Bimolecular Synergy Reshapes the Interfacial Reactivity of Wide‐Bandgap Perovskites

Wide-bandgap (WBG) perovskites have garnered significant attention owing to their extensive applicability in tandem photovoltaic devices. However, the conventional post-treatment strategy utilizing phenethylammonium iodide (PEAI) frequently results in the formation of two-dimensional (2D) perovskite structures with mixed n-values and the phase evolution under operating conditions, thereby constraining further enhancement of device performance and deteriorating the operation stability. Through intermolecular interactions and localized polarity modulation, this method facilitates cation exchange reactions between phenethylammonium (PEA⁺) and formamidinium (FA⁺) ions, enabling the formation of homogenized 2D perovskite layer with n = 2 on WBG perovskite surface. Benefiting from the precisely defined 2D passivation layer, the WBG single-junction device achieves a power conversion efficiency (PCE) of 20.98%, while the 2-terminal (2T) all-perovskite tandem device based on this strategy attains a PCE of 28.35%. This research underscores the benefits of bimolecular synergy in surface phase regulation and provides a promising pathway for advancing high-performance, stable perovskite photovoltaics.