A highly transparent laminated composite cathode for organic light-emitting diodes

Transparent conductive electrodes are one of the important components of organic optoelectronic devices. Mg:Ag alloys have been widely used as a semi-transparent cathode for top-emitting organic light-emitting diodes (OLEDs) or transparent OLEDs (TrOLEDs) due to their high conductivity and relatively low work function. However, their low transmittance makes the light-extraction efficiency to be comparatively low. In this study, a highly transparent (84.6% in the visible light range), conductive, laminated cathode with the structure of ZnO/Mg:Ag/ZnO was fabricated via atomic layer deposition (ALD). ALD-deposited ZnO is used to protect the vulnerable ultrathin Mg:Ag-alloy layer, while preserving its low work function for the entire structure. In particular, the bottom ZnO layer is used as an ETL, which can provide reasonable energy alignment between the cathode layer and the emission layer and prevent metal particles from penetrating into the underlying organic layer. The top-emission OLED, which is prepared in this study by using the laminated cathode, showed an 85.8% increase in brightness, with a current density of 11.2 mA/cm3 at 4.0 V, which is an ultra-low voltage compared with devices made with Mg:Ag cathodes. In addition, the total luminance of TrOLEDs increases 34.4% compared with devices using Mg:Ag cathodes. Furthermore, the average transmittance of the device was still as high as 77.19% for visible light.