Alternating-current Electroluminescent Research Articles

Electroluminescence of Zn2GeO4:Mn through SiC whisker electric field enhancement

Alternating current (AC) electroluminescence of thin film oxide phosphors is well known. However in this work electroluminescence of bulk oxide powder phosphors is achieved. A new type of AC Electroluminescent (ACEL) device has been created and developed by integrating SiC whiskers into a phosphor matrix composed of manganese-activated zinc germanate (Zn2GeO4:Mn2+). The conductive SiC whiskers enhance the average electric field in specific regions of the phosphor such that localized breakdown of the phosphor occurs, thus emitting green light. This field enhancement allows light emission to occur in thick film oxide powder phosphors and is notably the first time that bright and reasonably efficient electroluminescence of zinc germanate has been observed without using expensive thin film deposition techniques. Light emission has been achieved in thick pressed pellets using surface-deposited electrodes and the brightness-voltage characteristics of light emission are shown to be consistent with field emission of carriers from the embedded whiskers.

Highly stretchable and self-deformable alternating current electroluminescent devices.

A highly stretchable light-emitting device based on an alternating current electroluminescent (ACEL) material is fabricated by a simple and all-solution processable method. The ACEL device shows stable emission performance with strain up to 100%, which can be repetitively stretched to 80% strain. Furthermore, a novel self-deformable EL device that can be electrically driven into dynamic shapes is demonstrated by integrating the device with dielectric elastomer actuators. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Studies on the Orientation of ACEL ZnS:Cu Particles in Applied AC Fields

In the last sixty years the understanding of the mechanisms of alternating current electroluminescence (ACEL) in transition metal-doped zinc sulfides, in particular ZnS:Cu particles has been the goal of all workers in the field. An in depth understanding of the crystallography and uncommon hemimorphic nature of these particles has lead to some understanding as to mechanism of light emission under an alternating current field. Hydrochloric acid etching of ZnS:Cu particles has allowed an intimate study of the hemimorphic nature and high density of planar stacking faults present in the particles that are critical for ACEL to occur. This work using complimentary field emission scanning electron microscopy and digital optical microscopy shows that the alignment of the planar stacking faults of these particles relative to the applied electric field is critical for light emission. Perpendicular alignment of the stacking faults results in no emission of light; as the alignment gradually approaches parallel emission increases and at parallel reaches a maximum. Thus, for devices using these materials alignment of the particles with the electric field is most important to maximize light output.

Flexible Graphene-Based Electroluminescent Devices

For the first time, large-area CVD-grown graphene films transferred onto flexible PET substrates were used as transparent conductive electrodes in alternating current electroluminescence (ACEL) devices. The flexible ACEL device based on a single-layer graphene electrode has a turn-on voltage of 80 V; at 480 V (16 kHz), the luminance and luminous efficiency are 1140 cd/m(2) and 5.0 lm/W, respectively. The turn-on voltage increases and the luminance decreases with increasing stacked layers of graphene, which means the single-layer graphene is the best optimal choice as the transparent conductive electrode. Furthermore, it demonstrates that the graphene-based ACEL device is highly flexible and can work very well even under a very large strain of 5.4%, suggesting great potential applications in flexible optoelectronics.

Crystal structure of quenched and in-field electroluminescent phosphors

Electroluminescent zinc sulfide doped with copper and chloride (ZnS:Cu, Cl) powder was heated to 400°C and rapidly quenched to room temperature. Comparison between the quenched and non-quenched phosphors using synchrotron radiation X-ray powder diffraction (XRPD) (λ = 0.828692 Å) and X-ray absorption spectroscopy (XAS) was made. XRPD shows that the expected highly faulted structure is observed with excellent resolution out to 150° 2θ (or to (12 2 2) of the sphalerite phase). The quenched sample compared to the unheated sample shows a large change in peak ratios between 46.7° and 46.9°, which is thought to correspond to the wurtzite (0 0 6), (0 3 2) and sphalerite (3 3 3)/(5 1 1) peaks. Hence, a large proportion of this sphalerite diffraction is lost from the material upon rapid quenching, but not when the material is allowed to cool slowly. The Zn K-edge XAS data indicate that the crystalline structures are indistinguishable using this technique, but do give an indication that the electronic structure has altered due to changing intensity of the white line. It is noted that the blue electroluminescence (EL) emission bands are lost upon quenching: however, a large amount of total EL emission intensity is also removed, which is consistent with our findings. We report the XRPD of a working alternating-current electroluminescence device in the synchrotron X-ray beam, which exhibits a new diffraction pattern when the device is powered in an AC field even though the phosphor is fixed in the binder. Significantly, only a few crystals are required to yield the diffraction data because of the high flux X-ray source. These in panel data show multiple sharp diffraction lines spread out under the region, where capillary data show broad diffraction intensity indicating that the phosphor powder is comprised of unique crystals, each having different structures.

Materials for Powder-Based AC-Electroluminescence

At present, thick film (powder based) alternating current electroluminescence (AC-EL) is the only technology available for the fabrication of large area, laterally structured and coloured light sources by simple printing techniques. Substrates for printing may be based on flexible polymers or glass, so the final devices can take up a huge variety of shapes. After an introduction of the underlying physics and chemistry, the review highlights the technical progress behind this development, concentrating on luminescent and dielectric materials used. Limitations of the available materials as well as room for further improvement are also discussed.

Flexible and Transparent SWCNT Electrodes for Alternating Current Electroluminescence Devices

The application of transparent single-walled carbon nanotube (SWCNT) electrodes in rigid and flexible alternating current electroluminescence (ACEL) devices is demonstrated. SWCNT thin-film electrodes (50-160 nm) were made using a spray-coating process suitable for adjusting the transparency and sheet resistance. The dispersing procedure was optimized by comparing the transparency to sheet resistance ratio (T/R) of the electrodes. The emission intensity was as high as that for indium-tin oxide (ITO)-based ACEL devices with transparencies comparable to those of ITO-coated polymer slides.

Flexible and Transparent SWCNT Electrodes for Alternating Current Electroluminescence (ACEL) Devices

AbstractThe application of transparent single-walled carbon nanotube (SWCNT) electrodes in rigid and flexible alternating current electroluminescence (ACEL) devices is demonstrated. SWCNT thin film electrodes (50 – 160 nm) were made using a spray-coating process suitable for adjusting transparency and sheet resistance. The emission intensity was as high as for indium tin oxide (ITO) based ACEL devices with transparencies comparable to ITO coated polymer slides.

Alternating-current electroluminescence from an organic heterojunction sandwiched between two amorphous SiO2 layers

We report alternating-current electroluminescence (ACEL) from an organic heterojunction sandwiched between two amorphous SiO2(α-SiO2) layers, i.e., ITO\α-SiO2\poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene](MEH-PPV)\Alq3\α-SiO2\Al. Besides the exciton radiation from Alq3 (peak at 520 nm) and MEH-PPV (peak at 587 nm), an additional broad blue emission around 410 nm is observed in its ACEL spectrum. By studying the ACEL and brightness wave form of the device, we found both hot-electron-impact excitation and injection carriers recombination are combined in its emission mechanism for this hybrid organic-inorganic structure, and the origin of the broad blue emission around 410 nm is discussed.

Visible and near-infrared alternating-current electroluminescence from sputter-grown GaN thin films doped with Er

A demonstration of visible and near-infrared (NIR) alternating-current electroluminescence from sputter-grown GaN thin films doped with Er is reported. The alternating-current thin-film electroluminescent (ACTFEL) devices were constructed using a standard single-insulating structure, Al/GaN:Er/aluminum–titanium–oxide/indium–tin–oxide/Corning 7059 glass. Visible emissions peaked at 550 and 665 nm as well as NIR emissions centered at 1000 and 1550 nm were observed from the fabricated ACTFEL devices operating at room temperature. The visible and NIR emissions at 550, 665, 1000, and 1550 nm were attributed to the Er3+ 4f–4f intrashell transitions from the 4S3/2, 4F9/2, 4I11/2, and 4I13/2 excited-state levels to the 4I15/2 ground state, respectively. The green 550 nm emission had a larger dI/dV and a higher threshold voltage, Vth than the NIR 1550 nm emission, which could result from the need for higher electron impact energy to impact-excite the Er ion into the higher-energy excited-states for emission of green light.

Direct-current and alternating-current electroluminescence of MOS capacitors with Si-implanted SiO 2

The direct-current and alternating-current electroluminescence (d.c.- and a.c.-EL) characteristics of n +-polysilicon MOS capacitors with 50 nm Si-implanted SiO 2 have been measured to discuss EL stability and frequency response. While the d.c.-EL intensity increases with time t at small constant current density ( J G) and decreases for large J G, it decreases with t monotonously under constant gate voltage ( V G) condition. The time dependence of the d.c.-EL intensity has been modeled with two kinds of states in the oxide layer. The a.c.-EL intensity for samples with various Si implantation conditions increases with frequency except for samples without Si implantation or with low energy implantation. The above implies that Si implantation produces slow states near the oxide–Si interface. A.c.-operation remarkably improved the EL intensity and time dependence. It may be attributed to an increase of effective current due to charge stored under alternating bias conditions.

Tentative anatomy of ZnS-type electroluminescence

The paper reviews the electrical and optical mechanisms at work in sulfide-based thin-film electroluminescence display devices within the framework of general semiconductor physics. The electrical problem is twofold: (i) charge carriers are sourced at high electric field in a nominally insulating material, the carrier density increasing by almost eight orders of magnitude; (ii) the carriers are transported at high field, with an average energy largely exceeding the thermal one. (i) Carrier sourcing is best understood from direct-current-driven ZnS films, and is ascribed to partly filled deep donors transferring electrons to the conduction band by Fowler–Nordheim tunneling. The deep donors also act as carrier sinkers, and evidence for space charge is afforded by small-signal impedance analysis disclosing a markedly inductive behavior. The conduction picture obtained from dc-driven films is then used to clarify the operation of alternating-current electroluminescence structures where the sulfide is sandwiched between two blocking oxide layers. The electrostatics of the ac structure is investigated in detail including space charge and field nonuniformity, and external observables are related to internal quantities. The simple model of interfacial carrier sourcing and sinking is examined. (ii) High-field electronic transport is controlled by the electron-phonon interaction, and the modeling resorts to numerical simulations or the lucky-drift concept. At low electron energies the interaction with phonons is predominantly polar, while at optical energies it proceeds via deformation potential scattering. In spite of the uncertainties in transport models in that range, it is likely that ∼50% of the electrons overtake 2 eV at the usual operating fields in ZnS. Light emission is associated with impurity luminescence centers embedded in the sulfide host. They are excited while current is flowing, and the ensuing relaxation is partly radiative. We describe the two ways in which an impurity may be excited electrically, namely, impact excitation (internal promotion of the center to a state of higher energy) or impact ionization (with an electron released to the host conduction band). The actual excitation mechanism depends on the position of the impurity excited level relative to the host energy bands. A calculation of the excitation yield (number of excited centers per transferred electron) is detailed in the case of impact excitation. Lastly, a phenomenological description of the various relaxation channels is given in terms of formal kinetics, and the relative importance of radiative relaxation is assessed by means of the deexcitation yield (fraction of centers decaying radiatively), which is defined in the case of the impulse response.

Reducing noise and flash of guns: N. Flamel. (Scientific American Supplement, vol. lxxxiii, no. 2158, p. 300, May 12, 1917. Translated from La Nature)

The direct-current and alternating-current electroluminescence (d.c.- and a.c.-EL) characteristics of n+-polysilicon MOS capacitors with 50 nm Si-implanted SiO2 have been measured to discuss EL stability and frequency response. While the d.c.-EL intensity increases with time t at small constant current density (JG) and decreases for large JG, it decreases with t monotonously under constant gate voltage (VG) condition. The time dependence of the d.c.-EL intensity has been modeled with two kinds of states in the oxide layer. The a.c.-EL intensity for samples with various Si implantation conditions increases with frequency except for samples without Si implantation or with low energy implantation. The above implies that Si implantation produces slow states near the oxide–Si interface. A.c.-operation remarkably improved the EL intensity and time dependence. It may be attributed to an increase of effective current due to charge stored under alternating bias conditions.