Nickel Oxide Hole Injection/Transport Layers for Efficient Solution-Processed Organic Light-Emitting Diodes

Solution-processed nickel oxides (s-NiOx) are used as hole injection and transport layers in solution-processed organic light-emitting diodes (OLEDs). By increasing the annealing temperature, the nickel acetate precursor fully decomposes and the s-NiOx film shows larger crystalline grain sizes, which lead to better hole injection and transport properties. UV-ozone treatment on the s-NiOx surface is carried out to further modify its surface chemistry, improving the hole injection efficiency. The introduction of more dipolar species of nickel oxyhydroxide (NiO(OH)) is evidenced after the treatment. Dark injection space charge limited (DI-SCL) transient measurement was carried out to compare the hole injection efficiency of s-NiO and poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) hole injection layers (HIL). The UV-ozone treated s-NiOx shows significantly better hole injection, with a high injection efficiency of 0.8. With a p-type thin film transistor (TFT) configuration, the high-temperature annealed s-NiOx film shows a hole mobility of 0.141 cm(2) V-1 s(-1), which is significantly higher compared to conventional organic hole transport layers (HTLs). Because of their improved hole injection and transport properties, the solution-processed phosphorescent green OLEDs with NiOx HIL/HTL show a maximum power efficiency of 75.5 +/- 1.8 lm W-1, which is 74.6 + 2.1% higher than the device with PEDOT:PSS HIL. The device with NiOx HIL/HTL also shows a better shelf stability than the device with PEDOT:PSS HIL. The NiOx HIL/HTL is further compared with PEDOT:PSS HIL/N,N'-Di(1-naphthyl)-N,N'diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) HTL in the thermal-evaporated OLEDs. The device with NiOx HIL/HTL shows a comparable efficiency at high electroluminescence (EL) intensities.