Hole Injection Layer of Thermally Evaporated Copper Oxide for Top Emitting Organic Light Emitting Diodes.

We report the enhancement of the electroluminescent property of top emitting organic light emitting diodes (TEOLEDs) using thermally evaporated copper oxide (CuO_x) as a hole injection layer (HIL) between UV ozone-treated silver (Ag) anodes and 4,4'-bis[N-(l-naphthyl)-N-phenylamino]biphenyl (α-NPD). The operation voltage of TEOLEDs at 1 mA/cm² decreased from 6.2 to 5.0 V as a 2 nm thick CuO_x. layer was used as the HIL. α-NPD layers were separately in situ deposited on both Ag and thermally evaporated CuO_x-coated Ag (Ag/CuO_x and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of Ag/CuO_x. is higher by 0.53 eV than that of Ag. The corresponding interface dipole energies were -0.93 eV for Ag and -0.88 eV for Ag/CuO_x. As a result, CuO_x. plays a role in reducing the hole injection barrier from 1.63 to 0.96 eV, resulting in a decrease in the turn-on voltage of TEOLEDs. [ABSTRACT FROM AUTHOR]