Light emitting diodes of fully conjugated heterocyclic aromatic rigid-rod polymer dopedwith multi-wall carbon nanotubes

Abstract

Poly-p-phenylenebenzobisoxazole (PBO) and carbon nanotubes have a fully conjugated rod-likebackbone entailing excellent mechanical properties, thermo-oxidative stability and solventresistance. Rigid-rod PBO is commonly processed by dissolving in methanesulfonic acid orLewis acid. A multi-wall carbon nanotube (MWNT) was dissolved in an isotropic Lewisacid solution of PBO for dispersion of nanotubes and for processing into thin film. TheMWNT concentration in the films was up to 5 wt%. Compared to pure PBO film, allPBO + MWNT composite films retained the same but enhanced UV–Vis absorption peaks,showing that MWNT and PBO did not have overlapping electron orbitals affecting theirenergy gaps. The films were excited at 325 nm using a He–Cd laser for photoluminescence(PL) emission. All PL spectra had the maximum intensity at 540 nm, indicative of ayellow–green light emission. Single-layer light emitting diodes of MWNT doped PBOdecreased the threshold voltage by 2 V. Up to 0.1 wt% of MWNTs, the diode emissioncurrent increased by about two orders of magnitude over those of the diodes withoutMWNT. Further increase of MWNT concentration caused a successive decrease inelectroluminescence emission intensity, that was attributed to a quench effect fromaggregation of MWNTs.