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Measurement and modeling of carrier transport and exciton formation in blue polymer light emitting diodes

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 required.