  At low temperatures and high magnetic fields, electron and hole spins in an
organic light-emitting diode (OLED) become polarized so that recombination
preferentially forms molecular triplet excited-state species. For low device
currents, magnetoelectroluminescence (MEL) perfectly follows Boltzmann
activation, implying a virtually complete polarization outcome. As the current
increases, the MEL effect is reduced because spin polarization is suppressed by
the reduction in carrier residence time within the device. Under these
conditions, an additional field-dependent process affecting the spin-dependent
recombination emerges, which appears to relate to the build-up of triplet
excitons and the interaction with free charge carriers. Suppression of the EL
at high fields on its own does not, strictly, prove electronic spin
polarization. We therefore probe changes in the spin statistics of
recombination directly in a dual singlettriplet emitting OLED, which shows a
concomitant rise in phosphorescence intensity as fluorescence is suppressed.
Finite spin-orbit coupling in these materials gives rise to a microscopic
distribution in effective g factors of electrons and holes, $\Delta g$, i.e. a
distribution in Larmor frequencies, leading to singlet-triplet mixing within
the electronhole pair as a function of applied field. This $\Delta g$ effect in
the pair further suppresses singlet-exciton formation in addition to thermal
spin polarization of the individual carriers. Since the $\Delta g$ process
involves weakly bound carrier pairs rather than free electrons and holes, in
contrast to thermal spin polarization, the effect does not depend significantly
on current or temperature.
