Extending Anisotropy Dynamics of Light-Emitting Dipoles as Necessary Condition Toward Developing Highly-Efficient OLEDs

Designing in-plane-oriented light-emitting dipoles is known as a critical method to develop high-efficiency organic light-emitting diodes (OLEDs) by enhancing light extraction. However, in-plane-oriented light-emitting dipoles must demonstrate sufficient polarization memory extended into light emission lifetime window, generating extended anisotropy dynamics shown as the necessary condition to increase light extraction toward developing high-efficiency OLEDs. This paper reports experimental studies on anisotropy dynamics of light-emitting dipoles in both time and energy domains by using time-resolved and steady-state photoluminescence anisotropy measurements based on the in-plane oriented exciplex-heterostructured [BCzPh:CN-T2T] host dispersed with phosphorescent molecules. It is found that, when host–guest Coulomb scattering is suppressed by parallel placing of the in-plane-configured phosphorescent Ir(ppy)2(acac) molecules into the in-plane-oriented exciplex-heterostructured [BCzPh:CN-T2T] host, the anisotropy dynamics of light-emitting dipoles can be extended into microseconds time window comparable with its phosphorescence lifetime, satisfying the necessary condition in time domain to increase light out-coupling efficiency toward developing high external quantum efficiencies (EQEs) in Ir(ppy)2(acac):exciplex system. More importantly, by suppressing host–guest Coulomb scattering, the high-energy transition dipoles can still maintain extended anisotropy dynamics in the energy domain in Ir(ppy)2(acac):exciplex system while hot electrons are relaxing toward lowest unoccupied molecular orbital (LUMO). Consequently, the extended anisotropy dynamics of light-emitting dipoles demonstrate a high EQE of 34.01% in the Ir(ppy)2(acac):exciplex OLED.