Peyman Servati


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
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 (2010 - 2018)
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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Calcium-decorated boron-doped graphene for high-capacity hydrogen storage (2009)

Hydrogen has been viewed as a clean synthetic energy carrier that could replace fossil fuels, especially for transport applications. One bottleneck in developing a hydrogen economy is to find feasible and safe storage materials that can store hydrogen with high gravimetric and volumetric densities at ambient conditions. The U.S. depart ment of energy has set a system target of 6 wt.% hydrogen storage density by 2010 and 9 wt.% by 2015, which has not been met yet. In this thesis, hydrogen adsorption and storage in calcium-decorated boron-doped graphene is studied by ab initio calculations using density functional theory (DFT).We first consider pure graphene coated with calcium atoms on both sides, supposing that metal atoms are dispersed uniformly on the surface with a calcium coverage of 25%. We find that up to four hydrogen molecules can bind to a Ca atom, which results in a storage capacity of 8.32 wt.%. Then, we address the issue of metal adsorbate clustering. Our calculations show that Ca clustering takes place on pristine graphene because of the small binding energy of Ca to graphene. One way to enhance the metal adsorption strength on the graphene plane is to dope graphene with acceptors such as boron atoms. We show that upon boron doping with a concentration of 12 at.%, the clustering problem could be prevented and the resulting gravimetric capacity is 8.38 wt.% hydrogen.

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