Conductivity Enhancement of Nickel Oxide by Copper Cation Codoping for Hybrid Organic-Inorganic Light-Emitting Diodes

We demonstrate a Cu(I) and Cu(II) codoped nickel(II) oxide (NiO_x) hole injection layer (HIL) for solution-processed hybrid organic-inorganic light-emitting diodes (HyLEDs). Codoped NiO_x films show no degradation on optical properties in the visible range (400-700 nm) but have enhanced electrical properties compared to those of conventional Cu(II)-only doped NiO_x film. Codoped NiO_x film shows an over four times increased vertical current in comparison with that of NiO_x in conductive atomic force microscopy (c-AFM) configuration. Moreover, the hole injection ability of codoped NiO_x is also improved, which has ionization energy of 5.45 eV, 0.14 eV higher than the value of NiO_x film. These improvements are a consequence of surface chemical composition change in NiO_x due to Cu cation codoping. More off-stoichiometric NiO_x formed by codoping includes a large amount of Ni vacancies, which lead to better electrical properties. Density functional theory calculations also show that Cu doped NiO model structure with Ni vacancy contains diverse oxidation states of Ni based on both density of states and partial atomic charge analysis. Finally, HyLEDs of Cu codoped NiO_x HIL have higher performance comparing with those of pristine NiO_x. The current efficiency of devices with NiO_x and codoped NiO_x HIL are 11.2 and 15.4 cd/A, respectively. Therefore, codoped NiO_x is applicable to various optoelectronic devices due to simple sol-gel process and enhanced doping efficiency.