New rare-earth quinolinate complexes for organic light-emitting devices

Abstract

Because of its thermal and morphological stability and optical and electrical properties, tris(8-hydroxyquinoline) aluminum (Alq3) is one of the most widely used electron transporting materials in organic light-emitting devices (OLEDs). The search for substitutes for this compound constitutes an important field of research in organic electronics. We report on a study of a new rare-earth tetrakis 8-hydroxyquinoline complex. Synthesis of tris complexes with rare-earth metals and 8-hydroxyquinoline resulted in unstable compounds. However, the inclusion of an additional quinoline group stabilized these compounds. Li[RE(q)4] (where RE=La3+, Lu3+ and Y3+ and q=8-hydroxyquinoline) were synthesized and then used as the electron-transporting and emitting layer in OLEDs. Thin films were deposited in a high-vacuum environment by thermal evaporation on quartz and silicon substrates. Optical characterization of the RE complexes revealed emission in the 510–525nm range, the same as that observed for Alq3, while absorption was observed at wavelengths of 382nm for the Y/La complexes and 388nm for the Lu complex. The OLEDs were fabricated with an indium tin oxide layer (ITO) as the anode, (N,N′-bis (1-naphtyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine) NPB as the hole-transporting layer (25nm), Li[RE(q)4] as the electron-transporting and emitting layer (40nm) and aluminum as the cathode (120nm). The electroluminescence (EL) spectra showed a broad band from 520 to 540nm and green-colored emission associated with the 8-hydroxyquinoline ligand. There was an interesting dependence of the maximum energy peak position and half-width of the emission band in the EL spectra on the atomic radius of the RE ion used. The best luminance for the OLEDs produced in this study was achieved with the Li[RE(q)4] compound. The optical and electrical properties of this OLED were comparable to those of similar devices based on Alq3.