**1. Introduction**

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290 Optoelectronics - Advanced Materials and Devices

Displays. *Kobunshi Ronbunshu in Japanese*.

In the last decades a high interest has been paid to the field of organic materials for electron‐ ic and optoelectronic devices as potential candidates for replacing the more expensive, ener‐ gy consumer and polluting technologies involved by inorganic semiconductor devices.

One of the most important advantages of the organic materials is the possibility to modify and optimize their molecular structure using the advantages of the design at the molecular level and the versatility of the synthetic chemistry with the purpose to tune their properties and make them adequate for a well defined optic, electronic or optoelectronic application.

Organic light emitting diodes (OLED) are interesting for applications in the full-colour flat panel displays and new generation of lighting source as an alternative to incandescent bulbs and compact fluorescent lamps. The OLEDs based technology has a large area of applica‐ tions from small mobile phone displays to TV and monitors because they show two impor‐ tant advantages compared to the competitive liquid crystals based technology: high brightness and wide viewing angle.

Since the discovery of luminescence in anthracene [1], the crucial moment in the develop‐ ment of the OLEDs technology was the realisation of the first organic bilayer structure able to emit light at low applied voltages [2]. After that, different organic multilayer structures have been tested to improve carrier injection, carrier transport and radiative recombination with the purpose to increase the OLED efficiency and lifetime [3-5]. The performances of the devices can be enhanced either by the selection of an adequate architecture, such as multi‐ layer structure or by doping, by controlled impurification of the organics.

properly cited.

© 2013 Stanculescu and Stanculescu; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is © 2013 Stanculescu and Stanculescu; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The research was focalised on different topics such as: effect of doping of the organic semi‐ conductor to increase the "transparency" of the energetic barrier to the injection of electron from the contact [6], influence of the trapped and interfacial charges generated in multilayer organic heterostructures on the properties of the device [7], charge tunnelling in multilayer stack and at the interface between organic and anode [8], influence of the thickness and dop‐ ing of the emission layer on the properties of OLEDs [9], injection of the charge carriers from the electrodes and their migration in correlation with different types of cathodes [10-13], transport phenomena in organics [14; 15], stacked geometry for efficient double-sided emit‐ ting OLED [16], graded mixed layer as active layer to replace heterojunction in OLEDs [17]. The awarding in 2000 of the Nobel prize for researches in the field of conducting polymers has stimulated the development of OLEDs based on polymeric materials, application em‐ phasised years before [18], opening the way of reducing the applied voltages (< 10 V) and increasing the brightness and lifetime.

**2. Bulk aromatic derivatives for optoelectronic applications**

prove the emission properties and thermal stability of the organic.

by fluorescence emission.

crystalline perfection and chemical defects.

conductivity of organic compounds.

transparency domain and good fluorescence emission.

dopant sites.

Organic luminescent solids are attracting increasing interest in various field of application from optoelectronics to photonics. The interest in studying organic crystals is justified by the perspective to use these materials as a crystalline host matrix both for organic and inorganic guests (dopants) for developing new classes of materials combining the advantageous prop‐ erties of both components host and guest. The organic matrix can assure an efficient fluores‐ cence mechanism, can assure simple methods for processing and can contribute to electrical transport. On the other hand, the dopants could increase the charge carrier mobility and im‐

Aromatic Derivatives Based Materials for Optoelectronic Applications

http://dx.doi.org/10.5772/51675

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Organic molecules containing electrons occupying nonlocalised molecular orbitals and strongly conjugated systems such as aromatic compounds, dyes, show important lumines‐ cence in solid state. This radiative emission involves transitions inside very well shielded systems of π-electrons. By light absorption, an electron is transferred to an antibonding π orbital on the lowest singlet excited state with a lifetime of 10-6-10-9 s, from which it decays

The perspective to tailor the specific physical properties of a molecular solid by guest parti‐ cles (dopant) embedded in the crystalline organic matrix is very attractive, but not so acces‐ sible because some complications can appear both from the crystalline structure and/or

Special research has been devoted to the growth of organic crystals doped with rare earth metallic ions to prepare materials for luminescent and laser applications and benzil doped with Cd2+. The properties of the host/guest systems based on organic crystals depend on the

Growth of large and structural good organic crystals at good ratio cost/properties is very im‐ portant for theoretical understanding of the phenomena taking place in organic solid state and development of new organic-organic, organic-inorganic materials for a target applica‐ tion. The main limitations in large-scale using of aromatic derivatives as crystalline matrix are correlated with the requirements for crystals growth, which involves identification of particularised solutions to overpass the low melting point, supercooling and low thermal

Substituted aromatic molecules are a class of organic materials containing weakly coupled, strongly polarisable delocalised π electrons. Concerning the bulk organic crystals, our inter‐ est was focalised on aromatic derivatives that contain one and two aromatic rings and sub‐ stituent groups which disturb the symmetry of the π-electrons cloud, such as metadinitrobenzene (m-DNB)/ C6H4N2O4 and benzil/ C6H5-CO-CO-C6H5, characterised by large

The development of the technologies in the field of the structures for optoelectronic applica‐ tions based on organic compounds is dependent of the development of fundamental and ap‐ plied knowledge of all the optical and electrical processes involved, because many particularities of the organic solid state are not yet well known and understand. This is a re‐ al challenge because the number of the organic luminescent compounds is much larger than the number of inorganic compounds and it is continuously increasing. To increase the quan‐ tum efficiency, lifetime and thermal stability of these devices requires the separate optimisa‐ tion of the generation, injection and transport of the charge carriers and their controlled recombination in different layers. The electrical properties of the OLED are controlled by the mobility of the charge carriers and the heights of the barriers [19], whereas the optical prop‐ erties by the refractive index mismatches at the glass/air and organic/ITO interfaces that generate the trapping of a large fraction of the light by the mechanism of total internal re‐ flection into glass and ITO [20].

Therefore the organic materials must be designed and selected in such a way to show spe‐ cial properties to satisfy these requirements. Organic luminescent materials can be divided considering their molecular structure and the macro scale organization. From the first point of view there are low-molecular compounds characterised by the possibility of high purifi‐ cation, easy vacuum deposition, high quantum yield fluorescence and large variety and high-molecular compounds (oligomers and polymers) characterised by mechanical strength, flexibility and luminescence over various spectral regions from near UV to near IR but, by small quantum yield of fluorescence. From the second point of view, macro scale organiza‐ tion, there are bulk crystalline organic and organic thin films and heterostructures to be used in devices' fabrication.

A special attention will be paid in this chapter to investigate the properties of bulk and thin films organic compounds showing both good optical, including luminescent, and transport properties for potential optoelectronic applications.
