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.
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Dhoot, A.S., Greenham, N.C. Spin-Dependent Recombination in PPV and Polyfluorene LEDs. MRS Online Proceedings Library 725, 82 (2002). https://doi.org/10.1557/PROC-725-P8.2