The Chemistry, Physics and Engineering of Organic Light-Emitting Diodes

Semiconducting Polymers Chemistry, Physics and Engineering. Edited by G. Hadziioannou and P. R van Hutten Copyright 1999 WILEY-VCH Verlag GmbH, Weinheim ISBN 3-527-29507-0 

13 The Chemistry, Physics and Engineering of Organic Light-Emitting Diodes 

In Section 13.2, we introduce the materials used in OLEDs. The most obvious classification of the organic materials used in OLEDs is small molecule versus polymer. This distinction relates more to the processing methods used than to the basic principles of operation of the final device. Small molecule materials are typically coated by thermal evaporation in vacuum, whereas polymers are usually spin-coated from solution. Vacuum evaporation lends itself to easy coating of successive layers. With solution processing, one must consider the compatibility of each layer with the solvents used for coating subsequent layers. Increasingly, multilayered polymer devices are being described in the literature and, naturally, hybrid devices with layers of both polymer and small molecule have been made. 

The paper is organized to describe, first, the materials that have been used in OLEDs, then the device structures that have been evaluated. After a description of the methods used to characterize and evaluate materials and devices, we summarize the current state of understanding of the physics of device operation, followed by a discussion of the mechanisms which lead to degradation and failure. Finally, we present the issues that must be addressed to develop a viable flat-panel display technology using OLEDs. Space and schedule prevent a comprehensive review of the vast literature in this rapidly moving field. We have tried to 

A second major class of emissive polymer is based on polythiophene [8], Again, substitution in this case at the -3- position of the thiophene ring is used to solubilize the polymer and to time the emission wavelength [9]. Other heterocyclic conjugated polymers are represented by polypyridine [10] and polypyridi-nevinylene [11]. Since each of these contains electron-deficient aromatic groups, they lend to be better electron transporters than their phenylene analogs. They emit primarily in the blue. 

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