Evidence for charge-carrier mediated magnetic-field modulation of electroluminescence in organic light-emitting diodes

Electroluminescence (EL) from organic light-emitting diodes can be surprisingly sensitive $(\sim 20\%)$ to modest magnetic fields $B$ $(0–2\mathrm{T})$ . The origin of this magnetic-field effect has not been clearly identified, although the magnetic-field effect in some devices resembles that of delayed fluorescence in anthracene, which originates from magnetic-field-dependent singlet-exciton production via triplet-triplet annihilation (TTA). Here, we test the role of TTA at low magnetic fields $(\sim 80\mathrm{mT})$ by measuring transient EL and by employing dc drive levels so low that the bimolecular rate of TTA is unimportant. Under these conditions, we find enhancements of EL exceeding 14% at the lowest drives, which excludes TTA-mediated magnetic-field effects and indicates a role for charge-carrier pair states.