Enhancing Stability of n-Type Doped Organic Semiconductors through Low Boiling Point Solvent-Assisted Room Temperature Drying.

The development of n-type organic semiconductors (OSCs) has been lagged behind that of p-type OSCs, mainly due to the limited availability of the electron deficient π-conjugated backbones and facile electron trapping by ambient oxidants. Improving the performance of n-type OSCs through n-doping is essential for realizing p-n junction diodes and complementary circuits. Conventional vacuum deposition doping is costly and time-consuming, while solution doping risks thermal damage through necessary annealing. Therefore, the development of a simpler, more affordable n-doping method is crucial. In this study, we have developed a solution-processed n-doping method using an organic cationic dye in a low boiling point solvent that can be dried at room temperature in 1 h, which eliminates the need for annealing. The effects of different organic cationic dyes and reducing agents on the n-type OSC were evaluated. After n-doping, electron mobility and photoresponsivity in the sample increased by 5.5 and 20 times, respectively, compared to undoped samples. Furthermore, there was no significant degradation in the electron mobility of the n-doped samples under ambient conditions after 15 days. Studying n-doping with various organic cationic dyes in different OSC materials, embracing further research into their applications and mechanisms, would advance the field of organic electronics.

Electro-Optically Configurable Synaptic Transistors With Cluster-Induced Photoactive Dielectric Layer for Visual Simulation and Biomotor Stimuli.

The integration of visual simulation and biorehabilitation devices promises great applications for sustainable electronics, on-demand integration and neuroscience. However, achieving a multifunctional synergistic biomimetic system with tunable optoelectronic properties at the individual device level remains a challenge. Here, an electro-optically configurable transistor employing conjugated-polymer as semiconductor layer and an insulating polymer (poly(1,8-octanediol-co-citrate) (POC)) with clusterization-triggered photoactive properties as dielectric layer is shown. These devices realize adeptly transition from electrical to optical synapses, featuring multiwavelength and multilevel optical synaptic memory properties exceeding 3 bits. Utilizing enhanced optical memory, the images learning and memory function for visual simulation are achieved. Benefiting from rapid electrical response akin to biological muscle activation, increased actuation occurs under increased stimulus frequency of gate voltage. Additionally, the transistor on POC substrate can be effectively degraded in NaOH solution due to degradation of POC. Pioneeringly, the electro-optically configurability stems from light absorption and photoluminescence of the aggregation cluster in POC layer after 200 °C annealing. The enhancement of optical synaptic plasticity and integration of motion-activation functions within a single device opens new avenues at the intersection of optoelectronics, synaptic computing, and bioengineering.

Bioinspired Flexible and Low-Voltage Organic Synaptic Transistors for UV Light-Driven Vision Systems.

Neuromorphic vision systems, particularly those stimulated by ultraviolet (UV) light, hold great potential applications in portable electronics, wearable technology, biological analysis, military surveillance, etc. Organic artificial synaptic devices hold immense potential in this field due to their ease of processing, flexibility, and biocompatibility. In this work, we have fabricated a flexible organic field-effect transistor (OFET) that utilizes chitosan-silver nanoparticles (AgNPs) composite material as the active dielectric material. During UV light illumination, both silver nanoparticles and the pentacene layer generate a large number of charge carriers. The photogenerated carriers lead to a more significant hole accumulation at the pentacene interface, resulting in a current rise. In the absence of light, the trapped electron in the silver nanoparticles persists for a longer duration, preventing the instant recombination with holes. This extended retention of electrons leads to the observed synaptic performance of the transistor. The use of aluminum oxide (Al2O3) as one of the dielectric layers enables the device to operate effectively at low voltage (<1 V). The device mimics various crucial synaptic properties of the brain, including short-term potentiation and long-term potentiation (STP and LTP), paired-pulse facilitation (PPF), spike-duration dependent plasticity (SDDP), spike-number dependent plasticity (SNDP), and spike-rate dependent plasticity (SRDP), etc. This work introduces an approach to develop flexible organic synaptic transistors that operate efficiently at low voltages, paving the way toward high-performance, UV light-driven neuromorphic vision systems.

Unusual Cross-Conjugation in Cyclic Dipeptide-Based Building Blocks for Donor-Acceptor Semiconducting Conjugated Polymers.

Cross conjugation, though prevalent in many organic compounds, is typically considered less effective for electron delocalization compared to linear conjugation. Consequently, it is rarely used as the backbone structure for semiconducting conjugated polymers. In this study, we designed and synthesized a novel building block, TIDP, which features a central cyclic dipeptide with cross conjugation characteristics. Strong intramolecular hydrogen bonding interactions confer TIDP with a highly rigid and coplanar conformation. Importantly, theoretical calculations reveal that π electrons are well delocalized across the entire structure, despite its low aromaticity. Conjugated polymers incorporating TIDP exhibit high charge carrier mobilities, demonstrating the effective π electrons delocalization of this innovative building block. Our findings show that with rational design, cross conjugation can achieve effective π electrons delocalization, providing a valuable approach for developing high-performance conjugated polymers for organic electronic materials.

Dye-Containing Polymers with π-Extened Diketopyrropyrrole Derivatives for Semi-Transparent Organic Photovoltaics.

Dye-containing polymers P1 (PEDPP-OT-BDT) and P2 (PEDPP-OT-BDTT) including a π-extended diketopyropyrrole (DPP) derivative and electron-rich thiophene fused ring units (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene for P1 and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene for P2) were synthesized as narrow band gap dyes. A π-extended DPP (EDPP-OT-BrPh), fragment of the polymers P1 and P2, was obtained by extending the π-conjugation of DPP using Ru(III)-catalyzed C-H and N-H activation reported by Gonka et al. in 2019, exhibiting a high quantum yield (ϕem=0.84) and small HOMO-LUMO gap (Eg=1.69 eV) due to the spatial overlap of the HOMO and LUMO orbitals. The solubility of the π-extended DPP was improved by introducing four 2-octylthophene side chains around the periphery of the planer dye moiety, while maintaining the high planarity of the dye molecule, which is essential to the function of optoelectronic devices. As a result, P1 and P2, polymerized with the π-extended DPP and BDT derivatives, exhibit carrier mobility of approximately 10-5 cm2/Vs in organic field-effect transistors (OFETs). In bulk heterojunction (BHJ) solar cells with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), they demonstrate a power conversion efficiency (PCE) of 1.0 % with an average transmittance (AVTs) of around 60 %.

Fully Fused Indacenodithiophene-Centered Small-Molecule n-Type Semiconductors for High-Performance Organic Electronics.

Developing novel n-type organic semiconductors is an on-going research endeavour, given their pivotal roles in organic electronics and their relative scarcity compared to p-type counterparts. In this study, a new strategy was employed to synthesize n-type organic semiconductors featuring a fully fused conjugated backbone. By attaching two sets of adjacent amino and formyl groups to the indacenodithiophene-based central cores and triggering a tandem reaction sequence of a Knoevenagel condensation-intramolecular cyclization, DFA1 and DFA2 were realized. The solution-processed organic field effect transistors based on DFA1 exhibited unipolar n-type transport character with a decent electron mobility of ca. 0.10 cm2 V-1 s-1 (ca. 0.038 cm2 V-1 s-1 for DFA2 based devices). When employing DFA1 as a third component in organic solar cells, a high power conversion efficiency of 19.2 % can be achieved in ternary devices fabricated with PM6 : L8-BO : DFA1. This work provides a new pathway in the molecular engineering of n-type organic semiconductors, propelling relevant research forward.

Long-range polymer ordering by directional coating to remarkably enhance the charge carrier mobility in PCDTPT-based organic field-effect transistors.

With the wide potential of organic field-effect transistors in all the modern electronic circuitries, researchers are grappling with the challenge of poor charge transport and hence lower mobility in organic polymers. Low-charge carrier mobility is mainly due to disorder in the molecular packing of organic semiconductors along with other factors, such as impurities, defects and interactions between molecules. The current research work has been conducted to align the molecular chains of poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-|||b:5,4-|b']|dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT) using directional coating techniques such as dip coating and brush coating on nano-grooved substrates. Long-range order of polymer chains was clearly observed along the direction of brush coating and nanogrooves in optical and atomic force microscope (AFM) images while transmission spectra confirmed decreased pi-pi stacking for the polymer films deposited by this technique. By comparing the mobility performance of brush-coated devices with other techniques, we found a remarkable mobility enhancement of 90 times that of conventional spin-coated device and 24 times enhancement compared with the dip-coated device for the case when the alignment of polymer chains was parallel to the channel. All the fabrication and characterizations were performed in the ambient environment. This study demonstrates a potential approach to align the polymers on long and short ranges hence providing a route for high-performing devices in ambient conditions.

Nanostructured Molecular Packing of Polymer Films Formed on Water Surfaces.

In this study, we examined the nanostructured molecular packing and orientations of poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) films formed on water for the application of nanotechnology-based organic electronic devices. First, the nanoscale molecule-substrate interaction between the polymer and water was modulated by controlling the alkyl side chain length in NDI-based copolymers. Increasing alkyl side chain lengths induced a nanomorphological transition from face-on to edge-on orientation, confirmed by molecular dynamics simulations revealing nanostructural behavior. Second, the nanoscale intermolecular interactions of P(NDI2OD-T2) were controlled by varying the volume ratio of the high-boiling-point additive solvent in the binary solvent blends. As the additive solvent ratio increased, the nanostructured molecular orientation of the P(NDI2OD-T2) films on water changed remarkably from edge-on to bimodal with more face-on crystallites, thereby affecting charge transport. Our finding provides essential insights for precise nanoscale morphological control on water substrates, enabling the formation of high-performance polymer films for organic electronic devices.

Dual Optoelectronic Organic Field-Effect Device: Combination of Electroluminescence and Photosensitivity.

Merging the functionality of an organic field-effect transistor (OFET) with either a light emission or a photoelectric effect can increase the efficiency of displays or photosensing devices. In this work, we show that an organic semiconductor enables a multifunctional OFET combining electroluminescence (EL) and a photoelectric effect. Specifically, our computational and experimental investigations of a six-ring thiophene-phenylene co-oligomer (TPCO) revealed that this material is promising for OFETs, light-emitting, and photoelectric devices because of the large oscillator strength of the lowest-energy singlet transition, efficient luminescence, pronounced delocalization of the excited state, and balanced charge transport. The fabricated OFETs showed a photoelectric response for wavelengths shorter than 530 nm and simultaneously EL in the transistor channel, with a maximum at ~570 nm. The devices demonstrated an EL external quantum efficiency (EQE) of ~1.4% and a photoelectric responsivity of ~0.7 A W-1, which are among the best values reported for state-of-the-art organic light-emitting transistors and phototransistors, respectively. We anticipate that our results will stimulate the design of efficient materials for multifunctional organic optoelectronic devices and expand the potential applications of organic (opto)electronics.

Clarifying the Dominant Role of Crystallinity and Molecular Orientation in Differently Processed Thin Films of Regioregular Poly(3-hexylthiophene).

Conjugated polymers (CPs) offer the potential for sustainable semiconductor devices due to their low cost and inherent molecular self-assembly. Enhanced crystallinity and molecular orientation in thin films of solution-processable CPs have significantly improved organic electronic device performance. In this work, three methods, namely spin coating, dip coating, and unidirectional floating-film transfer method (UFTM), were utilized with their parametric optimization for fabricating RR-P3HT films. These films were then utilized for their characterization via optical and microstructural analysis to elucidate dominant roles of molecular orientation and crystallinity in controlling charge transport in organic field-effect transistors (OFETs). OFETs fabricated by RR-P3HT thin films using spin coating and dip coating displayed field-effect mobility (µ) of 8.0 × 10-4 cm2V-1s-1 and 1.3 × 10-3 cm2V-1s-1, respectively. This two-time enhancement in µ for dip-coated films was attributed to its enhanced crystallinity. Interestingly, UFTM film-based OFETs demonstrated µ of 7.0 × 10-2 cm2V-1s-1, >100 times increment as compared to its spin-coated counterpart. This superior device performance is attributed to the synergistic influence of higher crystallinity and molecular orientation. Since the crystallinity of dip-coated and UFTM-thin films are similar, ~50 times improved µ of UFTM thin films, this suggests a dominant role of molecular orientation as compared to crystallinity in controlling the charge transport.

Recent Progress of Fluorinated Conjugated Polymers.

In recent years, conjugated polymers have received widespread attention due to their characteristic advantages of light weight, favorable solution processability, and structural modifiability. Among various conjugated polymers, fluorinated ones have developed rapidly to achieve high-performance n-type or ambipolar polymeric semiconductors. The uniqueness of fluorinated conjugated polymers contains the high coplanarity of their structures, lower frontier molecular orbital energy levels, and strong nonbonding interactions. In this review, first the fluorinated building blocks, including fluorinated benzene and thiophene rings, fluorinated B←N bridged units, and fluoroalkyl side chains are summarized. Subsequently, different synthetic methods of fluorinated conjugated polymers are described, with a special focus on their respective advantages and disadvantages. Then, with these numerous fluorinated structures and appropriate synthetic methods bear in mind, the properties and applications of the fluorinated conjugated polymers, such as cyclopentadithiophene-, amide-, and imide-based polymers, and B←N embedded polymers, are systematically discussed. The introduction of fluorine atoms can further enhance the electron-deficiency of the backbone, influencing the charge carrier transport performance. The promising fluorinated conjugated polymers are applied widely in organic field-effect transistors, organic solar cells, organic thermoelectric devices, and other organic opto-electric devices. Finally, the outlook on the challenges and future development of fluorinated conjugated polymers is systematically discussed.

Interfacial Charge Transport Enhancement of Liquid-Crystalline Polymer Transistors Enabled by Ionic Polyurethane Dielectric.

In organic field-effect transistors (OFETs) using disordered organic semiconductors, interface traps that hinder efficient charge transport, stability, and device performance are inevitable. Benchmark poly(9,9-dioctylfuorene-co-bithiophene) (F8T2) liquid-crystalline polymer semiconductor has been extensively investigated for organic electronic devices due to its promising combination of charge transport and light emission properties. This study demonstrates that high-capacitance single-layered ionic polyurethane (PU) dielectrics enable enhanced charge transport in F8T2 OFETs. The ionic PU dielectrics are composed of a mild blending of PU ionogel and PU solution, thereby forming a solid-state film with robust interfacial characteristics with semiconductor layer and gate electrode in OFETs and measuring high capacitance values above 10 µF cm-2 owing to the combined dipole polarization and electric double layer formation. The optimized fabricated ionic PU-gated OFETs exhibit a low-voltage operation at -3 V with a remarkable hole mobility of over 5 cm2 V-1 s-1 (average = 2.50 ± 1.18 cm2 V-1 s-1), which is the highest mobility achieved so far for liquid-crystalline F8T2 OFETs. This device also provides excellent bias-stable characteristics in ambient air, exhibiting a negligible threshold voltage shift of -0.03 V in the transfer curves after extended bias stress, with a reduced trap density.

Ring Fusion Elevates the Electronic Mobility of Azabenzannulated Perylene Diimide.

The backwardness of n-type organic semiconductors still exists compared with the p-type counterparts. Thus, the development of high-performance n-type organic semiconductors is of great importance for organic electronic devices and their integrated circuits. In recent years, azabenzannulated perylene diimide (PDI), as one of immense bay-region-annulated PDI derivatives, has drawn considerable attentions. However, the electronic mobilities of azabenzannulated PDI derivatives are barely satisfactory. In this contribution, the peripheral benzene ring in azabenzannulated PDI 2 was fused to the ortho position by intramolecular C-H arylation cyclization. This endows the resultant azabenzannulated PDI 4 a planar configuration as well as electron deficient pentagonal ring. As a result, the electronic mobility of 4 is almost two orders of magnitude higher than that of the nonfused azabenzannulated PDI 2. This work shall pave a new avenue in elevating the performance of azabenzannulated PDI in organic electronics.

Tuning Electrode Work Function and Surface Energy for Solution Shearing High-Performance Organic Field-Effect Transistors.

A bottom-contact organic field-effect transistor (OFET) is easily adaptable to the standard lithography process because the contact electrodes are deposited before the organic semiconductor (OSC). However, the low surface energy of bare electrodes limits utilizing solution-processed single-crystal OSCs. Additionally, the bare electrode usually leads to a significant charge injection barrier, owing to its relatively low work function (WF). Here, we simultaneously improved the surface energy and WF of gold electrodes by conducting oxygen plasma treatment to achieve high-performance OFET based on solution-processed organic single crystals. We cultivated a thin layer of gold oxide on Au electrodes to increase the WF by ∼0.7 eV. The surface energy of Au electrodes was enhanced to the same as AlOx dielectric surface, enabling the seamless growth of large-area C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene) organic single-crystal thin films via solution shearing. This technique facilitates the production of high-performance OFETs with the highest carrier mobility of 6.7 cm2 V-1 s-1 and sharp switching characterized by a subthreshold swing of 63.6 mV dec-1. The bottom-contact OFETs exhibited a lower contact resistance of 1.19 kΩ cm than its F4-TCNQ-doped top-contact control device. This method offers a straightforward and effective strategy for producing high-quality single-crystal OFETs, which are potentially suitable for commercial applications.

Large-Language-Model-Based AI Agent for Organic Semiconductor Device Research.

Large language models (LLMs) have attracted widespread attention recently, however, their application in specialized scientific fields still requires deep adaptation. Here, an artificial intelligence (AI) agent for organic field-effect transistors (OFETs) is designed by integrating the generative pre-trained transformer 4 (GPT-4) model with well-trained machine learning (ML) algorithms. It can efficiently extract the experimental parameters of OFETs from scientific literature and reshape them into a structured database, achieving precision and recall rates both exceeding 92%. Combined with well-trained ML models, this AI agent can further provide targeted guidance and suggestions for device design. With prompt engineering and human-in-loop strategies, the agent extracts sufficient information of 709 OFETs from 277 research articles across different publishers and gathers them into a standardized database containing more than 10 000 device parameters. Using this database, a ML model based on Extreme Gradient Boosting is trained for device performance judgment. Combined with the interpretation of the high-precision model, the agent has provided a feasible optimization scheme that has tripled the charge transport properties of 2,6-diphenyldithieno[3,2-b:2',3'-d]thiophene OFETs. This work is an effective practice of LLMs in the field of organic optoelectronic devices and expands the research paradigm of organic optoelectronic materials and devices.

Structure-property relationships in dicyanopyrazinoquinoxalines and their hydrogen-bonding-capable dihydropyrazinoquinoxalinedione derivatives.

Presented here is the design, synthesis, and study of a variety of novel hydrogen-bonding-capable π-conjugated N-heteroacenes, 1,4-dihydropyrazino[2,3-b]quinoxaline-2,3-diones (DPQDs). The DPQDs were accessed from the corresponding weakly hydrogen-bonding dicyanopyrazinoquinoxaline (DCPQ) suspensions with excess potassium hydroxide, resulting in moderate to good yields. Both families of compounds were analyzed by UV-vis and NMR spectroscopy, where the consequences of hydrogen bonding capability could be assessed through the structure-property studies. Conversion of the DCPQs into hydrogen-bonding capable DPQDs results in modulation of frontier MO energies, higher molar extinction coefficients, enhanced crystallinity, and on-average higher thermal stability (where in some cases the 5% weight loss temperature is increased by up to 100 °C). Single crystal X-ray diffraction data could be obtained for three DPQDs. One reveals pairwise hydrogen bonding in the solid state as well as a herringbone packing arrangement rendering it a promising candidate for additional studies in the context of organic optoelectronic devices.

Enhancing Sensitivity in Gas Detection: Porous Structures in Organic Field-Effect Transistor-Based Sensors.

Gas detection is crucial for detecting environmentally harmful gases. Organic field-effect transistor (OFET)-based gas sensors have attracted attention due to their promising performance and potential for integration into flexible and wearable devices. This review examines the operating mechanisms of OFET-based gas sensors and explores methods for improving sensitivity, with a focus on porous structures. Researchers have achieved significant enhancements in sensor performance by controlling the thickness and free volume of the organic semiconductor layer. Additionally, innovative fabrication techniques like self-assembly and etching have been used to create porous structures, facilitating the diffusion of target gas molecules, and improving sensor response and recovery. These advancements in porous structure fabrication suggest a promising future for OFET-based gas sensors, offering increased sensitivity and selectivity across various applications.

Understanding Quasi-Static and Dynamic Characteristics of Organic Ferroelectric Field Effect Transistors.

Leveraging poly(vinylidene fluoride-trifluoroethylene) [(PVDF-TrFE)] as the dielectric, we fabricated organic ferroelectric field-effect transistors (OFe-FETs). These devices demonstrate quasi-static transfer characteristics that include a hysteresis window alongside transient phenomena that bear resemblance to synaptic plasticity-encapsulating excitatory postsynaptic current (EPSC) as well as both short-term and long-term potentiation (STP/LTP). We also explore and elucidate other aspects such as the subthreshold swing and the hysteresis window under dynamic state by varying the pace of voltage sweeps. In addition, we developed an analytical model that describes the electrical properties of OFe-FETs, which melds an empirical formula for ferroelectric polarization with a compact model. This model agrees well with the experimental data concerning quasi-static transfer characteristics, potentially serving as a quantitative tool to improve the understanding and design of OFe-FETs.

Nonhalogenated Solvent Processable and High-Density Photopatternable Polymer Semiconductors Enabled by Incorporating Hydroxyl Groups in the Side Chains.

Polymer semiconductors hold tremendous potential for applications in flexible devices, which is however hindered by the fact that they are usually processed by halogenated solvents rather than environmentally more friendly solvents. An effective strategy to boost the solubility of high-performance polymer semiconductors in nonhalogenated solvents such as tetrahydrofuran (THF) by appending hydroxyl groups in the side chains is herein presented. The results show that hydroxyl groups, which can be easily incorporated into the side chains, can significantly improve the solubility of typical p- and n-types as well as ambipolar polymer semiconductors in THF. Meanwhile, the thin films of these polymer semiconductors from the respective THF solutions show high charge mobilities. With THF as the processing and developing solvents these polymer semiconductors with hydroxyl groups in the side chains can be well photopatterned in the presence of the photo-crosslinker, and the charge mobilities of the patterned thin films are mostly maintained by comparing with those of the respective pristine thin films. Notably, THF is successfully utilized as the processing and developing solvent to achieve high-density photopatterning with ≈82 000 device arrays cm-2 for polymer semiconductors in which hydroxyl groups are appended in the side chains.

Low Contact Resistance Organic Single-Crystal Transistors with Band-Like Transport Based on 2,6-Bis-Phenylethynyl-Anthracene.

Contact resistance has become one of the main bottlenecks that hinder further improvement of mobility and integration density of organic field-effect transistors (OFETs). Much progress has been made in reducing contact resistance by modifying the electrode/semiconductor interface and decreasing the crystal thickness, however, the development of new organic semiconductor materials with low contact resistance still faces many challenges. Here, 2,6-bis-phenylethynyl-anthracene (BPEA) is found, which is a material that combines high mobility with low contact resistance. Single-crystal BEPA OFETs with a thickness of ≈20 nm demonstrated high mobility of 4.52 cm2 V-1 s-1, contact resistance as low as 335 Ω cm, and band-like charge transport behavior. The calculated compatibility of the EHOMO of BPEA with the work function of the Au electrode, and the decreased |EHOMO-ΦAu| with the increase of external electric field intensity from source to gate both contributed to the efficient charge injection and small contact resistance. More intriguingly, p-type BPEA as a buffer layer can effectively reduce the contact resistance, improve the mobility, and meanwhile inhibit the double-slope electrical behavior of p-channel 2,6-diphenyl anthracene (DPA) single-crystal OFETs.