Triplet formation and decay in conjugated polymer devices

We have studied infrared absorptions in working polymer light-emitting diodes due to the presence of polarons and triplet excitons. The polaron population is found to be proportional to the applied voltage, providing direct evidence for space-charge-limited currents. Measurement of triplet absorptions allows us to study the generation and decay rates for triplet excitons. At low temperatures, we find that 83±7% of excitons are formed as singlets rather than triplets, and we show that interaction with polarons is an important decay mechanism for triplet excitons, with a rate constant of 3×10−14 cm3 s−1.