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Title: Polythiophene nanowires for use in organic electronic applications
Author: Hampton, Mark D.
ISNI:       0000 0004 2733 7738
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2012
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The growth and formation of polythiophene nanowires (NWs) was studied using atomic force microscopy (AFM), UV-Vis spectroscopy, and X-ray diffraction (XRD). This was followed by the successful development of a novel technique for doping the nanowires, which led to the doped NWs being investigated in the active blend of organic photovoltaics (OPVs). Finally exploratory work was carried out on the feasibility of NWs as humidity and octylamine sensors, and as the semiconducting medium in water-gated organic field-effect transistors (WGOFETs). AFM revealed that the NWs were formed typically with heights of 1~10 nm, widths of 20~80 nm, and lengths exceeding 10 μm. The dimensions and the rate of growth of the NWs were affected by the polymer-solvent combination. XRD of the poly(3-hexylthiophene-2,5’-diyl) (P3HT) NWs suggested a structure that is in agreement with that of P3HT films found in the literature. There is vertical stacking of the alkyl-chains in the NWs, p- stacking of the thiophene chains along the long axis of the wires and the polymer back bones lie perpendicular to the length of the wire. It was seen that the NWs possessed a more compact alkyl chain stacking than the P3HT films. In situ thermal annealing of the NWs led to expansion of the alkyl chain stacking, an increase in domain sizes and a decrease in paracrstallinity. Poly(3,3”’-didodecyl-quarter-thiophene) (PQT-12) NWs displayed a meta-stable structure as cast, turning amorphous above 100°C, and recrystallising during cooling. Photoluminescence (PL) and combustion analysis provided indirect evidence to suggest successful intercalation of anthraquinone based dye molecules into P3HT and PQT-12 NWs during the growth phase, which is a novel and facile doping technique. Dye-intercalated P3HT-NW:PCBM OPVs showed an increase in power conversion efficiency (PCE) and open circuit voltage (VOC), and a decrease in short-circuit density (JSC) for one dye (2d) where the HOMO was deeper than that of both P3HT and PCBM. Octylamine and humidity chemiresistor sensors were successfully fabricated from P3HT NWs. The chemiresistors showed an increase in resistance upon exposure to as little as 3.6 ppm of octyl-amine, and a ~13-fold increase in resistance at 36 ppm. P3HT NW humidity sensors had a greater response than a P3HT film, with a 75-fold increase in conductance at 90% relative humidity (RH) compared to less than 25% increase for the film. Both film and P3HT NW devices had a threshold of 50% RH below which no significant change in conductance was measured. P3HT was successfully water-gated; NWs had a lower resistance than P3HT film water-gated organic field effect transistor (WGOFET) devices, while greater off-current was seen in the NW devices. P3HT-NW devices were found to be more sensitive than film devices indicated by a lower threshold voltage, and the electrochemical current to field-effect current, IOECT/IOFET, was lower for NWs compared to the film.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics