Light-emitting poly(N-naphthyl-2,7-carbazole)

Abstract

A blue-emitting poly(2,7-carbazole) having a naphthyl substituent at the N-position (PNCz) was newly synthesised by the Yamamoto method and used to fabricate an organic light emitting diode (OLED). PNCz was highly soluble in common organic solvent with the number average molecular weight (M_n) of 17600 and good thermal stability, with the decomposition temperature higher than 390 °C. PNCz exhibited intense blue photoluminescence with emission peak maxima at 415 nm in CHCl₃ and 428 nm in the film state, and the fluorescence quantum efficiency in CHCl₃ was extremely high (Φ_fl = 0.99). The HOMO level estimated by cyclic voltammetry was −5.50 eV and the band gap estimated from an absorption edge was 2.72 eV. The OLED device with a configuration of ITO/PEDOT:PSS/PNCz/CsF/Al exhibited bluish emission with several hundreds cd m⁻² and CIE index (x = 0.20, y = 0.21) at low operating voltages in the range of 6–7 V and greenish emission with about 3000 cd m⁻² and CIE index (0.25, 0.44) under high operating voltage around 11 V.