cm049473l - 4556 Chem. Mater. 2004, 16, 4556-4573 Electron...

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Electron Transport Materials for Organic Light-Emitting Diodes Abhishek P. Kulkarni, Christopher J. Tonzola, Amit Babel, and Samson A. Jenekhe* Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750 Received March 28, 2004. Revised Manuscript Received August 5, 2004 A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs) is presented. The structure - property - performance relationships of many classes of ETMs, both small- molecule- and polymer-based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential), thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage). Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron- and hole-transport properties are combined in one material to achieve efficient single-layer OLEDs is also discussed. The review concludes with a brief perspective on the challenges that future research should address. 1. Introduction The tremendous research effort following the discov- ery of efficient electroluminescence in organic and conjugated polymer thin films 1,2 has resulted in a vast literature on organic light-emitting diodes (OLEDs). 1 - 3 Reviews of different aspects of the field of OLEDs have appeared, including electroluminescent materials 4 and device physics and engineering. 5 Evidence of the con- siderable progress that has been made in the field is that flat-panel displays based on OLEDs are emerging in commercial products such as cell phones and digital cameras. 6 Major challenges remain, however, including the need to significantly improve the performance and durability of blue, green, red, and white OLEDs for displays and lighting. One of the key challenges on the path to developing the next generation of high-performance OLEDs is the design and synthesis of readily processible and ther- mally robust emissive and charge transport materials with improved multifunctional properties. OLEDs are double charge injection devices, requiring the simulta- neous supply of both electrons and holes to the elec- troluminescent (EL) material sandwiched between two electrodes (Figure 1). To achieve an efficient OLED with the single-layer configuration shown in Figure 1a, the organic EL material would ideally have a high lumi- nescence quantum yield and be able to facilitate injec- tion and transport of electrons and holes. This demand
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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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cm049473l - 4556 Chem. Mater. 2004, 16, 4556-4573 Electron...

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