B413249C - View Online FEATURE ARTICLE...

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Electron-transporting materials for organic electroluminescent and electrophosphorescent devices Gregory Hughes and Martin R. Bryce * Received 27th August 2004, Accepted 22nd October 2004 First published as an Advance Article on the web 18th November 2004 DOI: 10.1039/b413249c One of the requirements for efficient organic electroluminescent devices (OLEDs) is balanced charge injection from the two electrodes and efficient transport of both holes and electrons within the luminescent layer in the device structure. Many of the common luminescent conjugated polymers, e.g. derivatives of poly(phenylenevinylene) and poly(fluorene), are predominantly hole transporters ( i.e. p-dopable). This article gives a brief overview of organic electroluminescence and electrophosphorescence and provides a more detailed consideration of ways in which electron transport in these systems has been enhanced by the incorporation of electron-deficient ( i.e. n-dopable) small molecules and polymers into the devices, either as blends or by covalent attachment of sub-units to the luminophore or as an additional electron-transporting, hole-blocking (ETHB) layer adjacent to the cathode. The chemical structures of these systems are presented and their roles are assessed. Most of these ETHB molecules are electron-deficient aromatic nitrogen-containing heterocycles, e.g. derivatives of 1,3,4-oxadiazole, pyridine, pyrimidine, pyrazine, quinoline, etc . Non-aromatic thiophene- S,S -dioxide derivatives are also discussed. The article is written from an organic chemist’s perspective. 1. Introduction and background Electroluminescence (EL) from an organic material was first reported in 1963 by applying ca. 400 V across single crystals of anthracene. 1 In 1982, vacuum-deposited thin films of anthra- cene were shown to exhibit blue EL when subjected to lower drive voltages, e.g. 30 V. 2 However, the quantum efficiencies (defined as photons emitted/electrons injected) and the life- times of these devices were considerably lower than those based on inorganic systems which continued to be the focus of attention. It was the seminal work of Tang and VanSlyke with bilayers of low molecular weight organic molecules, reported in 1987, which initiated worldwide interest in the development of organic light-emitting devices (OLEDs). 3 The device structure was conceptually simple, comprising evaporated thin layers of organic compounds, which were chosen for their ability to act as fluorophores and to transport charge carriers, sandwiched between two electrodes one of which was transparent. Green EL was observed with devices comprising the bis(triarylamine) 1 as the hole-transporting layer and tris(8-hydroxyquinoline)aluminium (Alq 3 ) 2 as the emitter and electron-transporting layer in the configuration ITO/ 1 / 2 / Mg:Ag. A brightness of . 1000 cd m 2 2 at an operating voltage of , 10 V was achieved. The next fundamental breakthrough was reported by Friend
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This note was uploaded on 03/27/2011 for the course CHEM 2211L taught by Professor T.a. during the Spring '08 term at University of Georgia Athens.

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B413249C - View Online FEATURE ARTICLE...

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