Electroluminescent devices based on polymers forming hole-transporting layers. II. Polyimides containing ?-naphthyldiphenylamine units

Abstract

Two polyimides, PI(C1-BA) and PI(C1-6FDA), based on an aromatic di- amine compound and two aromatic dianhydrides (biphenyltetracarboxylic dianhydride (BA) and 4,49-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)) were newly syn- thesized. They were characterized by viscosity, Fourier transform infrared measure- ments, and X-ray analyses. X-ray results revealed that the crystallinity of PI(C1-BA) was lower than that of PI(C1-6FDA). The synthesized polyimides could be formed into stable and homogeneous thin films on an ITO (indium tin oxide) electrode, functioning as good hole-transport materials for organic electroluminescent (EL) devices. The double-layered EL devices, consisting of the hole-transport layer of these polyimides and an emitting layer of tris(8-quinolinolato)aluminum complex, exhibited a peak emission wavelength in the bright green at 524 -527 nm. Moreover, a maximum luminance of 355 cd/m 2 was achieved at a voltage of 13 V, with a current density of 600 mA/cm 2 for the EL device using PI(C1-BA) as a hole-transport layer. The phenomenon of an extended luminance of these EL devices was also found when the voltage was