Electroluminescence characteristics of aromatic polyimides by vapor deposition polymerization

Abstract

Light emitting polyimide (PI) thin films are successfully prepared from two kinds of diamines: 2,5-Bis(4-aminophenyl)-1,3,4-oxadiazole (BAO) and 4,4'-(9-Fluorenylidene)dianiline (BAPF) reacting with 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) by vapor deposition polymerization (VDP). We succeed to reduce the thickness of the thin polyimide film to 150 Angstrom, and the threshold voltage of BAO-6FDA and BAPF-6FDA PI light-emitting diodes to 4.5 V and 6.5 V respectively. The root mean square of the surface roughness of the BAO-6FDA and BAPF-6FDA PI films are 8.8 Angstrom and 4.7 Angstrom respectively. But that of the BAO-6FDA film from wet coating process is 37.1 Angstrom. Apparently, VDP process is the better way to produce smooth polyimide film. The BAO-6FDA LED film emits a broader electroluminscence (EL) band, covering the full range of visible light (400 nm to 700 nm), than the BAPF-6FDA LED. However, the electroluminescence efficiency of BAPF-6FDA LED device is better than BAO-6FDA LED device. It is due to the better balance on the hole and electron injection in the former and better intermolecular charge transfer.