Air Annealing‐Driven Preferential Growth of Low‐ <i>n</i> Phases and Vertical Orientation in Cesium‐Based 2D Perovskite Solar Cells

Cesium‐based two‐dimensional perovskite solar cells (2D PSCs) demonstrate better stability, expanding its application to single‐junction and tandem solar cells. However, the photovoltaic performance of cesium‐based 2D PSCs is hampered by inefficient carrier transport, arising from disordered phase distribution and unfavorable crystal orientation. Herein, we propose an air‐annealing process coupled with a precise humidity control strategy to prepare cesium‐based 2D perovskite films featuring regular low‐ n ‐value phase distribution and vertical crystal orientation. The moisture in air serves to facilitate the rapid decomposition of dimethylamine iodide (DMAI), while also accelerating the ordered insertion of spacer cations phenethylamine (PEA + ) by hydrogen bonding, ultimately achieving additive‐free and passivator‐free PEA 2 Cs 3 Pb 4 I 13 PSCs with a power conversion efficiency (PCE) of 15.52% (5.33% efficiency of PSCs in N 2 ). The air‐annealed devices can retain 90% of the initial PCE after 1300‐h storage in N 2 and over 80% of its initial PCE after 1200 h of continuous maximum power point tracking under illumination. This work provides a feasible strategy for optimizing the phase distribution and crystal orientation of 2D Cs‐based perovskites, which is crucial for the further development of high‐performance and stable cesium‐based 2D PSCs.