The first aggregation-induced emission fluorophore as a solution processed host material in hybrid white organic light-emitting diodes

A judicious structure design of bis(3,5-di(9H-carbazol-9-yl)phenyl)amine (CzPA) and 1,1,2,2-tetraphenylethene (TPE) hybrid generates the first aggregation-induced emission (AIE) fluorophore, CzPATPE, as a solution processed host material in a hybrid white organic light-emitting diode (OLED). Small molecule CzPATPE has been verified for the amorphous characteristics by a featureless halo in powder X-ray diffraction spectra and a glass transition temperature as high as 180 °C. Fluorescence images indicate that a homogenous thin film of CzPATPE is obtained from a solution drop-casting process. Solution (THF/water) fluorescence spectroscopy study clearly demonstrates typical AIE properties of CzPATPE. Whereas CzPATPE is virtually non-emissive in common organic solvents, thin film photoluminescence (PL) spectroscopy shows that CzPATPE has a strong sky blue emission around 495 nm with a relatively wide full width at half maximum (FWHM) about 95 nm and a reasonable PL quantum yield of 40%. Low temperature time-gated PL spectroscopy identifies CzPATPE's triplet optical gap energy E_T ∼ 2.05-2.16 eV, which is barely enough for the phosphorescent dopant, tris(2-phenylquinoline) iridium(iii) (Ir(2-phq)₃) (E_T ∼ 2.13 eV). CzPATPE is solution processable to be a sky blue emissive host material for an orange dopant in the hybrid white OLED, ITO/PEDOT:PSS (65 nm)/CzPATPE:Ir(2-phq)₃(99.9 : 0.1, ∼40 nm)/TmPyPB (50 nm)/CsF (2 nm)/Al (100 nm). This AIE hybrid white OLED displays white electroluminescence (EL) with a colour chromaticity of CIE_x,y(0.36, 0.43). Although the maximum EL efficiency is modest 3.4 cd A^-1 or 1.8 lm W^-1, the white OLED has a colour rendering index (CRI) of 72, which is unusually high considering the orange dopant and a two-colour white feature. We attribute such CRI to the relatively large FWHM of CzPATPE.