Red electroluminescence from a thin organometallic layer of europium

We investigate the performance of organic electroluminescence devices employing a fluorinated diketone complex of europium (ETP) as the emitter material. The architecture of our devices isolates the emitter from the injecting contacts by sandwiching a thin layer of ETP between a hole-transporting diamine layer and an electron-transporting aluminum complex layer, The organic layers are deposited in high vacuum with rate-controlled sources onto glass substrates coated with indium-tin-oxide, and the cell is completed by evaporation of aluminum or calcium-aluminum cathodes, By varying the thickness of ETP layer in increments of 6 Angstrom we demonstrate spatial confinement of the electroluminescence emission zone and optimal performance for an ETP thickness of 50 Angstrom. Both the optical and electrical characteristics of these cells follow steep power-law relationships with voltage, which are indicative of trap-modified, space-charge-limited conduction With aluminum cathodes we routinely achieve luminances up to 10 cd/m(2) with direct current densities near 40 mA/cm(2). The electroluminescence has a red-orange color and exhibits a narrow spectrum that is characteristic of trivalent europium ions. (C) 1996 American Institute of Physics.