Photoconductivity in conjugated polymers

Abstract

This thesis reports the results of photoconductivity experiments on films of the conjugated polymers poly(2-methoxy-5- (2'ethylhexyloxy)-l,4-phenylene vinylene) (MEH-PPV), poly(2,5-pyridinediyl) (PPY) and poly(9,9-bis(2-ethyl hexyl)fluorene-2,7-diyl) (PF2/6) sandwiched between gold and indium tin oxide on a glass substrate. The methods used to fabricate the sandwich structures, the photoconductivity methods used and the procedures used to normalise the data are described. Results indicate that, at photon energies appropriate to the fundamental absorption of the particular polymer, the absorption of photons leads to the formation of excitons. These excitons can be extrinsically dissociated at an electrode to give free charge carriers. These free charge carriers then give rise to a photocurrent. At photon energies above the fundamental absorption of the conjugated polymer, free charge carriers are intrinsically generated in the bulk polymer. The polymer film thickness and the relative mobilities of electrons and holes in the conjugated polymers influence the spectral response of the photocurrent In MEH-PPV, the hole mobility is one order of magnitude greater than the electron mobility. The effects of charge carrier mobility explain the temperature dependence of the photocurrent spectra for MEH-PPV samples. The charge carrier generation mechanism explains the electric field dependence of the photocurrent spectra for MEH-PPV samples. The electron mobility is greater than the hole mobility in PPY and this is confirmed by photoconductivity experiments. A comparison of the spectra obtained from photoconductivity experiments in PF2/6 and PPY suggest that excitations involving the nitrogen atom in the PPY molecule occur producing additional features in the photocurrent spectra