Novel star-shaped host materials for highly efficient solution-processed phosphorescent organic light-emitting diodes

Abstract

Two novel star-shaped host materials for solution processed blue phosphorescent organic light-emitting devices, 9-(5′,5‴-diphenyl[1,1′:3′,1″:3″,1‴:3‴,1⁗-quinquephenyl]-5″-diyl)-9H-carbazole (DQC) and 9,9′-(5′-phenyl[1,1′:3′,1″-terphenyl]-3,5-diyl)bis-9H-carbazole (PTC), were synthesized by the Suzuki coupling reaction. These compounds both exhibited high glass-transition temperatures (T_g ≥ 128 °C) and excellent film-forming ability. The nonplanar star-shaped configuration of DQC and PTC limited the effective extension of their π–conjugation, leading to the same triplet energy of 2.81 eV. The solution processed single layer devices using DQC and PTC as the host for the phosphorescence emitter iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) showed the maximum luminance efficiencies of 9.2 and 12.8 cd A⁻¹, respectively. By introducing a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) electron-transporting and exciton-confining layer, the maximum efficiencies of the solution processed devices based on DQC and PTC were further improved to 21.7 and 25.7 cd A⁻¹ with the maximum external quantum efficiencies up to 9.2% and 11.9%, respectively. Furthermore, the DQC- and PTC-based devices showed significantly high performance compared with the corresponding devices based on 1,3-bis(9-carbazolyl)benzene (mCP).