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

Poly-p-phenylenebenzobisoxazole (PBO) and carbon nanotubes have a fully conjugated rod-like backbone entailing excellent mechanical properties, thermo-oxidative stability and solvent resistance. Rigid-rod PBO is commonly processed by dissolving in methanesulfonic acid or Lewis acid. A multi-wall carbon nanotube (MWNT) was dissolved in an isotropic Lewis acid solution of PBO for dispersion of nanotubes and for processing into thin film. The MWNT concentration in the films was up to 5 wt%. Compared to pure PBO film, all PBO + MWNT composite films retained the same but enhanced UV–Vis absorption peaks, showing that MWNT and PBO did not have overlapping electron orbitals affecting their energy 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 a yellow–green light emission. Single-layer light emitting diodes of MWNT doped PBO decreased the threshold voltage by 2 V. Up to 0.1 wt% of MWNTs, the diode emission current increased by about two orders of magnitude over those of the diodes without MWNT. Further increase of MWNT concentration caused a successive decrease in electroluminescence emission intensity, that was attributed to a quench effect from aggregation of MWNTs.