School of Physical Science and Technology, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing, 400715, P. R. China.
Sci Rep. 2017 Mar 15;7:44396. doi: 10.1038/srep44396.
Non-emissive triplet excited states in devices that undergo thermally activated delayed fluorescence (TADF) can be up-converted to singlet excited states via reverse intersystem crossing (RISC), which leads to an enhanced electroluminescence efficiency. Exciton-based fluorescence devices always exhibit a positive magneto-electroluminescence (MEL) because intersystem crossing (ISC) can be suppressed effectively by an external magnetic field. Conversely, TADF devices should exhibit a negative MEL because RISC is suppressed by the external magnetic field. Intriguingly, we observed a positive MEL in TADF devices. Moreover, the sign of the MEL was either positive or negative, and depended on experimental conditions, including doping concentration, current density and temperature. The MEL observed from our TADF devices demonstrated that ISC in the host material and RISC in the guest material coexisted. These competing processes were affected by the experimental conditions, which led to the sign change of the MEL. This work gives important insight into the energy transfer processes and the evolution of excited states in TADF devices.
非辐射三重态激发态在经历热激活延迟荧光（TADF）的器件中可以通过反向系间窜越（RISC）上转换为单重态激发态，从而提高电致发光效率。基于激子的荧光器件总是表现出正磁电致发光（MEL），因为外部磁场可以有效地抑制系间窜越（ISC）。相反，TADF 器件应该表现出负的 MEL，因为 RISC 被外部磁场抑制。有趣的是，我们在 TADF 器件中观察到正的 MEL。此外，MEL 的符号是正的还是负的，取决于实验条件，包括掺杂浓度、电流密度和温度。我们的 TADF 器件观察到的 MEL 表明，主体材料中的 ISC 和客体材料中的 RISC 共存。这些竞争过程受实验条件的影响，导致 MEL 符号的变化。这项工作为 TADF 器件中的能量转移过程和激发态的演化提供了重要的见解。
