Light-emitting diode based on oligo-phenylene vinylene and butyl-PBD blends

We have fabricated light-emitting diodes (LEDs) using organic materials; a polymer blend dispersing oligo-phenylene vinylene (oligo-PV), 1,4-distyrylbenzene and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (butyl-PBD) as emissive materials into a soluble polyimide mixed with polyaniline (PANI) of emeraldine salt used as a hole transport material. These polymer dispersed materials were sandwiched between In and indium-tin-oxide (ITO) electrodes. In order to increase the electron injection into the emissive material, we have inserted a thin Mg layer between In and polymer blends. The electroluminescence (EL) spectra of LEDs showed noticeable enhancement of the oscillator strength of oligo-PV peak at 2.76 eV. This implies improved quantum efficiency of this blue light-emitting diode, resulting from the excitonic migration from butyl-PBD to oligo-PV. We have found that the EL device with host polymers, polyimide and PANI, displayed increasing device performance, lowering the turning point in I-V characteristics, compared to that of LED without PANI. Under normal illumination conditions, our devices with PANI showed visible blue-violet color at-room temperature after applying a bias exceeding 8 V. (C) 1997 Elsevier Science Ltd.