Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes
作者:Shi-Jian Su、Chao Cai、Junji Kido
DOI:10.1039/c2jm14151e
日期:——
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 (ΔEST), and improved bipolarity were also achieved with introducing heterocycles of pyridine and pyrimidine instead of benzene. In contrast to the slightly decreased triplet energy (ET), a significantly decreased ΔEST was achieved by introducing heterocycles of pyridine and pyrimidine as the core, and the more nitrogen atoms in the central heterocycle, the smaller ΔEST 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 ΔEST 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.
一系列含有各种芳基核心的三咔唑臂主材料,如苯(1,3,5-三(3-(咔唑-9-基)苯基)-苯,TCPB)、吡啶(2,4,6-三(3-(咔唑-9-基)苯基)-吡啶,TCPY)和嘧啶(2,4,6-三(3-(咔唑-9-基)苯基)-嘧啶,TCPM)、TCPY)和嘧啶(2,4,6-三(3-(咔唑-9-基)苯基)-嘧啶,TCPM),开发出了红色、绿色和蓝色磷光有机发光二极管(OLED)。与以苯为核心的 TCPB 相比,以吡啶和嘧啶为杂环核心的 TCPY 和 TCPM 观察到了分子内电荷转移,从而在光致发光光谱中产生了浴色偏移,能带隙也有所减小。此外,引入吡啶和嘧啶杂环而不是苯,还可获得更低能量的单线和三线激发态、更低的最低未占分子轨道(LUMO)能级、更小的单线-三线交换能(ΔEST)以及更好的双极性。与三重态能量(ET)略有降低相反,引入吡啶和嘧啶杂环作为核心可显著降低ΔEST,而且中心杂环中的氮原子越多,ΔEST 越小。利用 TCPY 和 TCPM 作为宿主,绿色和红色磷光 OLED 的驱动电压降低了,这是因为它们的ΔEST 减小了,LUMO 能级降低了,这证明必须通过宿主分子将更多的载流子注入发光层,而不是通过掺杂剂直接捕获载流子。此外,基于 TCPY 和 TCPM 的绿色和红色磷光 OLED 还提高了效率并抑制了效率衰减,这是因为它们提高了双极性,从而改善了载流子平衡。