Light-emitting Field-effect Transistors Research Articles

Halogenated Materials as Organic Semiconductors

Organic semiconductors have great potential as the active material in low-cost, large area plastic electronics, whether as light-emitting diodes (LEDs), field-effect transistors (FETs) or solar cells. Organic semiconducting materials retain the processability associated with polymers while maintaining good optoelectronic properties, for example, high absorption coefficients for photons in the visible, and field-effect mobilities comparable with that of amorphous silicon. The elucidation of important structure−property relationships is vital for the design of functional, high-performance organic semiconductors. In this short review, we summarize such relationships stemming from the halogenation of organic semiconductors. While it has been known in the past decade that fluorination lowers the energy levels in carbon based systems, induces stability and electron transport, less is known about the effect of the other halogens. Chlorination has recently been shown to be a viable route to n-type materials. The bandgap of conjugated compounds can also be decreased slightly by the addition of Cl, Br, and I to the aromatic core. The effect of the halogenated moieties on the packing of molecules is discussed.

Green light emission from the edges of organic single-crystal transistors

We have fabricated ambipolar light-emitting field-effect transistors made of 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) single crystals, which have 35% fluorescent quantum efficiency. The obtained hole and electron mobilities were 2.9×10−1 cm2/V s and 6.7×10−3 cm2/V s, respectively. These are the highest values among AC5 transistors. Importantly, although the light emission from the crystal surface was less than the detection level of the camera, we observed bright and polarized light emission from the edge of the single crystals. This polarized edge emission is attributed to the strong self-assembled light-confining nature and perfectly aligned transition dipole moments, which are advantageous for future laser devices.

Light-emitting field-effect transistors made of single crystals of an ambipolar thiophene/phenylene co-oligomer

We have developed an ambipolar thiophene/phenylene co-oligomer (TPCO) by introducing the trifluoromethyl group and the methoxy group to molecular terminals of the parent (unsubstituted) TPCO molecule. The crystals of the compound exhibit the ambipolar carrier transport on the field-effect transistors with homogeneous Au electrodes and heterogeneous MgAg and Au electrodes. The latter device definitively emits the light by applying sinusoidal-wave and square-wave alternating-current (AC) voltages to the gate electrode. Time-dependent emission measurements indicate that major emissions take place twice during each period of the AC gate bias.

Conjugated‐Polymer‐Based Lateral Heterostructures Defined by High‐Resolution Photolithography

AbstractSolution processing of polymer semiconductors provides a new paradigm for large‐area electronics manufacturing on flexible substrates, but it also severely restricts the realization of interesting advanced device architectures, such as lateral heterostructures with defined interfaces, which are easily accessible with inorganic materials using photolithography. This is because polymer semiconductors degrade, swell, or dissolve during conventional photoresist processing. Here a versatile, high‐resolution photolithographic method is demonstrated for patterning of polymer semiconductors and exemplify this with high‐performance p‐type and n‐type field‐effect transistors (FETs) in both bottom‐ and top‐gate architectures, as well as ambipolar light‐emitting field‐effect transistors (LEFETs), in which the recombination zone can be pinned at a photolithographically defined lateral heterojunction between two semiconducting polymers. The technique therefore enables the realization of a broad range of novel device architectures while retaining optimum materials performance.

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.

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

AbstractElectron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.

Synthesis, mesomorphic behaviour and optoelectronic properties of phosphorus-based thermotropic liquid crystalline dendrimers

A new series of thermotropic phosphorus-based liquid crystalline (LC) dendrimers based on a thiophosphoryl-phenoxymethyl(methylhydrazono) core (thiophosphoryl-PMMH) up to the fifth generation has been synthesised by solution condensation of aldehyde groups, surface-functionalised thiophosphoryl-PMMH dendritic substrates of generation numbers G0.5 to G5.5, with the appropriate molar equivalents of the pro-mesogenic n-hexadecylaniline mono-functional building block. Their chemical composition has been confirmed by 1H/13C/31P nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, elemental analysis. Optical properties have been studied by ultraviolet-visible absorption, photoluminescence spectroscopy and polarised optical microscopy, and thermal characteristics by differential scanning calorimetry. Electrical studies have been made using the current-voltage characteristics of organic light-emitting diodes consisting of multi-layered indium tin oxide/dendrimer/aluminium tris(8-hydroxyquinoline)Al architecture. It has been demonstrated that the molecular engineering approach adopted can successfully lead to phosphorus-containing dendritic organic semiconductors (OSCs) which show tunable mesomorphic behaviour (extension of the observed smectic mesophase) and (opto) electronic properties, owing to their peripheral decoration with a tunable number of azomethine-based optically active chromophoric units. This rare combination of ‘tunable by design’ properties makes this series of thermostable thiophosphoryl-PMMH-based LC dendrimers a particularly appealing class of OSCs for use in optically and/or electronically active layers of (opto)electronic devices such as light-emitting diodes, field-effect transistors, solar cells and lasers.

Conjugated polyelectrolytes for organic light emitting transistors

We report on solution-processed light emitting field-effect transistors (LEFETs) that incorporate symmetric high work function (WF) source and drain metal electrodes. A key architectural design is the incorporation of a conjugated polyelectrolyte (CPE) electron injection layer atop the emissive layer. The device structure also comprises a hole-transporting layer underneath the emissive layer. Both holes and electrons are injected from stable, high WF metal though the CPE layer leading to electroluminescence near the electron-injecting electrode. With the benefits of the simplicity in device fabrication, the LEFETs incorporating CPEs are interesting structures for integrated organic optoelectronic devices.

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.

Organic electrochemical light emitting field effect transistors

We report on the demonstration of an organic electrochemical light emitting field effect transistor. The device fabricated in bottom gate/bottom contact geometry using a conjugated polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) mixed with polymer electrolyte poly(ethylene oxide) including lithium trifluoromethanesulfonate (Li triflate) as active layer and poly(vinyl alcohol) as gate dielectric. Orange-yellow light emission is observed from the device. The output characteristics (source-drain current versus the source-drain voltage) and transfer characteristics (source-drain current versus the gate voltage) of the device are reported.

Ambipolar behavior of 2,5-diphenyl-1,4-distyrylbenzene based field effect transistors: An experimental and theoretical study

New organic semiconductor molecule 2,5-diphenyl-1,4-distyrylbenzene (trans-DPDSB) was synthesized and its single crystal was grown for light-emitting field effect transistors (FETs). This molecule forms J-aggregate crystals which show high luminescent efficiency compared with the corresponding H-aggregates. Although J-aggregates have a weak point of lower mobility, the FETs using trans-DPDSB single crystals and Au–Ca asymmetrical electrodes showed light emission from the channel. This study shows that the design and modification of organic molecular stacking modes are useful in realization of highly efficient light-emitting FETs.

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.

Single-Molecule Chemistry of Metal Phthalocyanine on Noble Metal Surfaces

To develop new functional materials and nanoscale electronics, researchers would like to accurately describe and precisely control the quantum state of a single molecule on a surface. Scanning tunneling microscopy (STM), combined with first-principles simulations, provides a powerful technique for acquiring this level of understanding. Traditionally, metal phthalocyanine (MPc) molecules, composed of a metal atom surrounded by a ligand ring, have been used as dyes and pigments. Recently, MPc molecules have shown great promise as components of light-emitting diodes, field-effect transistors, photovoltaic cells, and single-molecule devices. In this Account, we describe recent research on the characterization and control of adsorption and electronic states of a single MPc molecule on noble metal surfaces. In general, the electronic and magnetic properties of a MPc molecule largely depend on the type of metal ion within the phthalocyanine ligand and the type of surface on which the molecule is adsorbed. However, with the STM technique, we can use on-site molecular "surgery" to manipulate the structure and the properties of the molecule. For example, STM can induce a dehydrogenation reaction of the MPc, which allows us to control the Kondo effect, which describes the spin polarization of the molecule and its interaction with the complex environment. A specially designed STM tip can allow researchers to detect certain molecule-surface hybrid states that are not accessible by other techniques. By matching the local orbital symmetry of the STM tip and the molecule, we can generate the negative differential resistance effect in the formed molecular junction. This orbital symmetry based mechanism is extremely robust and does not critically depend on the geometry of the STM tip. In summary, this simple model system, a MPc molecule absorbed on a noble metal surface, demonstrates the power of STM for quantum characterization and manipulation of single molecules, highlighting the potential of this technique in a variety of applications.

Organic light emitting field effect transistors based on an ambipolar p-i-n layered structure

A bottom contact/top gate ambipolar “p-i-n” layered light emitting field effect transistor with the active medium inserted between two doped transport layers, is reported. The doping profile results crucial to the capability of emitting light, as well as to the electrical characteristics of the device. In this sense, high output current at relative low applied gate/drain voltage and light emission along the whole large area transistor channel are observed, putting the basis to full integration of organic light emitting field effect transistors in planar complex devices.

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.

Organic light emitting complementary inverters

We show that p- and n-type light emitting field-effect transistors (LEFETs) can be made using “superyellow” as a light-emitting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as a p-type material and a naphthalene di-imide as an n-type material. By connecting two of these LEFETs, we have demonstrated a light emitting complementary inverter (LECI). The LECI exhibited electrical and optical characteristics in the first and third quadrant of the transfer characteristics with voltage gain of 6 and 8, respectively.

One-Dimensional Group III-Nitrides: Growth, Properties, and Applications in Nanosensing and Nano-Optoelectronics

This review will give a brief introduction to the growth and characterization methods of both binary and ternary compounds, in particular those exhibiting one-dimensionality, of the family to orient the readers about the material system to be discussed. A section will deal with the size and shape selection in group III nitride nanomaterials with a stress on intriguing morphologies such as nanowires, nanotips, and nanobelts. Complex structures, such as hierarchical and core-shell structures, will be introduced. Optical, electrical, and mechanical property, such as hardness, will be discussed in a greater detail, distinguishing the bulk from the nano wherever possible. Available models of electrical conduction and photoconduction in nanomaterials and their dependence on the actual size of the objects will be presented and compared. Optical properties of ensemble and single nanostructures, wherever possible, will be addressed in detail. The section on application will focus mainly on the sensor applications, including chemical sensors, gas sensors, and biosensors, with a thrust on DNA sensing. Because popular applications such as light-emitting diodes (LEDs) and field effect transistors (FETs) have already been reviewed extensively, only major contributions to this field—for example, nano-LEDs—will be discussed. Some recent advances in the group III-nitride materials family will be presented that will indicate future directions of research in this area.