Matching Items (23)

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High-mobility Hydrogenated Indium Oxide without Introducing Water During Sputtering

Description

The key role of water to obtain high-mobility IO:H (hydrogenated indium oxide) layers has been well documented, but introducing the required tiny amount of water is technologically challenging. We first

The key role of water to obtain high-mobility IO:H (hydrogenated indium oxide) layers has been well documented, but introducing the required tiny amount of water is technologically challenging. We first use simulations to evidence the key role of high mobility for the transparent conductive oxide for high-efficiency crystalline silicon solar cells. Then, we investigate an approach to fabricate high-mobility IO:H that circumvent the introduction of water vapor, relying on water vapor from ambient air. A sputtering tool equipped with a residual gas analyzer allows partial pressure monitoring of hydrogen and water in the system, and to link the gas composition to the properties of the deposited films. To vary the residual water pressure, we varied the pumping time after opening the chamber and before starting the deposition to reach different base pressures (1. 10[superscript -5] mbar to 3. 10[superscript -7] mbar), which are mostly composed of residual water. An optimum base pressure around 3. 10[superscript -6] mbar—and not lower pressures—was found to yield the highest mobility values after annealing. An alternative approach by introducing a small flow of hydrogen together with argon and oxygen is also shown to provide promising results.

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  • 2016-09-23

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Plasma-initiated rehydrogenation of amorphous silicon to increase the temperature processing window of silicon heterojunction solar cells

Description

The dehydrogenation of intrinsic hydrogenated amorphous silicon (a-Si:H) at temperatures above approximately 300 °C degrades its ability to passivate silicon wafer surfaces. This limits the temperature of post-passivation processing steps during

The dehydrogenation of intrinsic hydrogenated amorphous silicon (a-Si:H) at temperatures above approximately 300 °C degrades its ability to passivate silicon wafer surfaces. This limits the temperature of post-passivation processing steps during the fabrication of advanced silicon heterojunction or silicon-based tandem solar cells. We demonstrate that a hydrogen plasma can rehydrogenate intrinsic a-Si:H passivation layers that have been dehydrogenated by annealing. The hydrogen plasma treatment fully restores the effective carrier lifetime to several milliseconds in textured crystalline silicon wafers coated with 8-nm-thick intrinsic a-Si:H layers after annealing at temperatures of up to 450 °C. Plasma-initiated rehydrogenation also translates to complete solar cells: A silicon heterojunction solar cell subjected to annealing at 450 °C (following intrinsic a-Si:H deposition) had an open-circuit voltage of less than 600 mV, but an identical cell that received hydrogen plasma treatment reached a voltage of over 710 mV and an efficiency of over 19%.

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  • 2016-07-19

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Optical Characterization of Silver-Doped Germanium-Chalcogenide Thin Films

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The purpose of this research is to optically characterize germanium-based chalcogenide thin films and evaluate how their properties change when the composition is altered. The composition changes based on if

The purpose of this research is to optically characterize germanium-based chalcogenide thin films and evaluate how their properties change when the composition is altered. The composition changes based on if the chalcogenide contains selenium or sulfur, if the film is 60 nanometers or 200 nanometers, and if the film is doped with silver (ranging from 0 nanometers to 30 nanometers). These amorphous germanium-chalcogenide thin films exhibit interesting properties when doped with silver, such as transporting ions within the film in addition to electron transport. Using optical characterization techniques such as UV-Vis spectroscopy, profilometry, and ellipsometry, parameters that describe the optical characteristics are found, including the absorption coefficient, refractive index, optical band gap energy, and information on the density of states. This research concludes that as silver content within the film increases, the optical bandgap energy decreases—this is a consistent trend in existing literature. Having a better understanding of the materials’ physical properties will be useful to aid in the creation of microsystems based on these materials by selecting optimal composition and growth conditions. Important applications using these materials are currently being researched, including variable capacitor devices relying on the ionic conductor behavior these materials display. The optical properties like the absorption coefficient and the optical bandgap energy are invaluable in designing these applications effectively.

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  • 2019-05

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Multi-Layer Optical Coatings Composed of Silicon Nanoparticles

Description

To compete with fossil fuel electricity generation, there is a need for higher efficiency solar cells to produce renewable energy. Currently, this is the best way to lower generation costs

To compete with fossil fuel electricity generation, there is a need for higher efficiency solar cells to produce renewable energy. Currently, this is the best way to lower generation costs and the price of energy [1]. The goal of this Barrett Honors Thesis is to design an optical coating model that has five or fewer layers (with varying thickness and refractive index, within the above range) and that has the maximum reflectance possible between 950 and 1200 nanometers for normally incident light. Manipulating silicon monolayers to become efficient inversion layers to use in solar cells aligns with the Ira. A Fulton Schools of Engineering research themes of energy and sustainability [2]. Silicon monolayers could be specifically designed for different doping substrates. These substrates could range from common-used materials such as boron and phosphorus, to rare-earth doped zinc oxides or even fullerene blends. Exploring how the doping material, and in what quantity, affects solar cell energy output could revolutionize the current production methods and commercial market. If solar cells can be manufactured more economically, yet still retain high efficiency rates, then more people will have access to alternate, "green" energy that does not deplete nonrenewable resources.

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Date Created
  • 2016-12

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Novel Applications to Si Heterojunction Solar Cells

Description

Proposed and tested were three different methods to deposit important layers of Silicon heterojunction solar cells (SHJs). If there were a shortage of Silver, Aluminum could be substituted for the

Proposed and tested were three different methods to deposit important layers of Silicon heterojunction solar cells (SHJs). If there were a shortage of Silver, Aluminum could be substituted for the contacts. If there were a shortage of Indium, Yttrium Zinc Oxide could be substituted. To improve the solar cell, the p and n type layers can be grown with hydrogenated nanocrystallline Silicon (nc-Si:H). 40% and 50% nc-Si:H has shown a maximum absorbance reduction of 5 times compared to hydrogenated amorphous Silicon (a-Si). The substitutions offer alternatives which increase the total possible amount of solar cell production, advancing toward completion of the Terrawatt challenge.

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Date Created
  • 2014-05

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Dry Aerosol Impaction of Mesoporous Silica Aerogels

Description

Aerogels are among the best known thermally insulating materials due their high porosities (>90%). This, in conjunction with their high transparency make them ideal candidates for highly insulating window coatings.

Aerogels are among the best known thermally insulating materials due their high porosities (>90%). This, in conjunction with their high transparency make them ideal candidates for highly insulating window coatings. However, current state of the art techniques involve time-consuming drying steps and poor mechanical robustness, severely limiting their wide-scale adaptation. By using a dry aerosol impaction process, synthesizing nanoparticles in a plasma, upstream of a slit-shaped nozzle and impacting these particles onto a substrate below, a novel way for producing mesoporous silica aerogels is shown. This removes the need for solution-based processing, improving the potential for high throughput. Thick (~100um), 90% mesoporous silica has been characterized showing low effective thermal conductivity (~0.02 W/mK) and high transparency (>90%). The morphology of these coatings were analyzed showing tight pore distributions. Film adhesion and stress have shown themselves to be major hurdles during the development of these coatings and will be the focus of future work.

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Date Created
  • 2018-05

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Balance of System Cost Analysis to Investigate Future Economic Competitiveness of Tandem Solar Cells

Description

As single junction silicon based solar cells approach their Shockley\u2014Queasier (SQ) conversion efficiency limits, tandem solar cells (TSC) provide an attractive prospect for higher efficiency cells. Although TSCs have been

As single junction silicon based solar cells approach their Shockley\u2014Queasier (SQ) conversion efficiency limits, tandem solar cells (TSC) provide an attractive prospect for higher efficiency cells. Although TSCs have been shown to be more efficient, their higher fabrication costs are a limiting factor for their economic competitiveness and large-scale integration in PV power systems. Current literature suggests that even with reduced costs of fabrication in the future, TSCs still offer no competitive benefit for integration in utility-scale systems and may yield minimal benefits only in places where area-related costs are high. This work investigates Balance of Systems (BoS) circumstances under which TSCs can attain economic viability in scenarios where the necessary technological advances are made to increase the efficiency of solar cells beyond the SQ limit.

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  • 2017-05

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Electrical and Optical Characterization of Wide Band Gap Materials in the Zinc and Tin Oxynitride Family

Description

The investigation into wide band gap semiconductors for use in tandem solar cells has become an increasingly more researched area with many new absorbers outlining the landscape. Pairing silicon with

The investigation into wide band gap semiconductors for use in tandem solar cells has become an increasingly more researched area with many new absorbers outlining the landscape. Pairing silicon with another cheap wide band gap semiconductor absorber can generate more efficient solar cell, which could continue to drive up the energy output from solar. One such recently researched wide band gap absorber is ZnSnN2. ZnSnN2 proves too difficult to form under most conditions, but has the necessary band gap to make it a potential earth abundant solar absorber. The deposition process for ZnSnN2 is usually conducted with Zn and Sn metal targets while flowing N2 gas. Due to restrictions with chamber depositions, instead ZnO and SnO2 targets were sputtered with N2 gas to attempt to form separate zinc and tin oxynitrides as an initial single target study prior to future combinatorial studies. The electrical and optical properties and crystal structure of these thin films were analyzed to determine the nitrogen incorporation in the thin films through X-ray diffraction, UV-Vis spectrophotometry, and 4-point probe measurements. The SnO2 thin films showed a clear response in the absorption coefficient leading but showed no observable XRD peak shift. Thus, it is unlikely that substantial amounts of nitrogen were incorporated into SnO¬2. ZnO showed a clear response increase in conductivity with N2 with an additional shift in the XRD peak at 300 °C and potential secondary phase peak. Nitrogen incorporation was achieved with fair amounts of certainty for the ZnO thin films.

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Date Created
  • 2019-05

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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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The design and demonstration of monocrystalline CdTe/MgCdTe double-heterostructure solar cells

Description

Cadmium Telluride (CdTe) possesses preferable optical properties for photovoltaic (PV) applications: a near optimum bandgap of 1.5 eV, and a high absorption coefficient of over 15,000 cm-1 at the band

Cadmium Telluride (CdTe) possesses preferable optical properties for photovoltaic (PV) applications: a near optimum bandgap of 1.5 eV, and a high absorption coefficient of over 15,000 cm-1 at the band edge. The detailed-balance limiting efficiency is 32.1% with an open-circuit voltage (Voc) of 1.23 V under the AM1.5G spectrum. The record polycrystalline CdTe thin-film cell efficiency has reached 22.1%, with excellent short-circuit current densities (Jsc) and fill-factors (FF). However, the Voc (~900 mV) is still far below the theoretical value, due to the large non-radiative recombination in the polycrystalline CdTe absorber, and the low-level p-type doping.

Monocrystalline CdTe/MgCdTe double-heterostructures (DHs) grown on lattice-matched InSb substrates have demonstrated impressively long carrier lifetimes in both unintentionally doped and Indium-doped n-type CdTe samples. The non-radiative recombination inside of, and at the interfaces of the CdTe absorbers in CdTe/MgCdTe DH samples has been significantly reduced due to the use of lattice-matched InSb substrates, and the excellent passivation provided by the MgCdTe barrier layers. The external luminescent quantum efficiency (η_ext) of n-type CdTe/MgCdTe DHs is up to 3.1%, observed from a 1-µm-thick CdTe/MgCdTe DH doped at 1017 cm-3. The 3.1% η_ext corresponds to an internal luminescent quantum efficiency (η_int) of 91%. Such a high η_ext gives an implied Voc, or quasi-Fermi-level splitting, of 1.13 V.

To obtain actual Voc, the quasi-Fermi-level splitting should be extracted to outside the circuit using a hole-selective contact layer. However, CdTe is difficult to be doped p-type, making it challenging to make efficient PN junction CdTe solar cells. With the use of MgCdTe barrier layers, the hole-contact layer can be defective without affecting the voltage. P-type hydrogenated amorphous silicon is an effective hole-selective contact for CdTe solar cells, enabling monocrystalline CdTe/MgCdTe DH solar cells to achieve Voc over 1.1 V, and a maximum active area efficiency of 18.8% (Jsc = 23.3 mA/cm2, Voc = 1.114 V, and FF = 72.3%). The knowledge gained through making the record-efficiency monocrystalline CdTe cell, particularly the n-type doping and the double-heterostructure design, may be transferable to polycrystalline CdTe thin-film cells and improve their competitiveness in the PV industry.

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Date Created
  • 2016