We report a new experimental method to measure the localization length of photo-generated carriers in an organic donor-acceptor photovoltaic blend by comparing their dielectric and electron spin-resonance susceptibilities which are simultaneously measured by monitoring the resonance frequency of a superconducting resonator. We show that at cryogenic temperatures excitons are dissociated into long lived states, but that these are confined within a separation of around $4;{rm nm}$. We determine the Debye and recombination times, showing the coexistence of a fast electrical response corresponding to delocalized motion, with glass-like recombination kinetics.
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