Productive harvesting of triplet excitons in anthracene-based emitters toward high-performance deep-blue nondoped organic light-emitting diodes

Developing efficient deep-blue non-doped organic light-emitting diodes (OLEDs) is of great significance in practical applications. Here, two highly efficient asymmetric anthracene-based fluorescent emitters, 1-phenyl-2-(4-(10-(4-(2-phenyl-1H-phenanthro[9,10-d]imidazole-1-yl)phenyl)anthracen-9-yl)phenyl)-1H-phenanthro [9,10-d]imidazole (PPI-An-NPPI) and 1-phenyl-2-(4-(10-(4-(2,4,5-triphenyl-1H-imidazole-1-yl)phenyl) anthracen-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (PPI-An-NPIM), have been designed and synthesized by introducing large steric hindrance imidazole moieties to regulate molecular excited states and photophysical properties. Experimental data show that they have high photoluminescence efficiencies, good thermal stabilities, and suitable energy levels for carrier injection. Theoretical calculations present that their high-lying excited states exhibit dominant locally excited-state characteristics with enhancing oscillator strength compared with anthracene core. The calculated transition dipole moments data show that two molecules are preferentially oriented along the horizontal direction. In addition, some hot exciton mechanism-like channels are also observed and confirmed, which are beneficial for the productive triplet-singlet exciton conversion. The non-doped OLED using PPI-An-NPPI as the emitting layer achieves a maximum external quantum efficiency (EQEmax) of 7.75% and Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.11), whereas PPI-An-NPIM gives a better color purity of CIE (0.14, 10) with an EQEmax of 7.48%. Moreover, all devices exhibit an insignificant efficiency roll-off at high luminescence and still yield an EQE of 7.61% and 7.14% at 1,000 cd/m2. This work provides an interesting insight into developing efficient deep-blue fluorescent emitters for high-performance non-doped OLEDs.