Biphasic Interfacial Swimming Top‐Down Growth of Ruddlesden‒Popper Halide Perovskite Single‐Crystal Sheets with Few Defects

Ruddlesden‐Popper (RP) hybrid perovskites have the characteristics of controllable quantum well thickness, excellent stability, and good photoelectric performance. However, traditional space‐limited methods easily induce additional defects at the crystal interface. Herein, a novel swimming top‐down method is proposed for growing high‐quality perovskite single‐crystal sheets in a biphasic liquid–liquid n‐dodecane/γ‐butyrolactone (GBL) system. The (BA) 2 MAPb 2 I 7 single‐crystal sheet exhibited the lowest defect density on the top surface because of interfacial features, including a flat biphasic confined interface, a large area for GBL molecule diffusion, and sufficient traction for swimming growth. As a typical result, (BA) 2 MAPb 2 I 7 single‐crystal‐based photodetectors exhibit outstanding weak‐light photoelectric performance with a responsivity of 1.02 A W −1 , an external quantum efficiency of 226.9%, and a detectivity of 8.7 × 10 12 Jones; these values are 310%, 310%, and 700%, respectively, as high as those of photodetectors obtained by the antisolvent vapor‐assisted crystallization method.