Composites of conjugated polymers with fullerenes are perspective materials for polymer photovoltaics. In the present Chapter, the Light-Induced EPR (LEPR) study of magnetic, relaxation and dynamic parameters of polaron-fullerene radical pairs photoinduced in bulk heterojunction of polymer/fullerene composites is described. The steady-state saturation method allowed to determine separately all magnetic resonance parameters for positively charged polarons and negatively charged fullerene ion-radicals photoinduced in the composites. Weak interaction in such radical pairs is shown to stipulate by a difference interaction of excitons with own microenvironment. Paramagnetic susceptibility of excitons reflects their activation dynamics and dipole-dipole interaction in the polymer/fullerene system. A decay of long-living spin pairs depends on the spatial distance between photoinduced charge carriers and on the energy of exciting photons. The 1D polaron diffusion along the polymer chain and the fullerene rotation near main molecular axis was shown to follow activation Elliot hopping model and to govern by the photon energy. The deviation in activation energies for dynamics of charge carriers and the difference in their dependence on the exciting photon energy prove a non-interacting character of the charge carriers photoinduced in the polymer/fullerene composites. Main magnetic, relaxation and dynamics parameters of excitations are governed by the energy of photons due to different domains inhomogeneously distributed in polymer/fullerene composites.

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