The fraction of excitons formed as singlets governs the efficiency of a polymer light-emitting diode (LED) (see Figure). By measuring absorptions due to triplet excitons in working poly(9,9-dioctylfluorene) devices, the singlet formation probability is calculated. A lower limit of 58 % is found, significantly larger than that predicted statistically. In devices based on the copolymer F8BT, no triplet exciton absorption can be detected.
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