Atomic-scale imaging of polyvinyl alcohol crystallinity using electron ptychography

Polyvinyl alcohol (PVA) is considered to have great potential in medical, pharmaceutical, and packaging applications because of its outstanding biocompatibility, water solubility, low density and relatively low cost. PVA crystallinity, central to the materials properties, has been studied by X-ray diffraction, but two possible crystal structures are mooted. Electron microscopic techniques can potentially image PVA at high resolution. Still, it is challenging for conventional electron microscopies because of the relatively low crystallinity of PVA, its severe beam sensitivity, and the poor contrast of light elements. Electron ptychography makes use of a 4D STEM dataset comprising the intensity in the STEM detector plane recorded as a function of each probe position and has lower sample damage and better phase-contrast compared to traditional techniques. Here, we use electron ptychography to image PVA crystallinity. The reconstructed images, which show good agreement in the unit cell dimension with X-ray diffraction data, can show how the atoms order in the materials, however, deviations from previous models derived from X-ray diffraction are observed. To interpret the data, we propose a series of changes based on previous models to formulate a description of PVA crystal structure. Simulated results from this new model accord well with the experimental images. This study manages to image both carbon and oxygen atoms in PVA, which has not previously been achieved by any conventional method. The results are expected to bring a new and deeper understanding of PVA crystal structure, and illustrate the opportunity presented by this approach for directly imaging molecular order in polymer crystals

Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

The low energy muon (LEM) technique has been used to probe local changes in the dynamical spectrum of thin film polymer samples taking place as a function of the temperature and the implantation depth below the free surface. The studies have been made on samples of polydimethylsiloxane (PDMS) and polybutadiene (PB) using the transverse magnetic field (TF) configuration and diamagnetic probe muons. In PDMS evidence is found for suppression of the glass transition temperature near the surface, along with significantly modified dynamics in the near-surface region as well as at depths significantly below the surface. For PB the LEM technique reveals well-defined layers of dynamical and spatial inhomogeneity at depths of order 0.1–0.2 μm below the free surface. These inhomogeneous regions may be assigned to nanopores produced by solvent streaming during preparation of spin-cast films. A thermal annealing procedure is shown to significantly reduce the thickness of these inhomogeneous layers. These results demonstrate that using LEM in the TF configuration provides a promising new method for studying surface-modified local dynamics of polymers that is also able to reveal nanostructured buried layers in polymer films

Twofold efficiency increase in nanocrystalline-TiO2/polymer photovoltaic devices by interfacial modification with a lithium salt

Modification of the interface of titanium dioxide/poly[2-(2-ethylhexyloxy)-5-methoxy-1,4,-phenylenevinylene] (TiO2/MEH-PPV) nanocomposite photovoltaic devices with a lithium salt, Li[CF3SO2](2)N, is shown to result in a twofold increase in device efficiency. The devices are of the type ITO/TiO2/MEH-PPV/Au. The TiO2 layer is deposited by doctor blading a colloidal anatase paste, and the polymer is then spin-coated on top followed by thermal evaporation of gold contacts. Careful control of manufacturing conditions and use of a 35 nm polymer layer leads to a device efficiency of 0.48% for un-modified devices. The increased efficiency following Li treatment is the result of a 40% increase in both the short-circuit current and fill factor, while the open-circuit voltage remains unchanged. A maximum efficiency of 1.05% has been achieved under 80% sun illumination. This represents a record efficiency for this type of cell. Photoconductivity experiments show a substantial increase in conductivity of the TiO2 layer following Li modification. Interfacial modification is done via a simple soaking procedure, and the effect of varying the concentration of Li[CF3SO2](2)N is discussed. We report investigations into optimization and the mechanism of such improvement, for example by varying processing parameters of the modification procedure or the ionic species themselves

Crystallization and phase separation in thin film polymers

Properties of polymers in thin films are distinct from those in the bulk due to the significant effects of free or substrate surfaces. The presence of a free surface allows an increased mobility of polymer chains in the near surface region, therefore, a lower glass transition temperature (Tg). With this lower surface Tg, a surface-specific crystallization phenomenon occurring at temperatures much lower than the bulk crystallization temperature (Tc) in polymers including PET, PEN and PVOH has previously been observed. However, whether or not this surface-specific crystallization is a phenomenon observable in all crystallizable polymers is still a question. Similarly, due to this greater mobility, phase separation may also be able to take place in the near-surface region of a polymer blend at a temperature much lower than the bulk phase separation temperature. Yet, no such investigation on polymer blends has been carried out. In addition, it is interesting to study the thin-film behaviours of a block copolymer that undergoes both phase separation and crystallization and compare these with corresponding bulk behaviour. In this thesis, the thin-film crystallization behaviour of polyamide 12 (PA12) in spin-cast films is presented together with some investigation of crystallization of polyamide 6 (PA6) and polystyrene. Polystyrene and poly(methyl methacrylate) (PS/PMMA) systems are used to illustrate the phase behaviours specific to the near-surface region. Finally, the microstructural evolution in high hard block content thermoplastic polyurethane (TPU) thin films on annealing has also been investigated. These TPUs have hard segments (HS) extended by 2 methyl 1,3 propanediol (2M13PD) or 1,5 pentanediol (15PD). With its flexible chains, PA12 crystallizes during spin coating forming as-spin-cast crystals with morphology that varies with solvent evaporation rate and film thickness. Despite the as spin-cast crystals, the free surface allows secondary surface crystallization of PA12 at an annealing temperature (Ta) roughly 20°C below the bulk Tc. The secondary surface crystals were indicated to exist in the most stable crystalline phase of PA12. Similar secondary surface crystallization has also been observed in the PA6 films but at a higher Ta due to the higher Tg of PA6. In addition, surface-specific crystals have been observed in PS (semicrystalline, likely due to some stereoregularity of composition), a polymer with bulky side groups. The PS surface crystals are, however, flat-on oriented showing the important effect of side groups on the morphology or growth shape of surface crystals. The discovery of these surface crystals supports the universality of surface specific crystallization. Using fast solvent quenching, it is possible to "freeze in" a structure containing both PS and PMMA in the near surface region. On annealing, surface-specific phase behaviours (observable as pits, undulations and aggregations) confined to the near-surface region take place first at temperatures around or just below the bulk polymer Tg, while bulk vertical phase separation and dewetting of PS to PMMA, forming holes, network structures and islands, occur at temperatures well above Tg. This surface specific phenomenon, being a result of the free surface, should be applicable to other phase separation systems with a free surface as well. An increase in the crystallinity of PS was found to promote the phase separation process, but the free surface effect is independent of the interplay between the crystallization and phase separation. Rather than having a two-phase morphology, as was previously observed in melt-quenched bulk samples, 2M13PD extended TPU spin-cast films showed a single-phase morphology as-spin-cast. However, the HS ordering, the formation of mesophase, the melting of HS ordered regions, and microphase mixing observed in thin films are consistent with the bulk results but with slightly different transition temperatures due to spatial confinement. With a more flexible chain extender, e.g. 15PD, the hard and soft phase separation is more limited. The thin film investigations have allowed a better understanding of the microstructural evolution in these high hard block content TPUs on annealing by imaging the morphology directly. A thin-film specific phenomenon: formation of large multilayer flat-on crystals, was also observed in these TPU thin films. These crystals are initially developed from preformed aggregations and are believed to be induced by the significant substrate effect in thin films and the free surface effect.This thesis is not currently available in ORA

Lead oxides for photovoltaics

This thesis investigates lead oxides as photovoltaic materials. Vacuum deposition methods and ex-situ annealing are used to produce different stoichiometries of lead oxide. The relationship between structure and the optoelectronic properties is then investigated. Following this, a number of photovoltaic devices are prototyped and a Kelvin probe used to determine and understand the band structure of devices. Thin films of PbO produced via air annealing of thermally evaporated lead consist of a mixture of two phases, orthorhombic and tetragonal, that determine the materials properties and effectiveness as absorber layer in a Schottky device. Films of higher tetragonal content are more photoactive, showing lower series resistance. Kelvin probe reveals that with an increasing work function of the PbO with increasing duration of the annealing, the Schottky barrier between PbO and Al increases, which results in a higher VOC. This trend is inverted when the Fermi level of PbO drops below that of ITO, creating an opposing junction. Reactively sputtered PbO2 films are highly conductive degenerate semiconductors. Increasing oxygen flow rate during deposition leads to increased resistivity and decreased mobility, resulting from a decrease in grain size. Alongside this an increase in carrier concentration is observed as the material gets less ordered at higher oxygen flow rates, which results in an increase in Fermi level. Due to its high conductivity the material is not photoactive, and the high work function between -5.6 and -5.8 eV does not allow the formation of a Schottky junction or a p-n junction with the evaporated p- type PbO. Post deposition annealing of the sputtered films leads to the formation of the more resistive Pb3O4 phase. This material shows lower carrier concentration and mobility, however, work functions are similarly high. The changes induced by the heat treatment are not substantial enough to be able to create a junction between the as-deposited and the annealed material, as is revealed by Kelvin probe and Hall Effect measurements. Heterojunctions between P3HT and Pb3O4 were made to test predictions made by KP measurements. A heat treatment on P3HT improved its electronic properties and raised the Fermi level, resulting in the transformation of a diode in to a photovoltaic device and a decrease in dark current.This thesis is not currently available in ORA

Behaviour of corrosion-protection coatings in light alloys

Anionic chromate (VI) compounds are inhibitive pigments and have been effectively incorporated into organic coatings to protect metal surfaces from aggressive ions, but their risk as a human carcinogen and being harmful to the environment has led to the search of suitable alternatives. Aluminium alloy, AA2024-T3, is the substrate metal alloy used in the experiments and can be found in aircraft fuselage structures due to their high strength-to-weight ratio. However, the presence of intermetallic particles increases susceptibility to localised corrosion. To investigate the protection mechanisms of primers on light alloys, many different factors must be taken into account; from aluminium alloy corrosion processes, the effects of intermetallic additions to coating chemistry, morphology and inhibitive pigments. The chemical environment in which the samples are tested in will also affect the corrosion mechanisms of the alloy as well as the performance of the coatings and release of pigments. It will be important to consider which factors are operating under particular conditions so that experimental results can then be best interpreted. As part of this project, potentiodynamic polarisation, electrochemical impedance spectroscopy and electrochemical noise analysis have been used to investigate the protective mechanisms in which chromate-based paints protect against corrosion and UV-Visible spectroscopy, scanning acoustic microscopy and optical microscopy have been used to investigate pigment release mechanism to identify what characteristics are important when developing new primers.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Advanced optoelectronic characterisation of solar cells

Optoelectronic characterisation techniques are assessed in their application to three solar cell systems. Charge injection barriers are found in PbS/ZnO colloidal quantum dot solar cells through the use of temperature dependent current-voltage and capacitance-voltage measurements. The injection barriers are shown to complicate the Mott-Schottky capacitance analysis which determines built-in bias and doping density. A model that incorporates depletion capacitance and a constant capacitance arising from the injection barriers is given to explain the Mott-Schottky plots. The junction mechanism at the PbS/ZnO interface is found to transition from excitonic to p-n behaviour based on the amount of UV photodoping the cell has received. External quantum efficiency analysis at different photodoping times reveals a growing charge collection region within the material, demonstrating the shift to p-n behaviour. This is further supported by the observance of depletion capacitance behaviour after, but not before, UV photodoping. Defects within GaAs cells containing InAs quantum dots are found to enhance the sub-bandgap performance of the cell using external quantum efficiency analysis. This is verified by illuminated current-voltage analysis using a 1000 nm high pass optical filter to block photons of larger energy than the bandgap. Using capacitance-voltage analysis, high temperature rapid thermal annealing is shown to induce defects in dilute nitride cells, which explains the drop in open circuit voltage compared to lower temperature annealed cells. The doping level of polymer solar cells exposed to air is found to increase with continued exposure using Mott-Schottky capacitance analysis. Current-voltage measurements show the formation of an Al2O3 barrier layer at the polymer/aluminium interface. The usefulness of capacitance-voltage measurements to probe the polymer/fullerene interface is investigated in thermally evaporated thiophene/C60 cells.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The Transitional Heterojunction Behavior of PbS/ZnO Colloidal Quantum Dot Solar Cells

The nature of charge separation at the heterojunction interface of solution processed lead sulphide-zinc oxide colloidal quantum dot solar cells is investigated using impedance spectroscopy and external quantum efficiency measurements to examine the effect of varying the zinc oxide doping density. Without doping, the device behaves excitonically with no depletion region in the PbS layer such that only charge carriers generated within a diffusion length of the PbS/ZnO interface have a good probability of being harvested. After the ZnO is photodoped such that the doping density is near or greater than that of the PbS, a significant portion of the depletion region is found to lie within the PbS layer increasing charge extraction (p–n operation)