Facile Graphene Oxide Modification Method via Hydroxyl-yne Click Reaction for Ultrasensitive and Ultrawide Monitoring Pressure Sensors.

Enhancing the durability and functionality of existing materials through sustainable pathways and appropriate structural design represents a time- and cost-effective strategy for the development of advanced wearable devices. Herein, a facile graphene oxide (GO) modification method via the hydroxyl-yne click reaction is present for the first time. By the click coupling between propiolate esters and hydroxyl groups on GO under mild conditions, various functional molecules are successfully grafted onto the GO. The modified GO is characterized by FTIR, XRD, TGA, XPS, and contact angle, proving significantly improved dispersibility in various solvents. Besides the high efficiency, high selectivity, and mild reaction conditions, this method is highly practical and accessible, avoiding the need for prefunctionalizations, metals, or toxic reagents. Subsequently, a rGO-PDMS sponge-based piezoresistive sensor developed by modified GO-P2 as the sensitive material exhibits impressive performance: high sensitivity (335 kPa-1, 0.8-150 kPa), wide linear range (>500 kPa), low detection limit (0.8 kPa), and long-lasting durability (>5000 cycles). Various practical applications have been demonstrated, including body joint movement recognition and real-time monitoring of subtle movements. These results prove the practicality of the methodology and make the rGO-PDMS sponge-based pressure sensor a real candidate for a wide array of wearable applications.

Field-effect bulk mobilities in polymer semiconductor films measured by sourcemeters.

Semiconducting polymers inherently exhibit polydispersity in terms of molecular structure and microscopic morphology, which often results in a broad distribution of energy levels for localized electronic states. Therefore, the bulk charge mobility strongly depends on the free charge density. In this study, we propose a method to measure the charge-density-dependent bulk mobility of conjugated polymer films with widely spread localized states using a conventional field-effect transistor configuration. The gate-induced variation of bulk charge density typically ranges within ±1018 cm-3; however, this range depends significantly on the energetic dispersion width of localized states. The field-effect bulk mobility and field-effect mobility near the semiconductor-dielectric interface along with their dependence on charge density can be simultaneously extracted from the transistor characteristics using various gate voltage ranges.

High-performance asymmetric electrode structured light-stimulated synaptic transistor for artificial neural networks.

Photonics neuromorphic computing shows great prospects due to the advantages of low latency, low power consumption and high bandwidth. Transistors with asymmetric electrode structures are receiving increasing attention due to their low power consumption, high optical response, and simple preparation technology. However, intelligent optical synapses constructed by asymmetric electrodes are still lacking systematic research and mechanism analysis. Herein, we present an asymmetric electrode structure of the light-stimulated synaptic transistor (As-LSST) with a bulk heterojunction as the semiconductor layer. The As-LSST exhibits superior electrical properties, photosensitivity and multiple biological synaptic functions, including excitatory postsynaptic currents, paired-pulse facilitation, and long-term memory. Benefitting from the asymmetric electrode configuration, the devices can operate under a very low drain voltage of 1 × 10-7 V, and achieve an ultra-low energy consumption of 2.14 × 10-18 J per light stimulus event. Subsequently, As-LSST implemented the optical logic function and associative learning. Utilizing As-LSST, an artificial neural network (ANN) with ultra-high recognition rate (over 97.5%) of handwritten numbers was constructed. This work presents an easily-accessible concept for future neuromorphic computing and intelligent electronic devices.

Enhanced Multiwavelength Response of Flexible Synaptic Transistors for Human Sunburned Skin Simulation and Neuromorphic Computation.

In biological species, optogenetics and bioimaging work together to regulate the function of neurons. Similarly, the light-controlled artificial synaptic system not only enhances computational speed but also simulates complex synaptic functions. However, reported synaptic properties are mainly limited to mimicking simple biological functions and single-wavelength responses. Therefore, the development of flexible synaptic devices with multiwavelength optical signal response and multifunctional simulation remains a challenge. Here, flexible organic light-stimulated synaptic transistors (LSSTs) enabled by alumina oxide (AlOX ), with a simple fabrication process, are reported. By embedding AlOX nanoparticles, the excitons separation efficiency is improved, allowing for multiple wavelength responses. Optimized LSSTs can respond to multiple optical and electrical signals in a highly synaptic manner. Multiwavelength optical synaptic plasticity, electrical synaptic plasticity, sunburned skin simulation, learning efficiency model controlled by photoelectric cooperative stimulation, neural network computing, "deer" picture learning and memory functions are successfully proposed, which promote the development for future artificial intelligent systems. Furthermore, as prepared flexible transistors exhibit mechanical flexibility with bending radius down to 2.5 mm and improved photosynaptic plasticity, which facilitating development of neuromorphic computing and multifunction integration systems at the device-level.

Optical interference on the measurement of film-depth-dependent light absorption spectroscopy and a correction approach.

Organic thin films usually feature vertical phase segregation, and film-depth-dependent light absorption spectroscopy is an emerging characterization method to study the vertical phase separation of active layer films in organic electronics field. However, the interference effects on thin films can lead to optical errors in their characterization results. In this work, the interference effects on fluctuations of peak intensity and peak position of film-depth-dependent light absorption spectroscopy are investigated. Subsequently, a numerical method based on inverse transfer matrix is proposed to obtain the optical constants of the active layer through the film-depth-dependent light absorption spectroscopy. The extinction coefficient error in the non-absorbing wavelength range caused by interference effect is reduced by ∼95% compared with the traditional film-depth-dependent light absorption spectroscopy measurement. Thus, the optical properties of the thin film and quantitative spectrographic analysis based on these optical constants largely avoid the effects of interference including fluctuations of peak intensity and peak position. It is concluded that for many morphologically homogenously films, the spatial (film-depth) resolution of this film-depth-dependent light absorption spectroscopy can be optimized to be <1 nm. Subsequently, this modified film-depth-dependent light absorption spectroscopy approach is employed to simulate the local optical properties within devices with a multilayer architecture.

Highly Conductive Ultrathin Layers of Conjugated Polymers for Metal-Free Coplanar Transistors with Single-Polymer Transport Layers.

Although metal or oxide conductive films are widely used as electrodes of electronic devices, organic electrodes would be more favorable for next-generation organic electronics. Here, using some model conjugated polymers as examples, we report a class of highly conductive and optically transparent polymer ultrathin layers. Vertical phase separation of semiconductor/insulator blends leads to a highly ordered two-dimensional (2D) ultrathin layer of conjugated-polymer chains on the insulator. Afterwards, the thermally evaporated dopants on the ultrathin layer lead to a conductivity of up to 103 S cm-1 and a sheet resistance 103 Ω/square for a model conjugated polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT). The high conductivity is due to the high hole mobility (∼ 20 cm2 V-1 s-1), although doping-induced charge density is still in the moderate range of 1020 cm-3 with a 1 nm thick dopant. Metal-free monolithic coplanar field-effect transistors using the same conjugated-polymer ultrathin layer with alternatively doped regions as electrodes and a semiconductor layer are realized. The field-effect mobility of this monolithic transistor is over 2 cm2 V-1 s-1 for PBTTT, one order higher than that of the conventional PBTTT transistor using metal electrodes. The optical transparency of the single conjugated-polymer transport layer is over 90%, demonstrating a bright future for all-organic transparent electronics.

In situ infrared spectroscopy depth profilometer for organic thin films.

Organic films are widely used in organic optoelectronics due to their flexibility, low-cost fabrication, and ability to be processed over large areas. Typically, the composition of these thin films varies along the film depth direction. In this work, we present a home-developed in situ instrument comprised of a capacitive coupled plasma generator in combination with a Fourier transform infrared spectrometer, to measure the composition distribution along the film-normal direction. During the measurement, the film is sequentially etched by the soft plasma and the evolution of the infrared spectra of the film is in situ monitored by a spectrometer, from which the film-depth-dependent infrared spectra are extracted. The film-depth resolution of this analytical method has been improved to ∼1 nanometer. Thus, it is possible to calculate the composition that varies with depth by utilizing this analysis method. This equipment, which can be applied effectively to the characterization of thin films for both conjugated and unconjugated organic molecules by directly measuring their distinctive molecular vibration signatures, is simple and clear to set up in a large number of laboratories.

Artificial Tactile Recognition Enabled by Flexible Low-Voltage Organic Transistors and Low-Power Synaptic Electronics.

The advancement of self-powered intelligent strain systems for human-computer interaction is crucial toward wearable and energy-saving applications. Simultaneously, lowering operating voltage and thus reducing power consumption are of particular interests. A brain-like smart synaptic hardware system is considered as a promising candidate for low-power, parallel computing and learning processes. However, the combination of low-voltage organic transistors and energy efficient smart synapse hardware systems driven by a tactile signal has been hindered by the limited materials and technology. Here, by employing an elastomeric copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with a high HFP content of 25 mol %, flexible, low-voltage transistors (|VG| ≤ 3 V) and a low energy consumption synapse ≤ 9.2 × 10-17 J are devised simultaneously, along with the lowest quality factor (R = Pw × VG, 2.76 × 10-16 J V). Furthermore, based on the low voltage and low power consumption characteristics, flexible artificial tactile recognition system and Morse code recognition are established without any computing supporting. Mechanical flexibility, cycling stability, image contrast enhancement functions, and simulated pattern recognition accuracy of the multilayer perceptron neural network are also simulated. This work recommends a route of exploiting low voltage, low power consumption synaptic systems and smart human-machine interfaces with low energy loss based on flexible organic synaptic transistors.

Spectroscopic depth profilometry of organic thin films upon inductively coupled plasma etching.

During the deposition and post-treatments of organic films, phase separation along the film-depth direction is a commonly observed phenomenon. Thus, film-depth profilometry of organic thin films and the corresponding scientific instruments are attracting extensive interest. Here, we propose spectroscopic film-depth profilometry of organic thin films upon inductively coupled plasma etching. Compared with capacitively coupled plasma, which usually generates inhomogeneous filamentous discharge, damaging films underneath the etched surface, inductively coupled plasma studied in this work refers to a so-called soft plasma source generated by a well-defined homogenous glow discharge. The absorption spectra of the etched films are monitored by using a spectrometer, from which the film-depth-dependent light absorption spectra are, thus, numerically obtained with a film-depth resolution better than 1 nm. This methodology is available not only for non-conjugated molecules but also for conjugated organic semiconductors, which are usually known as unstable materials for many ionic plasma sources. Organic films for solar cells and field-effect transistors are investigated as model materials to demonstrate the applications of this depth profilometry.

Enhanced Performance of Organic Field-Effect Transistors by a Molecular Dopant with High Electron Affinity.

Organic field-effect transistors (OFETs) are attractive for next-generation electronics, while doping plays an important role in their performance optimization. In this work, a soluble molecular dopant with high electron affinity, CN6-CP, is investigated to manipulate the performance of OFETs with a p-type organic semiconductor as the transport layer. The performance of the model 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT) bottom-gate top-contact (BGTC) OFETs is greatly optimized upon doping by CN6-CP, and the field-effect mobility is improved from 5.5 to 11.1 cm2 V-1 s-1, with a widely tunable threshold voltage from -40 to +5 V. Improvements in performance also appear in CN6-CP doped BGBC OFETs. As compared with commonly used molecular dopant F4-TCNQ, CN6-CP exhibits excellent doping effects and great potential for organic electronic applications.

Improving Charge Injection at Gold/Conjugated Polymer Contacts by Polymer Insulator-Assisted Annealing for Transistors.

The poor chemical miscibility between metal and organic materials usually leads to both structural and energetic mismatches at gold/organic interfaces, and thereby, high contact resistance of organic electronic devices. This study shows that the contact resistance of organic field-effect transistors is significantly reduced by one order of magnitude, by reforming the contact interface between gold electrodes and conjugated polymers upon a polymer insulator-assisted thermal annealing. Upon an optimized solution process, the conjugated polymer is homogenously distributed within the amorphous polymer insulator matrix with relatively low glass transition temperature, and thus, even a moderate annealing temperature can induce sufficient motion of conjugated polymer chains to simultaneously adjust the polymer orientation and improve the packing of gold atoms. Consequently, gold/conjugated polymer contact is reorganized after annealing, which improves both charge transport from bulk gold to interface and charge injection from gold into conjugated polymers. This method, with appropriate insulator matrix, is effective for improving the injection of both holes and electrons, and widely applicable for many unipolar and ambipolar conjugated polymers to optimize the device performance and simultaneously increase the optical transparency (over 80%). A frequency doubler and a phase modulator are demonstrated, respectively, using the ambipolar transistors with optimized charge injection properties.

P3HT-Based Organic Solar Cells with a Photoresponse to 1000 nm Enabled by Narrow Band Gap Nonfullerene Acceptors with High HOMO Levels.

Three narrow band gap (NBG) acceptors, namely, TTDTC-0F, TTDTC-2F, and TTDTC-4F, were synthesized by introducing a strong electron-donating unit as the central core. The enhanced intramolecular charge transfer endows the three acceptors with high-lying highest occupied molecular orbitals (HOMOs) of ∼-5.20 eV and ultranarrow band gaps (∼1.25 eV). When blended with poly(3-hexylthiophene) (P3HT), all organic solar cells (OSCs) exhibited a broad photoresponse from 300 to ∼1000 nm. Among them, P3HT:TTDTC-4F-based devices achieved the highest efficiency of 7.81% with a prominent Jsc exceeding 22 mA·cm-2. This study demonstrates that the conjugated molecules with high HOMOs can also function as acceptor materials for P3HT-based OSCs, which opens a window to increase PCEs of P3HT-based OSCs in the future to the level of the devices based on the current state-of-the-art polymer donor materials.

Crystallization Control of N,N'-Dioctyl Perylene Diimide by Amphiphilic Block Copolymers Containing poly(3-Hexylthiophene) and Polyethylene Glycol.

The preparation of micron- to nanometer-sized functional materials with well-defined shapes and packing is a key process to their applications. There are many ways to control the crystal growth of organic semiconductors. Adding polymer additives has been proven a robust strategy to optimize semiconductor crystal structure and the corresponding optoelectronic properties. We have found that poly(3-hexylthiophene) (P3HT) can effectively regulate the crystallization behavior of N,N'-dioctyl perylene diimide (C8PDI). In this study, we combined P3HT and polyethylene glycol (PEG) to amphiphilic block copolymers and studied the crystallization modification effect of these block copolymers. It is found that the crystallization modification effect of the block copolymers is retained and gradually enhanced with P3HT content. The length of C8PDI crystals were well controlled from 2 to 0.4 µm, and the width from 210 to 35 nm. On the other hand, due to the water solubility of PEG block, crystalline PEG-b-P3HT/C8PDI micelles in water were successfully prepared, and this water phase colloid could be stable for more than 2 weeks, which provides a new way to prepare pollution-free aqueous organic semiconductor inks for printing electronic devices.

In situ Measuring Film-Depth-Dependent Light Absorption Spectra for Organic Photovoltaics.

Organic donor-acceptor bulk heterojunction are attracting wide interests for solar cell applications due to solution processability, mechanical flexibility, and low cost. The photovoltaic performance of such thin film is strongly dependent on vertical phase separation of each component. Although film-depth-dependent light absorption spectra measured by non-in situ methods have been used to investigate the film-depth profiling of organic semiconducting thin films, the in situ measurement is still not well-resolved. In this work, we propose an in situ measurement method in combination with a self-developed in situ instrument, which integrates a capacitive coupled plasma generator, a light source, and a spectrometer. This in situ method and instrument are easily accessible and easily equipped in laboratories of the organic electronics, which could be used to conveniently investigate the film-depth-dependent optical and electronic properties.

Dark Current Reduction Strategy via a Layer-By-Layer Solution Process for a High-Performance All-Polymer Photodetector.

The ideal bulk-heterojunction for high-performance organic photodetectors prefers a morphology with a vertically gradient component to suppress the leaking current. Here, we demonstrate an all-polymer photodetector with a segregated bulk-heterojunction active layer. This all-polymer photodetector exhibits a dramatically reduced dark current density because of its built-in charge blocking layer, with a responsivity of 0.25 A W-1 at a wavelength of approximately 600 nm and specific detectivity of 5.68 × 1012 cm Hz1/2 W-1 as calculated from the noise spectra at 1 kHz. To our knowledge, this is among the best performances reported for photodetectors based on both polymeric donor and acceptor in the photoactive layer. These findings present a facile approach to improving the specific detectivity of polymer photodetectors via a layer-by-layer solution process.

Field-Effect Charge Transport in Doped Polymer Semiconductor-Insulator Alternating Bulk Junctions with Ultrathin Transport Layers.

Conjugated-polymer field-effect transistors are attractive for flexible electronics. However, relatively high chemical doping (oxidation) concentration of p-type polymer semiconductors is usually not compatible with good transistor performance, due to poor switching-off capability and short-channel performance. Here, we propose a combined simulation and experimental investigation on charge transport in a semiconductor-insulator alternating bulk junction composed of repeating semiconductor and insulator regions, which shows better transistor performance at higher doping levels, as compared with traditional planar transistors. Moreover, the doped semiconductor transport layers in the junction should be less than 2 nm thick to ensure sufficient pinch-off capability. Using some semiconductors including poly(3-hexylthiophene), we utilize a fast solvent evaporation approach to obtain semiconductor-insulator alternating bulk junctions with ultrathin (thickness < 2 nm) semiconductor crystallites and with vertical gradients of both morphology and electronic properties. Doping with a concentration of up to 1019 cm-3 simultaneously induces the improvement of field-effect mobility, on/off ratio, and subthreshold swing, which leads to long-term (>1 year) stability, without lowering the short-channel performance. Moreover, these heterojunctions are optically transparent, nearly colorless, and flexible, thus could be exploited for wide electronic and photonic applications.

Reconstructing Space- and Energy-Dependent Exciton Generation in Solution-Processed Inverted Organic Solar Cells.

Photon absorption-induced exciton generation plays an important role in determining the photovoltaic properties of donor/acceptor organic solar cells with an inverted architecture. However, the reconstruction of light harvesting and thus exciton generation at different locations within organic inverted device are still not well resolved. Here, we investigate the film depth-dependent light absorption spectra in a small molecule donor/acceptor film. Including depth-dependent spectra into an optical transfer matrix method allows us to reconstruct both film depth- and energy-dependent exciton generation profiles, using which short-circuit current and external quantum efficiency of the inverted device are simulated and compared with the experimental measurements. The film depth-dependent spectroscopy, from which we are able to simultaneously reconstruct light harvesting profile, depth-dependent composition distribution, and vertical energy level variations, provides insights into photovoltaic process. In combination with appropriate material processing methods and device architecture, the method proposed in this work will help optimizing film depth-dependent optical/electronic properties for high-performance solar cells.

Printing Semiconductor-Insulator Polymer Bilayers for High-Performance Coplanar Field-Effect Transistors.

Source-semiconductor-drain coplanar transistors with an organic semiconductor layer located within the same plane of source/drain electrodes are attractive for next-generation electronics, because they could be used to reduce material consumption, minimize parasitic leakage current, avoid cross-talk among different devices, and simplify the fabrication process of circuits. Here, a one-step, drop-casting-like printing method to realize a coplanar transistor using a model semiconductor/insulator [poly(3-hexylthiophene) (P3HT)/polystyrene (PS)] blend is developed. By manipulating the solution dewetting dynamics on the metal electrode and SiO2 dielectric, the solution within the channel region is selectively confined, and thus make the top surface of source/drain electrodes completely free of polymers. Subsequently, during solvent evaporation, vertical phase separation between P3HT and PS leads to a semiconductor-insulator bilayer structure, contributing to an improved transistor performance. Moreover, this coplanar transistor with semiconductor-insulator bilayer structure is an ideal system for injecting charges into the insulator via gate-stress, and the thus-formed PS electret layer acts as a "nonuniform floating gate" to tune the threshold voltage and effective mobility of the transistors. Effective field-effect mobility higher than 1 cm2 V-1 s-1 with an on/off ratio > 107 is realized, and the performances are comparable to those of commercial amorphous silicon transistors. This coplanar transistor simplifies the fabrication process of corresponding circuits.

Tailoring ultrasound-induced growth of perylene diimide nanowire crystals from solution by modification with poly(3-hexyl thiophene).

Tailoring nanocrystalline morphologies of organic semiconductors holds importance for organic electronics due to the influence of crystal characteristics on optoelectronic properties. Soluble additives that control crystal growth are commonly found in a variety of contexts such as biomineralization, pharmaceutical processing, and food science, while the use of ultrasound to modify crystal nucleation and growth has been routinely employed in producing crystals of food ingredients, biomolecules, pharmaceuticals, and inorganic materials. However, both methods have been applied to the growth of organic semiconductor crystals only in limited fashion. Here, we combine these two approaches to show that colloidally stable nanowire suspensions of a n-type small molecule, perylene diimide (PDI), can be prepared with well-controlled structures by sonocrystallization in the presence of a p-type polymer, poly(3-hexyl thiophene) (P3HT), as a soluble additive. By preferentially adsorbing on lateral crystal faces, P3HT dramatically reduces PDI crystal growth rate in the lateral directions relative to that along the nanowire axis, yielding nanocrystals with widths below 20 nm and narrow width distributions. With the use of uniform short PDI nanowires as seeds and extension with metastable solutions, controlled growth of PDI nanowires by "living crystallization" is demonstrated, providing access to narrowed length distributions and tailored branched crystal morphologies.

Bulk interpenetration network of thermoelectric polymer in insulating supporting matrix.

Thermoelectric properties of conjugated polymers are found to improve upon homogeneously distributing conjugated polymer into an insulating supporting matrix. The local one-dimensional charge transport along the interpenetration conductive network simultaneously leads to lower thermal conductivity, higher electrical conductivity without sacrifice of Seebeck coefficient, and thus a higher figure of merit ZT, as compared with neat conjugated polymer.