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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 (M
h ) 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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