Magneto-Electroluminescence (MEL) Study of Exciton Processes in Organic Light-Emitting Diodes (OLEDs)

Meanwhile, the magneto-electroluminescence (MEL) technique was employed to elucidate the evolution of the exciton process. As depicted in Figure 6a, the MEL curves exhibit a sharp increase at low external magnetic fields (0-50 mT), but remain unchanged at higher external magnetic fields (50-300 mT) in the undoped device under different current densities. Previous studies have reported that the triplet-triplet annihilation (TTA) process increases and then decreases at higher external magnetic fields. \n\nRegarding the physical mechanisms underlying the spin-mixing process for the hole-limited reverse intersystem crossing (HL-RISC) channel in OLEDs, two viewpoints have been proposed in the literature: hyperfine interaction (HFI) and spin-orbit coupling (SOC). Several years ago, Peng et al. proposed that the RISC process (referred to as HL-RISC) from the T2 to S2 states originated from HFI-induced spin mixing, based on their investigation of TPA-NZP-based doped and non-doped devices. They also observed characteristic negative MEL responses. Similarly, in 2015, Hu et al. summarized that the spin-mixing process between singlet and triplet states originated from HFI in organic materials. However, Xu et al., in their study of the PAC emitter containing π-π* and n-π* transition characters, considered that the RISC process from T2 to S1 originates from the SOC effect. \n\nIn the doped device, the MEL curve exhibits a similar trend at low current density compared to the neat D-TTT-H device induced by HL-RISC. However, at higher current density, the MEL curve is determined by the RISC process and displays inverted Lorentzian line shapes. Additionally, the MEL trace falls within the range of 5-20 mA cm-2.