High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers

Organic light-emitting diodes utilizing thermally activated delayed fluorescence sensitizers and multiple-resonance (MR) dopants may simultaneously offer high efficiencies and narrow-band emissions, but these devices still face intractable challenges with a lack of design rules for high-performance sensitizers. Here, sensitizers with ortho-arranged donor-acceptors on a (trifluoromethyl)benzene linker have been proposed, which not only facilitate relatively small molecular dipole moments but also combine through-bond and through-space charge transfers for fast reverse intersystem crossing (RISC). Their photophysical properties are further modulated by additional electron-deficient moieties to better understand the interplay between sensitizer and MR dopant. The highest maximum external quantum efficiency of 33.1% together with a full width at half maximum of 28 nm were obtained in the device utilizing the sensitizer featuring the fastest RISC though inferior emission spectra overlap (J) with MR-dopant absorption among all sensitizers. It has been shown that, other than a blue-shifted emission to enhance J, an ideal sensitizer should establish efficient photoluminescence and fast RISC to suppress exciton loss and annihilation, as well as a small dipole moment to maintain the narrow-band emission of the MR dopant. These findings pave the way for practical applications of all fluorescent devices with both high efficiency and high color purity.