Light-emitting Polymers

Structure of an OLED

An OLED consists of a substrate, a transparent electrode, one or more thin organic layers and a counter-electrode. This structure is protected through encapsulation, since both the organic layers and also some of the electrode materials are very sensitive to oxidation and react adversely with moisture. The substrate is glass as a rule but plastic films and glass/plastic laminates are also conceivable. A good diffusion barrier against oxygen and moisture is important here too. The transparent anode is ITO (Indium Tin Oxide) or similar oxide compounds.

An OLED functions much like an inorganic LED and is based on injection electroluminescence. The four elementary processes in an OLED are: the injection and transport of electrons and holes, the formation of electron-hole pairs and the radiative recombination of the charge carriers. When the applied voltage between the electrodes is high enough, charge carriers are injected into the organic layer. Under the influence of the electrical field, the injected charge carriers move to the opposite electrode. If electrons and holes meet, an electron-hole pair (exciton) can be formed, which can recombine radiatively and emit light. The emission spectrum and thus its color are determined by the energy of the exciton and thus by the organic material used. The spatial extension of the recombination zone depends on the mobility, the diffusion length and the energetic conditions, such as for example internal barriers. For high efficiency OLEDs the processes mentioned above must be synchronized with each other.