  Electric fields are central to the operation of optoelectronic devices based
on conjugated polymers since they drive the recombination of electrons and
holes to excitons in organic light-emitting diodes but are also responsible for
the dissociation of excitons in solar cells. One way to track the microscopic
effect of electric fields on charge carriers formed under illumination of a
polymer film is to exploit the fluorescence arising from delayed recombination
of carrier pairs, a process which is fundamentally spin dependent. Such
spin-dependent recombination can be probed directly in fluorescence, by
optically detected magnetic resonance (ODMR). Depending on the relative
orientation, an electric field may either dissociate or stabilize an
electron-hole carrier pair. We find that the ODMR signal in a polymer film is
quenched in an electric field, but that, at fields exceeding 1 MV/cm, this
quenching saturates. This finding contrasts the complete ODMR suppression that
was previously observed in polymeric photodiodes, indicating that
exciton-charge interactions---analogous to Auger recombination in crystalline
semiconductors---may constitute the dominant carrier-pair dissociation process
in organic electronics.
