New poly(4,4′-dicyano-4-vinyl-triphenylamine) host material for single-layer Ir complex phosphorescent light-emitting devices

We report a new host material for iridium complex light-emitting devices, poly(4,4'-dicyano-4 ''-vinyl-triphenylamine) (PCNVTPA), which was synthesized by nitroxide-mediated free radical polymerization. The incorporation of electron-withdrawing cyano groups led to a significant variation in electronic energy levels and luminescence characteristics in comparison with the parent poly(4-vinyltriphenylamine) (PVTPA). The prepared organosoluble PCNVTPA and PVTPA had number-average molecular weights (M-n) of 10 200 and 23400, respectively. PCNVTPA exhibited higher thermal stability (T-g=211 degrees C) and photoluminescence (PL) quantum efficiency (PLQY) (20%) compared with PVTPA (T-g=140 degrees C, PLQY=3%) because of enhanced rigidity from the cyano-substituted group. Cyano substitution also led to lower energy levels (HOMO, LUMO, unit: eV) in PCNVTPA (-5.63, -2.52) than in PVTPA (-5.35, -1.89). The emission peak of the Ir complexes was observed in the PL spectra of PVTPA or PCNVTPA/Ir complex blend through efficient energy transfer from the host polymer to the guest Ir complex. Single-layer phosphorescent electroluminescent devices of indium-tin oxide/PEDOT:PSS/PCNVTPA:Ir complexes/Ca:Al showed maximum luminance (2899 cd m(-2)) and luminance efficiency (8.84 cd A(-1)), respectively, which were much higher than those of PVTPA. Such an improvement was probably due to the more efficient hole trapping in Ir complexes by the lower HOMO level or better electron injection from the lower LUMO level of PCNVTPA. The results suggested that the new PCNVTPA could be a good host polymer for the electrophosphorescent device. Polymer Journal (2010) 42, 327-335; doi:10.1038/pj.2010.1; published online 27 January 2010