Tert-butyl-substituted bicarbazole as bipolar host material for efficient green and yellow PhOLEDs
For host materials, a high triplet energy(ET) value and good charge transporting ability with appropriate frontier orbital energy are vital properties to achieve efficient PhOLEDs. A novel host material, 3,3’,6,6’-tetra-tert-butyl-9,9’-bicarbazole (tcaz-tcaz), was designed and synthesized by intermolecular N–N coupling reaction of 9,9'-bicarbazoles. The compound displays excellent thermal and morphological stability with the decomposition temperature (Td) of 302 °C and small value of root-mean-square (RMS) roughness less than 0.670 nm for neat film, respectively. In addition, tcaz-tcaz possesses high ET value of 3.0 eV and suitable HOMO/LUMO of -5.71 eV/-2.08 eV. Tcaz-tcaz hosted devices exhibited maximum external quantum, power and current efficiencies of 15.5%, 40.0 lm W-1 and 56.1 cd A-1, respectively, with a low turn-on voltage of 2.4 V for Ir(ppy)3 and 11.0%, 35.6 lm W-1 and 35.9 cd A-1 for PO-01. The orange and red devices were also fabriacted showing inferior performance with a maximum external quantum efficiency of 3.81% for Ir(MDQ)2(acac) and 6.41% for Ir(piq)2(acac), respectively. Thus, proving its promising utilizations of tcaz-tcaz as a host material for the future exploration in multi-colour PhOLEDs.