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

Abstract

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.