  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.
