Berlin 2012 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 85: Organic Semiconductors: Transistors and OLEDS

HL 85.6: Talk

Thursday, March 29, 2012, 16:30–16:45, EW 203

Investigation of Triplet Exciton Dynamics in Fluorescent Polymer Light-Emitting Diodes — •Bodo Wallikewitz, Simon Gélinas, Dinesh Kabra, and Richard Friend — University of Cambridge, Optoelectronics Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, UK

We report on fluorescent, polymer light-emitting diodes with a high external quantum efficiency of 6.5 %. To understand the high efficiency of these PLEDs, we investigated excited state dynamics on working devices. Emissive and non-emissive excited states were monitored by their characteristic absorption and emission features using transient optical absorption spectroscopy and transient, time-resolved electroluminescence measurements. By fitting a model for triplet decay to the experimental triplet and electroluminescence dynamics, we are able to quantitatively characterize triplet-triplet annihilation as the dominant triplet decay mechanism. Singlet states generated from triplet-triplet annihilation contribute up to 33% of the total amount of singlets generated in these fluorescent devices. To model these results, we require that triplet states can undergo bimolecular annihilation several times. With this model, we show that singlets can reach a maximum fraction of 40% of all excitons generated by charge recombination, without violating spin statistics. Singlet states generated from triplet-triplet annihilation are one important explanation for high external quantum efficiencies found in these fluorescent devices.