No description available.

No description available.

No description available.

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

In this study, photoluminescence properties of as grown, Ge implanted GaSe and InSe crystals with doses 1013, 1014, and 1015 ions/cm2 and 1015 ions/cm2 Ge implanted and annealed GaSe and InSe single crystals grown by using 3-zone vertical Bridgman-Stockbarger system have been studied by photoluminescence spectroscopy (PL). PL spectra of as grown and implanted GaSe samples with three different doses have been studied in the ranges within the wavelength interval 570-850 nm and in the temperature ranges between 21 and 110 K. Temperature dependencies of all observed bands revealed that the peak with highest energy has excitonic origin and most of the others originate from donor-acceptor pair recombination. For GaSe samples implanted with 1013 and 1015 ions/cm2 Ge, PL spectra exhibited four emission bands while for as grown and the sample implanted with 1014 ions/cm2 v Ge had three bands. Variations of emission peaks were studied as a function of temperature. It was observed that centers of all bands shifted towards red continuously with temperature. The intensities of the emission peaks showed similarities with those obtained from as grown, 1013 and 1014 ions/cm2 Ge implanted GaSe while the peak intensities of the sample implanted with 1015 ions/cm2 Ge decreased with the temperature continuously. Using the temperature variation of the peak intensities and peak energy values activation energies were obtained and these results revealed that the two bands with low wavelength to be excitonic origin for the implanted samples with the doses 1013 and 1015 ions/cm2 Ge. Similar results were obtained for the implanted with 1015 ions/cm2 Ge and annealed sample. The other two peaks observed for these samples were attributed to donor acceptor pair transitions. In addition, direct band gaps were found to be 2.12 eV at 32 K for as grown, 2.121 eV at 25 K for 1013 ions/cm2 Ge implanted, 2.121 eV at 21 K for 1014 ions/cm2 Ge implanted, 2.124 eV at 33 K for 1015 ions/cm2 Ge implanted GaSe samples and lastly 2.113 eV at 28 K for 1015 ions/cm2 Ge implanted and annealed GaSe. PL spectra of as grown, 1013, 1014, 1015 ions/cm2 Ge implanted, and 1015 ion/cm2 Ge implanted and annealed InSe samples were also obtained at 20 K. Two broad bands were observed in the spectrum of all InSe crystals and considered to be due to impurity levels within the materials.

In this study, silicon (Si) rich silicon dioxide (SiO2) films and terbium (Tb) embedded in three different Si containing films has been produced by e-beam evaporation and magnetron sputtering techniques. Post deposition annealing was done for different temperatures and durations to study its effect on both Si nanocrystal formation and Tb luminescence. It was verified by X-ray diffraction technique (XRD) that Si nanocrystals were formed in Si rich matrices. Energy dispersive X-ray (EDS) spectroscopy analysis was carried out to determine the relative concentrations of the atoms inside the produced films. X-ray photoelectron spectroscopy (XPS) gave the evidence of different bonding structures inside the Tb-Si-O containing films. Depth profile measurements were carried out to analyze changes in the relative concentration during sputtering of the layers after annealing of the Tb containing film. Luminescence characteristics of Si nanocrystals and Tb3+ ions were studied by photoluminescence (PL) spectroscopy. It was observed that Tb3+ luminescence enhanced by an energy transfer from Si nanocrystals and trap levels in a matrix. This result supplies valuable information about the excitation paths of Tb3+ ion the way of intense luminescence.

Hybrid carbon fiber reinforced composites are a new breed of materials that are currently being explored and characterized for next generation aerospace applications. Through the introduction of secondary reinforcements, such as alumina nanoparticles, hybrid properties including improved mechanical properties and stress sensing capabilities can be achieved. In order to maximize these properties, it is necessary to achieve a homogeneous dispersion of particulate filler. Utilizing the photoluminescent properties of alumina, it is possible to compare local levels of particle concentration through emission intensities as a way to determine the effectiveness of the tested manufacturing parameters in increasing material homogeneity. Parameters of these photoluminescence emissions have been established to be stress dependent, which allows for in situ residual stress measurements. It is shown here that the application of silane coupling agents as particle surface treatments improves particle dispersion when compared to untreated samples. Reactive silane coupling agent (RSCA) treatments were found to provide for greater dispersion improvements when compared to non-reactive silane coupling agents (NRSCA). Higher resolution investigations into these samples found that treatment with a reactive coupling agent altered the stress state of particles concentrated around the fiber from a tensile stress state to a compressive stress state. This is proposed to result from bonding of the reactive groups on the coupling agent to the organic groups on the carbon fibers which adjusts the stress state of the particle. Future mechanical tests will verify the effects of the particle surface functionalization treatments on mechanical properties of the composites.

No description available.

This thesis reports studies done on single-walled carbon nanotubes (SWNTs) using optical spectroscopy as the primary investigative technique. It focuses on advances in sample preparation which have been made possible through improvements to the method of photo-luminescence excitation (PLE) mapping of nanotubes. An introduction to the field and some theoretical models are presented initially to provide a background to the experimental chapters which follow. A description of the standard procedure for sample preparation in aqueous surfactants is then followed by a detailed introduction to PLE mapping, including modeling of SWNT spectra. The next chapter discusses improvements to the sample preparation method by using organic polymer solutions instead of aqueous surfactants for suspending the nanotubes. The results show reductions in the distribution of SWNT species which are solubilised, leading to significant improvements in the resolution of the optical absorbance spectra and an increased photoluminescence yield. Two experiments which were performed on the novel polymer-SWNT systems are then described. The first shows (via PLE mapping) that energy is transfered to the SWNTs when the polymer is photo-excited. The possible mechanisms behind this, as well as the implications for using carbon nanotubes as an additive in polymer photovoltaics, are discussed. The second experiment details a recent magneto-PL study of SWNTs embedded in films produced from the polymer solutions. Here, the improved optical signatures and absence of strain at low temperatures have revealed a previously unseen high field intensity dependence. The behavior has been explained by the magnetic field induced mixing of the excitonic states.

DC and RF magnetron co-sputtering techniques are one of the most suitable techniques in fabrication of thin films with different compositions. In this work, Al2O3 and Si/Al2O3 thin films were fabricated by using magnetron co-sputtering technique. For Al2O3 films, the stoichiometric, optical and crystallographic analyses were performed. For Si contained Al2O3 films, the formation conditions of Si nanocrystals were investigated. To do so, these thin films were sputtered on Si (100) substrates. Post annealing was done in order to clarify the evolution of Al2O3 matrix and Si nanocrystals at different temperatures. Crystallographic properties and size of the nanocrystals were investigated by X-ray diffraction (XRD) method. The variation of the atomic concentrations and bond formations were investigated with X-ray photoelectron spectroscopy (XPS). The luminescent behaviors of Si nanocrystals and Al2O3 matrix were investigated with photoluminescence (PL) spectroscopy. Finally, the characteristic emissions from the matrix and the nanocrystals were separately identified.