Light-emitting Field-effect Transistors Research Articles

Light emitting field effect transistor with two self-aligned Si nanocrystal layers

Light emitting field effect transistors based on narrow layers of silicon nanocrystals (NCs) in the gate oxide were fabricated. Direct quantum mechanical electron and hole tunneling into NCs was achieved by self-alignment of NCs-interface-distances to ∼2 nm. The direct tunneling reduces oxide degradation, prolongs device lifetime and increases operation speed. Self-alignment occurs during thermal treatment of ion irradiated stacks of 50 nm polycrystalline silicon/15 nm SiO2/(001)Si substrate. An alternating voltage (ac) was applied to the gate to inject charges into the NCs. Due to injection by direct tunneling, electroluminescence extends to higher ac frequencies than reported so far.

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.

Simultaneous Optimization of Light Gain and Charge Transport in Ambipolar Light-Emitting Polymer Field-Effect Transistors

Ambipolar light-emitting field-effect transistors (LEFETs) based on poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) are able to combine electrical switching capability with the generation of light within a well-defined recombination zone inside the channel. Therefore, they are an attractive architecture not only for low-loss light signal transmission in optoelectronic integrated circuits but also for the potential realization of electrically pumped organic lasers. However, these applications require a simultaneous optimization of transistor performance and waveguiding of emitted light. Here we show that to achieve efficient waveguiding and optical gain the annealing temperature of the F8BT film has to be kept at 120 °C below its glass transition temperature. At such low processing temperatures it is difficult to achieve efficient ambipolar charge injection from gold source/drain electrodes and charge transport at the active transistor interface. We show that modification of the gold electrodes with ...

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.

Low Thresholds in Polymer Lasers on Conductive Substrates by Distributed Feedback Nanoimprinting: Progress Toward Electrically Pumped Plastic Lasers

A study was conducted to investigate the effect of optically pumped lasing in a 'super yellow' film cast onto a gate dielectric and gate conducting electrode. It was observed during the study that light-emitting field-effect-transistor (LEFET) architecture with a distributed feedback (DFB) resonator structure nanoimprinted into the gain medium helped to achieve an electrically pumped plastic laser. Nanoimprint lithography was applied to obtain 1D and 2D DFB grating resonator structures in multilayer films. The study showed that the 2D DFB laser provided a lower lasing threshold compare to 1D DFB laser under same conditions. The nanoimprinted 2D DFB laser showed a lasing threshold of 32nJ/pulse and slope efficiency of 1.2%. The 1D grating provided a high degree of chain orientation for the organic semiconducting polymers in the source-drain channel.

Substituent effects on the electronic structure of siloles

Siloles are attractive building blocks for the design and synthesis of organic semiconductors that exhibit potential applications in light-emitting diodes, solar cells, field-effect transistors, and sensors. Understanding how molecular engineering of the electronic structures can help control the properties of these materials has attracted significant research effort. In the present review we illustrate the current state-of-the-art of the molecular engineering of siloles, especially focusing on the effects of substituents and on the electronic structure of siloles.

Organic Materials for Large Area Electronics

Organic materials have been developed to operate as the active semiconductor in a wide range of semiconductor devices, including light-emitting diodes, LEDs, field-effect transistors, FETs, and photovoltaic diodes, PVs. The ability to process these materials as thin films over large areas makes possible a range of applications, currently in displays, as LEDs and as active matrix FET arrays, and solar cells. This article reviews developments in semiconductor physics of these materials and in their application in semiconductor devices

Electron–Hole Recombination in Uniaxially Aligned Semiconducting Polymers

AbstractA promising, general strategy for improving performance of optoelectronic devices based on conjugated polymer semiconductors is to make better use of the fast intrachain transport along the covalently bonded polymer backbone. Little is known, however, about how the recombination rate between electrons and holes would be affected in device structures in which current flow is primarily along the polymer chain. Here a light‐emitting field effect transistor (LFET) structure with a uniaxially aligned semiconducting polymer is used to show that the width and shape of the recombination zone depend strongly on polymer alignment. For alignment of the polymer parallel to the current the emission zone is 5–10 times wider than for perpendicular alignment. 2D drift‐diffusion modeling is used to show that such significant widening of the recombination zone in the case of parallel alignment implies that the recombination rate constant is more than 100 times lower than expected for standard Langevin recombination. On the basis of Monte Carlo modeling it is proposed that such unexpected weak recombination is a result of the significant mobility anisotropy of the aligned polymer. These results provide new fundamental insight into the recombination physics of polymer semiconductors.

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.

Low-Temperature Control of Nanoscale Morphology for High Performance Polymer Photovoltaics

Understanding and controlling nanoscale morphology is crucial to the performance of polymer bulk heterojunction solar cells, as well as other optoelectronic devices such as polymer light-emitting diodes, field-effect transistors, and sensors. In photovoltaic devices, optimum blend morphologies must be commensurate with the nanometer length scales of exciton diffusion and charge separation. We report on a generally applicable method of optimizing the phase segregation in polymer-polymer bulk heterojunctions based on tuning mixtures of low and high boiling point solvents. We have characterized the resulting blend morphologies with nanometer resolution using a transient absorption technique that probes the distribution of paths traveled by the excitons themselves prior to generating charges at an interface. Photovoltaic efficiencies are accounted for in terms of exciton diffusion, geminate pair separation, and polymer ordering, all of which are sensitive to the nanoscale morphology determined by the composition of the solvent mixture.

Finite difference time domain analysis of the light extraction efficiency in organic light-emitting field-effect transistors

The authors report on three-dimensional numerical optical simulations of the emission outcoupling efficiency in light-emitting devices with a field-effect carrier transport. The finite difference time domain method is applied to organic thin film structures on a silicon substrate with metal and metal oxide electrodes. Simulations are performed for Au, Ag, and indium tin oxide electrodes in a bottom gate, bottom contact geometry. Additional attention is paid to the dependence on electrode thickness and contact shape. We demonstrate that in unipolar driven devices with Si gate, silicon dioxide insulator, and 40 nm thick organic films, the maximum outcoupling efficiency is below 10%. This value can be increased by the implementation of a metal reflecting layer on the Si substrate. In further studies, the emission efficiency in the ambipolar regime is investigated. The result presents the dependence of light extraction on the light source-electrode distance for rectangular and wedge shaped contacts.

On the width of the recombination zone in ambipolar organic field effect transistors

The performance of organic light emitting field effect transistors is strongly influenced by the width of the recombination zone. We present an analytical model for the recombination profile. By assuming Langevin recombination, the recombination zone width W is found to be given byW=4.34dδ, with d and δ the gate dielectric and accumulation layer thicknesses, respectively. The model compares favorably to both numerical calculations and measured surface potential profiles of an actual ambipolar device.

A gate dielectric that enables high ambipolar mobilities in polymer light-emitting field-effect transistors

Polymer light-emitting field-effect transistors (LEFETs) require high and balanced electron and hole mobilities to achieve high current densities. Here we demonstrate a novel gate dielectric for polymer LEFETs that enables mobilities of 0.01cm2∕Vs for both electrons and holes. The low-k dielectric polycyclohexylethylene is placed in direct contact with the poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) semiconductor. A second dielectric layer comprising the high-k material poly(vinylidene fluoride-trifluoroethylene) is used to apply a high electric field onto the low-k dielectric layer. The attainable electron-hole recombination current in such optimized polymer LEFETs is measured and the implications for achieving electrically pumped lasing in a LEFET are discussed.

High performance light emitting transistors

Solution processed light emitting field-effect transistors (LEFETs) with peak brightness exceeding 2500cd∕m2 and external quantum efficiency of 0.15% are demonstrated. The devices utilized a bilayer film comprising a hole transporting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene) and a light emitting polymer, Super Yellow, a polyphenylenevinylene derivative. The LEFETs were fabricated in the bottom gate architecture with top-contact Ca∕Ag as source/drain electrodes. Light emission was controlled by the gate voltage which controls the hole current. These results indicate that high brightness LEFETs can be made by using the bilayer film (hole transporting layer and a light emitting polymer).

Theoretical comparative studies of charge mobilities for molecular materials: Pet versus bnpery

Charge mobility is the most important issue for organic semiconductors. We calculate the electron and hole mobilities for prototypical polycyclic hydrocarbon molecules, perylothiophene (pet) and benzo( g, h, i)-perylene (bnpery) using Marcus electron transfer theory coupled with a diabatic model and a homogeneous diffusion assumption to obtain the charge mobility. The first-principles DFT calculations show that the hole mobility is about an order of magnitude higher than the electron mobility in pet. However, we find that for bnpery, the electron and hole transports are balanced, namely, very close in mobility, indicating the possible application in light-emitting field-effect transistor. The crystal packing effects on the frontier orbital coupling are found to be essential to understand such differences in transport behaviors.

Quantum efficiency of ambipolar light-emitting polymer field-effect transistors

The emission characteristics and external quantum efficiencies of ambipolar polymer light-emitting field-effect transistors are investigated as a function of applied voltage, current density, and ratio of hole to electron mobility. Green-emitting poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) with balanced electron and hole mobilities and red-emitting poly((9,9-dioctylfluorene)-2,7- diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) with strongly unbalanced hole and electron mobilities as semiconducting and emissive polymers are compared. The current-voltage and light output characteristics of the two types of light-emitting transistors were found to be fundamentally alike independent of mobility ratio. Device modeling allowing for a single (Langevin-type) charge recombination mechanism was able to reproduce the device characteristics for both cases but could not replicate the experimentally observed dependence of external quantum efficiency on current density. The increase of quantum efficiency with current density up to a saturation value could be indicative of a trap-assisted nonradiative decay mechanism at the semiconductor-dielectric interface. Optical output modeling confirmed that the maximum external quantum efficiency of F8BT light-emitting transistors of 0.8% is consistent with complete recombination of all charges and a singlet exciton fraction of 25%.

Electroluminescence imaging and microstructure of organic light-emitting field-effect transistors

The effect of morphology and microstructure on the emission characteristics of ambipolar light-emitting field-effect transistors is studied using the polyfluorene copolymer F8BT [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)] as a model system. Different intensity distributions of the emission zones of amorphous, polycrystalline, and aligned F8BT films are demonstrated. Electroluminescence maps of the channel region are produced by overlaying a series of images recorded during gate voltage sweeps. They show a correlation to the microcrystalline structure of the F8BT and are assumed to visualize the current density distribution within the transistor channel.

High Current Density in Light-Emitting Transistors of Organic Single Crystals

We measured the external electroluminescence quantum efficiency (eta(ext)) in light-emitting field-effect transistors (LETs) made of organic single crystals and found that, in the ambipolar transport region, eta(ext) is not degraded up to several hundreds A/cm(2) current-density range, which is 2 orders of magnitude larger than that achieved in conventional organic light-emitting diodes. The present result indicates the single-crystal organic LET is a promising device structure that is free from various kinds of nonradiative losses such as exciton dissociation near electrodes and exciton annihilations.