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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,然而背后的物理机制缺乏充分的理解.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,M
h )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 mA减小到25 mA,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M h 曲线的低场效应(Mh L)都归因于被磁场调控的ISC过程,但是平衡器件中Mh L的幅值比非平衡器件的低~4倍.这两种器件中不同的MC、MEL和Mh 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 will influence the emission efficiency of exciplex-based organic light-emitting diodes (OLEDs), but physical mechanisms behind this phenomenon lack full understandings. Here, organic magnetic field effects (OMFEs) including magneto-conductance (MC), magneto-electroluminescence (MEL), and magneto-efficiency (Mh ) are used as fingerprint probing tools to study physical mechanisms of charge balances affecting 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 mA, 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 to RISC processes. Furthermore, although low-field effects of M h traces (Mh L) from both unbalanced and balanced devices are attributed to the B-mediated ISC process, Mh L values are ~4 times lower in the balanced device than the unbalanced one. These different MC, MEL, and Mh 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 upconvert dark triplet exciplex states to bright singlet exciplex states, the emission efficiency of the balanced device is higher than that of the unbalanced one. Obviously, this paper uses OMFEs to provide a new physical mechanism of charge balances affecting the emission efficiency of exciplex-based OLEDs.-
Keywords:
- organic light-emitting diodes /
- magneto-conductance /
- magneto-electroluminescence /
- magneto-efficiency
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