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Exploring processes for industrial large-scale manufacture of low-cost electronics devices is timely for the upcoming era of the Internet of Things. In this study, a roll-to-roll process using selective metallisation (a combination of flexographic printing of an oil liquid-mask and physical vapour deposition: sputtering and evaporation) is explored for the large-scale manufacture of flexible electronics, with a wearable thermoelectric generator as an exemplar device. Two post-treatments (electron beam and infrared) are trialled to remove the residual oil after selective metallisation. Both are effective for the removal of oil, however, the oil used in selective metallisation degrades the functional thermoelectric material severely, causing a dramatic increase in the internal resistance of the device. This is attributable to (1) the doping and oxidation issues in the semiconductor film; (2) the existence of oil at the interface between the two coatings. The fabricated thermoelectric generator with post-treatments shows a good voltage output comparable to a standard device fabricated using shadow mask, providing an option for portable devices requiring a voltage source. This study reports an industrially compatible roll-to-roll manufacturing process for large-scale manufacture of flexible electronics. Further studies are under way to optimise the device performance.

Strain-induced failure is a major concern in wearable electronics. Herein, a ~ 90-nm thick bismuth telluride film is fabricated on 125-μm thick polyethylene terephthalate as the basis for a flexible thermoelectric generator. To simulate induced mechanical strains during wearable operation and roll-to-roll manufacture, both tensile and buckling strains are studied. To improve the mechanical resilience, two types of substrate-to-coating interlayers with varying thicknesses are investigated: 0.4–7.7 μm-thick Poly-tripropyleneglycol diacrylate (acrylate) and 2.5–16.8 μm-thick Poly-dimethyl siloxane (PDMS). The thermoelectric performance of the coating is influenced through the stress, applied on the polymer substrate, being transferred through to the coating. Samples exposed to tensile testing recover once the load is released, mitigating against change on the film morphology and thermoelectric performance, in contrast to those measured in-situ under buckling strain. Both interlayers are shown to assist in maintaining the film performance, as fewer cracks are formed in the semiconductor. A thicker interlayer is shown to better mitigate the impact of deformation of the substrate. Furthermore, in fatigue-bending tests, the film grown on a PDMS interlayer shows better cyclical fatigue performance.

We report here on the time evolution of photoconductivity under continuous illumination in nanocrystalline TiO2 samples prepared by a sol gel method, and also on the conductivity decay once the illumination is switched off. We observe strong dependence of the photoconductivity on the illumination intensity for both processes. It is found that the conductivity decay after high-intensity illumination is slower than after low-intensity illumination, and we have attempted to explain these experimental results using a model involving hole trapping-detrapping processes.

We report the fabrication of a range of organic circuits produced by a high-yielding, vacuum-based process compatible with roll-to-roll production. The circuits include inverters, NAND and NOR logic gates, a simple memory element (set-reset latch), and a modified Wilson current mirror circuit. The measured circuit responses are presented together with simulated responses based on a previously reported transistor model of organic transistors produced using our fabrication process. Circuit simulations replicated all the key features of the experimentally observed circuit performance. The logic gates were capable of operating at frequencies in excess of 1 kHz while the current mirror circuit produced currents up to 18~μ A. © 2014 IEEE.

The mapping of chemically heterogeneous regions in PEA/PS film was achieved by reacting the film in a low pH environment and analyzing by AFM. In 70:30 blend, the domains were identified as PS rich regions and matrix as the PEA rich regions, based on AFM images, FTIR measurements, and chemical modification study. During the course of hydrolysis of PEA, pits were formed in isolated regions of the matrix, as characterized by AFM.

This communication reports on a study of the microstructure, mechanical and gas barrier properties of PECVD metal oxide coatings on heat stabilised (HS) PET [poly(ethylene terephthalate)], primer-coated HS PET, filled PET and PEN [poly(ethylene naphthalate)] substrates. The coatings are highly conformal and exhibit very few large scale defects. Internal stresses in the coatings are found to be compressive for all substrates. The substrate is found to influence some of the mechanical properties with the cohesive strength of the coating greatest for primer-coated HS PET and lowest on filled PET. Secondly, a primer layer also increases the adhesive strength between coating and substrate compared to plain PET and PEN. Substrate surface roughness is found to influence the morphology of the SiO x films but does not seem to adversely affect the barrier performance of the composites. We found that heat stabilised PET, with or without an additional acrylate primer layer and PEN produce superior composites than untreated PET film in terms of water vapour barrier. Using a mass spectrometry detection technique we measured a WVTR value as low as 2 × 10 -4 g m -2 day -1 for 1 μm thick coatings on PEN.

The transparent barrier films presently available on the market all have their strengths and weaknesses. At the same time there is consideration to use more plastic-based materials for different applications, e.g. displays and insulation panels. This situation has stimulated the industry to provide new, more efficient barrier solutions. Some of the innovations go along the following lines: nanocomposite materials; Ormocers®; hybrid PECVD coatings; PML structures; and 'super sapphire' sputtered layers. This extended abstract presents an overview of some of the different approaches, outlining the basic principle behind each barrier technology, its performance and the companies developing and producing the materials.

Time-resolved laser-pump X-ray-photoemission-probe spectroscopy of a ZnO (101 ⎯ ⎯ 0 \ud 101¯0\ud ) substrate with and without PbS quantum dots (QDs) chemically linked to the surface is performed, using laser photon energies resonant with and below the band gap energy of the substrate (λ = 372 and 640 nm, hν = 3.33 and 1.94 eV). Charge injection from the photoexcited QDs to ZnO is demonstrated through the change in the surface photovoltage of the ZnO substrate observed when the heterojunction is illuminated with 1.94 eV radiation. The measured carrier dynamics are limited by the persistent photoconductivity of ZnO, giving dark carrier lifetimes of the order of 200 μs in a depletion layer at the interface. The chemical specificity of soft X-rays is used to separately measure the charge dynamics in the quantum dots and the substrate, yielding evidence that the depletion region at the interface extends into the PbS QD layer.

This thesis studies lead suphide (PbS) colloidal quantum dots and their photovoltaic applications. Different sizes of PbS QDs were synthesised and characterised using absorption spectroscopy and transmission electron microscopes. PbS QD Schottky junction devices were fabricated with AM1.5 power conversion efficiency up to 1.8 %. The Schottky junction geometry limits the device performance. A semiconductor heterojunction using ZnO as an electron acceptor was built and the device efficiency increased to 3%. By studying the light absorption and charge extraction profile of the bilayer device, the absorber layer has a charge extraction dead zone which is beyond the reach of the built-in electric field. Therefore, strategies to create a QD bulk heterojunction were considered to address this issue by distributing the junction interface throughout the absorber layer. However, the charge separation mechanism of the QD heterojunction is not clearly understood: whether it operates as an excitonic or a depleted p-n junction, as the junction operating mechanism determines the scale of phase separation in the bulk morphology. This study shows a transitional behaviour of the PbS/ZnO heterojunction from excitonic to depletion by increasing the doping density of ZnO. To utilise the excitonic mechanism, a PbS/ZnO nanocrystal bulk heterojunction was created by blending the two nanocrystals in solution such that a large interface between the two materials could facilitate fast exciton dissociation. However, the devices show poor performance due to a coarse morphology and formation of germinate pairs. To create a bulk heterojunction where a built-in electric field could assist the charge separation, a TiO2 porous structure with the pore size matching with the depletion width was fabricated and successfully in-filled by PbS QDs. The porous device produces 5.7% power conversion efficiency, among one of the highest in literature. The enhancement comes from increased light absorption and suppression of charge recombination.

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.