Peyman Servati


Relevant Thesis-Based Degree Programs

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Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Design and evaluation of textile electrodes for biological signal monitoring (2023)

Developments in electronic textiles (e-textiles) have focused on integration into remote physiological monitoring systems. This has encompassed functionalized textile sensors for continuous biopotential signal collection. This research examines structural hierarchical effects in e-textiles to understand properties influencing the performance of e-textile sensors. Electrocardiography (ECG), a bioelectric signal associated with electrical activity of the heart muscle, was selected for evaluating e-textile electrode performance.The structure-property relationships of fibre and yarn to fabric systems were investigated for silver (Ag)-nylon textiles. Fibre geometry and yarn twist were found to impact yarn linear resistance, and electrode contact impedance at the fabric level up to limiting values. Influencing factors at the yarn level were explained by contact between fibres in the yarn structure, yarn specific volume, fibre volume fraction, and orientation angles. At the fabric level, influencing factors included yarn diameter, fibre packing, and apparent contact area of the electrode. Skin-electrode interfacial interaction, and relationship to biopotential signal quality were characterized through surface roughness and skin contact impedance measurements of knitted electrodes. Compatibility at the skin-electrode interface was impacted by skin hydration, roughness, and applied pressure. A high throughput roll-to-roll system was developed to produce electrochemically coated silver/silver-chloride (Ag/AgCl) yarns, and the process was characterized. Electrodes were fabricated through embroidery of Ag/AgCl yarns. The performance of Ag/AgCl e-textile electrodes was compared against embroidered Ag e-textiles, and standard Ag/AgCl electrodes. The Ag/AgCl e-textile electrodes demonstrated excellent ECG signal quality, and high stability based on polarization potential measurements when compared to Ag e-textiles. Durability of e-textiles for long-term use was addressed through the development of a stretchable, self-healing, biocompatible and recyclable chloride-based ionogel coating. The material was employed as a cast film, and sprayable e-textile coating, applied to strain sensing and biosignal recording, respectively. The ionogel film demonstrated high recovery of material properties after a 30-minute healing period at room temperature and reformed after abrasion to a coated e-textile surface by the addition of ethanol. Electrical properties of ionogel coated electrodes met specified performance ranges for e-textile electrodes. The methods and findings presented in this dissertation can be used to guide e-textile electrode materials selection and design.

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Chemical vapor deposited single layer graphene as transparent electrodes for flexible photovoltaic devices (2019)

Graphene has attracted intensive attention for various electronic applications in the past decades given its unique properties. The synthesis of graphene by chemical vapor deposition (CVD) on copper foil provides the opportunity to deliver large-area, high quality, and continuous graphene films. The metal foil can be removed with a wet etching process. The transferred CVD graphene films can be integrated into existing semiconductor device manufacturing platforms, or into low-cost roll-to-roll manufacturing of flexible electronics. Since graphene is a two-dimensional material, the optical, mechanical, and electrical properties can easily be altered with surface modification. Copper etchants used in graphene transfer process can lead to films with different levels of doping and mechanical strength. The topology and temperature dependent electrical properties of transferred graphene using three different etchants were investigated. All of the graphene samples demonstrate a doping level above 10¹³ cm-³. The graphene films prepared with cupric sulfate solution presents the most uniform and continuous layer, with the least density of defects. Metallic and organic residues, defects and grain boundaries, as well as intercalated water molecules, attribute to the variation in conductivity and permittivity of the films. By coating the films with charge selective materials, graphene sheets with improved sheet resistance and transparency of about 90% were fabricated. The hole-selective transparent conductors show about 50% reduction in resistivity. All the samples demonstrate high stability with repeated bending of over 800 cycles. Organic photovoltaic (OPV) devices using the hole-selective graphene transparent conductors as electrodes were fabricated on plastic substrates. Less than 5% fluctuation in power conversion efficiency (PCE) was noticed when the devices were bent up to 130 degrees. As an extension of this work, the photovoltaic characteristics of inverted OPV devices fabricated with AlxZn(1-x)O as an electron transport layer with Al fraction of up to 11% were reported. The light-soaking effect can be eliminated by using more than 4% of Al doping. All devices demonstrate PCE over 3.4% with air-stability of over 150 days. The light-soaking mechanism is investigated by employing a numerical simulation on the devices.

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Flexible film and breathable textile electrodes for electrodermal activity monitoring (2018)

The field of research on wearable systems that monitor human biological responses for healthcare applications is constantly advancing. Electrodermal activity (EDA) is related to the neurological system and is a result of the autonomic nervous system being stimulated, which produces sweat on the surface of the skin, thereby changing its electrical characteristics. The current clinical devices used to monitor EDA utilize rigid and non-breathable silver/silver chloride (Ag/AgCl) electrodes, possibly in combination with gels and irritating adhesives. The research detailed in this dissertation is focused on advancing our understanding of the design and development of comfortable, flexible and breathable EDA electrodes. Flexible dry Ag/AgCl electrodes were fabricated on a compliant substrate with various surface areas, distances between and geometries. The flexible electrodes were systematically characterized to determine their ability to detect EDA stimulus responses and these were compared to the responses simultaneously collected by rigid dry Ag/AgCl electrodes. The data demonstrated that surface area, spacing and geometry of electrodes affected the detection of the EDA stimulus response. The minimum number of sweat glands to be covered by flexible EDA electrodes has been estimated at 140 to maintain functionality. The optimal design of flexible electrodes is a serpentine geometry (0.15 cm² surface area, 0.20 cm distance).Ag/AgCl electronic yarns were developed through a novel roll-to-roll system and integrated into textile substrates of cotton, nylon and polyester. The EDA stimulus responses detected by dry electronic textile (e-textile) electrodes at various locations on the hand were compared to the EDA signals collected by dry solid Ag/AgCl electrodes. The cotton textile substrate with e-textile electrodes (0.12 cm² surface area, 0.40 cm distance) was the optimal material to detect the EDA stimulus responses. Also, differences with EDA waveforms recorded on various fingers were observed. Trends of long-term measurements showed that skin surface temperature affected EDA signals recorded by non-breathable electrodes more than when e-textile electrodes were used. The effects of electrode design and material for flexible and breathable EDA electrodes detailed in this dissertation can promote the development of effective and wearable EDA monitoring systems, which can help improve our knowledge of the human neurological system.

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Charge-selective transparent conductors for solution-processed organic solar cells (2017)

Organic solar cells (OSCs) become increasingly popular for harvesting solar energy, due to their potential for low-cost manufacturing and mechanical flexibility. As the efficiency of laboratory-scale devices increases, developing materials and processes that would enable low-cost roll-to-roll fabrication of such devices gains increasing research interest. In order to promote OSCs as a viable substitute for silicon-based solar cells, it is necessary to synthesize materials that can offer high performances, roll-to-roll processability, and potential for flexibility, via processes that are scalable, and do not rely heavily on costly fabrication conditions, such as high temperature, vacuum processing, or inert atmospheres.This research is focused on two related aspects within this goal. The first part of the research concerns the fabrication of highly stretchable transparent conductive electrodes (TCEs) as replacement for conventional indium tin oxide (ITO) TCEs. Sparse meshes of metallized polyacrylonitrile nanofibers (NFs) fabricated via the scalable electrospinnig method are used to realize TCEs with performances comparable to ITO electrodes (sheet resistance, Rs, of 155 Ω/□, with transparency, T, of 95%) and with unprecedented electromechanical stretchability (only 56% increase in resistance at 100% strain). Furthermore, by incorporating the metallized NFs into matrices of solution-processed, charge-selective layers, composite charge-selective TCEs are fabricated. Annealed at the appropriate temperature, these charge-selective TCEs achieve performances superior to ITO and on a par with the uncoated NF TCEs. Using ZnO as the matrix, electron-selective composite TCEs with Rs = 23 Ω/□ at T = 95% are fabricated. Using MoO₃, hole-selective TCEs with Rs = 35.5 Ω/□ at T = 94% are obtained. The second part of the research, involves the fabrication of OSCs using a scalable, low-temperature spray-coating process in air. By employing an accelerated air drying post-deposition stage, we were able to achieve large-area pinhole-free coatings of P3HT:PCBM layers with sub-nanometer surface roughness, through single-pass spray-coating at 25 °C substrate temperature. OSCs fabricated in air through this process, achieve power conversion efficiencies up to 2.57%, comparable to the reference devices fabricated via spin-coating in nitrogen atmosphere. The introduced process is successfully used to fabricate fully-sprayed, as well as large-area OSCs.

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Computationally Efficient Modeling and Analysis of Conductivity and Sensitivity of CNT/Polymer Composites (2015)

In this PhD thesis, an efficient model of conductivity for carbon nanotube (CNT)/polymer composites is developed, considering the effect of intertube tunneling through polymers, and the eletromechanical properties of the composites are estimated. The statistical nature of intertube distance is first investigated through numerical analysis for both two dimensional and three dimensional networks of CNTs. Considering theintertube distance as the key parameter of tunneling effect, analytical models are developed for tunneling conductivity at the CNT junctions and the overall conductivity of the composites. The model of composite conductivity provides significantly lower computational cost as compared to the numerical resistive network models, withreasonable accuracy. By incorporation of electron tunneling effects, this model also provides closer approximation to experimental results in comparison to the models based on percolation theory, which are highly relevant for filler/polymer composite applications designed around the percolation threshold.Using the conductivity model, the sensitivity of the composite films is estimated in presence of an organic gas. The change in the film resistance due to gas absorption is investigated for different CNT and gas concentrations. From the phase of reflected radio frequency (RF) signal, the wireless gas sensitivity is estimated for a lossless transmission system terminated with a composite film as the load. Films with lowerfiller concentration is found to have higher gas sensitivity and higher wireless sensitivity within a low range of gas pressure. This work is useful for design and development of biohazard gas sensors for real-time remote monitoring.Furthermore, the sensitivity of the composite films is estimated under the application of tensile strain. The influence of varying film thickness on the intertube distance in composite films is analyzed numerically. Then our analytical model is employed to estimate the composite conductivity and film sensitivity under mechanicalstrain. The partial alignment of CNTs introduced by the film thickness less than the CNT length is observed to have significant influence on the composite conductivity and strain sensitivity specially at low CNT concentration. The numerical results are compared with literature reports and experimental results. This work is helpful forstrain sensing and stretchable switching applications.

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Optical and Electronic Properties of GaAsBi Alloys for Device Applications (2015)

GaAs1-xBix is a new III-V semiconductor alloy that shows promise for many optoelectronic applications. In this thesis, several characterization techniques were used to explore the properties of molecular beam epitaxy grown GaAs1-xBix alloys in a wide range of Bi-content.The fundamental bandgap and the optical absorption coefficient of pseudomorphic GaAs1-xBix/GaAs films are studied by optical transmission and photoluminescence spectroscopies. All GaAs1-xBix films (0≤x≤17.8%) show direct optical bandgaps. The bandgap (Eg) decreases strongly with increasing Bi-content, reaching 0.52 eV (~2.4 µm) at 17.8% Bi. At Eg
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Zinc oxide nanostructures for sensing and photovoltaic devices (2014)

In this PhD thesis, vertical arrays of zinc oxide (ZnO) nanowires (NWs) are synthesized in a CVD system and then deposited on patterned electrodes using dielectrophoresis (DEP). The nanowire devices illustrate 4 orders of magnitude increase in conductivity when exposed to ultra violet (UV) irradiation of 1220 μW/cm². The UV response has a fast component, due to electron-hole generation, as well as a slower component, attributed to the release of oxygen. Moreover, due to the increased electron density in the presence of UV, the type of oxygen species on the surface of ZnO changes to more reactive negative ions. In addition, when the pressure is decreased to 0.05 mBar, the conductivity of the NWs increases ∼ 2 and 3.5 times for NWs with 300-nm and 100-nm diameter, respectively. For the first time, UV irradiation is used to improve the carbon monoxide (CO) sensing properties of ZnO. When exposed to 250 μW/cm² UV irradiation, not only the sensitivity increases more than 75%, but also a repeatable and recoverable response is obtained, which is due to formation of more reactive oxygen ions. For the same reason, when the temperature is elevated, higher sensitivity to CO is achieved. The devices demonstrate exponential sensitivities of more than 5 decades to 60% increase in relative humidity (RH) at room temperature, which is a record for ZnO NW based RH sensors.A novel, low-cost and simple technique is developed for fabrication of sensors based on solution processed ZnO nanoparticles (NPs) by simply sketching the electrode lines and painting the NP ink. Sensors show 2000 times increase in conductivity when exposed to 1220 μW/cm² UV irradiation and more than 200% increase in current when exposed to 5-mins of CO pulse at room temperature.Furthermore, this thesis presents efficient (3.8%) inverted organic photovoltaic devices based on a P3HT:PCBM bulk heterojunction blend with improved charge-selective layers. ZnO NP films with different thicknesses are deposited on the transparent electrodes as a nano-porous electron-selective contact layer. The optimized inverted devices show exceptional short circuit current, which is related to increased quantum efficiency.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Machine learning based gait parameter analysis for normal and pathological patterns (2023)

The pervasiveness of wearable sensors has contributed to plenty of daily activity data and greatly improved the ability to link data analytics with healthcare applications. Gait analysis is an important part of the healthcare field to help monitor the patient’s progress with gait disturbances such as Stroke and Parkinson’s. The traditional method to get gait parameters is the GaitRite walkway, which is cumbersome and requires professional training in setting up every time. With the assistance of wearable sensors, we are able to record the motion of the foot without those limitations. Zero Update Position and Timing (ZUPT) has been used to analysis of normal people’s walking for a long time. However, ZUPT has been validated on the walking of normal people only. In this article, we test the ZUPT on pathological patients. We compare the results of normal and abnormal walking patterns in terms of step recognition and gait parameter prediction accuracy. To help track and monitor patient disease progress with better accuracy, we propose Machine Learning (ML) algorithms to improve the accuracy of healthcare application estimation results based on the time series data. Those patients also suffer from gait impairments which may cause fatal dangers such as falls. In the meantime, gait parameters are also significant signs of the progress of the disease. To monitor the gait condition, we collect datasets with different gaits, comparing and predicting gait parameters for the patients. Compared to the ZUPT algorithm, the final prediction from ML algorithm of gait parameters achieves greater performance.

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A wearable sensor system to relate thermoregulatory signals to comfort (2022)

Several metrics for assessing human thermal response have been proposed in the literature, with skin temperature (TSkin) and skin relative humidity (RHSkin) being among them. These metrics aim to describe human thermal perception of their thermal environment. Traditional parameter analysis involves multiple expensive, complex, invasive devices that require unnatural adjustments to the user, may result in poor signal quality, and provide unreliable feedback on thermal status. We tested a wearable system containing TSkin and RHSkin sensors that can perform thermoregulatory evaluation that will circumvent the limitations of the traditional thermal analysis systems.19 healthy adults completed validation testing in a high intensity exercise protocol with our wearable sensor system. Band sensors were validated against two types of iButtons (Hygrochrons for relative humidity, Thermochrons for temperature). The comparisons were done between the Bands and three different averaged locations of iButtons: abdomen, torso, and whole body. Pearson’s correlation and Bland Altman Analysis were used to compare the Bands to the Hygrochrons. Except for the abdomen location, the RH’s calculated by our wearable sensor system showed high to very high correlation with the Hygrochrons (r = 0.862 to 0.967) but due to the large error cannot be used as a reference. For TSkin, a t-test found that the only non-significant differences were between the Bands and the whole-body averages of Thermochrons. One-Way ANOVA testing found that the location of Hygrochrons and Bands led to significantly differences in RH measurements (p
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Evaluation of stability and performance of an air-processed all-inorganic perovskite solar cell (2022)

In the past decade, perovskite solar cells have attracted many concerns with the features of easy fabrication methods, high energy conversion efficiency, and relatively lower fabrication cost. Remarkable progress was witnessed on the power conversion efficiency from 3.8% to 25.5%, but the degradation issue is still one of the main barriers to the commercialization of perovskite solar cells. Using the stable CsPbBr₃ as the photoactive material is regarded as a promising solution to improve device stability, however, degradation is also caused by other components in a device. The widely used Spiro-OMeTAD hole transporting material is expensive. It also needs the addition of p-dopants such as LiTFSI and 4-tBP to reach its peak performance. These additives exhibit poor stability against the harsh environment. An inexpensive p-type semiconductor CuSCN can be an ideal alternative, owing to its chemical stability and unique electronic properties. In this work, the potential of dopant-free CuSCN acting as a stable hole transporting material for CsPbBr3 solar cell was evaluated. High-quality CsPbBr3 film was prepared through an optimized two-step solution method and incorporated into all-inorganic perovskite solar cells with low-temperature solution-processed CuSCN film. The entire fabrication process was completed in an ambient environment. The best device delivered a power conversion efficiency of 5.55%, with superior air stability, ultraviolet stability, and a wide operating temperature from -20 °C to 160 °C. However, a faster degradation was witnessed during long-term thermal aging. This work demonstrates that improved stability and suitable photovoltaic performance can be achieved by using CuSCN as the hole transporting material of CsPbBr₃ perovskite solar cell, in comparison to other reported results in the literature.

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A wearable inertial sensor system for gait and balance monitoring (2021)

Healthy gait and balance are essential for independent living and maintaining a high quality of life. Unfortunately, people’s gait and balance may be compromised by disease, injury, and age-related factors. Immobility can lead to the loss of independence, social isolation, comorbidities, and a decreased quality of life. Traditional gait analysis requires expensive motion capture laboratories equipped with sophisticated optical motion capture systems that require the patient to be physically present in the lab. We developed a wearable sensor system containing inertial measurement units (IMU) and signal processing algorithms that are capable of performing gait analysis and balance monitoring that will circumvent the limitations of camera and floor-based gait analysis systems. 16 healthy adults completed validation testing with our wearable gait analysis system. Reliability (intraclass correlation coefficients) and validity testing (Pearson’s coefficients) were used to compare IMU measured values to ground truth values. The gait parameters calculated by our foot worn IMU system showed good to excellent reliability (ICC₂,₁ = 0.75 to 1.00) and high to very high validity (r = 0.70 to 1.00) for nearly every gait parameter compared to ground truth validated parameters. 13 healthy participants completed balance testing whereby IMUs on the upper and lower back measured 11 balance parameters during poses of increasing difficulty. One-Way ANOVA testing compared parameters between poses and Pearson’s coefficients and Mann-Whitney U testing compared parameters from IMUs on the upper and lower back. ANOVA testing revealed that for both IMUs, nearly all the balance parameters could detect significant (p
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The effect compressed carbon nanofibres made from lignin as electrodes have on the performance of a supercapacitor (2021)

Anthropogenic climate change entices civilisation to adopt renewable energy that requires electric storage due to its intermittent nature. Batteries are traditionally responsible for storing electricity because of their large density of energy but they are expensive because they degrade significantly faster than supercapacitors. However, supercapacitors store a lower density of energy. Like renewable energy, these devices must minimise their ecological footprint and enforce the concept of circular economy but the current production of both batteries and supercapacitors has much room for improvement. Increasing the area of supercapacitors' electrodes is key to producing more energy. These are made of nanofibres of carbon because they are conductive and hold a large surface. They are made from polymers traditionally produced from fossil fuels. However, recent research successfully replaced these expensive ingredients with a polymer wasted by the industry of pulp and paper. Instead of burning the waste to supply the industry with energy, and releasing greenhouse gases to the atmosphere, it is possible to simply convert the polymer to carbonic nanofibres at scales thanks to electrospinning and thermal treatment. However, their performance remains inferior to those reported in literature. Chemically doping the carbonic nanofibres is a popular option with extensive presence in literature but increasing the surface is a simple mean to also increase the capacity of storing energy. Electrodes made from nanofibres are so porous that much of their volume is void. Compressing the nanofibres to roughly half their volume prior to thermally treating the carbonic nanofibres translated in an increase of 2.21 fold in density after the thermal treatment. The surface of the samples per unit volume increased by 2.34 times yet the volumetric capacitance decreased by 78% even though the resistivity decreased by 56%. First material characterisation on the samples identifies the presence of carbonic nanofibres through measurements of conductivity, Raman spectroscopy, X-ray diffraction and Scanning Electron Microscopy. Then, electrochemical characterisation evaluates the performance of the samples in a device, including Cyclic Voltammetry for 10,000 charging and discharging cycles, Galvanostatic Charge and Discharge, and Electrical Impedance Spectroscopy to evaluate their performance in a supercapacitor.

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A thin-film of mechanochemically synthesized nanoparticles: an experimental and theoretical exploration of an absorber compound for photovoltaic devices (2017)

Nearly 200 years after the discovery of the photovoltaic effect, harvesting energyfrom the sun is finally becoming a price competitive marketing option forpower generation. Government and private investments, motivated by a socialawareness of environmental issues cause by prominent power generation methods,have helped create this opportunity to advance earth conscientious, greenenergy solutions. As inorganic nanoparticles in solar cell layers are one of theforefront areas of interest for solar cell research, mechanochemical material synthesishas been used for a scalable production of Fe₂GeS₄ nanoparticles carriedout via ball milling. The compound is composed of earth-abundant materials, andball milling allows for a solution free process, which minimizes chemical wastefrom material synthesis. The viability of this promising compound has been previouslymentioned and herein confirmed. X-Ray Powder Diffraction (XRD) showeda successful synthesis, and optical characterization confirmed favorable absorptionproperties for solar cell implementation. New methods were implementedin doping the nanoparticles, which lead to an observable photovoltaic responsefrom a simple prototype architecture implementing the Fe₂GeS₄ nanoparticles.The thin film deposition of the nanoparticles used for prototype implementationshould allow for cost effective and scalable manufacturing.Since ball milling is also cost effective and scalable, an empirical model implementingprobabilistic logic is developed and shown as capable to fit experimentaldata via measurable parameters. The eventual optimization possibilitiesfor minimizing manufacturing costs, as well as enhancement of general scientificunderstanding for an underrepresented branch of theory, mechanochemical solidstate reactions, motivated this work. Modeling of Fe₂GeS₄ production, as a solidstate chemical reaction, demonstrates a proof of principle application. Potentialapplications are not limited to mechanochemical synthesis. Extensions to otherreaction types are possible as the model utilizes chemical kinetics theory in a generalizedfashion. The demonstration focuses on a sigmoid trend, as observed inFe₂GeS₄ synthesis, though other profiles are attainable.

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Engineering high performance electrodes for energy storage devices from low-cost, sustainable and naturally abundant biomaterials (2017)

With the increased global push towards sustainable energy utilization, the need for advanced energy storage technologies has become increasingly important as countries seek to integrate rapidly advancing renewable energy technologies like wind and solar. At the same time, the burgeoning electric vehicle and wearable electronics industries are fuelling demand for lower-cost energy storage devices with high energy capacities and longer cycle lives. Currently, despite huge leaps in performance of Li-Ion batteries in recent years, the technology is approaching its predicted limits and new solutions will be needed to keep up with the demand of current and future electrical devices. At a time where scientific applications of nanomaterials and nanofabrication is on the rise, there exists an opportunity to take advantage of our increased understanding of nanotechnology to significantly improve existing energy storage devices and to unlock the potential of next-generation energy storage technologies. In this work, binder-free and porous graphitic nanofibre electrodes produced from low-cost and sustainable softwood kraft lignin are devised and proposed as a platform for the development of high-performance energy storage devices. Motivated by difficulties facing some key energy storage technologies, scalable electrospinning of lignin and polyethylene oxide (PEO) precursor materials, followed by a hydrothermal treatment and carbonization in an inert atmosphere yields free-standing interconnected nanofibre electrodes with tunable porosity, high conductivity and superior electrochemical performance. Electrical impedance spectroscopy measurements of the optimized porous nanofibre electrodes demonstrate a conductivity reaching 18.39 S cm-¹, while Brunauer-Emmett-Teller specific surface area measurements yield a specific surface area as high as 1258.41 m² g-¹. Supercapacitor devices revealed highly symmetric cyclic voltammetry results which demonstrated a gravimetric capacitance approaching 112 F g-¹ at a voltage scan rate of 5 mV s-¹. Galvanostatic charge/discharge experiments show reversible supercapacitor behaviour, a high capacity even at elevated voltage scan rates up to 200 mV s-¹ and exhibit excellent cyclic stability, retaining 91% of their initial capacity after 6000 cycles. This work demonstrates the use of sustainable and abundant softwood Kraft lignin as source for porosity-tunable electrodes with high capacitance and stability as a demonstration of nature sourced and high performance electronic devices.

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Highly flexible top-emitting phosphorescent organic light emitting diodes (OLEDs) (2014)

Organic Light Emitting Diodes (OLEDs) have become attractive for flat panel display industry, with applications ranging from mobile phone screens to TVs. They have several advantages over inorganic LEDs such as high contrast ratio, wide viewing angle, faster response time, scalable large area processing and most importantly mechanical flexibility. OLEDs on flexible substrates can endure certain level of mechanical deformation such as bending, rolling or folding without disruption of the performance. The current demonstrated flexibility for OLEDs is up to a few centimeters or millimeters bending radius, depending on the materials, substrates and device structures. More flexible OLEDs with bending radius of curvature on the order of microns will be needed for applications in wearable, roll-up, or foldable displays and bezel-free screens and flexible signage systems. This thesis presents the design, fabrication and characterization of highly flexible and foldable top-emitting OLEDs made on 50 micron thick polyimide (PI) plastic substrates, which can achieve approximately 200 microns bending radius of curvature (folding) without visible damage or impact on emission brightness and uniformity. To the best of our knowledge these are the most flexible phosphorescent OLEDs and first foldable OLEDs ever reported. We believe such flexibility is the benefit of the mechanical stability and low film thickness of the PI substrate. The surface roughness of PI had been the major limitation of its application as OLED substrates, and in this thesis a special side-angle evaporation method is proposed to improve the step coverage of deposited thin films of materials on PI without the requirement of buffer layers. The same method is also proved to be applicable for fabricating OLEDs on much rougher substrates such as Scotch tapes, and fiberglass and transparency sheets. The OLEDs fabricated on above substrates are also presented and characterized.

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Integrating graphene and nanofibers with silicon to form Schottky junction solar cells (2013)

Since the development of the world’s first practical solar cell in 1954 at Bell Laboratories, researches have been conducted to increase solar cell efficiencies and lower the fabrication cost. Traditional Schottky junction solar cells suffer from the low transparency of metal films and increasing cost of indium tin oxide. In this thesis, p-type and n-type silicon Schottky junction solar cells are fabricated by integrating novel materials with silicon in an attempt to overcome these limitations. The p-type solar cells integrate graphene and p-type silicon. Graphene is first synthesized using scotch tape exfoliation method, and then using chemical vapor deposition (CVD) of methane on copper foils to improve its quality. The CVD graphene growth system is custom built in our lab. Graphene films are optically and electrically characterized and solar cells are fabricated. Measured solar cell characteristics results are presented and reasons for the obtained parameters are discussed. Finally, methods for improving the solar cell performance are described. The n-type solar cells are fabricated by depositing gold coated Polyacrylonitrile (PAN) nanofiber mesh on top of n-type silicon. Schottky junctions are formed where the nanofibers are in contact with silicon surface, and each junction contributes to the total current. The nanofibers are economically produced by electrospinning and coated with gold by sputtering. The solar cells are characterized and the results suggest this structure can be a promising candidate for photovoltaic application. In addition to experimental work, we conduct numerical simulations of graphene based Schottky junction solar cells to identify possible future applications of graphene. Copper indium gallium diselenide, cadmium telluride, and amorphous silicon are chosen as the semiconductor bases because of their high absorption coefficient, high/tunable bandgap, and the possibility for economical fabrication as compared to single crystal silicon technology. The simulation is carried out using MATLAB with material properties obtained from textbooks and published literatures. The simulation results provide an estimate of the relevant photovoltaic parameters. It identifies graphene/p-type cadmium telluride as a potential Schottky junction solar cell that can achieve a conversion efficiency of 11.3%, if the graphene sheet resistance of 30 ohms/square and transmittance of 90% can be attained.

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Degradation of P3HT:PCBM-based conjugated polymer solar cells (2011)

This work examines the effect of regioregularity (RR) and zinc oxide (ZnO) nanoparticle doping on the degradation of poly(3-hexylthiophene) (P3HT):6,6-phenyl C₆₁-butyric acid methyl ester (PCBM) organic solar cells. This is done through application of semi-compact models that relate experimentally measured transport characteristics to structural properties. In this way, the contribution of regioregularity and ZnO nanoparticles to the change in structuralproperties can be quantified. These models allow interpretation of experimental data and insight into the underlying degradation mechanisms. In this thesis, the mobility edge model is used, and corresponding parameters such as effective electron and hole mobilities are extracted and compared. These results show that studying electron transport plays a critical role inunderstanding the degradation of P3HT:PCBM solar cells.Examination of regioregular devices reveals that the drop in effective electron mobility with annealing for the high RR devices is greater than that of the low RR ones. This is attributed to the greater tendency for crystallization-driven phase segregation in blends of 98% RR P3HT and PCBM. In hybrid polymer-ZnO devices, effective electron mobility improves with the addition of an optimal concentration of ZnO. The decline in electroneffective mobilities with annealing is smaller for the devices containing ZnO in comparison to devices without ZnO. Studying the morphology of these devices shows that the phase segregation is identical for devices with and without ZnO.

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