A CBP derivative as bipolar host for performance enhancement in phosphorescent organic light-emitting diodes

Abstract

N,N′-Dicarbazolyl-4,4′-biphenyl (CBP) is one of the most successful uni-polar host materials for phosphorescent organic light-emitting diodes (PhOLEDs). We report the synthesis and properties of one novel CBP derivative, CBP-CN, with two cyano groups (CN) at the 3-site of carbazole rings. The strong electron-withdrawing CN group was introduced with the expectation to promote electron-injecting/-transporting abilities and to achieve bipolar features for CBP-CN. In comparison with the parent CBP, CBP-CN possesses lowered HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels and dramatically increased T_g (glass transition temperature, 162 °C), but unaltered HOMO–LUMO band gap and triplet energy (2.69 eV). Green and red PhOLEDs were fabricated with CBP-CN as hosts for traditional iridium phosphors. The maximum luminance efficiency (η_L) of 80.61 cd A⁻¹ (23.13%) was achieved for the green PhOLED, and 10.67 cd A⁻¹ (15.54%) for the red one, which represent efficiency increases of 25–33% compared with those of the best devices with CBP host and are even among the best data for phosphorescent OLEDs reported so far. The theoretical calculation and the carrier-only devices investigation confirmed that the electron-injecting/-transporting character and the bipolar nature of CBP-CN should be responsible for the performance enhancements.