Carbazole oligomers revisited: new additions at the carbazole 1- and 8-positions

Abstract

In recent years low molecular weight carbazole-based materials have attracted much interest as hole-transporting and host materials for organic light emitting diode (OLED) applications. Herein we report the tri- and tetra-bromination of carbazole and the subsequent synthesis of novel carbazole oligomers substituted at the 1- and 8-positions. The oligomer series range from mono-carbazole (Cz-1) to quinque-carbazole (Cz-5) and we have carried out a comparative study of their thermal, optical, electrochemical and optoelectronic properties. The glass transition temperature increases as the carbazole core is enlarged (0 °C to 54 °C) and all oligomers exhibit excellent thermal stability with high thermal decomposition temperatures in the range of 442 °C to 494 °C. From optical data and Raman spectroscopy we observe negligible changes in the effective conjugation length as the number of peripheral carbazole units increases. However, the oxidation potential decreases as the number of carbazole units increase, in which Cz-5 exhibits the highest HOMO level (−5.19 eV) compared to other carbazole oligomers in the series. All carbazole materials emit deep blue photoluminescence ranging from 416 to 432 nm in the solid-state. Finally, electrophosphorescence was demonstrated from solution processable guest-host organic light- emitting diodes (OLEDs) fabricated using Cz(2–5) as the host materials with device configuration ITO/PEDOT/Cz1–5 : 10% Ir(mppy)₃/TPBi/LiF/Al. The device efficiency decreases from Cz-3 to Cz-5. The best OLED device that incorporates Cz-3 as the host exhibits a turn-on voltage of 2.8 V, maximum brightness of 2310 cd m⁻², maximum power efficiency of 33.6 lm W⁻¹, maximum current efficiency 35.5 cd A⁻¹ and EQE of 10.6%.