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. Anisotropy dynamics of light‐emitting dipoles in both time and energy domains are investigated to reveal extended anisotropy dynamics in exciplex:Ir(ppy)2(acac) light‐emitting system. The anisotropy‐enhanced system can take advantage of the high horizontally oriented dipole ratio of exciplex during the energy transfer process, contributing to the light out‐coupling efficiency in organic light‐emitting diodes (OLEDs). Consequently, Ir(ppy)2(acac):exciplex OLED demonstrates a high external quamtum efficiency of 34.01%.