The insert of zinc oxide thin film in indium tin oxide anode for organic electroluminescence devices

Abstract

Zinc oxide films were prepared by rf magnetron sputtering on glass substrates with designed ZnO target using high-purity of zinc oxide (99.99%) powder. Systematic study on dependence of target-to-substrate distance (Dts) on structural, electrical and optical properties of the as-grown ZnO films was mainly investigated in this work. XRD showed that highly preferred ZnO crystal in the [0 0 1] direction was grown in parallel to the substrate, while the Dts did not effect to the peak position of XRD. With decreasing Dts, the growth rate is increased while the electrical resistivity as well as crystal size in the ZnO films was decreased. The XPS data showed that the O/Zn ratio in ZnO films was increased with increasing Dts in the films. The as-grown ZnO films have an average transmittance of above 85% at the visible region. The optical band gap of the as-grown ZnO films was changed from 3.18 to 3.36 eV with Dts. With decreasing Dts, the electrical resistivity was decreased, while the growth rate was increased. A bilayer is used as an anode electrode for organic electroluminescent devices. The bilayer consists of an ultrathin ZnO layer adjacent to a hole-transporting layer and an Indium tin oxide (ITO) outerlayer. We tried to bring low the barrier between the devices as deposited ZnO films on ITO substrates. We fabricated the organic EL structure consisted of Al as a cathode, Al2O3 as an electro transport layer, Alq3 as a luminously layer, TPD as a hole transport layer and ZnO (1 nm)/ITO (150 nm) as an anode. The result of this experiment was not good compared with the case of using ITO, nevertheless, at this structure we obtained the lowest turn-on voltage as the value of 19 V and the good brightness (6200 cd/m2) of the emission light from the devices. Then the quantum efficiency was to be 1.0%.