Accelerating the Radiative Decay by Regulating the Long-Range and Short-Range Charge Transfer for Efficient Organic Light Emitting Diodes with Relieved Efficiency Roll-Off

Abstract

A precise modulation of charge-transfer characteristics is essential for optimizing the performance of thermally activated delayed fluorescence (TADF) materials. Herein, two novel TADF emitters, FIA-TR and FIA-BO, are rationally designed to explore the interplay between long-range charge transfer (LRCT) and short-range charge transfer (SRCT) in governing exciton dynamics. Both emitters feature a rigid donor-acceptor (D-A) architecture with large torsion angles, enabling high photoluminescence quantum yields (PLQYs > 95%) and short delayed lifetimes. Structural and theoretical analyses confirm well-separated frontier molecular orbitals and distinct CT characteristics, with FIA-TR exhibiting conventional LRCT, while FIA-BO presents a hybridized LRCT-SRCT nature. Photophysical measurements reveal that weakening LRCT via a less electron-donating donor significantly accelerates radiative decay (kr) but concurrently increases non-radiative losses, whereas incorporating an MR-type acceptor in FIA-BO promotes kr while suppressing non-radiative decay. Both emitters demonstrate excellent horizontal dipole orientation (θ‖ > 93%) and achieve high device efficiencies, with maximum external quantum efficiencies (EQEs) of 35.4% (FIA-TR) and 36.0% (FIA-BO), alongside mitigated efficiency roll-off. Importantly, EL performance analyses suggest that enhanced radiative decay may also accelerate triplet up-conversion, offering a new perspective for exciton management. This study provides a comprehensive structure-property correlation and proposes a viable strategy for achieving high-efficiency TADF emitters with suppressed roll-off, guiding future designs of practical organic light-emitting diodes.

Keywords: Organic light emitting diode, thermally activated delayed fluorescence, long-ranged charge transfer, short-ranged charge transfer, spiro-acridine