Boosting Carrier Transport in Carbon‐Based Hole‐Transport‐Layer‐Free Perovskite Solar Cells via 2D/3D Interfacial Structural Configuration

Interfacial nonradiative recombination and operational instability remain major barriers to the commercialization of perovskite solar cells (PSCs). Hole‐transport‐layer‐free carbon‐based PSCs (C‐PSCs) provide a promising low‐cost and scalable device configuration, yet their efficiency is strongly limited by defective and energetically mismatched perovskite/carbon interfaces. Here, a phenethylammonium chloride (PEACl)‐induced interfacial reconstruction strategy is developed to form in situ a Ruddlesden–Popper (RP) two‐dimensional (2D) capping layer on three‐dimensional (3D) perovskite absorbers. Analyses via low‐angle X‐ray diffraction (XRD), photoluminescence (PL), and X‐ray photoelectron spectroscopy (XPS) demonstrate that a 2D RP phase capping layer is formed. This layer passivates defects associated with undercoordinated lead, improves interfacial energy states, and enhances surface moisture resistance. Consequently, interfacial recombination is substantially suppressed, while hole extraction toward the carbon electrode is promoted and electron back‐transfer is inhibited. The optimized C‐PSCs deliver a power conversion efficiency (PCE) of 17.75% with a fill factor of 69.02%, together with enhanced open‐circuit voltage. Moreover, the unencapsulated device retains over 93% of its initial efficiency after 550 h of exposure to humid air, evidencing markedly improved stability. This work establishes 2D RP phase‐mediated interfacial reconstruction as an effective route toward efficient, stable, and low‐cost carbon‐based perovskite photovoltaics.