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

Abstract

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 × 104 and 1.90 × 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. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7960–7971, 2008