The Physics of Conjugated Polymers

2 The Physics of Electroluminescent Devices 5.2.1 The Physics of Conjugated Polymers 

Several reviews have been reported on the electronic properties of conjugated polymers [6-9] as well as on the physical mechanisms occurring in LED devices [10-12]. Here, a few aspects of this very wide field are summarised, focusing on the properties of the electronic excitations which are more relevant in the physics of an electrolnminescent device. 

Only singlet excitons, S, are created by photon absorption (direct triplet generation is forbidden from the spin selection) and recombine radiatively with a certain probability, expressed by the PL quantum efficiency, (the ratio between the number of photons emitted and the number of photons absorbed). When the exciton is created by the coalescence of two polarons, the probability of forming a singlet exciton is 1/4 while 

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Most research effort, therefore, concentrated on those polymers where it is possible to control morphology, to obtain chemically pure materials and to manipulate them to provide forms, e.g. chain oriented samples and thin films, suitable for basic research and applications. The development of synthetic routes that provide clean, reproducible samples has been critical in the progress achieved in understanding the physics of conjugated polymers. This in turn underpinned the rapid development of technological applications, which appeared on a much shorter timescale than for other, comparable technologies. The level of scientific and technical interest in polymers, such as PAc, PAni, PPy, PThs, PPVs and polyfluorenes, has resulted in the commercial production of them and their precursors. 

The purpose of this chapter is to illustrate the usefulness of PES for studying the electronic and chemical structure of conjugated polymer surfaces and interfaces. As this handbook deals with all aspects of conjugated polymers, the background information on conjugated polymers is kept to a minimum here. It is further assumed that the reader is familiar with some of the unique electronic structural issues in the physics of conjugated polymers, i.e., the concepts of solitons, polarons, and bipolarons. 

Another most notable aspect relates to the vibronic coupling, that is the coupling between electronic excitations and vibrational modes. As has been stressed many times in the literature, much of the rich and fascinating physics of conjugated polymers is based on the strong... 

An important point in the science of conjugated polymers is the dependence of their optical properties as a response to their physical state (solution, aggregated, liquid crystalline, solid state, thermal treatment) [50]. Processable PPEs, particularly dialkoxy-PPEs, have been known since the early 1990s [1], and their spectroscopic behavior has been studied both in... 

In view of the diversity in the morphology of conjugated polymers, it is essential that physical measurements be made on samples that are not only chemically pure but where the morphology has been established by thorough characterisation. 

J. L. Bredas, R. R. Chance, R. Silbey, Comparative theoretical study of the doping of conjugated polymers - polarons in polyacetylene and polypara-phenylene, Physical Review B 1982, 26, 5843. 

In the field of conjugated polymers, poly(/ -phenylene) (PPP) continues to receive considerable attention due to its outstanding physical and chemical properties. In particular, it is known for its exceptional thermal stability in the neutral state, its resistance to environmental oxidation and irradiation, its very wide conductivity range (from 10 S cm in the pristine form to more than 10 S cm in the doped state), the... 

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