Investigation of photophysical, electrochemical and electroluminescent properties of Iridium(III)bis[2-phenylbenzo[d]thiazolato-N,C^2′]-quinolin-8-olate for white organic light-emitting diodes application

The complex Iridium(III)bis[2-phenylbenzo[d]thiazolato-N,C²′]-quinolin-8-olate ((bt)₂Irq) was synthesized and identified by ¹H NMR and IR spectroscopy. Its photophysical, electrochemical and electroluminescent properties for White Organic Light-Emitting Diodes (WOLED) application were investigated. It was established that (bt)₂Irq irradiates yellow-orange light in CH₂Cl₂ (DCM) solution and solid film with maximum peaks, respectively, at 578 and 666 nm and shoulders at 628 and 720 nm. The complex was used as stand-alone emitter and a dopant in the matrixes of hole transporting layer (HTL) or electroluminescent layer (EL) in the range from 0 to 16.5 w%. The base OLED structure was Anode/HTL/EL/ETL/Cathode, where HTL is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) included in a matrix of poly(N-vinylcarbazole) (PVK), EL is Aluminium (III) bis(2-methyl-8-quninolinato)-4-phenylphenolate (BAlq) and ETL is an electron transporting layer of Tris-(8-hydroxyquinoline) aluminium (Alq₃) or Zinc (II) bis(2-(hydroxyphenyl) benzothiazole (Zn(bt)₂). It is found that the electroluminescent spectra of both types OLEDs were the sum of the greenish-blue and yellow-orange emissions of BAlq and Ir complex and CIE coordinates of the emitted light remain within the white range over wide dopant concentrations. The best current efficiency and CIE coordinates close to the ideal white (0.3300; 0.3300) demonstrate devices with 2 wt% doped HTL (6 Cd/A, (0.2735; 0.3613)) and 16.5 w% doped EL (5.1 Cd/A, (0.2721; 0.3284)). It was found that both types of doped devices operate through a charge trapping mechanism.