Luminescence of thin films of conjugated rigid-rod polymer PBT

Abstract

Heterocyclic aromatic polymer poly(p- phenylenebenzobisthiazole), PBT, is a rigid-rod polymer having a fully conjugated backbone as well as excellent dimensional, thermo-oxidative, and solvent stabilities. A PBT polymer with intrinsic viscosity of 18.0 dL/g was dissolved in methanesulfonic acid or Lewis acid. The PBT solution was spin- coated, doctor-bladed or extruded for freestanding films or onto an indium-tin-oxide (ITO) substrate. The acid was removed via coagulation resulted in PBT films of about 80 nm in thickness on the ITO substrate as determined by scanning electron microscopy. X-ray scattering demonstrated that the extruded freestanding films were uniaxial while the others were isotropic without long-range order. Both temperature and excitation power dependences of the photoluminescence measurements were performed. The laser excitation power dependence of the emission intensity is fitted well with a bimolecular recombination model. Light-emitting devices were fabricated with a structure of Al(Mg)/PBT/ITO/glass, and gave out green-yellow light. A threshold voltage as low as 1 V was achieved. Electroluminescence spectra showed a blue-shift with increasing voltage, which is ascribed to the band-filling effect. Vibrational structure emerged in the photoluminescence spectra at low temperatures, and was observed in the electroluminescence spectra at high voltages, which gives a vibrational mode spacing of 186 meV for the lowest levels.