Electroluminescence of Poly(vinylcarbazole) Doubly Doped with Two Phosphorescence Dyes:  Investigation of Spectral Change as a Function of Driving Voltage

The utilization of molecular phosphorescent dyes in polymer-based organic light emitting diodes (OLEDs) was investigated by codoping several phosphorescent dopants into poly(N-vinylcarbazole) (PVK)-based single layer light emitting diodes (LEDs). In particular, green fac-tris(2-phenylpyridine)iridium(III), [Ir(ppy)3], red tris(4,7-diphenyl-1,10-phenanthroline)rhenium dications, [Ru(4,7-Ph2-phen)3]2+, and (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin)platinum(II) (PtOEP) were used. In the phosphorescence dye doubly doped PVK system ca. x wt % [Ru(4,7-Ph2-phen)3]2+:1 wt % Ir(ppy)3:PVK(x = 0.5−5), the intensity ratio (Ri) of red emission vs green emission as a function of doping ratio (Rc) of [Ru(4,7-Ph2-phen)3]2+ vs Ir(ppy)3 is found according to a power-law function with the exponent of ∼2.0 ± 0.2 for photoluminescence, which obey the power law with Ri ∝ Rc2 predicated by the Förster energy transfer model modified by concentration quenching factor. The red emission of these systems in EL spectra is found to be distinctly decreased compared to the PL spectra at the same doping ratio, and also to be further decreased as driving voltage increased, resulting in a driving voltage dependent power law of Ri ∝ Rc1.5 at low voltage and Ri ∝ Rc1.0 at high voltage, respectively. The relationship between Ri and Rc in the EL spectrum shifted to a power law of Förster energy transfer is likely due to the presence of additional emission mechanism in EL process ca. carrier trapping.