The effects of UV-ozone treated ultra-thin Li2CO3-doped NiO film as the anode buffer layer on the electrical characteristics of organic light-emitting diodes

In this study, the efficiency of organic light-emitting diodes (OLEDs) was enhanced by depositing a Li2CO3-doped NiO (LNO) film as a buffer layer between the indium tin oxide (ITO) electrode and α-naphthylphenylbiphenyl diamine (NPB) hole transport layer, with the structure configuration ITO/NiO:Li2CO3 (X wt%, Y nm) (X = 0, 5 and 10 wt%; Y = 0.5, 1, and 1.5 nm)/NPB (40 nm)/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/lithium fluoride (LiF) (1 nm)/Al (150 nm). The enhancement mechanism was systematically investigated the X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy results revealed the formation of the UV-ozone treated Li2CO3-doped NiO film. The work function increased from 4.8 eV (standard ITO electrode) to 5.45 eV (1-nm-thick UV-ozone treated Li2CO3-doped NiO film deposited on the ITO electrode with 5 wt%). The surface roughness of the UV-ozone treated Li2CO3-doped NiO film was smoother than that of the standard ITO electrode. Further, the UV-ozone treated Li2CO3-doped NiO film increased both the surface energy and polarity as determined from contact angle measurement and admittance spectroscopy measurement showed that increased capacitance and conductance of the OLEDs. In addition, the carrier injection increased in the space-charge region when the UV-ozone treated Li2CO3-doped NiO buffer layer was inserted. Moreover, the turn-on voltage decreased from 4.2 V to 3.5 V at 1 mA/cm2, the luminance increased from 7588 cd/m2 to 23240 cd/m2, and the current efficiency increased from 3.2 cd/A to 3.4 cd/A when the 1-nm-thick UV-ozone treated Li2CO3-doped NiO film with 5 wt% was inserted of the OLEDs.