LECs Made of m-Lppp
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, L. HOLZER', U. SCHERF 1, Aj. HEEGER
2
and G.
LEISING' ' Institut fir Festkdrperphysik, Technische UniversitAt Graz, Petersgasse 16, A-8010 Graz, Austria
Institute for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara,California 93106-5090, USA 3 Max-Planck-Institut far Polymerforschung, Ackermannweg 10, D-55128, Mainz, Germany 2
Abstract We report the application of the blue light emitting conjugated polymer m-LPPP (methyl substituted laddertype poly(paraphenylene) ) in light emitting electrochemical cells. The active layer of the LEC consists of a blend of m-LPPP with the ionically conductive polymer PEO and LiCF 3SO3 as
ionic salt. Investigations of different concentrations of PEO and salt showed that the best LECs made of m-LPPP up to now where realised with an active layer consisting of a blend of m-LPPP:PEO:salt in the range of 20:10:3. In this case we are able to realise LECs with response times in the range of 30 ps. The I/U characteristics show low turn on voltages both for current and electroluminescence, but only in the case of ITO biased as a cathode. The initial electroluminescence spectra are quite the same as those for LEDs made of m-LPPP but turn into green after some time of operation.
Introduction Nowadays highly efficient light-emitting diodes based on conjugated polymers (PLEDs) can be realised over the whole visible spectral range.[ 1-5 ]There are a lot of possible applications for PLEDs, maybe the most attractive is their use in self-emissive flat panel displays. While low band band gap polymers show low turn on voltages, this is not the case for wide band gap ones, due to the fact, that for these polymers the charge injection is more difficult because of the higher energetic barriers at the interface polymer/electrode. However, in order to realise multicolor flat panel displays the fabrication of blue PLEDs of sufficient lifetimes is absolutely necessary. Moreover, one of the technologically most promising ways to produce multicolor displays is based on blue PLEDs, where the red and green light emission are realised by an
external color conversion technique [6-8].
Recently, a new type of polymer light-emitting device, the polymer light-emitting electrochemical cell (LEC) has been presented[9-10], which has the advantage, that the EL onset occurs for all emission colors at a bias voltage close to E./e [11 ] where E.• means the single particle energy gap and e is the elementary charge. The different performance of LECs compared to PLEDs is due to the fact that the charge carrier injection in these devices relies on different physical mechanisms. In contrast to PLEDs, in LECs a pin-junction is formed during operation[10].The formation of the pin-junction is not an instantaneous process but depends on the mobility of the ions in the blend of the ionically conductive polymer, the light-emitting polymer and the ionic salt. Longer response times are therefore necessary for LECs compared to PLEDs[ 12]. The response time strongly depends on the morphology
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