Dopant-MediatedActivation of the Hot Exciton Channelfor Efficient and Stable Deep-Blue Triplet–Triplet Fusion OLEDs

Triplet–triplet fusion (TTF)-based fluorescent OLEDs are promising for stable deep-blue emission, yet they exhibit a limited external quantum efficiency (EQE) of up to 12.5%. Although introducing a hot exciton layer further promotes triplet harvesting in TTF-based OLEDs, this configuration does not guarantee full activation of the hot exciton channel. Herein, we reveal that the charge-transport characteristics of the dopant in the TTF emissive layer act as a key factor for hot-exciton-channel activation. By comparing three dopants with distinct hole-transport properties, we demonstrate that integrating a highly hole-transporting dopant with a hole-dominated hot exciton material shifts the exciton recombination zone toward the hot exciton layer, allowing for full activation of the hot exciton channel. The optimized OLED achieves a record-high EQE of 14.8%, with a CIEy value of 0.080, and maintains 14.3% at 1000 cd/m2. Single-carrier devices, transient electroluminescence, and exciton dynamics further verify that dopant-mediated charge transport governs the activation of the hot exciton channel. The resulting device also achieves a 5.3-fold improvement in the operational lifetime. Defect-induced exciton quenching during the TTF process was identified as the main degradation mechanism. This work establishes a general paradigm for developing efficient deep-blue TTF-based OLEDs and provides insights into device physics.