Polyimide-Doped Indium-Gallium-Zinc Oxide-Based Transparent and Flexible Phototransistor for Visible Light Detection

We report a transparent and flexible polyimide (PI)-doped single-layer (PSL) phototransistor for the detection of visible light. The PSL was deposited on a SiO₂ gate insulator by a co-sputtering process using amorphous indium-gallium-zinc oxide (IGZO) and PI targets simultaneously. The PSL acted as both a channel layer and a visible-light absorption layer. PI is one of the few flexible organic materials that can be fabricated into sputtering targets. Compared with the IGZO phototransistor without PI doping, the PSL phototransistor exhibited improved optoelectronic characteristics under illumination with 635 nm red light of 1 mW/mm² intensity; the obtained photoresponsivity ranged from 15.00 to 575.00 A/W, the photosensitivity from 1.38 × 10¹ to 9.86 × 10⁶, and the specific detectivity from 1.35 × 10⁷ to 5.83 × 10¹¹ Jones. These improvements are attributed to subgap states induced by the PI doping, which formed decomposed organic molecules, oxygen vacancies, and metal hydroxides. Furthermore, a flexible PSL phototransistor was fabricated and showed stable optoelectronic characteristics even after 10,000 bending tests.

High-Responsivity Gate-Tunable Ultraviolet-Visible Broadband Phototransistor Based on Graphene-WS2 Mixed-Dimensional (2D-0D) Heterostructure

Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition-metal dichalcogenide (TMDC) quantum dots (QDs) with their broadband absorption spectra and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution-based processing, we demonstrate a superlarge (∼0.75 mm²), ultraviolet-visible (UV-vis) broadband (365-633 nm) phototransistor made of WS₂ QDs-decorated chemical vapor deposited (CVD) graphene as the active channel with extraordinary stability and durability under ambient conditions (without any degradation of photocurrent until 4 months after fabrication). Here, colloidal zero-dimensional (0D) WS₂ QDs are used as the photoabsorbing material, and graphene acts as the conducting channel. A high photoresponsivity (3.1 × 10² A/W), moderately high detectivity (∼8.9 × 10⁸ Jones), and low noise equivalent power (∼9.7 × 10^-11 W/Hz^0.5) are obtained at a low bias voltage (V_ds = 1 V) at an illumination of 365 nm with optical power as low as ∼0.8 μW/cm², which can be further tuned by modulating the gate bias. While comparing the photocurrent between two different morphologies of WS₂ [QDs and two-dimensional (2D) nanosheets], a significant enhancement of photocurrent is observed in the case of QD-based devices. Ab initio density functional theory (DFT)-based calculations further support our observation, revealing the role of quantum confinement in enhanced photoresponse. Our work reveals a strategy toward developing a scalable, cost-effective, high-performance hybrid mixed-dimensional (2D-0D) photodetector with graphene-WS₂ QDs for next-generation optoelectronic applications.

High-Performance Broadband Phototransistor Based on TeOx/IGTO Heterojunctions

Ultraviolet to infrared broadband spectral detection capability is a technological challenge for sensing materials being developed for high-performance photodetection. In this work, we stacked 9 nm-thick tellurium oxide (TeO_x) and 8 nm-thick InGaSnO (IGTO) into a heterostructure at a low temperature of 150 °C. The superior photoelectric characteristics we achieved benefit from the intrinsic optical absorption range (300-1500 nm) of the hexagonal tellurium (Te) phase in the TeO_x film, and photoinduced electrons are driven effectively by band alignment at the TeO_x/IGTO interface under illumination. A photosensor based on our optimized heterostructure exhibited a remarkable detectivity of 1.6 × 10¹³ Jones, a responsivity of 84 A/W, and a photosensitivity of 1 × 10⁵, along with an external quantum efficiency of 222% upon illumination by blue light (450 nm). Simultaneously, modest detection properties (responsivity: ∼31 A/W, detectivity: ∼6 × 10¹¹ Jones) for infrared irradiation at 970 nm demonstrate that this heterostructure can be employed as a broadband phototransistor. Furthermore, its low-temperature processability suggests that our proposed concept might be used to design array optoelectronic devices for wide band detection with high sensitivity, flexibility, and stability.

Photoconductivity of Thin Films Obtained from a New Type of Polyindole

The optoelectronic properties of a new poly(2-ethyl-3-methylindole) (MPIn) are discussed in this paper. The absorption and photoluminescence spectra were studied. The electronic spectrum of MPIn showed a single absorption maximum at 269 nm that is characteristic of the entire series of polyindoles. The fluorescence spectra show that the emission peaks of the test sample are centered around 520 nm. The photoconductivity of thin film samples of MPIn polyindole was studied by measuring the current-voltage characteristics under ultraviolet radiation with a wavelength of 350 nm. Samples of phototransistors were obtained, where thin films of MPIn polyindole were used as a transport layer, and their characteristics were measured and analyzed. The value of the quantum efficiency and the values of the mobility of charge carriers in thin polyindole films were estimated.

Layer-by-Layer Growth of AA-Stacking MoS2 for Tunable Broadband Phototransistors

The stacking configuration has been considered as an important additional degree of freedom to tune the physical property of layered materials, such as superconductivity and interlayer excitons. However, the facile growth of highly uniform stacking configuration is still a challenge. Herein, the AA-stacking MoS₂ domains with a ratio up to 99.5% has been grown by using the modified chemical vapor deposition through introducing NaCl molecules in the confined space. By tuning the growth time, MoS₂ domains would transit from an AA-stacking bilayer to an AAAAA-stacking five-layer. The epitaxial growth mechanism has been insightfully studied, revealing that the critical nucleation size of the AA-stacking bilayer is 5.0 ± 3.0 μm. Through investigation of the photoluminescence, the photoemission, especially the indirect photoexcitation, is dependent on both the stacking fashion and layer number. Furthermore, by studying the gate-tuned MoS₂ phototransistors, we found a significant dependence on the stacking configuration of MoS₂ of the photoexcitation and a different gate tunable photoresponse. The AAA-stacking trilayer MoS₂ phototransistor delivers a photoresponse of 978.14 A W^-1 at 550 nm. By correction of the external quantum efficiency with external field and illumination power density, it has been found that the photoresponse tunability is dependent on the layer number due to the strong photogating effect. This strategy provides a general avenue for the epitaxial growth of van der Waals film which will further facilitate the applications in a tunable photodetector.

Highly Sensitive and Ultrafast Organic Phototransistor Based on Rubrene Single Crystals

Rubrene single crystals have received a lot of attention for their great potential in electronic and wearable nanoelectronics due to their high carrier mobility and excellent flexibility. While they exhibited remarkable electrical performances, their intrinsic potential as photon detectors has not been fully exploited. Here, we fabricate a sensitive and ultrafast organic phototransistor based on rubrene single crystals. The device covers the ultraviolet to visible range (275-532 nm), and the responsivity and detectivity can reach up to ∼4000 A W^-1 and 10¹¹ jones at 532 nm, respectively. Furthermore, the response times are highly gate-tunable down to sub-90 μs, and the cutoff frequency is ∼4 kHz, which is one of the fastest organic material-based phototransistors reported so far. Equally important is that the fabricated device exhibits stable light detection ability even after 8 months, indicating great long-term stability and excellent environmental robustness. The results suggest that the high-quality rubrene single crystal may be a promising material for future flexible optoelectronics with its intrinsic mechanical flexibility.

Neuromorphic Active Pixel Image Sensor Array for Visual Memory

Neuromorphic engineering, a methodology for emulating synaptic functions or neural systems, has attracted tremendous attention for achieving next-generation artificial intelligence technologies in the field of electronics and photonics. However, to emulate human visual memory, an active pixel sensor array for neuromorphic photonics has yet to be demonstrated, even though it can implement an artificial neuron array in hardware because individual pixels can act as artificial neurons. Here, we present a neuromorphic active pixel image sensor array (NAPISA) chip based on an amorphous oxide semiconductor heterostructure, emulating the human visual memory. In the 8 × 8 NAPISA chip, each pixel with a select transistor and a neuromorphic phototransistor is based on a solution-processed indium zinc oxide back channel layer and sputtered indium gallium zinc oxide front channel layer. These materials are used as a triggering layer for persistent photoconductivity and a high-performance channel layer with outstanding uniformity. The phototransistors in the pixels exhibit both photonic potentiation and depression characteristics by a constant negative and positive gate bias due to charge trapping/detrapping. The visual memory and forgetting behaviors of the NAPISA can be successfully demonstrated by using the pulsed light stencil method without any software or simulation. This study provides valuable information to other neuromorphic devices and systems for next-generation artificial intelligence technologies.

Penta-PdPSe: A New 2D Pentagonal Material with Highly In-Plane Optical, Electronic, and Optoelectronic Anisotropy

Due to their low-symmetry lattice characteristics and intrinsic in-plane anisotropy, 2D pentagonal materials, a new class of 2D materials composed entirely of pentagonal atomic rings, are attracting increasing research attention. However, the existence of these 2D materials has not been proven experimentally until the recent discovery of PdSe₂ . Herein, penta-PdPSe, a new 2D pentagonal material with a novel low-symmetry puckered pentagonal structure, is introduced to the 2D family. Interestingly, a peculiar polyanion of [SePPSe]^4- is discovered in this material, which is the biggest polyanion in 2D materials yet discovered. Strong intrinsic in-plane anisotropic behavior endows penta-PdPSe with highly anisotropic optical, electronic, and optoelectronic properties. Impressively, few-layer penta-PdPSe-based phototransistor not only achieves excellent electronic performances, a moderate electron mobility of 21.37 cm² V^-1 s^-1 and a high on/off ratio of up to 10⁸ , but it also has a high photoresponsivity of ≈5.07 × 10³ A W^-1 at 635 nm, which is ascribed to the photogating effect. More importantly, penta-PdPSe also exhibits a large anisotropic conductance (σ_max /σ_max  = 3.85) and responsivity (R_max /R_min  = 6.17 at 808 nm), superior to most 2D anisotropic materials. These findings make penta-PdPSe an ideal material for the design of next-generation anisotropic devices.

Lateral Structured Phototransistor Based on Mesoscopic Graphene/Perovskite Heterojunctions

Due to their outstanding optical properties and superior charge carrier mobilities, organometal halide perovskites have been widely investigated in photodetection and solar cell areas. In perovskites photodetection devices, their high optical absorption and excellent quantum efficiency contribute to the responsivity, even the specific detectivity. In this work, we developed a lateral phototransistor based on mesoscopic graphene/perovskite heterojunctions. Graphene nanowall shows a porous structure, and the spaces between graphene nanowall are much appropriated for perovskite crystalline to mount in. Hot carriers are excited in perovskite, which is followed by the holes’ transfer to the graphene layer through the interfacial efficiently. Therefore, graphene plays the role of holes’ collecting material and carriers’ transporting channel. This charge transfer process is also verified by the luminescence spectra. We used the hybrid film to build phototransistor, which performed a high responsivity and specific detectivity of 2.0 × 10³ A/W and 7.2 × 10¹⁰ Jones, respectively. To understand the photoconductive mechanism, the perovskite’s passivation and the graphene photogating effect are proposed to contribute to the device’s performance. This study provides new routes for the application of perovskite film in photodetection.

Dual-gate MoS2phototransistor with atomic-layer-deposited HfO2as top-gate dielectric for ultrahigh photoresponsivity

An asymmetric dual-gate (DG) MoS₂field-effect transistor (FET) with ultrahigh electrical performance and optical responsivity using atomic-layer-deposited HfO₂as a top-gate (TG) dielectric was fabricated and investigated. The effective DG modulation of the MoS₂FET exhibited an outstanding electrical performance with a high on/off current ratio of 6 × 10⁸. Furthermore, a large threshold voltage modulation could be obtained from -20.5 to -39.3 V as a function of the TG voltage in a DG MoS₂phototransistor. Meanwhile, the optical properties were systematically explored under a series of gate biases and illuminated optical power under 550 nm laser illumination. An ultrahigh photoresponsivity of 2.04 × 10⁵AW^-1has been demonstrated with the structure of a DG MoS₂phototransistor because the electric field formed by the DG can separate photogenerated electrons and holes efficiently. Thus, the DG design for 2D materials with ultrahigh photoresponsivity provides a promising opportunity for the application of optoelectronic devices.

Vertical Channel Inorganic/Organic Hybrid Electrochemical Phototransistors with Ultrahigh Responsivity and Fast Response Speed

Organic phototransistors (OPTs) have attracted enormous attention because of their promising applications in sensing, communication, and imaging. Currently, most OPTs reported utilize field-effect transistors (FETs) with relative long channel length which usually has undesired amplification because of their inherent low transconductance originated from their low channel capacitance, limiting the further improvement of performance. Herein, a vertical channel hybrid electrochemical phototransistor with a nanoscale channel and large transconductance (VECPT) is invented for the first time to achieve ultrahigh photoresponsivity along with a fast response speed. Benefiting from the nanoscale channel length and large transconductance, the photo-generated carriers in channel can be efficiently dissociated, transported, and amplified into the enlarged photocurrent output. Therefore, the devices deliver substantially improved optoelectronic performances with a photoresponsivity as high as ≈2.99 × 10⁷ A/W, detectivity of ≈1.49 × 10¹³ Jones, and fast-speed response of ≈73 μs under a low voltage of 1 V, which are superior to those of the reported OPTs based on FETs. Moreover, the in situ Kelvin probe microscopy is performed to characterize the surface potential of device systems for better elucidating the photosensing mechanism. Furthermore, taking advantage of its excellent optoelectronic performance, an ultraviolet light monitoring system is constructed by integrating VECPT with a light-emitting diode, which also shows the real-time, high-sensitive, and controllable photoresponse threshold properties. All these results demonstrate the great potential of these electrochemical phototransistors and provide valuable insights into the design of the nanoscale channel length device system for high-performance photodetection.

High-Performance Phototransistors Based on MnPSe3 and Its Hybrid Structures with Au Nanoparticles

Layered metal thiophosphates with a general formula MPX₃ (M is a group VIIB or VIII element and X is a chalcogen) have emerged as a novel member in a two-dimensional (2D) family with fascinating physical and chemical properties. Herein, the photoelectric performance of the few-layer MnPSe₃ was studied for the first time. The multilayer MnPSe₃ shows p-type conductivity and its field-effect transistor delivers an ultralow dark current of about 0.1 pA. The photoswitching ratio reaches ∼10³ at a wavelength of 375 nm, superior to that of other thiophosphates. A responsivity and detectivity of 392.78 mA/W and 2.19 × 10⁹ Jones, respectively, have been demonstrated under irradiation of 375 nm laser with a power intensity of 0.1 mW/cm². In particular, the photocurrent can be remarkably increased up to 30 times by integrating a layer of Au nanoparticle array at the bottom of the MnPSe₃ layer. The metal-semiconductor interfacial electric field and the strain-induced flexoelectric polarization field caused by the underlying nanorugged Au nanoparticles are proposed to contribute together to the significant current improvement.

Scalable Integration of Coplanar Heterojunction Monolithic Devices on Two-Dimensional In2Se3

The formation of lateral heterojunction arrays within two-dimensional (2D) crystals is an essential step to realize high-density, ultrathin electro-optical integrated circuits, although the assembling of such structures remains elusive. Here we demonstrated a rapid, scalable, and site-specific integration of lateral 2D heterojunction arrays using few-layer indium selenide (In₂Se₃). We use a scanning laser probe to locally convert In₂Se₃ into In₂O₃, which shows a significant increase in carrier mobility and transforms the metal-semiconductor junctions from Schottky to ohmic type. In addition, a lateral p-n heterojunction diode within a single nanosheet is demonstrated and utilized for photosensing applications. The presented method enables high-yield, site-specific formation of lateral 2D In₂Se₃-In₂O₃-based hybrid heterojunctions for realizing nanoscale devices with multiple advanced functionalities.

Low-Power and Ultra-Thin MoS2 Photodetectors on Glass

Integration of low-power consumer electronics on glass can revolutionize the automotive and transport sectors, packaging industry, smart building and interior design, healthcare, life science engineering, display technologies, and many other applications. However, direct growth of high-performance, scalable, and reliable electronic materials on glass is difficult owing to low thermal budget. Similarly, development of energy-efficient electronic and optoelectronic devices on glass requires manufacturing innovations. Here, we accomplish both by relatively low-temperature (<600 °C) metal-organic chemical vapor deposition growth of atomically thin MoS₂ on multicomponent glass and fabrication of low-power phototransistors using atomic layer deposition (ALD)-grown, high-k, and ultra-thin (∼20 nm) Al₂O₃ as the top-gate dielectric, circumventing the challenges associated with the ALD nucleation of oxides on inert basal planes of van der Waals materials. The MoS₂ photodetectors demonstrate the ability to detect low-intensity visible light at high speed and low energy expenditure of ∼100 pico Joules. Furthermore, low device-to-device performance variation across the entire 1 cm² substrate and aggressive channel length scalability confirm the technology readiness level of ultra-thin MoS₂ photodetectors on glass.

Optoelectronic Property Modulation in Chiral Organic Semiconductor/Polymer Blends

Organic phototransistors (OPTs) have been widely used in biomedical sensing, optical communications, and imaging. Charge-trapping effect has been utilized as an effective strategy for enhancing their photoresponsivity by effectively decreasing the dark current. The combination of organic semiconductors (OSCs), especially chiral OSCs, with insulating polymers has rarely been carried out for optoelectronic applications. Here, we fabricated OPTs containing both enantiopure and racemic air-stable n-type perylene diimide derivatives, CPDI-CN2-C6, and insulating biopolymer polylactide (PLA) and evaluated their photoresponsive properties. The PLA-blended systems exhibited greatly enhanced optoelectronic performances owing to the intense charge-trapping effect. Interestingly, the racemic system showed 3 times higher electron mobility and 12 times higher specific detectivity (1.3 × 10¹³ jones) compared with the enantiopure systems due to the more aggregated morphologies and larger grains, indicating that chiral composition can be used as a tuning parameter in optoelectronic devices. Our systematic study provides a feasible and effective method for producing high-performance n-type OPTs under ambient conditions.

Implementing an Insect Brain Computational Circuit Using III-V Nanowire Components in a Single Shared Waveguide Optical Network

Recent developments in photonics include efficient nanoscale optoelectronic components and novel methods for subwavelength light manipulation. Here, we explore the potential offered by such devices as a substrate for neuromorphic computing. We propose an artificial neural network in which the weighted connectivity between nodes is achieved by emitting and receiving overlapping light signals inside a shared quasi 2D waveguide. This decreases the circuit footprint by at least an order of magnitude compared to existing optical solutions. The reception, evaluation, and emission of the optical signals are performed by neuron-like nodes constructed from known, highly efficient III-V nanowire optoelectronics. This minimizes power consumption of the network. To demonstrate the concept, we build a computational model based on an anatomically correct, functioning model of the central-complex navigation circuit of the insect brain. We simulate in detail the optical and electronic parts required to reproduce the connectivity of the central part of this network using previously experimentally derived parameters. The results are used as input in the full model, and we demonstrate that the functionality is preserved. Our approach points to a general method for drastically reducing the footprint and improving power efficiency of optoelectronic neural networks, leveraging the superior speed and energy efficiency of light as a carrier of information.

Ultrasensitive Phototransistor Based on WSe2-MoS2 van der Waals Heterojunction

Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe₂ and MoS₂ was developed. The MoS₂ was utilized as the channel for a phototransistor, whereas the WSe₂-MoS₂ PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 10⁶. The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 × 10¹¹ Jones, and response time of 17 ms. The proposed scheme in conjunction with the large-area synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.

Organic Single-Crystal Transistor with Unique Photo Responses and Its Application as Light-Stimulated Synaptic Devices

Tremendous progress has been achieved on organic transistor-based photodetectors; however, because of the nonpositive correlation relationship between the photo/dark current ratio (P) and the gate voltage, the claimed best P, R (photoresponsivity), and D* (detectivity) can hardly be obtained simultaneously at a given gate voltage, which severely compromises the device performance. Here, a light and voltage dually gated transistor based on an organic semiconducting single crystal of 2,6-dithienylanthracene (DTAnt) is developed. Attributing to its very low on/off ratio in the dark and the remarkable increment of mobilities under illumination, this phototransistor shows good performance with a P of 3.83 × 10³, R of 1.32 A W^-1, and D* of 1.94 × 10¹² Jones achieved simultaneously at V_g = -100 V. Besides, the good reversibility and repeatability of its light-responsive behavior allows for the construction of an artificial photonic neuromorphic device with demonstrated synaptic functions, including excitatory postsynaptic current, short/long-term memory , and pair-pulse facilitation/depression.

Photoinduced Tuning of Schottky Barrier Height in Graphene/MoS2 Heterojunction for Ultrahigh Performance Short Channel Phototransistor

Two-dimensional (2D) layered materials with properties such as a large surface-to-volume ratio, strong light interaction, and transparency are expected to be used in future optoelectronic applications. Many studies have focused on ways to increase absorption of 2D-layered materials for use in photodetectors. In this work, we demonstrate another strategy for improving photodetector performance using a graphene/MoS₂ heterojunction phototransistor with a short channel length and a tunable Schottky barrier. The channel length of sub-30 nm, shorter than the diffusion length, decreases carrier recombination and carrier transit time in the channel and improves phototransistor performance. Furthermore, our graphene/MoS₂ heterojunction phototransistor employed a tunable Schottky barrier that is only controlled by light and gate bias. It maintains a low dark current and an increased photocurrent. As a result, our graphene/MoS₂ heterojunction phototransistor showed ultrahigh responsivity and detectivity of 2.2 × 10⁵ A/W and 3.5 × 10¹³ Jones, respectively. This is a considerable improvement compared to previous pristine MoS₂ phototransistors. We confirmed an effective method to develop phototransistors based on 2D materials and obtained ultrahigh performance of our phototransistor, which is promising for high-performance optoelectronic applications.

A Graphene/Polycrystalline Silicon Photodiode and Its Integration in a Photodiode-Oxide-Semiconductor Field Effect Transistor

In recent years, the characteristics of the graphene/crystalline silicon junction have been frequently discussed in the literature, but study of the graphene/polycrystalline silicon junction and its potential applications is hardly found. The present work reports the observation of the electrical and optoelectronic characteristics of a graphene/polycrystalline silicon junction and explores one possible usage of the junction. The current–voltage curve of the junction was measured to show the typical exponential behavior that can be seen in a forward biased diode, and the photovoltage of the junction showed a logarithmic dependence on light intensity. A new phototransistor named the “photodiode–oxide–semiconductor field effect transistor (PDOSFET)” was further proposed and verified in this work. In the PDOSFET, a graphene/polycrystalline silicon photodiode was directly merged on top of the gate oxide of a conventional metal–oxide–semiconductor field effect transistor (MOSFET). The magnitude of the channel current of this phototransistor showed a logarithmic dependence on the illumination level. It is shown in this work that the PDOSFET facilitates a better pixel design in a complementary metal–oxide–semiconductor (CMOS) image sensor, especially beneficial for high dynamic range (HDR) image detection.

Unsymmetrical Small Molecules for Broad-Band Photoresponse and Efficient Charge Transport in Organic Phototransistors

Organic photosensitizers have been investigated as effective light-sensing elements that can promote strong absorption with high field-effect mobility in organic phototransistors (OPTs). In this study, a novel organic photosensitizer is synthesized to demonstrate broad-band photoresponse with enhanced electrical performance. An unsymmetrical small molecule of a solubilizing donor (D_sol)-acceptor (A)-dye donor (D_dye) type connected with a twisted conjugation system is designed for broad-band detection (ranging from 250 to 700 nm). This molecule has high solubility, thereby facilitating the formation of uniformly dispersed nanoparticles in an insulating polymer matrix, which is deposited on top of OPT semiconductors by a simple solution process. The broad-band photodetection shown by the organic photosensitizer is realized with improved mobility close to an order of magnitude and high on/off current ratio (∼10⁵) of the organic semiconductor. Furthermore, p-type charge transport behavior in the channel of the OPT is enhanced through the intrinsic electron-accepting ability of the organic photosensitizer caused by the unique molecular configuration. These structural properties of organic photosensitizers contribute to an improvement in broad-band photosensing systems with new optoelectronic properties and functionalities.

Organic Transistor Nonvolatile Memory with Three-Level Information Storage and Optical Detection Functions

By the current processing technology, it is a challenge to obtain ultrahigh-density information storage in the conventional binary floating-gate-based organic field-effect transistor (FG-OFET) nonvolatile memories (NVMs). To develop a multilevel memory in one cell is a feasible solution. In this work, we demonstrate FG-OFET NVMs with an integrated polymer floating-gate/tunneling (I-FG/T) layer consisting of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and polystyrene. The photoelectric effect of organic/polymer semiconductors is used to improve the controllability of the polarity and the number of the charges stored in the floating-gate. The FG-OFET NVMs integrate light sensitivity and nonvolatile information storage functions. By selecting suitable optical and electrical programming/erasing conditions, three-level information storage states, corresponding to electron storage, approximate neutrality, and hole storage in the floating-gate, are achieved and freely switched to each other. The memory mechanism and the dependence of the memory performances on the F8BT contents in I-FG/T layers are investigated. As a result, good memory performances, with mobility larger than 1.0 cm² V^-1 s^-1, reliable three-level switching endurance over 100 cycles, and stable three-level retention capability over 20 000 s, are achieved in our memory. Furthermore, an imaging system with a nonvolatile information storage function is demonstrated in a 16 × 5 array of FG-OFET NVMs.

Highly Efficient Photo-Induced Charge Separation Enabled by Metal-Chalcogenide Interfaces in Quantum-Dot/Metal-Oxide Hybrid Phototransistors

Quantum dot (QD)-based optoelectronics have received great interest for versatile applications because of their excellent photosensitivity, facile solution processability, and the wide range of band gap tunability. In addition, QD-based hybrid devices, which are combined with various high-mobility semiconductors, have been actively researched to enhance the optoelectronic characteristics and maximize the zero-dimensional structural advantages, such as tunable band gap and high light absorption. However, the difficulty of highly efficient charge transfer between QDs and the semiconductors and the lack of systematic analysis for the interfaces have impeded the fidelity of this platform, resulting in complex device architectures and unsatisfactory device performance. Here, we report ultrahigh detective phototransistors with highly efficient photo-induced charge separation using a Sn₂S₆^4--capped CdSe QD/amorphous oxide semiconductor (AOS) hybrid structure. The photo-induced electron transfer characteristics at the interface of the two materials were comprehensively investigated with an array of electrochemical and spectroscopic analyses. In particular, photocurrent imaging microscopy revealed that interface engineering in QD/AOS with chelating chalcometallate ligands causes efficient charge transfer, resulting in photovoltaic-dominated responses over the whole channel area. On the other hand, monodentate ligand-incorporated QD/AOS-based devices typically exhibit limited charge transfer with atomic vibration, showing photo-thermoelectric-dominated responses in the drain electrode area.

Structure-Property Relation in Organic-Metal Oxide Hybrid Phototransistors

We report an optoelectronic device consisting of a solution-processed indium gallium zinc oxide (IGZO) thin-film transistor and vacuum-deposited small organic molecules. Depending on the configurations of the organic materials, either bulk heterojunction or planar heterojunction (PHJ), the device assumes the functionality of either a photosensor or a photoinduced memory, respectively. Under λ = 625 nm light illumination, the photosensor shows response and recovery time of ∼50 ms, responsivity of ∼5 mA/W, sensitivity above 10⁴, and a linear response. The mechanism of the photoinduced memory is studied experimentally and verified using a device simulation. We find that the memory is due to long charge retention time at the organic PHJ interface which is stable for over 9 days. It is correlated with the low leakage current found in ordered organic junctions having low subgap tail states. The presented integration of the PHJ with the transistor constitutes a new design of write-once-read-many-times memory device that is likely to be attractive for low-cost applications.

One-Step Coating Processed Phototransistors Enabled by Phase Separation of Semiconductor and Dielectric Blend Film

Fabrication of organic thin-film transistors (OTFTs) via high throughput solution process routes have attracted extensive attention. Herein, we report a simple one-step coating method for vertical phase separation of the poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) blends as semiconducting and dielectric layers in OTFTs. These OTFTs can be used as phototransistors for ultraviolet (UV) light detection, where the phototransistors exhibited great photosensitivity of 597.6 mA/W and detectivity of 4.25 × 10¹⁰ Jones under 1 mW/cm² UV light intensity. Studies of the electrical properties in these phototransistors suggested that optimized P3HT contents in the blend film can facilitate the improvement of film morphology, and therefore form optimized vertical phase separation of the PMMA and P3HT. These results indicate that the simple one-step fabrication method creates possibilities for realizing high throughput phototransistors with great photosensitivity.