Work function-tunable ZnO/Ag/ZnO film as an effective hole injection electrode prepared via nickel doping for thermally activated delayed fluorescence-based flexible blue organic light-emitting diodes

Metal-doped ZnO, as an alternative to indium tin oxide, has been extensively investigated; however, its widespread use has been limited because of its low conductivity and work function. In this study, ZnO was Ni-doped via co-sputtering to achieve work function tuning and improve its conductivity. The Ni-doped ZnO could be used as an effective hole injection electrode for organic light-emitting diodes (OLEDs). Herein, we prepared a Ni-doped ZnO/Ag/ZnO (Ni:ZAZ) electrode for thermally activated delayed fluorescence-based flexible blue OLEDs. The Ni:ZAZ electrode exhibited outstanding performance with a surface resistivity of 6.33 Ω/sq, high transmittance of 93.3% at 484 nm, and work function of up to 0.6 eV, along with a smooth surface and excellent mechanical durability. In addition, the Ni:ZAZ-based OLEDs exhibited excellent performance with a turn-on voltage of 3.0 V and a maximum current efficiency of 17.2 cd/A owing to its improved hole injection property. Furthermore, on flexible substrates, these OLEDs exhibited improved mechanical stability even after being subjected to 2000 bending cycles at a radius of curvature of 6 mm. Thus, the synthesis approach proposed in this study will pave a new pathway for the development of transparent electrodes for flexible and wearable opto-electronic devices.