Mark Harrison
NOW PUBLISHED!!!
In this chapter, we discuss a number of optical and opto-electronic device applications such as light-emitting diodes [LEDs], photovoltaic and photoconductive devices, field-effect optical modulator devices and all-optical modulator devices. Following an introduction to the basic operating principles of each device, we will assess progress in the development of each type of device and focus on the underlying semiconductor physics issues.
The field of organic semiconductors, has existed for several decades. Molecular crystals of acenes, phthalocyanines, small molecules and metal-organic complexes such as Alq3 were studied because of their photoconductive and semiconducting properties and also as an approach to probe the optoelectronic properies of biological membranes. Small organic semiconductor molecules blended in polymer hosts have already found a major application in xerography. Recently, there has been renewed commercial interest and research activity into organic semiconductors with the development of conducting and semiconducting conjugated polymers. These offer scope for preparing large area conducting films for lightweight conductors, electromagnetic shielding and large area semiconducting films for displays, solar cells and transistor arrays.
In this chapter we will not dwell on the more established small molecular organic semiconductors but will focus on oligomers which can be viewed as finite model systems of recognisable conjugated polymers. We discuss oligomers of poly(thiophene) [PT], because of their high field-effect mobilities. Structures of some oligothiophenes are shown. Poly(para-phenylene vinylene) [PPV] shows highly efficient yellow-green emission and this family of polymers is used extensively in polymer light-emitting diodes. We therefore discuss its oligomers, such as stilbene and distyrlbenzene, which are also highly fluorescent. Thirdly, we study oligomers of poly(para-phenylene) [PPP], a polymer similar to PPV but with a larger semiconductor gap, leading to blue emission. While the parent polymer is rather insoluble, its oligomers, such as para-sexiphenyl [6P] and ladder oligophenyls are rather more amenable to the formation of thin films.
We identify two main reasons for studying oligomers of conjugated polymers:
Firstly, oligomers represent model systems for understanding the fundamental electronic properties of the corresponding polymer. Oligomers can be synthesised with a well-defined molecular length, as shown in the structural formula of the extensively studied oligomer, alpha-sexithiophene [alpha-6T] or para-sexiphenyl [6P]. Oligomers have therefore been recognised for some time as model systems for theoretical and experimental investigations aimed at extrapolating physical properties of finite oligomers to the corresponding ideal polymer of infinite length. In marked contrast, real conjugated polymers exhibit a distribution of lengths, along which pi-conjugation is effective. The coherent conjugated segments of the polymer chain are interrupted by defects, which may be of a conformational nature (e.g. twisting of the chain so that it is no longer planar) or of a chemical nature, such as a saturated sp3-hybridised carbon atom located somewhere along the chain. Extrapolations of quantitative characteristics from studies of oligomers can therefore also yield estimates of the effective conjugation length in real polymers.
Oligomers are well-defined systems of monodisperse (uniform-length) molecules, with greatly reduced occurrence of defects within the molecular chains, in comparison with polymers. They therefore offer the possibility of better ordering of the molecules and consequently more well-defined optical properties. This renders them particularly appealing for both theoretical and experimental investigations into a number of issues, which cannot be so readily assessed in polymeric systems. These include the following:
(i) dependence of the energies and equilibria of neutral and charged excitations as a function of the coherence (molecular) length.
(ii) substitution of oligomers, either with electro-active groups or with the aim of inducing order or disorder.
(iii) the role of inter-molecular processes.
When trying to understand the behaviour of oligomers, it is helpful to consider concepts employed for conjugated polymers and also those from the more established field of molecular semiconductors and charge-transfer salts, since inter-chain processes can be more easily observed in thin films of oligomers.
Secondly, in some cases, oligomers have already been shown to exhibit characteristics superior to those currently found in many conjugated polymers. In the first part of this chapter, the high field-effect mobilities for thin film transistors employing oligothiophenes are shown to be due to very effective inter-molecular charge transport. In this part of the chapter, we focus mainly on electroluminescence. Since the discovery of blue electroluminescence from anthracene, there has been interest in using short oligomers, particularly to achieve the blue emission required for full-colour displays. The energies of optical transitions of oligomers often vary linearly with the reciprocal of the oligomer length, since the length of the molecule confines the spatial extent of many of the charged and neutral excitations of the oligomer. Therefore, in short oligomers the lowest excited state of the singlet exciton is more confined than in long polymers, so higher excitation energies can be achieved, leading to blue emission. We also include discussion of an all-optical spatial light modulator prototype, which could have major applications in rapid image-processing.
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Mark Harrison mgh@who.net, Marburg, May 3, 1998
