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

Abstract

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 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 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 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 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.

Passivation of Metal Oxide Surfaces for High-Performance Organic and Hybrid Optoelectronic Devices

“…1 Nickel acetate tetrahydrate (Ni(CH 3 COO) 2 · 4H 2 O) was dissolved in ethanol with monoethanolamine (NH 2 CH 2 CH 2 OH) (0.1−0.2 mol L −1 ) at a mole ratio of 1:1. The VO x solution precursor was synthesized by mixing vanadium oxytriisopropoxide (VO(O(CH 3 ) 2 ) 3 ) with isopropanol solvent at a volume ratio of 1:50.…”

“…15,41 The presence of the dipolar NiOOH species on NiO x surface is evidenced from our previous work. 1 The strong NiOOH dipoles could facilitate the nonradiative decay leading to a high exciton quenching rate at long distances. 42 …”

“…The energy band diagram is shown in Figure 2a. 1,43,44 The current density−voltage (J−V) curves of all five devices are shown in Figure 2b, the corresponding luminescence-voltage (L−V) and current efficiency curves are shown in parts c and d of Figure 2, respectively. The electroluminescence (EL) spectra of the OLEDs with NiO x HTLs are given in the Supporting Information.…”

Investigation of charge transport between nickel oxide nanoparticles and CdSe/ZnS alloyed nanocrystals

“…The crystallite size, as calculated from Scherrer's equation, is 5 -6 nm indicating that the formed NiO is nanocrystalline. NiO thin films and nanoparticles reported so far are nonstoichiometric with nickel vacancies [9,10]. Presence of these defects are identified by the shift in the diffraction peaks, specifically the (200) and (220) planes, when compared to stoichiometric NiO or bunsenite [9].…”

“…The peak centered at 854.2 eV binding energy corresponds to the Ni 2+ oxidation state of Ni from stoichiometric NiO. The higher binding energy peak at 856.0 eV corresponds to the Ni 3+ oxidation state, which is a result of non-stoichiometric Ni 2 O 3 or NiOOH formed due to nickel vacancies [9,10]. Finally, the peak at 861 eV is due to the shake up process [9].…”