Organic Light-emitting Field-effect Transistors Research Articles

Synthesis of a Novel D-A Type Photoelectrical Material: 5,5'-(9,9-Dioctyl-9H-Fluorene-2,7-Diyl)bis(Thiophene-2-Carbaldehyde)

A donor-acceptor organic molecule based on fluorene unit as an electron donor and aldehyde group as an electron acceptor has been demonstrated in high yields over four steps. This approach offers a much milder and more efficient route to synthesize the target compound via the Vilsmeier-Haack reaction. Optical spectra show that the electron-accepting groups induce an intermolecular charge transfer, resulting in a shift of the absorbance maximum toward longer wavelength. Such D-A type intermediate compounds as organic molecules display a significantly improved property profile in photoelectrical materials for applications in organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs).

Study on copper phthalocyanine and perylene-based ambipolar organic light-emitting field-effect transistors produced using neutral beam deposition method

The neutral cluster beam deposition (NCBD) method has been applied to the production and characterization of ambipolar, heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, multi-digitated, long-channel geometry. Organic thin films of n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide and p-type copper phthalocyanine were successively deposited on the hydroxyl-free polymethyl-methacrylate (PMMA)-coated SiO2 dielectrics using the NCBD method. Characterization of the morphological and structural properties of the organic active layers was performed using atomic force microscopy and X-ray diffraction. Various device parameters such as hole- and electron-carrier mobilities, threshold voltages, and electroluminescence (EL) were derived from the fits of the observed current-voltage and current-voltage-light emission characteristics of OLEFETs. The OLEFETs demonstrated good field-effect characteristics, well-balanced ambipolarity, and substantial EL under ambient conditions. The device performance, which is strongly correlated with the surface morphology and the structural properties of the organic active layers, is discussed along with the operating conduction mechanism.

Light-emitting field-effect transistors combining organic and metal oxide layers with partitioned heterogeneous source and drain electrodes

We fabricated organic light-emitting field-effect transistors (OLEFETs) characterized by partitioned heterogeneous source and drain contacts along with an aluminum-doped zinc oxide (AZO) layer inserted between an organic layer and a gate insulator. We elaborated such contacts so that each contact was made of a metal(s) suitable for injecting either electrons or holes. We fabricated the devices by choosing two of three kinds of metals (Au, Al, and MgAg) and one of three organic semiconductor materials. In the devices with the Au source and MgAg drain contacts, we observed drain currents at both positive and negative drain voltages. Those currents were predominant at negative drain voltages in the devices with Al drain contacts. The most intense current-injected emissions arose from the vicinity of the electron injection contact edges near channels on the AZO layers. Taking into account the energy level consideration of the materials and the effect of the partitioned contacts, we discussed these electrical and emission properties.

Light-emitting organic field-effect transistors based on highly luminescent single crystals of thiophene/phenylene co-oligomers

A series of thiophene/phenylene co-oligomers containing π-conjugated anthracene, naphthalene, and biphenyl central cores have been developed as new organic laser active materials. Light-emitting organic field-effect transistors (LE-OFETs) based on 2,6-bis(5-phenylthiophen-2-yl)anthracene (BPTA), 2,6-bis(5-phenylthiophen-2-yl)naphthalene (BPTN), and 2,6-bis(5-phenylthiophen-2-yl)-1,1′-biphenyl (BPTB) single crystals were fabricated. A clear laser oscillation was observed for BPTN and BPTB single crystals. Especially for BPTB, a low amplified spontaneous emission threshold of 1.8 ± 0.2 μJ cm−2 was achieved. In addition, all of the devices showed ambipolar transport characteristics, in which both electron and hole carriers are transported in a single FET device, and electroluminescence was clearly observed. An obvious difference in the light emission direction was observed for LE-OFET devices, which is attributed to the difference in transition dipole moment arrangement of individual molecules within the crystal.

Organic single-crystal light-emitting field-effect transistors

Growth and characterisation of single crystals constitute a major field of materials science. In this feature article we overview the characteristics of organic single-crystal light-emitting field-effect transistors (OSCLEFETs). The contents include the single crystal growth of organic semiconductors and their application to transistor devices. First, we describe various single crystal growth methods that produce different morphologies and geometries of crystals. Using these single crystals we highlight construction and performance of the devices. The single crystal approach not only allows us to study the device performance that reflects the intrinsic nature of the organic semiconductors but also is advantageous to enhancement in the steady device operation. A current-injected laser oscillation in an electronic device configuration remains as a big challenge to be achieved. In this context we briefly mention the spectrally narrowed emissions as well as the possibility of light amplification in the OSCLEFETs.

Annealing effect on light-emitting FET characteristics of π-conjugated liquid crystalline polymer

Thermal annealing effects on organic light-emitting field-effect transistor (OLEFET) characteristics of a liquid crystalline π-conjugated polymer were investigated using spin-coated films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophenione] (F8T2). Well-balanced ambipolar carrier injection was attained in the homogeneous polycrystalline film grown from the isotropic phase at 350 °C, resulting in the highest light emission under AC-gate operation. On the other hand, the film with microcrystalline grains quenched from the nematic phase at 290 °C was less electron-transporting so that the light emission intensity was lower than the amorphous film annealed at 250 °C.

A Highly Stretchable, Fiber‐Shaped Supercapacitor

Flexible and portable devices are a mainstream direction in modern electronics and related multidisciplinary fields. To this end, they are generally required to be stretchable to satisfy various substrates. As a result, stretchable devices, such as electrochemical supercapacitors, lithium-ion batteries, organic solar cells, organic light-emitting diodes, field-effect transistors, and artificial skin sensors have been widely studied. However, these stretchable devices are made in a conventional planar format that has largely hindered their development. For the portable applications, the devices need to be lightweight and small, though it is difficult for them to be made into efficient microdevices. In particular, it is challenging or even impossible for them to be used in electronic circuits and textiles that are urgently required also in a wide variety of other fields, such as microelectronic applications. Recently, some attempts have been made to fabricate wire-shaped microdevices, such as electrochemical supercapacitors. They have been generally produced by twisting two fiber electrodes with electrolytes coated on the surface. Several examples have been also successfully shown to make fiber-shaped supercapacitors with a coaxial structure. Compared with their planar counterparts, the wire or fiber shape enables promising advantages such as being lightweight and woven into textiles. Although the wire and fiber-shaped supercapacitors are also flexible with high electrochemical performance, they are not stretchable, which is critically important for many applications. For instance, the resulting electronic textiles could easily break during the use if they were not stretchable. To the best of our knowledge, herein we have, for the first time, developed a novel family of highly stretchable, fibershaped high-performance supercapacitors. Aligned carbon nanotube (CNT) sheets that are sequentially wrapped on an elastic fiber serve as two electrodes. The use of aligned CNT sheets offers combined remarkable properties including high flexibility, tensile strength, electrical conductivity, and mechanical and thermal stability. As a result, the fibershaped supercapacitor maintains a high specific capacitance of approximately 18 F/g after stretch by 75% for 100 cycles. Spinnable CNT arrays were first synthesized by chemical vapor deposition. A scanning electron microscopy (SEM) image of the array with height of 230 mm is shown in the Supporting Information, Figure S1, and the CNT shows a multi-walled structure with diameter of about 10 nm (Supporting Information, Figure S2). Aligned CNT sheets could be then continuously drawn from the array and easily attached to various substrates. Elastic fibers were used herein to offer the stretchability in the resulting supercapactiors, and rubber fibers have been mainly studied as a demonstration. For a typical fabrication on the fiber-shaped supercapacitor (Figure 1), a rubber fiber was first coated with a thin layer of

Determination of Thermal Transition Depth Profiles in Polymer Semiconductor Films with Ellipsometry

Geometric confinement and interface effects can significantly alter the thermodynamic properties of thin polymer films. Phase transition temperatures have been shown to strongly depend on film thickness below a critical thickness threshold. It has been suggested that this behavior is due to an interface-induced continuous variation in phase transition temperatures throughout the depth of the films. Here we employ variable-temperature spectroscopic ellipsometry to demonstrate the existence of these depth profiles. We examine four different polymer semiconductors that are of interest for organic light-emitting diodes, solar cells, and field-effect transistors. In contrast to insulating polymers, these light-absorbing materials provide detailed information about structural changes as a function of depth due to wavelength-dependent attenuation. This concept enables us to investigate a broad range of thermodynamic processes including the glass transition, crystallization as well as crystalline and liquid-cryst...

Electronic-Structure Theory of Organic Semiconductors: Charge-Transport Parameters and Metal/Organic Interfaces

We focus this review on the theoretical description, at the density functional theory level, of two key processes that are common to electronic devices based on organic semiconductors (such as organic light-emitting diodes, field-effect transistors, and solar cells), namely charge transport and charge injection from electrodes. By using representative examples of current interest, our main goal is to introduce some of the reliable theoretical methodologies that can best depict these processes. We first discuss the evaluation of the microscopic parameters that determine charge-carrier transport in organic molecular crystals, i.e., electronic couplings and electron-vibration couplings. We then examine the electronic structure at interfaces between an organic layer and a metal or conducting oxide electrode, with an emphasis on the work-function modifications induced by the organic layer and on the interfacial energy-level alignments.

Light‐Emitting Field‐Effect Transistors Having Combined Organic Semiconductor and Metal Oxide Layers

A new organic light-emitting field-effect transistor characterized by a metal oxide layer inserted between the organic layer and the gate insulator is proposed. The metal oxide is indirectly connected with source and drain electrodes through the organic layer. Upon increasing the potential difference between the source and drain electrodes, the emission becomes exceedingly strong and the emission region encompasses the whole channel zone.

Organic Light-Emitting Field-Effect Transistors Based upon Pentacene and Perylene

Air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, multidigitated, long-channel geometry were produced, and the current–voltage–light emission (I–V–L) characteristics were systematically examined. Two active layers of p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetra carboxylic diimide (P13) as well as a protecting layer of 2,5-bis(4-biphenyl) thiophene (BP1T) were successively deposited using the neutral cluster beam deposition method. On the basis of the growth of high-quality, well-packed crystalline thin films, the OLEFETs demonstrated good field-effect characteristics, well-balanced ambipolarity, operational stability, and electroluminescence (EL) under ambient conditions. The operating conduction and EL mechanisms responsible for the observed recombination zone are discussed with the aid of light-emission images obtained using a charge-coupled device.

Ambipolar organic light-emitting electrochemical transistor based on a heteroleptic charged iridium(III) complex

High performance organic light-emitting electrochemical transistor (OLECT) based on a phosphorescent heteroleptic charged iridium(III) complex has been developed with low-cost solution processing technique. The new OLECT showed good ambipolar behavior with balanced hole and electron mobilities of 0.20 cm2 V–1 s–1 and 0.22 cm2 V–1 s–1, respectively. Furthermore, light emission has been observed from the OLECT device and modulated by the gate. All these results suggest that charged iridium(III) complexes could be a promising candidate for single-component multifunctional organic light-emitting field-effect transistor.

Single crystal biphenyl end-capped furan-incorporated oligomers: influence of unusual packing structure on carrier mobility and luminescence

We report the synthesis and characterization of two new furan-based biphenyl end-capped oligomers, 2-([1,1′-biphenyl]-4-yl)-5-(5-([1,1′-biphenyl]-4-yl)thiophen-2-yl)furan (BPFT) and 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bifuran (BP2F) as candidate semiconductors for organic light-emitting field effect transistors (OLETs). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed the high thermostability of these furan-based semiconductors. X-Ray crystallography of single crystals grown by physical vapor transfer (PVT) method revealed a complicated herringbone packing of BPFT stacking with unusual flat and bent structures, which is different from that of BP2F and the bithiophene-based analogue 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bithiophene (BP2T). BPFT single crystal showed a higher absolute quantum yield (51%) compared to that of BP2F and BP2T. Density Functional Theory (DFT) calculations showed that the different excitation energies between flat and bent structures led to the asymmetric transition dipoles in dark state of BPFT H-aggregates, which explains the highest PLQY of BPFT single crystal. Single crystal FET based on BPFT showed an ambipolar characteristic with high hole and electron mobilities, while single crystal FET based on BP2F exhibited p-type characteristic with a high hole mobility. Light emission was observed from the single-crystal FET based on BPFT.

An Alternative Convenient Protocol for the Preparation of Regioregular Poly(3-hexylthiophene) via Thienylzincate Complex

Regioregular head-to-tail poly(3-hexylthiophene), P3HT, has been found to be an unusual class of conducting polymer with good solubility, processibility, environmental stability, and high electrical conductivity. It has been used for a broad range of chemical devices such as photovoltaics, electrochromic devices, optical sensors, organic light-emitting diodes and field-effect transistors. Due to the significant role of this polymer in the field of functionalized material chemistry, many procedures have been developed and utilized for its production. To this aim, in general, two synthetic routes such as electrochemical methods and chemical methods have been developed. Chemical methods have extensively used for the synthesis of regioregular P3HT mainly because these methods generally provide high regioregularity of the polymer in excellent yields and can be readily carried out in large scale production. Among the chemical synthetic methods, organometallic reagents have been frequently employed as key intermediates. Among those, thienylzinc halides and thienylmagnesium halides are the most common reagents. In addition, a few other reports have exploited the Suzuki and Stille reagents. Although each protocol has provided the corresponding polymer with good qualities, the use of thienylzinc halide as an organometallic reagent has been proven to be the most reliable procedure for the preparation of regioregular P3HT. Rieke demonstrated the use of thienylzinc bromide intermediates prepared by the direct insertion of highly active zinc to 2,5-dibromo-3-alkylthiophenes. McCullough also reported the synthesis of the regioregular polythiophenes utilizing both thienylmagnesium halides and thienylzinc halides prepared by the Grignard metathesis followed by transmetallation with zinc halides. In our continuing studies on the preparation of P3HT, we next focused on the development of more practical synthetic procedures especially utilizing thienylzinc reagents. One of our studies was to use 2-bromo-3-hexyl-5-iodothiophene along with the dialkylzinc reagents. Recently, Higashihara also reported the use of 2-bromo-3-hexyl-5-iodothiophene along with a zincate complex of tBu4ZnLi2 for the synthesis of P3HT. Even though the use of 2-bromo-3-hexyl-5iodothiophene provides a satisfactory route, it still requires the regio-selective halogenation which is consisted of two separate steps. Thus, to alleviate this difficulty, the use of 2,5-dibromothiophene derivatives as a monomer would be highly recommended especially in large scale production. Herein, we report a more practical synthetic route for the preparation of thienylzinc reagents and its subsequent polymerization in the presence of a Ni-catalyst affording the high head-to-tail regioregular P3HT. The schematic diagram for the preparation of P3HT used in this study is described in Scheme 1. As depicted in Scheme 1, the monomer and trialkylzincate complex should be prepared prior to polymerization. 2,5Dibromo-3-hexylthiophene was prepared by the literature procedure; treatment of n-hexyl Grignard reagent with 3bromothiophene in the presence of Ni(dppe)Cl2, followed by bromination with NBS yielded 1. Simple distillation afforded the pure product (1) in good yield (Scheme 2). The trialkylzincate complex was also easily prepared as shown in Scheme 3. When zinc chloride was treated with 1.0

Dual-transduction-mode sensing approach for chemical detection

Abstract Smart devices such as electronic nose have been developed for application in many fields like national security, defense, environmental regulation, health care, pipeline monitoring and food analysis. Despite a large array of individual sensors, the resolving power of these devices can still be improved to identify a target at a very low concentration out of a mixture of odors, if different types of transductions are employed in concert as a set of sensing responses to distinguish one odor from another. Here, we propose a new sensor architecture enabling different types of transduction signals in parallel on the same individual sensor. We demonstrate this architecture using a light emitting organic field-effect transistor (LEOFET) operated at a dual-transduction mode, as a proof-of-concept. Sensing response has been observed on both electrical and optical output signals from a green LEOFET upon exposure to an explosive taggant, with optical signal exhibiting much higher sensitivity. This new sensor architecture opens a field of devices of synergic capabilities to distinguish chemical and biological targets.

Superstructures Based upon n- and p-Type Organic Semiconductors: Toward Light-Emitting Device Applications

We have made organic light-emitting field-effect transistors (OLEFETs) using superstructures composed of layered n- and p-type organic semiconductor crystals. The superstructures are fabricated by lamination of the crystals via physical adsorption. The drain and source electrodes made of different kinds of metal are attached to both the upper and lower surfaces of each crystal. Under application of direct-current voltages to the drain and source electrodes, the superstructure OLEFETs emit bright light. The result indicates that the superstructures based on both types of organic crystals are useful for light-emitting devices.

Light-Emitting Field-Effect Transistors Having Metal Electrodes Modified with an Organic Thin Film

We have improved the emission properties and the carrier mobilities of organic light-emitting field-effect transistors (OLEFETs) by modifying the metal electrode(s) with a thin film of n-type thiophene/phenylene co-oligomer (TPCO). Their semiconductor layer was a p-type TPCO crystal. When we used the modified electrode for electron injection, the device exhibited eight times higher emission intensity than a device with unmodified electrodes. By contrast, employing the modified electrode as the hole injection contact, we achieved the maximum hole mobility of 0.11 cm2·V-1·s-1 under the hole-enhancement mode. The modified electrodes effectively functioned for injecting both electrons and holes into the p-type crystal. The origin of this is briefly discussed.

Light-Emitting Field-Effect Transistors Having Metal Electrodes Modified with an Organic Thin Film

We have improved the emission properties and the carrier mobilities of organic light-emitting field-effect transistors (OLEFETs) by modifying the metal electrode(s) with a thin film of n-type thiophene/phenylene co-oligomer (TPCO). Their semiconductor layer was a p-type TPCO crystal. When we used the modified electrode for electron injection, the device exhibited eight times higher emission intensity than a device with unmodified electrodes. By contrast, employing the modified electrode as the hole injection contact, we achieved the maximum hole mobility of 0.11 cm2·V-1·s-1 under the hole-enhancement mode. The modified electrodes effectively functioned for injecting both electrons and holes into the p-type crystal. The origin of this is briefly discussed.

π-Conjugated Cyanostilbene Derivatives: A Unique Self-Assembly Motif for Molecular Nanostructures with Enhanced Emission and Transport

π-Conjugated organic molecules represent an attractive platform for the design and fabrication of a wide range of nano- and microstructures for use in organic optoelectronics. The desirable optical and electrical properties of π-conjugated molecules for these applications depend on their primary molecular structure and their intermolecular interactions such as molecular packing or ordering in the condensed states. Because of the difficulty in satisfying these rigorous structural requirements for photoluminescence and charge transport, the development of novel high-performance π-conjugated systems for nano-optoelectronics has remained a challenge. This Account describes our recent discovery of a novel class of self-assembling π-conjugated organic molecules with a built-in molecular elastic twist. These molecules consist of a cyano-substituted stilbenic π-conjugated backbone and various terminal functional groups, and they offer excellent optical, electrical, and self-assembly properties for use in various nano-optoelectronic devices. The characteristic "twist elasticity" behavior of these molecules occurs in response to molecular interactions. These large torsional or conformational changes in the cyanostilbene backbone play an important role in achieving favorable intermolecular interactions that lead to both high photoluminescence and good charge carrier mobility in self-assembled nanostructures. Conventional π-conjugated molecules in the solid state typically show concentration (aggregation) fluorescence quenching. Initially, we describe the unique photoluminescence properties, aggregation-induced enhanced emission (AIEE), of these new cyanostilbene derivatives that elegantly circumvent these problems. These elastic twist π-conjugated backbones serve as versatile scaffolds for the preparation of well-defined patterned nanosized architectures through facile self-assembly processes. We discuss in particular detail the preparation of 1D nanowire structures through programmed self-assembly. This Account describes the importance of utilizing AIEE effects to explore optical device applications, such as organic semiconducting lasers (OSLs), optical memory, and sensors. We demonstrate the rich electronic properties, including the electrical conductivity, field-effect carrier mobility, and electroluminescence of highly crystalline 1D nanowire and coaxial donor-acceptor nanocable structures composed of elastic twist π-conjugated molecules. The electronic properties were measured using various techniques, including current-voltage (I-V), conducting-probe atomic force microscopy (CP-AFM), and space-charge-limited-current (SCLC) measurements. We prepared and characterized several electronic device structures, including organic field-effect transistors (OFETs) and organic light-emitting field-effect transistors (OLETs).

Multi-layer ambipolar light-emitting organic field-effect transistors

Mutli-layer light-emitting organic field-effect transistors (OLETs) are shown to have high internal quantum efficiencies approaching 5%, a value much higher than the conventional organic light-emitting diodes (OLEDs). This work re-examines some data reported in the literature on OLETs and put forward a model that explains the charge transport and light emission process. Our analyses suggest that the reported improvements on the internal quantum efficiency of OLETs are directly linked to charge recombination and light emission and is independent of the drain-source current as well as the gate-induced charge density in the accumulation layer. Such independence allows the internal quantum efficiency to increase as the drain-source current decreases. The process differs from the charge transport in OLEDs where recombination and light emission are directly tied to the injected space charge densities thereby preventing the internal quantum efficiency of OLEDs to increase even when the device current is lowered.