Hazel E. Assender

Crystallinity in poly(vinyl alcohol). 1. An X-ray diffraction study of atactic PVOH

Abstract The significant level of crystallinity in atactic poly(vinyl alcohol) results from the comparatively modest size of the OH group enabling it to assume d and l positions in an atactic chain within a single crystal lattice. The structure is highly hydrogen bonded. An X-ray diffraction investigation has been made of the crystal structure to determine the distribution of hydrogen bonds, to understand the crystalline thermal expansion and to determine the influence of water on the polymer crystals and crystallisation process. Thermal annealing treatments have been developed to obtain samples of the maximum level of crystallinity with minimum disorder. The well-resolved diffraction peaks from such samples are analysed to measure the thermal expansion coefficients of the crystal unit cell which are related to the hydrogen bonding network in the crystal structure. Finally, the effect on the crystals of absorbing water into polymer of various levels of crystallinity and also during the crystallisation process is investigated, leading to the hypothesis that water molecules may be incorporated within the random crystal structure resulting in a change in the relative intensities of the diffraction peaks.

Heterodyne force microscopy of PMMA/rubber nanocomposites: nanomapping of viscoelastic response at ultrasonic frequencies

We present measurements of the nanoscale elastic and viscoelastic properties of samples of poly(methylmetacrylate) (PMMA)/rubber nanocomposites. For these studies we have used a new technique based on atomic force microscopy (AFM) with ultrasonic excitation, heterodyne force microscopy (HFM), which provides a means of testing the viscoelastic response of polymeric materials locally (in tip-probed regions) at MHz frequencies. Phase-HFM contrast distinguishes local differences in the dynamic response of PMMA/rubber composites. Comparison of HFM with other AFM-based techniques (ultrasonic force microscopy, friction force microscopy and force modulation microscopy), while imaging the same surface region, emphasizes the unique capabilities of HFM for these kinds of studies, and reveals key nanostructural characteristics of the composites. Some of the toughening particles appear to be broken down, with areas of PMMA detached from the surrounding matrix.

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).

Crystallinity in poly(vinyl alcohol) 2. Computer modelling of crystal structure over a range of tacticities

In Part 1 of this series, the behaviour of the crystalline phase in atactic poly(vinyl alcohol) was discussed. This paper examines the detail of the crystal structure by use of molecular mechanics modelling, and extends the prediction of crystal structure across the whole range of tacticity, the extremes of which have yet to be synthesised. The variation in the crystal structure with changes in tacticity is modelled in the light of experimentally observed changes in physical properties. As the extremes of stereoregularity are approached, novel crystal structures are considered and extensive comparisons made with existing X-ray diffraction data for the limited tacticity range available experimentally. A crystal structure is proposed for a recently synthesised isotactic rich polymer which was observed to have a distinctly different X-ray powder diffraction trace from the atactic material. Finally, the possibility that water may be incorporated into the atactic crystal structure is discussed and the change in the X-ray diffraction behaviour if water is included is simulated for comparison with experimental data.

A numerical model for explaining the role of the interface morphology in composite solar cells

We have developed a numerical model that simulates the operation of organic∕inorganic photovoltaic devices. Using this model, we have investigated the effect of the interface morphology and have shown that for a given system, there is both a most efficient device thickness and the interfacial feature size for overall power conversion. The variation of current-voltage (I‐V) curves with differing recombination rates, anode barrier height, and light intensity has been simulated with reducing the recombination rate and lowering the anode barrier height shown to lead to improved open circuit voltages and fill factors. Through this model, we show that the increase in fill factor observed when the lithium salt Li[CF3SO2]2N is added to devices can be explained by an increase in the polymer hole mobility.

Nucleation and growth of gas barrier aluminium oxide on surfaces of poly(ethylene terephthalate) and polypropylene: effects of the polymer surface properties

The nucleation and initial stages of growth of aluminium oxide deposited on two different polymer surfaces [poly(ethylene terephthalate), (PET) and amorphous polypropylene, (PP)] have been studied by atomic force microscopy (AFM). The permeation of water vapor and oxygen through the films has been measured. The initial stages of the growth of the oxide consisted of separated islands on the polymer surface. Further growth of oxide depends strongly on the surface morphology and chemical nature of the polymer surface. Growth on PET follows a layer-by-layer mechanism that maintains the native surface roughness of the polymer substrate. Growth on PP, however, follows an island mode, which leads to an increase in surface roughness. This may be due to a lack of chemical bonding between the polymer and the arriving metal-oxygen particles. The oxide layer on PET grows more densely than on PP, providing superior barrier to gas permeation

Elastic mapping of heterogeneous nanostructures with ultrasonic force microscopy (UFM)

Ultrasonic force microscopy (UFM) is an atomic force microscopy (AFM)-related technique originally introduced to study the surface elastic properties of stiff materials. We report elastic images of heterogeneous nanostructures with a lateral resolution of the order of a few nanometres. One of the main intentions of this paper is not only to show the capability of UFM to allow one to image surface elastic properties of stiff materials but also to show that UFM can be applied to relatively soft materials with reproducible and interpretable results. The samples presented were chosen over a wide range of stiffness values (with Youngs modulus E = 0.1-400 GPa): very stiff silicon carbide fibres embedded in a mullite matrix, less stiff carbon fibres embedded in an epoxy matrix and relatively compliant rubber inclusions in a polymethylmethacrylate matrix. A discussion of the conditions required to obtain unambiguous data is also provided. Results obtained using the more traditional force modulation mode are also presented and compared with the UFM images of the same samples.

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Controlling the optical scattering of plasmonic nanoparticles using a thin dielectric layer

The effect of a thin dielectric film on the plasmonic behaviour of metal nanoparticles (MNPs) above a high refractive index substrate is explored. Using finite-difference time domain simulations, the optical properties of Ag nanoparticles are investigated as a function of film thickness, refractive index, and particle position within the film. We demonstrate that the addition of a film around a MNP at the air interface of a high-index substrate, where nair<nfilm<nsubstrate, will always increase the fraction of light coupled to the substrate (Fsubs). It is found that placement within a layer that does not conform to nair<nfilm<nsubstrate can lead to reduced enhancements in Fsubs. The principal application for this work is for light-trapping in thin-film solar cells. We show that the inclusion of a thin film can increase the fraction of radiation coupled into the substrate by up to 30% for solar wavelengths. Additional potential benefits of the film structure, such as greater tunability of scattering resona...

Colloidal synthesis of lead oxide nanocrystals for photovoltaics

Lead oxide nanocrystals are synthesised by injecting oxygen gas into an air and moisture free complex of Pb oleylamine and oleic acid in octadecene. Using various characterization methods including fabrication and testing of photovoltaic devices we explore the material properties and photovoltaic application of lead oxide nanocrystal films.