New electroactive polymers with electronically isolated 3,6,9-triarylcarbazole units as efficient hole transporting materials for organic light emitting diodes

Abstract

Polyethers containing electronically isolated 3,6,9-triarylcarbazole moieties have been synthesized by the multi-step synthetic procedures. A polymer containing unsubstituted carbazole rings was also prepared for comparison of properties. Full characterization of the materials is presented. The new polymers represent materials of high thermal stability with initial thermal degradation temperatures exceeding 367 °C. The high glass transition temperatures of the amorphous materials were in the rage of 133–146 °C. The electron photoemission spectra of thin layers of the polymers showed ionization potentials in the range of 5.52–5.6 eV. Hole-transporting properties of the electroactive materials were tested in the structures of organic light emitting diodes (OLEDs) with tris(quinolin-8-olato)aluminium as the green emitter and electron transporting material. The OLED devices in general exhibited rather low turn-on voltages of 3.2–4.2 V, high maximum brightness of 4910–7500 cd/m2 and luminous efficiency of 1.7–3.1 cd/A. It was confirmed that the devices containing the polymers with 3,6,9-triarylcarbazole moieties demonstrate considerably better OLED performance than that of device with polymer containing unsubstituted carbazole rings due to its higher ionization potential and accordingly worse hole injecting properties. The device containing hole-transporting layer of polymer with 3,6-di(1-naphthyl)-9-phenylcarbazole units exhibited the best overall performance with rather low turn on voltage of 4.4 V, maximum photometric efficiency exceeding 3.1 cd/A and maximum brightness of about 7500 cd/m2.