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

Nickel oxide (NiO_x) 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 NiO_x NP HIL, we compared the performance of OLEDs with three types of HILs: spin-coated PEDOT:PSS, thermally evaporated HAT-CN, and spin-coated NiO_x NP films. The considerably high component ratio of NiOOH on the air-annealed NiO_x NP film surface results in an enhanced hole injection functionality even without UV-ozone treatment. Consequently, the OLEDs using the NiO_x 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 NiO_x 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.