Low-temperature OLED magnetoelectroluminescence: separating excitonic effects from carrier-pair singlet-triplet mixing

Magnetic field effects provide a powerful spectroscopic tool for probing charge and exciton recombination processes in OLEDs. We probe the magneto-electroluminescence (MEL) of two polymer OLEDs, with protonated and deuterated variants of the same polymer MEH-PPV, in a wide temperature range down to 1.5 K, and analyze the MEL line shapes theoretically. A narrow MEL structure is observed around zero magnetic field, between ±3 mT, which is inverted upon deuteration. As the prime effect of deuteration is the reduction of hyperfine coupling, the narrow MEL structure is assigned to the hyperfine-mediated electron-hole spin mixing. At larger fields of around ±50 mT, a broader MEL feature showing a discrete substructure is identified. The broader feature is assigned to the zero-field splitting of the triplet exciton. The resolution of the discrete substructure is enhanced by deuteration. Numerical modelling of the MEL by solving the stochastic Liouville equation in the density-matrix formalism provides excellent agreement with the experimental observations 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 triplet excitations in the amorphous π-conjugated polymer are close to parallel to each other, providing a new insight into the underlying physics of TTA.