Single-crystal Field-effect Transistors Research Articles

Crystal-Structure Control of Molecular Semiconductors by Methylthiolation: Toward Ultrahigh Mobility.

ConspectusThe crystal structure of organic semiconductors has been regarded as one of the crucial factors for realizing high-performance electronic devices, such as organic field-effect transistors. However, although the control of crystal structures of organic semiconductors has been examined in the last two decades of intensive efforts of the development of organic semiconductors, active measures to control crystal structures enabling high carrier mobility are still limited. In 2016, our research group noticed that regioselective methylthiolation could provide a selective crystal structure change from an ordinary herringbone structure to a pitched π-stacking structure, similar to the crystal structure of rubrene, in the benzo[1,2-b:4,5-b']dithiophene (BDT) system. Following this serendipitous finding, our group systematically investigated the relationship between the molecular and crystal structures of a range of methylthiolated aromatic and heteroaromatic compounds.This Account provides a comprehensive overview of our research efforts and advancements in the development of methylthiolated small-molecule-based organic semiconductors (molecular semiconductors). We first describe the outline of the past development of molecular semiconductors, focusing on the types of crystal structures of high-performance molecular semiconductors. Then, we describe our findings on the drastic crystal structure change in the BDT system upon methylthiolation, detailing the causes of the change in terms of the intermolecular contacts and intermolecular interaction energies. This is followed by the confirmation of the generality of the crystal-structure change by methylthiolation of a series of acene and heteroacenes, where the herringbone structure in the parent system is unexceptionally transformed into the pitched π-stacking structure, a promising crystal structure for high-mobility molecular semiconductors well exemplified by the prototypical molecular semiconductor, rubrene. In fact, the methylthiolated anthradithiophene afforded comparable high mobility to rubrene in single-crystal field-effect transistors. Then, we demonstrate that the sandwich herringbone structures of peri-condensed polycyclic aromatic hydrocarbons, including pyrene, perylene, and peropyrene, change into brickwork crystal structures upon methylthiolation and that, among these compounds, very promising molecular semiconductors, methylthiolated pyrene and peropyrene, showing ultrahigh mobility of 30 cm2 V s-1, are realized.Through the studies, by gaining insights into the underlying mechanisms driving the crystal structure changes, we lay a strong foundation for tackling challenges related to controlling the crystal structures and developing high-performance molecular semiconductors. This will be a distinct approach from the past activities in the development of molecular semiconductors that mainly focused on molecules themselves, including their synthesis, properties, and characterization. We thus anticipate that our findings and the present Account will open the door to a new era of the development of molecular semiconductors.

Crystal-structure simulation of methylthiolated peri-condensed polycyclic aromatic hydrocarbons for identifying promising molecular semiconductors: discovery of 1,3,8,10-tetrakis(methylthio)peropyrene showing ultrahigh mobility.

The crystal structures of molecular semiconductors critically affect their carrier transport properties. One of the promising crystal structures that afford high carrier mobility is a brickwork structure recently reported for 1,3,6,8-tetrakis(methylthio)pyrene (MT-pyrene) showing ultrahigh mobility. However, such ultrahigh mobility was not realized in other methylchalocogenated pyrenes, owing to subtle differences in the molecular positions in their crystal structures. This means that, for developing superior molecular semiconductors, it is desirable to simulate the crystal structure with sufficient quality before time-consuming and labor-intensive synthetic trials. To realize this, we developed a new computational approach to simulating crystal structures of all methylchalocogenated pyrenes, which was then applied to MT-pyrene-related methylthiolated peri-condensed polycyclic aromatic hydrocarbons including perylene, peropyrene, and terrylene derivatives. Among these, 1,3,8,10-tetrakis(methylthio)peropyrene (MT-peropyrene) was expected to show high mobility based on the simulated crystal structures. We thus chose MT-peropyrene as the synthetic target and developed a new peropyrene synthesis method. Thus synthesized MT-peropyrene has virtually the same crystal structure as the simulated one, and its single-crystal field-effect transistors showed mobility as high as 30 cm2 V-1 s-1 and band-like transport behaviors. These results indicate that the present crystal-structure simulation is a powerful tool for exploring promising molecular semiconductors. This article is protected by copyright. All rights reserved.

Effects of source/drain electrode thickness on water-induced instability of laminated organic single-crystal field-effect transistors

Water is known to be the main cause of operational instability in organic electronic devices under ambient conditions. The water-induced hysteresis of lithography-friendly bottom-gate-bottom-contact-type organic field-effect transistors (FETs) with a laminated single-crystal channel was investigated using varying electrode thicknesses. As the amount of water in the air gap enveloped by the semiconductor layer and insulating substrate, adjacent to the electrode, increases, the mechanism dominating the hysteresis phenomenon shifted in the following order: (1) current increase owing to the decreasing charge-injection barrier because of a drain-induced change in the electrode work function, (2) current decrease because of the gate bias stress, and (3) current increase because of the gate enhancement effect owing to the dipole orientation of the water molecules penetrating the semiconductor-insulator interface. Changes in the electrode thickness had the same effect on the hysteretic behavior as the relative humidity of the experimental environment. Thus, this study provides a comprehensive understanding of the water-induced operational instability of laminated organic single-crystal FETs and should aid the realization of stable-operation organic devices, as well as foreign-molecule sensors, by controlling the electrode thickness.

Phase Transition Field Effect Transistor Observed in an α-(BEDT-TTF)2I3 Single Crystal

The metal–insulator transition induced by the gate electric field in the charge order phase of the α-(BEDT-TTF)2I3 single-crystal field-effect transistor (FET) structure was clearly observed near the phase transition temperature. An abrupt increase in the electrical conductance induced by the applied gate electric field was evident, which corresponds to the partial dissolution of the charge order phase triggered by the gate electric field. The estimated nominal dissolved charge order region (i.e., the gate-induced metallic region) was overestimated in 130–150 K, suggesting additional effects such as Joule heating. On the other hand, in the lower temperature region below 120 K, the corresponding dissolved charge order was several monolayers of BEDT-TTF, suggesting that it is possible to dissolve the charge order phase within the bistable temperature region.

π-Extended Pyrrole-Fused Heteropine: Synthesis, Properties, and Application in Organic Field-Effect Transistors.

Sulfur-embedded polycyclic aromatic compounds have been used as building blocks for numerous organic semiconductors over the past few decades. While the success is based on thiophene-containing compounds, aromatic compounds that contain thiepine, a sulfur-containing seven-membered-ring arene, has been less well investigated. Here we report the synthesis and properties of π-extended pyrrole-fused heteropine compounds such as thiepine and oxepine. A π-extended pyrrole-fused thiepine exhibited a "pitched π-stacking" structure in the crystal, and exhibited a high charge carrier mobility of up to 1.0 cm2 V-1 s-1 in single-crystal field-effect transistors.

A new dithieno[3,2-b:2',3'-d]thiophene derivative for high performance single crystal organic field-effect transistors and UV-sensitive phototransistors.

Organic phototransistors (OPTs), as the basic unit for organic image sensors, are emerging as one of the most promising light signal detectors. High performance UV-sensitive phototransistors are highly desired for the detection of UV light. Herein, by introducing the anthracene group to the 2,6-positions of dithieno[3,2-b:2',3'-d]thiophene, we designed and synthesized a new dithieno[3,2-b:2',3'-d]thiophene derivative, 2,6-di(anthracen-2-yl)dithieno[3,2-b:2',3'-d]thiophene (2,6-DADTT). The single crystal structure of 2,6-DADTT presents classical herringbone packing with multiple intermolecular interactions, including S⋯S (3.470 Å), S⋯C (3.304 Å, 3.391 Å, 3.394 Å) and C-H⋯π (2.763 Å, 2.822 Å, 2.846 Å, 2.865 Å, 2.885 Å, 2.890 Å) contacts. Single crystal organic field-effect transistors (SC-OFETs) based on 2,6-DADTT reach a highest mobility of 1.26 cm2 V-1 s-1 and an average mobility of 0.706 cm2 V-1 s-1. 2,6-DADTT-based single crystal organic phototransistors (OPTs) demonstrate photosensitivity (P) of 2.49 × 106, photoresponsivity (R) of 6.84 × 103 A W-1 and ultrahigh detectivity (D*) of 4.70 × 1016 Jones to UV light, which are among the best figures of merit for UV-sensitive OPTs. These excellent comprehensive performances indicate its good application prospects in integrated optoelectronics.

Synthesis and properties of 10,20-bis(triisopropylsilylethynyl)-tetrabenzo-5,15-diazaporphine

A 10,20-bis(triisopropylsilylethynyl)tetrabenzo-5,15-diazaporphine (TIPS-TBDAP) was synthesized by the metal-template aza-annulation reaction. TIPS-TBDAP showed intense far-red fluorescence at 723 nm with a fluorescence quantum yield of 31% in chloroform solution. X-ray diffraction analysis revealed that TIPS-TBDAP forms a one-dimensional slipped [Formula: see text]-stacking structure in the single crystal state. TIPS-TBDAP is soluble in common organic solvents owing to the bulky substituents and solution-processed field-effect transistors (FETs) could be fabricated. Dip-coating and single-crystal FETs showed the maximum hole mobilities of 1.5× 10[Formula: see text] and 0.16 cm2 V[Formula: see text] s[Formula: see text], respectively.

Uncovered Effects of thieno[2,3-b]thiophene Substructure in a Tetrathienoacene Backbone: Reorganization Energy and Intermolecular Interaction

Although tetrathienoacene, thieno[2″,3″:4′,5′]thieno[2′,3′-d]thieno[3,2-b]thiophene (4TA), has been used as a representative building block for organic semiconductors and polymers in the last two decades, its structural isomer, thieno[3″,2″:4′,5′]thieno[2′,3′-d]thieno[2,3-b]thiophene (i4TA), is still an unexplored structure. We are interested in the i4TA framework as a potential building block for organic semiconductors because the theoretically calculated reorganization energy (λ) of i4TA (149 meV) is significantly lower than that of 4TA (325 meV). In this study, we performed the synthesis and characterization of parent i4TA and its methylthio derivatives and found that several features of i4TA derivatives differ from those of 4TA. Parent i4TA and its derivatives tend to crystallize into the pitched π-stacking structure, which is a promising crystal structure for high-mobility organic semiconductors. The single-crystal field-effect transistor of β-methylthio-i4TA (β-MT-i4TA) showed typical transistor behaviors including as high as 3.5 cm2 V–1 s–1 hole mobility and band-like transport. This is understood as an outcome of the pitched π-stacking structure and the low λ originating in the i4TA framework. The origins of the low λ and the tendency to adopt the pitched π-stacking structure were investigated by theoretical analyses. The cross-conjugation path resulting from the thieno[2,3-b]thiophene substructure in the i4TA framework plays a pivotal role in reducing λ, meaning that the incorporation of the thieno[2,3-b]thiophene substructure is a potential strategy to reduce λ. On the other hand, the tendency to adopt the pitched π-stacking structure is due to the favorable σ*C–S–π intermolecular interaction of the i4TA framework, and the intermolecular interaction is also regarded as an outcome of the thieno[2,3-b]thiophene substructure. On the basis of these experimental and theoretical results, we conclude that the i4TA framework and the thieno[2,3-b]thiophene substructure are promising but overlooked structures as building blocks for organic semiconductors.

Sila-annulated terrylene diimides for balanced ambipolar transporting

The key building blocks, tetrachlorinated terrylene diimides and the targeted sila-annulated terrylene diimides (Si-TDIs and 2Si-TDIs) were synthesized for the first time. Single-crystal analysis verified the almost planar molecular configurations of both Si-TDIs and 2Si-TDIs. They exhibited intriguing optical properties including red-shifted absorption and near-infrared emission properties with excellent fluorescence quantum yields, as well as precisely controlled HOMO/LUMO energy levels by Si-heteroannulation. The single-crystal organic field-effect transistors based on 2Si-TDI 5a featuring long and branched alkyl chains demonstrated well-balanced ambipolar transporting properties with electron/hole mobilities of 0.10/0.18 cm2 V−1 s−1.

Mixed-Orbital Charge Transport in N-Shaped Benzene- and Pyrazine-Fused Organic Semiconductors.

The hole-carrier transport of organic semiconductors is widely known to occur via intermolecular orbital overlaps of the highest occupied molecular orbitals (HOMO), though the effect of other occupied molecular orbitals on charge transport is rarely investigated. In this work, we first demonstrate evidence of a mixed-orbital charge transport concept in the high-performance N-shaped decyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10–DNBDT–NW), where electronic couplings of the second HOMO (SHOMO) and third HOMO (THOMO) also contribute to the charge transport. We then present the molecular design of an N-shaped bis(naphtho[2′,3′:4,5]thieno)[2,3-b:2′,3′-e]pyrazine (BNTP) π-electron system to induce more pronounced mixed-orbital charge transport by incorporating the pyrazine moiety. An effective synthetic strategy for the pyrazine-fused extended π-electron system is developed. With substituent engineering, the favorable two-dimensional herringbone assembly can be obtained with BNTP, and the decylphenyl-substituted BNTP (C10Ph–BNTP) demonstrates large electronic couplings involving the HOMO, SHOMO, and THOMO in the herringbone assembly. C10Ph–BNTP further shows enhanced mixed-orbital charge transport when the electronic couplings of all three occupied molecular orbitals are taken into consideration, which results in a high hole mobility up to 9.6 cm2 V–1 s–1 in single-crystal thin-film organic field-effect transistors. The present study provides insights into the contribution of HOMO, SHOMO, and THOMO to the mixed-orbital charge transport of organic semiconductors.

"Spine Surgery" of Perylene Diimides with Covalent B-N Bonds toward Electron-Deficient BN-Embedded Polycyclic Aromatic Hydrocarbons.

BN-embedded polycyclic aromatic hydrocarbons (PAHs) with unique optoelectronic properties are underdeveloped relative to their carbonaceous counterparts due to the lack of suitable and facile synthetic methods. Moreover, the dearth of electron-deficient BN-embedded PAHs further hinders their application in organic electronics. Here we present the first facile synthesis of novel perylene diimide derivatives (B2N2-PDIs) featuring n-type B-N covalent bonds. The structures of these compounds are fully confirmed through the detailed characterizations with NMR, MS, and X-ray crystallography. Further investigation shows that the introduction of BN units significantly modifies the photophysical and electronic properties of these B2N2-PDIs and is further understood with the aid of theoretical calculations. Compared with the parent perylene diimides (PDIs), B2N2-PDIs exhibit deeper highest occupied molecular orbital energy levels, new absorption peaks in the high-energy region, hypsochromic shift of absorption and emission maxima, and decrement of photoluminescent quantum yields. Single-crystal field-effect transistors based on B2N2-PDIs showcase an electron mobility up to 0.35 cm2 V-1 s-1, demonstrating their potential application in optoelectronic materials.

Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules.

The magnitude of the field-effect mobility μ of organic thin-film and single-crystal field-effect transistors (FETs) has been overestimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the μ values using the effective field-effect mobility, μeff, a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of μeff depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of μeff for each FET. The FET performance based on the values of μeff obtained in this study is discussed in detail. In particular, the μeff values of single-crystal FETs are almost consistent with the μ values that were reported previously, but the μeff values of thin-film FETs were much lower than those previously reported for μ, owing to a high absolute threshold voltage, |Vth|. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the μ values of single-crystal FETs with phenacene molecules, is well reproduced from the μeff values. The FET performance is discussed based on the newly evaluated μeff values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

Single crystal field-effect transistor of tetrabenzoporphyrin with a one-dimensionally extended columnar packing motif exhibiting efficient charge transport properties

The single crystal field-effect transistor of 5,15-bis(triisopropylsilyl)ethynyltetrabenzoporphyrin exhibited better hole mobility than its metal complexes, with efficient charge transport through π–π stacking along tetrabenzoporphyrin units.

Giant Enhancement of Excitonic Electro-optic Response in Trap-Reduced Organic Transistors

Organic field-effect transistors (FETs) exhibit excellent switching characteristics when they involve trap-eliminated semiconductor interfaces with highly hydrophobic gate dielectric layers. In this study, we investigate the excitonic electro-optic response by delocalized carrier accumulation at the trap-eliminated interfaces of pentacene single-crystal FETs. We find that gate-modulation (GM) imaging, which sensitively visualizes the variation in optical microscope images between the gate-on and gate-off states, exclusively reveals the unique enhancement of electro-optic response under the application of drain voltages (${V}_{d}$). The ${V}_{d}$-unbiased GM image exhibits a uniform spatial distribution, which is consistent with the accumulated carrier density in the channel. In contrast, the ${V}_{d}$-biased GM image presents a peculiar spatial distribution with fairly sharp increases around the edges of the source and drain electrodes. Furthermore, the following intriguing features are observed: (1) The sharp increase in the GM signal distribution around the electrode edge is similar to the lateral electric field distribution as measured by Kelvin probe force microscopy, and (2) the GM spectra, extracted from the respective GM images measured at different wavelengths, present a second-derivative-like shape that implies the broadening of exciton absorption. Based on these observations, we investigate the origin of this unique effect in terms of the enhanced violation of exciton coherence by delocalized carrier accumulations under drain bias. The gate-induced holes that are weakly bound to shallow traps should be detrapped by lateral electric fields, which eventually generate valence band holes and thus enhance the electro-optic response. These findings should elucidate the spatial coherence of the molecular excitons that are responsible for the various unique photoelectric characteristics of organic electronic devices.

Organic Single Crystals with High Photoluminescence Quantum Yields Close to 100% and High Mobility for Optoelectronic Devices.

Organic single crystals with excellent optical and electrical properties are critical for the development of organic optoelectronics. Herein, two compounds 9,10-bis([N,N-diphenyl]-4'-phenylethynyl)anthracene (TPA-An) and 9,10-bis([1',3'-diphenyl]-5'-phenylethynyl)anthracene (TBA-An) are synthesized by introducing two different luminescent groups, triphenylamine and 1,3-diphenylbenzene, at the 9,10 positions of anthracene via triple bond connection. Single crystals based on TPA-An and TBA-An with a ribbon morphology obtained through the slow solvent-evaporation method exhibit high photoluminescence quantum yields (PLQYs) of 98% and 99% at room temperature, and remarkable hole mobilities of 0.45 and 0.15 cm2 V-1 s-1 in single-crystal organic field-effect transistors (SC-OFETs). Furthermore, UV phototransistors based on the two single crystals obtain photosensitivities of 1.03 × 103 and 3.45 × 104 , ultrahigh photoresponsivities of 7.19 × 105 and 1.50× 105 A W-1 , and the detectivities exceeding 1.40 × 1016 and 1.60 × 1017 Jones.

"Manipulation" of Crystal Structure by Methylthiolation Enabling Ultrahigh Mobility in a Pyrene-Based Molecular Semiconductor.

Control and prediction of crystal structures of molecular semiconductors are considered challenging, yet they are crucial for rational design of superior molecular semiconductors. It is here reported that through methylthiolation, one can rationally control the crystal structure of pyrene derivatives as molecular semiconductors; 1,6-bis(methylthio)pyrene keeps a similar sandwich herringbone structure to that of parent pyrene, whereas 1,3,6,8-tetrakis(methylthio)pyrene (MT-pyrene) takes a new type of brickwork structure. Such changes in these crystal structures are explained by the alteration of intermolecular interactions that are efficiently controlled by methylthiolation. Single crystals of MT-pyrene are evaluated as the active semiconducting material in single-crystal field-effect transistors (SC-FETs), which show extremely high mobility (32 cm2 V-1 s-1 on average) operating at the drain and gate voltages of -5V. Moreover, the band-like transport and very low trap density are experimentally confirmed for the MT-pyrene SC-FETs, testifying that the MT-pyrene is among the best molecular semiconductors for the SC-FET devices.

Acridino[2,1,9,8-klmna]acridine Bisimides: An Electron-Deficient π-System for Robust Radical Anions and n-Type Organic Semiconductors.

We report the synthesis and properties of acridino[2,1,9,8-klmna]acridine bisimide (AABI), a nitrogen-doped anthanthrene with two imide functionalities. AABI exhibits excellent electron affinity as evident by its low-lying LUMO level (-4.1 eV vs. vacuum). Single-electron reduction of one AABI derivative afforded the corresponding radical anion, which was stable under ambient conditions. Photoconductivity measurements suggest that the intrinsic electron mobility of an N-phenethyl AABI derivative obeys a band-transport model. Accordingly, an electron mobility of 0.90 cm2 V-1 s-1 was attained with the corresponding single-crystal organic field-effect transistor (OFET) device. The vacuum-deposited OFET device consisting of a polycrystalline sample exhibited high electron mobility of up to 0.27 cm2 V-1 s-1 even in air. This study demonstrates that dual incorporation of both imide substituents and imine-type nitrogen atoms is an effective strategy to create novel electron-deficient π-systems.

Synthesis, structure, and charge transport properties of a novel donor-acceptor complex of coronene and DTTCNQ

The rise of interest to organic charge-transfer (CT) systems in the last decade provides new opportunities for their practical applications in organic electronics. Herein, a charge-transfer complex of polyaromatic hydrocarbon (PAH) and 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo-[1,2-b:4,5-b′]-dithiophene (DTTCNQ) was reported, grown by liquid-liquid interfacial precipitation method, with a 1:1 donor-acceptor stoichiometry and mixed stacking binary structure. The single-crystal field-effect transistors based on coronene-DTTCNQ microcrystals showed good electron mobility of 0.07 ​cm2 ​V−1 ​s−1, which was consistent with the quantum calculations. This good electronic characteristic suggests that this PAH based CT complex family of organic semiconductor can be promisingly used for high performance organic electronic devices.

High-yield and sustainable synthesis of quinoidal compounds assisted by keto-enol tautomerism.

The classical synthesis of quinoids, which involves Takahashi coupling and subsequent oxidation, often gives only low to medium yields. Herein, we disclose the keto–enol-tautomerism-assisted spontaneous air oxidation of the coupling products to quinoids. This allows for the synthesis of various indandione-terminated quinoids in high isolated yields (85–95%). The origin of the high yield and the mechanism of the spontaneous air oxidation were ascertained by experiments and theoretical calculations. All the quinoidal compounds displayed unipolar n-type transport behavior, and single crystal field-effect transistors based on the micro-wires of a representative quinoid delivered an electron mobility of up to 0.53 cm2 V−1 s−1, showing the potential of this type of quinoid as an organic semiconductor.

2D Ruddlesden–Popper Perovskite Single Crystal Field‐Effect Transistors

Abstract2D Ruddlesden–Popper perovskites (2D PVKs) have attracted huge interest because of their excellent optoelectronic properties, yet the understanding of their electrical properties is inadequate due to the difficulties in obtaining 2D PVK field‐effect transistors (FETs) with decent performance. Herein, the fabrication and characterization of 2D PVK ((BA)2(MA)n−1PbnI3n+1) single crystal FETs are reported, which exhibit reliable field effect electrical characteristics at low temperatures. Kelvin probe force microscopy (KPFM) results reveal that both ion migration and contact resistance seriously degrade device performance. While ion migration can be suppressed at low temperatures, contact resistance seems to fundamentally determine device performance. On one hand, Schottky contacts are observed to form at the metal/2D PVK interface because of Fermi level pinning, resulting in significant charge injection resistance, although this can be remarkably improved by replacing Au electrodes with Ca. On the other hand, the out‐of‐plane mobility is found to be three orders of magnitude lower than the in‐plane mobility in 2D PVKs, causing large interlayer transport resistance. Thus, a low work‐function metal and a thin crystal are important for achieving high device performance. This work provides important experimental insights into fabrication and electrical properties of 2D PVK FETs.