Scharff Tobias, Ratzke Wolfram, Zipfel Jonas, Klemm Philippe, Bange Sebastian, Lupton John M
Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Regensburg, Germany.
Nat Commun. 2021 Apr 6;12(1):2071. doi: 10.1038/s41467-021-22191-3.
At low temperatures and high magnetic fields, electron and hole spins in an organic light-emitting diode become polarized so that recombination preferentially forms molecular triplet excited-state species. For low device currents, magnetoelectroluminescence perfectly follows Boltzmann activation, implying a virtually complete polarization outcome. As the current increases, the magnetoelectroluminescence 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, possibly related to the build-up of triplet excitons and their interaction with free charge carriers. Suppression of the EL alone does not prove electronic spin polarization. We therefore probe changes in the spin statistics of recombination directly in a dual singlet-triplet emitting material, 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, Δg, i.e., a distribution in Larmor frequencies. This Δg effect in the pair, which mixes singlet and triplet, further suppresses singlet-exciton formation at high fields in addition to thermal spin polarization of the individual carriers.
在低温和强磁场条件下，有机发光二极管中的电子和空穴自旋会发生极化，从而使复合优先形成分子三重态激发态物种。对于低器件电流，磁电致发光完全遵循玻尔兹曼激活，这意味着几乎完全的极化结果。随着电流增加，磁电致发光效应会降低，因为器件内载流子停留时间的减少会抑制自旋极化。在这些条件下，出现了一个额外的与场相关的过程，该过程影响自旋相关的复合，可能与三重态激子的积累及其与自由电荷载流子的相互作用有关。仅抑制电致发光并不能证明电子自旋极化。因此，我们在一种双单重态 - 三重态发光材料中直接探测复合自旋统计的变化，该材料在荧光被抑制时磷光强度会随之增加。这些材料中的有限自旋 - 轨道耦合导致电子和空穴的有效g因子出现微观分布，即Δg，也就是拉莫尔频率的分布。这种成对中的Δg效应会混合单重态和三重态，除了单个载流子的热自旋极化外，还会在高场下进一步抑制单重态激子的形成。
