Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes

A series of three-carbazole-armed host materials containing various arylene cores, like benzene (1,3,5-tris(3-(carbazol-9-yl)phenyl)-benzene, TCPB), pyridine (2,4,6-tris(3-(carbazol-9-yl) phenyl)-pyridine, TCPY), and pyrimidine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyrimidine, TCPM), were developed for red, green, and blue phosphorescent organic light-emitting diodes (OLEDs). An intramolecular charge transfer was observed for TCPY and TCPM with heterocyclic cores of pyridine and pyrimidine, giving bathochromic shifts in the photoluminescent spectrum and reduced energy band gaps in comparison with TCPB with a benzene core. In addition, lower energy singlet and triplet excited states, reduced lowest unoccupied molecular orbital (LUMO) energy level, smaller singlet-triplet exchange energy (Delta E-ST), and improved bipolarity were also achieved with introducing heterocycles of pyridine and pyrimidine instead of benzene. In contrast to the slightly decreased triplet energy (E-T), a significantly decreased Delta E-ST was achieved by introducing heterocycles of pyridine and pyrimidine as the core, and the more nitrogen atoms in the central heterocycle, the smaller Delta E-ST is achieved. Reduced driving voltages were achieved for the green and red phosphorescent OLEDs by utilizing TCPY and TCPM as the host due to their decreased Delta E-ST and lower-lying LUMO energy level, proving that more carriers must be injected into the emitting layer through the host molecules rather than direct carrier trapping by the dopant. Moreover, improved efficiency and suppressed efficiency roll-off were also achieved for the green and red phosphorescent OLEDs based on TCPY and TCPM due to their improved bipolarity and thus improved carrier balance.