A green organic light emitting diode based on a rare-earth Terbium complex

Abstract

Tris(8-hydroxyquinoline) aluminum (Alq) has been widely used as the electron transporting layer as well as the green emitter layer in organic light emitting diodes (OLEDs) since it is thermally and morphologically stable to evaporate into thin films. Alq exhibits strong green emission but its spectrum is broad (∼85 nm FWHM) which results in inferior colour purity. On the other hand, rare-earth metal ions exhibit sharp spectral band which corresponds to 5DX-FX transitions. In the case of terbium, this results in a sharp emission in green. In this work, organic light emitting diodes based on a rare-earth metal complex [Tris(3-methyl-1-phenyl-4trimethyl-acetyl-5-pyrazoline) terbium III] were fabricated by thermal evaporation. The basic device structure is indium tin oxide(ITO) / 4,4′-N,N'-dicarbazole-biphenyl (CBP) / 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) / tris(8-hydroxyquinoline) aluminum (Alq) / Lithium Floride(LiF) / Aluminium(Al). The terbium (Tb) complex was doped into the hole transporting material (CBP) and the electron transporting material (Alq) to investigate the effect of dopant in different layers on the device performance. The effects of different dopant concentration on the photoluminescence (PL) and electroluminescence (EL) emission spectra were also investigated. Sharp emission in green (545 nm) was observed for optimum device structure and doping concentration.