Enhanced luminescence of top-emission organic light emitting diodes with ZnS/Ag/ZnS structure

Abstract

The effect of ZnS/Ag/ZnS multiple-layer coating on the top-emitting top-cathode organic light emitting diodes (OLED) was studied. The OLED device consisted of Ag/CuPc:F4-TCNQ/NPB/Alq3/BCP/LiF/Al layers. All organic layers and electrodes were fabricated by thermal evaporation. F4-TCNQ was doped in the hole-injection layer (CuPc) to enhance hole injection, since the energy barrier between Ag and CuPc was high. ZnS layer was first deposited on the top cathode (Al) and found to enhance the light emission of the OLED by 50% (from 10,000 cd/m 2 to 15,000 cd/m 2 ). The high-refractive index dielectric material as a capping layer enhances light output for the semitransparent cathode. ZnS/Ag/ZnS multi-layer cathode with photon tunneling characteristics were added on top of Al cathode, and found to further enhance the light emission up to 20,000 cd/m 2 at 13V for Al/ZnS/Ag/ZnS (17/37/8/37 nm) layers with maximum current efficiency of 2.6 cd/A. Coupling of surface plasmon modes may occur in the ZnS/Ag/ZnS structure. By increasing Ag layer thickness to compensate the reduction of Al layer thickness, the Al/ZnS/Ag/ZnS (7/37/15/37 nm) cathode was used, and found to achieve the maximum brightness of 31,000cd/m 2 at 15V and a maximum current efficiency at 5.6 cd/A. The increase of luminescence efficiency is likely due to high photon tunneling efficiency of Ag as well as its high electric conductivity improving the electron injection. Keywords: OLED, top emission, top cathode, tunneling, surface plasmon.