Fabrication Of Organic Field-effect Transistors Research Articles

Tunable Solid‐State Properties and Anisotropic Charge Mobility in Hydrogen‐Bonded Diketopyrolopyrrole Polymers via Automated Device Fabrication and Characterization

AbstractThe optoelectronic properties of semiconducting polymers and device performance rely on a delicate interplay of design and processing conditions. However, screening and optimizing the relationships between these parameters for reliably fabricating organic electronics can be an arduous task requiring significant time and resources. To overcome this challenge, Polybot is developed—a robotic platform within a self‐driving lab that can efficiently produce organic field‐effect transistors (OFETs) from various semiconducting polymers via high‐throughput blade coating deposition. Polybot not only handles the fabrication process but also can conduct characterization tests on the devices and autonomously analyze the data gathered, thus facilitating the rapid acquisition of data on a large scale. This work leverages the capabilities of this platform to investigate the fabrication of OFETs using hydrogen bonding‐containing semiconducting polymers. Through high‐throughput fabrication and characterization, various data trends are analyzed, and large extents of anisotropic charge mobility are observed in devices. The materials are thoroughly characterized to understand the role of processing conditions in solid state and electronic properties of these organic semiconductors. The findings demonstrate the effectiveness of automated fabrication and characterization platforms in uncovering novel structure–property relationships, facilitating refinement of rational chemical design, and processing conditions, ultimately leading to new semiconducting materials.

Liquid-Crystalline Naphthalene and Perylene Bisimides with a D2–π2–A Architecture for High-Mobility Organic Field-Effect Transistors

We designed and synthesized four naphthalene and perylene bisimide-based imidazole-containing liquid-crystalline (NI and PI) derivatives based on a donor−π–acceptor−π–donor (D2–π2–A) architecture. These compounds bear 10 flexible peripheral chains to ensure liquid crystallinity and solution processability making them suitable to use as organic semiconductors. All four compounds stabilized different columnar phases. Higher thermal stability and lower band gaps were also observed in both NI and PI derivatives. NI derivatives showed larger Stokes shifts, while PI derivatives exhibited very high molar extinction coefficients. Two solution-processable liquid-crystalline compounds (NI-12 and PI-12) were utilized for the fabrication of organic field-effect transistors, exhibiting a hole mobility of 1.99 cm2/V·s for NI-12 and 0.63 cm2/V·s for PI-12 derivatives at room temperature in normal air conditions.

Ribbon-Type Boron-Doped Polycyclic Aromatic Hydrocarbons: Conformations, Dynamic Complexation and Electronic Properties.

Molecular ribbons (MRs), one-dimentional topological polycyclic aromatic hydrocarbons (PAHs), are of importance for synthetic chemistry and material sciences. Herein, we disclose an effective strategy to develop boron-doped MRs, i.e. photochemical cyclization on conjugated organoboranes for rapid π-extension. A series of ribbon-type boron-doped PAHs that own multiple cove edges were synthesized using Mallory photoreaction in solution. Two of them feature isomeric C68 B2 π-skeletons with 2.2 nm in length, thus representing a new kind of boron-doped MRs. The boron atoms endow them with sufficient Lewis acidity, and notably, the formed Lewis acid-base adducts based on boron-doped MRs display the photo-induced dual-dissociation behavior in the excited state and thus photochromism property. Moreover, despite of the highly contorted topological conformations, their potential utility as organic semiconductor was demonstrated by fabrication of organic field-effect transistors.

Polyethylene and Semiconducting Polymer Blends for the Fabrication of Organic Field-Effect Transistors: Balancing Charge Transport and Stretchability

Polyethylene is amongst the most used polymers, finding a plethora of applications in our lives owing to its high impact resistance, non-corrosive nature, light weight, cost effectiveness, and easy processing into various shapes from different sizes. Despite these outstanding features, the commodity polymer has been underexplored in the field of organic electronics. This work focuses on the development of new polymer blends based on a low molecular weight linear polyethylene (LPE) derivative with a high-performance diketopyrrolopyrrole-based semiconducting polymer. Physical blending of the polyethylene with semiconducting polymers was performed at ratios varying from 0 to 75 wt.%, and the resulting blends were carefully characterized to reveal their electronic and solid-state properties. The new polymer blends were also characterized to reveal the influence of polyethylene on the mechanical robustness and stretchability of the semiconducting polymer. Overall, the introduction of LPE was shown to have little to no effect on the solid-state properties of the materials, despite some influence on solid-state morphology through phase separation. Organic field-effect transistors prepared from the new blends showed good device characteristics, even at higher ratios of polyethylene, with an average mobility of 0.151 cm2 V−1 s−1 at a 25 wt.% blend ratio. The addition of polyethylene was shown to have a plasticizing effect on the semiconducting polymers, helping to reduce crack width upon strain and contributing to devices accommodating more strain without suffering from decreased performance. The new blends presented in this work provide a novel platform from which to access more mechanically robust organic electronics and show promising features for the utilization of polyethylene for the solution processing of advanced semiconducting materials toward novel soft electronics and sensors.

New metallophthalocyanines including benzylphenoxy groups and investigation of their organic-field effect transistor (OFET) features

In this study, metal and metal-free novel phthalocyanines containing peripheral and non-peripheral tetra 2-benzylphenoxy groups were synthesized. The compounds were characterized by UV–Vis, FT-IR, 1H NMR, and MALDI-TOF mass spectrometry as well as elemental analysis. These new phthalocyanines exhibited excellent solubility in most organic solvents, and their redox behavior was investigated in different solvents such as dimethyl sulfoxide (DMSO) and dichloromethane (DCM). The redox behavior of the peripheral and non-peripheral phthalocyanine compounds 1a-c and 2a-c was determined by cyclic voltammetry and in situ spectroelectrochemistry. According to organic field-effect transistors (OFETs) measurements, the peripheral and non-peripheral phthalocyanine-cobalt complexes which have higher mobility than others were utilized top-gate bottom-contact OFETs fabrication. The output characteristics of the device show that its mobility is approximately 5 × 10−2 cm2/Vs with p-type accumulation.

Extreme Orientational Uniformity in Large-Area Floating Films of Semiconducting Polymers for Their Application in Flexible Electronics.

Layer-by-layer fabrication of uniformly oriented thin films over large areas by cost-effective solution-based approaches can open new horizons for the realization of high-performance organic circuits in various applications. In this work, fabrication of a large-area ≈40 cm2 film with uniform orientation is reported for poly(3,3‴-dialkylquaterthiophene) (PQT) using a unidirectional floating film transfer method (UFTM). Orientation characteristics and charge transport anisotropy were analyzed using polarized UV-vis spectral mapping and fabrication of bottom-gated organic field-effect transistors (OFETs) from different regions. Films were found to be highly oriented with an optical dichroic ratio of ca. 15. Orientation characteristics reveal that films were highly oriented along the width of the film, covering >70% of the area, and angle-dependent field-effect mobilities are in good agreement with the orientation of the polymer backbones. These highly oriented films resulted in charge transport anisotropy of 8.9. An array of bottom-gated OFETs fabricated along the length of single large-area (≈15 × 2.5 cm2) thin film demonstrated the average field-effect mobility of 0.0262 cm2/(V s) with a very narrow standard deviation of 12.6%. We also demonstrated that film thickness can be easily tuned from 5.6 to 45 nm by increasing the PQT concentration, and field-effect mobility is highly reproducible even when the film thickness is 10 nm. Microstructural characterization of the thus-prepared large-area thin films revealed the edge-on stacked polymer backbones and surface roughness of <1 nm as probed by grazing incidence X-ray diffraction and atomic force microscopy, respectively. Flexible OFETs with bottom-gate top-contact geometry were also fabricated, having average field-effect mobility of 0.0181 cm2/(V s). There was no considerable change in mobility after bending the flexible devices at different radii.

Enhancing the Solubility of Semiconducting Polymers in Eco-Friendly Solvents with Carbohydrate-Containing Side Chains.

Semiconducting polymers are at the forefront of next-generation organic electronics due to their robust mechanical and optoelectronic properties. However, their extended π-conjugation often leads to materials with low solubilities in common organic solvents, thus requiring processing in high-boiling-point and toxic halogenated solvents to generate thin-film devices. To address this environmental concern, a natural product-inspired side-chain engineering approach was used to incorporate galactose-containing moieties into semiconducting polymers toward improved processability in greener solvents. Novel isoindigo-based polymers with different ratios of galactose-containing side chains were synthesized to improve the solubilities of the organic semiconductors in alcohol-based solvents. The addition of carbohydrate-containing side chains to π-conjugated polymers was found to considerably impact the intermolecular aggregation of the materials and their microstructures in the solid state as confirmed by atomic force microscopy and grazing-incidence wide-angle X-ray scattering. The charge transport characteristics of the new semiconductors were evaluated by the fabrication of organic field-effect transistors prepared from both toxic halogenated and greener alcohol-based solvents. Importantly, the incorporation of carbohydrate-containing side chains was shown to have very little detrimental impact on the electronic properties of the polymer when processed from green solvents.

Improving Ideality of P‐Type Organic Field‐Effect Transistors via Preventing Undesired Minority Carrier Injection

AbstractElectron injection plays a crucial role in arousing the double‐slope characteristics for p‐type organic field‐effect transistors (OFETs) with narrow‐bandgap organic semiconductors (OSCs). This issue will not only result in the misrepresentation of OFET performance but also may cause device instability, hence impeding their further development in real‐world applications. A facile and highly efficient approach is developed to circumvent this issue by implementing modification on the electrode/organic semiconductor interface. An ultrathin layer of wide‐bandgap OSC with suitable energy levels is introduced to block the undesirable electron injection. By this means, typical double‐slope behaviors and bias stress stability in the p‐type OFETs can be significantly improved. Using 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene‐based OFETs the double‐slope behaviors of as‐fabricated devices are effectively converted to near‐ideal behaviors after modification, leading to a dramatic improvement of average reliability from 65.11% to 91.76%. Furthermore, the positive drift of transfer curves under prolonged bias stress is also successfully suppressed. This strategy demonstrates good universality and can provide a new guideline for the fabrication of OFETs with ideal behaviors.

Facile and reliable route to ensure chemical-environmental stability of pen-printed organic transistors with blended polymer Semiconductor–Insulator

We have introduced an efficient pen-printing method for solution-process fabrication of organic field-effect transistors (OFETs). Although polymer semiconductors used in this method are promising materials that provide electrical properties with mechanical flexibility, they have drawbacks such as poor long-term driving stability or dramatically decreased electrical performance under chemical environments. Herein we applied the spontaneous phase separation in blended polymers of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and polystyrene (PS) as model semiconducting and protective polymers, respectively. A protective PS layer spontaneously formed on top of the PTAA layer, preventing direct exposure to chemical molecules and thus greatly improving long-term driving stability under ambient, high-humidity, and even ethanol vapor conditions. Even under the harshest ethanol vapor condition, the average field-effect mobility (μFET) of the PTAA + PS-blend FETs were maintained at 76% or more, and the threshold voltage showed only a small change of ±1.1 V over the range of vacuum, ambient, 75% humidity, and ethanol vapor conditions. Furthermore, μFETs of the PTAA + PS-blend FETs showed a low range of variation of about ±10% during 50 repeated measurements over 100 min under all conditions. The result suggests an efficient way forming a protective layer without an additional deposition step to secure the chemical and environmental stabilities of practical electronics.

Perfluoroalkylated alternating copolymer possessing solubility in fluorous liquids and imaging capabilities under high energy radiation.

A highly fluorinated alternating polymer, P(RFMi-St), possessing improved thermal properties and patterning capabilities over perfluoroalkyl polymethacrylates under high energy radiation was achieved with semi-perfluorododecyl maleimide (RFMi) and styrene (St). RFMi could be synthesised efficiently via a Mitsunobu reaction condition and copolymerised with St by free radical and reversible-deactivation radical polymerisation protocols. P(RFMi-St) showed a satisfactory glass-transition temperature (108 °C) and intermolecular cross-linking behaviour under electron-beam and commercially more important extreme UV (λ = 13.5 nm) irradiation. The exposed regions lost their solubility, resulting in the successful formation of mechanically non-deteriorated negative-tone images down to 50 nm. In addition, P(RFMi-St) could be solution-processed with chemically non-damaging fluorous liquids, which enabled the polymer to be applied effectively on top of an organic semiconductor layer as a dielectric material (dielectric constant 2.7) for the organic field-effect transistor fabrication.

Orientation Control of Semiconducting Polymers Using Microchannel Molds

The molecular orientation of organic semiconductors (OSCs) is of fundamental importance to anisotropic electrical behavior as well as superior properties in practical applications. Here, a simple and effective method is demonstrated to fabricate highly oriented semiconducting polymers, poly(3-hexylthiophene) (P3HT) and poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)), by mass transfer effect under microchannel molds by diffusion and convection. Furthermore, parallel or perpendicular molecular arrangements relative to the channel direction were achieved by varying the widths of the microchannels, which are directly observed using polarized optical microscopy and two-dimensional grazing-incidence X-ray diffraction experiments. The method could enable the fabrication of organic field-effect transistors that exhibit anisotropic electrical properties indicating inter- or intrachain charge transport. The resulting platform will provide a simple approach for multidirectional orientations of anisotropic OSCs.

Synthesis of SnO&lt;sub&gt;2&lt;/sub&gt; Nanoparticles Using &lt;i&gt;Ficus religiosa &lt;/i&gt;Leaf Extract and their Application in Fabrication of OFETs for Glucose Monitoring

Organic field effect transistors (OFETs) as a sensor have gained interest of researchers due to its portable size and less expensive design in the field of medical diagnostic, food monitoring, chemical detection, wearable sensors, etc. In this present research work, we demonstrate the fabrication of OFETs from organic-inorganic SnO2 nanoparticles tailored pentacenefor glucose detection. SnO2 nanoparticles were synthesized bygreen method using Ficus religiosa leaf extract. The as-synthesized SnO2 NPs with cassiterite crystal structure was analysed using X-Ray Diffraction (XRD) and the energy bandgap of ~3.8 eV was calculated using Tauc relation with absorption spectra so obtained from UV-vis spectroscopy (UV-vis). The structure and morphological analysis of SnO2 NPs with size of ~15 nm was confirmed by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) analysis. The sensor characteristics of OFET device fabricated using pentacene layer (soluble pentacene precursor of DMP) and as-synthesized SnO2 nanoparticles confirmed the aqueous glucose (glucose in water) detection at room temperature (27 °C). The extracted electrical parameters such as mobility (μ), On-current (Ion), saturation current (IDsat) and the sensor response were discussed to support the sensor characteristics.

Crack propagation and electronic properties of semiconducting polymer and siloxane-urea copolymer blends

A persistent challenge in the field of organic electronics is balancing the optoelectronic properties of π-conjugated semiconducting polymers with their thermomechanical properties. A popular and effective approach to resolve this dichotomy is to blend π-conjugated polymers with amorphous, stretchy elastomers. In this work, poly(diketopyrrolopyrrole-co-thienovinylthiophene) was blended with an easily-prepared poly(dimethylsiloxane)-based phenylurea copolymer (PDMS-PU) to further explore this approach. Interestingly, the differing surface energy and polarity of this soft amorphous copolymer in comparison to other common siloxane-based polymers blended with conjugated polymers showed little impact on the solid-state morphology. Various techniques were used to evaluate the properties of the polymer blends, including atomic force microscopy, UV–vis spectroscopy, and x-ray diffraction. An in-depth morphological evaluation was performed on the blend at varying strain, elucidating the formation of cracks at the nanoscale. The results show a significant decrease in crystallinity and increase in crack onset with increased PDMS-PU content. Fabrication of organic field-effect transistors (OFETs) utilizing the new polymer blends exhibited charge mobility up to 8.2 × 10−2 cm2 V−1 s−1, and charge transfer characteristics up to 75 wt.% PDMS-PU content. The study shows the promise of PDMS-PU/conjugated polymer blends for use in OFETs, and towards the large-scale preparation of mechanically robust and stretchable electronic devices.

Non-lithographic direct patterning of carbon nanomaterial electrodes via electrohydrodynamic-printed wettability patterns by polymer brush for fabrication of organic field-effect transistor

In this study, we report a new approach for patterning of carbon nanomaterials via the solution route through wettability patterning using electrohydrodynamic (EHD) printing. Differences in wettability were successfully obtained by treating dimethylchlorosilane-terminated polystyrene (PS-brush) with the micro-dripping mode of EHD printing up to 6-μm line width and scaled fine patterns with various shapes at 6-μm printing conditions. We obtained sub-micrometer scaled carbon nanomaterial patterns, which were utilized as source and drain (S/D) electrodes for organic field-effect transistors (OFETs), without using any lithographic process. In addition, the PS-brush treatment modified the surface hydrophobicity of the substrates that served as the PS-brush and SiO2 bilayer dielectrics to guarantee high stability during the OFET operation. As a result, the OFETs employing S/D electrodes based on carbon nanomaterial patterns exhibited the electrical performance of typical p-type OFETs with negligible hysteresis. Additionally, we confirmed that a solution processing organic semiconductor forms a more suitable crystal structure than a vacuum process based organic semiconductor by several tools like CPOM, AFM, and 2D-GIXD.

Surface-modified quantum-dot floating layer using novel thiol with large dipole moment for improved feasibility of light-erasable organic transistor memory applications

In this study, a new functional thiol with a pentafluorophenyl group was synthesized for the surface modification of CdSe quantum-dot floating layers; this was aimed at the fabrication of organic field-effect transistors (OFETs). The dipole moments and surface properties of the fluorinated thiols were used to control the transistor operations; these thiols acted as surface modifiers of the CdSe quantum-dot floating layers in the OFETs. Further, the new functional thiol exhibited a larger dipole moment than that of the commercial 2,3,4,5,6-pentafluorothiophenol. The OFET comprising the new functional thiol with the pentafluorophenyl group functioned as a normally ON transistor and exhibited bistable current states during nondestructive reading. In addition, it exhibited sensitive responses to electrical-only and light-only biases, which demonstrates its feasibility for light-responsive flash memory applications.

Synthesis of Soluble Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) Derivatives: One-Step Functionalization of 2-Bromo-DNTT.

For developing dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) derivatives as solution processable organic semiconductors, we synthesized 2-brominated DNTT (Br-DNTT) as a common precursor to 2-substituted DNTT derivatives. The synthesis of Br-DNTT features chemoselective metalation and cross-coupling reactions that enable us to keep the 2-bromo group intact from the starting material, 2-bromo-6-methoxynaphthalene, to Br-DNTT. We demonstrated one-step functionalization of Br-DNTT by various palladium- and copper-catalyzed cross-coupling reactions to introduce a variety of substituents, including ethynyl, aryl, heteroaryl, alkyl, alkoxy, and alkylthio groups, in yields of 73 to 98%. The resulting 12 examples of 2-substituted DNTT derivatives, which have bulky or flexible solubilizing groups, have improved solubilities of up to 200 times the solubility of unsubstituted DNTT. Some of the soluble 2-substituted DNTT derivatives were applied to the solution-processed fabrication of organic field-effect transistor (OFET) devices. Most of the OFET devices exhibited average hole mobilities in the order of 10-1 to 10-2 cm2 V-1 s-1. Among the DNTT derivatives, the one substituted with 4-(2-(2-methoxyethoxy)ethoxy)butyl group has the highest solubility of 8.45 g L-1 and also exhibited the highest average hole mobility of 0.28 cm2 V-1 s-1 in the OFET devices.

Solution-processed organic field-effect transistors using directed assembled carbon nanotubes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT)

Achieving low-cost fabrication of organic field-effect transistors (OFETs) has long been pursued in the semiconductor industry. Solution-based process allows the fabrication of OFETs cost-effective because of its merit of vacuum-free and room temperature operation. Here, we show a facile and scalable fabrication of solution-processed OFETs using carbon nanotube (CNT) as source/drain electrodes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as semiconducting layer on silicon as well as on flexible and transparent polyethylene terephthalate (PET) substrates. The CNT electrodes and the C8-BTBT film are fabricated using a dip coating-based directed assembly process, and two dip coating parameters, the pulling speed and the solution concentration, are carefully chosen so that the thickness of the C8-BTBT film is close to that of the CNT electrodes. The fabricated OFET devices show typical p-channel behavior. Low-cost, ease of processing, wafer level scalability and good compatibility with various substrates make the fabrication process presented in this paper well suited for next-generation electronics and sensors.

Mitigating Meniscus Instabilities in Solution-Sheared Polymer Films for Organic Field-Effect Transistors.

Semiconducting donor-acceptor copolymers are considered to be a promising material class for solution-coated, large-scale organic electronic applications. A large number of works have shown that the best-performing organic field-effect transistors (OFETs) are obtained on low-surface-energy substrates. The meniscus instabilities that occur when coating on such surfaces considerably limit the effective deposition speeds. This represents a limiting factor for the upscaling of device fabrication for mass production, an issue that needs to be addressed if organic electronic devices are ever to become commercially relevant. In this work, we present a method to increase the accessible window of coating speeds for the solution shearing of donor-acceptor semiconductor polymers for the fabrication of OFETs. By incorporating a piezo crystal that is capable of producing high-frequency vibrations into the coating head, we are able to mitigate contact line instabilities due to the depinning of the contact line, thereby suppressing the commonly encountered "stick-and-slip" phenomenon.

Organic Semiconductor/Polymer Blend Films for Organic Field‐Effect Transistors

AbstractThe development of low‐cost printed organic electronics entails the processing of active organic semiconductors (OSCs) through solution‐based techniques. However, the preparation of large‐area uniform and reproducible films based on OSC inks can be very challenging due to the low viscosity of their solutions, which causes dewetting problems, the low stability of OSC polymer solutions, or the difficulty in achieving appropriate crystal order. To circumvent this, a promising route is the use of blends of OSCs and insulating binding polymers. This approach typically gives rise to films with an enhanced crystallinity and organic field‐effect transistors (OFETs) with significantly improved device performance. Recent progress in the fabrication of OFETs based on OSC/binding polymer inks is reviewed, highlighting the main morphological and structural features that play a major role in determining the final electrical properties and some future perspectives. Undoubtedly, the use of these types of blends results in more reliable and reproducible devices that can be fabricated on large areas and at low cost and, thus, this methodology brings great expectations for the implementation of OSCs in real‐world applications.

Enhanced gate-bias stress stability of organic field-effect transistors by introducing a fluorinated polymer in semiconductor/insulator ternary blends

Solution-processed polymer semiconductors are key materials in the fabrication of lightweight, low-cost, and flexible electronic devices without using the high-vacuum process. For practical applications, reliable device operations based on these materials are required. In this study, we propose a strategy to improve the operation stability of organic field-effect transistors (OFETs) using solution-processed polymer semiconductor/insulator blends as the active channel prepared by introducing a fluorinated insulating polymer in the blends. The semiconducting polymer forms nanowire networks in spin-coated ternary blend films, which serve as charge transport pathways in the insulating polymer matrix consisting of the fluorinated polymer and polystyrene. Owing to its high surface potential attributed to the strongly electron-withdrawing structure, the fluorinated polymer provides a large energy barrier for suppression of the hole trapping at the semiconductor/insulator interface. Consequently, OFETs based on the ternary blend films with an optimized polymer composition exhibit almost hysteresis-free transfer and output characteristics and superior electrical stabilities under sustained gate-bias stresses in both N2 and air atmospheres. We believe that our study provides a practical route to the fabrication of OFETs based on polymer semiconductor/insulator blend systems with high operation stabilities.