Matching Items (20)
Description
Increasing the conversion efficiencies of photovoltaic (PV) cells beyond the single junction theoretical limit is the driving force behind much of third generation solar cell research. Over the last half century, the experimental conversion efficiency of both single junction and tandem solar cells has plateaued as manufacturers and researchers have optimized various materials and structures. While existing materials and technologies have remarkably good conversion efficiencies, they are approaching their own limits. For example, tandem solar cells are currently well developed commercially but further improvements through increasing the number of junctions struggle with various issues related to material interfacial defects. Thus, there is a need for novel theoretical and experimental approaches leading to new third generation cell structures. Multiple exciton generation (MEG) and intermediate band (IB) solar cells have been proposed as third generation alternatives and theoretical modeling suggests they can surpass the detailed balance efficiency limits of single junction and tandem solar cells. MEG or IB solar cell has a variety of advantages enabling the use of low bandgap materials. Integrating MEG and IB with other cell types to make novel solar cells (such as MEG with tandem, IB with tandem or MEG with IB) potentially offers improvements by employing multi-physics effects in one device. This hybrid solar cell should improve the properties of conventional solar cells with a reduced number of junction, increased light-generated current and extended material selections. These multi-physics effects in hybrid solar cells can be achieved through the use of nanostructures taking advantage of the carrier confinement while using existing solar cell materials with excellent characteristics. This reduces the additional cost to develop novel materials and structures. In this dissertation, the author develops thermodynamic models for several novel types of solar cells and uses these models to optimize and compare their properties to those of existing PV cells. The results demonstrate multiple advantages from combining MEG and IB technology with existing solar cell structures.
ContributorsLee, Jongwon (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Bowden, Stuart (Committee member) / Roedel, Ronald (Committee member) / Goodnick, Stephen (Committee member) / Schroder, Dieter (Committee member) / Arizona State University (Publisher)
Created2014
Description
Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si heterojunction (SHJ) have a potential to overcome the limitations of the conventional diffused junction solar cells and become the next industry standard manufacturing technology of solar cells. A brand feature of SHJ technology is ultrapassivated surfaces with already demonstrated 750 mV open circuit voltages (Voc) and 24.7% efficiency on large area solar cell. Despite very good results achieved in research and development, large volume manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main objectives of this work were to develop a SHJ solar cell fabrication flow using industry compatible tools and processes in a pilot production environment, study the interactions between the used fabrication steps, identify the minimum set of optimization parameters and characterization techniques needed to achieve 20% baseline efficiency, and analyze the losses of power in fabricated SHJ cells by numerical and analytical modeling. This manuscript presents a detailed description of a SHJ solar cell fabrication flow developed at ASU Solar Power Laboratory (SPL) which allows large area solar cells with >750 mV Voc. SHJ cells on 135 um thick 153 cm2 area wafers with 19.5% efficiency were fabricated. Passivation quality of (i)a-Si:H film, bulk conductivity of doped a-Si films, bulk conductivity of ITO, transmission of ITO and the thickness of all films were identified as the minimum set of optimization parameters necessary to set up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured wafers to minimize the concentration of surface impurities and to avoid epitaxial growth of a-Si films was found to be a key challenge in achieving a repeatable and uniform passivation. This work resolved this issue by using a multi-step cleaning process based on sequential oxidation in nitric/acetic acids, Piranha and RCA-b solutions. The developed process allowed state of the art surface passivation with perfect repeatability and negligible reflectance losses. Two additional studies demonstrated 750 mV local Voc on 50 micron thick SHJ solar cell and < 1 cm/s effective surface recombination velocity on n-type wafers passivated by a-Si/SiO2/SiNx stack.
ContributorsHerasimenka, Stanislau Yur'yevich (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Bowden, Stuart G (Thesis advisor) / Tracy, Clarence (Committee member) / Vasileska, Dragica (Committee member) / Holman, Zachary (Committee member) / Sinton, Ron (Committee member) / Arizona State University (Publisher)
Created2013
Description
Multiple quantum well (MQW) structures have been employed in a variety of solid state devices. The InGaAs/GaAs material system is of special interest for many optoelectronic applications. This study examines epitaxial growth and defect creation in InGaAs/GaAs MQWs at its initial stage. Correlations between physical properties, crystal perfection of epitaxial structures, and growth conditions under which desired properties are achieved appear as highly important for the realization and final performance of semiconductor based devices.
Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature Tdep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response.
Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature Tdep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response.
ContributorsKarow, Matthias (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Faleev, Nikolai N (Committee member) / Ning, Cun-Zheng (Committee member) / Arizona State University (Publisher)
Created2014
Description
In the nano-regime many materials exhibit properties that are quite different from their bulk counterparts. These nano-properties have been shown to be useful in a wide range of applications with nanomaterials being used for catalysts, in energy production, as protective coatings, and in medical treatment. While there is no shortage of exciting and novel applications, the world of nanomaterials suffers from a lack of large scale manufacturing techniques. The current methods and equipment used for manufacturing nanomaterials are generally slow, expensive, potentially dangerous, and material specific. The research and widespread use of nanomaterials has undoubtedly been hindered by this lack of appropriate tooling. This work details the effort to create a novel nanomaterial synthesis and deposition platform capable of operating at industrial level rates and reliability.
The tool, referred to as Deppy, deposits material via hypersonic impaction, a two chamber process that takes advantage of compressible fluids operating in the choked flow regime to accelerate particles to up several thousand meters per second before they impact and stick to the substrate. This allows for the energetic separation of the synthesis and deposition processes while still behaving as a continuous flow reactor giving Deppy the unique ability to independently control the particle properties and the deposited film properties. While the ultimate goal is to design a tool capable of producing a broad range of nanomaterial films, this work will showcase Deppy's ability to produce silicon nano-particle films as a proof of concept.
By adjusting parameters in the upstream chamber the particle composition was varied from completely amorphous to highly crystalline as confirmed by Raman spectroscopy. By adjusting parameters in the downstream chamber significant variation of the film's density was achieved. Further it was shown that the system is capable of making these adjustments in each chamber without affecting the operation of the other.
The tool, referred to as Deppy, deposits material via hypersonic impaction, a two chamber process that takes advantage of compressible fluids operating in the choked flow regime to accelerate particles to up several thousand meters per second before they impact and stick to the substrate. This allows for the energetic separation of the synthesis and deposition processes while still behaving as a continuous flow reactor giving Deppy the unique ability to independently control the particle properties and the deposited film properties. While the ultimate goal is to design a tool capable of producing a broad range of nanomaterial films, this work will showcase Deppy's ability to produce silicon nano-particle films as a proof of concept.
By adjusting parameters in the upstream chamber the particle composition was varied from completely amorphous to highly crystalline as confirmed by Raman spectroscopy. By adjusting parameters in the downstream chamber significant variation of the film's density was achieved. Further it was shown that the system is capable of making these adjustments in each chamber without affecting the operation of the other.
ContributorsFirth, Peter (Author) / Holman, Zachary C (Thesis advisor) / Kozicki, Michael (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2015
Description
Silicon photovoltaics (PV) is approaching its theoretical efficiency limit as a single-junction technology. To break this limit and further lower the PV-generated levelized cost of electricity, it is necessary to engineer a silicon-based “tandem” technology in which a solar cell of another material is stacked on top of silicon to make more efficient use of the full solar spectrum.
This dissertation understands and develops four aspects of silicon-based tandem PV technology. First, a new “spectral efficiency” concept is proposed to understand how tandem cells should be designed and to identify the best tandem partners for silicon cells. Using spectral efficiency, a top-cell-design guide is constructed for silicon-based tandems that sets efficiency targets for top cells with various bandgaps to achieve targeted tandem efficiencies.
Second, silicon heterojunction solar cells are tuned to the near-infrared spectrum to enable world-record perovskite/silicon tandems both in two- and four-terminal configurations. In particular, for the 23.6%-efficient two-terminal tandem, a single-side textured silicon bottom cell is fabricated with a low-refractive-index silicon nanoparticle layer as a rear reflector. This design boosts the current density to 18.5 mA/cm2; this value exceeds that of any other silicon bottom cell and matches that of the top cell.
Third, “PVMirrors” are proposed as a novel tandem architecture to integrate silicon cells with various top cells. A strength of the design is that the PVMirror collects diffuse light as a concentrating technology. With this concept, a gallium-arsenide/silicon PVMirror tandem is demonstrated with an outdoor efficiency of 29.6%, with respect to the global irradiance.
Finally, a simple and versatile analytical model is constructed to evaluate the cost competitiveness of an arbitrary tandem against its sub-cell alternatives. It indicates that tandems will become increasingly attractive in the market, as the ratio of sub-cell module cost to area-related balance-of-system cost—the key metric that will determine the market success or failure of tandems—is decreasing.
As an evolution of silicon technology, silicon-based tandems are the future of PV. They will allow more people to have access to clean energy at ultra-low cost. This thesis defines both the technological and economic landscape of silicon-based tandems, and makes important contributions to this tandem future.
This dissertation understands and develops four aspects of silicon-based tandem PV technology. First, a new “spectral efficiency” concept is proposed to understand how tandem cells should be designed and to identify the best tandem partners for silicon cells. Using spectral efficiency, a top-cell-design guide is constructed for silicon-based tandems that sets efficiency targets for top cells with various bandgaps to achieve targeted tandem efficiencies.
Second, silicon heterojunction solar cells are tuned to the near-infrared spectrum to enable world-record perovskite/silicon tandems both in two- and four-terminal configurations. In particular, for the 23.6%-efficient two-terminal tandem, a single-side textured silicon bottom cell is fabricated with a low-refractive-index silicon nanoparticle layer as a rear reflector. This design boosts the current density to 18.5 mA/cm2; this value exceeds that of any other silicon bottom cell and matches that of the top cell.
Third, “PVMirrors” are proposed as a novel tandem architecture to integrate silicon cells with various top cells. A strength of the design is that the PVMirror collects diffuse light as a concentrating technology. With this concept, a gallium-arsenide/silicon PVMirror tandem is demonstrated with an outdoor efficiency of 29.6%, with respect to the global irradiance.
Finally, a simple and versatile analytical model is constructed to evaluate the cost competitiveness of an arbitrary tandem against its sub-cell alternatives. It indicates that tandems will become increasingly attractive in the market, as the ratio of sub-cell module cost to area-related balance-of-system cost—the key metric that will determine the market success or failure of tandems—is decreasing.
As an evolution of silicon technology, silicon-based tandems are the future of PV. They will allow more people to have access to clean energy at ultra-low cost. This thesis defines both the technological and economic landscape of silicon-based tandems, and makes important contributions to this tandem future.
ContributorsYu, Zhengshan (Author) / Holman, Zachary C (Thesis advisor) / Zhang, Yong-Hang (Committee member) / Bowden, Stuart G (Committee member) / King, Richard R (Committee member) / Arizona State University (Publisher)
Created2018
Description
Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps.
III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells.
III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of
ii
InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation.
In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated.
III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells.
III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of
ii
InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation.
In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated.
ContributorsVadiee, Ehsan (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Doolittle, William A (Thesis advisor) / Arizona State University (Publisher)
Created2019
Description
Semiconductor nanostructures are promising building blocks for light management in thin silicon solar cells and silicon-based tandems due their tunable optical properties. The present dissertation is organized along three main research areas: (1) characterization and modeling of III-V nanowires as active elements of solar cell tandems, (2) modeling of silicon nanopillars for reduced optical losses in ultra-thin silicon solar cells, and (3) characterization and modeling of nanoparticle-based optical coatings for light management.
First, the recombination mechanisms in polytype GaAs nanowires are studied through photoluminescence measurements coupled with rate equation analysis. When photons are absorbed in polytype nanowires, electrons and holes quickly thermalize to the band-edges of the zinc-blende and wurtzite phases, recombining indirectly in space across the type-II offset. Using a rate equation model, different configurations of polytype defects along the nanowire are investigated, which compare well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects. The presented analysis is a path towards predicting the performance of nanowire-based solar cells.
Following this topic, the optical mechanisms in silicon nanopillar arrays are investigated using full-wave optical simulations in comparison to measured reflectance data. The simulated electric field energy density profiles are used to elucidate the mechanisms contributing to the reduced front surface reflectance. Strong forward scattering and resonant absorption are observed for shorter- and longer- aspect ratio nanopillars, respectively, with the sub-wavelength periodicity causing additional diffraction. Their potential for light-trapping is investigated using full-wave optical simulation of an ultra-thin nanostructured substrate, where the conventional light-trapping limit is exceeded for near-bandgap wavelengths.
Finally, the correlation between the optical properties of silicon nanoparticle layers to their respective pore size distributions is investigated using optical and structural characterization coupled with full-wave optical simulation. The presence of
scattering is experimentally correlated to wider pore size distributions obtained from nitrogen adsorption measurements. The correlation is validated with optical simulation of random and clustered structures, with the latter approximating experimental. Reduced structural inhomogeneity in low-refractive-index nanoparticle inter-layers at the metal/semiconductor interface improves their performance as back reflectors, while reducing parasitic absorption in the metal.
First, the recombination mechanisms in polytype GaAs nanowires are studied through photoluminescence measurements coupled with rate equation analysis. When photons are absorbed in polytype nanowires, electrons and holes quickly thermalize to the band-edges of the zinc-blende and wurtzite phases, recombining indirectly in space across the type-II offset. Using a rate equation model, different configurations of polytype defects along the nanowire are investigated, which compare well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects. The presented analysis is a path towards predicting the performance of nanowire-based solar cells.
Following this topic, the optical mechanisms in silicon nanopillar arrays are investigated using full-wave optical simulations in comparison to measured reflectance data. The simulated electric field energy density profiles are used to elucidate the mechanisms contributing to the reduced front surface reflectance. Strong forward scattering and resonant absorption are observed for shorter- and longer- aspect ratio nanopillars, respectively, with the sub-wavelength periodicity causing additional diffraction. Their potential for light-trapping is investigated using full-wave optical simulation of an ultra-thin nanostructured substrate, where the conventional light-trapping limit is exceeded for near-bandgap wavelengths.
Finally, the correlation between the optical properties of silicon nanoparticle layers to their respective pore size distributions is investigated using optical and structural characterization coupled with full-wave optical simulation. The presence of
scattering is experimentally correlated to wider pore size distributions obtained from nitrogen adsorption measurements. The correlation is validated with optical simulation of random and clustered structures, with the latter approximating experimental. Reduced structural inhomogeneity in low-refractive-index nanoparticle inter-layers at the metal/semiconductor interface improves their performance as back reflectors, while reducing parasitic absorption in the metal.
ContributorsVulic, Natasa (Author) / Goodnick, Stephen M (Thesis advisor) / Honsberg, C. (Christiana B.) (Committee member) / Holman, Zachary C (Committee member) / Smith, David J. (Committee member) / Arizona State University (Publisher)
Created2019
Description
High photoluminescence (PL) quantum yields reported from amorphous (a-Si) and crystalline (c-Si) nanoparticles have opened up lots of possibilities for use of silicon in optical applications such as light emitting diodes (LEDs), photonics and solar cells with added processing and cost benefits. However, the PL response and the mechanisms behind it are highly dependent on the matrix in which the nanoparticles are grown and on the growth method. While, the bottom-up approach for deposition of free standing nanoparticles seem to be perfectly suited for large area deposition for LED and solar cell applications, the dominant growth techniques (laser ablation and pyrolysis) have been shown to suffer from limitations in control over size distribution of nanoparticles and the requirement of equipment capable of withstanding high temperature. This led to the exploration of plasma based synthesis methods in this work.
Towards this effort, the development and automation of a novel tool “Anny” for synthesis of silicon nanoparticles using non-thermal plasma chamber is reported. These nanoparticles are then accelerated due to choked flow through a nozzle leading to substrate independent deposition. The nanoparticle properties are characterized against precursor gas flow rates and RF power to identify the optimum growth conditions for a stable, continuous deposition. It is found that amorphous nanoparticles offer a wide variety of chamber conditions for growth with a high throughput, stable plasma for continuous, long term operations.
The quantum confinement model for crystalline and spatial confinement models for amorphous nanoparticles in our size regime (6-8nm) are suggested for free standing nanoparticles and we report a high PL output from well passivated amorphous nanoparticles.
The PL output and its dependence on stability of surface hydrogen passivation is explored using Fourier Transform Infrared spectroscopy (FTIR). It is shown that the amorphous nanoparticles have a better and more stable passivation compared to crystalline nanoparticles grown under similar conditions. Hence, we show a-Si nanoparticles as exciting alternatives for optical applications to c-Si nanoparticles.
Towards this effort, the development and automation of a novel tool “Anny” for synthesis of silicon nanoparticles using non-thermal plasma chamber is reported. These nanoparticles are then accelerated due to choked flow through a nozzle leading to substrate independent deposition. The nanoparticle properties are characterized against precursor gas flow rates and RF power to identify the optimum growth conditions for a stable, continuous deposition. It is found that amorphous nanoparticles offer a wide variety of chamber conditions for growth with a high throughput, stable plasma for continuous, long term operations.
The quantum confinement model for crystalline and spatial confinement models for amorphous nanoparticles in our size regime (6-8nm) are suggested for free standing nanoparticles and we report a high PL output from well passivated amorphous nanoparticles.
The PL output and its dependence on stability of surface hydrogen passivation is explored using Fourier Transform Infrared spectroscopy (FTIR). It is shown that the amorphous nanoparticles have a better and more stable passivation compared to crystalline nanoparticles grown under similar conditions. Hence, we show a-Si nanoparticles as exciting alternatives for optical applications to c-Si nanoparticles.
ContributorsGarg, Prateek (Author) / Holman, Zachary C (Thesis advisor) / Zhang, Yong H (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2015
Description
Solar energy is leading in renewable energy sources and the aspects surrounding the efforts to harvest light are gaining importance. One such aspect is increasing the light absorption, where heliotropism comes into play. Heliotropism, the ability to track the sun across the sky, can be integrated with solar cells for more efficient photon collection and other optoelectronic systems. Inspired by plants, which optimize incident sunlight in nature, several researchers have made artificial heliotropic and phototropic systems. This project aims to design, synthesize and characterize a material system and evaluate its application in a phototropic system. A gold nanoparticle (Au NP) incorporated poly(N-isopropylacrylamide) (PNIPAAm) hydrogel was synthesized as a photo-thermo-responsive material in our phototropic system. The Au NPs generate heat from the incident via plasmonic resonance to induce a volume phase change of the thermo-responsive hydrogel PNIPAAm. PNIPAAm shrinks or swells at temperature above or below 32°C. Upon irradiation, the Au NP-PNIPAAm micropillar actuates, specifically bending toward the incident light and precisely following the varying incident angle.
Swelling ratio tests, bending angle tests with a static incident light and bending tests with varying angles were carried out on hydrogel samples with varying Au NP concentrations. Swelling ratios ranging from 1.45 to 2.9 were recorded for pure hydrogel samples and samples with very low Au NP concentrations. Swelling ratios of 2.41 and 3.37 were calculated for samples with low and high concentrations of Au NPs, respectively. A bending of up to 88° was observed in Au NP-hydrogel pillars with a low Au NP concentration with a 90° incident angle. The light tracking performance was assessed by the slope of the pillar Bending angle (response angle) vs. Incident light angle plot. A slope of 1 indicates ideal tracking with top of the pillar being normal to the incident light, maximizing the photon absorption. Slopes of 0.82 and 0.56 were observed for the low and high Au NP concentration samples. The rapid and precise incident light tracking of our system has shown the promise in phototropic applications.
Swelling ratio tests, bending angle tests with a static incident light and bending tests with varying angles were carried out on hydrogel samples with varying Au NP concentrations. Swelling ratios ranging from 1.45 to 2.9 were recorded for pure hydrogel samples and samples with very low Au NP concentrations. Swelling ratios of 2.41 and 3.37 were calculated for samples with low and high concentrations of Au NPs, respectively. A bending of up to 88° was observed in Au NP-hydrogel pillars with a low Au NP concentration with a 90° incident angle. The light tracking performance was assessed by the slope of the pillar Bending angle (response angle) vs. Incident light angle plot. A slope of 1 indicates ideal tracking with top of the pillar being normal to the incident light, maximizing the photon absorption. Slopes of 0.82 and 0.56 were observed for the low and high Au NP concentration samples. The rapid and precise incident light tracking of our system has shown the promise in phototropic applications.
ContributorsGopalakrishna, Hamsini (Author) / He, Ximin (Thesis advisor) / Holman, Zachary C (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2016
Description
A basic theory and terminology that comprehensively applies to all different types
of contacts in silicon solar cells has, thus far, been elusive. While the well established diode model has been applied to many of the complex contacts, the theory is not adequate to intuitively describe the characteristics of novel contacts. This thesis shows that the many desirable characteristics of contacts that are discussed in the literature—carrier selectivity, passivation, and low majority carrier conductance, key among them—originate from the resistance to electrons and holes in the contact. These principles are applied to describe a few popular contact technologies in order to pave the path to envisioning novel contacts. Metrics for contact performance is introduced to quantify each of the above characteristics using the two carrier resistances. The the validity of the proposed metrics is explored using extensive PC-1D simulations.
of contacts in silicon solar cells has, thus far, been elusive. While the well established diode model has been applied to many of the complex contacts, the theory is not adequate to intuitively describe the characteristics of novel contacts. This thesis shows that the many desirable characteristics of contacts that are discussed in the literature—carrier selectivity, passivation, and low majority carrier conductance, key among them—originate from the resistance to electrons and holes in the contact. These principles are applied to describe a few popular contact technologies in order to pave the path to envisioning novel contacts. Metrics for contact performance is introduced to quantify each of the above characteristics using the two carrier resistances. The the validity of the proposed metrics is explored using extensive PC-1D simulations.
ContributorsKoswatta, Priyaranga L (Author) / Holman, Zachary C (Thesis advisor) / King, Richard (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2016