Electric-field quenching of optically detected magnetic resonance in a π-conjugated polymer

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.