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

Solution-processed nickel oxides (s-NiO x ) 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-NiO x film shows larger crystalline grain sizes, which lead to better hole injection and transport properties. UV–ozone treatment on the s-NiO x surface is carried out to further modify its surface chemistry, improving the hole injection efficiency. The introduction of more dipolar species of nickel oxy-hydroxide (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 x and poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) hole injection layers (HIL). The UV–ozone treated s-NiO x 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-NiO x film shows a hole mobility of 0.141 cm2 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 NiO x 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 NiO x HIL/HTL also shows a better shelf stability than the device with PEDOT:PSS HIL. The NiO x 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 NiO x HIL/HTL shows a comparable efficiency at high electroluminescence (EL) intensities.