Magnetic-field dependence of the electroluminescence of organic light-emitting diodes: a competition between exciton formation and spin mixing

We explore the magnetoelectroluminescence (MEL) of organic light-emitting diodes by evaluating the magnetic-field dependent fraction of singlet excitons formed. We use two- and multisite polaron-hopping models with spin mixing by hyperfine fields and different singlet and triplet exciton formation rates kS and kT. A huge MEL is predicted when exciton formation is in competition with spin mixing and when kT is significantly larger than kS. This competition also leads to a low-field structure in the MEL that is in agreement with recent experiments.