Single-crystal Organic Field-effect Transistors Research Articles

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

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

Spin-Distribution-Directed Regioselective Substitution Strategy for Highly Stable Olympicenyl Radicals.

The synthesis of bench-stable conjugated π-radicals is challenging owing to the lack of modular approaches, which greatly hampers their practical material screens and applications. Here, we demonstrate a spin-distribution-directed regioselective substitution strategy to introduce substituents into the specific positions of an olympicenyl radical in a stepwise manner, resulting in a series of highly stable radical species. The substituents can also adjust the crystal packing by means of steric and electronic factors, enabling the changing from a π-dimer to a pseudo-one-dimensional chain. The first single crystal organic field-effect transistor device based on a graphenic radical is fabricated in air, showing a hole mobility of up to 0.021 cm2 V-1 s-1 and excellent device stability. This approach may be generalized to diverse spin-delocalized open-shell organic radicals.

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.

Synthesis and Properties of a Fused Antiaromatic Hydrocarbon By Oxidative Transformation of Carbacorrole Analogue

Polycyclic aromatic hydrocarbons (PAHs) containing fused pyrrole moieties are known as unique nitrogen-doped pi-conjugated molecules, e.g., azacoronenes, which possess a large π-conjugated planar structure and show rich redox properties. The related molecules are thus expected to be applied to redox-responsible optoelectronic materials. Furthermore, PAH-based compounds containing [4n]π-electron antiaromatic segments, such as pentalene and indacene are contributed to the ambipolar semiconducting material applications due to the distinct high HOMO and low LUMO energy levels.In this work, we have synthesized and characterized a novel N-doped poly-antiaromatic hydrocarbon (PAH) by the oxidative transformation of a dibenzocarbacorrole isomer, namely, dibenzocorrorin (1). X-ray crystal structure of the fused product (2) revealed a highly planar pi-conjugated structure containing a unique diaza-dicyclopentaazulene moiety. The distinct 12pi antiaromatic resonance structure led to the emergence of a near-infrared band beyond 1000 nm in the absorption spectrum and facile redox properties determined in the cyclic voltammetry. Furthermore, crystalline compound 2 showed carrier mobility in the single-crystal organic field effect transistor.

Manipulating Crystal Stacking by Sidechain Engineering for High‐Performance N‐Type Organic Semiconductors

AbstractY‐series non‐fullerene acceptors (NFAs) have achieved great progress in organic solar cells (OSCs). Most research attention is currently paid to their molecular engineering to improve the efficiency of OSCs. However, as n‐type organic semiconductors, the relationship between their molecular packing structures and charge transport properties is mostly ignored. Herein, it is clarified how the molecular packing of Y‐series NFAs fundamentally determines their charge transport properties by manipulating their crystal stacking via sidechain engineering. Therefore, branched alkyl/alkoxy substitutions are taken on a reference NFA (Y6‐1O), affording three derivatives, namely 1OBO‐1, 1OBO‐2, and 1OBO‐3. Results show that while the replacement of branched alkyl/alkoxy sidechains has little impact on optical properties and energy levels, it can change crystal stacking motifs significantly. The single crystal of Y6‐1O with all linear sidechains forms a 2D‐brickwork structure and shows lower mobility. In contrast, 1OBO‐2 with all branched sidechains exhibits a favorable 3D interpenetrating porous network, displaying an electron mobility of 1.42 cm2 V−1 s−1 in single‐crystal organic field‐effect transistors (SC‐OFETs). This value is among the highest for NFA‐based n‐type OFETs. This study not only reveals the fundamental structure–property relationships of Y‐series NFAs, but also suggests the potential of Y‐series NFAs for high‐performance n‐type organic semiconductors.

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.

Polarity Switching in Organic Electronic Devices via Terminal Substitution of Active-Layer Molecules

Organic electronic devices often suffer from poor charge injection limiting their performance. Specifically, high-performance electronic devices usually need Ohmic contacts, but it is not easy to realize them in junctions of the organic semiconductor with the electrodes because of the contact problems. In this work, polarity switching in organic field-effect transistors (OFETs) that is independent of the electrode work function is demonstrated─the switching of charge injection from the p-type to ambipolar and to the n-type─via modification of the donor/acceptor character of the molecular terminal substituents. By using three thiophene–phenylene co-oligomers with the same conjugated core, similar crystal packings, but different terminal substituents (methyl, trimethylsilyl, and trifluoromethyl), the polarity switching in both thin-film and single-crystal OFETs is demonstrated. The ultraviolet photoelectron spectroscopy studies and electronic structure calculations justify a definitive role of the interface dipole stemming from the terminal groups in controlling the heights of charge injection barriers and hence in charge injection into the OFET active layer. The results obtained are expected to facilitate rational design of organic semiconductors for high-performance electronic devices.

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.

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.

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.

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.

Correlation between the static and dynamic responses of organic single-crystal field-effect transistors

Transistors, the most important logic elements, are maintained under dynamic influence during circuit operations. Practically, circuit design protocols and frequency responsibility should stem from a perfect agreement between the static and dynamic properties. However, despite remarkable improvements in mobility for organic semiconductors, the correlation between the device performances achieved under static and dynamic circumstances is controversial. Particularly in the case of organic semiconductors, it remains unclear whether parasitic elements that relate to their unique molecular aggregates may violate the radiofrequency circuit model. Thus, we herein report the manufacture of micrometre-scale transistor arrays composed of solution-processed organic semiconductors, which achieve near very high-frequency band operations. Systematic investigations into the device geometrical factors revealed that the radiofrequency circuit model established on a solid-state continuous medium is extendable to organic single-crystal field-effect transistors. The validity of this radiofrequency circuit model allows a reliable prediction of the performances of organic radiofrequency devices.

High-performance n- and p-type organic single-crystal field-effect transistors with an air-gap dielectric towards anti-ambipolar transport

By combining high-performance n- and p-type single crystals with an air-gap dielectric, excellent anti-ambipolar transport with a small hysteresis was achieved.

Single-crystal field-effect transistors based on a fused-ring electron acceptor with high ambipolar mobilities

A fused-ring electron acceptor Y6 was used to fabricate single-crystal organic field-effect transistors, providing high ambipolar mobilities.

Investigation of charge transport properties of [1]Benzothieno[3,2-b][1]-benzothiophene single-crystals in field-effect transistor configuration

Abstract Single-crystals of unsubstituted [1]Benzothieno[3,2-b][1]-benzothiophene (BTBT) were prepared by physical vapor transport deposition (VTP). The packing structure and morphology of the crystals were studied by X-ray diffraction (XRD), polarized optical microscopy (POM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The charge transport properties of BTBT single-crystals were also investigated via bottom contact/bottom gate (BC/BG) organic field-effect transistors (OFETs) on both SiO2 and n-octadecyltrichlorosilane (OTS) treated surfaces. A maximum hole mobility value of 0.032 cm2V−1s−1 was measured on the OTS substrate. In addition, single-crystal OFETs with ion gel top gate (TG) configuration were also investigated for low voltage operation. This work represents the first investigation of charge carrier mobility of a simple BTBT in transistor configuration and highlights the essential role of the BTBT substitution in charge transport properties.

Relieving the Photosensitivity of Organic Field-Effect Transistors.

It is generally believed that the photoresponse behavior of organic field-effect transistors (OFETs) reflects the intrinsic property of organic semiconductors. However, this photoresponse hinders the application of OFETs in transparent displays as driven circuits due to the current instability resulting from the threshold voltage shift under light illumination. It is necessary to relieve the photosensitivity of OFETs to keep the devices stable. 2,6-diphenyl anthracene thin-film and single-crystal OFETs are fabricated on different substrates, and it is found that the degree of molecular order in the conducting channels and the defects at the dielectric/semiconductor interface play important roles in determining the phototransistor performance. When highly ordered single-crystal OFETs are fabricated on polymeric substrates with low defects, the photosensitivity (P) decreases by more than 105 times and the threshold voltage shift (ΔVT ) is almost eliminated compared with the corresponding thin-film OFETs. This phenomenon is further verified by using another three organic semiconductors for similar characterizations. The decreased P and ΔVT of OFETs ensure a good current stability for OFETs to drive organic light-emitting diodes efficiently, which is essential to the application of OFETs in flexible and transparent displays.

Quadruply B←N-Fused Dibenzo-azaacene with High Electron Affinity and High Electron Mobility.

For many years, organoboron compounds have been expected to show excellent electron-injecting and -transporting properties. However, lowest unoccupied molecular orbital (LUMO) energy levels (ELUMO) of B-containing π-conjugated molecules are mostly higher than -4.0 eV and their electron mobilities are usually less than 10-2 cm2 V-1 s-1. In this work, we experimentally prove the remarkably high electron affinity and high electron mobility of organoboron compounds. Our strategy is to incorporate multiple boron-nitrogen coordination bonds (B←N) into azaacenes. We synthesized quadruply B←N-fused dibenzo-azaacene (QBNA) through one-pot multifold borylation cyclization reaction. The incorporation of four B←N units greatly changes the electronic structures and properties and significantly downshifts the electronic energy levels of QBNA. QBNA shows a ELUMO of as low as -4.58 eV, which is among the lowest for n-type organic semiconductors. Single-crystal organic field-effect transistors of QBNA display unipolar n-type characteristic with an electron mobility of up to 1.60 cm2 V-1 s-1 together with excellent ambient stability. This study thus provides a design strategy for high-performance n-type organic semiconductors and high electron-affinity π-systems based on organoboron chemistry.

Self-assembled monolayers induced performance difference in organic single crystal field-effect transistors

Abstract In this report, 2,7-bis(4-methoxyphenyl)[1]benzothieno [3,2-b][1]benzothiophene. (DBOP-BTBT) organic single crystal field-effect transistors (OSCFETs) based on octyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS) treated substrates have been systematically investigated. During all three consecutive scans of all transistors with different orientation in a-b plane, transistors modified with HMDS exhibit negligible hysteresis, while those modified with OTS show large hysteresis. Meanwhile, a high mobility up to 12.9 cm2 V-1 s-1 is obtianed for HMDS devices, compared to 7.7 cm2V-1 s-1 for devices modified with OTS. The hysteresis difference can be explained by the different trap densities between forward scan and reverse scan. Furthermore, the discrepancy in mobilities of forward and reverse scans is interpreted using a scattering model. Finally, after comparing the surface energies HMDS and OTS treated substrates, we attribute the performance difference between HMDS and OTS modified devices to the difference in intermolecular interaction between the SAMs and water. The work here indicates the importance of fine selection of SAMs for OSCFETs.

Investigating the single crystal OFET and photo-responsive characteristics based on an anthracene linked benzo[b]benzo[4,5]thieno[2,3-d]thiophene semiconductor

Abstract Organic single crystals have great potential in optoelectronic devices owing to their grain-boundary free nature and long exciton diffusion length. In this report, an anthracene linked benzo[b]benzo[4,5]thieno[2,3-d]thiophene semiconductor, 2-An-BTBT was designed and synthesized. Belt like organic single crystals based on 2-An-BTBT were grown via physical vapor transport method. Using an ultrathin wire as the shadow mask, organic field effect transistor (OFET) with a channel length of around 15 μm was fabricated and the highest mobility of 0.9 cm2V−1s−1 and average mobility of 0.58 cm2/V were obtained. Furthermore, photo-response characteristics of the 2-An-BTBT based device were also investigated. Photo-response of as high as 7136 AW-1 was achieved via 17.7 μW/cm2 @380 nm mono wavelength light illumination which is ascribed to the long exciton diffusion length of organic single crystal and ultra-short channel length of the fabricated OFET. Stable switching behavior with rising time and decline time of several seconds is attained which indicates the existence of shallow traps at the insulator/semiconductor interface.

Organic Single-Crystalline p–n Heterojunctions for High-Performance Ambipolar Field-Effect Transistors and Broadband Photodetectors

Organic semiconducting single crystals are ideal building blocks for organic field-effect transistors (OFETs) and organic photodetectors (OPDs) because they can potentially exhibit the best charge transport and photoelectric properties in organic materials. Nevertheless, it is usual for single-crystal OFETs to be built from one kind of organic material in which the dominant transport is either electron or hole; such OFETs show unipolar charge transport. Furthermore, single-crystal OPDs present high performance only in restricted regions because of the limited absorption of one-component single crystals. In an ideal situation, devices which comprise both electron- and hole-transporting single crystals with complementary absorptions, such as single-crystalline p-n heterojunctions (SCHJs), can permit broadband photoresponse and ambipolar charge transport. In this paper, a solution-processing crystallization strategy to prepare an SCHJ composed of C60 and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) was shown. These SCHJs demonstrated ambipolar charge-transport characteristics in OFETs with a balanced performance of 2.9 cm2 V-1 s-1 for electron mobility and 2.7 cm2 V-1 s-1 for hole mobility. This demonstration is the first of single-crystal OFETs in which both electron and hole mobilities were over 2.5 cm2 V-1 s-1. OPDs fabricated upon as-prepared SCHJs exhibited highly sensitive photoconductive properties ranging from ultraviolet to visible and further to near-infrared regions as a result of complementary absorption between C60 and TIPS-PEN, thereby attaining photoresponsivities that are among the highest reported values within the OPDs. This work would provide valuable references for developing novel SCHJ systems to achieve significant progress in high-performance ambipolar OFETs and broadband OPDs.