Low-temperature magnetoelectroluminescence of organic light-emitting diodes: Separating excitonic effects from carrier-pair singlet-triplet mixing

Low-temperature magnetoelectroluminescence (MEL) of organic light-emitting diodes (OLEDs) reveals a near-complete suppression of electroluminescence at strong magnetic fields due to the high degree of thermal spin polarization (TSP) arising when the Zeeman energy exceeds the thermal energy. In addition to TSP, spin mixing within the Coulombically bound carrier pairs can arise, as can interactions between triplet excitons or triplet excitons and charge carriers. These effects also depend on the applied magnetic field strength. We report on the surprisingly nonmonotonic MEL in the intermediate magnetic-field region of up to 230 mT at temperatures down to 1.5 K, and explore the effect of deuteration to distinguish between triplet-excitonic and carrier-pair effects. A narrow MEL feature is observed in the field region of ±3 mT, which is inverted upon deuteration and can therefore be clearly assigned to spin mixing mediated by the hyperfine fields. At larger fields, a broader MEL feature is identified, which shows discrete substructure assigned to the zero-field splitting of the triplet exciton. The resolution of this substructure is enhanced by deuteration. Quantitative modeling of the MEL by solving the stochastic Liouville equation in the density-matrix formalism provides excellent agreement with the experimental results and demonstrates that the triplet excitonic feature arises from delayed fluorescence generated by triplet-triplet annihilation (TTA). The microscopic simulations reveal that TTA occurs preferentially when the axes of the two triplets in the amorphous $\pi$-conjugated polymer are close to parallel to each other, illustrating an alternative spectroscopic approach to investigating the underlying physics of TTA.