Oled

<p><img src="http://lilsamobi.mywapblog.com/files/lg-77oledtv8-480-80.jpg" alt="lg-77oledtv8-480-80.jpg" /></p><div style="text-align:center;"></div><div style="text-align:center;"><p>A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode , all deposited on a substrate . The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over part or all of the molecule. These materials have conductivity levels ranging from insulators to conductors, and are therefore considered organic semiconductors . The highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO ) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors.</p></div><div style="text-align:right;"><p>Originally, the most basic polymer OLEDs consisted of a single organic layer. One example was the first light-emitting device synthesised by J. H. Burroughes et al. , which involved a single layer of poly(p-phenylene vinylene). However multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency. As well as conductive properties, different materials may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, [22] or block a charge from reaching the opposite electrode and being wasted. [23] Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer. More recent developments in OLED architecture improves quantum efficiency (up to 19%) by using a graded heterojunction. [24] In the graded heterojunction architecture, the composition of hole and electron-transport materials varies continuously within the emissive layer with a dopant emitter. The graded heterojunction architecture combines the benefits of both conventional architectures by improving charge injection while simultaneously balancing charge transport within the emissive region. [25] During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. Anodes are picked based upon the quality of their optical transparency, electrical conductivity, and chemical stability. [26] A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton , a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region . The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.</p></div>