Organic Light-emitting Field-effect Transistors Research Articles

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.

Solution-processed polyfluorene–ZnO nanoparticles ambipolar light-emitting field-effect transistor

We report on a solution-processed polyfluorene (PFO)–ZnO nanoparticles composite light-emitting organic field-effect transistor (LE-OFET). The behavior of absorption, photoluminescence spectra and electroluminescence intensity of the PFO:ZnO hybrid films with different concentration of ZnO nanoparticles is analyzed. By changing the PFO/ZnO nanoparticles concentration ratio the PFO:ZnO OFET shows either unipolar or ambipolar behavior of current–voltage characteristics and operates in the hole/electron accumulation mode with current saturation behavior. The field effect mobility of charge carriers depends on the ZnO concentration. For the ambipolar PFO:ZnO OFET with modest PFO/ZnO nanoparticles ratio (1:0.2), well balanced electron and hole mobility values at 300 K are ∼0021 and ∼0029 cm 2/Vs, respectively, whereas for films with high ZnO concentration (1:1) the mobility (∼2 cm 2/Vs) is close to that of polycrystalline ZnO. The ambipolar PFO:ZnO LE-OFET emits light at both positive and negative gate bias. The working mechanism of the PFO:ZnO LE-OFET is investigated.

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.

Influence of gate dielectrics on the performance of single-layered organic transistors and bi-layered organic light-emitting transistors prepared by the neutral cluster beam deposition method

The influence of two different SiO2 and polymethylmethacrylate (PMMA) gate dielectrics on the performance of single-layered organic field-effect transistors and bi-layered organic light-emitting field-effect transistors was examined. Organic active layers of p-type α,ω-dihexylsexithiophene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide were prepared using the neutral cluster beam deposition method. Characterization of surface morphology, contact angles, structural properties and temperature dependence of field-effect mobilities measured over the temperature range of 10–300 K revealed that compared to the SiO2 dielectrics, the hydroxyl-free PMMA dielectrics provided better conditions for crystalline film growth. The PMMA-based device characteristics exhibited excellent field-effect mobilities, stress-free operational stability, and electroluminescence through efficient carrier transport and well-balanced ambipolarity under ambient conditions.

Spectrally-narrowed emissions from a layered organic transistor equipped with a diffraction grating

We propose a construction of an organic light-emitting field-effect transistor for producing spectrally-narrowed emissions (SNEs) under current injection. The device is characterized by the followings: a diffraction grating fabricated within the channel region, homogeneous source and drain electrodes made of a low work-function metal and a layered structure of organic semiconductor materials. The organic layered structure was composed of a p-type crystal and an n-type thin film. The latter film was deposited so as to cover the grating on the channel. We observed SNEs from the device when it was operated by applying square-wave alternating gate voltages. The observed spectra were peaking as a dominant line around 578nm with a full width at half maximum less than 4nm.

25 Years Full of Chemical Discovery

25 Years Full of Chemical Discovery

Probing of contact formation via light emission from organic field-effect transistors

The recent progress on ambipolar organic light-emitting field-effect transistors has opened new possibilities for characterizing the basic physical processes in organic field-effect transistors (OFETs). Here, a way of investigating the contact formation from the light emission of OFETs is presented. In general, in the ambipolar transport regime a spatially controllable recombination zone can be observed in the channel of the transistor by light emission. In the unipolar regimes no light emission is expected due to the fact that only one charge carrier type is accumulated in the channel region and, thus, charge carriers cannot recombine. However, our results indicate that light emission in the unipolar transport regimes is possible due to thermal injection of charge carries at the contacts. It will be demonstrated that emission in the unipolar regime is present in devices providing ohmic contacts and that it can be totally suppressed utilizing non-ohmic contacts which can be achieved by the use of suited metals. Further, employing a double layer of different color emitting acenes in the transistor channel, the vertical movement of the recombination zone can be probed by a color change in the light emission when passing from the unipolar to the ambipolar regime. Both the dependence of the light emission on the utilized contact metal and the color change in the double layer device clearly demonstrate that different conditions apply for the light emission in the unipolar and ambipolar regimes.

A light-emitting field-effect transistor based on a polyfluorene–ZnO nanoparticles film

We report on a light-emitting organic field-effect transistor (LE-OFET) based on a polyfluorene (PFO)–ZnO nanoparticles film. We have analysed the behaviour of absorption, photoluminescence spectra and electroluminescence (EL) intensity of the PFO : ZnO hybrid films with different ZnO concentrations prepared in the FET configuration. We have found that the PFO : ZnO LE-OFET exhibits the current–voltage characteristics of a unipolar FET with saturation behaviour and operates in the hole accumulation mode. The FET mobility values in the PFO : ZnO films with a high ZnO concentration are close to the mobility of polycrystalline ZnO. A significant increase in the EL intensity of the hybrid films has been observed with an increase in the concentration of ZnO nanoparticles and with the application of gate voltages. The working mechanism of the LE-OFET device is discussed.

A Color‐Tuneable Organic Light‐Emitting Transistor

An organic light-emitting field-effect transistor whose emission color can be changed by the applied voltage is presented. The transistor is based on a parallel layer stack of acenes serving as organic transport and emission layers. During operation, the position of the recombination zone can be moved by a proper change in the drain and gate bias from one organic semiconductor of the stack to the other, inducing a change in the emission color from green to red.

Emission Color Tuning in Ambipolar Organic Single‐Crystal Field‐Effect Transistors by Dye‐Doping

AbstractThe effect of dye‐doping in ambipolar light‐emitting organic field‐effect transistors (LE‐OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye‐doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE‐OFETs. A rather high external EL quantum efficiency (∼0.64%) of one order of magnitude higher than that of a pure p‐distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene‐doped P3V2 crystal involves direct carrier recombination on the tetracene molecules.

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

AC-biased organic light-emitting field-effect transistors from naphthyl end-capped oligothiophenes

The photoluminescence and electroluminescence of two devices made from naphthyl end-capped oligothiophenes are investigated based on an organic light-emitting field-effect transistor (OLEFET) configuration. A novel method to light up an active organic layer using a sinusoidal alternating current (AC) gate voltage is examined. The electroluminescence spectra from both materials are very similar to their photoluminescence spectra indicating that the light emission is caused by the same electronic transitions. The intensity of the electroluminescence emission increases with increasing frequency of the AC gate voltage, and it is non-linearly dependent on the amplitude of the AC gate voltage. Moreover, light emission on both sides of the charge transporting channel is revealed by an optical microscope when the channel is appropriately wide, e.g. 10 μm. A simple model involving sequential injection of both charge-carrier types from one electrode into the organic material explains the results.

Characterization of Perylene and Tetracene-Based Ambipolar Light-Emitting Field-Effect Transistors

Herein is presented systematic analysis of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors (OLEFETs). Top-contact OLEFETs with multidigitated, long channel-width geometry were produced by the successive deposition of electron-transporting N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) and hole-transporting tetracene layers, using the neutral cluster beam deposition (NCBD) method. The morphological, structural, and photoluminescence properties of the untreated and thermally post-treated P13/tetracene active layers were examined by atomic force microscopy, X-ray diffraction, and laser scanning confocal microscopy. From the comparative analysis of the NCBD thin films, the neutral cluster beams led to the preparation of smooth, uniform bilayer films consisting of well-packed grain crystallites. The OLEFETs demonstrated good field-effect characteristics, stress-free operational stability, and electroluminescence under ambient conditions. The operating ...

Organic-Crystal Light-Emitting Field-Effect Transistors Driven by Square-Wave Gate Voltages

We have improved the operation method of organic light-emitting field-effect transistors by applying a square wave to the gate electrode. A thiophene/phenylene co-oligomer crystal was used as the organic layer. Compared with the sinusoidal wave gate bias application, the square-wave bias produces the emission intensity ten times as large as that of the former. The effective emissions take place through electrons injection from the source contact when the gate bias traverses 0 V so as to be positive. When asymmetric electrodes were used for the source and drain contacts, the resulting emission exhibited the narrowed spectral line at 491.5 nm with its FWHM approximately 1.1 nm. The line narrowing is expected to be a consequence of the emission intensity increment caused by the enhanced electrons injection from the Ag source contact. The location of the emission line is closely related to those of the multimodes due to the laser oscillation by cavity resonance.

The impact of contact formation on the light emission from ambipolar transistors

In this letter the effect of the charge carrier injection on the performance of ambipolar light-emitting organic field-effect transistors will be investigated. For the analysis, the light output and spatial information of the recombination zone from different devices will be compared. The three investigated devices provide either Ohmic contacts for one or both charge carrier types or hindered injection for both. It will be demonstrated that the light emission in the different operation regimes of the transistor can be used to characterize the contact properties at source and drain.

Tuning of threshold voltage by interfacial carrier doping in organic single crystal ambipolar light-emitting transistors and their bright electroluminescence

Organic light-emitting field-effect transistors, based on a p-bis[(p-styryl)styryl] benzene (P5V4) single crystal, that possess high mobilities of over 0.1 cm2/V s for both electrons and holes were fabricated. For a small charge injection barrier and successive formation of high exciton density in the carrier recombination zone, the hole accumulation threshold voltage was significantly reduced by interfacial hole doping based on electron transfer from P5V4 molecules to a molybdenum oxide layer. The threshold voltage for hole accumulation was drastically decreased from −80±3 to 2±3 V, leading to dual charge injection and accumulation of a very high current density of J>100 A cm−2 with intense edge electroluminescence.

Fabrication and characterization of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors

We present our first application of the neutral cluster beam deposition (NCBD) method to fabricate bilayer heterojunction-based organic light-emitting field-effect transistors (OLEFETs) by superimposing two layers of α, ω-dihexylsexithiophene (DH6T) and N, N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) successively. Based upon well-balanced ambipolarity (hole and electron field-effect mobilities of 2.22 × 10 −2 and 2.78 × 10 −2 cm 2/Vs), the air-stable OLEFETs have demonstrated good field-effect characteristics, stress-free operational stability and electroluminescence under ambient condition.

Integration of a Rib Waveguide Distributed Feedback Structure into a Light‐Emitting Polymer Field‐Effect Transistor

AbstractAmbipolar light‐emitting organic field‐effect transistors (LEFETs) possess the ability to efficiently emit light due to charge recombination in the channel. Since the emission can be made to occur far from the metal electrodes, the LEFET structure has been proposed as a potential architecture for electrically pumped organic lasers. Here, a rib waveguide distributed feedback structure consisting of tantalum pentoxide (Ta2O5) integrated within the channel of a top gate/bottom contact LEFET based on poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) is demonstrated. The emitted light is coupled efficiently into the resonant mode of the DFB waveguide when the recombination zone of the LEFET is placed directly above the waveguide ridge. This architecture provides strong mode confinement in two dimensions. Mode simulations are used to optimize the dielectric thickness and gate electrode material. It is shown that electrode absorption losses within the device can be eliminated and that the lasing threshold for optical pumping of the LEFET structure with all electrodes (4.5 µJ cm−2) is as low as that of reference devices without electrodes. These results enable quantitative judgement of the prospects for realizing an electrically pumped organic laser based on ambipolar LEFETs. The proposed device provides a powerful, low‐loss architecture for integrating high‐performance ambipolar organic semiconductor materials into electrically pumped lasing structures.

Encapsulation of organic light-emitting devices using a perfluorinated polymer

Films of Cytop™, a perfluorinated polymer, are spin cast as a single barrier layer for evaluation of barrier properties on organic light-emitting devices and on Ca thin films. Cytop™ is water repellant, resulting in encapsulated organic light-emitting field effect transistors and organic light-emitting diodes (OLEDs), which remain active even after immersion into water or exposure to water droplets on the Cytop™ surface. OLEDs encapsulated with Cytop™ exhibit up to five times longer continuous operation under identical environmental and driving conditions compared with devices that are not encapsulated with Cytop™.

Organic Light‐Emitting Field‐Effect Transistors Operated by Alternating‐Current Gate Voltages

Organic light-emitting field-effect transistors are operated by a novel method characterized by the application of ac voltages to the gate electrode. This prompts carrier injection from both source and drain electrodes into the organic layer (a thin film or a crystal) without altering the device constitution. Stronger emissions are observed from the device with increasing gate voltage frequencies.