Boosting EQE to nearly 35% in acridone-based TADF materials via HOMO delocalization and nonradiation suppression

Abstract

As weak donors, carbazole (Cz) derivatives are particularly suitable for blue thermally activated delayed fluorescence (TADF) emitters. However, due to the large singlet–triplet energy gap (ΔE_ST) and excessively weak charge transfer state, these are unfavorable for efficient TADF emitters. Herein, a series of blue and sky-blue TADF materials, tCz-mPAO, BCz-mPAO and BCz-PAO, were developed by incorporating tert-butyl-carbazole (tCz) or 3,9′-bicarbazole (BCz) as a donor and 10-pyridyl-acridone as an acceptor. Focusing on simultaneously ensuring efficient reverse intersystem crossing (RISC) and increasing the photoluminescence quantum yields (PLQYs), two molecular design strategies related to the donor and acceptor units were deliberately employed, respectively. By varying the donor from tCz to BCz, the HOMO distribution was significantly delocalized while maintaining a minimal HOMO–LUMO overlap, resulting in an enhanced k_r and optimal ΔE_ST. Furthermore, strategic elimination of the flexible methyl group on the acceptor effectively suppresses energy dissipation, thereby restricting the non-radiative transition rate constant (k_nr). Combining these strategies, BCz-PAO demonstrated an optimal k_r of 9.3 × 10⁷ s⁻¹ and a high PLQY of 98.2% in doped films. The sky-blue OLED exhibited a remarkable maximum external quantum efficiency of 34.4% without light out-coupling enhancement or a TADF-sensitized structure. These synergistic strategies establish a generalizable design paradigm for high-performance TADF emitters.