Spin-Dependent Recombination in PPV and Polyfluorene LEDs
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Spin-Dependent Recombination in PPV and Polyfluorene LEDs Anoop S. Dhoot and Neil C. Greenham Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, United Kingdom ABSTRACT In a polymer light-emitting diode, the fraction of excitons formed as singlets is of crucial importance in determining the quantum efficiency. We have shown that it is possible to measure excited state absorptions due to triplet excitons and polarons in working polymer LEDs, and we are able to quantify the triplet generation rate by measuring the strength of the triplet absorption. Here, we show that by careful study of singlet emission and triplet absorption in an LED based on a poly(p-phenylenevinylene) derivative we can obtain an accurate value of 83±7% for the singlet formation probability, significantly higher than the value of 25% predicted by simple spin statistics. We extend these measurements to devices based on poly(dioctyl-fluorene), where we find similarly high values for the singlet formation probability. In devices using the polyfluorene copolymer F8BT, we find that the triplet absorption is extremely small, consistent with even higher singlet formation probabilities. INTRODUCTION Exciton formation in polymer light-emitting diodes (LEDs) occurs by recombination of electrons and holes injected from opposite electrodes. Excitons may be formed in either the singlet or triplet spin state, and only singlet excitons can usually undergo radiative decay. Formation of triplet states therefore represents an important loss mechanism which limits the efficiency of polymer LEDs. It has typically been assumed that exciton formation occurs on a statistical basis, leading to a singlet formation probability of only 25%. However, recent measurements of LED efficiency have suggested that this limit is broken in conjugated polymers [1,2]. Also, magnetic resonance measurements have demonstrated spin-dependent exciton formation rates [3], and electroluminescence measurements in platinum-containing polymers have measured a singlet formation probability of 57% [4]. In this paper, we aim to study triplet formation in conjugated polymer LEDs using spectroscopic techniques, without the need to introduce phosphorescent materials to observe triplet emission. Although triplet excitons are not strongly coupled to the ground-state, there is an allowed optical transition to a higher-lying triplet state. Measurement of this triplet absorption allows triplet populations to be quantified in a working device, and hence the ratio of singlet and triplet exciton formation can be estimated. We have studied triplet populations in three different polymers: a poly(p-phenylenevinylene) derivative, and two polyfluorene derivatives, all of which are technologically important in polymer displays.
P8.2.1 Downloaded from https://www.cambridge.org/core. La Trobe University, on 20 Jun 2020 at 11:55:26, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-725-P8.2
EXPERIMENT The experimental apparatus is shown
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