Properties of light-emitting diodes fabricated from self-assembled multilayer heterostructures of poly(p-pyridyl vinylene)

Abstract

Heterostructure multilayer thin films of electroactive polymers such as protonated poly(p-pyridyl vinylene) (PHPyV) and sulphonated polyaniline (SPAn) have been fabricated via processes based on the spontaneous self-assembly of conjugated poly-ions on to a substrate. The heterostructure thin films formed from these polymers exhibited a perfectly linear relationship between the absorbance and the number of SPAn/PHPyV bilayers deposited. A greenish - yellow electroluminescent (EL) diode utilizing a SPAn/PHPyV multilayer thin film has been fabricated and the EL properties of this device are discussed. The EL emission of this device can be qualitatively interpreted in terms of radiative recombination of the singlet polaron exciton formed by the injection of electrons and holes. The EL intensity of this light-emitting diode is weak. This result is discussed in terms of the quenching centre of cationic sites on the PHPyV polymer's main chain at the heterojunction. The Stokes shift due to the absorption and emission of a phonon is clearly observed in the EL spectrum of this device. A small shift between the EL and PL spectra of SPAn/PHPyV is observed. It was assumed that the EL emission takes place in a narrow region at the interface whereas the PL emission probes the bulk of the film. The SPAn/PHPyV monolayer LED emits yellow light, but the SPAn/PHPyV multilayer LED emits greenish - yellow light and there is a slight shift between the two spectra of these devices. These results can be interpreted in terms of the effect of confinement of carriers in the superlattice structure constituting the SPAn/PHPyV multilayer system.