Spin-dependent kinetics of polaron pairs in organic light-emitting diodes studied by electroluminescence detected magnetic resonance dynamics

We describe a method for characterizing the spin-dependent kinetics of polaron pairs (PP) in polymer organic light-emitting diodes (OLEDs) made from a derivative of poly(phenylene-vinylene), using the dynamic response of spin- 1/2 electroluminescence detected magnetic resonance (ELDMR) compared with the response of the current-detected magnetic resonance (CDMR). We found that at 10 K the in-phase ELDMR and CDMR responses are positive at low microwave modulation frequency f, but both change sign at a frequency f0 that depends on the microwave power, current density, and device architecture. The similarity between ELDMR and CDMR response dynamics shows that the two phenomena share a common origin. We identify the underlying ELDMR mechanism as due to current-density increase under resonance conditions that is caused by enhanced PP effective recombination in the device, in agreement with a recently proposed model for explaining the magnetoconductivity in OLEDs. Our data are in disagreement with previous models for ELDMR such as polaron-electroluminescence quenching and triplet-polaron interaction. From a model fit to the data that involves both spin singlet and triplet PP dynamics, we obtained their effective recombination and spin-lattice relaxation rates. We found that the spin-lattice relaxation rate in the active layer increases with the current density in the device, showing the importance of spin-spin interaction in OLEDs.