Indolo[3,2-b]carbazole/benzimidazole hybrid bipolar host materials for highly efficient red, yellow, and green phosphorescent organic light emitting diodes

By incorporating electron-accepting benzimidazole and electron-donating indolo[3,2-b]carbazole into one molecule, two novel donor-acceptor bipolar host materials, TICCBI and TICNBI, have been synthesized. The photophysical and electrochemical properties of the hybrids can be tuned through the different linkages (C- or N-connectivity) between the electronic donor and acceptor components. The promising physical properties of these two new compounds made them suitable for use as hosts doped with various Ir or Os-based phosphors for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). PhOLEDs using TICCBI and TICNBI as hosts incorporated with Ir-based emitters such as green (PPy)(2)Ir(acac), yellow (Bt)(2)Ir(acac), and two new red emitters (35dmPh-6Fiq) Ir-2(acac) (i3) and (4tBuPh-6Fiq)(2)Ir(acac) (i6) accomplished high external quantum efficiencies ranging from 14 to 16.2%. Nevertheless, the red PhOLED device incorporating TICNBI doped with the red emitter osmium(II) bis[3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate] dimethylphenylphosphine [Os(bpftz)(2)(PPhMe2)(2)] achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 22%, 28 cd A(-1), and 22.1 lm W-1, respectively, with CIE coordinates of (0.65,0.35). The external quantum efficiency remained high (20%) as the brightness reached to 1000 cd m(-2), suggesting balanced charge fluxes within the emitting layer, rendering devices with limited efficiency roll-off.