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Organic electronics is the field of study concerning organic substances with desirable electronic properties. Conventional materials used in electronic equipments are inorganic. Organic electronics has led to finding alternatives to them which are organic or carbon containing materials. This is a very important contribution to the field of electronics for this paves way for cheaper, lighter and more energy efficient electronic devices Organic electronic equipments have already entered market and the field is expected to be critical for growth of the industry. Organic Eelectroncis is a field of material science.
Since the discovery of electricity and its conduction research on conductive materials focused primarily on inorganic materials for a very long time. It was only in the second half of the nineteenth century that a significant finding about conducting organic materials took place with Henry Letheby prepared polyaniline. Almost nine decades later researches became more active with the discovery of charge transfer by halogen salts of polycyclic aromatic compounds. Consequently many organic materials with conducting, semiconducting and insulating properties were discovered. By the end of the twentieth century conductive plastics were produced industrially and plastic electronics technology became popular.
The semiconductor property of organic solids can be explained by considering those with pi-electrons. In addition to the coplanar -bonds there exists pi-bonds between perpendicular orbitals. Molecular orbitals are formed with filled bonding and empty anti—bonding orbitals separated by a small energy gap. This is similar to the valence band and conduction band separated by an energy gap in the inorganic semiconductors. The difference is that gap between the bonding and anti-bonding states is a little lower than the band gap energy of inorganic semiconductors. This is because of the presence of weak van der Waals forces that hold the molecules of organic solids together. In inorganic solids molecules engage in covalent or ionic bonds persistent throughout the solids. These bonds influence the electrical behaviour of the crystalline solids. In the amorphous organic solids the van der Waals forces have little effect on the electrical behaviour of these solids but they do keep the band gap smaller.
Organic semiconductors can be either small molecules or polymers and also these are most important part of organic electronics. Small molecules like polycyclic aromatic compounds are used in the construction of organic semiconductors. Organic Light Emitting Diodes produced from anthracene are examples of such semiconductors. Organic polymers are intrinsic conductors with some of them having conductivity in the range of metals. Such polymers called Intrinsically Conducting Polymers (ICP) are easier to manufacture and can be modified so that they attain desirable electrical properties. Certain polymers of aromatic cyclic compounds, doubly bonded compounds and doubly bonded aromatic cyclic compounds are found to be ICPs.
OLEDs are constructed by exploiting electroluminescence in organic semiconductors. Electroluminescence is the generation of light other than black body radiation by semiconductors when excited electrically. Early discoveries of electroluminescence in organic materials were the results of application of high alternating voltages. By early 1960s electroluminescence from application of direct currents was observed. Consequent advances in research resulted in the discovery of electroluminescence in polymer films. OLEDs generally consist of two electrodes and the organic semiconducting material between them. The organic semiconductor may be single layer, bilayer or multi-layer. OLEDs with multiple layers of organic semiconductor are more efficient and offer better tuning of properties. Simpler bilayer OLEDs with a conductive and an emissive layer are common but graded heterojunction OLEDs are also gaining popularity.
Organic Field Effect Transistors can be developed from both small molecules and polymer organic semiconductors. Their structure and working are similar to the Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) but differ slightly. OFETs, like MOSFETs have a source and a drain with a channel between them that facilitates movement of charge carriers. The difference is that unlike MOSFETs with pn-junctions OFETs have conducting, insulating and an organic semiconductor to which the electrodes are directly attached.
Organic electronics is primarily applied in developing display devices like computer or television screens. The electroluminescent OLEDs are used in producing modern light weight displays with greater viewing angles and better contrast display devices. A layer of Thin Film Transistor (TFT), a type of OFET is also used in these devices. Organic solar cells are another application of organic electronics.
To understand Active Matrix OLEDs one needs to know about addressing schemes of display devices by which a pixel is set to a value. There are three types of addressing schemes – direct, matrix and raster. In direct addressing each pixel is control by a separate signal. Raster addressing requires scanning the screen. A matrix scheme requires only control signals from rows and columns of displays. AMOLED displays use active matrix mode which requires capacitors to maintain cell states. Passive matrix displays are used in low cost applications where stability is achieved by persistence of vision.
Thin Film Transistors also find applications in display technology. TFTs were already in use for making TFT LCDs before using in AMOLED displays. Formerly TFTs are made by using inorganic compound semiconductors and metal oxides, but now organic TFTs are being developed by depositing a thin film of insulator between an organic semiconductor and metallic contact. Being a type of OFET the source and drain electrodes are deposited in the semiconductor. Conducting channel is formed when the semiconductor is under positive bias due to bending of energy bands. Under zero bias the bands bend in the opposite direction.
Organic solar cells are one of the impotant filed of organic electronics, these are made from organic small molecules and polymers offering better flexibility and cost effectiveness over the conventional inorganic semiconductors. The operation of organic solar cells is based on the breaking up of excitons, quasiparticles which are the bound states of electrons and holes, into free electron-hole pairs. To understand how this is done a single layer organic solar cell is considered which is the simplest of the organic solar cells consisting of sandwiched organic semiconductors between two metallic conductors of different work functions. Excitons are generated when organic solar cells absorb photons. The difference in work functions forces excitons to split into electrons and holes.
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