Matching Items (5)

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Insights into Crack Dynamics Governing Surface Quality during Spalling of Semiconductors

Description

The rationale of this thesis is to provide a thorough understanding of spalling for semiconductor materials and develop a low temperature spalling technology that reduces the surface roughness of the spalled wafers for Photovoltaics applications.

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Date Created
  • 2020

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Advanced Characterization of Aerogel Films Deposited via Aerosol Impaction-Driven Assembly

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A new nanoparticle deposition technique, Aerosol Impaction-Driven Assembly (AIDA), was extensively characterized for material structures and properties. Aerogel films can be deposited directly onto a substrate with AIDA without the

A new nanoparticle deposition technique, Aerosol Impaction-Driven Assembly (AIDA), was extensively characterized for material structures and properties. Aerogel films can be deposited directly onto a substrate with AIDA without the long aging and drying steps in the sol-gel method. Electron microscopy, pore size analysis, thermal conductivity, and optical measurements show the nanoparticle (NP) films to be similar to typical silica aerogel. Haze of nanoparticle films modeled as scattering sites correlates strongly with pore size distribution. Supporting evidence was obtained from particle sizes and aggregates using electron microscopy and small-angle X-ray scattering. NP films showed interlayers of higher porosity and large aggregates formed by tensile film stress.

To better understand film stress and NP adhesion, chemical bonding analyses were performed for samples annealed up to 900 °C. Analysis revealed that about 50% of the NP surfaces are functionalized by hydroxyl (-OH) groups, providing for hydrogen bonding. Ellipsometric porosimetry was used to further understand the mechanical properties by providing a measure of strain upon capillary pressure from filling pores. Upon annealing to 200 °C, the films lost water resulting in closer bonding of NPs and higher Young’s modulus. Upon further annealing up to 900 °C, the films lost hydroxyl bonds while gaining siloxane bonds, reducing Young’s modulus. The application of ellipsometric porosimetry to hydrophilic coatings brings into question the validity of pore size distribution calculations for materials that hold onto water molecules and result in generally smaller calculated pore sizes.

Doped hydrogenated microcrystalline silicon was grown on crystalline silicon NPs, as a test case of an application for NP films to reduce parasitic absorption in silicon heterojunction solar cells. Parasitic absorption of blue light could be reduced because microcrystalline silicon has a mix of direct and indirect bandgap, giving lower blue absorption than amorphous silicon. Using Ultraviolet Raman spectroscopy, the crystallinity of films as thin as 13 nm was determined rapidly (in 1 minute) and non-destructively. A mono-layer of nanocrystals was applied as seeds for p-doped microcrystalline silicon growth and resulted in higher crystallinity films. Applications of the method could be explored for other nanocrystalline materials.

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Date Created
  • 2020

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Reactive Ink Metallization for Next Generation Photovoltaics

Description

In order to meet climate targets, the solar photovoltaic industry must increase photovoltaic (PV) deployment and cost competitiveness over its business-as-usual trajectory. This requires more efficient PV modules that use

In order to meet climate targets, the solar photovoltaic industry must increase photovoltaic (PV) deployment and cost competitiveness over its business-as-usual trajectory. This requires more efficient PV modules that use less expensive materials, and longer operational lifetime. The work presented here approaches this challenge with a novel metallization method for solar PV and electronic devices.

This document outlines work completed to this end. Chapter 1 introduces the areas for cost reductions and improvements in efficiency to drive down the cost per watt of solar modules. Next, in Chapter 2, conventional and advanced metallization methods are reviewed, and our proposed solution of dispense printed reactive inks is introduced. Chapter 3 details a proof of concept study for reactive silver ink as front metallization for solar cells. Furthermore, Chapter 3 details characterization of the optical and electrical properties of reactive silver ink metallization, which is important to understanding the origins of problems related to metallization, enabling approaches to minimize power losses in full devices. Chapter 4 describes adhesion and specific contact resistance of reactive ink metallizations on silicon heterojunction solar cells. Chapter 5 compares performance of silicon heterojunction solar cells with front grids formed from reactive ink metallization and conventional, commercially available metallization. Performance and degradation throughout 1000 h of accelerated environmental exposure are described before detailing an isolated corrosion experiment for different silver-based metallizations. Finally, Chapter 6 summarizes the main contributions of this work.

The major goal of this project is to evaluate potential of a new metallization technique –high-precision dispense printing of reactive inks–to become a high efficiency replacement for solar cell metallization through optical and electrical characterization, evaluation of durability and reliability, and commercialization research. Although this work primarily describes the application of reactive silver inks as front-metallization for silicon heterojunction solar cells, the work presented here provides a framework for evaluation of reactive inks as metallization for various solar cell architectures and electronic devices.

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Date Created
  • 2019

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Efficiency-limiting recombination mechanisms in high-quality crystalline silicon for solar cells

Description

Recent technology advancements in photovoltaics have enabled crystalline silicon (c-Si) solar cells to establish outstanding photoconversion efficiency records. Remarkable progresses in research and development have been made both on the

Recent technology advancements in photovoltaics have enabled crystalline silicon (c-Si) solar cells to establish outstanding photoconversion efficiency records. Remarkable progresses in research and development have been made both on the silicon feedstock quality as well as the technology required for surface passivation, the two dominant sources of performance loss via recombination of photo-generated charge carriers within advanced solar cell architectures.

As these two aspects of the solar cell framework improve, the need for a thorough analysis of their respective contribution under varying operation conditions has emerged along with challenges related to the lack of sensitivity of available characterization techniques. The main objective of my thesis work has been to establish a deep understanding of both “intrinsic” and “extrinsic” recombination processes that govern performance in high-quality silicon absorbers. By studying each recombination mechanism as a function of illumination and temperature, I strive to identify the lifetime limiting defects and propose a path to engineer the ultimate silicon solar cell.

This dissertation presents a detailed description of the experimental procedure required to deconvolute surface recombination contributions from bulk recombination contributions when performing lifetime spectroscopy analysis. This work proves that temperature- and injection-dependent lifetime spectroscopy (TIDLS) sensitivity can be extended to impurities concentrations down to 109 cm-3, orders of magnitude below any other characterization technique available today. A new method for the analysis of TIDLS data denominated Defect Parameters Contour Mapping (DPCM) is presented with the aim of providing a visual and intuitive tool to identify the lifetime limiting impurities in silicon material. Surface recombination velocity results are modelled by applying appropriate approaches from literature to our experimentally evaluated data, demonstrating for the first time their capability to interpret temperature-dependent data. In this way, several new results are obtained which solve long disputed aspects of surface passivation mechanisms. Finally, we experimentally evaluate the temperature-dependence of Auger lifetime and its impact on a theoretical intrinsically limited solar cell. These results decisively point to the need for a new Auger lifetime parameterization accounting for its temperature-dependence, which would in turn help understand the ultimate theoretical efficiency limit for a solar cell under real operation conditions.

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Date Created
  • 2018

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Correlative X-ray microscopy studies of CuIn₁-xGaxSe₂ solar cells

Description

It is well known that the overall performance of a solar cell is limited by the worst performing areas of the device. These areas are usually micro and nano-scale defects

It is well known that the overall performance of a solar cell is limited by the worst performing areas of the device. These areas are usually micro and nano-scale defects inhomogenously distributed throughout the material. Mitigating and/or engineering these effects is necessary to provide a path towards increasing the efficiency of state-of-the-art solar cells. The first big challenge is to identify the nature, origin and impact of such defects across length scales that span multiple orders of magnitude, and dimensions (time, temperature etc.). In this work, I present a framework based on correlative X-ray microscopy and big data analytics to identify micro and nanoscale defects and their impact on material properties in CuIn1-xGaxSe2 (CIGS) solar cells.

Synchrotron based X-ray Fluorescence (XRF) and X-ray Beam Induced Current (XBIC) are used to study the effect that compositional variations, between grains and at grain boundaries, have on CIGS device properties. An experimental approach is presented to correcting XRF and XBIC quantification of CIGS thin film solar cells. When applying XRF and XBIC to study low and high gallium CIGS devices, it was determined that increased copper and gallium at grain boundaries leads to increased collection efficiency at grain boundaries in low gallium absorbers. However, composition variations were not correlated with changes in collection efficiency in high gallium absorbers, despite the decreased collection efficiency observed at grain boundaries.

Understanding the nature and impact of these defects is only half the battle; controlling or mitigating their impact is the next challenge. This requires a thorough understanding of the origin of these defects and their kinetics. For such a study, a temperature and atmosphere controlled in situ stage was developed. The stage was utilized to study CIGS films during a rapid thermal growth process. Comparing composition variations across different acquisition times and growth temperatures required the implementation of machine learning techniques, including clustering and classification algorithms. From the analysis, copper was determined to segregate the faster than indium and gallium, and clustering techniques showed consistent elemental segregation into copper rich and copper poor regions. Ways to improve the current framework and new applications are also discussed.

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Date Created
  • 2018