  We report transient electron paramagnetic resonance (trEPR) measurements with
sub-microsecond time resolution performed on a P3HT:PCBM blend at low
temperature. The trEPR spectrum immediately following photoexcitation reveals
signatures of spin-correlated polaron pairs. The pair partners (positive
polarons in P3HT and negative polarons in PCBM) can be identified by their
characteristic g-values. The fact that the polaron pair states exhibit strong
non-Boltzmann population unambiguously shows that the constituents of each pair
are geminate, i.e. originate from one exciton. We demonstrate that coupled
polaron pairs are present even several microseconds after charge transfer and
suggest that they embody the intermediate charge transfer complexes which form
at the donor/acceptor interface and mediate the conversion from excitons into
free charge carriers.
