Energy Gain and Loss Processes in OLEDs with Spacer-Separated Electron-Hole Pairs Investigated by Magnetic Field Effects

Interfacial or diluted exciplex organic light emitting diodes (OLEDs) with spacer-separated electron-hole pairs have emerged as a promising approach for precise tuning the energy level of charge-transfer (CT) excitons and the improvement in device performance. However, their microscopic energy gain and loss mechanisms are not well understood. In this study, magnetic field effects (MFEs) including magneto-electroluminescence (MEL), magneto-conductance (MC), and magneto-efficiency (M eta) in two series of donor-spacer-acceptor type exciplex-based OLEDs, which exhibited opposite distance-dependent device performances, are studied to explore the microscopic dynamics of spacer-separated electron-hole pairs. Intersystem crossing (ISC) between CT excitons and between polaron-pairs, reverse intersystem crossing (RISC) between CT excitons, triplet-triplet annihilation (TTA), scattering and dissociation channels of triplet-charge annihilation (TQA), and additional Dexter energy transfer (DET) processes are identified. The decreased external quantum efficiency (EQE) and MEL with increasing thickness of space layer in Configuration II devices are attributed to an activation of the potential DET process from the lowest triplet of spacer-separated CT excitons to the lowest triplet of Bphen caused by their smaller energy gaps. The study offers in-depth insights into the exciton utilization and energy loss, which may allow better design and optimization of space-modulated OLEDs.