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.

Wavelength and Polarization Sensitive Synaptic Phototransistor Based on Organic n-type Semiconductor/Supramolecular J-Aggregate Heterostructure

“…Most interestingly, at higher V G ≈ 50 V, I DS increased up to a blue illumination intensity of 320 μW cm –2 and then finally decreased, while I DS instantly decreased under red illumination (NPC; negative photocurrent) as shown in Figure b, c. Although the absolute photosensitivity varies from device to device, all devices show both PPC and NPC characteristics (Figure S7 and Table S2). The bidirectional photoresponse is a prerequisite for retinomorphic applications for motion detection and recognition (Figure S1c) and has so far been rarely reported in organic phototransistors. , …”

“…The bidirectional photoresponse is a prerequisite for retinomorphic applications for motion detection and recognition (Figure S1c) 45 and has so far been rarely reported in organic phototransistors. 26,46 The operating mechanism of PPC can be explained via an energy diagram and a floating gate transistor architecture of the Cl 2 -NDI/core−shell PBI-1 heterostructure (Figure 2d, f). Cyclic voltammetry results show that Cl 2 -NDI and PBI-1 form a type-II heterostructure; the lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) energies are at −4.29 eV/−7.20 eV and −3.90 eV/−5.93 eV for Cl 2 -NDI and PBI-1, respectively (Figure S8).…”

“…Although an organic semiconductor-based optoelectronic synapse has successfully demonstrated persistent photocurrent characteristics, , less progress has been made to discriminate spectropolarimetric information as well as a bidirectional photoresponse ( i.e. , positive and negative photoresponse) . Bidirectional spectropolarimetric neural networks will be able to recognize and track target objects with polarization information as well as shape and color based on extended input layers (Figure S1).…”

Solution-Processed Organic–Inorganic Semiconductor Heterostructures for Advanced Hybrid Phototransistors

“…49 In addition, the negative response is attained in the 1-dimensional/2-dimensional molecular crystal (1DMC, 2DMC) p−n heterojunction by the charge transfer from the p-type 1DMC to n-type 2DMC channel at positive bias. 50 HPT with wavelength-selective bidirectional photoresponse has attracted great attention by possessing the potential applicability of wavelength differentiation and photoelectric logic units. 46 The heterojunction provides complementary sensitive wavelength and tunable band alignment, enabling wavelength-selective bidirectional photoresponse.…”

Near-infrared heterojunction field modulated phototransistors with distinct photodetection/photostorage switching features for artificial visuals

“…More recently, methods for controlling the diffusion interface layer and 1D/2D molecular crystal p–n heterojunctions of organics have improved the light sensitivity, giving great potential on this issue. 21,22 However, the absence of a blocking layer may cause energy losses due to the recombination at the graphene/acceptor interface and the lack of molecular packing orientation restricts efficient charge transfer, suggesting that there is still space to improve.…”