Three-Dimensional MOS Process Development

A novel MOS technology for three-dimensional integration of electronic circuits on silicon substrates was developed. Selective epitaxial growth and epitaxial lateral overgrowth of monocrystalline silicon over oxidized silicon were employed to create locally restricted silicon-on-insulator device islands. Thin gate oxides were discovered to deteriorate in ambients typically used for selective epitaxial growth. Conditions of general applicability to silicon epitaxy systems were determined under which this deterioration was greatly reduced. Selective epitaxial growth needed to be carried out at low temperatures. However, crystalline defects increase as deposition temperatures are decreased. An exact dependence between the residual moisture content in epitaxial growth ambients, deposition pressure, and deposition temperature was determined which is also generally applicable to silicon epitaxy systems. The dependences of growth rates and growth rate uniformity on loading, temperature, flow rates, gas composition, and masking oxide thickness were investigated for a pancake type epitaxy reactor. A conceptual model was discussed attempting to describe the effects peculiar to selective epitaxial growth. The newly developed processing steps were assembled to fabricate three dimensional silicon-on-insulator capacitors. These capacitors were electrically evaluated. Surface state densities were in the order of 1O11cm-2 eV-1 and therefore within the range of applicability for a practical CMOS process. Oxidized polysilicon gates were overgrown with silicon by epitaxial lateral overgrowth. The epitaxial silicon was planarized and source and drain regions were formed above the polysilicon gates in Silicon-on-insulator material. The modulation of the source-drain current by bias changes of the buried gate was demonstrated

Silicon Quasi‐One‐Dimensional Nanostructures for Photovoltaic Applications

Thanks to the silicon abundance, stability, non-toxicity and well known electronic properties, Si based solar cells have represented the leading actors in the photovoltaic market and future projections confirm this predominance. However, half of the module cost is due to the material consumption and processing. In order to decrease the costs, a cut in the Si consumption must be operated, with consequent decrement in the optical absorption, generated current and device efficiency. To keep the performance level, a proper Si surface design with the objective to trap the light, has been developed. One of the most popular approaches is to use silicon nanowires embedded in the solar cell emitter where they play the role of optically and electrically active layer, thanks to their excellent optical absorption properties. However, also another material has been the terminus of the light-trapping materials, the silicon nanoholes. Their mechanical robustness is superior, making their integration inside the cell easier and cost-effective. The review will bring about all of the most common methods to fabricate these two types of nanostructures when used for solar cells applications, their optical properties and some critical aspects related to their high surface to volume ratio which modify the recombination processes

Solar Cells

Over the last decade, photovoltaic (PV) technology has shown the potential to become a major source of power generation for the world - with robust and continuous growth even during times of financial and economic crisis. That growth is expected to continue in the years ahead as worldwide awareness of the advantages of PV increases. However, cost remains as the greatest barrier to further expansion of PV-generated power, and therefore cost reduction is the prime goal of the PV and solar cell investigation. This book intends to contribute to such a purpose by covering a wide range of modern research topics in the solar cell physics and technology fields. The already established -1st generation- silicon solar cell technology, the 2nd generation thin film and the 3rd generation dye sensitized solar cells, including new technologies with very high perspectives for reducing the cost of solar electricity such as CZTS, organic polymer and tandem solar cells based on III-V compounds -under concentrated sunlight- are studied in this book by experts in the field from around the world. At the end, two chapters are also dedicated to the systems engineering, providing a complete PV energy research and application perspectives panoram

Influence of defects and impurities on solar cell performance

Multicrystalline silicon (mc-Si) solar cells exhibit high impurity content and higher density of crystal defects such as grain boundaries, dislocations, stacking faults and impurity precipitates. Even though the effect of dislocations on mc-Si solar cell performance has been studied, a severe lack of understanding of the quantitative effects of dislocations on cell parameters still exists. Some correlation has been reported under the assumption of a uniform distribution of dislocation density and a negligible effect of front and back surface recombination velocity. This assumption can cause a significant error as the current mc-Si technology provides good surface passivation by SiN:H and very effective back surface fields. This work is an extension of previous models that use Green Function to include the influence of front (S1) and back (S2) surface recombination velocities. The three dimensional continuity equation of the minority carriers has been solved in a solar cell having periodic array of dislocations and with front and back surface recombination. Each dislocation is considered to be a space charge cylinder perpendicular to the surface and extending through the entire cell. The calculations show that low dislocation densities (\u3c 104 cm-2) have very little effect on the cell performance. This is in agreement with the previously published data. The results of calculated dependencies of cell parameters on the dislocation density for different recombination activities are discussed in this work

Synthesis and Characterization of Functional One Dimensional Nanostructures

One- dimensional (1D) nanostructures have received growing interest due to their unique physical and chemical properties and promising nanodevice applications, as compared with their bulk counterparts. Complex 1D nanostructures with tunable properties and functionalities have been successfully fabricated and characterized in this thesis. I will show our recent efforts on precise controlled 1D nanostructures by template- assisted electrochemical synthesis as well as fundamental understanding of their physical behavior and growth mechanism of as-synthesized nanostructures. Particularly, three topics are presented: Firstly, a constant current (CC) based anodization technique is newly demonstrated to fabricate and control the structure of an anodic aluminum oxide (AAO) template. This technique has enabled the formation of long- range self- ordered hexagonal nanopore patterns with broad range of tunability of interpore distance (Dint). In addition, the combination of CC based anodization and conventional CV anodization can offer a fast, simple, and flexible methodology to achieve new degrees of freedom for engineering planar nanopore structures. This work also facilitates our understanding of the self- ordering mechanism of alumina membranes and complex nanoporous structure. Secondly, functional 1D nanostructures including pure metallic, magnetic and semiconducting nanowires and their heterostructure are demonstrated by versatile template- based electrochemical deposition under feasible control. This study has enabled the creation of high quality and well- controlled 1D nanostructures that can be applied as a model system for understanding unique 1D physics. Some preliminary investigations including exciton confinement, anisotropic magnetism and surface plasmon resonance are also presented. Lastly, a novel and universal non-epitaxial growth of metal-semiconductor core-shell lattice-mismatched hybrid heterostructures is presented. Importantly, a new mechanical stress driven crystalline growth mechanism is developed to account for non-epitaxial shape and monocrystalline evolution kinetics

Fracture Mechanics of Silicon: From durability of photovoltaic modules to the production of thin film solar cells

Nowadays the photovoltaic research is focused on increasing the performances, the durability and reducing the cost of production of solar cells, such as PV modules. These are the paromount fields to make photovoltaics more attractive for the energetic market. In this dissertation two of these aspects are investigated: the durability and the cost reduction issues. The fracture mechanics of the Silicon, the standard material used for the solar cells, is the main subject of the presented study. In the recent and next years the relevance of the durability studies is expected to increase more and more because of the developing of a new segment of PV, the building integrated Photovoltaic (BIPV). These new products incorporating PV modules in the building materials are curtains, walls, windows, sloped roofs, flat roofs, facades, shading systems and roofing shingles. In the new generation of BIPV systems, PV modules replace parts of the building structure, providing functional considerations and lowering costs. In this market the thin-film PV is the most promising technology because of its superior flexibility, minimal weight, and the ability to perform in variable lighting conditions. The issues of this particular PV market are not only the energy production but also the structural safety and performance in addition to architectural specifics as the shadowing. In this framework the durability, the degradation and new technology to achieve a cost reduction are of fundamental importance. In this thesis, experimental diagnostic techniques and interpretative models based on linear and nonlinear fracture mechanics for studying the phenomena of fracture in Silicon are presented. In particular the development and the use of techniques for the quantitative analysis of electroluminescence signals, for the detection of cracks in Silicon caused by thermo-elastic stresses, have been developed. The experimental results have been obtained during an extensive experimental campaign conducted at Politecnico di Torino. For the interpretation of the experimental evidence it has been proposed an original onedimensional electrical model for predicting the eect of cracks on the distribution of electric current. Subsequently, the electric field has been coupled to the mechanical, introducing an electric resistance located at the level of the crack and dependent on the crack itself. In parallel, a numerical analysis has been carried out, using the finite element codes FRANC2D and FEAP, on the phenomenon of peeling in mono-crystalline Silicon induced by thermoelastic stresses. This study, which can be very important in applications because it may allow the production of ultra-thin solar cells with a significant saving of material, is carried out in collaboration with the Institute for Solar Energy Research (ISFH), Hamelin, Germany. This process exploits the thermo-mechanical stresses due to the contrast between the elastic proviii perties of Silicon and Aluminium in line with earlier studies of the school of Harvard. It has been proposed a broad campaign experimental and numerical in order to optimize the process

A new metallization technology for solar cells application

This Ph.D. thesis is focused on the development and optimization of front and rear side metallization of industrial silicon solar cells. The commonly adopted screen-printed silver metallization has several well-known issues, such as low contact resistance, moderate bulk conductivity and high cost. The approach of this work allows complete silver replacement, both on the front and the rear sides. The development of such a new technology is divided into several parts, each resulting in appropriate feedback in terms of solar cell operation parameters. A detailed investigation of the aluminum-silicon interdiffusion that occurs during the firing process of screen-printed aluminum layer usually deposited onto the rear of solar cells is reported. This process is very important because it affects solar cell operation and performance through back-surface field passivation. In this study different screen-printing aluminum pastes, differing one from each other in aluminum particle dimensions and glass frit composition, are evaluated in terms of their bulk resistivity, contact resistance to silicon, back surface field depth and solar cell performance. Finally, this study allowed to reveal certain dependences between pastes parameters and their effect on solar cells and to develop useful recommendations for better solar cell performance. In this work, a new metallization technology is based on an electroplating technique, which for a real industrial application, however, has some critical issues as throughput, floor space, quantity of liquid to manage and the necessity to use some masking technique, such as photolithography. These issues are strongly influencing the metallization technology cost, making it not economically convenient respect silver screen-printing technology. For this purpose, the proposed metallization technique is based on a novel dynamic liquid drop/meniscus (DLD/DLM) technique able to solve both issues. In this work DLD/DLM technique is studied for possible application in a new rear side metallization technology for solar cells, allowing localized formation of solder pads without any use of photolithography, limiting the cost of the process mainly to the cost of materials, such as nickel and tin, which are significantly cheaper than a silver counterpart that is currently adopted by the industry. The cost reduction is not a single advantage of the proposed technology. An efficiency improvement of up to 0.5 %abs is obtained due to a better back-surface field conditions. The development of a new front side metallization is based on a new approach which introduces a layer of mesoporous silicon helpful for further creation of nickel-copper electrical contacts to the emitter region of a solar cell. Process conditions of mesoporous silicon formation and further electroplating steps are studied and optimized in terms of contact resistance and adhesion of such a contacts, in order to guarantee a beneficial influence for solar cells fabricated with the new metallization approach. As for combination of both front and rear side metallization technologies, together, they result in complete silver removal from a metallization technology of a solar cell with a feasible efficiency enhancement of up to 1 %abs

Loss analysis of back-contact back-junction thin-film monocrystalline silicon solar cells

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Thin Film Silicon Solar Cell Prepared By Thermal Evaporation On Polyimide Substrate

Conventional wafer-based silicon (Si) technology still dominates around 90% of the photovoltaic (PV) market with 15 - 20% conversion efficiency due to its abundance (~25% of silica in the earth’s crust), non-toxicity besides having close to ideal band gap (1.12 eV) for photoconversion process. But, this technology suffers from high cost/Wattp (USD 2 - 3/Wattp at present) that impedes its widespread to be an alternative power generation technique at present. This stems from high processing and purification costs of the Si material (single crystal costs about USD 400/kg) besides high material consumption (300 - 500 μm/wafer). This work explored the feasibility of fabricating thin film Si solar cells on low-cost polyimide (PI) substrates via thermal evaporation method in order to bring down the costs of the Si PV technology to below USD 1/Wattp. The solar cells were fabricated in substrate-configuration with p-n and p-i-n junction structures. Various light trapping strategies such as aluminium (Al) back contact reflector, PI surface texturing, zinc oxide (ZnO) anti-reflective coating (ARC) and white paint back surface reflector (BSR) have been evaluated to increase optical path length of the incident light and to reduce reflection losses. A 1.5 μm thick p-type Si absorber was used in the p-n junction cells while 800 nm intrinsic Si was adopted in the p-i-n junction cells

Urea Pre-treatment of N2-annealed Transition Metal Oxides for Low Cost and Efficient Counter Electrodes in Dye-sensitized Solar Cell

Photovoltaic cells have shown great promise as an alternative to fossil fuel-based energy sources. Dye-sensitized solar cells (DSSCs) have shown potential as low-cost replacement to silicon solar cells owing to their reduced material costs and simple fabrication techniques. Platinum (Pt) was used as a catalyst in the counter electrode for DSSCs. Metal oxides have been used as an alternative material to Pt. The introduction of oxygen vacancies inside metal oxides helps to facilitate electron transport to the electrolyte to enhance the reduction process of triiodide ions. Annealing n-type metal oxides under a reducing agent gas such as hydrogen (H2) at temperature ≥400 oC helps to introduce more oxygen vacancies. In this dissertation, a novel method was developed to convert the electrocatalytically inactive commercial n-type WO3, SnO2, and ZnO into highly active WO3-x, SnO2-x and ZnO1-x as counter electrodes (CEs) for DSSCs. These new metal oxides replaced Pt by controlling the number of introduced oxygen vacancies. All the metal oxides including WO3, SnO2, and ZnO were pre-treated with urea at different wt% before annealing under N2 environment at 470 oC for 2 hr. At high temperatures (e.g., 300-400 oC), urea easily decomposes to ammonia which then decomposes to H2 and N2 gases. Higher wt% of urea leads to more reducing H2 gas and hence helps to create more oxygen vacancies. The urea treatment significantly improved the catalytic activity of all metal oxides, and solar cell power conversion efficiency (PCE) of DSSCs was increased by urea pre-treatment. All other characterizations including SEM, EDS, and Mott-Schottky performed for urea pre-treatment of WO3, SnO2 and ZnO support the hypothesis that urea treatment helps create oxygen vacancies (shallow defects states) in metal oxides. These oxygen vacancies facilitate the redox process in the iodide/triiodide electrolyte. The density of these oxygen vacancies can be engineered by controlling the urea wt% during the treatment. Compared to previous work by only annealing metal oxides in N2 or H2 without any pre-treatment, this new method using urea pre-treatment is more efficient to maximize the performance of DSSC devices