Magneto-Electroluminescence (MEL) and Spin-Mixing in OLEDs: Investigating the Role of HFI and SOC

This study investigates the role of hyperfine interaction (HFI) and spin-orbit coupling (SOC) in the spin-mixing process in OLEDs using magneto-electroluminescence (MEL) measurements. The MEL curves show a sharp increase at low external magnetic field (0–50 mT) but remain unchanged at higher external magnetic field (50–300 mT) based on undoped device under different current density. Previous studies have shown that TTA would increase and then decrease at higher external magnetic field. There are two viewpoints proposed in the literature regarding the physical mechanisms behind the spin-mixing process for the HL-RISC channel in OLEDs: HFI and spin-orbit coupling (SOC). Peng et al. proposed that the RISC process from T2 to S2 states were originated from HFI-induced spin mixing and had fingerprint negative MEL responses. Hu et al. also summarized that the spin-mixing process between singlet and triplet states originated from the HFI in organic materials. However, Xu et al. considered that the RISC process from T2 to S1 originates from the SOC effect. For the doped device, MEL curve shows similar trend at low current density compared to hRISC-induced neat D-TTT-H device. However, RISC-determined inverted Lorentzian line-shapes at higher current density. In addition, the MEL trace in the range of 5-20 mA cm-2 can be fitted by a single peak Lorentzian line-shape, indicating the dominance of HL-RISC process. This is consistent with the previous reports on doped devices, where the RISC process is enhanced due to the presence of dopants. Overall, these results suggest that both HFI and SOC play important roles in the spin-mixing process in OLEDs, and their contributions can be modulated by external magnetic field and current density.