Semiconductors In Organic Field-effect Transistors Research Articles

Anthracene-[1]benzothieno[3,2-b][1]benzothiophene (BTBT) dyad and triads as p-type semiconductors for organic field-effect transistors and phototransistors

[1]Benzothieno[3,2-b][1]benzothiophene (BTBT) and anthracene (An) are the most promising planar building blocks for constructing p-type small molecules for high-performance organic field-effect transistors (OFETs).

Rational Design of a Multifunctional Benzothiadiazole Derivative in Organic Photonics and Electronics.

In order to achieve a multifunctional compound with potential application in organic photonics and electronics, a multidonor benzothiadiazole derivative was rationally designed and synthesized employing microwave irradiation as energy source, increasing the process efficiency about yields and reaction times in comparison with conventional conditions. This powerful compound displayed solvatochromism and showed efficient behavior as red optical waveguide with low OLC around 10-2 dB μm-1 and with the capacity of light transmission in two directions. In addition, the proposed derivative acted as efficient p-type semiconductor in organic field-effect transistors (OFETs) with hole mobilities up 10-1 cm2 V-1 s-1 . This corroborates its multifunctional character, thus making it a potential candidate to be applied in hybrid organic field-effect optical waveguides (OFEWs).

Solution-Processable Indenofluorenes on Polymer Brush Interlayer: Remarkable N-Channel Field-Effect Transistor Characteristics under Ambient Conditions.

The development of solution-processable n-type molecular semiconductors that exhibit high electron mobility (μe ≥ 0.5 cm2/(V·s)) under ambient conditions, along with high current modulation (Ion/Ioff ≥ 106-107) and near-zero turn on voltage (Von) characteristics, has lagged behind that of other semiconductors in organic field-effect transistors (OFETs). Here, we report the design, synthesis, physicochemical and optoelectronic characterizations, and OFET performances of a library of solution-processable, low-LUMO (-4.20 eV) 2,2'-(2,8-bis(3-alkylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile small molecules, β,β'-Cn-TIFDMTs, having varied alkyl chain lengths (n = 8, 12, 16). An intriguing correlation is identified between the solid-isotropic liquid transition enthalpies and the solubilities, indicating that cohesive energetics, which are tuned by alkyl chains, play a pivotal role in determining solubility. The semiconductors were spin-coated under ambient conditions on densely packed (grafting densities of 0.19-0.45 chains/nm2) ultrathin (∼3.6-6.6 nm) polystyrene-brush surfaces. It is demonstrated that, on this polymer interlayer, thermally induced dispersive interactions occurring over a large number of methylene units between flexible alkyl chains (i.e., zipper effect) are critical to achieve a favorable thin-film crystallization with a proper microstructure and morphology for efficient charge transport. While C8 and C16 chains show a minimal zipper effect upon thermal annealing, C12 chains undergo an extended interdigitation involving ∼6 methylene units. This results in the formation of large crystallites having lamellar stacking ((100) coherence length ∼30 nm) in the out-of-plane direction and highly favorable in-plane π-interactions in a slipped-stacked arrangement. Uninterrupted microstructural integrity (i.e., no face-on (010)-oriented crystallites) was found to be critical to achieving high mobilities. The excellent crystallinity of the C12-substituted semiconductor thin film was also evident in the observed crystal lattice vibrations (phonons) at 58 cm-1 in low-frequency Raman scattering. Two-dimensional micrometer-sized (∼1-3 μm), sharp-edged plate-like grains lying parallel with the substrate plane were observed. OFETs fabricated by the current small molecules showed excellent n-channel behavior in ambient with μe values reaching ∼0.9 cm2/(V·s), Ion/Ioff ∼ 107-108, and Von ≈ 0 V. Our study not only demonstrates one of the highest performing n-channel OFET devices reported under ambient conditions via solution processing but also elucidates significant relationships among chemical structures, molecular properties, self-assembly from solution into a thin film, and semiconducting thin-film properties. The design rationales presented herein may open up new avenues for the development of high-electron-mobility novel electron-deficient indenofluorene and short-axis substituted donor-acceptor π-architectures via alkyl chain engineering and interface engineering.

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.

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.

Theoretical evaluation of charge transport properties and mobility of tetraphenyldipyranylidene derivatives in organic field-effect transistors

Density functional theory calculations were performed to calculate the structural, electronic, physical, and hole/electron charge transport properties of three possible pathways of Tetraphenyldipyranylidene (DPPh) derivatives including electron-acceptor atoms and electron-donor groups as the organic semiconductors in organic field-effect transistors (OFETs). Additionally, the reduced density gradient-based non-covalent interaction (RDG-NCI) analysis was performed to investigate the presence of non-covalent intermolecular interactions. The results of electronic and physical properties show that all compounds are stable in environmental conditions and intermolecular interactions between the three possible dimers of each compound are different. The maximum mobility (µ(max)) and anisotropic mobility (µφ(max)) of the hole (0.16 cm2 V−1 s−1) is greater than that of the electron in DPPh, which is in good agreement with the experiment. The mobility of the hole decreases by introducing the electron-acceptor atoms and electron-donor groups. However, µ(max) and µφ(max) values of the electron in the case of DPPh derivatives increase including C(CH3)3, Br, OCH3, CH3, Cl, and –OCH2CH3, respectively. According to different analyses, DPPh-CH3, DPPh-C(CH3)3, DPPh-Br, and DPPh-Cl semiconductors are proposed as the preferred candidates for use in the OFET devices.

Modification of alkyl side chain on thiophene-containing benzothieno[3,2-b]benzothiophene-based organic semiconductors for organic field-effect transistors

In this study, three new organic semiconductors based on benzothieno[3,2-b]benzothiophene with double thiophene rings and different types of alkyl side chains were synthesized and characterized as active layers for organic field-effect transistors. Physicochemical properties of the newly synthesized organic compounds including thermal, optical, and electrochemical properties were characterized through thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), UV–vis spectroscopy, and cyclic voltammetry (CV). The thin film surface morphology and texture of all three compounds fabricated by solution-shearing process were analyzed via atomic force microscopy (AFM) and θ-2θ X-ray diffraction (XRD). All semiconductor thin films showed p-channel activity and the compound with monoalkylated 2-ethylhexyl group as a substituent exhibited the highest electrical performance with carrier mobility of 0.033 cm2/Vs and current on/off ratio of ∼106.

Side chain engineering of [1]benzothieno[3,2-b]benzothiophene (BTBT)-based semiconductors for organic field-effect transistors

[1]benzothieno[3,2-b]benzo-thiophene (BTBT)-based compounds with different side chains, 2-(2-ethylhexyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (compound 1), 2-(2-ethylhexyl)−7-(thiophen-2-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (compound 2), 2-(2-ethylhexyl)−7-(5-octylthiophen-2-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (compound 3), and 2-(2-ethylhexyl)−7-(5-(2-ethylhexyl)thiophen-2-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (compound 4), were synthesized and characterized as solution-processable small molecular organic semiconductors for organic field-effect transistors (OFETs). The corresponding compounds were employed as active layers for top-contact/bottom-gate OFETs via solution-shearing method and the electrical properties were investigated. The resulting devices exhibited p-channel activity and, especially, thin films of monoalkylated BTBT with additional thiophene ring showed the highest hole mobility up to 0.12 cm2/Vs and current on/off ratio over 107. The analysis of atomic force microscopy (AFM) and X-ray diffraction (XRD) were carried out to correlate surface morphology and microstructure of thin films with the device performance.

Cover Feature: Synthesis and n‐Type Semiconducting Properties of Bis(dioxaborin) Compounds Containing a π‐Extended 2,2′‐Bithiophene Structure (Chem. Asian J. 3/2022)

Looking at crystal structures makes us feel as if we are in a museum. Molecules are self-assembled to form beautiful crystal structures. Herein, bis(dioxaborin) compounds containing 2,2′-bithiophene derivatives were synthesized as n-type semiconductors for organic field-effect transistors (OFETs). An OFET fabricated using one of these compounds having a layer-by-layer crystal structure exhibited n-type semiconductor behavior with a low threshold voltage. This OFET performance was studied by theoretical calculations based on the crystal structure. More information can be found in the Full Paper by Katsuhiko Ono et al.

Surface modification with a fluorinated N-heterocyclic carbene on Au: effect on contact resistance in organic field-effect transistors

A fluorinated N-heterocyclic carbene is chemically bonded to Au and employed as a surface modifier to improve contact between electrodes and semiconductors in organic field-effect transistors, showing a significant improvement in device performance.

Optimized Charge Transport in Molecular Semiconductors by Control of Fluid Dynamics and Crystallization in Meniscus‐Guided Coating (Adv. Funct. Mater. 2/2022)

Optimized Charge Transport In article number 2107976, Jasper J. Michels, Wojciech Pisula, Tomasz Marszalek, and co-workers identify an optimal processing window for zone-casting of a molecular semiconductor for organic fieldeffect transistors. Computational modelling of the fluid dynamics and the crystallization explains the occurrence of three morphological subregimes: i) isotropic domain-like, ii) unidirectional crystal stripes and iii) corrugated dendritic. Control of the semiconductor film morphology allows to optimize the in-plane charge carrier transport.

Dithienylnaphthalenes and quaterthiophenes substituted with electron-withdrawing groups as n-type organic semiconductors for organic field-effect transistors

The synthesis and subsequent complex study of two related series of derivatives with various acceptor groups reveals the first thienylnaphthalene-based n-type semiconductors and compares them with similarly effective quaterthiophene derivatives.

High-performance five-ring-fused organic semiconductors for field-effect transistors.

Organic molecular semiconductors have been paid great attention due to their advantages of low-temperature processability, low fabrication cost, good flexibility, and excellent electronic properties. As a typical example of five-ring-fused organic semiconductors, a single crystal of pentacene shows a high mobility of up to 40 cm2 V-1 s-1, indicating its potential application in organic electronics. However, the photo- and optical instabilities of pentacene make it unsuitable for commercial applications. But, molecular engineering, for both the five-ring-fused building block and side chains, has been performed to improve the stability of materials as well as maintain high mobility. Here, several groups (thiophenes, pyrroles, furans, etc.) are introduced to design and replace one or more benzene rings of pentacene and construct novel five-ring-fused organic semiconductors. In this review article, ∼500 five-ring-fused organic prototype molecules and their derivatives are summarized to provide a general understanding of this catalogue material for application in organic field-effect transistors. The results indicate that many five-ring-fused organic semiconductors can achieve high mobilities of more than 1 cm2 V-1 s-1, and a hole mobility of up to 18.9 cm2 V-1 s-1 can be obtained, while an electron mobility of 27.8 cm2 V-1 s-1 can be achieved in five-ring-fused organic semiconductors. The HOMO-LUMO levels, the synthesis process, the molecular packing, and the side-chain engineering of five-ring-fused organic semiconductors are analyzed. The current problems, conclusions, and perspectives are also provided.

Synthesis and n-Type Semiconducting Properties of Bis(dioxaborin) Compounds Containing a π-Extended 2,2'-Bithiophene Structure.

Bis(dioxaborin) compounds containing π-conjugated systems have been studied as n-type semiconductors for organic field-effect transistors (OFETs). In this study, with the aim of investigating the effect of the extension of the π-conjugation on the n-type semiconducting properties and stability of bis(dioxaborin) compounds, we synthesized new compounds containing 2,2'-bithiophene derivatives extended with an olefin or an acetylene spacer. The absorption maxima of the compounds containing olefin spacers were greatly red-shifted compared with those of the original compound without a π-spacer. The newly synthesized compounds exhibited high electron affinity, and the olefin spacers effectively reduced the on-site Coulomb repulsion in the two-electron reduction of the compounds. An OFET fabricated using one of these compounds having a layer-by-layer crystal structure exhibited n-type semiconductor behavior with a low threshold voltage, most likely due to the small on-site Coulomb repulsion. The electron-transporting properties were investigated by theoretical calculations based on the Marcus theory.

Synthesis and characterization of benzo[b]thieno[2,3-d]thiophene (BTT) derivatives as solution-processable organic semiconductors for organic field-effect transistors

In this study, novel benzo[b]thieno[2,3-d]thiophene (BTT) derivatives, 2-(thiophene-2-yl)benzo[b]thieno[2,3-d]thiophene (compound 1), 2-(5-(2-ethylhexyl)thiophene-2-yl)benzo[b]thieno[2,3-d]thiohpene (compound 2), and 2-(5-octylthiophene-2-yl)benzo[b]theino[2,3-d]thiohpene (compound 3), were synthesized and characterized as solution-processable organic semiconductors (OSCs) for organic field-effect transistors (OFETs). Developed organic compounds were analyzed by thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), UV–vis spectroscopy, and cyclic voltammetry for physicochemical characteristics. All BTT-derivatives were coated by solution-shearing method and fabricated films were characterized by atomic force microscopy (AFM) and θ-2θ X-ray diffraction (XRD). To investigate the electrical performance of newly synthesized BTT-derivatives, bottom-gate/top-contact OFETs were fabricated and characterized in ambient conditions. All compounds exhibited p-channel activity and especially, the resulting OFETs based on films of compound 3 showed the highest hole mobility of 0.11 cm2/Vs and current on/off ratio of 8.7 × 106.

Development of Dithieno[3,2-b:2′,3′-d]thiophene (DTT) Derivatives as Solution-Processable Small Molecular Semiconductors for Organic Thin Film Transistors

Novel solution-processable dithieno[3,2-d:2′,3′-d]thiophene (DTT) derivatives with alkylated thiophene or alkyl chain substituents, 2,6-bis(5-octylthiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 1), 2,6-bis(5-(2-ethylhexyl)thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 2), and 2,6-dioctyldithieno[3,2-b:2′,3′-d]thiophene (compound 3), have been synthesized and employed as small molecular organic semiconductors for organic field-effect transistors (OFETs). All compounds exhibited good thermal stability over 290 °C, while different side groups of DTT compounds afforded different melting temperatures. The molecular orbital energy levels were experimentally and theoretically calculated, and their trend was almost the same. The developed compounds were employed as active layers for top-contact/bottom-gate OFETs with average charge carrier mobility as high as 0.10 cm2/Vs and current on/off ratio > 107 in ambient atmosphere. Notably, DTT derivative with linear alkyl chain (-octyl) substituents showed the best device performance. High device performance could be attributed to the large grains and continuous surface coverages as well as high film texture of the corresponding semiconductor films.

Crystal Engineering of Angular-Shaped Heteroarenes Based on Cyclopenta[b]thiopyran for Controlling the Charge Carrier Mobility.

Cyclopenta[b]thiopyran, isomeric to benzo[b]thiophene while isoelectronic to azulene, is involved as a building block to construct soluble organic semiconductors for field-effect transistors. Two series of angular-shaped heteroarenes based on cyclopenta[b]thiopyran, that is, Cn-SS (n = 4, 6, 8, 10) with different linear alkyl groups and C8-SS-Clm (m = 2, 3, 4) with chlorides substituted at different positions, have been straightforward synthesized. The obtained seven S-heteroarenes exhibit intriguing and similar photophysical and electrochemical properties, such as near-infrared absorption and high-energy levels of the highest occupied molecular orbitals. Nevertheless, the S-heteroarenes with identical π-conjugated skeletons demonstrate completely different molecular packing structures, which is proofed to be the key determinate factor for the charge carrier mobilities. Upon the engineering of the pendant alkyl lengths, the highest hole mobility in the Cn-SS series is achieved for C8-SS (1.1 cm2 V-1 s-1) with moderate alkyl length. The further incorporation of chlorides on C8-SS results in the shortened intermolecular H···S contacts and the interplane distances. Most interestingly, when chlorine-containing chloroform and chlorobenzene are used as crystallization solvents, single crystals of C8-SS-Clm with different packing structures are produced owing to the intermolecular interactions among the solute and solvent molecules. Upon further engineering of the chlorination position and the crystallization solvent, the maximum hole mobility in the ambient air improves to 2.7 cm2 V-1 s-1 for C8-SS-Cl2 crystallized from chlorobenzene, suggesting that the introduction of the accessible chlorides is a feasible pathway to engineering the crystal structures and controlling the charge transport characteristics.

Isomeric Dibenzoheptazethrenes for Air‐Stable Organic Field‐Effect Transistors

Singlet diradicaloids hold great potential as semiconductors for organic field-effect transistors (OFETs). However, their relative low material and device stabilities impede the practical applications. Here, to achieve balanced stability and performance, two isomeric dibenzoheptazethrene derivatives with singlet diradical character were synthesized in a concise manner. Benefitting from the aromatic stabilization, both compounds display a small diradical character and large singlet-triplet gap, as corroborated by variable-temperature electron paramagnetic resonance spectra, single-crystal analysis, and theoretical calculations. OFET devices based on single crystals showed a high hole mobility of 0.15 cm2 V-1 s-1 , which is the highest for zethrene-based semiconductors. Both isomers exhibited remarkable material stability in air-saturated solutions as well as excellent bias-stress and storage stability in device under ambient air.

Synthesis of D-π-A high-emissive 6-arylalkynyl-1,8-naphthalimides for application in Organic Field-Effect Transistors and optical waveguides

Donor-π-Acceptor (D-π-A) compounds based on a 1,8-naphthalimide core branched with different alkynyl donor groups have been synthesized, characterized and tested in Organic Field-Effect Transistors (OFETs) and as optical waveguides. The aim was to design materials with promising applications in new organic optoelectronic devices, namely Organic Field-Effect Optical Waveguides (OFEW). Computational studies showed that these compounds possess suitable electronic properties to behave as efficient semiconductors in OFETs and as optical waveguides. The former application is evidenced by the optimal energetic values of the frontier molecular orbitals and intramolecular reorganization energies, whereas the latter application is possible due to the high Stoke shifts and strong tendency to self-assemble. This last feature has been corroborated by NMR aggregation studies in solution. The experimental results indicate that the three studied semiconductors are active in OFETs, with electron field effect mobilities close to 10−2 cm2V−1s−1 for compound 1c. Furthermore, compound 1b also shows excellent behavior as an optical waveguide, thus making it a good candidate to be applied in OFEWs.

Organic Semiconductor Field-Effect Transistors Based on Organic-2D Heterostructures

In the past three decades, organic semiconductor field-effect transistors (OFETs) have drawn intense attentions as promising candidates for drive circuits of flat panel display, radio frequency identifications, chemical/bio-sensors and other devices. Generally, the key parameters of OFETs, carrier mobility, threshold voltage and on/off current ratio, are closely related to the degree of order and surface/interface electronic structure of organic semiconductor (OSC) films. The ordering of the films is crucially determined by the molecule-substrate interactions. On inert substrates (such as SiO2) OSC films can hardly reach high order of degree without growth templates, while traditional single crystal surfaces usually force the OSC molecules deviate their favorite assemble manner resulting in an unstable structure. Recently, the rise of two-dimensional materials (2D) provides a possible solution. The in-plane lattice of 2D materials can offer possible epitaxy templates for OSCs while the weak van der Waals (vdWs) interaction between OSC and 2D layers allows more flexibility to realize the epitaxy growth of OSCs with their favored assemble manner. In addition, the various band structures tuned by layer numbers of 2D materials encourage widely modified OSC electronic structures by interface doping between the OSC and 2D layers, which benefits to obtain high-performance OFETs. In this review, we emphasize and discuss the recent advances of OSC-2D hybrid OFETs. The OSC-2D heterostructures not only promote the OFET device performances by film morphology/structure optimization and channel electronic structure modification, but also offer platforms for basic organic solids physics investigations and further functional optoelectronic devices.