Molecular light emitting heterojunctions based on fully conjugated heterocyclic aromatic rigid-rod polymer poly- p-phenylenebenzobisthiazole

Abstract

Poly- p-phenylenebenzobisthiazole, PBT, is a heterocyclic aromatic polymer having a fully conjugated rigid-rod backbone, excellent stabilities and opto-electronic properties. Multilayer heterojunctions of PBT and conjugated dyes of 3-(2-benzothiazoly)-7-(diethylamino), Coumarin 6 (or C 6), and tris-(8-hydroxyquinoline) aluminum, Alq 3, were processed using PBT as a hole injection and transport layer. A PBT with an intrinsic viscosity of 18.0 dL/g was dissolved in acids and spun onto an indium-tin-oxide (ITO) substrate followed by coagulation to remove the acid. The low molar dyes were evaporated directly to, or onto the PBT film adhered to the ITO substrate. UV–Vis absorption spectra showed direct band gaps for PBT, C 6 and Alq 3 at correspondingly 2.6, 2.4, and 2.9 eV. Scanning electron microscope revealed that the heterojunction layer for PBT and the dyes were, respectively, about 23 and 50 nm in thickness. Aluminum was evaporated onto the heterojunction as electron injector for light-emitting diode (LED). By varying the layer ordering and composition, it was evidenced that PBT was a p-type material of high hole mobility; both C 6 and Alq 3 were n-type materials of a low and a high electron mobility, respectively. Significantly reduced LED threshold voltage and enhanced electroluminescence intensity were resulted from the dyes by introducing a PBT layer showing that the holes were the minority carrier for the heterojunction LEDs.