Asymptotic enumeration of sparse uniform hypergraphs, with applications

A hypergraph is a generalisation of a graph where the edges may contain more than two vertices. This thesis concentrates on asymptotic enumeration of some families of sparse uniform hypergraphs, when the number of vertices is sufficiently large and each vertex degree is given. Our first result gives an asymptotic formula for the number of simple uniform hypergraphs with a given degree sequence which contain no edges of another specified hypergraph. This formula holds under some restrictions on the number of forbidden edges, and the maximum degree and edge size of the hypergraph. We apply a combinatorial argument known as the switching method to obtain our estimates.This formula allows us to calculate the probability that a random uniform hypergraph with given degrees contains a specified set of edges. As a result, we find an asymptotic formula for the expected number of perfect matchings and the expected number of loose Hamilton cycles in random regular uniform hypergraphs. As another application of our first result, we study the average number of uniform spanning hypertrees.Finally, we give a new proof of Blinovsky and Greenhill's asymptotic formula for the number of sparse linear uniform hypergraphs with given degrees. Our proof uses a more complicated switching and extends the range of degrees and edge size for which the formula holds

Tutorials-Based System to Assist Qatari Children with Special Needs

The objective of this project is to develop an Arabic predictive communication system for physically disabled people with speech and language impairments who are resident at the Shafalah Qatari center for children with special needs in Doha. We aim to improve the learning and understanding skills of children with special needs. We aim to develop graphics-based and interactive tutorials. The main topics of these tutorials include: human body, transportation means, time and date, animals, food and plants. We design some intelligent puzzles to be assembled by the disabled children. Greedy algorithms will used to guide the potential users to reach a solution. The disabled children will be asked to arrange a set of simple words and phrases to form a meaningful sentence on the tutorials' topics that they learnt. The existing augmentative systems for disabled children are solely designated for western children (design, interface and contents). Thus the development of an Arabic augmentative communication system taking into consideration the culture and specificity of Qatari disabled children will be an important tool that helps them to overcome communication problems and access independently digital information

Thermally-Induced Degradation in PM6:Y6-Based Bulk Heterojunction Organic Solar Cells

Thermally induced degradation of organic photovoltaic devices hinders the commercialization of this emerging PV technology. Thus, a precise understanding of the origin of thermal device instability, as well as identifying strategies to circumvent degradation is of utmost importance. Here, it investigates thermally-induced degradation of state-of-the-art PBDB-T-2F (PM6):BTP (Y6) bulk heterojunction solar cells at different temperatures and reveal changes of their optical properties, photophysics, and morphology. The open-circuit voltage and fill factor of thermally degraded devices are limited by dissociation and charge collection efficiency differences, while the short-circuit current density is only slightly affected. Energy-resolved electrochemical impedance spectroscopy measurements reveal that thermally degraded samples exhibit a higher energy barrier for the charge-transfer state to charge-separated state conversion. Furthermore, the field dependence of charge generation, recombination, and extraction are studied by time-delayed collection field and transient photocurrent and photovoltage experiments, indicating significant bimolecular recombination limits device performance. Finally, coupled optical-electrical device simulations are conducted to fit the devices’ current-voltage characteristics, enabling us to find useful correlations between optical and electrical properties of the active layers and device performance parameters

Influence of thermal annealing on microstructure, energetic landscape and device performance of P3HT:PCBM-based organic solar cells

Thermal annealing alters the morphology of organic donor-acceptor bulk-heterojunction thin films used in organic solar cells. Here, we studied the influence of thermal annealing on blends of amorphous regio-random (RRa) and semi-crystalline regio-regular (RR) poly (3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C _60 -butyric acid methyl ester. Since the P3HT:PCBM blend is one of the most studied in the OPV community, the existing research provides a solid foundation for us to compare and benchmark our innovative characterization techniques that have been previously under-utilized to investigate bulk heterojunction organic thin films. Here, we combine advanced novel microscopies and spectroscopies, including polarized light microscopy, photo-deflection spectroscopy, hyperspectral photoluminescence imaging, and energy resolved-electrochemical impedance spectroscopy, with structural characterization techniques, including grazing-incidence wide-angle x-ray scattering, grazing-incidence x-ray diffraction, and Raman spectroscopy, in order to reveal the impact of thermal annealing on the microstructural crystallinity and morphology of the photoactive layer in organic solar cells. Coupled transfer matrix and drift-diffusion simulations were used to study the impact of the density of states on the solar cells’ device performance parameters, namely the short-circuit current ( J _SC ), open circuit voltage ( V _OC ), fill factor (FF), and power conversion efficiency (PCE)