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电荷平衡会影响激基复合物有机发光二极管的发光效率, 然而对其背后的物理机制却缺乏充分的理解. 本文利用有机磁效应包括磁电导(magneto-conductance, MC)、磁电致发光(magneto-electroluminescence, MEL)和磁效率(magneto-efficiency, Mη)作为指纹式探测工具来研究电荷平衡影响激基复合物器件发光效率的物理机制. 实验发现, 非平衡器件的MC曲线中快速上升的低场效应(low-field effects, MCL, |B| ≤ 10 mT)和缓慢下降的高场效应(high-field effects, MCH, 10 < |B| ≤ 300 mT)分别归因于被磁场调控的系间窜越(intersystem crossing, ISC)过程和三重态激基复合物与多余电荷之间的三重态-电荷湮灭(triplet-charge annihilation, TCA)过程. 与非平衡器件不同, 平衡器件中快速下降的MCL和快速饱和的MCH分别归因于被磁场调控的反向系间窜越(reverse intersystem crossing, RISC)过程和平衡的载流子注入. 随着注入电流从200 μA减小到25 μA, 非平衡器件中MEL曲线的低场效应(MELL)始终反映被磁场调控的ISC过程, 然而平衡器件的MELL呈现从ISC向RISC过程的转换(ISC → RISC). 另外, 虽然非平衡和平衡器件中Mη曲线的低场效应(MηL)都归因于被磁场调控的ISC过程, 但是平衡器件中MηL的幅值为非平衡器件的~1/4. 这两种器件中不同的MC、MEL和Mη曲线揭示平衡的载流子注入会通过减弱TCA过程来增加三重态激基复合物的数量, 从而增强RISC过程. 因为RISC可以将不能退激辐射的三重态激基复合物转换为能退激辐射的单重态激基复合物, 所以平衡器件的发光效率比非平衡器件的更高. 显然, 本文利用有机磁效应对电荷平衡影响激基复合物器件发光效率这个现象提出了一种新的物理机制.Charge balances can influence the emission efficiency of exciplex-based organic light-emitting diodes (OLEDs), but so far, the physical mechanism behind this phenomenon is not fully understood. Here, organic magnetic field effects (OMFEs) including magneto-conductance (MC), magneto-electroluminescence (MEL), and magneto-efficiency (Mη) are used as fingerprint probing tools to study physical mechanism of influence of charge balanceon the emission efficiency of exciplex-based OLEDs. Specifically, low- and high-field effects of MC traces [MCL (|B| ≤ 10 mT) and MCH (10 < |B| ≤ 300 mT)] from the unbalanced device are separately attributed to the magnetic field (B)-mediated intersystem crossing (ISC) process and the B-mediated triplet-charge annihilation (TCA) process between triplet exciplex states and excessive charge carriers, whereas those from the balanced device are respectively attributed to the B-mediated reverse intersystem crossing (RISC) process and the balanced carrier injection. As the injection current decreases from 200 to 25 μA, low-field effects of MEL traces (MELL) form the unbalanced device always reflect the B-mediated ISC process, but those from the balanced device exhibit a conversion from ISC process to RISC process. Furthermore, although low-field effects of Mη traces (MηL) from unbalanced device and balanced device are attributed to the B-mediated ISC process, MηL value in the balanced device is approximately one-fourth of that in the unbalanced device. These different MC, MEL, and Mη traces reveal that the balanced carrier injection can increase the number of triplet exciplex states via weakening the TCA process, which leads to the enhanced RISC process. Because RISC can convert dark triplet exciplex states into bright singlet exciplex states, the emission efficiency of the balanced device is higher than that of the unbalanced one. Obviously, in this work OMFEs are used to provide a new physical mechanism for charge balance that influences the emission efficiency of exciplex-based OLEDs.
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Keywords:
- organic light-emitting diodes /
- magneto-conductance /
- magneto-electroluminescence /
- magneto-efficiency
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图 1 (a), (b)能级排布图; (c)有机材料的分子结构图; (d)有机半导体薄膜的PL谱和器件的EL谱; (e), (f)电流随电压的变化关系和EL强度随电流的变化关系
Fig. 1. (a), (b) Energy-level diagrams; (c) molecular structures of organic materials; (d) PL spectra of organic semiconductor films and EL spectra of devices; (e), (f) current as a function of voltage and EL intensity as a function of current.
图 2 (a), (b)器件1和器件2中MC曲线的电流依赖关系
Fig. 2. (a), (b) Current-dependent MC traces of devices 1 and 2.
图 5 (a), (b)器件1和器件2中PP态和EX态的形成和演变通道
Fig. 5. (a), (b) Formation and evolution channels of PP and EX states in devices 1 and 2.
图 3 (a), (b) 器件1和器件2中MEL曲线的电流依赖关系
Fig. 3. (a), (b) Current-dependent MEL traces of devices 1 and 2.
图 4 (a), (b)器件1和器件2中Mη曲线的电流依赖关系
Fig. 4. (a), (b) Current-dependent Mη traces of devices 1 and 2.
图 6 器件1、器件2和器件3的电流效率-电压特性曲线
Fig. 6. Current efficiency-voltage characteristics of devices 1, 2, and 3.
图 7 器件3的能级结构、光电特性和有机磁效应 (a)能级结构; (b) EL谱; (c)亮度-电流特性; (d)—(f) MC, MEL和Mη曲线的电流依赖关系
Fig. 7. Energy-level structure, photoelectric properties, and OMFEs of device 3: (a) Energy-level structure; (b) EL spectrum; (c) brightness-current characteristic; (d)–(f) current-dependent MC, MEL, and Mη traces.
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