Host to Guest Energy Transfer Mechanism in Phosphorescent and Fluorescent Organic Light-Emitting Devices Utilizing Exciplex-Forming Hosts

The use of exciplex-forming hosts has recently emerged as an avenue to obtain very high efficiency with phosphorescent dopants in organic light-emitting devices (OLEDs). The exact electroluminescence (EL) mechanism, however, is still not clearly understood. In this work, we use time-resolved photoluminescence measurements and find that the EL mechanism is primarily based on efficient energy transfer from the exciplex to the emitter guest. By altering the distance between the guest and the exciplex-formation interface, we further uncover that this energy transfer occurs mostly by the Dexter mechanism. The results explain the high efficiency of phosphorescent OLEDs based on an exciplex-forming host and shed light on the reasons behind the significant difference in the efficiency of OLEDs utilizing exciplex-forming hosts when used with phosphorescent versus fluorescent emitters.