Intragrain Lateral Funneling as a Hidden Photovoltage Loss Channel in 2D Ruddlesden–Popper Perovskite Solar Cells

Two-dimensional Ruddlesden-Popper perovskites (2D RPPs) show great potential for next-generation photovoltaics. While the vertical phase distribution in 2D RPP films has been extensively studied, the impact of lateral heterogeneity within individual grains on carrier dynamics and photovoltaic performance remains elusive. Herein, we reveal that a lateral phase gradient across the grain generates a funnel-like energy landscape, which induces detrimental carrier relaxation toward the grain center before extraction. This intragrain lateral funneling is identified as a direct and previously overlooked loss channel that affects the open-circuit voltage (V_OC). As a proof-of-concept, we show that solvent engineering can help homogenize the lateral phase distribution, leading to an enhanced V_OC and power conversion efficiency in 2D perovskite solar cells. This work highlights the critical role of subgrain-scale phase uniformity and underscores the importance of regulating lateral phase heterogeneity for high-performance 2D RPP optoelectronics.