Magneto‐Electroluminescence Studies on the Role of Intermolecular Spin–Orbital Coupling Processes for the Transition between Singlet and Triplet Excitons in Exciplex‐Based Phosphorescent Organic Light‐Emitting Diodes

Abstract Phosphorescent organic light‐emitting diodes (PhOLEDs) using exciplexes as host have achieved unprecedented breakthrough. The energy transfer process between host and guest is the popular way to effectively utilize excitons in this kind of devices. Actually, the intermolecular spin–orbital coupling (ISOC) interaction between the heavy‐atom phosphorescent molecule and exciplex also plays an important role in exciton utilization, thus device performance, but this role is always ignored. Here, the synergistic effects of energy transfer and ISOC interaction on device efficiency are clearly demonstrated in exciplex‐based PhOLEDs by magneto‐electroluminescence (MEL) measurements. It can be seen that the MEL line shape at high field component is significantly wider due to the ISOC effect inducing the accelerated transition process between singlet and triplet states of exciplex host. In this case, the energy transfer process from exciplex host to phosphorescent molecule is more efficient, which is confirmed by the different line widths of MELs. Ultimately, a high‐efficiency exciplex‐based green PhOLED with the maximum external quantum efficiency of 23% is obtained at a low phosphorescent doping concentration of 2 wt%.