Guest Editors Nick Takaki, Alexander Lin, Fatima Toor, and Matthew E.L. Jungwirth introduce the Special Section on Education and Training in Optical Instrumentation and Lens/Illumination.
KEYWORDS: Solar cells, Photovoltaics, Solar energy, Silicon, Perovskite, Manufacturing, Sustainability, Dye sensitized solar cells, Design, Energy efficiency
This report provides a snapshot of emerging photovoltaic (PV) technologies. It consists of concise contributions from experts in a wide range of fields including silicon, thin film, III-V, perovskite, organic, and dye-sensitized PVs. Strategies for exceeding the detailed balance limit and for light managing are presented, followed by a section detailing key applications and commercialization pathways. A section on sustainability then discusses the need for minimization of the environmental footprint in PV manufacturing and recycling. The report concludes with a perspective based on broad survey questions presented to the contributing authors regarding the needs and future evolution of PV.
Significance: Mid-infrared (MIR) light refers to wavelengths ranging from 3 to 30 μm and is the most attractive spectral region for ablation of soft and hard tissues. This is because building blocks of biological tissue, such as water, proteins, and lipids, exhibit molecular vibrational modes in the MIR wavelengths that result in strong MIR light absorption. To date, researchers investigating MIR lasers for surgical applications have used bulky light sources, such as free electron lasers, nonlinear light generators, and carbon dioxide lasers. We demonstrate the use of a tiny (a few microns wide, a few millimeters long) MIR interband cascade laser (ICL) for surgical thermal ablation applications.
Aim: Our goal is to demonstrate the use of an ICL for surgical thermal ablation and demonstrate its efficacy in ablating normal fibroblasts and primary undifferentiated pleomorphic sarcoma tumor cells (C1619).
Approach: We conducted Fourier transform infrared spectroscopy analysis of healthy and cancerous tissue samples, which indicated that the absorption of tumor tissue is higher than healthy tissue around 3.3-μm wavelength. These results enabled us to select an ICL emission wavelength, λ, of 3.3 μm to probe normal fibroblast and primary undifferentiated pleomorphic sarcoma cell survival after ICL exposure.
Results: We show that the absorption of tumorous tissue is higher than that of healthy tissues around the 3-μm MIR wavelength. We demonstrate that the ICL is able to ablate cancer cells at very low-power levels that can be clinically implemented but that this effect does not appear to be specific to C1619 when compared to normal fibroblasts.
Conclusions: Our study demonstrates that ICLs may represent an exciting new avenue toward precise laser-based thermal ablation.
Terahertz (THz) optoelectronics have great potentials in communication, imaging, sensing and security applications. However, the state-of-the-art fabrication processes for THz devices are costly and time-consuming. In this work, we present a novel laser-based metamaterial fabrication (LMF) process for high-throughput fabrication of transparent conducting surfaces on dielectric substrates such as quartz and transparent polymers to achieve tunable THz bandpass filtering characteristics. The LMF process comprises two steps: (1) applying ultrathin-film metal deposition, with a typical thickness of 10 nm, on the dielectric substrate; (2) creating periodic surface pattern with a feature size of ~100 microns on the metal film using nanosecond pulsed laser ablation. Our results demonstrate the LMF-fabricated ultra-thin metal film exhibits newly integrated functionalities: (a) highly conductive with sheet resistance of ~20 Ω/sq; (b) optically transparent with visible transmittance of ~70%; (c) tunable bandpass filtering effect in the THz frequency range; and (d) extraordinary mechanical durability during repeated fatigue bending cycles. The scientific findings from this work will render an economical and scalable manufacturing technique capable of treating large surface area for multi-functional THz metamaterials.
KEYWORDS: Solar energy, Water, Solar cells, Photovoltaics, Planets, Neural networks, Evolutionary algorithms, Detection and tracking algorithms, Solar concentrators, Geometrical optics
Guest editors introduce the articles in the Special Section on Solar Energy Solutions for Electricity and Water Supply in Rural Areas, published in Volume 9, Issue 4 of the Journal of Photonics for Energy.
Nanostructured black silicon (bSi) exhibits a broadband antireflection (AR) response due to graded-index and scattering effects, unlike traditional quarter-wavelength dielectric AR coatings. We present various techniques to improve the front- and back-surface performance of nanostructured bSi solar cells. Ammonium dihydrogen phosphate (ADP) is used for proximity doping to reduce the physical impact on the bSi nanostructures during front-surface emitter formation. An optimum concentration of 2 wt. % of ADP is found to result in a typical solar cell emitter sheet resistivity of 50 Ω / sq. Potassium hydroxide is used to etch off the highly doped region of the bSi solar cell front emitter, which results in lower surface recombination and up to a 23% increase in short wavelength (400 to 600 nm) internal quantum efficiency of the bSi solar cell. To reduce the series resistance and enhance surface passivation, forming gas anneal is employed, improving bSi cell’s overall efficiency by over 31%. By optimizing the back-surface-field formed by sputtered aluminum (Al), the backside recombination rate is reduced, improving external quantum efficiency by up to 11% in the long wavelength (>900 nm) region.
KEYWORDS: Solar cells, Silicon, Solar energy, Tandem solar cells, Electrical engineering, Energy efficiency, Computer engineering, Physics, Doping, Optics
InAs nanowires directly integrated on Si platform show great promise in fabricating next generation mid-infrared optoelectronic devices. In this study we demonstrated the growth of catalyst-free, selective-area InAs nanowire arrays on electron beam patterned Si3N4/Si(111) by molecular beam epitaxy. Growth parameters were studied, and nanowire growth kinetics dependence on patterned mask opening diameter and interwire distance was investigated. Under certain growth conditions, nanowire diameter was found to be relatively independent of nanohole diameter and pitch. We also realized the growth of randomly-nucleated, self-assembled nanowires on Si(111) and investigated the temperature, flux influence on nanowire morphology.
In this work we characterize the thermal conductivity properties of nanoprous ‘black silicon’ (bSi). We fabricate the nanoporous bSi using the metal assisted chemical etching (MACE) process utilizing silver (Ag) metal as the etch catalyst. The MACE process steps include (i) electroless deposition of Ag nanoparticles on the Si surface using silver nitrate (AgNO3) and hydrofluoric acid (HF), and (ii) a wet etch in a solution of HF and hydrogen peroxide (H2O2). The resulting porosity of bSi is dependent on the ratio of the concentration of HF to (HF + H2O2); the ratio is denoted as rho (ρ). We find that as etch time of bSi increases the thermal conductivity of Si increases as well. We also analyze the absorption of the bSi samples by measuring the transmission and reflection using IR spectroscopy. This study enables improved understanding of nanoporous bSi surfaces and how they affect the solar cell performance due to the porous structures’ thermal properties.
In this work we present the design of a novel ophthalmic prismatic contact lens to correct for
strabismus. Strabismus, colloquially called "crossed-eyes" or "wall eyes," is a condition in which the
eyes are not properly aligned with each other. To our knowledge there are no contact lenses that
allow for strabismus correction. To address this, we have designed a poly methyl methacrylate
(PMMA) based prismatic correction contact lens. Therefore, we modeled a Fresnel lens with the
appropriate optical properties and a human eye in COMSOL Multiphysics Ray Optics module. Our
first design was created by mapping Fresnel lenses onto the curved surface of the eye, the focus of
light on retina was suboptimal. Next we determined two more potential solutions and improved the
light focus on the retina but there were still some issues. A small fraction of light (~5%) diverged
and could not be focused. Due to dispersive characteristic of PMMA, chromatic aberration was
present. We will use our ray optics solution and convert into a metasurface nanophotonic lens that
has the identical behavior and mitigates the issues related with prismatic lens.
In this work, we utilize electrochemical impedance spectroscopy (EIS) to study the electronic characteristics of nanostructured silicon (Si) fabricated using the metal-assisted chemical etched (MACE) process. The nanostructured Si fabricated using the MACE process results in a density graded surface that reduces the broadband surface reflection of Si making it appear almost black, which coins it the name ‘black Si’ (bSi). We study two bSi samples prepared using varying MACE times (20s and 40s) and a reference bare silicon sample using EIS between 1 MHz and 1 Hz frequencies. At an illumination intensity created with the use of a tungsten lamp source calibrated to output an intensity of 1-Sun (1000 W/m2), the impedance behavior at bias potentials in both the forward and reverse bias ranging between -1 V and 1 V are studied. We also study the effect of illumination wavelength by using bandpass filters at 400 nm and 800 nm. The results indicate that the charge transfer resistance (Rct) decreases as the surface roughness of the electrodes increases and as the illumination wavelength increases. We also find that the constant phase element (CPE) impedance of the electrodes increases with increasing surface roughness. These results will guide our future work on high efficiency bSi solar cells.
Metasurfaces are boundaries between two media that are engineered to induce an abrupt phase shift in propagating light over a distance comparable to the wavelength of the light. Metasurface applications exploit this rapid phase shift to allow for precise control of wavefronts. The phase gradient is used to compute the angle at which light is refracted using the generalized Snell’s Law. [1] In practice, refractive metasurfaces are designed using a relatively small number of phaseshifting elements such that the phase gradient is discrete rather than continuous. Designing such a metasurface requires finding phase-shifting elements that cover a full range of phases (a phase range) from 0 to 360 degrees. We demonstrate an analytical technique to calculate the refraction angle due to multiple metasurfaces arranged in series without needing to account for the effect of each individual metasurface. The phase gradients of refractive metasurfaces in series may be summed to obtain the phase gradient of a single equivalent refractive metasurface. This result is relevant to any application that requires a system with multiple metasurfaces, such as biomedical imaging [2], wavefront correctors [3], and beam shaping [4].
KEYWORDS: Solar cells, Data modeling, Thin film solar cells, Nanostructuring, Thin films, Optimization (mathematics), Silicon solar cells, MATLAB, Physics, Perovskite, Silicon, External quantum efficiency, Manufacturing, Reflectivity, Tandem solar cells
Several research groups are developing solar cells of varying designs and materials that are high efficiency as well as cost competitive with the single junction silicon (Si) solar cells commercially produced today. One of these solar cell designs is a tandem junction solar cell comprised of perovskite (CH3NH3PbI3) and silicon (Si). Loper et al.1 was able to create a 13.4% efficient tandem cell using a perovskite top cell and a Si bottom cell, and researchers are confident that the perovskite/Si tandem cell can be optimized in order to reach higher efficiencies without introducing expensive manufacturing processes. However, there are currently no commercially available software capable of modeling a tandem cell that is based on a thin-film based bottom cell and a wafer-based top cell. While PC1D2 and SCAPS3 are able to model tandem cells comprised solely of thin-film absorbers or solely of wafer-based absorbers, they result in convergence errors if a thin-film/wafer-based tandem cell, such as the perovskite/ Si cell, is modeled. The Matlab-based analytical model presented in this work is capable of modeling a thin-film/wafer-based tandem solar cell. The model allows a user to adjust the top and bottom cell parameters, such as reflectivity, material bandgaps, donor and acceptor densities, and material thicknesses, in order to optimize the short circuit current, open circuit voltage, and quantum efficiency of the tandem solar cell. Using the Matlab-based analytical model, we were able optimize a perovskite/Si tandem cell with an efficiency greater than 30%.
We demonstrate a solar control window film consisting of metallic nanoantennas designed to reflect infrared (IR) light while allowing visible light to pass through. The film consists of a capacitive frequency-selective surface (CFSS) which acts as a band-stop filter, reflecting only light at target wavelengths. The designed CFSS when installed on windows will lower air conditioning costs by reflecting undesired wavelengths of light and thus reduce the amount of heat that enters a building. State-of-the-art commercial solar control films consist of a multilayer stack which is costly (~$13/m2 to $40/m2) to manufacture and absorbs IR radiation, causing delamination or glass breakage when attached to windows. Our solar control film consists of a nanostructured metallic layer on a polyethylene terephthalate (PET) substrate that reflects IR radiation instead of absorbing it, solving the delamination problem. The CFSS is also easy to manufacture with roll-to-roll nanoimprint lithography at a cost of <$12/m2. We design the CFSS using the COMSOL Wave Optics module to solve for electromagnetic wave propagation in optical media via the finite element method. The simulation domain is reduced to a single unit cell with periodic boundary conditions to account for the symmetries of the planar, periodic CFSS. The design is optimized using parametric sweeps around the various geometric components of the metallic nanoantenna. Our design achieves peak reflection of 80% at 1000 nm and has a broadband IR response that will allow for optimum solar control without significantly affecting the transmission of visible light.
The ν1+ν3 combination band of uranium hexafluoride (UF6) is targeted to perform analytical enrichment measurements
using laser absorption spectroscopy. A high performance widely tunable EC-QCL sources emitting radiation at 7.74 μm
(1291 cm-1) is employed as an UF6-LAS optical source to measure the unresolved rotational-vibrational spectral
structure of several tens of wavenumbers (cm-1). A preliminary spectroscopic measurement based on a direct laser
absorption spectroscopy of methane (CH4) as an appropriate UF6 analyte simulant, was demonstrated.
We report on a study to determine the effect of waveguide side-wall roughness on Quantum Cascade (QC) laser
performance, such as threshold current density, slope efficiency, far-field beam pattern and group refractive index, using
two two-wavelength heterogeneous cascade QC laser structures, one with emission wavelengths of 7.0 μm/11.2 μm, and
the other with 8.7 μm /12.0 μm. For the range of roughness standard deviation values from about 0.4 μm to 1.0 μm for
which all four QC lasers were operating, the threshold current density increases by 12%-15% and the slope efficiency
decreases by 30%-70% with stronger performance degradation for the shorter wavelength lasers, which is in agreement
with a model based on Rayleigh scattering. Moreover, no significant change in the far-field beam patterns for different
σrough values was observed, and the group effective index values of the four wavelengths have several values for each
rough waveguide indicative of multiple transverse modes in the waveguides.
Quantum Cascade (QC) wafer quality testing requires intensive processing and characterization. Here, we demonstrate a
minimally invasive technique that gives rapid feedback on wafer quality. A mesa is fabricated using only a single etch
and metallization step. The device is electrically pumped and optically and electrically characterized. The peak
wavelength position and the full width at half maximum (FWHM) as a function of applied electric field, turn-on voltage,
maximum operating current density and threshold current density of the mesas are measured. Results of the mesa and
lasers processed from the same wafer are compared and differed by less than 10 %.
Diode lasers supply high power densities at wavelengths from 635-nm to 2000-nm, with different applications enabled
by providing this power at different wavelengths. As the range of available wavelengths broadens, many novel medical
and atmospheric applications are enabled. Traditional quantum well lasers provide high performance in the range 635-
nm to 1100-nm range for GaAs-based devices and 1280-nm to 2000-nm for InP, leaving a notable gap in the 1100 to
1280-nm range. There are many important medical and sensing applications in this range and quantum dots produced
using Stranski-Krastanow self-organized MBE growth on GaAs substrates provide an alternative high performance
solution. We present results confirming broad area quantum dot lasers can deliver high optical powers of 16-W per
emitter and high power conversion efficiency of 35% in this wavelength range. In addition, there are growing
applications for high power sources in wavelengths > 1500-nm. We present a brief review of our current performance
status in this wavelength range, both with conventional quantum wells in the 1500-nm to 2500-nm range and MOCVD
grown quantum cascade lasers for wavelengths > 4000-nm. At each wavelength, we review the designs that deliver this
performance, prospects for increased performance and the potential for further broadening the availability of novel
wavelengths for high power applications.
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