Single-crystal Organic Field-effect Transistors Research Articles

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.

An Ethynylene-Bridged Pentacene Dimer: Two-Step Synthesis and Charge-Transport Properties.

A rigid and planar ethynylene-bridged pentacene dimer (PenD) was synthesized from pentacenequinone in two steps, skipping the conventional stepwise approach. A brickwork motif in the single crystal shows two-dimensionally extended electronic interaction in the solid state. Highly crystalline dip-coated films exhibited average hole mobility of 0.24 cm2 V-1 s-1 , comparable to that of the single-crystal organic field-effect transistors. This discovery and understanding of the reaction for the facile synthesis of ethynylene-bridged π-conjugated systems enables to the synthesis of a wide range of organic semiconducting materials.

Solution-crystallized n-type organic thin-film transistors: An impact of branched alkyl chain on high electron mobility and thermal durability

Abstract Single-crystal organic thin-film field-effect transistors (OTFTs) have been actively studied because they are regarded as an ideal platform for charge carrier transport phenomena in small-molecule organic materials. Moreover, recently developed solution-based techniques to grow single-crystalline thin films have attracted industrial attention beyond fundamental physics. In this work, one of the solution-based techniques, namely, the edgecasting method, was employed to study single-crystal OTFTs of n-type organic semiconductors, dimerized naphtho[2,3-b]thiophene diimides, with linear and branched alkyl chains. A high electron mobility of >1 cm2 V−1 s−1 and an excellent thermal durability were observed in the branched alkyl-chain derivative, while the OTFT based on linear alkyl-substituted one showed clear degradation of mobility after thermal annealing. Therefore, this work will demonstrate a potential availability of branched alkyl chains for improving carrier mobility and thermal durability with large-scalable printing methods.

Direct Printing of Graphene Electrodes for High-Performance Organic Inverters.

Scalable fabrication of high-resolution electrodes and interconnects is necessary to enable advanced, high-performance, printed, and flexible electronics. Here, we demonstrate the direct printing of graphene patterns with feature widths from 300 μm to ∼310 nm by liquid-bridge-mediated nanotransfer molding. This solution-based technique enables residue-free printing of graphene patterns on a variety of substrates with surface energies between ∼43 and 73 mN m-1. Using printed graphene source and drain electrodes, high-performance organic field-effect transistors (OFETs) are fabricated with single-crystal rubrene (p-type) and fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIF-CN2) (n-type) semiconductors. Measured mobilities range from 2.1 to 0.2 cm2 V-1 s-1 for rubrene and from 0.6 to 0.1 cm2 V-1 s-1 for PDIF-CN2. Complementary inverter circuits are fabricated from these single-crystal OFETs with gains as high as ∼50. Finally, these high-resolution graphene patterns are compatible with scalable processing, offering compelling opportunities for inexpensive printed electronics with increased performance and integration density.

Effect of the Deformation State on the Response of a Flexible H2S Sensor Based on a Ph5T2 Single-Crystal Transistor

Flexible H2S sensors were fabricated based on ultrathin dinaphtho [3, 4-d:3’, 4’-d’] benzo [1, 2-b:4, 5b’] dithiophene (Ph5T2) single-crystal organic field-effect transistors, and response performances at different deformation states were investigated. The flexible devices exhibited strikingly different sensing behaviors under different deformation states toward H2S at room temperature. In a tensile state with ${r} = \textsf {7}$ mm, the flexible sensor presented an unprecedented response as high as 400% at 1 ppm H2S, which is almost one order of magnitude higher than the response in a flat state and surpasses all reported flexible H2S sensors. Such a surprising improvement provides an effective way to enhance the response of gas sensors and shows the potential advantage of using flexible sensors in tensile states.

Intrinsic charge carrier mobility in single-crystal OFET by “fast trapping vs. slow detrapping” model

Abstract This study investigates the gate stress-induced mobility discrepancy in p-type single-crystal organic field-effect transistors (OFETs) on an octyltrichlorosilane (OTS)-modified SiO2/Si substrate. During measurements in atmosphere, anti-clockwise hysteresis was observed in the transfer curve, and the mobility calculated from the forward sweep was smaller than that calculated from the reverse sweep. Hysteresis has often been observed for OFETs but the mobility discrepancy has not been clearly understood. We formulated a “fast trapping vs. slow detrapping” model and suggested that the mobility values calculated from the reverse sweep represent the intrinsic property of the material. To verify the validity of this model, we investigated mobility anisotropy of an air-stable organic semiconductor, 2,7-bis(4-methoxyphenyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (DBOP-BTBT). By measuring the single-crystal OFET characteristics of many crystals with different orientations, we observed anisotropic hole mobility calculated from the reverse sweep. The mobility along the b-axis, which corresponds to the π-π stacking direction, was 13.9 cm2 V−1 s−1, and that along the a-axis was 6.2 cm2 V−1 s−1. However, we did not see clear anisotropy when mobility was calculated from the forward sweep due to a variation in the data. The threshold voltage from the reverse and the forward sweeps showed isotropic characteristics within the range from −60 to −70 V and from −50 to −60 V, respectively. These results indicate that the numbers of filled traps were different between the reverse and the forward sweeps at the interface, and confirm the validity of our model.

Solution processed air-stable p-channel organic crystal field-effect transistors of Aminobenzodifuranone

A novel donor-acceptor type aminobenzodifuranone based chromophore molecule, coded as ABDF, with a benzodifuranone (BDF) core as acceptor and an N-ethylamino side chain as donor moiety was synthesized and used in single crystal organic field-effect transistors (OFETs). The optical, thermal, electrochemical properties of ABDF were also investigated. Compared to other BDF or halogenated BDF analogs, the solution absorption of ABDF exhibits large bathochromic shift over 100 nm, indicating the strong intramolecular charge transfer intensity within the molecules. In particular, the incorporation of amino groups into the BDF core also enables the formation of intermolecular hydrogen-bonding interactions in the solid states of ABDF as proved by the FT-IR results. The crystalline ABDF based OFET devices was simply prepared by drop-casting the ABDF solution on to the electrode-prepatterned substrates, giving a hole mobility as high as 0.42 cm2/V·s without large degradation after store in ambient environment for up to two months. ABDF is a promising chromophore for high performance organic field-effect transistors.

Polarization-Dependent Photoinduced Bias-Stress Effect in Single-Crystal Organic Field-Effect Transistors

Photoinduced charge transfer between semiconductors and gate dielectrics can occur in organic field-effect transistors (OFETs) operating under illumination, leading to a pronounced bias-stress effect in devices that are normally stable while operating in the dark. Here, we report an observation of a polarization-dependent photoinduced bias-stress effect in two prototypical single-crystal OFETs, based on rubrene and tetraphenylbis(indolo{1,2-a})quinolin. We find that the decay rate of the source-drain current in these OFETs under illumination is a periodic function of the polarization angle of incident photoexcitation with respect to the crystal axes, with a periodicity of π. The angular positions of maxima and minima of the bias-stress rate match those of the optical absorption coefficient of the corresponding crystals. The analysis of the effect shows that it stems from a charge transfer of "hot" holes, photogenerated in the crystal within a very short thermalization length (≪μm) from the semiconductor-dielectric interface. The observed phenomenon is a type of intrinsic structure-property relationship, revealing how molecular packing affects parameter drift in organic transistors under illumination. We also demonstrate that a photoinduced charge transfer in OFETs can be used for recording rewritable accumulation channels with an optically defined geometry and resolution, which can be used in a number of potential applications.

Methyl substituent effect on structure, luminescence and semiconducting properties of furan/phenylene co-oligomer single crystals

Single crystals of furan/phenylene co-oligomers are among the most promising highly-emissive materials for applications in various optoelectronic devices. In this work, we synthesized and studied furan/phenylene co-oligomers with the same conjugated core 1,4-bis(5-phenylfuran-2-yl)benzene and methyl substituents at p- and m-positions of the terminal phenyls. The effect of substituents on the crystal packing, charge transport and luminescence of the single crystals was studied. Compared to the unsubstituted compound, the methyl-substituted co-oligomers demonstrated improved thermostability and enhanced photoluminescence, which we assign to J-aggregation resulting from the strong inclination of the molecules against the main crystal facet. The charge mobility in single crystal organic field-effect transistors decreased upon the inclination of the molecules. We conclude that the molecular tilt angle, intermolecular distances and interactions in crystals of heteroaryl-containing linear conjugated oligomers can be controlled by the introduction of end methyl groups in the appropriate positions.

Methylthionated benzo[1,2-b:4,5-b']dithiophenes: a model study to control packing structures and molecular orientation in thienoacene-based organic semiconductors.

Methylthionation on benzo[1,2-b:4,5-b']dithiophene (BDT) was examined as a promising molecular functionalization strategy to tune the packing structure and molecular orientation on the substrates. β-Methylthionated BDT with a rubrene-like "pitched" π-stacking showed higher mobility than its α-counterpart with a herringbone packing in single-crystal organic field-effect transistors (SC-OFETs).

Enhanced mobility in organic field-effect transistors due to semiconductor/dielectric interface control and very thin single crystal

A perfect organic crystal while keeping high quality semiconductor/dielectric interface with minimal defects and disorder is crucial for the realization of high performance organic single crystal field-effect transistors (OSCFETs). However, in most reported OSCFET devices, the crystal transfer processes is extensively used. Therefore, the semiconductor/dielectric interface is inevitably damaged. Carrier traps and scattering centers are brought into the conduction channel, so that the intrinsic high mobility of OSCFET devices is entirely disguised. Here, very thin pentacene single crystal is grown directly on bare SiO2 by developing a ‘seed-controlled’ pentacene single crystal method. The interface quality is controlled by an in situ fabrication of OSCFETs. The interface is kept intact without any transfer process. Furthermore, we quantitatively analyze the influence of crystal thickness on device performance. With a pristine interface and very thin crystal, we have achieved the highest mobility: 5.7 cm2 V−1 s−1—more than twice the highest ever reported pentacene OSCFET mobility on bare SiO2. This study may provide a universal route for the use of small organic molecules to achieve high performance in lamellar single crystal field-effect devices.

Suppressing molecular vibrations in organic semiconductors by inducing strain.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm2 V−1 s−1 by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.

Organic Single Crystals: An Essential Step to New Physics and Higher Performances of Optoelectronic Devices

Organic Single Crystals: An Essential Step to New Physics and Higher Performances of Optoelectronic Devices

Synthesis and transistor application of the extremely extended phenacene molecule, [9]phenacene.

Many chemists have attempted syntheses of extended π-electron network molecules because of the widespread interest in the chemistry, physics and materials science of such molecules and their potential applications. In particular, extended phenacene molecules, consisting of coplanar fused benzene rings in a repeating W-shaped pattern have attracted much attention because field-effect transistors (FETs) using phenacene molecules show promisingly high performance. Until now, the most extended phenacene molecule available for transistors was [8]phenacene, with eight benzene rings, which showed very high FET performance. Here, we report the synthesis of a more extended phenacene molecule, [9]phenacene, with nine benzene rings. Our synthesis produced enough [9]phenacene to allow the characterization of its crystal and electronic structures, as well as the fabrication of FETs using thin-film and single-crystal [9]phenacene. The latter showed a field-effect mobility as high as 18 cm2 V−1 s−1, which is the highest mobility realized so far in organic single-crystal FETs.

Determination of optimal ionic liquid for organic single-crystal field-effect transistors

We investigate organic single-crystal field-effect transistors with various ionic liquids as gate dielectric. We find that the mobility of the field-effect transistors for both p-type and n-type organic semiconductors increases with decreasing total capacitance of the ionic liquid. However, it does not depend on the ion species at the interface between the organic semiconductor and the ionic liquid. By choosing an appropriate ionic liquid, a high carrier mobility of 12.4 cm2/V s in rubrene single crystals (p-type) and 0.13 cm2/V s in 7.7.8.8-Tetracyanoquinodimethane single crystals (n-type) are achieved. This study clarifies the influence of ionic liquids on the device performance of organic field-effect transistors and shows a way to maximize carrier mobility at the solid/liquid interface.

2-Positional pyrene end-capped oligothiophenes for high performance organic field effect transistors.

Three new 2-positional pyrene end-capped oligothiophene co-oligomers, BPynT (n = 1, 2, 3), have been synthesized for application in organic field effect transistors (OFETs). BPy2T showed the highest hole mobility of 3.3 cm(2) V(-1) s(-1) in a single crystal OFET and a good photoluminescence efficiency of 32% in the crystalline state. A green light emission was observed for the OFET based on a BPy2T single crystal.

Organic single-crystal transistors and circuits on ultra-fine Au wires with diameters as small as 15 μm via jigsaw puzzle method

A novel jigsaw puzzle method was developed to fabricate the high-performance CuPc and F16CuPc organic single-crystal field-effect transistors and circuits on ultra-fine and flexible Au wires with diameters as small as $15~\mu \text{m}$ . This method weakens the effect of fiber size on the device-fabrication process and provides a simple way to realize organic single-crystal complementary circuits on ultra-fine fibers. The resulting high-performance miniaturized fiber-shaped devices with extremely small diameters have strong potential for the applications in wearable and implantable electronics.

Organic field-effect transistors based on single-crystalline active layer and top-gate insulator consistently fabricated by electrostatic spray deposition

An electrostatic spray deposition (ESD) method was applied to prepare both crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and insulating films of poly(methyl methacrylate) (PMMA) for fabricating top-gate single-crystal organic field-effect transistors (OFETs). The electrical characteristics of the top-gate device were compared to those of the bottom-gate one (SiO2 bottom-gate insulator) with the same active layer, and the lower charge-trap density at the interface between the top-gate insulator and single-crystalline active layer was demonstrated. The drain current compression in the output characteristics of the top-gate device, however, occurred due to the large parasitic resistance between the source/drain electrodes and accumulation channel. Reducing the thickness of the single-crystalline active layer resulted in a high charge-carrier mobility of 0.29cm2/Vs (channel length of 5μm).

All solution-processed organic single-crystal transistors with high mobility and low-voltage operation

High-mobility organic single-crystal field-effect transistors of 3,11-didecyldinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]-dithiophene (C10-DNBDT) operating at low driving voltage are fabricated by an all-solution process. A field-effect mobility as high as 6.9cm2/Vs is achieved at a driving voltage below 5V, a voltage as low as in battery-operated devices, for example. A low density of trap states is realized at the surface of the solution-processed organic single-crystal films, so that the typical subthreshold swing is less than 0.4V/decade even on a reasonably thick amorphous polymer gate dielectrics with reliable insulation. The high carrier mobility and low interface trap density at the surface of the C10-DNBDT crystals are both responsible for the development of the high-performance all-solution processed transistors.