Research on physical mechanism of influence of charge balances on emission efficiency of exciplex-based OLEDs by using organic magnetic field effects

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 (M<i>η</i>) are used as fingerprint probing tools to study physical mechanism of influence of charge balance on the emission efficiency of exciplex-based OLEDs. Specifically, low- and high-field effects of MC traces [MC<sub>L</sub> (|<i>B</i>| ≤ 10 mT) and MC<sub>H</sub> (10 < |<i>B</i>| ≤ 300 mT)] from the unbalanced device are separately attributed to the magnetic field (<i>B</i>)-mediated intersystem crossing (ISC) process and the <i>B</i>-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 <i>B</i>-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 (MEL<sub>L</sub>) form the unbalanced device always reflect the <i>B</i>-mediated ISC process, but those from the balanced device exhibit a conversion from ISC process to RISC process. Furthermore, although low-field effects of M<i>η</i> traces (M<i>η</i><sub>L</sub>) from unbalanced device and balanced device are attributed to the <i>B</i>-mediated ISC process, M<i>η</i><sub>L</sub> value in the balanced device is approximately one-fourth of that in the unbalanced device. These different MC, MEL, and M<i>η</i> 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.