Solution-processed Organic Semiconductors Research Articles

Retina-Inspired Large-Area Solution-Processed flexible Multi-Band organic neuromorphic synaptic transistor arrays

The proliferation of high-efficacy, pliable organic neuromorphic synaptic transistors is quintessential for the integration into wearable artificial intelligence ecosystems. Nevertheless, their development is hindered by substantial energy demands and suboptimal charge carrier mobility. To surmount these obstacles, we exploit solution-processed poly(amic acid) (PAA) dielectrics and organic semiconductors to engineer pliable arrays of organic neuromorphic synaptic transistors atop a polyethylene terephthalate (PET) matrix. The polar group and nanogroove structure of PAA obtain excellent semiconductor crystal structure, and the excellent dielectric properties enable these engineered arrays to show a charge carrier mobility peak of 29.83 cm2 V−1 s−1 an aggregate mean mobility of 15.87 cm2 V−1 s−1 (standard deviation is 6.71) at a voltage of −3 V and the energy expenditure per synaptic transaction was only 0.26 fJ. Additionally, the transistors possess the aptitude for photodetection across ultraviolet–visible to near-infrared spectral bands. The successful execution of flexible imaging, Pavlovian classical conditioning reflex associative learning, coupled with a 93.9 % accuracy in handwritten numeral recognition, corroborates the viability of these high-mobility, low-power organic neuromorphic synaptic transistor arrays.

Advances in Charge Carrier Mobility of Diketopyrrolopyrrole-Based Organic Semiconductors

In recent years, the charge carrier mobility study of organic semiconductors has seen significant progress and surpassed that of amorphous silicon thanks to the development of various molecular engineering, solution processing, and external alignment methods. These advances have allowed the implementation of organic semiconductors for fabricating high-performance organic electronic devices. In particular, diketopyrrolopyrrole-based small-molecular and polymeric organic semiconductors have garnered considerable research interest due to their ambipolar charge-carrier properties. In this article, we focus on conducting a comprehensive review of previous studies that are dedicated to the external alignment, thermal annealing, and molecular engineering of diketopyrrolopyrrole molecular structures and side-chain structures in order to achieve oriented crystal orientation, optimized thin-film morphology, and enhanced charge carrier transport. By discussing these benchmark studies, this work aims to provide general insights into optimizing other high-mobility, solution-processed organic semiconductors and sheds lights on realizing the acceleration of organic electronic device applications.

Hall measurements reveal band-like transport in high-mobility solution-processed organic semiconductor films.

Hall measurements reveal band-like transport in high-mobility solution-processed organic semiconductor films.

Ultranarrow-bandgap small-molecule acceptor enables sensitive SWIR detection and dynamic upconversion imaging.

Short-wavelength infrared (SWIR) light detection plays a key role in modern technologies. Emerging solution-processed organic semiconductors are promising for cost-effective, flexible, and large-area SWIR organic photodiodes (OPDs). However, the spectral responsivity (R) and specific detectivity (D*) of SWIR OPDs are restricted by insufficient exciton dissociation and high noise current. In this work, we synthesized an SWIR small molecule with a spectral coverage of 0.3 to 1.3 micrometers peaking at 1100 nanometers. The photodiode, with optimized exciton dissociation, charge injection, and SWIR transmittance, achieves a record high R of 0.53 ampere per watt and D* of 1.71 × 1013 Jones at 1110 nanometers under zero bias. The D* at 1 to 1.2 micrometers surpasses that of the uncooled commercial InGaAs photodiode. Furthermore, large-area semitransparent all-organic upconversion devices integrating the SWIR photodiode realized static and dynamic SWIR-to-visible imaging, along with excellent upconversion efficiency and spatial resolution. This work provides alternative insights for developing sensitive organic SWIR detection.

High-performance fully solution-processed OLEDs based on carbazole-benzonitrile TADF emitter with long alkyl chains

The introduction of alkyl chains into the luminescent molecules will increase solubility and film-forming performance for fabricating solution-processed organic semiconductor devices. In this work, 5CzBN-9CSP and 5CzBN-12CSP with long alkyl chains were synthesized based on the typical thermally activated delayed fluorescence (TADF) molecule 5CzBN as core, and bipolar 9,9′-spirobi[fluorene] (SP) groups as peripheral dendrons. Not only the excellent solution processability is invested, but also the uniform photophysical property is sustained, including small singlet-triplet splitting energy (ΔEST) and high reverse intersystem crossing (RISC) rate. However, excessive length of alkyl chains degrading thermodynamic stability and carrier transport and balance, results in a maximum external quantum efficiency (EQEmax) of 17.6% for 5CzBN-12CSP-based fully solution-processed non-doped device, which is lower than that for 5CzBN-9CSP counterpart. Furthermore, TADF sensitized (TSF) solution-processed device obtains a high EQEmax of 25.9% within 5CzBN-12CSP host and multiple-resonance (MR) final emitter thanks to an efficient energy transfer.

Novel solution-processed 2D organic semiconductor crystals for high-performance OFETs

2D organic semiconductor crystals have the advantage of ultrathin thickness, long-range ordered molecular structures, the absence of grain boundaries, and low defect and impurity densities. They are of great significance for preparing high-performance OFET devices.

Diselenophene-Dithioalkylthiophene Based Quinoidal Small Molecules for Ambipolar Organic Field Effect Transistors.

This work presents a series of novel quinoidal organic semiconductors based on diselenophene-dithioalkylthiophene (DSpDST) conjugated cores with various side-chain lengths (-thiohexyl, -thiodecyl, and -thiotetradecyl, designated DSpDSTQ-6, DSpDSTQ-10, and DSpDSTQ-14, respectively). The purpose of this research is to develop solution-processable organic semiconductors using dicyanomethylene end-capped organic small molecules for organic field effect transistors (OFETs) application. The physical, electrochemical, and electrical properties of these new DSpDSTQs are systematically studied, along with their performance in OFETs and thin film morphologies. Additionally, the molecular structures of DSpDSTQ are determined through density functional theory (DFT) calculations and single-crystal X-ray diffraction analysis. The results reveal the presence of intramolecular S (alkyl)···Se (selenophene) interactions, which result in a planar SR-containing DSpDSTQ core, thereby promoting extended π-orbital interactions and efficient charge transport in the OFETs. Moreover, the influence of thioalkyl side chain length on surface morphologies and microstructures is investigated. Remarkably, the compound with the shortest thioalkyl chain, DSpDSTQ-6, demonstrates ambipolar carrier transport with the highest electron and hole mobilities of 0.334 and 0.463 cm2 V-1 s-1 , respectively. These findings highlight the excellence of ambipolar characteristics of solution-processable OFETs based on DSpDSTQs even under ambient conditions.

Solution-Processable liquid crystalline organic semiconductors based on 6-(Thiophen-2-yl)benzo[b]thiophene for organic thin-film transistors

Liquid crystalline organic semiconductors have garnered significant attention in the realm of organic thin-film transistors (OTFTs) due to their inherent controllability of phase and electrical properties with temperature variations. Typically, liquid crystalline semiconductors possess distinct structures comprising rigid anisotropic cores and flexible chains. Here, we sought to identify liquid crystalline semiconductor materials by appending a thiophene ring and octyl chain to the anisotropic 6-(thiophen-2-yl)benzo[b]thiophene (TBT) backbone. Among the investigated TBT derivatives, the compound featuring additional thiophene ring and octyl chains on both sides exhibited pronounced liquid crystalline characteristics, specifically the SmE phase. The optical, thermal, and electrical characteristics as well as thin film properties of the TBT derivatives were investigated. The TBT compound was employed through solution-shearing as an active layer for top-contact/bottom-gate OTFTs. The liquid-crystalline TBT (2,6-bis(5-octylthiophen-2-yl)benzo[b]thiophene) thin film annealed at low temperature (30 °C) showed hole mobility as high as 0.025 cm2/Vs and current on/off ratio of > 106. This results in a significant enhancement by an order of magnitude compared to the sample annealed at 70 °C in the SmE phase, and nearly a two-fold improvement compared to the performance observed in the sample annealed at 130 °C in the transition phase. The utilization of the TBT derivative as a p-channel material represents a significant step toward the development of novel liquid crystalline semiconductors.

Optoelectronic Synaptic Transistors Based on Solution-Processable Organic Semiconductors and CsPbCl3 Quantum Dots for Visual Nociceptor Simulation and Neuromorphic Computing.

Increasing attention has been paid to organic electronics emulating the characteristics and functions of the peripheral nervous system (PNS) and central nervous system (CNS) in living organisms, which has important implications for exploring electronics from the underlying architecture to emulate biological sensory synapses/neurons and develop brain-like chips. However, the vast majority of current research has been limited to biomimetic electronics that implement the functionality of a single PNS or CNS. Here, we develop solution-processed optoelectronic synaptic transistors (OSTs) that simultaneously simulate the visual nociceptor perception functions of the PNS as well as the memorizing and computing functions of the CNS, where CsPbCl3 quantum dots (QDs) and organic semiconductor serve as the photoactive layer and channel layer, respectively. Benefiting from the distinctive absorption characteristic of CsPbCl3 QDs, the OSTs illustrate decent ultraviolet light selectivity, which can be utilized to mimic visual nociceptor behavior triggered by ultraviolet irradiation for pain perception. Furthermore, the OSTs successfully achieve 1000 conductance states, which also confirms the outstanding conductivity regulation ability of the OSTs and their potential in pattern recognition based on artificial neural networks. This work provides a pathway for the development of future artificial vision and neuromorphic computing using OSTs based on solution-processable organic semiconductors and QDs.

Effects of Processing-Induced Contamination on Organic Electronic Devices.

Organic semiconductors are a family of pi-conjugated compounds used in many applications, such as displays, bioelectronics, and thermoelectrics. However, their susceptibility to processing-induced contamination is not well understood. Here, it is shown that many organic electronic devices reported so far may have been unintentionally contaminated, thus affecting their performance, water uptake, and thin film properties. Nuclear magnetic resonance spectroscopy is used to detect and quantify contaminants originating from the glovebox atmosphere and common laboratory consumables used during device fabrication. Importantly, this in-depth understanding of the sources of contamination allows the establishment of clean fabrication protocols, and the fabrication of organic field effect transistors (OFETs) with improved performance and stability. This study highlights the role of unintentional contaminants in organic electronic devices, and demonstrates that certain stringent processing conditions need to be met to avoid scientific misinterpretation, ensure device reproducibility, and facilitate performance stability. The experimental procedures and conditions used herein are typical of those used by many groups in the field of solution-processed organic semiconductors. Therefore, the insights gained into the effects of contamination are likely to be broadly applicable to studies, not just of OFETs, but also of other devices based on these materials.

Solution-Processable Benzo[b]thieno[2,3-d]thiophene Derivatives as Organic Semiconductors for Organic Thin-Film Transistors

Two new benzo[b]thieno[2,3-d]thiophene (BTT) derivatives, 2-(benzo[b]thiophen-5-yl)benzo[b]thieno[2,3-d]thiophene (compound 2), and 2-(benzo[b]thieno[2,3-d]thiophene-2yl)dibenzo[b,d]thiophene (compound 3) have been synthesized and utilized as solution-processable small molecular organic semiconductors for organic field-effect transistors (OFETs). The physicochemical characteristics of the recently created substances were analyzed using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and UV-vis spectroscopy. Subsequently, the above-mentioned substances were employed as semiconductor layers in bottom-gate/top-contact OFETs through solution shearing methods for device fabrication, and their electrical performances were meticulously evaluated. The outcoming OFET device displayed p-channel behavior, demonstrating hole mobility of up to 0.005cm2/Vs and a current on/off ratio higher than 106.

Meniscus‐Guided Deposition of Organic Semiconductor Thin Films: Materials, Mechanism, and Application in Organic Field‐Effect Transistors

Solution-processable organic semiconductors are one of the promising materials for the next generation of organic electronic products, which call for high-performance materials and mature processing technologies. Among many solution processing methods, meniscus-guided coating (MGC) techniques have the advantages of large-area, low-cost, adjustable film aggregation, and good compatibility with theroll-to-roll process, showing good research results in the preparation of high-performance organic field-effect transistors. In this review, the types of MGC techniques are first listed and the relevant mechanisms (wetting mechanism, fluid mechanism, and deposition mechanism) are introduced. The MGC processes are focused and the effect of the key coating parameters on the thin film morphology and performance with examples is illustrated. Then, the performance of transistors based on small molecule semiconductors and polymer semiconductor thin films prepared by various MGC techniques is summarized. In the third section, various recent thin film morphology control strategies combined with the MGCs are introduced. Finally, the advanced progress of large-area transistor arrays and the challenges for roll-to-roll processes are presented using MGCs. Nowadays, the application of MGCs is still in the exploration stage, its mechanism is still unclear, and the precise control of film deposition still needs experience accumulation.

Controlling the molecular orientation of a novel diketopyrrolopyrrole-based organic conjugated polymer for enhancing the performance of organic field-effect transistors

A novel organic conjugated copolymer p(DPP-BZC) was utilized as a solution-processable organic semiconductor and its large area of the unidirectionally oriented thin film was fabricated by the Ribbon-shaped floating film transfer (FTM) method. Organic field-effect transistors (OFETs fabricated using oriented thin films of this polymer demonstrated a clear p-type semiconducting behaviour and anisotropic charge carrier transport. A maximum orientation with a dichroic ratio of 5.1 was obtained in the FTM-processed thin film under optimized conduction with liquid substrate viscosity, temperature and polymer solution concentration of 6.12 cst, 50 °C and 3% (w/v), respectively. Out-of-plane XRD and in-plane GIXD results revealed that oriented thin films of this copolymer are semicrystalline and adopt purely edge-on on conformation. The implication of surface modification of Si/SiO 2 gate dielectric utilizing HMDS. OTS and CYTOP on the charge carrier transport revealed that substrate with high-surface-energy hampers the on/off ratio, operating voltage and charge carrier mobility (μ). On the other hand, Si/SiO 2 substrate treated with CYTOP having relatively lower surface energy demonstrated the best device performance with the highest μ of 2.7 x 10 −2 cm 2 /Vs, which is > 30 times higher as compared to that of its non-oriented spin-coated device counterparts an μ of 8.0 x 10 −4 cm 2 /Vs. • Synthesis of novel conjugated polymer p(DPP-BZC) and utilization of its oriented thin films for OFET fabrication. • Utilizing the FTM technique an oriented thin film with DR of 5.1 was achieved under optimized conditions. • Purely edge-on orientation in the oriented film was confirmed by XRD and GIXD. • Surface passivation was found to be desired and amongst various surface modifiers used, CYTOP exhibited the best performance. • OFETs utilizing oriented thin films exhibited the mobility of 2.7 x 10 −2 cm 2 /Vs, which was >30 times higher than corresponding spin-coated devices.

High-speed hybrid complementary ring oscillators based on solution-processed organic and amorphous metal oxide semiconductors

Solution-processed single-crystal organic semiconductors (OSCs) and amorphous metal oxide semiconductors (MOSs) are promising for high-mobility p- and n-channel thin-film transistors (TFTs), respectively. Organic−inorganic hybrid complementary circuits hence have great potential to satisfy practical requirements. However, some chemical incompatibilities between OSCs and MOSs, such as heat and chemical resistance, make it difficult to rationally integrate TFTs based on solution-processed OSC and MOS onto the same substrates. Here, we report a rational integration method based on the solution-processed semiconductors by carefully managing the device configuration and the deposition and patterning techniques from a materials point of view. The balanced high performances as well as the uniform fabrication of the TFTs led to densely integrated five-stage ring oscillators with a short propagation delay of 1.3 µs per stage.

Characterization of [1]Benzothieno[3,2-b]benzothiophene (BTBT) Derivatives with End-Capping Groups as Solution-Processable Organic Semiconductors for Organic Field-Effect Transistors

Solution-processable [1]benzothieno[3,2-b]benzothiophene (BTBT) derivatives with various end-capping groups, 2-(phenylethynyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (Compound 1), 2-octyl-7-(5-(phenylethynyl)thiophen-2-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (Compound 2), and triisopropyl((5-(7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)thiophen-2-yl)ethynyl)silane (Compound 3), have been synthesized and characterized as active layers for organic field-effect transistors (OFETs). Thermal, optical, and electrochemical properties of the newly synthesized compounds were characterized using thermogravimetric analysis (TGA), a differential scanning calorimeter (DSC), UV–vis spectroscopy, and cyclic voltammetry (CV). Thin films of each compound were formed using the solution-shearing method and the thin film surface morphology and texture of the corresponding films were characterized using atomic force microscopy (AFM) and θ–2θ X-ray diffraction (XRD). All semiconductors exhibited p-channel characteristics in ambient and Compound 1 showed the highest electrical performance with a carrier mobility of ~0.03 cm2/Vs and current on/off ratio of ~106.

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor-Acceptor-Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization.

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules TRA-IID-TRA and TRA-TIID-TRA comprising an electron-donating triarylamine (TRA) and two different electron-withdrawing cores, isoindigo (IID) and thienoisoindigo (TIID), respectively, were synthesized and characterized. Replacing the phenylene rings of central IID A with thiophene gives a TIID core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that TRA-TIID-TRA exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like TRA-TIID-TRA crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm2 V-1 s-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm2 V-1 s-1) of TRA-IID-TRA with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.

Development and Characterization of Solution-Processable Dithieno[3,2-b : 2',3'-d]thiophenes Derivatives with Various End-capped Groups for Organic Field-Effect Transistors.

In this paper, four organic materials based on dithieno[3,2-b : 2',3'-d]thiophene (DTT) core structure with end-capping groups (phenyl and thienyl) and linker (acetylenic and olefinic) between DTT-core and end-capping groups were synthesized and characterized as solution-processable organic semiconductors (OSCs) for organic field-effect transistors (OFETs). Thermal, optical, and electrochemical properties of the corresponding materials were determined. Next, all DTT-derivatives were coated by solution-shearing method, and the thin-film microstructures and morphologies were investigated. To investigate the electrical performance of four newly synthesized DTT-derivatives, bottom-gate/top-contact OFETs were fabricated and characterized in ambient condition. It was found that substitution of acetylenic for olefinic linkers between DTT-cores and end-capping groups enhanced device performance. Especially, the resulting OFETs based on the compound containing phenylacetylene exhibited the highest hole mobility of 0.15 cm2 /Vs and current on/off ratio of ∼106 , consistent with film morphology and texture showing long range interconnected crystalline grains and strong diffraction peaks.

Effects of alkyl chain on the liquid crystalline properties of [1]benzothieno[3,2-b][1]benzothiophene-based organic semiconductors

New [1]benzothieno[3,2- b ] [1]benzothiophene (BTBT)-based small molecules with different substituents were synthesized and employed as solution-processable organic semiconductors for organic thin-film transistors (OTFTs). Two types of alkyl side chains (octyl/ethylhexyl) were selected for the solubility enhancement and analysis of material phase transition. Thermal, optical, and electrical properties of new compounds were analyzed and characterized. Synthesized BTBT derivatives exhibited completely different phase transition features depending on the attached side chains; linearly alkylated 2-octyl-7-(5-octylthiophen-2-yl)benzo[ b ]benzo [4,5]thieno[2,3- d ]thiophene (compound 1 ) showed decent carrier mobility up to 0.26 cm 2 /Vs at highly ordered smectic X phase, while 2-(5-(2-ethylhexyl)thiophen-2-yl)-7-octylbenzo[ b ]benzo [4,5]thieno[2,3- d ]thiophene (compound 2 ) with branched alkyl chain showed poor electrical performance. Thin film morphology and microstructure were investigated using atomic force microscopy and X-ray diffraction to reveal the dramatic property difference between the two dialkylated BTBT derivatives. • Solution-processable BTBT-based organic semiconductors were synthesized. • Analysis on alkyl side chain type; linear and branch-like substituents was conducted. • Linearly alkylated compound showed smectic liquid crystalline properties. • Organic transistors with LC phase showed mobility up to 0.26 cm 2 /Vs.

Chlorine‐Substituted N‐Heteroacene Analogues Acting as Organic Semiconductors for Solution‐Processed n‐type Organic Field‐Effect Transistors

High performance solution processable n-type organic semiconductor is an essential element to realize low-cost, all organic and flexible composite logic circuits. In the design of n-type semiconducting materials, tuning the LUMO level of compounds is a key point. As a strong electron withdrawing unit, the introduction of chlorine atom into the chemical structure can increase the electron affinity of the material and reduce the LUMO energy level. Here, a series chlorine substituted N-heteroacene analogues of 6,7,8,9-tetrachloro-4,11-bis(4-((2-ethylhexyl)oxy)phenyl)-[1,2,5]thiadiazolo[3,4-b]phenazine (O4Cl), 6,7,8,9-tetrachloro-4,11-bis(4-((2-ethylhexyl)thio)phenyl)-[1,2,5]thiadiazolo[3,4-b]phenazine (S4Cl), 1,2,3,4,8,9,10,11-octachloro-6,13-bis(4-((2-ethylhexyl)oxy)phenyl)quinoxalino[2,3-b]phenazine (8Cl) and 12Cl have been synthesized and characterized. Solution-processed organic field-effect transistors (OFETs) based on these four compounds exhibit good electron mobilities of 0.04 cm2 V-1 s-1 , 0.01 cm2 V-1 s-1 , 2×10-3 cm2 V-1 s-1 and 3×10-3 cm2 V-1 s-1 , respectively, under ambient conditions. The results suggest that these chlorine substituted π-conjugated N-heteroacene analogues are promising n-type semiconductors in OFET applications.

Molecular Engineering of Printed Semiconducting Blends to Develop Organic Integrated Circuits: Crystallization, Charge Transport, and Device Application Analyses.

Solution-based printing has contributed to the facile deposition of various types of materials, including the building blocks of printed electronics. In particular, solution-processable organic semiconductors (OSCs) are regarded as one of the most fascinating candidates for the fabrication of printed electronics. Herein, we report electrohydrodynamic (EHD) jet-printed p- and n-type OSCs, namely 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) and 6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14-tetraazapentacene (TIPS-TAP), and their use as single-OSC layers and as OSC mixed p-n layers to fabricate solution-processed p-, n-, and ambipolar-type organic field-effect transistors (OFETs). Use of the dragging mode of EHD jet printing, a process driven under a low electrostatic field with a short nozzle-to-substrate distance, was found to provide favorable conditions for growth of TIPS-PEN and TIPS-TAP crystals. In this way, the similar molecular structures of TIPS-PEN and TIPS-TAP yielded a homogeneous solid solution and showed ambipolar transport properties in OFETs. Therefore, the combination of single- and mixed-OSC layers enabled the preparation of various charge-transported devices from unit to integrated devices (NOT, NAND, NOR, and multivalued logic). Therefore, this fabrication technology can be useful for assisting in the production of OSC layers for practical applications in the near future.