High‐Mobility 2D Molecular Crystals Enable Field‐Effect X‐Ray Detectors with Record Volumetric Sensitivity

Direct X-ray detectors that are lightweight, low-cost, and ultra-sensitive are essential for next-generation portable imaging, wearable monitoring, and low-dose medical diagnostics. Conventional high-performance detectors use thick single-crystal active layers to boost sensitivity, but this increases device volume, raises dark current, and complicates integration with flexible and portable electronics. Here, we break from this paradigm with an ultrathin, high-mobility 2D molecular crystal (2DMC) integrated into a lateral field-effect transistor. This design employs a highly confined in-plane carrier transport path coupled with strong gate-electrostatic control to achieve complete carrier depletion and sub-pA dark currents. Thanks to high-mobility and efficient charge collection in the ultrathin channel, the detector achieves a record volumetric sensitivity of 5.91 × 10¹⁰ µC Gy^-1 cm^-3 and a detection limit of 1.43 nGy s^-1, surpassing all reported organic detectors and competing with state-of-the-art inorganic ones. Importantly, we show that in the ultrathin regime, high-mobility outweighs atomic number in governing sensitivity, overturning conventional design logic. We further fabricate large-area, uniform 2DMC arrays with excellent operational stability, enabling high-contrast imaging at doses as low as 10.17 nGy s^-1. This work establishes a new paradigm for lightweight, low-dose organic X-ray imaging and introduces mobility-centric design rules for next-generation detectors.