Heterojunction OLEDs fabricated by Eu ternary complexes with conducting secondary ligands

A series of Eu ternary complexes with charge conducting secondary ligands were examined as potential light-emitting materials for organic light-emitting diodes (OLEDs). The Eu complex contains an imidazole-based secondary ligand and is electron-conducting. Simple heterojunction OLEDs can be fabricated with a structure ITO/CuPc/TPD/Eu complex/cathode where ITO = indium-tin-oxide, CuPc = copper phthalocyanine, TPD = N,N-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine. The Eu complex plays the dual role of electron-transporting and light-emitting layer. Sharp red emission can be observed at about 612 nm. The OLEDs have a current efficiency of about 0.5 cd/A (at about 20 mA/cm2) and a maximum luminance between 150 and 200 cd/m2. The effects of various substituted imidazole moieties on the electrical properties of the on the OLEDs will be discussed. It is shown that the Eu complex with unsubstituted imidazole has electron mobility ∼10−5 cm2 V−1 s−1, and should be a very useful material for fabricating red OLEDs with simple device structure.