Light-emitting Transistors Research Articles

Organic single crystal-based light-emitting devices by template stripping technique

Organic single crystals have attracted extensive interests in the field of optoelectronic and electronic devices for their high chemical purity, high charge-carrier mobility and high luminescent quantum efficiency. The motivation for research on organic single crystals is their highly arranged packing structure, which provides an ideal model to investigate the basic interactions between the molecules and stacking modes and optoelectronic performance. Because of advantages of these crystals, they have been employed in much functional optoelectronic devices such as organic field-effect transistors (OFETs), light-emitting organic field-effect transistors (LE-OFETs), optically pumped lasers, photovoltaic cells and so on. However, the reports on organic single crystal based organic light emitting devices (OLEDs) are scarce. There is a big difficulty in the fabrication of organic single crystal-based OLEDs because of their intrinsic properties like soft, fragile and sensitive to organic solvent. In this work, a simple and non-destructive method template stripping has been applied into the single crystal-based OLEDs to improve the contact problem and realize its bright emission. Based on this method, the physical vapour transport grown crystals are firstly transferred to the modified silicon substrate. Then an anode of 100 nm gold film is deposited on the top of the crystal surface by thermal evaporation with metal mask. After dropping a microliter of photoresist polymer on the device, a glass is pressed on top by its own weight. And the polymer will spread to the edge of the glass and totally cover the device. Then it will be cured after exposed to a UV-light source with a power of 500 W. The device will be transferred to the glass with the help of a knife for separation. Finally the cathode of Ca/Ag will be evaporated on the other side of crystal. The sandwiched structure devices can be achieved by this method. Both anodes and cathodes of the crystal-based OLEDs can be deposited onto the opposite surface of the organic single crystals by thermal evaporation on both sides, so that a much improved contact between the crystals and the electrodes can be realized, which enhance the carrier injection into the active layer. Homogenous light emission can be observed from these crystal-based OLEDs. And the molecules of BP2T are arranged layer by layer which are nearly perpendicular to the crystal plane. As we known, the directions of the transition dipole moments lie along the long axis of the molecule. The light in the crystal will transmit to the edge of the crystal which recognized as self-waveguide effect. In the same way, edge-emitting can be seen from the crystal-based OLEDs which attributed to this self-waveguide. In order to clarify the template stripping method can work well on the fabrication of single crystal based OLEDs. We compare the performance of both devices between the lamination method and template stripping method. Based on lamination method, the single crystals are simply laminated onto the top of the anode by van der Waals force which results in an incomplete contact. Compared to the lamination method, the devices show higher carrier injection and low voltage threshold by this compacted contact between crystals and electrodes without any breakages. It is expectable that this technique would support broad applications of the organic single crystals in the crystal-based optoelectronic devices.

Emission Control for Organic Light-emitting Electrochemical Cell Using Ruthenium Complex by Redox Potential

A novel concept for the light-emitting electrochemical transistor (LECT) which consists of the electrochemical light-emitting cell (LEC) and the electrochemical transistor (ECT) was introduced. The four-electrode electrochemical system using interdigitated array (IDA) electrodes was employed to demonstrate the concept of LECT in solution. Cyclic voltammetric (CV) measurement was carried out in solution containing Ru(bpy)3(PF6)2 as redox species and a light-emitting material. We confirmed that the light emission was controlled at the electrode potential difference between both fingers of IDA electrodes.

Molecular Materials That Can Both Emit Light and Conduct Charges: Strategies and Perspectives.

Molecular materials with concomitant light-emissive and semiconducting properties have received increasing attention in recent years. Such dual functional materials ensure the development of multifunctional devices (e.g., organic light-emitting transistors) and the emergence of new technologies. However, owing to the fact that intermolecular interactions and dense packing have opposite effects on photoluminescence and charge-carrier mobility, it is still rather challenging to rationally design high-performance molecular materials that exhibit both semiconducting and light-emissive properties. In fact, only a limited number of such dual functional materials are available, and most of their performances need to be further improved. In this concept article we discuss the design strategies and perspectives of this challenging area with the introduction of representative examples of such dual functional materials reported in recent years.

Organic light-emitting transistors: a key enabling photonic device

Organic light-emitting transistors: a key enabling photonic device

ChemInform Abstract: Electronic Structure, Aromaticity and Spectra of Hetero[8]circulenes

AbstractReview: 159 refs.

Low-Temperature Plasma-Assisted Atomic Layer Deposition of Silicon Nitride Moisture Permeation Barrier Layers.

Encapsulation of organic (opto-)electronic devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells, and field-effect transistors, is required to minimize device degradation induced by moisture and oxygen ingress. SiNx moisture permeation barriers have been fabricated using a very recently developed low-temperature plasma-assisted atomic layer deposition (ALD) approach, consisting of half-reactions of the substrate with the precursor SiH2(NH(t)Bu)2 and with N2-fed plasma. The deposited films have been characterized in terms of their refractive index and chemical composition by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). The SiNx thin-film refractive index ranges from 1.80 to 1.90 for films deposited at 80 °C up to 200 °C, respectively, and the C, O, and H impurity levels decrease when the deposition temperature increases. The relative open porosity content of the layers has been studied by means of multisolvent ellipsometric porosimetry (EP), adopting three solvents with different kinetic diameters: water (∼0.3 nm), ethanol (∼0.4 nm), and toluene (∼0.6 nm). Irrespective of the deposition temperature, and hence the impurity content in the SiNx films, no uptake of any adsorptive has been observed, pointing to the absence of open pores larger than 0.3 nm in diameter. Instead, multilayer development has been observed, leading to type II isotherms that, according to the IUPAC classification, are characteristic of nonporous layers. The calcium test has been performed in a climate chamber at 20 °C and 50% relative humidity to determine the intrinsic water vapor transmission rate (WVTR) of SiNx barriers deposited at 120 °C. Intrinsic WVTR values in the range of 10(-6) g/m2/day indicate excellent barrier properties for ALD SiNx layers as thin as 10 nm, competing with that of state-of-the-art plasma-enhanced chemical vapor-deposited SiNx layers of a few hundred nanometers in thickness.

Hybrid Area-Emitting Transistors: Solution Processable and with High Aperture Ratios.

Area emission is realized in all-solution-processed hybrid light-emitting transistors (HLETs). A new HLET design is presented with increased aperture ratio, and optical and electrical characteristics are shown.

Graphene-based electrodes for flexible electronics

As ‘flexibility’ has emerged as an important issue in next-generation electronics, many efforts to find new classes of materials have been devoted to realizing stretchable, bendable and foldable electronic devices. For these devices to be realized, graphene has been considered as one of the most promising candidates for flexible electrodes due to its extraordinary electrical, optical and mechanical properties. Particularly, recent developments in the fabrication and modification of graphene point to a bright future for graphene electrodes in flexible electronics. This mini-review summarizes the recent progress in graphene films as flexible electrodes for various applications such as solar cells, organic light-emitting diodes, touchscreens, transistors and supercapacitors. © 2015 Society of Chemical Industry

Pixeled Electroluminescence from Multilayer Heterostructure Organic Light-Emitting Transistors

Improved performance of multilayer heterostructure organic light-emitting transistors (OLETs) was observed in brightness and external quantum efficiency (EQE) by inserting an ultrathin MoOx layer a...

1.1-μm InAs/GaAs quantum-dot light-emitting transistors grown by molecular beam epitaxy.

In this Letter, we report the enhanced radiative recombination output from an AlGaAs/GaAs heterojunction bipolar transistor with InAs quantum dots embedded in the base region to form a quantum-dot light-emitting transistor (QDLET) grown by molecular beam epitaxy systems. For the device with a 100 μm×100 μm emitter area, we demonstrate the dual output characteristics with an electrical output and an optical output when the device is operating in the common-emitter configuration. The quantum-dot light-emitting transistor exhibits a base recombination radiation in the near-infrared spectral range with a dominant peak at λ of 1100nm.

In-plane electroluminescence from microcavity organic light-emitting transistors

We report the observation of in-plane emission beneath the drain electrode in multilayer heterostructure organic light-emitting transistors (OLETs). A novel modification method for the interface between the hole transport layer and the emission layer has been proposed, which brought a great enhancement for the light power and external quantum efficiency. Further, distributed Bragg reflector was incorporated to the in-plane-emitted OLETs, which combined with the top thin layer of Au, forming a vertical microcavity. The electroluminescence spectra were significantly altered by the microcavity and much narrower linewidth was obtained. The results will help to develop high color purity and white OLETs with high performance, which would be useful for multifunctional displays.

A New Electroluminescent Organic Dual-Gate Field-Effect Transistor

In this letter, we show a new device architecture, which we call field-induced contacts light-emitting transistor (FICOLET), to fabricate high-efficiency electroluminescent dual-gate organic field-effect transistors. The FICOLET is a dual-gate transistor in which two accumulated regions, not completely overlapped, one of electrons and the other of holes, are formed. When a positive bias voltage is applied to the drain contact, electrons and holes are pushed by the combined action of the carrier diffusion and of the electric field to the opposite side of the semiconductor. They recombine far from the electrodes with a theoretical recombination efficiency of 100%, low exciton quenching, and current densities that, in high injection conditions, can exceed the lasing threshold.

Lead iodide perovskite light-emitting field-effect transistor.

Despite the widespread use of solution-processable hybrid organic–inorganic perovskites in photovoltaic and light-emitting applications, determination of their intrinsic charge transport parameters has been elusive due to the variability of film preparation and history-dependent device performance. Here we show that screening effects associated to ionic transport can be effectively eliminated by lowering the operating temperature of methylammonium lead iodide perovskite (CH3NH3PbI3) field-effect transistors. Field-effect carrier mobility is found to increase by almost two orders of magnitude below 200 K, consistent with phonon scattering-limited transport. Under balanced ambipolar carrier injection, gate-dependent electroluminescence is also observed from the transistor channel, with spectra revealing the tetragonal to orthorhombic phase transition. This demonstration of CH3NH3PbI3 light-emitting field-effect transistors provides intrinsic transport parameters to guide materials and solar cell optimization, and will drive the development of new electro-optic device concepts, such as gated light-emitting diodes and lasers operating at room temperature.

Charge transport and recombination in heterostructure organic light emitting transistors

Light-emitting field effect transistors (LEFETs) are a class of organic optoelectronic device capable of simultaneously delivering the electrical switching characteristics of a transistor and the light emission of a diode. We report on the temperature dependence of the charge transport and emissive properties in a model organic heterostructure LEFET system from 300K to 135K. We study parameters such as carrier mobility, brightness, and external quantum efficiency (EQE), and observe clear thermally activated behaviour for transport and injection. Overall, the EQE increases with decreasing temperature and conversely the brightness decreases. These contrary effects can be explained by a higher recombination efficiency occurring at lower temperatures, and this insight delivers new knowledge concerning the optimisation of both the transport and emissive properties in LEFETs.

Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers.

Efficient charge injection is critical for flexible organic electronic devices such as organic light-emitting diodes (OLEDs) and field-effect transistors (OFETs). Here, we investigated conjugated polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) as solution-processable charge-injection layers in ambipolar organic field-effect transistors with poly(thienylenevinylene-co-phthalimide)s. The interlayers were prepared using poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) or poly(9,9-dioctylfluorene) (PFO) to wrap s-SWNTs. In the contact-limited ambipolar OFETs, the interlayer led to significantly lower contact resistance (Rc) and increased mobilities for both holes and electrons. The resulting PTVPhI-Eh OFETs with PFO-wrapped s-SWNT interlayers showed very well-balanced ambipolar transport properties with a hole mobility of 0.5 cm2V-1S-1 and an electron mobility of 0.5 cm2V-1S-1 in linear regime. In addition, the chirality of s-SWNTs and kind of wrapping of conjugated polymers are not critical to improving charge-injection properties. We found that the improvements caused by the interlayer were due to the better charge injection at the metal/organic semiconductor contact interface and the increase in the charge concentration through a detailed examination of charge transport with low-temperature measurements. Finally, we successfully demonstrated complementary ambipolar inverters incorporating the interlayers without excessive patterning.

Nanowire Lasers

Abstract:We review principles and trends in the use of semiconductor nanowires as gain media for stimulated emission and lasing. Semiconductor nanowires have recently been widely studied for use in integrated optoelectronic devices, such as light-emitting diodes (LEDs), solar cells, and transistors. Intensive research has also been conducted in the use of nanowires for subwavelength laser systems that take advantage of their quasione- dimensional (1D) nature, flexibility in material choice and combination, and intrinsic optoelectronic properties. First, we provide an overview on using quasi-1D nanowire systems to realize subwavelength lasers with efficient, directional, and low-threshold emission. We then describe the state of the art for nanowire lasers in terms of materials, geometry, andwavelength tunability.Next,we present the basics of lasing in semiconductor nanowires, define the key parameters for stimulated emission, and introduce the properties of nanowires. We then review advanced nanowire laser designs from the literature. Finally, we present interesting perspectives for low-threshold nanoscale light sources and optical interconnects. We intend to illustrate the potential of nanolasers inmany applications, such as nanophotonic devices that integrate electronics and photonics for next-generation optoelectronic devices. For instance, these building blocks for nanoscale photonics can be used for data storage and biomedical applications when coupled to on-chip characterization tools. These nanoscale monochromatic laser light sources promise breakthroughs in nanophotonics, as they can operate at room temperature, can potentially be electrically driven, and can yield a better understanding of intrinsic nanomaterial properties and surface-state effects in lowdimensional semiconductor systems.

Ambipolar light-emitting organic single-crystal transistors with a grating resonator.

Electrically driven organic lasers are among the best lasing devices due to their rich variety of emission colors as well as other advantages, including printability, flexibility, and stretchability. However, electrically driven lasing in organic materials has not yet been demonstrated because of serious luminescent efficiency roll-off under high current density. Recently, we found that the organic ambipolar single-crystal transistor is an excellent candidate for lasing devices because it exhibits less efficient roll-off, high current density, and high luminescent efficiency. Although a single-mode resonator combined with light-emitting transistors (LETs) is necessary for electrically driven lasing devices, the fragility of organic crystals has strictly limited the fabrication of resonators, and LETs with optical cavities have never been fabricated until now. To achieve this goal, we improved the soft ultraviolet-nanoimprint lithography method and demonstrated electroluminescence from a single-crystal LET with a grating resonator, which is a crucial milestone for future organic lasers.

Using d-limonene as the non-aromatic and non-chlorinated solvent for the fabrications of high performance polymer light-emitting diodes and field-effect transistors

Aiming to environment protection, green solvents are crucial for commercialization of solution-processed optoelectronic devices. In this work, d-limonene, a natural product, was introduced as the non-aromatic and non-chlorinated solvent for processing of polymer light-emitting diodes (PLEDs) and organic field effect transistors (OFETs). It was found that d-limonene could be a good solvent for a blue-emitting polyfluorene-based random copolymer for PLEDs and an alternating copolymer FBT-Th4(1,4) with high hole mobility (μh) for OFETs. In comparisons to routine solvent-casted films of the two conjugated polymers, the resulting d-limonene-deposited films could show comparable film qualities, based on UV–vis absorption spectra and observations by atomic force microscopy (AFM). With d-limonene as the processing solvent, efficient blue PLEDs with CIE coordinates of (0.16, 0.16), maximum external quantum efficiency of 3.57%, and luminous efficiency of 3.66cd/A, and OFETs with outstanding μh of 1.06cm2 (Vs)−1 were demonstrated. Our results suggest that d-limonene would be a promising non-aromatic and non-chlorinated solvent for solution processing of conjugated polymers and molecules for optoelectronic device applications.

An Organic Device with Thin Film Transistor Merged with Light Emitting Diode through Use of an Accumulation Layer in TFT As an Electrode

A light-emitting device structure is proposed which includes an organic thin-film structure merged with an organic light-emitting diode structure by utilizing a part of the electron accumulation layer in the organic thin-film transistor as a common electrode for each structure. The organic thin-film structure and the organic light-emitting diode structure each include an organic semiconductor material in which hole mobility is greater in bulk region of the material than electron mobility in the bulk region but less in the channel region. The advantages of such a light-emitting device include less complex processing and a simpler pixel circuit structure in comparison to separately fabricating OTFT and OLED structures and subsequently interconnecting them to form a pixel. Furthermore, relative to a light-emitting transistor, the proposed structure offers the advantage of a broader light emission area which is more suitable for use in display devices.

Analysis of Patterned Defects on Graphene Using Micro-Raman Spectroscopy and Liquid Crystals

Graphene has been used in various applications such as light-emitting diodes, solar cells, transistors, and chemical sensors because of its intriguing properties. Since defects in graphene can affect its intrinsic properties, defect characterizations in graphene are crucial. Micro-Raman spectroscopy has been considered one of the powerful and non-destructive techniques to analyze fundamental properties of graphene. Since fast characterization of large-area graphene is critical for its commercialization, large-area graphene was analyzed using liquid crystal with polarized optical microscopy at different temperatures. Chemical-vapor-deposited graphene was transferred onto pre-patterned SiO2/Si substrate by using conventional wet-transfer method. Defects were intentionally generated by oxygen plasma for various treatment times. The details of our experiments and results will be discussed in the meeting.