An experimental and modelling study has been carried out of the current-voltage-luminance (J-V-L) characteristics of blue
polyfluorene-based organic light emitting devices, with a PEDOT:PSS anode and a Ba/Al cathode. The polymer contains copolymerized
hole transporting units that facilitate hole injection. The luminous efficacy for perpendicular emission as a
function of the voltage shows a pronounced peak; for an 80 nm thick device, it is equal to 3.3 cd/A at 8 V. At the peak
voltage, the external quantum efficiency is 2.2 %. We are working on a comprehensive device model that should provide a
framework within which these results can be understood, and present in this paper our intermediate results. Hole and electron
transport were studied using devices with a Au and Al cathode and anode, respectively. For hole-only devices a fair
description of the temperature and layer thickness dependent J-V curves could be obtained by using a 'conventional' model
for the mobility, involving a Poole-Frenkel factor for the field-dependence. For electron-only devices, the analysis is
complicated by the presence of an approximately 0.5 eV injection barrier. We have found a parametrization scheme that
provides a good description of the experimental J-V curves. A double carrier model that is based on the results of these
studies of single-carrier devices provides a good description of the J-V curves of double carrier devices. We have developed
a numerical model for the light outcoupling from the optical cavity. For the model parameters assumed, the calculated peak
position and shape of the lumunous efficacy as a function of V are in good agreement with the experimental results at room
temperature. An analysis is given of the factors that determine the peak height. We argue that a solid physical basis for the
model used to describe the electron injection and mobility is still lacking, so that continued electron transport studies will be
We present results and a discussion of highly efficient polymer Light-Emitting Diodes (polymer LEDs, PLEDs). The external quantum efficiency in current standard devices reaches up to 2-4% only. We have explored two routes to enhance this value. In the first route, PEDOT/PSS is replaced with a novel anode or hole injection layer. The efficiency with some Light Emitting Polymers (LEP) is improved significantly, resulting in an efficacy of 35 cd/A for a yellow emitting poly-(para-phenylene-vinylene) and 20 cd/A for a blue emitting poly-(spirobifluorene). We attribute the major improvement compared to standard devices, where about 10 and 5 cd/A are obtained, respectively, to a combination of improved exciton formation efficiency and light out-coupling efficiency, and to less quenching of the radiative decay under actual device operating conditions. In the second route, we developed a new host polymer with high triplet energy such that transition metal-based green-emitting phosphorescent dyes can be used without significant back transfer of triplet excitons to the polymer host. First results using this system showed about 25 cd/A using a soluble green Ir-based emitter. Importantly, all data are obtained in a standard two-layer device of a hole transport/injection layer and the LEP.
The status of the development of full-color polymer light emitting diodes will be presented. The focus of current materials research is on state-of-the-art red, green, and blue light-emitting polymers (LEP) with high efficiency, optimum color points, low driving voltages and long lifetimes in devices. A general overview of the progress of red, green and blue LEP lifetimes and efficiencies will be given and compared to requirements for both full-color passive and active matrix-displays for mobile display applications. Further, the status of ink-jet printing of LEPs for the industrialization of full-color displays will be discussed, and a comparison of the performance of spin coated and inkjet printed devices will be presented. In addition, two material-related topics studied recently will be discussed; namely, the lifetime of blue light-emitting devices correlated to processing, anodes, cathodes and the blue polymers themselves, and second, the consequences of pulsed-driving schemes on efficiency and lifetime.
An overview of the requirements for full color passive matrix displays and their implications for the light emitting materials will be presented. Using the performance of light emitting polymers tested in Philips devices the status of the light emitting polymers is reviewed. It will be shown that the performance of light emitting polymers is at the edge of being acceptable for practical applications. Red and green light emitting polymers can already be used for certain monochrome applications. However, for the high-resolution displays used in mobile telecom applications the efficacy for red and the lifetime for green are still somewhat low. Optimization routes for further improvement in terms of efficacies and lifetimes for red and green are identified. The peformance of blue light emitting polymers has rapidly improved over the last year, but the lifetime is still too short for full color applications. Improvement routes for the blue light emitting polymers and its device structure are outlined.