Effects of unipolar and bipolar charges on the evolution of triplet excitons in π-conjugated PLED

Abstract

Understanding and modulating the spin-pair correlation of conjugated polymer materials in π-conjugated polymer light-emitting devices (PLEDs) plays a crucial role in the development of their applications. We investigated the relationships between the internal hyperfine interactions (HFIs), triplet–triplet annihilation (TTA), and triplet–charge annihilation (TCA) spin evolution processes in π-conjugated PLED. Research has shown that in a unipolar π-conjugated PLED, the “M” shaped ultra-small magnetic field effect is due to the HFI between hole polarons and the spin-mixing process of charge carriers. Under high magnetic fields (15 mT<|B|<350 mT), the TCA process dominates the negative magneto-conductance (MC), and its intensity and sign are not controlled by temperature. In polar π-conjugated PLED, excess carriers can provide conditions for the generation of TCA. Moreover, π-conjugated polymers (emission layers) have relatively strong electron–phonon coupling, which can capture triplet excitons into adjacent organic layers to obtain higher triplet exciton concentrations, resulting in the TTA process. Under low-temperature conditions, excess carriers induce the magneto-electroluminescence (MEL) effect to undergo TTA and form P-type delayed fluorescence. This study can provide a new mechanism explanation for the HFI in PLED devices and a new approach for the effective utilization of triplets.