High-efficiency transparent organic light-emitting devices

We demonstrate organic light-emitting devices (OLEDs) employing highly transparent cathodes comprised of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) and an ultrathin film of Li capped with radio-frequency magnetron-sputtered indium–tin–oxide. The cathodes are incorporated onto a conventional bilayer small-molecule OLED. The operating voltages and the total device external quantum efficiencies emitted from the top and substrate surfaces $\text{(1.0±0.05)\%}$ are comparable to the best conventional undoped OLEDs employing thick metallic cathodes. The device characteristics are independent of the position of Li within the compound cathode, suggesting that Li readily diffuses through BCP to enhance electron injection. An increase of a factor ∼3.5 in the external quantum efficiency is observed compared to devices containing no Li. These results suggest that Li donates electrons to the BCP, increasing its conductivity to the point that band bending occurs to aid in the injection of charge.