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

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 Cs₃Bi₂I₉ 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 Cs₃Bi₂I₉ SC-sheets, achieving a device On-Off ratio of 3.3 × 10⁴ and specific detectivity of 2.55 × 10¹³ Jones under single-photon signals. Notably, experimental validation reveals that 1 µm-thick Cs₃Bi₂I₉ 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.