Structural characterization of the Ag/Ca transparent electrode for organic light emitting diodes

Abstract

The Ag/Ca double layer, a candidate material for a transparent cathode in top-emitting organic light emitting diodes, becomes semitransparent upon exposure to air. It was found that the bottom Ca layer becomes a Ca(OH)(2) layer, and the top Ag layer becomes an Ag (35% Ca) layer. The origin of the higher transmittance of the Ag/Ca double layer compared to the Ag single layer is explained as follows: The oxidation of the Ca layer to Ca(OH)(2) leads to compressive stress on the bottom layer and tensile stress on the top layer. Due to the stress gradient, the unreacted Ca in the bottom layer is pushed into the top Ag layer to form a Ag-Ca solid solution, reducing the conductivity in the top layer and increasing the transmittance of the top layer. A high-transmittance and low-resistivity top electrode can be designed when the Ag layer is connected continuously and it partially covers the surface of the bottom layer.