A Vertical Organic Transistor Architecture for Fast Nonvolatile Memory

Universal electrode for ambipolar charge injection in organic electronic devices

Ambipolar transistors, i.e. transistors with symmetrical n- and p-type performances, open new avenues for the design and integration of high-density, efficient and versatile circuits for advanced technologies. Their performance requires two processes: efficient injection of holes and electrons from the metal electrodes into the semiconductor; and transport of both carriers through the semiconductor. Organic semiconductors (OSCs) support ambipolar transport, but charge injection is strongly asymmetric due to inherent misalignment of the electrode work function with both conducting levels of the OSC. Here we introduce a new electrode concept capable of efficiently injecting both types of charge carriers into OSCs. The electrode has a mosaic-like structure composed of islands of two metals with high and low work functions, in this case Al and Au, respectively. Under suitable applied bias the Au (Al) domains in direct contact with the OSC allow efficient hole (electron) injection into the HOMO (LUMO) level. Implementing this electrode as both the source and drain in an organic field effect transistor (OFET) led to fully balanced ambipolar performance while maintaining high ON/OFF ratios. We then used the ambipolar OFETs to significantly simplify the circuit design and fabricate digital and analogue elements, i.e. a digital inverter and an analogue phase shifter using one type of transistor only. Finally, we demonstrate that a single ambipolar OFET can replace several unipolar transistors to fabricate digital transmission gate circuits. The new electrode design concept can include other metal combinations and compositions to balance ambipolar injection, and the use of the mosaic electrodes can be extended to other electronic devices that require ambipolar charge injection such as light emitting transistors, memory devices etc.

Flexible Artificial Optoelectronic Synapse based on Lead-Free Metal Halide Nanocrystals for Neuromorphic Computing and Color Recognition

Optoelectronic synapses combining optical-sensing and synaptic functions are playing an increasingly vital role in the neuromorphic computing systems development, which can efficiently process visual information and complex recognition, memory, and learning. Metal halides are considered promising candidates for synaptic devices due to their excellent optoelectronic properties. However, the toxicity of lead and the further development of device functions are the recognized problems at present. Herein, a flexible optoelectronic synapses system based on high-quality lead-free Cs₃ Bi₂ I₉ nanocrystals is demonstrated, in which the carrier confinement caused by the band mismatching between the Cs₃ Bi₂ I₉ and the organic semiconductor layer provides the possibility to simulate synaptic behaviors. The synaptic functions including long/short-term memory and learning-forgetting-relearning are demonstrated in this device and visual perception, visual memory, and color recognition functions are successfully implemented. Additionally, the flexible device exhibits excellent robustness and can realize imaging of light distribution under curved hemispheres similar to the human eye. Finally, through the simulation based on an artificial neural network algorithm, the device successfully realizes the high-precision recognition of handwritten digital images and possesses a strong fault tolerant capability even in bending states. These results are expected to drive the practical progress of metal halide for neuromorphic computing.

Doped Vertical Organic Field-Effect Transistors Demonstrating Superior Bias-Stress Stability

Bias-stress stability is essential to the practical applications of organic field-effect transistors (OFETs), yet it remains a challenge issue in conventional planar OFETs. Here, the feasibility of achieving high bias-stress stability in vertical structured OFETs (VOFETs) in combination with doping techniques is demonstrated. VOFETs with silver nanowires as source electrodes are fabricated and the device performance is optimized by understanding the influence of device parameters on performance. Then, the bias-stress stability of the optimized PDVT-10 VOFETs is investigated and found to be superior to the corresponding planar OFETs, which is attributed to reduced trapping effects of gate dielectrics in the VOFETs. Moreover, the bias-stress stability can be further improved by doping PDVT-10 to passivate bulk traps. Consequently, the characteristic time of doped PDVT-10 VOFETs extracted from stretched exponential equation is found to be over four times larger than that of the planar PDVT-10 OFETs under the same bias-stress conditions. These results present the promising applications of VOFETs as well as an effective strategy to achieve highly bias-stress stable OFETs.

Ambipolar Charge Storage in Type-I Core/Shell Semiconductor Quantum Dots toward Optoelectronic Transistor-Based Memories

Efficient charge storage media play a pivotal role in transistor-based memories and thus are under intense research. In this work, the charge storage ability of type-I InP/ZnS core/shell quantum dots is well revealed through studying a pentacene-based organic transistor with the quantum dots (QDs) integrated. The quantum well-like energy band structure enables the QDs to directly confine either holes or electrons in the core, signifying a dielectric layer-free nonvolatile memory. Especially, the QDs in this device can be charged by electrons using light illumination as the exclusive method. The electron charging process is ascribed to the photoexcitation process in the InP-core and the hot holes induced. The QDs layer demonstrates an electron storage density of ≈5.0 × 1011  cm-2 and a hole storage density of ≈6.4 × 1011  cm-2 . Resultingly, the output device shows a fast response speed to gate voltage (10 µs), large memory window (42 V), good retention (>4.0 × 104 s), and reliable endurance. This work suggests that the core/shell quantum dot as a kind of charge storage medium is of great promise for optoelectronic memories.

Comparative studies on vertical-channel charge-trap memory thin-film transistors using In-Ga-Zn-O active channels deposited by sputtering and atomic layer depositions

Vertical-channel charge-trap memory thin film-transistors (V-CTM TFTs) using oxide semiconductors were fabricated and characterized, in which In-Ga-Zn-O (IGZO) channels were prepared by sputtering and atomic-layer deposition (ALD) methods to elucidate the effects of deposition process. The vertical-channel gate stack of the fabricated device was verified to be well implemented on the vertical sidewall of the spacer patterns due to excellent step-coverage and self-limiting mechanisms of ALD process. The V-CTM TFTs using ALD-IGZO channel exhibited a wide memory window (MW) of 15.0 V at a V_GS sweep of ±20 V and a large memory margin of 1.6 × 10² at a program pulse duration as short as 5 ms. The programmed memory margin higher than 10⁵ did not experience any degradation with time evolution for 10⁴ s. The mechanical durability was also evaluated after the delamination process of polyimide (PI) film. There were no marked variations in charge-trap-assisted MW even at a curvature radius of 1 mm and programmed memory margin even after repeated program operations of 10⁴ cycles. The introduction of ALD process for the formation of IGZO active channel was suggested as a main process parameter to ensure the excellent memory device characteristics of the V-CTM TFTs.

Perovskite/Organic Semiconductor-Based Photonic Synaptic Transistor for Artificial Visual System

Artificial visual system with information sensing, processing, and memory function is promoting the development of artificial intelligence techniques. Photonic synapse as an essential component can enhance the visual information processing efficiency owing to the high propagation speed, low latency, and large bandwidth. Herein, photonic synaptic transistors based on organic semiconductor poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT) and perovskite CsPbBr₃ quantum dots are fabricated by a simple solution process. The device can simulate fundamental synaptic behaviors, including excitatory postsynaptic current, pair-pulse facilitation, the transition of short-term memory to long-term memory, and "learning experience" behavior. Combining the advantages of the high photosensitivity of perovskites and relatively high conductivity of DPPDTT, the device can exhibit excellent synaptic performances at a low voltage of -0.2 V. Even under an ultralow operation voltage of -0.0005 V, the device can still show obvious synaptic responses. Tunable synaptic integration behaviors including "AND" and "OR" light logic functions can be realized. An artificial visual system is successfully emulated by illuminating the synaptic arrays employing light of different densities. Therefore, low-voltage synaptic devices based on organic semiconductor and CsPbBr₃ quantum dots with a simple fabrication technique present high potential to mimic human visual memory.

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.

Electret-Based Organic Synaptic Transistor for Neuromorphic Computing

Neuromorphic computing inspired by the neural systems in human brain will overcome the issue of independent information processing and storage. An artificial synaptic device as a basic unit of a neuromorphic computing system can perform signal processing with low power consumption, and exploring different types of synaptic transistors is essential to provide suitable artificial synaptic devices for artificial intelligence. Hence, for the first time, an electret-based synaptic transistor (EST) is presented, which successfully shows synaptic behaviors including excitatory/inhibitory postsynaptic current, paired-pulse facilitation/depression, long-term memory, and high-pass filtering. Moreover, a neuromorphic computing simulation based on our EST is performed using the handwritten artificial neural network, which exhibits an excellent recognition accuracy (85.88%) after 120 learning epochs, higher than most reported organic synaptic transistors and close to the ideal accuracy (92.11%). Such a novel synaptic device enriches the diversity of synaptic transistors, laying the foundation for the diversified development of the next generation of neuromorphic computing systems.

Edge-driven nanomembrane-based vertical organic transistors showing a multi-sensing capability

The effective utilization of vertical organic transistors in high current density applications demands further reduction of channel length (given by the thickness of the organic semiconducting layer and typically reported in the 100 nm range) along with the optimization of the source electrode structure. Here we present a viable solution by applying rolled-up metallic nanomembranes as the drain-electrode (which enables the incorporation of few nanometer-thick semiconductor layers) and by lithographically patterning the source-electrode. Our vertical organic transistors operate at ultra-low voltages and demonstrate high current densities (~0.5 A cm⁻²) that are found to depend directly on the number of source edges, provided the source perforation gap is wider than 250 nm. We anticipate that further optimization of device structure can yield higher current densities (~10 A cm⁻²). The use of rolled-up drain-electrode also enables sensing of humidity and light which highlights the potential of these devices to advance next-generation sensing technologies.

For vertical organic field-effect transistors (VOFETs) to reach their potential for display and sensor applications, further improvements to fabrication methods are required. Here, the authors report microfabricated VOFETs featuring rolled-up metallic nanomembrane electrodes and displaying multi-sensing capability.

Developments of Diketopyrrolopyrrole-Dye-Based Organic Semiconductors for a Wide Range of Applications in Electronics

In recent times, fused aromatic diketopyrrolopyrrole (DPP)-based functional semiconductors have attracted considerable attention in the developing field of organic electronics. Over the past few years, DPP-based semiconductors have demonstrated remarkable improvements in the performance of both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices due to the favorable features of the DPP unit, such as excellent planarity and better electron-withdrawing ability. Driven by this success, DPP-based materials are now being exploited in various other electronic devices including complementary circuits, memory devices, chemical sensors, photodetectors, perovskite solar cells, organic light-emitting diodes, and more. Recent developments in the use of DPP-based materials for a wide range of electronic devices are summarized, focusing on OFET, OPV, and newly developed devices with a discussion of device performance in terms of molecular engineering. Useful guidance for the design of future DPP-based materials and the exploration of more advanced applications is provided.

Low-Voltage Organic Nonvolatile Memory Transistors with Water-Soluble Polymers Containing Thermally Induced Radical Dipoles

A water-soluble acidic polymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA), was applied as a gate-insulating layer for organic field-effect transistors (OFETs). Before depositing the poly(3-hexylthiophene) (P3HT) channel layers, the PAMPSA layers were subjected to thermal treatment at various temperatures from 140 to 230 °C. The OFET performance was greatly enhanced by thermal treatment between 140 and 170 °C, whereas it became very poor at higher temperatures (200-230 °C). In particular, the transfer curves showed pronounced hysteresis phenomena at 170 °C. Various measurements including thermogravimetric analysis and X-ray photoelectron spectroscopy disclosed that the PAMPSA chains underwent thermal degradation from ca. 160 °C and could generate carbon radicals leading to the formation of dipoles with the nitrogen lone pair electrons. The carbon-nitrogen dipoles delivered hysteresis phenomena to the OFETs with the PAMPSA layers treated at 170 °C, which exhibited excellent memory retention characteristics up to 10 000 cycles even at -1 V. Hence, it is expected that the thermally treated PAMPSA layers can be used as one of the viable gate-insulating memory materials for low-voltage transistor-type organic memory devices (TOMDs).

Engineering Asymmetric Charge Injection/Extraction to Optimize Organic Transistor Performances

The introduction of an appropriate functionality on the electrode/active layer interface has been found to be an efficient methodology to enhance the electrical performances of organic field-effect transistors (OFETs). Herein, we efficiently optimized the charge injection/extraction characteristics of source/drain (S/D) electrodes by applying an asymmetric functionalization at each individual electrode/organic semiconductor (OSC) interface. To further clarify the functionalizing effects of the electrode/OSC interface, we systematically designed five different OFETs: one with pristine S/D electrodes (denoted as pristine S/D) and the remaining ones made by symmetrically or asymmetrically functionalizing the S/D electrodes with up to two different self-assembled monolayers (SAMs) based on thiolated molecules, the strongly electron-donating thiophenol (TP) and electron-withdrawing 2,3,4,5-pentafluorobenzenethiol (PFBT). Both the S and D electrodes were functionalized with TP (denoted as TP-S/D) in one of the two symmetric cases and with PFBT in the other (PFBT-S/D). In each of the two asymmetric cases, one of the S/D electrodes was functionalized with TP and the other with PFBT (to produce PFBT-S/TP-D and TP-S/PFBT-D OFETs). The vapor-deposited p-type dinaphtho[2,3- b:2',3'- f]thieno[3,2- b]thiophene was used as the OSC active layer. The PFBT-S/TP-D case exhibited a field-effect mobility (μ_FET) of 0.86 ± 0.23 cm² V^-1 s^-1, about three times better than that of the pristine S/D case (0.31 ± 0.12 cm² V^-1 s^-1). On the other hand, the μ_FET of the TP-S/PFBT-D case (0.18 ± 0.10 cm² V^-1 s^-1) was significantly lower than that of the pristine case and even lower than those of the TP-S/D (0.23 ± 0.07 cm² V^-1 s^-1) and PFBT-S/D (0.58 ± 0.19 cm² V^-1 s^-1) cases. These results were clearly correlated with the additional hole density, surface potential, and effective work function. In addition, the contact resistance ( R_C) for the asymmetric PFBT-S/TP-D case was 10-fold less than that for the TP-S/PFBT-D case and more than five times lower than that for the pristine case. The results contribute a meaningful step forward in improving the electrical performances of various organic electronics such as OFETs, inverters, solar cells, and sensors.

A novel structure of grid spirofluorene: a new organic semiconductor with low reorganization energy

The lower charge mobility of organic semiconductors relative to that of inorganic semiconductors is a thorny problem that still has not been resolved.

Inkjet-Printed Vertical Organic Field-Effect Transistor Arrays and Their Image Sensors

Vertical organic field-effect transistors (VOFETs) have been explored with a higher current density, a faster switch speed, and a better air stability than conventional OFETs, which dramatically enhance the capability of driving an AMOLED backplane. Unfortunately, the state-of-the-art of the fabrication of solution-processed VOFETs is still very complicated, which can only focus at a single-cell level. In this work, with the assistance of the inkjet print, the fabrication process of a solution-processed VOFET was significantly simplified, and a solution-processed VOFET array was fabricated for the first time, which exhibited excellent device performance and outstanding mechanical stability. More importantly, the VOFET arrays exhibited excellent photodetector properties, and a flexible image sensor based on VOFET arrays with multipoint visible photodetection and image recognition was demonstrated for the first time. Therefore, this novel process dramatically simplified the VOFET device fabrication process and a successfully realized array, which promoted the commercialization of VOFET and showed great potential in flexible display, multifunctional sensors, and wearable integrated circuits.

Programmable Negative Differential Resistance Effects Based on Self-Assembled Au@PPy Core-Shell Nanoparticle Arrays

The negative differential resistance (NDR) effect observed in conducting polymer/Au nanoparticle composite devices is not yet fully clarified due to the random and disordered incorporation of Au nanoparticles into conducting polymers. It remains a formidable challenge to achieve the sequential arrangement of various components in an optimal manner during the fabrication of Au nanoparticle/conducting polymer composite devices. Here, a novel strategy for fabricating Au nanoparticle/conducting polymer composite devices based on self-assembled Au@PPy core-shell nanoparticle arrays is demonstrated. The interval between the two Au nanoparticles can be precisely programmed by modulating the thickness of the shell and the size of the core. Programmable NDR is achieved by regulating the spacer between two Au nanoparticles. In addition, the Au/conducting polymer composite device exhibits a reproducible memory effect with read-write-erase characteristics. The sequentially controllable assembly of Au@PPy core-shell nanoparticle arrays between two microelectrodes will simplify nanodevice fabrication and will provide a profound impact on the development of new approaches for Au/conducting polymer composite devices.

Diverse resistive switching behaviors of AlN thin films with different orientations

Aluminum nitride (AlN) thin films with different orientations (i.e., amorphous, (100)- and (002)-oriented) are deposited on Pt/Ti/SiO₂/Si substrates via the radio-frequency (RF) sputtering method.