Organic Light-emitting Transistors Research Articles

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.

Top-gate type, ambipolar, phosphorescent light-emitting transistors utilizing liquid-crystalline semiconducting polymers by the thermal diffusion method

In order to obtain phosphorescent emission from organic field-effect transistors, thermal diffusion of a phosphorescent dye at the interface between the active layer and gate insulator was controlled by utilizing the fact that liquid-crystalline semiconducting polymers self-organize due to reorientation of the molecules and increase in the size of the crystalline regions during thermal annealing. For top-gate type devices using poly(alkylfluorene) doped with a red emissive phosphorescent material, ambipolar characteristics and red emission from the phosphorescent material were clearly observed. We demonstrate the possibility of producing phosphorescent organic light-emitting transistors using a liquid-crystalline semiconducting polymer doped with phosphorescent emissive dopants by the thermal diffusion method.

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.

2,2′]Bi[naphtho[2,3-b]furanyl]: a versatile organic semiconductor with a furan–furan junction

[2,2']Bi[naphtho[2,3-b]furanyl] was synthesized, characterized, and examined as an organic semiconductor for thin-film OFETs, bilayer OPVs, and organic light-emitting transistors (OLETs). In the devices, the material acted as a p-type semiconductor, showing moderately high mobility in OFETs, good photo conversion efficiency in OPVs, and blue-green emission in OLETs.

Spatial Control of p–n Junction in an Organic Light-Emitting Electrochemical Transistor

Low-voltage-operating organic electrochemical light-emitting cells (LECs) and transistors (OECTs) can be realized in robust device architectures, thus enabling easy manufacturing of light sources using printing tools. In an LEC, the p-n junction, located within the organic semiconductor channel, constitutes the active light-emitting element. It is established and fixated through electrochemical p- and n-doping, which are governed by charge injection from the anode and cathode, respectively. In an OECT, the electrochemical doping level along the organic semiconducting channel is controlled via the gate electrode. Here we report the merger of these two devices: the light-emitting electrochemical transistor, in which the location of the emitting p-n junction and the current level between the anode and cathode are modulated via a gate electrode. Light emission occurs at 4 V, and the emission zone can be repeatedly moved back and forth within an interelectrode gap of 500 μm by application of a 4 V gate bias. In transistor operation, the estimated on/off ratio ranges from 10 to 100 with a gate threshold voltage of -2.3 V and transconductance value between 1.4 and 3 μS. This device structure opens for new experiments tunable light sources and LECs with added electronic functionality.

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).

Universal energy-level alignment of molecules on metal oxides

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

Correlation among Morphology, Crystallinity, and Charge Mobility in OFETs Made of Quaterthiophene Alkyl Derivatives on a Transparent Substrate Platform

In this Article, we present a comprehensive study of organic field-effect transistors (OFETs) made of thin films of methyl, n-butyl, and n-hexyl end-substituted quaterthiophenes on a transparent substrate platform. This particular platform has been already used for organic light-emitting diodes (OLEDs) but rarely employed for OFETs. In perspective, this is a very promising route for the development of field-effect photonic applications such as organic light-emitting transistors (OLETs). A systematic characterization of the organic films has been made by means of atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate morphology, crystallinity and charge mobility to the alkyl chain length. In particular, a charge mobility value of 0.09 cm2/(V s) has been obtained in transparent OFETs with a large area channel for DH4T grown at room temperature. This mobility exceeds the one obtained on silicon-oxide substrates and is likely due to a more favorable interaction of the DH4T molecules with the PMMA layer employed as gate dielectric.

Electro‐optical Materials: Solution‐Processed Organic Light‐Emitting Transistors Incorporating Conjugated Polyelectrolytes (Adv. Funct. Mater. 19/2011)

Electro‐optical Materials: Solution‐Processed Organic Light‐Emitting Transistors Incorporating Conjugated Polyelectrolytes (Adv. Funct. Mater. 19/2011)

The photonic perspective of organic light‐emitting transistors

AbstractThis article will give an overview of the current state‐of‐the‐art of OLETs from the point of view of their photonic characteristics. In particular, the different device structures realized, the materials used and the strategies implemented to integrate optical resonators and waveguiding structures into light‐emitting field‐effect transistors will be reviewed and the main findings discussed. (Picture: Courtesy of E. T. C. srl)

Solution‐Processed Organic Light‐Emitting Transistors Incorporating Conjugated Polyelectrolytes

AbstractImproved performance of p‐type organic light‐emitting transistors (OLETs) is demonstrated by introducing a conjugated polyelectrolyte (CPE) layer and symmetric high work function (WF) source and drain metal electrodes. The OLET comprises a tri‐layer film consisting of a hole transporting layer, an emissive layer, and a CPE layer as an electron injection layer. The thickness of the CPE layer is critical for achieving good performance and provides an important structural handle for consideration in future optimization studies. We also demonstrate for the first time, good performance solution‐processed blue‐emitting OLETs. These results further demonstrate the simplification of device fabrication and improved performance afforded by integrating CPE interlayers into organic optoelectronic devices.

Charge-carrier injection assisted by space-charge field in AC-driven organic light-emitting transistors

Organic light-emitting transistors can be operated by an alternating gate voltage to provide high light output intensity. We propose a model for the light generation process in such light-emitting transistors based on systematic measurements of how the light output intensity depends on the biasing parameters. Following injection of holes, which form a positively charged space-charge region, subsequent electron tunneling from the same metal electrode eventually leads to light emission from the organic emitter. The electron injection is found to depend on the positive space-charge region, and the hole injection efficiency therefore strongly influences the emission intensity. Low temperature measurements show increased emission intensity as the temperature decreases indicating that the light generation is dependent on thermally activated charge injection, i.e., Fowler–Nordheim tunneling theory is not applicable. Further metal/semiconductor interface modification could result in optimized charge injection under AC biasing, thus leading to more efficient organic light-emitting devices.

Observation of electron behavior in ambipolar polymer-based light-emitting transistor by optical second harmonic generation

By using the optical second harmonic generation (SHG) measurement, we directly visualized the carrier behavior leading to carrier recombination and electroluminescence (EL) in ambipolar polymer-based organic light-emitting transistor (OLET) with an active layer of poly 9,9-di-n-octylfluorene-alt-benzothiadiszole (F8BT). Eliminating photoluminescence generated at 560 nm by a two-photon absorption process, the dynamical carrier motion in the F8BT-OLET was visualized by the electric field induced SHG induced at 420 nm. Diffusion-like electron transport that starts from the drain electrode was directly caught as the transits of the SHG images. Accordingly, EL was obtained at the edge of the source electrode. The electron mobility was estimated from the visualized carrier motion as 9.2×10-4cm2/Vs, which was larger than that obtained from the transfer curve of the OLET.

Spotlight on organic transistors

The use of carbon nanotube electrodes and new device geometries is enhancing the performance of organic light-emitting transistors.

Four-Coordinate Organoboron Compounds with a π-Conjugated Chelate Ligand for Optoelectronic Applications

Four-coordinate organoboron compounds with a π-conjugated chelate backbone have emerged recently as highly attractive materials for a number of applications including use as emitters and electron-transport materials for organic light-emitting diodes (OLEDs) or organic field transistors, photoresponsive materials, and sensory and imaging materials. Many applications of this class of boron compounds stem from the electronic properties of the π-conjugated chelate backbone. Charge-transfer transitions from an aromatic substituent attached to the boron center of the π-conjugated chelate backbone and steric congestion have also been found to play important roles in the luminescent and photochromic properties of the four-coordinate boron compounds. This article provides an update-to-date account on the application aspects of this important class of compounds in materials science with the emphasis on OLED applications and photochromic switching.

Influence of the substrate platform on the opto-electronic properties of multi-layer organic light-emitting field-effect transistors

In this paper, we present a study of the effects of the influence of the substrate platform on the properties of a three-layer vertical hetero-junction made of thin films of α, ω-diperfluorohexyl-4T (DHF4T), a blend of tris(8-hydroxyquinoline)aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and α, ω-dihexyl-quaterthiophene (DH4T). The hetero-junction represents the active component of an organic light-emitting transistor (OLET). The substrate platforms investigated in this study are glass/indium-tin-oxide/poly(methyl-methacrylate) (PMMA) and Si++/silicon oxide (SiO2)/PMMA. The first platform is almost completely transparent to light and therefore is very promising for use in OLET applications. The second one has been chosen for comparison as it employs standard microelectronic materials, i.e. Si++/SiO2. We show how different gate materials and structure can affect the relevant field-effect electrical characteristics, such as the charge mobility and threshold voltage. By means of an atomic force microscopy analysis, a systematic study has been made in order to correlate the morphology of the active layers with the electrical properties of the devices.

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Low-Voltage, Low-Power, Organic Light-Emitting Transistors for Active Matrix Displays

Intrinsic nonuniformity in the polycrystalline-silicon backplane transistors of active matrix organic light-emitting diode displays severely limits display size. Organic semiconductors might provide an alternative, but their mobility remains too low to be useful in the conventional thin-film transistor design. Here we demonstrate an organic channel light-emitting transistor operating at low voltage, with low power dissipation, and high aperture ratio, in the three primary colors. The high level of performance is enabled by a single-wall carbon nanotube network source electrode that permits integration of the drive transistor and the light emitter into an efficient single stacked device. The performance demonstrated is comparable to that of polycrystalline-silicon backplane transistor-driven display pixels.

Multiple colour emission from an organic light-emitting transistor

The unique property of ambipolar organic light-emitting field-effect transistors that the position of the light emission zone within the transistor channel can be controlled by the applied biases opens novel aspects for applications. In this contribution we demonstrate a transistor with electrically tuneable emission colour by taking advantage of the controlled displacement of the light emission zone. The colour change is achieved due to a colour conversion layer covering partially the transistor channel. Depending on the position of charge carrier recombination the emitted light of the transport layer or the converted light from the colour conversion layer can be detected.