Balancing charge transport and C–N bond strength in stability-oriented host design for blue TADF-OLEDs

Abstract

Achieving stable and efficient blue thermally activated delayed fluorescence (TADF) OLEDs remains a critical challenge due to high-energy excitons and polarons that induce bond cleavage in host materials, limiting operational lifetimes. Here, a series of carbazole–biphenyl hosts is developed with tuned charge transport from ambipolar to strongly electron-transporting, while maintaining relatively high triplet energies (2.77–2.85 eV in neat films), enabling the systematic probing of charge balance and degradation mechanisms in blue TADF-OLEDs. Incorporating blue TADF emitters of different triplet energies (2.79 eV and 2.62 eV) at optimized doping levels (7–40 wt%), the devices exhibit maximum external quantum efficiencies up to 18% with low efficiency roll-off for the most suitable host-dopant combinations. Operational stability assessments at 1000 cd m⁻² reveal that, within the same device architecture and fabrication protocol, OLEDs using the N-phenylated host consistently outperform the non-phenylated analogues by 1.6–12 times in terms of LT₅₀ lifetimes. Density functional theory calculations link this enhanced stability to the higher anionic-state bond dissociation energy (BDE) of the weakest exocyclic C–N bond (2.22 eV vs. 0.73–0.75 eV in the non-phenylated hosts), achieved by shifting LUMO density away from the carbazole core via N-phenyl substitution. These findings establish anionic-state C–N bond strength as a key molecular parameter for robust high-triplet-energy hosts, providing chemically grounded design guidelines to mitigate polaron-induced degradation while retaining efficiency under the studied device conditions.