Three new solution processable small molecular host materials based on the bis- [3,5-di(9H-carbazol-9-yl) phenyl] structural moiety have been developed for blue phosphorescent (FIrpic dopant) organic lightemitting diodes. All three host materials have been characterized as having high glass transition temperatures (T g s), 155-175 C, indicative of good morphological stability of their amorphous thin films prepared from the solution process. Whereas N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]methylamine (CzPAMe) has the highest solid state triplet energy gap (E T ) of 2.73 eV, tetrakis-[3,3 0 ,5,5 0 -(9H-carbazol-9yl)]triphenylphosphine oxide (CzPPO) and N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]pyrimidin-2-amine (CzPAPm) are two host materials which are potentially bipolar for charge transport due to the electron deficient units of phenylphosphine oxide and pyrimidine, respectively. Due to the insufficient E T (2.56 eV) of CzPAPm, CzPPO or CzPAMe devices are significantly better than CzPAPm devices with or without a 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) co-host. Particularly, having no OXD-7 co-host and no vacuum thermal-deposited extra electron transporting layer, single-layer devices of CzPPO surpass CzPAMe devices and reach current efficiencies as high as 9.32 cd A À1 (or power efficiency of 4.97 lm W À1 ), one of the highest efficiencies among small molecular devices with the same fabrication process and same device configuration.
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 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 o 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 a typical AIE property 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 ET ~2.05-2.16 eV, which is barely enough for phosphorescence dopant, tris(2phenylquinoline) iridium (III) (Ir(2-phq)3) (ET ~2.13 eV). CzPATPE is solution processable to be a sky blue emissive host material for 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 colour chromaticity of CIEx,y(0.36, 0.43). Although the maximum EL efficiency is modest 3.4 cd/A or 1.8 lm/W, the white OLED has a colour rendering index (CRI) of 72, which is unusually high considering the orange dopant and two-colour white feature. We attribute such CRI to the relatively large FWHM of CzPATPE.
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