Thermally Independent Interfacial Reconstruction for Phase‐Pure <i>n</i> = 1 2D Perovskites With Mixed‐Orientation Surface Architecture

ABSTRACT Two‐dimensional (2D) perovskites are widely employed to enhance the efficiency and stability of perovskite solar cells (PSCs); however, their formation typically relies on thermal annealing, which can degrade the underlying three‐dimensional (3D) perovskite and lead to poorly defined phases. In addition, their intrinsically low out‐of‐plane conductivity imposes a trade‐off between improved stability and efficient charge transport. Here, a thermally independent interfacial reconstruction strategy is reported to enable the phase‐selective formation of phase‐pure n = 1 2D perovskites without thermal activation. This approach suppresses thermally induced degradation and prevents the formation of higher‐n or mixed‐phase intermediates. The resulting 2D perovskite exhibits a mixed‐orientation architecture, comprising domains parallel and tilted relative to the underlying 3D lattice. This structural configuration simultaneously enables effective surface passivation and ion‐blocking while maintaining efficient vertical charge transport, thereby overcoming the stability‐transport trade‐off. As a result, the optimized PSCs achieve a champion efficiency of 26.61% and retain over 98% of their initial performance after 2000 h of continuous maximum power point tracking. In contrast, the control devices exhibit inferior efficiency and accelerated degradation under identical conditions. This work establishes a nonthermal pathway to reconcile stability and charge transport in perovskite optoelectronics.