Photoactive Biaxial Ferroelectric Polarizations in Flexible Hybrid Metal‐Halide Single‐Crystal Thin Films Driving Multidirectional Self‐Powered Optical Communications

ABSTRACT Flexible optoelectronic devices hold great potential in fields such as health monitoring, human‐machine interactions and light communications. The emerging two‐dimensional (2D) hybrid metal‐halide perovskite ferroelectrics are highly attractive for optoelectronic applications owing to their unique spontaneous polarization, multiaxial ferroelectricity and superior mechanical flexibility. However, inducing multiaxial ferroelectric polarizations to construct flexible multidirectional self‐powered optoelectronic devices remains a significant challenge. Herein, we have designed a photoactive perovskite ferroelectric (BZ) 2 (FA)Pb 2 I 7 (BZ = benzylammonium, FA = formamidinium), which exhibits biaxial ferroelectricity with a high Curie temperature ∼435 K and a spontaneous polarization of 3.5 µC/cm 2 . Notably, high‐quality single‐crystal thin films (SCFs) were prepared by mechanical exfoliation, exhibiting a bulk photovoltaic effect (BPVE) of 0.72 V along the b ‐ and c ‐axes, attributed to the inherent biaxial spontaneous polarization. Driven by this exceptional BPVE, the flexible photodetectors achieve impressive self‐powered photodetection performance. Furthermore, the photodetector can recognize ASCII codes in multiple directions, demonstrating its potential for self‐powered optical communications. Notably, after 1000 bending cycles, the photocurrent of the flexible photodetector remained at 88.2% of its initial value, demonstrating excellent fatigue resistance. This study provides new insights for advanced flexible optoelectronic devices based on 2D perovskite ferroelectric SCFs.