High-efficiency transparent organic light-emitting devices

We demonstrate organic light-emitting devices (OLEDs) employing highly tran sparent cathodes comprised of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) and an ultrathin film of Li capped with radio-frequency magnetron-sp uttered indium-tin-oxide. The cathodes are incorporated onto a conventional bilayer small-molecule OLED. The operating voltages and the total device e xternal quantum efficiencies emitted from the top and substrate surfaces (1 .0 +/- 0.05)% are comparable to the best conventional undoped OLEDs employi ng thick metallic cathodes. The device characteristics are independent of t he position of Li within the compound cathode, suggesting that Li readily d iffuses through BCP to enhance electron injection. An increase of a factor similar to 3.5 in the external quantum efficiency is observed compared to d evices 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. (C) 2000 American Institute of Physics. [S0003-6951(00)05015-4].