Dynamic spin polarization in organic semiconductors with intermolecular exchange interaction

It is shown that in organic semiconductors where organic magnetoresistance (OMAR) is observed, the exchange interaction between electrons and holes localized at different molecules leads to dynamic spin polarization in the direction of the applied magnetic field. The polarization appears even at room temperature due to the nonequilibrium conditions. The strong spin polarization requires the exchange energy to be comparable to the Zeeman energy in the external field and to be larger than or comparable to the energy of hyperfine interaction of electron and nuclear spins. The exchange interaction also modifies the shape of OMAR.