Fabrication and subband gap optical properties of silicon supersaturated with chalcogens by ion implantation and pulsed laser melting

Topographically flat, single crystal silicon supersaturated with the chalcogens S, Se, and Te was prepared by ion implantation followed by pulsed laser melting and rapid solidification. The influences of the number of laser shots on the atomic and carrier concentration-depth profiles were measured with secondary ion mass spectrometry and spreading resistance profiling, respectively. We found good agreement between the atomic concentration-depth profiles obtained from experiments and a one-dimensional model for plane-front melting, solidification, liquid-phase diffusion, with kinetic solute trapping, and surface evaporation. Broadband subband gap absorption is exhibited by all dopants over a wavelength range from 1 to 2.5 microns. The absorption did not change appreciably with increasing number of laser shots, despite a measurable loss of chalcogen and of electronic carriers after each shot.One of the authors M.T. acknowledges the financial support of the Fulbright Program. This research was supported in part by the U.S. Army ARDEC under Contract No. W15QKN-07- P-0092

How cigarette smoking may increase the risk of anxiety symptoms and anxiety disorders : a critical review of biological pathways

Multiple studies have demonstrated an association between cigarette smoking and increased anxiety symptoms or disorders, with early life exposures potentially predisposing to enhanced anxiety responses in later life. Explanatory models support a potential role for neurotransmitter systems, inflammation, oxidative and nitrosative stress, mitochondrial dysfunction, neurotrophins and neurogenesis, and epigenetic effects, in anxiety pathogenesis. All of these pathways are affected by exposure to cigarette smoke components, including nicotine and free radicals. This review critically examines and summarizes the literature exploring the role of these systems in increased anxiety and how exposure to cigarette smoke may contribute to this pathology at a biological level. Further, this review explores the effects of cigarette smoke on normal neurodevelopment and anxiety control, suggesting how exposure in early life (prenatal, infancy, and adolescence) may predispose to higher anxiety in later life. A large heterogenous literature was reviewed that detailed the association between cigarette smoking and anxiety symptoms and disorders with structural brain changes, inflammation, and cell-mediated immune markers, markers of oxidative and nitrosative stress, mitochondrial function, neurotransmitter systems, neurotrophins and neurogenesis. Some preliminary data were found for potential epigenetic effects. The literature provides some support for a potential interaction between cigarette smoking, anxiety symptoms and disorders, and the above pathways; however, limitations exist particularly in delineating causative effects. The literature also provides insight into potential effects of cigarette smoke, in particular nicotine, on neurodevelopment. The potential treatment implications of these findings are discussed in regards to future therapeutic targets for anxiety. The aforementioned pathways may help mediate increased anxiety seen in people who smoke. Further research into the specific actions of nicotine and other cigarette components on these pathways, and how these pathways interact, may provide insights that lead to new treatment for anxiety and a greater understanding of anxiety pathogenesis

Material Development for Highly Processable Thin Film Solar Cells

The ability of a photovoltaic cell to convert incident photons into electrical power is determined by the properties of its constituent materials and on their ability to function in concert with one another. Thin film solar cell materials benefit from the use of thin absorber layers that are relatively tolerant of a variety of structural defects. This allows for absorber layers to be made from polycrystalline films fabricated using raw materials that do not need to be refined to incredible levels of purity, as is generally required for single crystalline solar materials. Each of these traits represents significant logistical advantages during the industrial scale-up of thin film technologies, but they can be severely offset if scarce, expensive, or toxic materials are required during device fabrication. The various studies contained in the following chapters are dedicated to the exploration of next generation material systems that are being developed to resolve material issues that could potentially inhibit the large-scale implementation of existing thin film solar cell technologies.Silver nanowire networks stand as a potential replacement for transparent conductors made from doped metal oxide films. They exhibit excellent optical and electronic performance, and can be deposited in minutes from benign solutions with little damage to underlying device layers. When combined with an appropriately chosen matrix material to surround and encapsulate the wires, the resulting wire/matrix nanocomposite becomes a highly versatile electrode that can be integrated into a variety of thin film devices. Much of this dissertation is dedicated to the study and analysis of silver nanowire networks and partner materials and their applications in Cu(In,Ga)Se2 (CIGS) and amorphous silicon (a-Si) photovoltaics, starting from material synthesis and ink formulation and ending with device fabrication and characterization. In addition, the last chapter is dedicated to a discussion of heterojunction and space-charge formation in CZTSe solar cells, which is quickly becoming understood as a far more sensitive process than in its various chalcogenide analogues such as CIGS and CdTe. Together this set of materials would pave the way for the arrival of next generation thin film devices that can be fabricated quickly and with minimal reliance on indium, tellurium, or any other elements that would prevent their widespread commercial adoption

Material Development for Highly Processable Thin Film Solar Cells

The ability of a photovoltaic cell to convert incident photons into electrical power is determined by the properties of its constituent materials and on their ability to function in concert with one another. Thin film solar cell materials benefit from the use of thin absorber layers that are relatively tolerant of a variety of structural defects. This allows for absorber layers to be made from polycrystalline films fabricated using raw materials that do not need to be refined to incredible levels of purity, as is generally required for single crystalline solar materials. Each of these traits represents significant logistical advantages during the industrial scale-up of thin film technologies, but they can be severely offset if scarce, expensive, or toxic materials are required during device fabrication. The various studies contained in the following chapters are dedicated to the exploration of next generation material systems that are being developed to resolve material issues that could potentially inhibit the large-scale implementation of existing thin film solar cell technologies.Silver nanowire networks stand as a potential replacement for transparent conductors made from doped metal oxide films. They exhibit excellent optical and electronic performance, and can be deposited in minutes from benign solutions with little damage to underlying device layers. When combined with an appropriately chosen matrix material to surround and encapsulate the wires, the resulting wire/matrix nanocomposite becomes a highly versatile electrode that can be integrated into a variety of thin film devices. Much of this dissertation is dedicated to the study and analysis of silver nanowire networks and partner materials and their applications in Cu(In,Ga)Se2 (CIGS) and amorphous silicon (a-Si) photovoltaics, starting from material synthesis and ink formulation and ending with device fabrication and characterization. In addition, the last chapter is dedicated to a discussion of heterojunction and space-charge formation in CZTSe solar cells, which is quickly becoming understood as a far more sensitive process than in its various chalcogenide analogues such as CIGS and CdTe. Together this set of materials would pave the way for the arrival of next generation thin film devices that can be fabricated quickly and with minimal reliance on indium, tellurium, or any other elements that would prevent their widespread commercial adoption

Highly Robust Silver Nanowire Network for Transparent Electrode

Solution-processed silver nanowire networks are one of the promising candidates to replace a traditional indium tin oxide as next-generation transparent and flexible electrodes due to their ease of processing, moderate flexibility, high transparency, and low sheet resistance. To date, however, high stability of the nanowire networks remains a major challenge because the long-term usages of these electrodes are limited by their poor thermal and chemical stabilities. Existing methods for addressing this challenge mainly focus on protecting the nanowire network with additional layers that require vacuum processes, which can lead to an increment in manufacturing cost. Here, we report a straightforward strategy of a sol–gel processing as a fast and robust way to improve the stabilities of silver nanowires. Compared with reported nanoparticles embedded in nanowire networks, better thermal and chemical stabilities are achieved via sol–gel coating of TiO₂ over the silver nanowire networks. The conformal surface coverage suppressed surface diffusion of silver atoms and prevented chemical corrosion from the environment. These results highlight the important role of the functional layer in providing better thermal and chemical stabilities along with improved electrical properties and mechanical robustness. The silver nanowire/TiO₂ composite electrodes were applied as the source and drain electrodes for In₂O₃ thin-film transistors (TFTs) and the devices exhibited improved electrical performance annealed at 300 °C without the degradation of the electrodes. These key findings not only demonstrated a general and effective method to improve the thermal and chemical stabilities of metal nanowire networks but also provided a basic guideline toward rational design of highly efficient and robust composite electrodes

Molecular Solution Approach To Synthesize Electronic Quality Cu₂ZnSnS₄ Thin Films

Successful implementation of molecular solution processing from a homogeneous and stable precursor would provide an alternative, robust approach to process multinary compounds compared with physical vapor deposition. Targeting deposition of chemically clear, high quality crystalline films requires specific molecular structure design and solvent selection. Hydrazine (N₂H₄) serves as a unique and powerful medium, particularly to incorporate selected metallic elements and chalcogens into a stable solution as metal chalcogenide complexes (MCC). However, not all the elements and compounds can be easily dissolved. In this manuscript, we demonstrate a paradigm to incorporate previously insoluble transitional-metal elements into molecular solution as metal–atom hydrazine/hydrazine derivative complexes (MHHD), as exemplified by dissolving of the zinc constituent as Zn­(NH₂NHCOO)₂(N₂H₄)₂. Investigation into the evolution of molecular structure reveals the hidden roadmap to significantly enrich the variety of building blocks for soluble molecule design. The new category of molecular structures not only set up a prototype to incorporate other elements of interest but also points the direction for other compatible solvent selection. As demonstrated from the molecular precursor combining Sn-/Cu-MCC and Zn-MHHD, an ultrathin film of copper zinc tin sulfide (CZTS) was deposited. Characterization of a transistor based on the CZTS channel layer shows electronic properties comparable to CuInSe₂, confirming the robustness of this molecular solution processing and the prospect of earth abundant CZTS for next generation photovoltaic materials. This paradigm potentially outlines a universal pathway, from individual molecular design using selected chelated ligands and combination of building blocks in a simple and stable solution to fundamentally change the way multinary compounds are processed

Nanoscale Joule Heating and Electromigration Enhanced Ripening of Silver Nanowire Contacts

Solution-processed metallic nanowire thin film is a promising candidate to replace traditional indium tin oxide as the next-generation transparent and flexible electrode. To date however, the performance of these electrodes is limited by the high contact resistance between contacting nanowires; so improving the point contacts between these nanowires remains a major challenge. Existing methods for reducing the contact resistance require either a high processing power, long treatment time, or the addition of chemical reagents, which could lead to increased manufacturing cost and damage the underlying substrate or device. Here, a nanoscale point reaction process is introduced as a fast and low-power-consumption way to improve the electrical contact properties between metallic nanowires. This is achieved <i>via</i> current-assisted localized joule heating accompanied by electromigration. Localized joule heating effectively targets the high-resistance contact points between nanowires, leading to the automatic removal of surface ligands, welding of contacting nanowires, and the reshaping of the contact pathway between the nanowires to form a more desirable geometry of low resistance for interwire conduction. This result shows the interplay between thermal and electrical interactions at the highly reactive nanocontacts and highlights the control of the nanoscale reaction as a simple and effective way of turning individual metallic nanowires into a highly conductive interconnected nanowire network. The temperature of the adjacent device layers can be kept close to room temperature during the process, making this method especially suitable for use in devices containing thermally sensitive materials such as polymer solar cells

Spatial Element Distribution Control in a Fully Solution-Processed Nanocrystals-Based 8.6% Cu₂ZnSn(S,Se)₄ Device

A fully solution-processed high performance Cu₂ZnSn(S,Se)₄ (CZTSSe, kesterite) device has been demonstrated. It is based on the rational engineering of elemental spatial distributions in the bulk and particularly near the surface of the film from nanocrystal precursors. The nanocrystals are synthesized through a modified colloidal approach, with excellent solubility over a large compositional window, followed by a selenization process to form the absorber. The X-ray photoluminescence (XPS) depth profiling indicates an undesirable Sn-rich surface of the selenized film. An excessive Zn species was quantitatively introduced through nanocrystals precursor to correct the element distribution, and accordingly a positive correlation between the spatial composition in the bulk/surface film and the resulting device parameter is established. The enhanced device performance is associated with the reduced interfacial recombination. With a Zn content 1.6 times more than the stoichiometry; the optimized device, which is fabricated by employing a full solution process from the absorber to the transparent top electrode, demonstrates a performance of 8.6%. This composition-control approach through stoichiometric adjustments of nanocrystal precursors, and the developed correlation between the spatial composition and device performance may also benefit other multielement-based photovoltaics