Experimental study on transparent polymer light emitting diodes using LiF/Ag/indium tin oxide cathode

In this work, we successfully demonstrate the transparent polymer light emitting diodes (PLEDs) with LiF/Ag/indium tin oxide (ITO) cathode that can efficiently improve the operation voltage and enhance light output. The devices with Ag insertion layer all perform the lower turn-on voltage in the range from 4.8 to 5.8 V at 1 mA/cm². Furthermore, these devices all show the superior luminance than the device without Ag buffer layer. However, the transparency of cathode is strongly dependent on the Ag thickness. Devices with a 1 nm-thick layer Ag exhibits a higher transparency than other devices with Ag layers, thus showing the highest luminance from the top cathode. In addition, the Ag insertion layer also plays a role to prevent the emitting layer from sputtering bombard damage. According to X-ray photoemission spectroscopy (XPS) measurement, it is observed that the Ag layer will sufficiently protect the emitting layer from sputtering penetration damage under a sputtering power of 50 W. In this paper, we present a practical structure for realizing transparent PLEDs.