Light-induced EPR spectroscopy of charge carriers photoinduced in polymer/fullerene bulk heterojunctions

Abstract

Composites of conjugated polymers with fullerenes are perspective materials for polymer photovoltaics. Light-induced EPR (LEPR) study of magnetic, relaxation, and dynamic parameters of polaron-fullerene radical pairs photoinduced in fullerene-modified poly(3-alkylthiophenes) is described. Weak interaction between positively charged polaron and negatively charged fullerene ion radical in the pairs allowed to determine separately all their magnetic, relaxation, and dynamics parameters. Paramagnetic susceptibility of charge carriers reflects their activation dynamics and spin exchange in the composite. Decay of long-living polaron and fullerene anion radical depends on a spatial distance between them and the energy of exciting photons. One-dimensional polaron diffusion along the polymer chain and fullerene rotation near its own main molecular axis were shown to follow the activation Elliot hopping model and to be governed by photon energy. The deviation in activation energies for dynamics of charge carriers and the difference in their dependence on the exciting photon energy prove the noninteracting character of charge carriers photoinduced in the polymer/fullerene composite. The dependence of the main magnetic, relaxation, and dynamics parameters of charge carriers on the phonon energy was ascribed to inhomogeneous distribution of polymer and fullerene domains in bulk heterojunctions. This inhomogeneity decreases by the annealing of the composite.