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We report a study of the triplet exciton dynamics and their effect on the performance of fluorescent organic light-emitting diodes. These polymer light-emitting diodes comprise metal oxide, injection electrodes, and poly(9,9'-dioctylfluorene-co-benzothiadiazole) as the emissive material and exhibit external quantum efficiencies up to 6.5%. Transient optical absorption measurements following a short (0.5 to 50 µs) electrical drive pulse were used to monitor triplet dynamics during device operation. Triplet generation and decay processes were modeled, and we find that triplet-triplet annihilation is the dominant triplet decay mechanism. Singlet states, generated from triplet-triplet annihilation were monitored as delayed electroluminescence after the end of the drive pulse. From the delayed electroluminescence dynamics, we determine monomolecular as well as bimolecular triplet decay rates and estimate the triplet-charge annihilation rate. Singlet states generated from bimolecular 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. [ABSTRACT FROM AUTHOR]