Negative Phototransistors with Ultrahigh Sensitivity and Weak‐Light Detection Based on 1D/2D Molecular Crystal p–n Heterojunctions and their Application in Light Encoders

Abstract 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.