Optical probing of polarons and triplet excitons in conjugated polymer devices

Polarons and triplet excitons in conjugated polymers exhibit sub-gap absorptions which allow them to be identified spectroscopically. We have used quasi-steady-state induced absorption techniques on working polymer light-emitting diodes to study charge carriers and triplet excitons in devices. We identify absorptions due to charges, and at low temperatures can also resolve features due to triplet excitons. From the magnitude of the absorption features, we study the charge and triplet densities as a function of applied voltage. We obtain a value for the triplet generation rate at low temperatures and an estimate of the singlet exciton formation probability. Analysis of the triplet lifetime as a function of charge density reveals the presence of triplet-polaron interactions and we obtain a rate constant for this triplet annihilation process.