Polymers for Organic Light Emitting Devices/Diodes (OLEDs)
Facing the 21st century, the development of new techniques that are able to display data faster, more detailed and in mobile applications, is one of the prospering scientiﬁc ﬁelds. One approach for lightweight, ﬂexible, power-eﬃcient full-color displays are organic light emitting diodes (OLEDs). Such devices with their low driving voltage, bright color and high repetition rate (e.g. for video-application) are ideal for usage in miniature displays as well as in large area screen [1-3]. The basic principle of these devices are electroluminescent ‘semiconducting’ organic materials packed between two electrodes. After charge injection from the electrodes into the organic layer and charge migration within this layer, electrons and deﬁcient electrons (so called ‘holes’) can recombine to form an excited singlet state. Light emission of the latter is then a result of relaxation processes [4-6]. To achieve high electroluminescence eﬃciencies, the materials have to fulﬁll several speciﬁc requirements including low injection barriers at the interface between electrodes and organic material, balanced electron-and hole-density and mobility and high luminescence eﬃciency. Furthermore, the recombination zone should be located away from the metal cathode to prevent annihilation of the exited state. Since no material known to date is able to meet all these criteria, modern OLEDs consist — besides the transparent substrate (e.g., glass, PET), anode (most commonly indium tin oxide, ITO) and metal cathode (e.g., Mg-Ag-alloy) — of several organic layers for charge injection, transport and/or emission [7,8] (the principal set-up is shown in Scheme 1).