Inorganic-Organic Hybrid Phototransistor Array with Enhanced Photogating Effect for Dynamic Near-Infrared Light Sensing and Image Preprocessing

An Active-Matrix Synaptic Phototransistor Array for In-Sensor Spectral Processing

The human retina perceives and preprocesses the spectral information of incident light, enabling fast image recognition and efficient chromatic adaptation. In comparison, it is reluctant to implement parallel spectral preprocessing and temporal information fusion in current complementary metal-oxide-semiconductor (CMOS) image sensors, requiring intricate circuitry, frequent data transmission, and color filters. Herein, an active-matrix synaptic phototransistor array (AMSPA) is developed based on organic/inorganic semiconductor heterostructures. The AMSPA provides wavelength-dependent, bidirectional photoresponses, enabling dynamic imaging and in-sensor spectral preprocessing functions. Specifically, near-infrared light induces inhibitory photoresponse while UV light results in exhibitory photoresponse. With rational structural design of the organic/inorganic hybrid heterostructures, the current dynamic range of phototransistor is improved to over 90 dB. Finally, a 32 × 64 AMSPA (128 pixels per inch) is demonstrated with one-switch-transistor and one-synaptic phototransistor (1-T-1-PT) structure, achieving spatial chromatic enhancement and temporal trajectory imaging. These results reveal the feasibility of AMSPA for constructing artificial vision systems.

Demonstration of a 3D-Assembled Dual-Mode Photodetector Based on Tubular Graphene/III-V Semiconductors Heterostructure and Coplanar Three Electrodes

3D assembly technology is a cutting-edge methodology for constructing high-performance and multifunctional photodetectors since some attractive photodetection features such as light trapping effect, omnidirectional ability, and high spatial resolution can be introduced. However, there has not been any report of 3D-assembled multimode photodetectors owing to the lack of design and fabrication guideline of electrodes serving for 3D heterostructures. In this study, a 3D-assembled dual-mode photodetector (3DdmPD) was realized successfully via the clever electrical contact between the rolled-up tubular graphene/GaAs/InGaAs heterostructure and planar metal electrode. Arbitrary switching of three coplanar electrodes makes the as-fabricated tubular 3D photodetector work at the unbiased photodiode mode, which is suitable for energy conservation high-speed photodetection, or the biased photoconductive mode, which favors extremely weak light photodetection, fully showing the advantages of multifunctional detection. In more detail, the Ilight/Idark ratio reached as high as 2 × 104, and a responsivity of 42.3 mA/W, a detectivity of 1.5 × 1010 Jones, as well as a rising/falling time (τr/τf) of 360/370 μs were achieved under the self-driven photodiode mode. Excitingly, 3DdmPD shows omnidirectional photodetection ability at the same time. When 3DdmPD works at the photoconductive mode with 5 V bias, its responsivity is extremely high as 7.9 × 104 A/W and corresponding detectivity is increased to 1.0 × 1011 Jones. Benefiting from the totally independent coplanar electrodes, 3DdmPD is much more easily integrated as arrays that are expected to offer the function of high-speed omnidirectional image-sensing with ultralow power consumption than the planar counterparts which share communal bottom electrodes. We believe that our work can contribute to the progress of 3D-assembled optoelectronic devices.

Organic Synaptic Transistors with Environmentally Friendly Core/Shell Quantum Dots for Wavelength-Selective Memory and Neuromorphic Functions

Organic transistors based on organic semiconductors together with quantum dots (QDs) are attracting more and more interest because both materials have excellent optoelectronic properties and solution processability. Electronics based on nontoxic QDs are highly desired considering the potential health risks but are limited by elevated surface defects, inadequate stability, and diminished luminescent efficiency. Herein, organic synaptic transistors based on environmentally friendly ZnSe/ZnS core/shell QDs with passivating surface defects are developed, exhibiting optically programmable and electrically erasable characteristics. The synaptic transistors feature linear multibit storage capability and wavelength-selective memory function with a retention time above 6000 s. Various neuromorphic applications, including memory enhancement, optical communication, and memory consolidation behaviors, are simulated. Utilizing an established neuromorphic model, accuracies of 92% and 91% are achieved in pattern recognition and complicated electrocardiogram signal processing, respectively. This research highlights the potential of environmentally friendly QDs in neuromorphic applications and health monitoring.

Spacer Engineering Enables Fine-Tuned Thin Film Microstructure and Efficient Charge Transport for Ultrasensitive 2D Perovskite-Based Heterojunction Phototransistors and Optoelectronic Synapses

2D Ruddlesden-Popper phase layered perovskites (RPLPs) hold great promise for optoelectronic applications. In this study, a series of high-performance heterojunction phototransistors (HPTs) based on RPLPs with different organic spacer cations (namely butylammonium (BA+), cyclohexylammonium (CyHA+), phenethylammonium (PEA+), p-fluorophenylethylammonium (p-F-PEA+), and 2-thiophenethylammonium (2-ThEA+)) are fabricated successfully, in which high-mobility organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene is adopted to form type II heterojunction channels with RPLPs. The 2-ThEA+-RPLP-based HPTs show the highest photosensitivity of 3.18 × 107 and the best detectivity of 9.00 × 1018 Jones, while the p-F-PEA+-RPLP-based ones exhibit the highest photoresponsivity of 5.51 × 106 A W-1 and external quantum efficiency of 1.32 × 109%, all of which are among the highest reported values to date. These heterojunction systems also mimicked several optically controllable fundamental characteristics of biological synapses, including excitatory postsynaptic current, paired-pulse facilitation, and the transition from short-term memory to long-term memory states. The device based on 2-ThEA+-RPLP film shows an ultra-high PPF index of 234%. Moreover, spacer engineering brought fine-tuned thin film microstructures and efficient charge transport/transfer, which contributes to the superior photodetection performance and synaptic functions of these RPLP-based HPTs. In-depth structure-property correlations between the organic spacer cations/RPLPs and thin film microstructure/device performance are systematically investigated.

Chiroptical Synaptic Heterojunction Phototransistors Based on Self-Assembled Nanohelix of π-Conjugated Molecules for Direct Noise-Reduced Detection of Circularly Polarized Light

High-performance chiroptical synaptic phototransistors are successfully demonstrated using heterojunctions composed of a self-assembled nanohelix of a π-conjugated molecule and a metal oxide semiconductor. To impart strong chiroptical activity to the device, a diketopyrrolopyrrole-based π-conjugated molecule decorated with chiral glutamic acid is newly synthesized; this molecule is capable of supramolecular self-assembly through noncovalent intermolecular interactions. In particular, nanohelix formed by intertwinded fibers with strong and stable chiroptical activity in a solid-film state are obtained through hydrogen-bonding-driven, gelation-assisted self-assembly. Phototransistors based on interfacial charge transfer at the heterojunction from the chiroptical nanohelix to the metal oxide semiconductor show excellent chiroptical detection with a high photocurrent dissymmetry factor of 1.97 and a high photoresponsivity of 218 A W-1 . The chiroptical phototransistor demonstrates photonic synapse-like, time-dependent photocurrent generation, along with persistent photoconductivity, which is attributed to the interfacial charge trapping. Through the advantage of synaptic functionality, a trained convolutional neural network successfully recognizes noise-reduced circularly polarized images of handwritten alphabetic characters with better than 89.7% accuracy.

Photogating Effect-Driven Photodetectors and Their Emerging Applications

Rather than generating a photocurrent through photo-excited carriers by the photoelectric effect, the photogating effect enables us to detect sub-bandgap rays. The photogating effect is caused by trapped photo-induced charges that modulate the potential energy of the semiconductor/dielectric interface, where these trapped charges contribute an additional electrical gating-field, resulting in a shift in the threshold voltage. This approach clearly separates the drain current in dark versus bright exposures. In this review, we discuss the photogating effect-driven photodetectors with respect to emerging optoelectrical materials, device structures, and mechanisms. Representative examples that reported the photogating effect-based sub-bandgap photodetection are revisited. Furthermore, emerging applications using these photogating effects are highlighted. The potential and challenging aspects of next-generation photodetector devices are presented with an emphasis on the photogating effect.