Effect of Li₂O/Al Cathode in Alq₃ Based Organic Light-Emitting Diodes

An effect of bilayer cathode Li₂O/Al was studied in Alq₃ based organic light-emitting diodes with a variation of Li₂O layer thickness. The current-luminance-voltage characteristics of ITO/TPD/Alq₃/Li₂O/Al device were measured at ambient condition to investigate the effect of Li₂O/Al. It was found that when the thickness of Li₂O layer is in the range of 0.5∼1 nm, there are improvements in luminance, efficiency, and turn-on voltage of the device. A current density and a luminance are increased by about 100 times, a turn-on voltage is lowered from 6 V to 3 V, a maximum current efficiency is improved by a factor of 2.3, and a maximum power efficiency is improved by a factor of 3.2 for a device with a use of thin Li₂O layer compared to those of the one without the Li₂O layer. These improvements are thought to be due to a lowering of electron-barrier height for electron injection from the cathode to the emissive layer.