Guest concentration, bias current, and temperature-dependent sign inversion of magneto-electroluminescence in thermally activated delayed fluorescence devices

Abstract

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.

Understanding degradation of organic light-emitting diodes from magnetic field effects

“…This was the result of modulating the ratio of singlet and triplet polaron pairs ( 1 PP and 3 PP). Numerous studies regarding the mechanism of magnetic field effects on OLEDs based on fluorescent 11,12 , phosphorescent 13 , exciplex 14 , and TADF emitters 15,16 were investigated to unveil the underlying dynamics of exciton generation, radiation, and annihilation processes. PP 11,17 , triplet-polaron interactions (TPI) 16,18 , and TTA mechanisms 12,16 were mainly used to explain magnetic field effects.…”

Abnormal magnetic field effects in doped organic light-emitting devices

“…TADF材料具有特殊的分子结构, 其分子最高 占据轨道(HOMO)和最低未占据轨道(LUMO)的重叠 较 小 , 从 而 形 成 分 子 内 电 荷 转 移 态 , 并 且 三 重 态 ( 3 CT)和单重态( 1 CT)之间能级差(ΔE ST )较小, 三重态 激子吸收环境热量通过反向系间窜越(reverse intersystem crossing, RISC)转化为单重态激子的过程增 强, 单重态激子退激发光 [18,20~23] , 从而可以较好地提 高器件的发光效率. 因此, TADF材料通常作为客体 应用于主客体掺杂型器件, 并采用三线态能量较高 的材料(如CBP和mCP)作为掺杂主体 [16,24] , 有效束缚 TADF材料中的三线态激子的能量, 从而充分利用 RISC过程将三线态激子转变成单线态激子, 显著增 强器件的效率.…”

“…PEDOT : PSS/m-MTDATA/Alq 3 : x% 4CzTPN-Ph/BCP/ LiF/Al的器件(其中, ITO为铟锡氧化物(indium tin oxide), [16] , 使 得 4CzTPN-Ph…”

“…4CzTPN-Ph中高效率进行 [17] , 器件能够产生更强的 发光(即MEL=ΔEL/EL中分母变大), 这将直接导致测 到的MEL幅值较小(图2(c)). 由于本文主要讨论器件 中MEL和MC幅值大小的差异, 因此对参考器件中 MEL随电流改变出现的正负转变不作讨论, 具体原 因可参考本研究组 [16]…”

“…发光增强 [1] , 表现为在外加较小磁场下快速增加的 MEL; 同时, 由于 1 PP较 3 PP的离子性更强, 1 PP还可以 解离为自由电子空穴, 从而使器件电流增加 [29] , 表 http://engine.scichina.com/doi/10.1360/N972018-00434 材 料 和 客 体 材 料 之 间 存 在 Förster和 Dexter能 量 转 移 [16] . 因此 [31,32] .…”