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

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

Propagation des charges électriques sur les pellicules faiblement conductrices « problème plan »

Dans une étude précédente sur les ondes de brillance en électrophotoluminescence, on observait des décalages anormaux si les électrodes étaient insuffisamment conductrices; pour obvier à cet inconvénient il avait été nécessaire de recourir à des électrodes liquides de conductibilité élevée. L'auteur étend aux électrodes circulaires l'étude théorique des pellicules faiblement conductrices, problème qu'il avait traité dans cette revue dans le cas d'une propagation linéaire. Il montre qu'on peut avoir ici des décalages importants entre la tension au centre et la tension à la périphérie, bien que la chute de tension soit insignifiante. L'expérience confirme les prévisions du calcul