Exciton engineering based on star-shaped blue thermally activated delayed fluorescence emitters for efficient white organic light-emitting diodes

Abstract

White organic light-emitting diodes (OLEDs) are promising as the next-generation lighting sources, for which thermally activated delayed fluorescence (TADF) materials provide alternative features, low cost and environmental sustainability, but require accurate exciton engineering for efficient white emission. Herein, we demonstrate that based on triphenylphosphine oxide (TPPO) acceptor and 9,9-diphenylacridine (DPAC) donors, three blue TADF emitters xDPACPO featuring star-shaped configurations can be used to effectively manage the exciton allocation for blue and yellow emissions. The intramolecular charge transfer (ICT) and steric effects of xDPACPO are in direct proportion to the DPAC number. Therefore, TDPACPO with three DPAC donors achieved the strongest TADF and host characteristics. Besides the highest photoluminescence quantum yield, the suitable first singlet (S₁) and triplet energy levels (T₁) of TDPACPO (∼3.1 eV) support efficient energy transfer to a yellow TADF dopant 4CzTPNBu, and the largest steric hindrances of its three peripheral DPAC groups simultaneously prevent excessive charge and energy transfer. 4CzTPNBu with higher reverse intersystem crossing (RISC) ability remedies the insufficiency of TDPACPO in triplet harvesting. Moreover, the balanced and effective incorporation of TDPACPO in both singlet and triplet harvesting further mitigates exciton accumulation induced quenching, rendering the highest performance among xDPACPO for its single-emissive-layer pure-TADF WOLEDs.