Sub‐Second Long Lifetime Triplet Exciton Reservoir as Assistant Host for Highly Efficient and Stable Organic Light‐Emitting Diode

Abstract In organic light‐emitting diodes (OLEDs), the confinement of triplet excitons is essential for achieving efficient and stable devices. Recently, an electron‐transporting material (ETM) with sub‐second triplet lifetime is reported that can effectively achieve triplet exciton confinement, even with a lower triplet energy (E T ) of 0.32 eV than that of the phosphorescent emitter, which is named the long lifetime triplet exciton reservoir (LTER) effect. Due to the challenge that confining triplet excitons in the emitting layer (EML) typically requires host materials with higher energy level, which leads to accelerated degradation, the possibility of LTER effect in the EML is further explored. The results show that directly using LTER molecule as the host only leads to severe quenching. However, when doped at low‐concentration (e.g., 1 wt.%) as assistant host in the carrier recombination zone (RZ), device performance is improved unexpectedly by the LTER effect. Besides, the RZ of carriers is shifted and expanded within the EML, contributing to improved carrier balance due to its intrinsic electron transport properties. As a result, an increase in device external quantum efficiency (EQE) to 24.5% is achieved, along with a 1.5‐fold increase in device lifetime.