New host copolymers containing pendant triphenylamine and carbazole for efficient green phosphorescent OLEDs

Abstract

A series of vinyl copolymers (P1–P6) containing pendant hole-transporting triphenylamine (11–88mol%) and carbazole chromophores were synthesized by radical copolymerization to investigate the influence of triphenylamine groups upon optoelectronic properties. The copolymers were readily soluble in common organic solvents and their weight-average molecular weights (Mws) were between 1.41×104 and 2.24×104. They exhibited moderate thermal stability with Td=402–432°C at 5% weight loss. The emission spectra (both PL and EL) of the blends [P1–P6 with 4wt% Ir(ppy)3] showed dominant green emission (517nm) attributed to Ir(ppy)3 due to efficient energy transfer from P1–P6 to Ir(ppy)3. The HOMO levels of P1–P6, estimated from onset oxidation potentials in cyclic voltammeter, were −5.42 to −5.18eV, which are much higher than −5.8eV of conventional poly(9-vinylcarbazole) (PVK) host owing to high hole-affinity of the triphenylamine groups. The optoelectronic performances of phosphorescent EL devices, using P1–P6 as hosts and Ir(ppy)3 as dopant (ITO/PEDOT:PSS/P1–P6:Ir(ppy)3 (4wt%):PBD (40wt%)/BCP/Ca/Al), were greatly improved relative to that of PVK. The best performance was obtained with P4 device, in which the maximum luminance and luminance efficiency were 11 501cd/m2 and 10.6cd/A, respectively.