VIBRATIONAL SPECTROSCOPIE STUDIES ON LECs

Research project P 14170 Vibrational spectroscopic studies on LECs Günther LIESING 06.03.2000 The application of conjugated polymers as active layers in polymer light emitting diodes (PLEDs) is currently in the focus of world wide research due to their high potential for flat panel display application. Several attempts were reported within the last years to increase the efficiencies of these devices and to overcome the problem of the barrier heights at the electrodes for charge injection, especially for wide band gap polymers. One of these promising concepts is the use of the so called light emitting electrochemical cells, where the conjugated polymer is blended with an polymeric solid state electrolyte. The goal of this project is to investigate the physical aspects of these mixed ionic-electronic conductors and their applications in light emitting devices, since there is still a debate on the working principle of these LECs. While one model suggests electrochemical doping and therefore the formation of p- and n doped regions close to the electrodes to occur, is the other model based on an ionic space charge assisted charge injection. These investigations will be performed by vibrational spectroscopy studies of -devices under operation, since vibrational spectroscopy is a well known method to study ion dissociation and ion-ion interaction in solid state electrolytes and also to study the changes of the vibrational spectra of conjugated polymers by doping. One therefore can obtain detailed informations about ion dissociation, ion distribution and ion-polymer interactions in these devices under operation and can unambiguosly display if doping occurs or not. Microscopic and X-ray spectroscopic studies will be performed in order to investigate the morphologies of the blends, since the morphology was found to have drastic influence on the performance of the devices. This project involves a characterisation of the electronic and the optical excitations of these mixed ionic electronic conductiors. The results of this project have a deep impact on other applications, for example in electrochromic displays - especially since it was recently shown that blending the conjugated polymer with an ionically conducting polymer can increase the doping kinetics and even the difference in the transmittances of the doped and the bleached states. In order to decrease the degree of phase separation also special designed copolymers, unifying electrical and ionically conducting properties, will be investigated. These copolymers have attracted attention due to the formation of supramolecular structures on the nanometer scale.