Anomalous negative phototransistors in which the channel current decreases under light illumination hold potential to generate novel and multifunctional optoelectronic applications. Although a variety of design strategies have been developed to construct such devices, NPTs still suffer from far lower device performance compared to well‐developed positive phototransistors (PPTs). In this work, a novel 1D/2D molecular crystal p–n heterojunction, in which p‐type 1D molecular crystal (1DMC) arrays are embedded into n‐type 2D molecular crystals (2DMCs), is developed to produce ultrasensitive NPTs. The p‐type 1DMC arrays act as light‐absorbing layers to induce p‐doping of n‐type 2DMCs through charge transfer under illumination, resulting in ineffective gate control and significant negative photoresponses. As a result, the NPTs show remarkable performances in photoresponsivity (P) (1.9 × 108) and detectivity (D*) (1.7 × 1017 Jones), greatly outperforming previously reported NPTs, which are one of the highest values among all organic phototransistors. Moreover, the device exhibits intriguing characteristics undiscovered in PPTs, including precise control of the threshold voltage by controlling light signals and ultrasensitive detection of weak light. As a proof‐of‐concept, the NTPs are demonstrated as light encoders that can encrypt electrical signals by light. These findings represent a milestone for negative phototransistors, and pave the way for the development of future novel optoelectronic applications.

The intermolecular interactions of cocrystals can be controlled via selecting the donors and acceptors, and then the TPA properties can be selectively adjusted, promoting the development of TPA materials prepared via cocrystal engineering.

Light‐stimulated optoelectronic synaptic devices are fundamental compositions of the neuromorphic vision system. However, there are still huge challenges to achieving both bidirectional synaptic behaviors under light stimuli and high performance. Herein, a bilayer 2D molecular crystal (2DMC) p‐n heterojunction is developed to achieve high‐performance bidirectional synaptic behaviors. The 2DMC heterojunction‐based field effect transistor (FET) devices exhibit typical ambipolar properties and remarkable responsivity (R) of 3.58×104 A W−1 under weak light as low as 0.008 mW cm−2. Excitatory and inhibitory synaptic behaviors are successfully realized by the same light stimuli under different gate voltages. Moreover, a superior contrast ratio (CR) of 1.53×103 is demonstrated by the ultrathin and high‐quality 2DMC heterojunction, which transcends previous optoelectronic synapses and enables application for the motion detection of the pendulum. Furthermore, a motion detection network based on the device is developed to detect and recognize classic motion vehicles in road traffic with an accuracy exceeding 90%. This work provides an effective strategy for developing high‐contrast bidirectional optoelectronic synapses and shows great potential in the intelligent bionic device and future artificial vision.