Arturo Ayon | University of Texas at San Antonio (original) (raw)

Papers by Arturo Ayon

2019 Symposium on Design, Test, Integration & Packaging of MEMS and MOEMS (DTIP)

The computational modeling of a 3D Metamaterial beam array has been performed employing a commerc... more The computational modeling of a 3D Metamaterial beam array has been performed employing a commercially available Finite Differences Time Domain (FDTD) software package, to determine the prospective suitability of the proposed geometry in the demonstration of filters and resonators operating in the THz regime. The modeled structure comprises arrays of metallic, horizontal beams supported by vertical anchors placed on a semi-infinite silicon substrate with a refractive index of n=3.42+i0.01 with the boundary conditions considered periodic in the directions parallel to the beams and a Perfectly Matched Layer (PML) in the direction perpendicular to them. The simulations indicate a high pass filter behavior with a strong dependence on the unit cell length such that the cutoff frequency decreases as the length of the beams increase. Furthermore, due to the symmetry built in the system, the Metamaterial array is insensitive to the use of transverse electric (TE) or transverse magnetic (TM) polarization (Abstract)

Micro and Nano Engineering, 2022

2017 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2017

Terahertz Time-Domain Spectroscopy (THz-TDS) has become an extremely important technique to chara... more Terahertz Time-Domain Spectroscopy (THz-TDS) has become an extremely important technique to characterize the optical properties of materials of interest in this regime. Specifically, UV photoresponsive semiconductors (i.e. wide-bandgap semiconductors) have attracted an ever increasing attention due to their prospective application in reconfigurable devices. We report a systematic error due to the presence of UV photons in a TDS set up, and we describe a systematic correction to overcome this error and discuss a preliminary explanation for the observed effect. An additional experiment employing Carbon Quantum Dots (QDs) was carried out to verify the aforementioned corrective method.

2017 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2017

We discuss the synthesis of zinc oxide (ZnO) and cadmium telluride (CdTe) quantum dots (QDs), the... more We discuss the synthesis of zinc oxide (ZnO) and cadmium telluride (CdTe) quantum dots (QDs), the characterization of their optical properties on their own as well as combined, and their influence on the power conversion efficiency of photovoltaic structures when the QD blends were dispersed in a polymeric solution and deployed on the window surface of polycrystalline Si solar cells as down-shifting photoluminescent (PL) coatings. Upon the incorporation of increasing amounts of ZnO QDs into a colloidal solution of CdTe, the characteristic CdTe PL peak location was monotonically red-shifted from 598 nm to 611 nm. A 1:3 volumetric combination CdTe:ZnO QDs exhibited a the power conversion efficiency increase from 14.71% for an uncoated solar cell to 15.02% with the QD layer, that is, an approximate improvement of approximately 2%.

The present study details the synthesis and characterization of photoluminescent ZnO quantum dots... more The present study details the synthesis and characterization of photoluminescent ZnO quantum dots (QDs) and their effect in the performance of in-house-fabricated solar cells. The colloidal ZnO quantum dots were synthetized in an ethanol-based solution, where the growing dynamics was controlled by the pH of the precursor solutions with fixed reaction times. The down-shifting effects of the colloidal quantum dots were characterized by absorption and photoluminescence spectra. Additionally, the crystallographic characterization was performed employing X-ray diffraction (XRD). Planar single-crystal silicon solar cells were fabricated, and their window side was spin-cast with different pH-tuned ZnO quantum dots dispersed in polymethylmethacrylate (PMMA). To evaluate the impact of the aforementioned ZnO QDs films on solar cells, the power conversion efficiency (PCE) values were obtained from the J-V curves generated in a solar simulator and the short-current density (Jsc) was corroborate...

Solar Cells and Light Management, 2020

Abstract The present chapter describes the influence of quantum dots (QDs) on the performance of ... more Abstract The present chapter describes the influence of quantum dots (QDs) on the performance of commercially available, as well as in-house fabricated planar and nanotexturized Si-based solar cells. The experimental observations indicate that higher power conversion efficiency values can be achieved by exploiting the Stokes or down-shifting effect of different varieties of QDs. The down-shifting effect of the aforementioned nanocrystals is reflected in the external quantum efficiency that exhibits a better response in the UV range, due to the absorption of the UV photons by the QDs involved, and the subsequent emission at longer wavelengths that are more suitable for polycrystalline and single crystal silicon (c-Si) absorption. In view of the availability of precursors for a number of photoluminescent QDs, as well as the robustness and relative simplicity of the associated synthesis schemes, it is warranted to conclude that semiconductor QDs can be construed as a viable alternative to improve the performance of photovoltaic devices.

Journal of Materials Science: Materials in Electronics, 2021

In this study, we report the synthesis and characterization of zinc sulfide quantum dots (ZnS QDs... more In this study, we report the synthesis and characterization of zinc sulfide quantum dots (ZnS QDs) coated with different concentrations of polyvinylpyrrolidone (PVP), as well as their deployment as luminescent films on the window side of previously characterized commercial silicon solar cells to quantify their influence on the power conversion efficiency (PCE). The synthesis of the semiconductor nanoparticles was carried out by the reaction of zinc nitrate with sodium sulfide in an aqueous solution at room temperature. XRD measurements indicated a cubic sphalerite phase of the QDs crystal structure, which was not modified by the addition of PVP in the synthesis process. However, the PVP concentration was a key parameter to modulate the size distribution and the luminescent intensity of the QDs, suggesting that an increase in the PVP concentration produced a slight decrease in the QDs size and improved their luminescent properties desired for the photovoltaic applications. The obtained nanoparticles presented great absorption of photons with energies above 3.72 eV and a broad intense blue photoluminescent emission centered at around 450 nm, under excitation of ultraviolet light of 325 nm. The implementation of the synthesized ZnS QDs as spectral response enhancer produced improvements on the performance of solar cells, leading to increases of 0.7%, 1.9%, and 6.1% on the efficiencies of commercial polycrystalline solar cells after the deposition of ZnS QDs synthesized without PVP, with 0.05 mM PVP and with 0.10 mM PVP, respectively.

Microscopy and Microanalysis, 2018

Open Journal of Inorganic Non-metallic Materials, 2019

The utilization of UV excitation to verify the terahertz (THz) wave modulation of hexagonal-shape... more The utilization of UV excitation to verify the terahertz (THz) wave modulation of hexagonal-shaped metamaterial (MM) arrays coated with synthesized photoluminescent, down-shifting ZnO quantum dots (QDs) of two different radius sizes, namely, 3.00 nm (pH 10) and 2.12 nm (pH 12), respectively is reported. In order to characterize the behavior of the MM before and after deployment of the ZnO QDs, THz time domain spectroscopy in transmission mode was employed. Upon exposure to UV excitation, the collected amplitude modulation values were 9.21% for the pH 12 and 4.55% for the pH 10 ZnO QDs, respectively. It is anticipated that the ability to actively tune the performance of otherwise passive structures will promote the proliferation of THz signal modulation devices in the near future.

Microsystem Technologies, 2019

We report an anomalous Stokes shift effect observed in colloidal solutions containing down-shifti... more We report an anomalous Stokes shift effect observed in colloidal solutions containing down-shifting Carbon quantum dots (CQDs) of different sizes that is expected to have a positive influence on the power conversion efficiency of photovoltaic structures. Specifically, with an excitation wavelength of 390 nm, individual colloidal solutions of CQDs whose diameter was determined by the applied current during synthesis, exhibited photoluminescent (PL) emission wavelength peaks centered at 420 nm. However, the colloidal solution comprising the mixture of all the previously synthesized CQDs of different diameters was observed to have an anomalous PL Stokes shift centered at 515 nm. Furthermore, the aforementioned anomalous SSE was also observed in CdTe QDs when added to the CQD mixed-solution (CMS). Thus, whereas a mixture of CdTe QDs of different sizes, exhibited a down-shifted photoluminescence centered at 555 nm, the peak was observed to have an anomalous Stokes shift centered at 580 nm when combined with the CMS. Quantum dot characterization included crystal structure analysis as well as photon absorption and photoluminescence wavelengths. Subsequently, the synthesized QDs were dispersed in a polymeric layer of PMMA and incorporated on functional and previously characterized solar cells, to quantify their influence in the electrical performance of the photovoltaic devices. The observations indicate an improvement in the PCE of 4.6% when incorporating Carbon QDs, 2.9% with CdTe QDs and 4.8% when employing both C and CdTe QDs.

Materials for Renewable and Sustainable Energy, 2019

This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and the... more This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and their use, either on their own or combined, as photoluminescent (PL) down-shifting nanostructured coatings aimed to enhance the photovoltaic efficiency of polycrystalline silicon solar cells. To this end, the front face of a set of silicon cells was coated with different volume ratios of the above-mentioned QDs, or some of its mixtures, dispersed in PMMA layers. Previously, the absorption and the PL (exc = 380 nm) response of the dispersions of the QDs were measured. It was observed that the PL response of the mixtures was strongly affected in location, spread, and intensity of the emission peak according to the volume ratio involved. As compared to the unmixed CdTe samples, a notorious red-shift of the main peak location was obtained for a couple of mixed QDs' dispersions, which was one of the project objectives given that Si solar cells respond better to photons with wavelengths in the 650-700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells covered with and without nanostructured PMMA/QDs layers tested under AM 1.5G solar simulator conditions. It was found that the use of the proposed mixtures of QDs gave an increase of 1.53% in solar cell power conversion efficiency.

Journal of Physics: Conference Series, 2016

We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) t... more We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) that exhibit absorption in the UV range of the solar spectrum and emit photons with wavelengths centered around 625 nm, a wavelength that is well suited for silicon absorption and electron-hole pair generation. We also report the fabrication and characterization of single crystal silicon (c-Si) solar cells with and without the aforementioned photo luminescent, down-shifting CdSe/CdS- QDs. The incorporation of these nanostructures triggered improvements in the performance of the devices, particularly in the open circuit voltage (Voc) and short circuit current density (Jsc) for which the measured values showed an increase from 543 to 546 mV and from 32.5 to 37.0 mA/cm2, respectively. The combined effect of the improved values led to an increment in the power conversion efficiency (PCE) from 12.01 to 13.54%. This increase represents a 12.7% improvement in the PCE of the fabricated devices. The effort described herein is considered a good fit to the generalized trend to improve the efficiency of solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

Journal of Physics: Conference Series, 2016

We report the synthesis of Zinc Oxide (ZnO) quantum dots (QDs) and their influence on the power c... more We report the synthesis of Zinc Oxide (ZnO) quantum dots (QDs) and their influence on the power conversion efficiency of photovoltaic devices. With an excitation wavelength of 340 nm the down-shifted emission peaks were observed to be located at 510 and 540 nm for colloidal solutions with pH values of 10 and 12, respectively. The largest PCE variation was observed to increase from 14.60% to 15.49% when dispersing the QDs extracted from a 4 mL colloidal solution that were subsequently dispersed in PMMA. This represents an improvement of ~6.1%.

2015 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2015

The synergistic utilization of nanoparticles of Au, Ag, and Au/Ag alloys, in combination with a p... more The synergistic utilization of nanoparticles of Au, Ag, and Au/Ag alloys, in combination with a photon capture scheme and selected thin films, has enabled the demonstration of photovoltaic structures on single-crystal silicon substrates with an efficiency of 15%. Additionally, hybrid solar cells (i.e organic/inorganic) have also been considered as a viable alternative to develop cost effective photovoltaic devices because the Schottky union between organic and inorganic materials can be formed employing low-temperatures fabrication methods. In this presentation we specifically describe a hybrid solar cell based on an ordered array of silicon nanopillars and the conductive polymer Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS). The proposed device comprises a thin layer of the aforementioned PEDOT:PSS on an array of silicon nanopillars that was formed utilizing electroless metal-assisted-chemical-etching methods. The performance characteristics of the produced solar cells was analyized in function of the height of said nanopillars. A maximum power conversion efficiency of 9.65% was observed for an optimized height of 400 nm, even without the utilization of antireflecting films on the front surface. The effect of ultrathin films of aluminum oxide (Al2O3) realized employing an atomic layer deposition tool was also included in this study and its utilization further increased the measured efficiency to 10.56%. Finally, with the specific aim of lowering the cost of solar cell manufacturing, additional tests have been carried out on structures with a total thickness of less than 20 μm in which the efficiency was observed to reach 7.7% when used in conjunction with multi-spiked nanoparticles of Au/Ag alloys. The utilization of CdTe quantum dots [1-3] on nanotexturized, relatively thin samples (150 μm) has permitted achieving an efficiency of 15%. Thus, the structures discussed in this presentation are considered promising towards the realization of high efficiency solar cells.

Microscopy and Microanalysis, 2018

International Journal of Sciences: Basic and Applied Research, 2017

An array of interdigitated microelectrodes was modeled, optimized, and tested for the contactless... more An array of interdigitated microelectrodes was modeled, optimized, and tested for the contactless alignment of living biological cells by negative dielectrophoresis (nDEP). The modeling focused on optimizing the x component, , of potential gradient to maximize the aligning force at minimal electric field to protect fragile cells. The optimized array was tested with micron-sized, hard polymeric beads and soft C2C12 mouse myoblast cells. The array completely aligned the initially random arrangement of beads after 15 s of applying 450 kHz sinewave electrical signal to the electrodes. Under the same conditions, the array was unable to move C2C12 cells due to their strong adhesion to the surface. However, the array was effective at directing the orientation of confluent C2C12 cells during their differentiation into myotubes for the formation of muscle fibers. The aligned myotubes were alive and capable of growth. The proposed approach has a potential for growing continuously orien...

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

Microsystem Technologies, 2015

In order to promote the widespread utilization of photovoltaic structures it is critical to incre... more In order to promote the widespread utilization of photovoltaic structures it is critical to increase their conversion efficiency and reduce their manufacturing cost. With these two goals in mind, planar and nanotexturized c-Si photovoltaic devices were built and their radiation incident side was covered with CdTe quantum dots (QDs) dispersed in a poly-methyl-methacrylate thin film ~100 nm thick. The measured performance with and without the CdTe QD layer indicates that the down-shifting photoluminescence (DSPL) of the synthesized QDs triggers an increase in the open circuit voltage and the short circuit current, resulting in an overall improvement of ~1.5 % in the power conversion efficiency of the produced devices. This observation can be understood as a result of decreasing the number of high energy photons that are prone to unproductive near surface absorption in c-Si p-n junction solar cells and the re-emission of lower energy photons that are more effectively bulk-absorbed in c-Si solar cells. The result of lowering the near surface photon absorption represents a reduction of carrier losses due to the high surface recombination velocity as well as the low lifetime that normally plague the diffused side of an active junction device. Additionally, solar cells with a layer of CdTe QDs exhibit a higher external quantum efficiency when compared to devices without the aforementioned quantum dots. Layers of CdTe QDs are economically attractive, highly photostable and their absorption and emission spectra have a strong dependence on particle size, therefore, they lend themselves to providing a broad DSPL spectrum and promoting the proliferation of photovoltaic structures.

2017 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2017

We report the synthesis and characterization of cadmium-telluride (CdTe) quantum dots (QDs) and t... more We report the synthesis and characterization of cadmium-telluride (CdTe) quantum dots (QDs) and their performance at Terahertz frequencies employing THz time-domain spectroscopy (THZ-TDS). The data was analyzed using a combination of Fast Fourier Transform (FFT) and effective medium theory to obtain the optical characteristics of the aforementioned CdTe QDs. Additionally, we performed similar measurements while exposing the QDs to a UV excitation with a wavelength of 302 nm and observed a noticeable variation in both the real and imaginary components in the index of refraction that could prove crucial in the demonstration of reconfigurable devices operating in the THz regime.

Applied Physics Letters, 2021

We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO subs... more We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO substrates at the relatively low temperatures of 120 o C, 150 o C, and 200 o C. The as-grown ZnO films are studied with aberration-corrected transmission electron microscopy and diffraction contrast, photoluminescence (PL) and surface photovoltage (SPV) spectroscopies. We find that the homoepitaxial films have monocrystalline structure with the density of basal stacking faults comparable to that of the substrate (∼ 10 11 cm −2) and that the stacking faults can induce high lattice strain due to their interaction with the inversion domain boundaries. The narrow excitonic PL linewidth (2 meV at 8 K) and sharp SPV bandgap transition confirm the high quality of the ZnO films. Despite similarities in the film properties, the growth temperature has an effect on the density and spatial distribution of intrinsic defects. Our results demonstrate a considerable potential of ALD ZnO homoepitaxy for fabricating high-quality ZnO nanostructures and attaining viable p-type ZnO. ZnO is a widely studied semiconductor with a unique set of physical properties and tremendous potential for various applications [1, 2]. Its wide bandgap (3.37 eV)

2019 Symposium on Design, Test, Integration & Packaging of MEMS and MOEMS (DTIP)

The computational modeling of a 3D Metamaterial beam array has been performed employing a commerc... more The computational modeling of a 3D Metamaterial beam array has been performed employing a commercially available Finite Differences Time Domain (FDTD) software package, to determine the prospective suitability of the proposed geometry in the demonstration of filters and resonators operating in the THz regime. The modeled structure comprises arrays of metallic, horizontal beams supported by vertical anchors placed on a semi-infinite silicon substrate with a refractive index of n=3.42+i0.01 with the boundary conditions considered periodic in the directions parallel to the beams and a Perfectly Matched Layer (PML) in the direction perpendicular to them. The simulations indicate a high pass filter behavior with a strong dependence on the unit cell length such that the cutoff frequency decreases as the length of the beams increase. Furthermore, due to the symmetry built in the system, the Metamaterial array is insensitive to the use of transverse electric (TE) or transverse magnetic (TM) polarization (Abstract)

Micro and Nano Engineering, 2022

2017 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2017

Terahertz Time-Domain Spectroscopy (THz-TDS) has become an extremely important technique to chara... more Terahertz Time-Domain Spectroscopy (THz-TDS) has become an extremely important technique to characterize the optical properties of materials of interest in this regime. Specifically, UV photoresponsive semiconductors (i.e. wide-bandgap semiconductors) have attracted an ever increasing attention due to their prospective application in reconfigurable devices. We report a systematic error due to the presence of UV photons in a TDS set up, and we describe a systematic correction to overcome this error and discuss a preliminary explanation for the observed effect. An additional experiment employing Carbon Quantum Dots (QDs) was carried out to verify the aforementioned corrective method.

2017 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2017

We discuss the synthesis of zinc oxide (ZnO) and cadmium telluride (CdTe) quantum dots (QDs), the... more We discuss the synthesis of zinc oxide (ZnO) and cadmium telluride (CdTe) quantum dots (QDs), the characterization of their optical properties on their own as well as combined, and their influence on the power conversion efficiency of photovoltaic structures when the QD blends were dispersed in a polymeric solution and deployed on the window surface of polycrystalline Si solar cells as down-shifting photoluminescent (PL) coatings. Upon the incorporation of increasing amounts of ZnO QDs into a colloidal solution of CdTe, the characteristic CdTe PL peak location was monotonically red-shifted from 598 nm to 611 nm. A 1:3 volumetric combination CdTe:ZnO QDs exhibited a the power conversion efficiency increase from 14.71% for an uncoated solar cell to 15.02% with the QD layer, that is, an approximate improvement of approximately 2%.

The present study details the synthesis and characterization of photoluminescent ZnO quantum dots... more The present study details the synthesis and characterization of photoluminescent ZnO quantum dots (QDs) and their effect in the performance of in-house-fabricated solar cells. The colloidal ZnO quantum dots were synthetized in an ethanol-based solution, where the growing dynamics was controlled by the pH of the precursor solutions with fixed reaction times. The down-shifting effects of the colloidal quantum dots were characterized by absorption and photoluminescence spectra. Additionally, the crystallographic characterization was performed employing X-ray diffraction (XRD). Planar single-crystal silicon solar cells were fabricated, and their window side was spin-cast with different pH-tuned ZnO quantum dots dispersed in polymethylmethacrylate (PMMA). To evaluate the impact of the aforementioned ZnO QDs films on solar cells, the power conversion efficiency (PCE) values were obtained from the J-V curves generated in a solar simulator and the short-current density (Jsc) was corroborate...

Solar Cells and Light Management, 2020

Abstract The present chapter describes the influence of quantum dots (QDs) on the performance of ... more Abstract The present chapter describes the influence of quantum dots (QDs) on the performance of commercially available, as well as in-house fabricated planar and nanotexturized Si-based solar cells. The experimental observations indicate that higher power conversion efficiency values can be achieved by exploiting the Stokes or down-shifting effect of different varieties of QDs. The down-shifting effect of the aforementioned nanocrystals is reflected in the external quantum efficiency that exhibits a better response in the UV range, due to the absorption of the UV photons by the QDs involved, and the subsequent emission at longer wavelengths that are more suitable for polycrystalline and single crystal silicon (c-Si) absorption. In view of the availability of precursors for a number of photoluminescent QDs, as well as the robustness and relative simplicity of the associated synthesis schemes, it is warranted to conclude that semiconductor QDs can be construed as a viable alternative to improve the performance of photovoltaic devices.

Journal of Materials Science: Materials in Electronics, 2021

In this study, we report the synthesis and characterization of zinc sulfide quantum dots (ZnS QDs... more In this study, we report the synthesis and characterization of zinc sulfide quantum dots (ZnS QDs) coated with different concentrations of polyvinylpyrrolidone (PVP), as well as their deployment as luminescent films on the window side of previously characterized commercial silicon solar cells to quantify their influence on the power conversion efficiency (PCE). The synthesis of the semiconductor nanoparticles was carried out by the reaction of zinc nitrate with sodium sulfide in an aqueous solution at room temperature. XRD measurements indicated a cubic sphalerite phase of the QDs crystal structure, which was not modified by the addition of PVP in the synthesis process. However, the PVP concentration was a key parameter to modulate the size distribution and the luminescent intensity of the QDs, suggesting that an increase in the PVP concentration produced a slight decrease in the QDs size and improved their luminescent properties desired for the photovoltaic applications. The obtained nanoparticles presented great absorption of photons with energies above 3.72 eV and a broad intense blue photoluminescent emission centered at around 450 nm, under excitation of ultraviolet light of 325 nm. The implementation of the synthesized ZnS QDs as spectral response enhancer produced improvements on the performance of solar cells, leading to increases of 0.7%, 1.9%, and 6.1% on the efficiencies of commercial polycrystalline solar cells after the deposition of ZnS QDs synthesized without PVP, with 0.05 mM PVP and with 0.10 mM PVP, respectively.

Microscopy and Microanalysis, 2018

Open Journal of Inorganic Non-metallic Materials, 2019

The utilization of UV excitation to verify the terahertz (THz) wave modulation of hexagonal-shape... more The utilization of UV excitation to verify the terahertz (THz) wave modulation of hexagonal-shaped metamaterial (MM) arrays coated with synthesized photoluminescent, down-shifting ZnO quantum dots (QDs) of two different radius sizes, namely, 3.00 nm (pH 10) and 2.12 nm (pH 12), respectively is reported. In order to characterize the behavior of the MM before and after deployment of the ZnO QDs, THz time domain spectroscopy in transmission mode was employed. Upon exposure to UV excitation, the collected amplitude modulation values were 9.21% for the pH 12 and 4.55% for the pH 10 ZnO QDs, respectively. It is anticipated that the ability to actively tune the performance of otherwise passive structures will promote the proliferation of THz signal modulation devices in the near future.

Microsystem Technologies, 2019

We report an anomalous Stokes shift effect observed in colloidal solutions containing down-shifti... more We report an anomalous Stokes shift effect observed in colloidal solutions containing down-shifting Carbon quantum dots (CQDs) of different sizes that is expected to have a positive influence on the power conversion efficiency of photovoltaic structures. Specifically, with an excitation wavelength of 390 nm, individual colloidal solutions of CQDs whose diameter was determined by the applied current during synthesis, exhibited photoluminescent (PL) emission wavelength peaks centered at 420 nm. However, the colloidal solution comprising the mixture of all the previously synthesized CQDs of different diameters was observed to have an anomalous PL Stokes shift centered at 515 nm. Furthermore, the aforementioned anomalous SSE was also observed in CdTe QDs when added to the CQD mixed-solution (CMS). Thus, whereas a mixture of CdTe QDs of different sizes, exhibited a down-shifted photoluminescence centered at 555 nm, the peak was observed to have an anomalous Stokes shift centered at 580 nm when combined with the CMS. Quantum dot characterization included crystal structure analysis as well as photon absorption and photoluminescence wavelengths. Subsequently, the synthesized QDs were dispersed in a polymeric layer of PMMA and incorporated on functional and previously characterized solar cells, to quantify their influence in the electrical performance of the photovoltaic devices. The observations indicate an improvement in the PCE of 4.6% when incorporating Carbon QDs, 2.9% with CdTe QDs and 4.8% when employing both C and CdTe QDs.

Materials for Renewable and Sustainable Energy, 2019

This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and the... more This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and their use, either on their own or combined, as photoluminescent (PL) down-shifting nanostructured coatings aimed to enhance the photovoltaic efficiency of polycrystalline silicon solar cells. To this end, the front face of a set of silicon cells was coated with different volume ratios of the above-mentioned QDs, or some of its mixtures, dispersed in PMMA layers. Previously, the absorption and the PL (exc = 380 nm) response of the dispersions of the QDs were measured. It was observed that the PL response of the mixtures was strongly affected in location, spread, and intensity of the emission peak according to the volume ratio involved. As compared to the unmixed CdTe samples, a notorious red-shift of the main peak location was obtained for a couple of mixed QDs' dispersions, which was one of the project objectives given that Si solar cells respond better to photons with wavelengths in the 650-700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells covered with and without nanostructured PMMA/QDs layers tested under AM 1.5G solar simulator conditions. It was found that the use of the proposed mixtures of QDs gave an increase of 1.53% in solar cell power conversion efficiency.

Journal of Physics: Conference Series, 2016

We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) t... more We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) that exhibit absorption in the UV range of the solar spectrum and emit photons with wavelengths centered around 625 nm, a wavelength that is well suited for silicon absorption and electron-hole pair generation. We also report the fabrication and characterization of single crystal silicon (c-Si) solar cells with and without the aforementioned photo luminescent, down-shifting CdSe/CdS- QDs. The incorporation of these nanostructures triggered improvements in the performance of the devices, particularly in the open circuit voltage (Voc) and short circuit current density (Jsc) for which the measured values showed an increase from 543 to 546 mV and from 32.5 to 37.0 mA/cm2, respectively. The combined effect of the improved values led to an increment in the power conversion efficiency (PCE) from 12.01 to 13.54%. This increase represents a 12.7% improvement in the PCE of the fabricated devices. The effort described herein is considered a good fit to the generalized trend to improve the efficiency of solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

Journal of Physics: Conference Series, 2016

We report the synthesis of Zinc Oxide (ZnO) quantum dots (QDs) and their influence on the power c... more We report the synthesis of Zinc Oxide (ZnO) quantum dots (QDs) and their influence on the power conversion efficiency of photovoltaic devices. With an excitation wavelength of 340 nm the down-shifted emission peaks were observed to be located at 510 and 540 nm for colloidal solutions with pH values of 10 and 12, respectively. The largest PCE variation was observed to increase from 14.60% to 15.49% when dispersing the QDs extracted from a 4 mL colloidal solution that were subsequently dispersed in PMMA. This represents an improvement of ~6.1%.

2015 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2015

The synergistic utilization of nanoparticles of Au, Ag, and Au/Ag alloys, in combination with a p... more The synergistic utilization of nanoparticles of Au, Ag, and Au/Ag alloys, in combination with a photon capture scheme and selected thin films, has enabled the demonstration of photovoltaic structures on single-crystal silicon substrates with an efficiency of 15%. Additionally, hybrid solar cells (i.e organic/inorganic) have also been considered as a viable alternative to develop cost effective photovoltaic devices because the Schottky union between organic and inorganic materials can be formed employing low-temperatures fabrication methods. In this presentation we specifically describe a hybrid solar cell based on an ordered array of silicon nanopillars and the conductive polymer Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS). The proposed device comprises a thin layer of the aforementioned PEDOT:PSS on an array of silicon nanopillars that was formed utilizing electroless metal-assisted-chemical-etching methods. The performance characteristics of the produced solar cells was analyized in function of the height of said nanopillars. A maximum power conversion efficiency of 9.65% was observed for an optimized height of 400 nm, even without the utilization of antireflecting films on the front surface. The effect of ultrathin films of aluminum oxide (Al2O3) realized employing an atomic layer deposition tool was also included in this study and its utilization further increased the measured efficiency to 10.56%. Finally, with the specific aim of lowering the cost of solar cell manufacturing, additional tests have been carried out on structures with a total thickness of less than 20 μm in which the efficiency was observed to reach 7.7% when used in conjunction with multi-spiked nanoparticles of Au/Ag alloys. The utilization of CdTe quantum dots [1-3] on nanotexturized, relatively thin samples (150 μm) has permitted achieving an efficiency of 15%. Thus, the structures discussed in this presentation are considered promising towards the realization of high efficiency solar cells.

Microscopy and Microanalysis, 2018

International Journal of Sciences: Basic and Applied Research, 2017

An array of interdigitated microelectrodes was modeled, optimized, and tested for the contactless... more An array of interdigitated microelectrodes was modeled, optimized, and tested for the contactless alignment of living biological cells by negative dielectrophoresis (nDEP). The modeling focused on optimizing the x component, , of potential gradient to maximize the aligning force at minimal electric field to protect fragile cells. The optimized array was tested with micron-sized, hard polymeric beads and soft C2C12 mouse myoblast cells. The array completely aligned the initially random arrangement of beads after 15 s of applying 450 kHz sinewave electrical signal to the electrodes. Under the same conditions, the array was unable to move C2C12 cells due to their strong adhesion to the surface. However, the array was effective at directing the orientation of confluent C2C12 cells during their differentiation into myotubes for the formation of muscle fibers. The aligned myotubes were alive and capable of growth. The proposed approach has a potential for growing continuously orien...

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

Microsystem Technologies, 2015

In order to promote the widespread utilization of photovoltaic structures it is critical to incre... more In order to promote the widespread utilization of photovoltaic structures it is critical to increase their conversion efficiency and reduce their manufacturing cost. With these two goals in mind, planar and nanotexturized c-Si photovoltaic devices were built and their radiation incident side was covered with CdTe quantum dots (QDs) dispersed in a poly-methyl-methacrylate thin film ~100 nm thick. The measured performance with and without the CdTe QD layer indicates that the down-shifting photoluminescence (DSPL) of the synthesized QDs triggers an increase in the open circuit voltage and the short circuit current, resulting in an overall improvement of ~1.5 % in the power conversion efficiency of the produced devices. This observation can be understood as a result of decreasing the number of high energy photons that are prone to unproductive near surface absorption in c-Si p-n junction solar cells and the re-emission of lower energy photons that are more effectively bulk-absorbed in c-Si solar cells. The result of lowering the near surface photon absorption represents a reduction of carrier losses due to the high surface recombination velocity as well as the low lifetime that normally plague the diffused side of an active junction device. Additionally, solar cells with a layer of CdTe QDs exhibit a higher external quantum efficiency when compared to devices without the aforementioned quantum dots. Layers of CdTe QDs are economically attractive, highly photostable and their absorption and emission spectra have a strong dependence on particle size, therefore, they lend themselves to providing a broad DSPL spectrum and promoting the proliferation of photovoltaic structures.

2017 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2017

We report the synthesis and characterization of cadmium-telluride (CdTe) quantum dots (QDs) and t... more We report the synthesis and characterization of cadmium-telluride (CdTe) quantum dots (QDs) and their performance at Terahertz frequencies employing THz time-domain spectroscopy (THZ-TDS). The data was analyzed using a combination of Fast Fourier Transform (FFT) and effective medium theory to obtain the optical characteristics of the aforementioned CdTe QDs. Additionally, we performed similar measurements while exposing the QDs to a UV excitation with a wavelength of 302 nm and observed a noticeable variation in both the real and imaginary components in the index of refraction that could prove crucial in the demonstration of reconfigurable devices operating in the THz regime.

Applied Physics Letters, 2021

We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO subs... more We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO substrates at the relatively low temperatures of 120 o C, 150 o C, and 200 o C. The as-grown ZnO films are studied with aberration-corrected transmission electron microscopy and diffraction contrast, photoluminescence (PL) and surface photovoltage (SPV) spectroscopies. We find that the homoepitaxial films have monocrystalline structure with the density of basal stacking faults comparable to that of the substrate (∼ 10 11 cm −2) and that the stacking faults can induce high lattice strain due to their interaction with the inversion domain boundaries. The narrow excitonic PL linewidth (2 meV at 8 K) and sharp SPV bandgap transition confirm the high quality of the ZnO films. Despite similarities in the film properties, the growth temperature has an effect on the density and spatial distribution of intrinsic defects. Our results demonstrate a considerable potential of ALD ZnO homoepitaxy for fabricating high-quality ZnO nanostructures and attaining viable p-type ZnO. ZnO is a widely studied semiconductor with a unique set of physical properties and tremendous potential for various applications [1, 2]. Its wide bandgap (3.37 eV)