OLED Material technologies

Material technologie
s Small molecules Alq3,[18] commonly used in small molecule OLEDs Efficient OLEDs using small molecules were first developed by Dr. Ching W. Tang et al.[18] at Eastman Kodak. The term OLED traditionally refers specifically to this type of device, though the term SM-OLED is also in use. Molecules commonly used in OLEDs include organometallic chelates (for example Alq3, used in the organic light-emitting device reported by Tang et al.), fluorescent and phosphorescent dyes and conjugated dendrimers. A number of materials are used for their charge transport properties, for example triphenylamine and derivatives are commonly used as materials for hole transport layers.[29] Fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene, rubrene and quinacridone derivatives are often used.[30] Alq3 has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes. The production of small molecule devices and displays usually involves thermal evaporation in a vacuum. This makes the production process more expensive and of limited use for large-area devices than other processing techniques. However, contrary to polymer-based devices, the vacuum deposition process enables the formation of well controlled, homogeneous films, and the construction of very complex multi-layer structures. This high flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high efficiencies of the small molecule OLEDs. Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the pulsed regime, has been demonstrated.[31] The emission is nearly diffraction limited with a spectral width similar to that of broadband dye lasers.[32] [edit]Polymer light-emitting diodes poly(p-phenylene vinylene), used in the first PLED[19] Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an electroluminescent conductive polymer that emits light when connected to an external voltage. They are used as a thin film for full-spectrum colour displays. Polymer OLEDs are quite efficient and require a relatively small amount of power for the amount of light produced. Vacuum deposition is not a suitable method for forming thin films of polymers. However, polymers can be processed in solution, and spin coating is a common method of depositing thin polymer films. This method is more suited to forming large-area films than thermal evaporation. No vacuum is required, and the emissive materials can also be applied on the substrate by a technique derived from commercial inkjet printing.[33][34] However, as the application of subsequent layers tends to dissolve those already present, formation of multilayer structures is difficult with these methods. The metal cathode may still need to be deposited by thermal evaporation in vacuum. An alternative method to vacuum deposition is to deposit a Langmuir-Blodgett film. Typical polymers used in PLED displays include derivatives of poly(p-phenylene vinylene) and polyfluorene. Substitution of side chains onto the polymer backbone may determine the colour of emitted light[35] or the stability and solubility of the polymer for performance and ease of processing.[36] While unsubstituted poly(p-phenylene vinylene) (PPV) is typically insoluble, a number of PPVs and related poly(naphthalene vinylene)s (PNVs) that are soluble in organic solvents or water have been prepared via ring opening metathesis polymerization