Enhancing Reverse Intersystem Crossing via Secondary Acceptors: toward Sky-Blue Fluorescent Diodes with 10-Fold Improved External Quantum Efficiency

How to simply but effectively facilitate reverse intersystem crossing (RISC) transition is always the key issue for developing high-performance thermally activated delayed fluorescence dyes. In this work, as a proof of concept, a feasible strategy named “acceptor enhancement” is demonstrated with a series of ternary blue emitters (xCz_mPO_nTPTZ) using diphenylphosphine oxide (PO) as secondary acceptors. Compared with its PO-free binary analogue, such a simple introduction of PO groups in pCzPO₂TPTZ dramatically enhances its RISC rate constant (k_RISC) by 10 times the level of ∼10⁵ s^–1, accompanied by RISC efficiency (η_RISC) of 92%, which further improves the triplet-to-singlet upconversion for effective triplet harvesting in its devices. As a result, on the basis of a trilayer device structure, pCzPO₂TPTZ realized a state-of-the-art external quantum efficiency beyond 20% with a 10-fold improvement.