Light-emitting Transistors Research Articles

One-Dimensional Zinc Oxide Nanomaterials for Application in High-Performance Advanced Optoelectronic Devices

Unlike conventional bulk or film materials, one-dimensional (1D) semiconducting zinc oxide (ZnO) nanostructures exhibit excellent photoelectric properties including ultrahigh intrinsic photoelectric gain, multiple light confinement, and subwavelength size effects. Compared with polycrystalline thin films, nanowires usually have high phase purity, no grain boundaries, and long-distance order, making them attractive for carrier transport in advanced optoelectronic devices. The properties of one-dimensional nanowires—such as strong optical absorption, light emission, and photoconductive gain—could improve the performance of light-emitting diodes (LEDs), photodetectors, solar cells, nanogenerators, field-effect transistors, and sensors. For example, ZnO nanowires behave as carrier transport channels in photoelectric devices, decreasing the loss of the light-generated carrier. The performance of LEDs and photoelectric detectors based on nanowires can be improved compared with that of devices based on polycrystalline thin films. This article reviews the fabrication methods of 1D ZnO nanostructures—including chemical vapor deposition, hydrothermal reaction, and electrochemical deposition—and the influence of the growth parameters on the growth rate and morphology. Important applications of 1D ZnO nanostructures in optoelectronic devices are described. Several approaches to improve the performance of 1D ZnO-based devices, including surface passivation, localized surface plasmons, and the piezo-phototronic effect, are summarized.

Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors.

We report the development of low operating voltages in inorganic-organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrO x gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer "Super Yellow" acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10-2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered.

High-Performance, Solution-Processed Quantum Dot Light-Emitting Field-Effect Transistors with a Scandium-Incorporated Indium Oxide Semiconductor.

Light-emitting field-effect transistors (LEFETs) have attained great attention due to their special characteristics of both the switching capacity and the electroluminescence capacity. However, high-performance LEFETs with high mobility, high brightness, and high efficiency have not been realized due to the difficulty in developing high electron and hole mobility materials with suitable band structures. In this paper, quantum dot hybrid LEFETs (QD-HLEFETs) combining high-luminous-efficiency quantum dots (QDs) and a solution-processed scandium-incorporated indium oxide (Sc:In2O3) semiconductor were demonstrated. The red QD-HLEFET showed high electrical and optical performance with an electron mobility of 0.8 cm2 V-1 s-1, a maximum brightness of 13 400 cd/m2, and a maximum external quantum efficiency of 8.7%. The high performance of the QD-HLEFET is attributed to the good energy band matching between Sc:In2O3 and QDs and the balanced hole and electron injection (less exciton nonradiative recombination). In addition, incorporation of Sc into In2O3 can suppress the oxygen vacancy and free carrier generation and brings about excellent current and optical modulation (the on/off current ratio is 105 and the on/off brightness ratio is 106).

High Efficiency Light-Emitting Transistor with Vertical Metal-Oxide Heterostructure.

The monolithic integration of light-emission with a standard logic transistor is a much-desired multifunctionality. Here, a high-efficiency light-emitting transistor (LET) employing an inorganic quantum dots (QDs) emitter and a laser-annealed vertical metal-oxide heterostructure is reported. The experimental results show that the peak efficiency and luminance of this QDs LET (QLET) are 11% and 8000 cdm-2 , respectively at a monochromatic emitting light wavelength of 585 nm. As far as it is known, these are among the highest values ever achieved for LETs. More importantly, the QLET exhibits an ultrahigh electron mobility of up to 25 cm2 V-1 S-1 , a lower efficiency roll-off (7% at high 3000 cdm-2 ), and excellent stability with long-duration gate stress switching cycles. Additionally, this approach is compatible with those used in conventional large-area silicon electronic manufacturing and can enable a scalable and cost-effective procedure for future integrated versatile displays and lighting applications.

Nanocage-Confined Synthesis of Fluorescent Polycyclic Aromatic Hydrocarbons in Zeolite.

Polycyclic aromatic hydrocarbons (PAHs) attract much attention for applications to organic light-emitting diodes, field-effect transistors, and photovoltaic cells. The current synthetic approaches to PAHs involve high-temperature flash pyrolysis or complicated step-by-step organic reactions, which lead to low yields of PAHs. Herein, we report a facile and scalable synthesis of PAHs, which is carried out simply by flowing acetylene gas into zeolite under mild heating, typically at 400 °C and generates the products of 0.30 g g-1 zeolite. PAHs are synthesized via acetylene polymerization inside Ca2+-ion-exchanged Linde type A (LTA) zeolite, of which the α-cage puts a limit on the product molecular size as a confined-space nanoreactor. The resultant product after the removal of the zeolite framework exhibits brilliant white fluorescence emission in N-methylpyrrolidone solution. The product is separated into four different color emitters (violet, blue, green, and orange) by column chromatography. Detailed characterizations of the products by means of various spectroscopic methods and mainly mass spectrometric analyses indicate that coronene (C24H12) is the main component of the blue emitter, while the green emitter is a mixture of planar and curved PAHs. The orange can be attributed to curved PAHs larger than ovalene, and the violet to smaller molecules than coronene. The PAH growth mechanism inside Ca2+-exchanged LTA zeolite is proposed on the basis of mass spectral analyses and density functional theory calculations.

Tunneling Modulation of Transistor Lasers: Theory and Experiment

The coherent photons generated at the base quantum wells in the transistor laser (TL) interact with the collector field and “assist” electron tunneling from the valence band of the base to the conduction band states of the collector. The cavity coherent photon intensity effect on photon-assisted tunneling in the TL has resulted in the realization of a novel photon-field enhanced optical absorption. This intra-cavity photon-assisted tunneling (ICPAT) in the TL or the light-emitting transistor is the unique property of voltage (field) modulation and the basis for ultrahigh speed direct tunneling photon modulation and switching. In addition, a new tunneling current modulation gain in a “new form” of transistor is discovered based on the collector tunneling holes feedback to the base and dielectric relaxation base transport. The TL, owing to its unique three-terminal configuration and the complementary nature of its optical and electrical collector output signals, enables fast base recombination, collector tunneling, and OEO feedback, which has resulted in the realization of compact electro-optical applications, such as non-linear signal-mixing, frequency multiplication, OE tunneling transistor, and electrical and optical bistability.

Optical absorption and oxygen passivation of surface states in III-nitride photonic devices

III-nitride surface states are expected to impact high surface-to-volume ratio devices, such as nano- and micro-wire light-emitting diodes, transistors, and photonic integrated circuits. In this work, reversible photoinduced oxygen desorption from III-nitride microdisk resonator surfaces is shown to increase optical attenuation of whispering gallery modes by 100 cm−1 at λ = 450 nm. Comparison of photoinduced oxygen desorption in unintentionally and n+-doped microdisks suggests that the spectral changes originate from the unpinning of the surface Fermi level, likely taking place at etched nonpolar III-nitride sidewalls. An oxygen-rich surface prepared by thermal annealing results in a broadband Q improvement to state-of-the-art values exceeding 1 × 104 at 2.6 eV. Such findings emphasize the importance of optically active surface states and their passivation for future nanoscale III-nitride optoelectronic and photonic devices.

Terahertz light-emitting graphene-channel transistor toward single-mode lasing

AbstractA distributed feedback dual-gate graphene-channel field-effect transistor (DFB-DG-GFET) was fabricated as a current-injection terahertz (THz) light-emitting laser transistor. We observed a broadband emission in a 1–7.6-THz range with a maximum radiation power of ~10 μW as well as a single-mode emission at 5.2 THz with a radiation power of ~0.1 μW both at 100 K when the carrier injection stays between the lower cutoff and upper cutoff threshold levels. The device also exhibited peculiar nonlinear threshold-like behavior with respect to the current-injection level. The LED-like broadband emission is interpreted as an amplified spontaneous THz emission being transcended to a single-mode lasing. Design constraints on waveguide structures for better THz photon field confinement with higher gain overlapping as well as DFB cavity structures with higher Q factors are also addressed towards intense, single-mode continuous wave THz lasing at room temperature.

Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies

Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.

Semi-transparent vertical organic light-emitting transistors

Abstract Vertical organic light-emitting transistor (VOLET) having an organic light-emitting diode integrated with a vertical thin-film transistor is promising for transparent electronics because the vertical device structure potentially offers a display with a large aperture ratio and a low power consumption. However, making a transparent VOLET has been challenging due to the requirements for all transparent electrodes including fabrication of a porous source electrode for current modulation in the device. Here, we report a semi-transparent VOLET with a large modulation of light emitted through the top and bottom electrodes using a nano-porous indium-tin oxide (ITO) source electrode, a Mg:Ag drain electrode, and an ITO gate electrode. The porous ITO source electrode is not only important for luminance modulation, but the nano-textured film morphology also enhances light extraction from the device. Finally, we show that the off current of the VOLET can be suppressed with an electron transporting layer (C60), leading to a large luminance on/off ratio of 104.

A study of effects of electrode contacts on performance of organic-based light-emitting field-effect transistors

Herein is presented a comparative performance analysis of heterojunction organic-based light-emitting field-effect transistors (OLEFETs) with symmetric (Au only) and asymmetric (Au and LiF/Al) electrode contacts. The devices had a top source–drain contact with long-channel geometry and were produced by sequentially depositing p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) using a neutral cluster beam deposition apparatus. The spectroscopic, structural and morphological properties of the organic thin films were examined using photoluminescence (PL) spectroscopy, X-ray diffraction (XRD) method, laser scanning confocal and atomic force microscopy (LSCM, AFM). Based upon the growth of high-quality, well-packed crystalline thin films, the devices demonstrated ambipolar field-effect characteristics, stress-free operational stability, and light emission under ambient conditions. Various device parameters were derived from the fits of the observed characteristics. The hole mobilities were nearly equal irrespective of the electrode contacts, whereas the electron mobilities of the transistors with LiF/Al drain electrodes were higher due to the low injection barrier. For the OLEFETs with symmetric electrodes, electroluminescence (EL) occurred only in the vicinity of the hole-injecting electrode, whereas for the OLEFETs with asymmetric electrodes, the emission occurred in the vicinity of both hole- and electron-injecting electrodes. By tuning the carrier injection and transport through high- and low-work function metals, the hole-electron recombination sites could be controlled. The operating conduction and light emission mechanism are discussed with the aid of EL images obtained using a charge-coupled device (CCD) camera.

Anthracene-based semiconductors for organic field-effect transistors

A review of materials design strategies and efficient study methods for modeling of anthracene derivatives, with a description of their significant application in light-emitting transistors.

Synthesis and Optical Properties of Donor-Acceptor-Type Benzo[&lt;i&gt;b&lt;/i&gt;]phosphole Oxides

Benzo[b]phosphole oxides exhibit attractive optical and electronic properties for applications as materials for organic light-emitting devices, photovoltaics and field-effect transistors. Recently, benzo[b]phosphole oxides introduced an electron-donating substituent have been emerged. These molecules show strong fluorescence and solvatochromism depending on the electron-donating substituent. This review highlights recent progress in developing donor-acceptor-type benzo[b]phosphole oxides and their photophysical properties.

Polymerization of acrylates based on n-type organic semiconductors using Cu(0)-RDRP

Three acrylic monomers have been prepared based on organic semiconductor motifs commonly used as n-type materials in organic light-emitting diodes (OLEDs) and organic thin-film transistors (OTFTs) and polymerized by Cu(0)-RDRP.

N-Type organic light-emitting transistors with high mobility and improved air stability

The first example of n-type organic light-emitting transistors operated in air was demonstrated using perylene diimide as a charge-transporting and light-emitting layer.

Electrolyte-gated light-emitting transistors: working principle and applications

The material selection, working mechanism and target application in the development of electrolyte-gated light-emitting transistors are summarized.

Organic crystal light-emitting transistors with top-gate configuration

We applied the top-gate configuration to organic light-emitting transistors (OLETs) having an organic semiconductor crystal and an aluminum-doped zinc oxide (AZO) layer. The AZO layer was inserted between a quartz substrate and the organic crystal. The devices had the top-contact configuration where gold and an alloy of magnesium and silver were used for the contacts. Silver and Parylene C® were used for a gate contact and a gate insulator for the top-gate configuration, respectively. These OLETs showed more efficient current injection at low voltages than both the devices without a gate contact and the devices having the bottom-gate configuration. The present results indicate that the top-gate configuration is effective for improving the crystal OLET characteristics.

Aromatic Extension at 2,6-Positions of Anthracene toward an Elegant Strategy for Organic Semiconductors with Efficient Charge Transport and Strong Solid State Emission.

Organic semiconductors integrating excellent charge transport with efficient solid emission are very challenging to be attained in the construction of light-emitting transistors and even for realization of electrically pumped organic lasers. Herein, we introduce naphthyl units at 2,6-positions of anthracene to achieve 2,6-di(2-naphthyl)anthracene (dNaAnt), which adopts J-aggregated mode in the solid state as a balanced strategy for excellent charge transporting and efficient solid state emission. Single crystal field-effect transistors show mobility up to 12.3 cm2·V-1·s-1 and a photoluminescence quantum yield of 29.2% was obtained for dNaAnt crystals. Furthermore, organic light-emitting transistors (OLETs) based on dNaAnt single crystals distribute outstanding balanced ambipolar charge transporting property (μh = 1.10 cm2·V-1·s-1, μe = 0.87 cm2·V-1·s-1) and spatially controllable emission, which is one of the best performances for OLETs.

Organic planar pn heterojunction light-emitting diodes: Materials and devices

Organic light-emitting diodes (OLEDs) have been widely applicable in the flat-panel display products, such as mobile phones and television. They also possess more outstanding application advantages in the area of wearable display products in the future and have a great potential to serve as a key competitor in the next-generation solid-state lighting technology. For the commercial OLED products, phosphorescent materials that contain the rare metals are essential. This is because that phosphorescent OLEDs can give more efficient radiation, as the result of harvesting both singlet and triplet excitons for radiative utilization, in contrast to traditional fluorescent emitters. However, because of the limited resources of the rare metals, e.g., indium, on the earth, the phosphorescent materials are rather expensive, and their sustainable supply are also questionable. Besides, the long-term stability of blue phosphorescent emitters are also challenging. To further lower the fabrication cost of materials and devices, numerous research efforts have been devoted to developing rare-metal-free, purely organic light-emitting materials, which could be more low-cost and still exhibit excellent electroluminescence performances. In this review, we firstly introduce the evolution of OLED materials and devices. The OLED materials could be roughly classified into three generations. For the 1st generation traditional fluorescent emitters, three quarters of electro-generated excitons are wasted as heat release, due to the spin forbidden rule of triplets. This suggests that the OLEDs employing traditional fluorescent emitters could only achieve a maximum internal quantum efficiency of 25%. For the 2nd generation phosphorescent emitters, the spin orbital coupling due to heavy atom effect results in the mixing of singlet and triplet energy levels, and make triplet radiative transition feasible. Thereby, phosphorescent OLEDs can obtain a maximum internal quantum efficiency close to 100%. Recently, the 3rd generation hyperfluorescence emitters with the thermally activated delayed fluorescence (TADF) character could also make the devices thereof give an internal quantum efficiency close to 100%. This paves a road to realize low-cost and high-performance OLEDs based on purely organic semiconductors. We also outline the evolution of the OLED device configuration. For single-, double- and multi-layer device configuration of conventional OLEDs, their performances are mainly dependent on the emission layer (EML). The current methodology to develop high-performance OLEDs is to maximize the light emission ability of EML. Based on the understanding of the excitonic property of organic semiconductors, we show that efficient EML-free planar pn heterojunction light-emitting diodes (pn-OLEDs) should be realizable, given that the planar heterojunction property is well modulated. This indicates a way to directly construct efficient fluorescent OLEDs without an EML from the perspective of planar heterojunction composed of p type and n type charge transport materials. As following, the progress of OLEDs based on the layer-to-layer charge transfer mechanism using purely organic semiconductors is introduced. Main discussion focuses on pn-OLEDs, including that how this concept was proposed and the progress of relevant materials and devices. We also discuss the challenges of developing high performance pn-OLEDs and their potential applications in other optoelectronic areas, such as organic light-emitting transistors and electrically-pumped organic laser.

Metal-Halide Perovskite Transistors for Printed Electronics: Challenges and Opportunities.

Following the unprecedented rise in photovoltaic power conversion efficiencies during the past five years, metal-halide perovskites (MHPs) have emerged as a new and highly promising class of solar-energy materials. Their extraordinary electrical and optical properties combined with the abundance of the raw materials, the simplicity of synthetic routes, and processing versatility make MHPs ideal for cost-efficient, large-volume manufacturing of a plethora of optoelectronic devices that span far beyond photovoltaics. Herein looks beyond current applications in the field of energy, to the area of large-area electronics using MHPs as the semiconductor material. A comprehensive overview of the relevant fundamental material properties of MHPs, including crystal structure, electronic states, and charge transport, is provided first. Thereafter, recent demonstrations of MHP-based thin-film transistors and their application in logic circuits, as well as bi-functional devices such as light-sensing and light-emitting transistors, are discussed. Finally, the challenges and opportunities in the area of MHPs-based electronics, with particular emphasis on manufacturing, stability, and health and environmental concerns, are highlighted.