Semitransparent cathodes for organic light emitting devices

We optimize transparent organic light emitting devices (TOLEDs) using compound cathodes consisting of a thermally evaporated metal contact layer capped with indium-tin-oxide (ITO). The ITO is sputtered at rates of up to 1.6 Angstrom/s using a high power radio frequency magnetron process. With a Mg:Ag contact layer, we demonstrate a TOLED with 50% transparency and an operating voltage within 0.3 V of a device with identical organic layers and a conventional Mg:Ag cathode. The operational lifetime of the TOLED is shown to be equal to that of a similar, nontransparent device. We also study the effects of using different contact metals, including Ca, Al and LiF, on the operating characteristics of the TOLEDs. With a thin Ca contact layer, undoped TOLEDs with > 80% peak transparency operating at (5.9 +/- 0.1) V at a brightness of > 100 cd/m(2) are demonstrated. These devices have application to transparent, head-up displays and to full color, stacked organic light emitting devices. (C) 2000 American Institute of Physics. [S0021-8979(00)07705-7].