New host homopolymers containing pendant triphenylamine derivatives: Synthesis, optical, electrochemical properties and its blend with Ir(ppy)₃ for green phosphorescent organic light-emitting devices

Two vinyl homopolymers poly(N-(4-(4-(4-vinylbenzyloxy)styryl)phenyl)-N-phenylbenzenamine) (PVST) and poly(4-vinyltriphenylamine) (PTPA) containing pendant hole-transporting triphenylamine and 4-oxystyryltriphenylamine groups, respectively, were synthesized by radical polymerization and employed as hosts for tris(2-phenylpyridine) iridium [Ir(ppy)₃] phosphor. Structural influences of the hole-transporting groups upon optoelectronic properties were investigated by photophysical, electrochemical, and electroluminescent methods. The polymers were readily soluble in common organic solvents and their weight-average molecular weights (Mw) were 5.68 x 10⁴ and 1.90 x 10⁴, respectively. The emission spectra (both photoluminescence, PL and electroluminescent, EL) of the blends [PTPA with 4 wt % Ir(ppy)₃] showed dominant green emission (517 nm) attributed to Ir(ppy)₃ due to efficient energy transfer from PTPA to Ir(ppy)₃. The HOMO levels of PVST and PTPA, estimated from onset oxidation potentials in their cyclic voltammograms, were -5.14 and -5.36 eV, which are much higher than -5.8 eV of the conventional poly(9-vinylcarbazole) (PVK) host owing to high hole-affinity of the triphenylamine groups. The optoelectronic performances of phosphorescent EL devices, using PVST and PTPA as hosts and Ir(ppy)₃ as dopant (indium tin oxide, ITO/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)/PVST or PTPA:Ir(ppy)₃(4 wt %):PBD(40 wt %)/BCP/Ca/Al), were investigated. The maximum luminance and luminance efficiency of the PTPA device were 9220 cd/m² and 6.1 cd/A, respectively, which were significantly improved relative to those of PVK and PVST.