Equivalent circuits and efficacy of single-layer ACPEL devices

This article has been made available through the Brunel Open Access Publishing Fund.A series resistance - capacitance equivalent circuit has been used to describe a single layer AC Powder Electroluminescence (ACPEL) lamp in operation. The two crucial components of this practical equivalent circuit are frequency and voltage dependent and have been independently determined for a single layer ACPEL device over a range of 50-800 Hz and 10-150V. The organic binder containing a ferroelectric component is mainly responsible for determining the capacitive element since it acts in series with a larger capacitative contribution mainly from the phosphor. The series resistive element will be determined by mainly the phosphor particles, and the remarkable changes in the effective series resistance and capacitance of the lamp structure are shown to be brought about by the activation of the ZnS phosphor. The effective resistance is consistent with a model where conductivity is governed by the average charge recombination time under given internal field and frequency conditions. Using the effective resistance and capacitance values of our equivalent circuit, the average rate of energy dissipation can be easily calculated as a function of applied voltage and frequency. For sinusoidal waveforms, first indications are that efficacy will be optimized at low voltages, but only weakly dependent on frequency. © The Author(s) 2014

Studies on the orientation of ACEL ZnS:Cu particles in applied AC fields

This article has been made available through the Brunel Open Access Publishing Fund.In the last sixty years the understanding of the mechanisms of alternating current electroluminescence (ACEL) in transition metaldoped 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. © 2013 The Electrochemical Society. All rights reserved

Optimizing the internal electric field distribution of alternating current driven organic light-emitting devices for a reduced operating voltage

This work was funded with financial means of the European Social Fund and the Free State of Saxony through the OrthoPhoto project.The influence of the thickness of the insulating layer and the intrinsic organic layer on the driving voltage of p-i-n based alternating current driven organic light-emitting devices (AC-OLEDs) is investigated. A three-capacitor model is employed to predict the basic behavior of the devices, and good agreement with the experimental values is found. The proposed charge regeneration mechanism based on Zener tunneling is studied in terms of field strength across the intrinsic organic layers. A remarkable consistency between the measured field strength at the onset point of light emission (3-3.1 MV/cm) and the theoretically predicted breakdown field strength of around 3 MV/cm is obtained. The latter value represents the field required for Zener tunneling in wide band gap organic materials according to Fowler-Nordheim theory. AC-OLEDs with optimized thickness of the insulating and intrinsic layers show a reduction in the driving voltage required to reach a luminance of 1000 cd/m2 of up to 23% (8.9 V) and a corresponding 20% increase in luminous efficacy.Publisher PDFPeer reviewe

Photoelectroluminescence of Single Crystals of Manganese‐Activated Zinc Sulfide

Photoelectroluminescence, which involves the control of electroluminescence by electromagnetic radiation, is reported for single crystals of manganese-activated zinc sulfide. Both plane-parallel and point-contact electrode structures were used. The latter resulted in greater homogeneity of luminescent emission with the applied voltage mainly across the spreading resistance regions. The field distribution was probed. The observations are explained with essentially the same basic theory as was earlier used to explain observations on vapor-deposited films. However, from the analysis of the data on single crystals we deduce that carrier multiplication is occurring in the cathode region and to a lesser extent in the anode region, and also obtain values for the local fields for acceleration of carriers and for collision excitation. © 1966 The American Institute of Physics

Structure and morphology of ACEL ZnS:Cu,Cl phosphor powder etched by hydrochloric acid

© The Electrochemical Society, Inc. 2009. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version is available at the link below.Despite many researches over the last half century, the mechanism of ac powder electroluminescence remains to be fully elucidated and, to this end, a better understanding of the relatively complex structure of alternate current electroluminescence (ACEL) phosphors is required. Consequently, the structure and morphology of ZnS:Cu,Cl phosphor powders have been investigated herein by means of scanning electron microscopy (SEM) on hydrochloric acid-etched samples and X-ray powder diffraction. The latter technique confirmed that, as a result of two-stage firing during their synthesis, the phosphors were converted from the high temperature hexagonal (wurtzite) structure to the low temperature cubic (sphalerite) polymorph having a high density of planar stacking faults. Optical microscopy revealed that the crystal habit of the phosphor had the appearance of the hexagonal polymorph, which can be explained by the sphalerite pseudomorphing of the earlier wurtzite after undergoing the hexagonal to cubic phase transformation during the synthesis. SEM micrographs of the hydrochloric-etched phosphor particles revealed etch pits, a high density of planar stacking faults along the cubic [111] axis, and the pyramids on the (111) face. These observations were consistent with unidirectional crystal growth originating from the face showing the pyramids.EPSRC, DTI, and the Technology Strategy Board-led Technology Program

Highly efficient broadband conversion of light polarization by composite retarders

Driving on an analogy with the technique of composite pulses in quantum physics, we propose highly efficient broadband polarization converters composed of sequences of ordinary retarders rotated at specific angles with respect to their fast-polarization axes.Comment: 5 pages, 3 figures; check JOSA A 201

A.C.Electroluminescent Lamps: Shedding some light on their mysteries

A.C.powder electroluminescent lamps have been known and used for many years, but their mechanism of operation is still debated. Many thousands of phosphors are known, but the vast majority are not electroluminescent. A number of materials do exhibit the effect. Of these, however, ZnS doped with Cu is absolutely in a class of its own, and is the only material from which viable lamps can be made. In this work studies have been made of the performance of devices under a range of pulsed and continuous excitation conditions and new hypotheses presented which attempt to explain the behavior of this unique material