Highly efficient red and green phosphorescent OLEDs based on benzonitrile and carbazole as bipolar host materials

Abstract

In this work, two novel bipolar host materials 3''-(9H-carbazol-9-yl)-5'-(2,6-diphenylpyridin-4-yl)-[1,1':3′,1″-terphenyl]-4-carbonitrile (p-PyCNCz) and 3''-(9H-carbazol-9-yl)-5'-(2,6-diphenylpyridin-4-yl)-[1,1':3′,1″-terphenyl]-3-carbonitrile (m-PyCNCz) were designed and synthesized by incorporating diphenylpyridine, benzonitrile and phenylcarbazole. Both materials exhibit excellent thermal stability with their glass transition temperatures (Tg) of exceeding 120 °C and thermal decomposition temperatures (Td) of reaching approximately 490 °C. The p-PyCNCz and m-PyCNCz not only possess a high ET of 2.75 eV and 2.84 eV, but their HOMO/LUMO are completely separated. Moreover, red and green phosphorescent OLEDs (PhOLEDs) hosted by the two materials were prepared. The m-PyCNCz exhibits better device performance, with the Ƞc,max, Ƞp,max and EQEmax of achieving 88.32 cd/A, 98.19 lm/W and 24.3 % for the green PhOLED, and 22.19 cd/A, 24.38 lm/W and 25.3 % for the red PhOLED, respectively.