BiPh-m-BiDPO as a Hole-Blocking Layer for Organic Light-Emitting Diodes: Revealing Molecular Structure-Properties Relationship

We report a simple hole-blocking material (biphenyl-3,3'-diyl)bis(diphenylphosphine oxide) (BiPh-m-BiDPO) based on our recent advance. The bis(phosphine oxide) compound shows HOMO/LUMO levels of \sim-6.71/-2.51 eV. Its phosphorescent spectrum in a solid film features two major emission bands peaking at 2.69 and 2.4 eV, corresponding to 0–0 and 0–1 vibronic transitions, respectively. The measurement of the electron-only devices reveals that BiPh-m-BiDPO possesses electron mobility of 2.28\times10^-9–3.22\times10^-8 cm^2V^-1s^-1 at E=2–5\times10^5 V/cm. The characterization of the sky blue fluorescent and red phosphorescent pin organic light-emitting diodes (OLEDs) utilizing BiPh-m-BiDPO as the hole blocker shows that its shallow LUMO level as well as the low electron mobility affects significantly the power efficiency and hence operational stability, relative to the luminous efficiency, especially at high luminance. In combination with our recent results, the present study provides an indepth insight on the molecular structure-property correlation in the organic phosphinyl-containing hole-blocking materials.