2D In‐Plane Molecular Superlattice Heterojunctions for High‐Performance Ambipolar Electronics and Low‐Dose X‐Ray Sensing

Two‐dimensional in‐plane molecular superlattice heterojunctions, where distinct semiconducting components are integrated laterally within a single crystalline plane, offer an ideal architecture for controlling charge separation and transport in optoelectronic devices. However, realizing such structures with molecular‐level precision, long‐range periodicity, and sharp interfaces in organic semiconductors remains a major challenge. Here, the first 2D organic heterojunction featuring long‐range, in‐plane donor–acceptor superlattice via cocrystal engineering is presented. By employing phase‐separated molecular design for the donor (TIPS‐PEN) and acceptor (PDI‐FCN), strong in‐plane cohesion is decoupled from out‐of‐plane steric repulsion, inducing a periodic –D–A–D– arrangement within the crystal plane. The resulting high‐density, lattice‐defined heterointerfaces facilitate exciton dissociation, directional charge transport, and efficient extraction. The ultrathin crystal enables effective gate‐field control, yielding ambipolar OFETs with exceptional on/off ratios of 10 8 (holes) and 10 7 (electrons). Capitalizing on the narrow bandgap and the ordered 2D heterointerface, this material demonstrates outstanding high‐energy photon conversion efficiency. As an X‐ray detector, it exhibits high sensitivity of 4.21 × 10 4 µC Gy −1 cm −2 and low detection limit 5.73 nGy s −1 , enabling clear imaging at 14.77 nGy s −1 ultralow dose rate. This work provides new ideas for constructing 2D multicomponent organic heterostructures and unlocks potential for next‐generation flexible electronics and low‐dose radiation sensing.