Molecular Doping of Flexible Lead‐Free Perovskite‐Polymer Thick Membranes for High‐Performance X‐Ray Detection

X‐ray detectors have broad applications in medicine and industry. Although flexible lead‐free perovskite films are competitive because of their lightweight and low toxicity, they are less efficient due to low charge transport. Herein, we report low‐toxicity, flexible X‐ray detectors based on p‐type doped MA 3 Bi 2 I 9 (MA=methylammonium) perovskite‐filled membranes (PFMs). Strong coordination between dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and MA 3 Bi 2 I 9 and the establishment of charge‐transfer complex (CPX) improved the conductivity by four times. The flexible X‐ray detector achieved a high sensitivity of 2065 μC Gy air −1 cm −2 and an ultra‐low detection limit of 2.71 nGy air s −1 , which is among the highest values in all environmentally friendly flexible X‐ray detectors. Importantly, the PFMs retained excellent charge transport under mechanical stress. All of those make flexible MA 3 Bi 2 I 9 membranes more competitive as next‐generation X‐ray detection.