Electronic Transport Properties of Linear Organic Semiconductors

Abstract

The electronic transport properties of certain organii c semIi conductors are expected to exhibit a quasi one-dimensional nature. Pulsed laser techniques have been used to study transient photoconductivity in a number of such linear molecular systems. This thesis explores carrier motion of zeolite encapsulated conjugated polymers such as polyacetylene and polypropyne, columnar discotic liquid crystals and single walled carbon nanotubes. At the time of writing, this thesis presents the first observations of transient photoconductivity for carbon nanotubes. In the systems studied: electric field, temperature and spectral dependencies are explored and the results are used to calculate a number of parameters, such as: carrier mobilities, carrier range and quantum efficiencies. Also, the effect of sample preparation has been investigated. A variation on the Auston switch technique has enabled picosecond time resolved photocurrents to be measured on carbon nanotubes, with a rise time of the order of 100ps. A similar technique was utilised to study the encapsulated polymers, but no measurable effect was observed. The Kepler-LeBlanc Time of Flight technique has been employed to find the carrier mobility in a number of columnar discotic liquid crystals along with the quantum efficiency for carrier generation in those systems. The results presented in this thesis have led to a greater understanding of charge transport on carbon nanotubes from which a ID bimolecular recombination model has been proposed. We have demonstrated a novel polymeric DLC where electrons are the majority carrier and demonstrated a photogeneration mechanism controlled by Poole- Frenkel barrier lowering. We have also been able to refute the proposal that the 3D Onsager model is applicable for describing the photogeneration mechanism in most DLC's.