Mixing of molecular exciton and excimer phosphorescence to tune color and efficiency of organic LEDs

Abstract

We report highly efficient, variable-color light-emitting diodes (LEDs) realized via mixing of molecular exciton and excimer phosphorescent emissions from a new organic phosphor platinum N^C 2^N-[1,3-di(4-methoxy-pyrid-2-yl)-4,6-difluorobenzene] chloride (Pt L 26Cl), a member of the Pt(N^C^N) complex series, where N^C^N represents a cyclometallating tridentate ligand based on 1,3-dipyridylbenzene, L. Pt L 26Cl-doped TCTA blends [TCTA = 4,4′,4′′-tris( N-carbazolyl-triphenylamine)] have been used as either the low-concentration bluish-green (molecular) phosphorescence emitter or high-concentration red (excimer) phosphorescence emitter. By adjusting the relative amount of blue and red emissive species, the color of the light emission was tuned from bluish-green through green and white up to red. The concentration-optimized devices can easily reach high brightness up to B = 10,000 cd/m 2, achieving extremely high external quantum efficiency (EQE) up to 20% at low-current densities (<10 −2 mA/cm 2) and up to 17% at B = 500 cd/m 2. High purity white light emission [CIE coordinates ( x = 0.34, y = 0.35)] was realized at this B, QE = 11.5%, power efficiency of 6.8 lm/W and color rendering index (CRI) = 74, though CRI = 81 has been achieved with CIE coordinates ( x = 0.42, y = 0.38). The reported performance data are among the best achievements reported for color and white emission organic LEDs.