Spin-dependent dynamics of polaron pairs in organic semiconductors

We present a theoretical investigation of the effect of spin manipulation of polaron pairs (PPs) on the conductivity of organic semiconductors. Control of the PP spin state is achieved using pulsed electron-spin resonance. We demonstrate that manipulation of PPs will result in changes in the free-polaron density in the material, with corresponding changes in the conductivity due to the contribution of PP dissociation to the free-carrier density. The time-dependent form of this conductivity change following spin resonant perturbation is determined, and the effect of a number of experimental variables investigated. We find that, under certain conditions, these time-dependent current measurements reveal the dynamics of PP intersystem crossing. We compare these predictions with previous experiments on organic light-emitting diodes made of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] and conclude that PP intersystem crossing times tau(isc) in this material may exceed 10 mu s at low temperatures.