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Description
A primary motivation of research in photovoltaic technology is to obtain higher efficiency photovoltaic devices at reduced cost of production so that solar electricity can be cost competitive. The majority of photovoltaic technologies are based on p-n junction, with efficiency potential being much lower than the thermodynamic limits of individual technologies and thereby providing substantial scope for further improvements in efficiency. The thesis explores photovoltaic devices using new physical processes that rely on thin layers and are capable of attaining the thermodynamic limit of photovoltaic technology. Silicon heterostructure is one of the candidate technologies in which thin films induce a minority carrier collecting junction in silicon and the devices can achieve efficiency close to the thermodynamic limits of silicon technology. The thesis proposes and experimentally establishes a new theory explaining the operation of silicon heterostructure solar cells. The theory will assist in identifying the optimum properties of thin film materials for silicon heterostructure and help in design and characterization of the devices, along with aiding in developing new devices based on this technology. The efficiency potential of silicon heterostructure is constrained by the thermodynamic limit (31%) of single junction solar cell and is considerably lower than the limit of photovoltaic conversion (~ 80 %). A further improvement in photovoltaic conversion efficiency is possible by implementing a multiple quasi-fermi level system (MQFL). A MQFL allows the absorption of sub band gap photons with current being extracted at a higher band-gap, thereby allowing to overcome the efficiency limit of single junction devices. A MQFL can be realized either by thin epitaxial layers of alternating higher and lower band gap material with nearly lattice matched (quantum well) or highly lattice mismatched (quantum dot) structure. The thesis identifies the material combination for quantum well structure and calculates the absorption coefficient of a MQFl based on quantum well. GaAsSb (barrier)/InAs(dot) was identified as a candidate material for MQFL using quantum dot. The thesis explains the growth mechanism of GaAsSb and the optimization of GaAsSb and GaAs heterointerface.
ContributorsGhosha, Kuṇāla (Author) / Bowden, Stuart (Thesis advisor) / Honsberg, Christiana (Thesis advisor) / Vasileska, Dragica (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2011
Description
As crystalline silicon solar cells continue to get thinner, the recombination of carriers at the surfaces of the cell plays an ever-important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. The focus of this work is to understand the properties of charges present in the SiNx films and then to develop a mechanism to manipulate the polarity of charges to either negative or positive based on the end-application. Specific silicon-nitrogen dangling bonds (·Si-N), known as K center defects, are the primary charge trapping defects present in the SiNx films. A custom built corona charging tool was used to externally inject positive or negative charges in the SiNx film. Detailed Capacitance-Voltage (C-V) measurements taken on corona charged SiNx samples confirmed the presence of a net positive or negative charge density, as high as +/- 8 x 1012 cm-2, present in the SiNx film. High-energy (~ 4.9 eV) UV radiation was used to control and neutralize the charges in the SiNx films. Electron-Spin-Resonance (ESR) technique was used to detect and quantify the density of neutral K0 defects that are paramagnetically active. The density of the neutral K0 defects increased after UV treatment and decreased after high temperature annealing and charging treatments. Etch-back C-V measurements on SiNx films showed that the K centers are spread throughout the bulk of the SiNx film and not just near the SiNx-Si interface. It was also shown that the negative injected charges in the SiNx film were stable and present even after 1 year under indoor room-temperature conditions. Lastly, a stack of SiO2/SiNx dielectric layers applicable to standard commercial solar cells was developed using a low temperature (< 400 °C) PECVD process. Excellent surface passivation on FZ and CZ Si substrates for both n- and p-type samples was achieved by manipulating and controlling the charge in SiNx films.
ContributorsSharma, Vivek (Author) / Bowden, Stuart (Thesis advisor) / Schroder, Dieter (Committee member) / Honsberg, Christiana (Committee member) / Roedel, Ronald (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2013
Description
The purpose of the solar powered quadcopter is to join together the growing technologies of photovoltaics and quadcopters, creating a single unified device where the technologies harmonize to produce a new product with abilities beyond those of a traditional battery powered drone. Specifically, the goal is to take the battery-only flight time of a quadcopter loaded with a solar array and increase that flight time by 33% with additional power provided by solar cells. The major concepts explored throughout this project are quadcopter functionality and capability and solar cell power production. In order to combine these technologies, the solar power and quadcopter components were developed and analyzed individually before connecting the solar array to the quadcopter circuit and testing the design as a whole. Several solar copter models were initially developed, resulting in multiple unique quadcopter and solar cell array designs which underwent preliminary testing before settling on a finalized design which proved to be the most effective and underwent final timed flight tests. Results of these tests are showing that the technologies complement each other as anticipated and highlight promising results for future development in this area, in particular the development of a drone running on solar power alone. Applications for a product such as this are very promising in many fields, including the industries of power, defense, consumer goods and services, entertainment, marketing, and medical. Also, becoming a more popular device for UAV hobbyists, such developments would be very appealing for leisure flying and personal photography purposes as well.
ContributorsMartin, Heather Catrina (Author) / Bowden, Stuart (Thesis director) / Aberle, James (Committee member) / Electrical Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2015-12
Description
This is a lectures series on photovoltaics. As the need for electrical energy rises, mankind has struggled to meet its need in a reliable lasting way. Throughout this struggle, solar energy has come to the foreground as a complete solution. However, it has many drawbacks and needs a lot of development. In addition, the general public is unaware of how solar energy works, how it is made, and how it stands economically. This series of lectures answering those three questions.
ContributorsLeBeau, Edward Sanroma (Author) / Goryll, Michael (Thesis director) / Bowden, Stuart (Committee member) / Dauksher, Bill (Committee member) / Electrical Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2017-05
Description
As global population and demand for electrical power increase, humanity is faced with the growing challenge of harnessing and distributing enough energy to sustain the developing world. Currently, fossil fuels (coal
atural gas) are our main sources of electricity. However, their cost is increasing, they are nonrenewable, and they are very harmful to the environment. Thus, capacity expansion in the renewable energy sector must be realized to offset higher energy demand and reduce dependence on fossil fuels. Solar energy represents a practical solution, as installed global solar capacity has been increasing exponentially over the past 2 decades. However, even with government incentives, solar energy price ($/kWh) continues to be highly dependent on political climate and raw material (silicon and silver) cost. To realistically and cost effectively meet the projected expansions within the solar industry, silver must be replaced with less costly and more abundant metals (such as copper) in the front-grid metallization process of photovoltaic cells. Copper, while offering both higher achievable efficiencies and a raw material cost nearly 100 times cheaper than silver, has inherent disadvantages. Specifically, copper diffuses rapidly into the silicon substrate, requires more complex and error-prone processing steps, and tends to have less adhesive strength, reducing panel robustness. In this study, nickel deposition via sputtering was analyzed, as well as overall potential of nickel as a seed layer for copper plating, which also provides a barrier layer to copper diffusion in silicon. Thermally-formed nickel silicide also reduces contact resistivity, increasing cell efficiency. It was found that at 400 \u00B0C, ideal nickel silicide formation occurred. By computer modeling, contact resistivity was found to have a significant impact on cell efficiency (up to 1.8%). Finally, sputtering proved useful to analyze nickel silicide formation, but costs and time requirements prevent it from being a practical industrial-scale metallization method.
atural gas) are our main sources of electricity. However, their cost is increasing, they are nonrenewable, and they are very harmful to the environment. Thus, capacity expansion in the renewable energy sector must be realized to offset higher energy demand and reduce dependence on fossil fuels. Solar energy represents a practical solution, as installed global solar capacity has been increasing exponentially over the past 2 decades. However, even with government incentives, solar energy price ($/kWh) continues to be highly dependent on political climate and raw material (silicon and silver) cost. To realistically and cost effectively meet the projected expansions within the solar industry, silver must be replaced with less costly and more abundant metals (such as copper) in the front-grid metallization process of photovoltaic cells. Copper, while offering both higher achievable efficiencies and a raw material cost nearly 100 times cheaper than silver, has inherent disadvantages. Specifically, copper diffuses rapidly into the silicon substrate, requires more complex and error-prone processing steps, and tends to have less adhesive strength, reducing panel robustness. In this study, nickel deposition via sputtering was analyzed, as well as overall potential of nickel as a seed layer for copper plating, which also provides a barrier layer to copper diffusion in silicon. Thermally-formed nickel silicide also reduces contact resistivity, increasing cell efficiency. It was found that at 400 \u00B0C, ideal nickel silicide formation occurred. By computer modeling, contact resistivity was found to have a significant impact on cell efficiency (up to 1.8%). Finally, sputtering proved useful to analyze nickel silicide formation, but costs and time requirements prevent it from being a practical industrial-scale metallization method.
ContributorsBliss, Lyle Brewster (Author) / Bowden, Stuart (Thesis director) / Karas, Joseph (Committee member) / Chemical Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2017-05
Description
This is a lectures series on photovoltaics. As the need for electrical energy rises, mankind has struggled to meet its need in a reliable lasting way. Throughout this struggle, solar energy has come to the foreground as a complete solution. However, it has many drawbacks and needs a lot of development. In addition, the general public is unaware of how solar energy works, how it is made, and how it stands economically. This series of lectures answering those three questions. After two years doing photovoltaic research, and an undergraduate degree in Electrical Engineering, enough expertise has been acquired present on at a late high-school to early college level. Education is key to improving the popularity of using solar energy and the popularity of investing in photovoltaic research. Solar energy is a viable option to satisfy our energy crisis because the materials it requires can quickly be acquired, and there is enough of material to provide a global solution. In addition, the amount of solar energy that hits the surface of the earth in a day is orders of magnitude more than the amount of energy we require. The main goal of this project is to have an effective accessible tool to teach people about solar. Thus, the lectured will be posted on pveducation.com, YouTube, the Barrett repository, and the QUSST website. The content was acquired in four ways. The first way is reading up on the current papers and journals describing the new developments in photovoltaics. The second part is getting in contact with Stuart Bowden and Bill Daukser at Arizona State University's Solar Power Lab as well as the other faculty associated with the Solar Power Lab. There is quite a bit of novel research going on at their lab, as well as a student run pilot line that is actively building solar cells. The third way is reading about solar device physics using device physics textbooks and the PVEducation website made by Stuart Bowden. The forth way is going into ASU's solar power lab.
ContributorsLeBeau, Edward (Author) / Goryll, Michael (Thesis director) / Bowden, Stuart (Committee member) / Dauksher, William (Committee member) / Barrett, The Honors College (Contributor)
Created2017-05
Description
This study focuses on the implications of a high reverse bias breakdown in silicon heterojunction cells (SHJ). In relevant literature, there is a lack of explicit investigation which
compares high breakdown voltage cells (commonly SHJ) to low breakdown voltage cells
(commonly silicon homojunctions) in an installation setting. In addition, their relationship with
shading and how they react with bypass diodes are also not very prevalent. Therefore, my project
dives into how shading impacts a string of high breakdown voltage cells and a string of low
breakdown voltage cells, as well as how those cells interact with a bypass diode.
In order to conduct this investigation, I used the simulation software LTSpice XVII to
create an accurate simulation model of a SHJ cell with a 21 V reverse breakdown voltage. With
this cell model, I strung 10 cells together, and varied the shading on a single cell while measuring
the string’s output current, voltage, and power. Next, I attached a bypass diode to the shaded
cell, and continued to increase the number of cells attached to the bypass diode while continuing
to examine the string’s output. Once I gathered this data, I modified the original cell model to
have a lower reverse breakdown voltage of 5 V. From here, I strung 10 cells together again, and
repeated the same measurements from the 21 V string.
Upon completing these measurements, I found that the SHJ cells were in fact harder to
force into reverse bias than the cells with the lower reverse breakdown voltage, suggesting that
solar installation owners should consider transitioning to SHJ-based modules. When bypass
diodes are being considered, my results demonstrated that heavy shading (about 65% and
higher) was required for the bypass diodes to have an observable impact on the string’s power
output. Therefore, owners should consider how severe the shading their installation may receive
before investing in bypass diodes. If owners do find the need for the bypass diodes, my findings
also show that the diodes should be used sparingly and in a compromise with output power and
cost.
ContributorsAvalos, Christian (Author) / Honsberg, Christiana (Thesis advisor) / Bowden, Stuart (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2023
Description
This comprehensive library of photovoltaic functions (PVSimLib) is an attempt to help the photovoltaics community to solve one of its long-lasting problems, the lack of a simple, flexible and comprehensive tool that can be used for photovoltaic calculations. The library contains a collection of useful functions and detailed examples that will show the user how to take advantage of the resources present in this library. The results will show how in combination with other Python libraries (Matplotlib), this library becomes a powerful tool for anyone involved in solar power.
ContributorsReguera, Pedro (Author) / Honsberg, Christiana (Thesis advisor) / King, Richard (Committee member) / Bowden, Stuart (Committee member) / Arizona State University (Publisher)
Created2018
Fill Factor Loss Mechanisms: Analysis and Basic Understanding in Silicon Hetero-junction Solar Cells
Description
The objective of this thesis is to achieve a detailed understanding of the loss mechanisms in SHJ solar cells. The working principles of these cells and what affects the cell operation, e.g. the IV characteristics at the maximum power point (MPP) and the correspondingly ll factor (FF) are investigated. Dierent loss sources are analyzed separately, and the weight of each in the total loss at the MPP are evaluated. The total series resistance is measured and then compared with the value obtained through summation over each of its components. In other words, series resistance losses due to recombination, vertical and lateral carrier transport, metalization, etc, are individually evaluated, and then by adding all these components together, the total loss is calculated. The concept of ll factor and its direct dependence on the loss mechanisms at the MPP of the device is explained, and its sensitivity to nearly every processing step of the cell fabrication is investigated. This analysis provides a focus lens to identify the main source of losses in SHJ solar cells and pave the path for further improvements in cell efficiency.
In this thesis, we provide a detailed understanding of the FF concept; we explain how it can be directly measured; how it can be calculated and what expressions can better approximate its value and under what operating conditions. The relation between FF and cell operating condition at the MPP is investigated. We separately analyzed the main FF sources of losses including recombination, sheet resistance, contact resistance and metalization. We study FF loss due to recombination and its separate components which include the Augur, radiative and SRH recombination is investigated. We study FF loss due to contact resistance and its separate components which include the contact resistance of dierent interfaces, e.g. between the intrinsic and doped a-Si layers, TCO and a-Si layers. We also study FF loss due to lateral transport and its components that including the TCO sheet resistance, the nger and the busbars resistances.
In this thesis, we provide a detailed understanding of the FF concept; we explain how it can be directly measured; how it can be calculated and what expressions can better approximate its value and under what operating conditions. The relation between FF and cell operating condition at the MPP is investigated. We separately analyzed the main FF sources of losses including recombination, sheet resistance, contact resistance and metalization. We study FF loss due to recombination and its separate components which include the Augur, radiative and SRH recombination is investigated. We study FF loss due to contact resistance and its separate components which include the contact resistance of dierent interfaces, e.g. between the intrinsic and doped a-Si layers, TCO and a-Si layers. We also study FF loss due to lateral transport and its components that including the TCO sheet resistance, the nger and the busbars resistances.
ContributorsLeilaeioun, Mohammadmehdi (Ashling) (Author) / Goodnick, Stephen (Thesis advisor) / Goryll, Michael (Thesis advisor) / Bertoni, Mariana (Committee member) / Bowden, Stuart (Committee member) / Stuckelberger, Michael (Committee member) / Arizona State University (Publisher)
Created2018
Description
Achieving high efficiency in solar cells requires optimal photovoltaics materials for light absorption and as with any electrical device—high-quality contacts. Essentially, the contacts separate the charge carriers—holes at one terminal and electrons at the other—extracting them to an external circuit. For this purpose, the development of passivating and carrier-selective contacts that enable low interface defect density and efficient carrier transport is critical for making high-efficiency solar cells. The recent record-efficiency n-type silicon cells with hydrogenated amorphous silicon (a-Si:H) contacts have demonstrated the usefulness of passivating and carrier-selective contacts. However, the use of a-Si:H contacts should not be limited in just n-type silicon cells.
In the present work, a-Si:H contacts for crystalline silicon and cadmium telluride (CdTe) solar cells are developed. First, hydrogen-plasma-processsed a-Si:H contacts are used in n-type Czochralski silicon cell fabrication. Hydrogen plasma treatment is used to increase the Si-H bond density of a-Si:H films and decrease the dangling bond density at the interface, which leads to better interface passivation and device performance, and wider temperature-processing window of n-type silicon cells under full spectrum (300–1200 nm) illumination. In addition, thickness-varied a-Si:H contacts are studied for n-type silicon cells under the infrared spectrum (700–1200 nm) illumination, which are prepared for silicon-based tandem applications.
Second, the a-Si:H contacts are applied to commercial-grade p-type silicon cells, which have much lower bulk carrier lifetimes than the n-type silicon cells. The approach is using gettering and bulk hydrogenation to improve the p-type silicon bulk quality, and then applying a-Si:H contacts to enable excellent surface passivation and carrier transport. This leads to an open-circuit voltage of 707 mV in p-type Czochralski silicon cells, and of 702 mV, the world-record open-circuit voltage in p-type multi-crystalline silicon cells.
Finally, CdTe cells with p-type a-Si:H hole-selective contacts are studied. As a proof of concept, p-type a-Si:H contacts enable achieving the highest reported open-circuit voltages (1.1 V) in mono-crystalline CdTe devices. A comparative study of applying p-type a-Si:H contacts in poly-crystalline CdTe solar cells is performed, resulting in absolute voltage gain of 53 mV over using the standard tellurium contacts.
In the present work, a-Si:H contacts for crystalline silicon and cadmium telluride (CdTe) solar cells are developed. First, hydrogen-plasma-processsed a-Si:H contacts are used in n-type Czochralski silicon cell fabrication. Hydrogen plasma treatment is used to increase the Si-H bond density of a-Si:H films and decrease the dangling bond density at the interface, which leads to better interface passivation and device performance, and wider temperature-processing window of n-type silicon cells under full spectrum (300–1200 nm) illumination. In addition, thickness-varied a-Si:H contacts are studied for n-type silicon cells under the infrared spectrum (700–1200 nm) illumination, which are prepared for silicon-based tandem applications.
Second, the a-Si:H contacts are applied to commercial-grade p-type silicon cells, which have much lower bulk carrier lifetimes than the n-type silicon cells. The approach is using gettering and bulk hydrogenation to improve the p-type silicon bulk quality, and then applying a-Si:H contacts to enable excellent surface passivation and carrier transport. This leads to an open-circuit voltage of 707 mV in p-type Czochralski silicon cells, and of 702 mV, the world-record open-circuit voltage in p-type multi-crystalline silicon cells.
Finally, CdTe cells with p-type a-Si:H hole-selective contacts are studied. As a proof of concept, p-type a-Si:H contacts enable achieving the highest reported open-circuit voltages (1.1 V) in mono-crystalline CdTe devices. A comparative study of applying p-type a-Si:H contacts in poly-crystalline CdTe solar cells is performed, resulting in absolute voltage gain of 53 mV over using the standard tellurium contacts.
ContributorsShi, Jianwei (Author) / Holman, Zachary (Thesis advisor) / Bowden, Stuart (Committee member) / Bertoni, Mariana (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2018