Performance improvement of blue TADF top-emission OLEDs by tuning hole injection barriers using a nickel-doped silicon dioxide buffer layer

The severe exciton self-quenching and twisted structure of typical thermally activated delayed fluorescence (TADF) emitters result in a low external quantum efficiency (EQE) and broad emission spectrum, especially in blue organic light-emitting diodes (OLEDs). These challenges have been overcome by employing complex device fabrication steps and complex molecular structures requiring fine stoichiometric adjustments. However, only a few attempts have been made to improve the performance of OLEDs through device structure engineering, particularly in top-emission OLEDs (TEOLEDs). Herein, we report blue TADF TEOLEDs fabricated by employing a second-order microcavity structure. These TEOLEDs simultaneously exhibit a high EQE (∼20.2 %) and narrow full width at half maximum (29 nm) owing to the incorporation of a Ni-doped SiO₂ buffer layer that balances the injection charges. The wide bandgap of SiO₂ prevents rapid hole injection from the anode and modifies the anode–organic layer interface, while tuning of the Ni doping concentration reduces the turn-on voltage of the device through co-sputtering. The charge balance mechanism is elucidated in detail by analyzing tunneling effects across the buffer layer and the bonding states of Ni atoms in the SiO₂ film. This study can promote the development of TADF top-emission devices with high efficiency and high color purity.