External quantum efficiency of exciplex-based OLEDs: Emission mechanisms revealed by magnetic field effects

Exciplex-based organic light-emitting diodes (EX-OLEDs) include interfacial- and bulk-heterojunction devices, and the external quantum efficiency (EQE) of the bulk device is typically higher than that of the interfacial one when fabricated with the same donor and acceptor materials. Although photoluminescence (PL) and transient PL were used to find that this higher EQE is attributed to the enlarged electron-hole recombination zone, its further reasons are still vague. Here, organic magnetic field $(B)$ effects (OMFEs), including magneto-electroluminescence (MEL), magneto-conductance (MC), and magneto-efficiency ($\mathrm{M}\ensuremath{\eta}$), are used to find new reasons for the improved EQE. Specifically, low-field effects ($|B|\ensuremath{\le}10\text{ }\mathrm{mT}$) of both MEL and $\mathrm{M}\ensuremath{\eta}$ traces from the interfacial and bulk devices, respectively, show the $B$-mediated intersystem crossing (ISC) and reverse ISC (RISC) features. Furthermore, although both low-field effects of their MC traces exhibit the $B$-mediated ISC feature, the ISC magnitude of the bulk device is lower than that of the interfacial one under the same injection currents. That is, the low-field effects of their OMFE traces reflect that the RISC process from triplet to singlet EX states (${\text{EX}}_{3}\ensuremath{\rightarrow}{\text{EX}}_{1}$) in the bulk device is stronger than that in the interfacial one since these low-field effects are determined by the superposition of the $B$-mediated ISC and RISC processes. Moreover, all high-field effects ($10<|B|\ensuremath{\le}300\text{ }\mathrm{mT}$) of their OMFE traces show the $B$-mediated triplet-charge annihilation (TCA) between ${\text{EX}}_{3}$ and free charges (${\text{EX}}_{3}+q\ensuremath{\rightarrow}{\text{S}}_{0}+{q}^{\ensuremath{'}}$), but the TCA magnitudes of the bulk device are lower than those of the interfacial one under the same injection currents, indicating a weaker TCA process in the bulk device than that in the interfacial one. This is because the enlarged electron-hole recombination zone increases the quantity of the intermolecular contacts between donor and acceptor materials, causing a higher recombination efficiency in the bulk device than that in the interfacial one. The improved recombination efficiency will increase the quantity of ${\text{EX}}_{3}$ and reduce that of free charges, which results in the enhanced RISC and weakened TCA, respectively. The weakened TCA still increases the quantity of ${\text{EX}}_{3}$ and then further enhances RISC, leading to the elevated singlet/triplet ratio and the improved device EQE. Therefore, this work not only uses OMFEs to reveal novel physical mechanisms of the higher EQE from the bulk device than that from the interfacial one, but also paves the way for further developing high-efficiency bulk-heterojunction EX-OLEDs.