectronic characterization, including X-ray Photoelectron Spectroscopy, steady-state and time-resolved photoluminescence, surface photovoltage spectroscopy, conductive atomic force microscopy, and device-level analysis, reveals that passivation effect and charge extraction are not intrinsically correlated [3]. While all molecules effectively reduce non-radiative recombination by passivating interfacial trap states, their impact on carrier extraction differs dramatically. Through a detailed investigation of the perovskite/HSL interface, we identify a correlation between the terminal functional group, the formation of quasi-2D Ruddlesden–Popper (RP) interlayers, and key photovoltaic performance metrics, including open-circuit voltage (V_OC​) and fill factor.

Single-functional group passivating agents promote efficient hole extraction, yielding enhanced V_OC​, fill factor, and overall power conversion efficiency. In contrast, the bifunctional molecule, despite exhibiting strong defect passivation, introduces an interfacial barrier that suppresses hole transport, leading to extraction-limited device performance. In particular, ammonium-functionalized molecules exhibit superior performance by selectively promoting the formation of n = 2 RP phases, thereby enabling effective surface passivation and efficient charge extraction. By decoupling defect passivation from carrier extraction, this work contributes in providing molecular design rules for next-generation interfacial materials in perovskite photovoltaics and other emerging thin-film optoelectronic devices where interfacial losses dominate performance.