Oligothiophenes optics materials

Oligothiophenes containing donor and acceptor substituents are of interest for the investigation of the electronic interaction between the two end groups, and their use as nonlinear optical materials. 

R. D. Myrex, G. M. Gray, A. G. VanEngen Spivey and C. M. Lawson, Synthesis and characterization of transition metal systems containing phosphino-oligothiophene ligands for nonlinear optical materials, Organometallics, 25, 5045-5050 (2006). 

The molecular structure of selected V-shaped oligothiophenes is shown in Scheme 5, while the corresponding electro-optical characteristics are reported in Table 2. The luminance vs. voltage plots and the electroluminescence spectra of the devices fabricated with 9,10 and 11 as the active materials, are shown in Figure 4. 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

During the last decade, there have been countless studies devoted to the synthesis of different thiophene-based polymers and oligomers for photovoltaic applications and various photoconversion efficiencies ranging from 0.1 to 5 % have been reported for devices based on such materials. We have chosen some references that cover a small part of the literature on polythiophene-based [40-54] and oligothiophene-based [55-60] devices with relatively successful photovoltaic performance. Figure 18.3 shows the molecular structures of some of the most successful materials, and their optical bandgaps and PCE are given in Table 18.1. 

The other aspect is related to the basic studies of the charge-transport processes and mechanisms in the molecular conductive materials. This is because fundamental transport properties such as mobility can be easily defined on these electronic devices. Optical processes can be appropriately dealt with similarly. In this section, we highlight the thin-film electronic devices of FETs and LEDs based on the oligothiophenes in relation to the devices using other organic materials. Details of the fabrication and action characteristics of the devices using those materials can be seen in recent papers and reports. 

Although the electronic properties of oligothiophenes have already been reviewed [29, 30], the picture of the nature and ordering of the electronic states in the solid state is still diffuse and incoherent. Therefore we will cover recent results on the optical properties especially in the soUd state and the outcome may lead to a clearer picture of the excited state ordering. This may even contribute to a better understanding of the electronic states for other rigid-rod-like conjugated materials. 

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