Solution-processed nickel oxide nanoparticles with NiOOH for hole injection layers of high-efficiency organic light-emitting diodes

Abstract

Nickel oxide (NiOx) nanoparticles (NPs) were synthesized by a solution-based method and NP films were used as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). To evaluate the hole injection functionality of the NiOx NP HIL, we compared the performance of OLEDs with three types of HILs: spin-coated PEDOT:PSS, thermally evaporated HAT-CN, and spin-coated NiOx NP films. The considerably high component ratio of NiOOH on the air-annealed NiOx NP film surface results in an enhanced hole injection functionality even without UV-ozone treatment. Consequently, the OLEDs using the NiOx NP HILs show significantly higher performances than those of the OLEDs using PEDOT: PSS along with a more than doubled lifetime. Moreover, the OLEDs using the NiOx NP layers show higher external quantum efficiency (EQE), and current and power efficiency values than those of the OLEDs using HAT-CN at a high luminance level. Most notably, the device shows considerably higher current and power efficiency values than those of the recently reported state-of-the-art OLEDs using other types of metal-oxide or metal-based HILs.