High‐Anisotropic Flexible Photodetector Based on Centimeter‐Sized Cs₃Bi₂I₉ Single‐Crystal Sheet

Abstract Bismuth(Bi)‐based 2D quasi‐layered perovskite single crystals (SCs) have emerged as promising candidates for next‐generation photodetector (PD) due to their anisotropic charge transport characteristics, suppressed dark current, robust thermo‐chemical stability, and intrinsic ion migration resistance. However, current millimeter‐scale lateral dimensions of SC sheets (SC‐sheets) fall short of practical requirements for large‐area device integration, while their longitudinal mechanical flexibility remains un‐quantified—a critical limitation that hinders flexible optoelectronic applications. This study demonstrates a breakthrough in on‐chip fabrication of centimeter‐scale Cs 3 Bi 2 I 9 SC‐sheets. By employing a self‐developed organic polymer solution curing transfer technology, using poly(methyl methacrylate) (PMMA), centimeter‐sized SC‐sheets are successfully transferred intact onto flexible polyethylene terephthalate (PET) electrode substrate to construct a PMMA/Cs₃Bi₂I₉/Au/PET multilayered lateral PD. This architecture fully exploits the highly anisotropic electrical properties of Cs 3 Bi 2 I 9 SC‐sheets, achieving a device On‐Off ratio of 3.3 × 10⁴ and specific detectivity of 2.55 × 10¹ 3 Jones under single‐photon signals. Notably, experimental validation reveals that 1 µm‐thick Cs 3 Bi 2 I 9 SC‐sheets exhibit a critical longitudinal bending radius of ≈2 mm. Even under super‐threshold bending, fragmented crystal strips remain adhered to the electrode surface under PMMA/PET dual‐layer confinement, sustaining PD functionality. Meanwhile, the confined encapsulation simultaneously confers 100% waterproofing.