Asymmetrically difunctionalized dibenzo[b,d]furan-based hole blocking materials for high-performance blue phosphorescent organic light-emitting diodes

Many researchers have reported dibenzo[b,d]furan segment as a building block of organic light-emitting diode (OLED) materials because it has high thermal stability and triplet energy. However, most of the research has focused on symmetrically substituting the same functional groups at 2-position due to easy functionalization or substituting the same functional groups at different positions of dibenzo[b,d]furan. Herein, we design and synthesize three new hole blocking materials based on asymmetrically difunctionalized dibenzo[b,d]furan, diphenyl (2-(pyridin-3-yl)dibenzo[b,d]furan-6-yl)phosphine oxide (DBFPO-Py), diphenyl(2-(pyrimidin-5-yl)dibenzo[b,d]furan-6-yl)phosphine oxide (DBFPO-Pyr), and diphenyl(2-(4-(triphenylsilyl)phenyl)dibenzo[b,d]furan-6-yl)phosphine oxide (DBFPO-Si) for high-performance phosphorescent OLEDs. Phosphine oxide, tetraphenylsilane, pyridine, and pyrimidine segments are successfully introduced into the asymmetric position of a dibenzo[b,d]furan. It is found that DBFPO-Py, DBFPO-Pyr, and DBFPO-Si possess high thermal stability; high triplet energies of 2.96, 2.98, and 2.80 eV; and deep highest occupied molecular orbital (HOMO) energy levels of −7.13, −7.23 and −7.07 eV; respectively. Blue phosphorescent OLEDs with DBFPO-Py, DBFPO-Pyr, and DBFPO-Si show low turn-on voltages, high current and power efficiencies, and superior external quantum efficiencies. Blue phosphorescent OLEDs with DBFPO-Py and DBFPO-Pyr showed improved performance in terms of current and power efficiencies, etc, compared with the device with 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB), which is commonly used as a hole blocking layer. More importantly, the blue phosphorescent OLEDs with DBFPO-Pyr showed the best performance with maximum external quantum efficiency of 23.6%, current efficiency of 29.8 cd A⁻¹, power efficiency of 26.0 lm W⁻¹, and low efficiency roll-off of 6.38%. Novel hole blocking materials based on asymmetrically difunctionalized dibenzo[b,d]furan are expected to make a significant contribution to the development of blue phosphorescent OLEDs.