This PDF file contains the front matter associated with SPIE Proceedings Volume 10759, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Hybrid organic-inorganic lead halide perovskite solar cells have made rapid advancements in efficiency, approaching and overtaking those of other thin-film technologies. Before commercialization can be achieved, however, the stability of perovskite solar cells must be improved. While moisture exposure can be mitigated through careful encapsulation, the thermal stability of the cell, with respect to both intrinsic degradation of the absorber material and extrinsic reactions with other layers, is critical. We evaluate thermal stability of semitransparent FA0.83Cs0.17Pb(I0.83Br0.17)3 and MAPbI3 perovskite solar cells at 85C in a nitrogen environment for up to 1000 hours and show that the primary factor in cell degradation is reaction with a metal contact. Using depth profiling in x-ray photoelectron spectroscopy, we show that silver contacts not only create a driving force for iodine migration from the perovskite, but also surprisingly have the potential to diffuse through a sputtered tin-doped indium oxide (ITO) window layer, an atomic layer deposited (ALD) tin oxide layer, and an evaporated fullerene electron transport layer into the perovskite, harming the performance of the perovskite solar cell. The poor barrier quality of the transparent conducting oxide (TCO) is due largely to diffusion channels in domain boundaries created by a proliferation of the existing rough perovskite morphology, shown with scanning electron microscopy (SEM). We investigate several solutions, including spin-coating the fullerene layer and using amorphous indium zinc oxide (IZO) as an alternative TCO. We discuss the performance and viability of each solution as well as implications for perovskite solar cell design.

The performance and degradation rate of photovoltaic (PV) modules primarily depend on the technology type, module design and field operating conditions. The metastability is a known phenomenon in the CIGS (copper indium gallium diselenide) module technology and it depends on the light exposure and operating temperature. This work aims to understand the metastability influence on the performance of CIGS modules exposed outdoor at three different operating temperatures at a fixed insolation over three years. Two types of CIGS modules from two different manufacturers have been investigated in this study. The three different temperatures were achieved by placing three CIGS modules per manufacturer at three different airgaps on a south facing mock rooftop tilted at 20°. The airgaps were 3”, 1.5” and 0”, and the 0” airgap module was thermally insulated to obtain a higher operating temperature. Throughout the test period over three years, all the modules were maintained at maximum power point using a setup containing optimizers and power resistors. The performance characterizations were carried out before and after exposure using both outdoor natural sunlight and indoor solar simulator. The influence of superstrate type and installation height on the soiling loss have also been investigated.

In order to accelerate the PID tests of commercial CIGS modules outdoors, water was regularly sprayed on modules during the PID test. We could increase the leakage current more than 10 times by water spraying. Comparison with indoor tests is to be presented at the conference, which is expected to provide supportive information for the related IEC standards currently under development.

Over the course of their lifetime, photovoltaic (PV) modules develop defects and experience performance degradation due to local environmental stresses. The defect type and rate of degradation depend upon cell technology, module construction type, module manufacturing quality control, installer workmanship, and the installed environment. Defects can be purely cosmetic, can cause performance degradation and/or can cause safety risks. Testing labs and other applied researchers typically report the type and number/distribution of defects observed in each PV plant they have investigated. Simply reporting the observed number of defect types and their percent distribution in a plant is of little use to stakeholders, unless each defect is quantitatively correlated with the corresponding degradation rate per year or safety risk. A quantitative correlation can be achieved using a risk priority number (RPN) approach to assess the risk associated with module defects and determine the appropriate action, such as panel removal for safety reasons or warranty claims for material defects. Understanding the climate dependence of degradation rates and defects is valuable for predicting power output and assessing the financial risk of future projects in specific climatic regions. In this study, the influence of climatic condition on RPN for different types of defects, including encapsulant discoloration and solder bond degradation, has been analyzed. The performance degradation rate data and visual inspection data obtained from seven crystalline-silicon PV plants, aged between 3 and 18 years, were used to calculate the RPN for each defect in three climatic conditions (hot-dry, cold-dry, and temperate). The RPN data were, in turn, used to identify the defects with the greatest effect on performance in each of the three climatic regions

Failure modes and degradation rates of PV modules in a specific climate are primarily dictated by the module design and field-specific climate stressors such as temperature, UV and humidity. To identify the long-term design issues and predict lifetime of PV modules, the plant owners, investors and researchers typically utilize long-term indoor accelerated tests such as extended/modified IEC 61215 tests. Though the indoor accelerated tests can appropriately be designed for the environmental stressors of a specific climate, several challenges are encountered and they include: capital and operating costs of multiple walk-in environmental and weathering chambers for commercial size modules; only statistically insignificant number of commercial modules can be tested at a time due to size limitation of the chambers, and; multiple climate-specific temperatures and multiple humidity profiles used in the long-term accelerated tests prevent performing conventional IEC 61215 test profiles inside the same chamber. All the above-mentioned challenges can be adequately addressed using a novel climate-specific field accelerated testing setup presented in this work. This test program has been designed specifically for the hot-dry desert climate where the environmental stressors are temperature and UV with little or no influence from humidity. This program can easily be modified for the other climatic conditions, e.g. test setup for a hot-humid condition can include temperature, UV and humidity. In the current outdoor accelerated test program for hot-dry desert climate, the temperature acceleration is achieved by inserting heavy thermal insulators on the backside of the modules and the UV acceleration at higher operating temperatures are achieved by using a V-trough solar concentrator on the thermally insulated PV modules installed on a 2-axis tracker. An acceleration factor of about 12-15 is expected depending on the activation energy of the climate-specific degradation mechanism, e.g. encapsulant browning and solder bond degradation.

Potential-induced degradation (PID) has been one of the critical reliability issues in solar photovoltaic (PV) industry last several years. There are several PID mechanisms, but most well-known failure mechanism is the junction shunting, called PID-s. Cell p-n junction is shunted by sodium ion migration from PV module glass, which is due to leakage current caused by high potential difference between solar cell and aluminum frame of the module. Various methods preventing or reducing PID-s have been developed and used by the PV industry; however, those methods can be applied only at the manufacturing plants. We present a method of suppressing or preventing PID by interrupting surface conductivity of the glass, which can be applied to the field installed PV modules. In our previous study, we chose flexible Corning Willow Glass strips with ionomer adhesive to interrupt the surface conductivity of one-cell PV modules and multi-cell commercial PV modules. By applying the flexible Corning Willow Glass strips on the glass surface close to the frame inner edges, we experimentally demonstrated that PID-s can be practically eliminated in the full size commercial modules. In the current study, we investigated the surface conductivity interrupting technique by applying hydrophobic materials (instead of Corning Willow Glass) on the glass surface close to the inner edges of the frame. The module without any hydrophobic material suffered with 29% of power loss after the PID stress test whereas the module with hydrophobic material suffered with only 15% of power loss after the PID stress test. The current investigation indicates that the PID degradation can be significantly reduced using the hydrophobic materials but not eliminated as observed with the flexible Corning Willow Glass.

Interconnect Metallization System (IMS) degradation of photovoltaic (PV) modules is one among the major degradation modes in the field caused by higher operating temperatures and daily/seasonal/cloud cyclic temperatures. Usually, the acceleration factor (AF) and activation energy (E_a) of IMS degradation are determined based on power degradation data. Using power degradation data may not be fully representative of a specific mechanism since the power drop could be caused by multiple degradation mechanisms. In this paper, we have used the series resistance (R_s) increase, instead of power degradation, to obtain the AF and E_a for IMS degradation mechanism in the damp heat test (85°C/85% RH). The degradation data were sourced from our qualification damp heat test database with 94 crystalline silicon modules, and two field databases of Arizona and New York climates with 615 and 236 crystalline silicon modules, respectively. A 3-step approach was implemented to determine the AF for the damp heat testing. First, the AF for the field-to-field degradation was determined based on R_s degradation rates of the modules in Arizona and New York. Second, the E_a was determined based on the AF, and hourly differences in field-to-field module temperature and relative humidity. Third, the AF was determined based on R_s increase in the damp heat test and the E_a determined using the field data in the second step. A model based on modified Peck’s equation was used to determine a generic AF for the R_s increase in qualification damp heat testing. This approach is useful to predict the service lifetime and reliability of PV modules for specific climatic regions.

An understanding of the exposure environment of any material is necessary for the use of accelerated stress testing to evaluate different designs, materials, and configurations. For photovoltaic modules there is a wide range of environments and mounting conditions, each with a unique combination of temperature and humidity profiles. This is further compounded by differences in the microenvironment within a module, e.g. the humidity in the front EVA is not the same as the humidity in the backside EVA, and the presence of seasonal and diurnal changes in water content. We demonstrate how one can model the temperature and humidity environments of representative climates and use this to estimate the amount of moisture present in a PV module. To compare the relative degradation in different environments and to compare this to indoor testing, one must consider kinetically weighted parameters to characterize an environment. With some understanding of the kinetics, better choices for stress testing conditions can be made to minimize the uncertainty in correlating chamber results to the field allowing for better rank ordering of material and better service life prediction. This more general analysis highlights the fact that within reasonable limits a single humidity can represent a given climate. Thus, when a lower representative humidity is used, one can focus testing conditions on temperature effects and/or UV radiation. This can significantly simplify testing when very little is known about the humidity dependence of degradation processes.

Accelerated laboratory testing has been recognized as a common means to simulate in-field exposure in a time-saving way, however, caution is needed as highly accelerated stresses can cause degradation which is never observed in the field. In this work, the effects of the environmental factors on backsheet degradation have been investigated using a commercial PPE backsheet (polyethylene terephthalate (PET/PET/ethylene-vinyl acetate (EVA)). The PPE films were exposed to NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) under different UV intensities, wavelengths, temperatures and humidities. The chemical and optical degradation of the backsheets were examined by FTIR and UV-vis spectroscopy. The reciprocity law and action spectrum were studied and the activation energy for PPE degradation was calculated. A preliminary statistical model for predicting the service life of the PPE backsheet has been established based on the SPHERE exposure data and further validated by the initial degradation of the fielded PPE in Florida, Arizona and Maryland.

Glass-Glass modules are gaining popularity for bifacial application and have believed advantages over PV modules with polymeric backsheets. Frameless glass-glass modules are promoted as PID-free, resistant to solvents, fire, and load stress, and capable of higher system voltages. We have found glass-glass modules run at higher operating temperature than Glass-Flex modules, and this reduces power output. Field power output results will be presented. Impermeable glass traps chemical byproducts, and faster power degradation from corrosion has been documented. Delamination has been observed in the field with glass-glass modules. A new accelerated test replicates this delamination. PID testing results will be presented comparing Glass-Glass and Glass-Flex modules.

Polyamide (PA)-based backsheet as an emerging product has been developed in recent years to partially substitute fluoropolymer materials under the intensified cost-reduction pressure. However, a number of reports indicate that backsheet cracks have been observed in fielded modules in a few locations after only a few years of exposure. An in-depth analysis on degradation and crack formation and their dependence on climatic conditions is needed. In this work, the field PV modules with PA-based backsheet under five different climatic conditions up to six years were retrieved and analyzed, including humid subtropical climate (Changshu, China), dry-summer subtropical climate (Rome, Italy), marine west coast climate (Bergamo, Italy), desert climate (Arizona, United States) and tropical climate (Thailand). Macroscopic cracks in backsheet were observed for modules aged in Italy and Thailand, while only hairline cracks showed up in backsheet from Changshu, and no cracks could be seen in Arizona. Backsheet in Changshu also experienced much higher yellowing than other sites, while the gloss loss of the backsheet in Italy is the highest. Spectroscopic analyses were also performed to identify various degradation products and to understand the possible changes in degradation mechanism of backsheets under different climates. The intercorrelations between various degradation modes of PA-based backsheet and weathering factors will be further established, providing a valuable information on the material selection and lifetime prediction for the backsheet.

A single layer polymeric backsheet material has been evaluated for use in silicon PV modules and as a luminescent solar collector. Physical and optical properties of the backsheet have been examined after damp heat, thermal cycling, and UV exposure for two different polymeric grades and three different pigment options including white, transparent, and photo-luminescent. As the importance of developing a white backsheet that is cost-effective, recyclable, and reliable is evident, the increased availability and use of bifacial cells is bound to increase the demand for transparent backsheets as well. In today’s market, the cost of a clear backsheet is significantly higher than a white backsheet which is motivation for research and development of new material options in this field. Results from these extensive tests reveal the need for continued research in white pigments and positive results in transparent and luminescent backsheet options.

This work focused on the technology of luminescent down shift (LDS), with a primary aim to identify and investigate a methodology to introduce the luminescent organic dye into PVB polymer encapsulant as emergent material for photovoltaic application. For this goal, we propose to study the feasibility to implement the LDS functionality and to identify suitability of available luminescent to be incorporated into the host polymer encapsulant material. The first step to this direction was through a comprehensive optical study of Violet 570 (V) organic dye in ethanol solvent. The methodology and experimental conditions such as laboratory polymer preparation and luminescence dye concentration were presented. Also, the emergent polymer encapsulant sheets were characterized by using optical and thermal analysis techniques. The absorption spectrum of the prepared PVB material shifts towards longer wavelengths, with increasing organic dye concentration.

The purpose of the experiment was to better understand the changes due to thermal transitions and the molecular organizations of PVB encapsulant material after cross linking process by thermal analysis methods as DSC, TSC and DMTA. DSC experiments on EVA show a glass transition at about -33.1°C, which is characteristic of crystalline phase and an endothermic peak at temperature of 55°C characteristic of amorphous phase. The basic results by TSC technique is that there are two relaxations that are reproducibly observed in crosslinked EVA encapsulant material. At temperature polarization 60 °C, a low temperature relaxation occurs at temperature -24.4°C and a high temperature relaxation occurs at temperature 30.4°C. DMTA results exhibit two tand peaks located at 14.9°C and 66.6°C. In addition, our results reveal that the glass transition temperature determined by TSC experiments in depolarization mode is more relevant than DSC and DMTA methods. TSC was chosen due to its low equivalent frequency consideration, useful to study encapsulant material exhibiting multiple relaxations.

Metasurfaces have emerged as elegant engineered interfaces capable of controlling optical phases and amplitudes within ultra-flat form factors. Recently, there has been an increasing effort to achieve reconfigurable metasurfaces incorporating various tuning mechanisms, including electrical, optical, mechanical or thermal driving forces. In particular, electronic tuning has previously been shown to provide potential control over virtually a full range of optical phases. However, practical implementation is limited by the maximum doping that can be achieved by applying bias, and by the inherent losses of the constituent materials. In this work, we apply electrically-tuned reconfigurable metasurfaces to achieve dynamically-controlled thermal sources. Kirchhoff’s law of thermal radiation suggests possible active control of spectral and angular properties of radiated heat in carefully designed metasurfaces. This goal can be achieved by coupling optical resonances that imply spectral and angular selectivity, to 2D plasmonic resonances in active structured 2D surfaces. We discuss the potential of different 2D materials, such as graphene, black phosphorus and transition metals dichalcogenides, with respect to their respective optical properties, bandgaps and inherent losses. The ultimate goal is to achieve maximal absorption in a dynamically selected direction at a given wavelength, by exciting surface-confined modes. Enabling active beam steering of coherent thermal sources may provide low-cost alternatives to existing infrared sources for applications such as sensing and thermal management.

Here we present our recent developments in temperature dependent ellipsometry, FTIR and emittance measurements of flat and structured vanadium dioxide (VO2) surfaces allowing significant control of switchable radiative cooling beyond that attainable via traditional VO2 surfaces. VO2 undergoes a metal-insulator transition at a critical temperature of ~ 68°C; previous work has investigated tuning of this critical temperature over a wide range of temperatures. Here we exploit the shift in optical properties to produce surfaces with various emittance temperature profiles that modulate the thermal radiative transfer to/from a surface.

Designing surfaces with different temperature emittance profiles requires accurate optical/thermal characterisation of materials. VO2 is produced by sputtering of vanadium followed by post deposition annealing in a 0.1Torr to 0.3Torr Air atmosphere at 450°C to 550°C, in-situ optical monitoring allows for accurate termination of the annealing process once the desired optical response is achieved.

The super Planckian features of radiative heat transfer in the near-field are known to depend strongly on both material and geometric properties. However, the relative importance and interplay of these two facets, and the degree to which they can be used to modify heat transfer, remains an open question. In this talk, we show that inverse design techniques can be exploited to resonantly enhance heat transfer between complex, structured surfaces. In particular, high loss metals such as tungsten can be structured to realize heat-transfer rates that come within 80% of the rate exhibited by an ideal pair of resonant lossless metals at selective frequencies and for separations as small as two hundredths of the design wavelength. We observe that the scaling of the enhancement factor with respect to material susceptibility follows that of recently derived bounds based on energy conservation. This and related work demonstrating highly modified non-equilibrium thermal effects in nonlinear media suggest the possibility of significant geometric and material tunability over radiative effects, beyond common approaches based on linear media and/or far-field emission.

We derive the fundamental limits of energy harvesting from the outgoing thermal radiation from the ambient to the outer space. The derivations are based on a duality relation between thermal engines that harvest solar radiation and those that harvest outgoing thermal radiation. We also derive the ultimate limit for harvesting outgoing thermal radiation, analogous to the Landsberg limit for solar energy harvesting, and show that the ultimate limit far exceeds what was previously thought to be possible.

Tailoring the emission spectra of a thermophotovoltaic emitter away from that of a blackbody has the potential to minimize transmission and thermalization loss in a photovoltaic receiver. Selective thermophotovoltaic emitters could lead to solar energy conversion with efficiency greater than the Shockley-Queisser limit and could facilitate the generation of useful energy from waste heat. We introduce a new design to radically tune thermal emission that leverages the interplay between two resonant phenomena in a simple planar structure – absorption in weakly-absorbing thin films and reflection in multi-layer dielectric stacks. We will discuss a virtual screening approach based on Pareto optimality to identify a small number of promising structures for a selective thermal emitter from a search space of millions, several of which approach the ideal values of a step-function selective thermal emitter. We will also discuss the experimental realization of several simple and optimal structures, estimates of their device-level performance, and ongoing efforts to close the gap between theoretical estimates and experimentally-realized performance of these structures.

This talk will introduce some of our recent work on using nanostructures to tailor thermal radiation with applications from solar and thermal energy for electrical generation and storage, to desalination. We fabricated solar photovolatic cells with efficiency >15% using 10 micron-thick crystalline silicon films. We demonstrated that aerogels can be used for concentrated solar thermal power, eliminating the need for vacuum and wavelength selective coatings. Photovoltaic cells can couple to terrestrial heat sources to convert thermal radiation into electricity, at an efficiency higher than photovoltaics. Moving to lower temperature range, we show that fabrics can be made to radiate out human body heat while remain opaque to visible light. We also demonstrate that by localizing solar energy on water surface, we can boil water and even achieve superheated steam under one sun. The talk will end with a discussion of the entropy of light and how we exploit the understanding to design better thermal-to-electrical energy converters.

III-V multijunction solar cells have demonstrated the highest efficiencies of any photovoltaic technology, in part because of their outstanding material quality and in part because of the ability to reduce thermalization losses while spanning a wide range of the solar spectrum. In this talk I will describe our on-going effort to demonstrate a 50% efficient solar cell using inverted metamorphic technology, and our work on solar cells that operate at temperatures up to 400°C. Along the way, we have learned to enhance photon recycling, to exploit luminescent coupling between junctions, and to fabricate internal reflectors. I will discuss how we are applying these cells to CPV, TPV and hydrogen production applications.

A replication system and method for fabricating volume hologram arrays is reviewed in this paper. The replication system can be used to fabricate high-efficiency transmission volume holographic lens arrays that are well-suited for spectrum-splitting photovoltaic applications. As in the well-known contact-copy replication technique, the new technique uses a master hologram to generate the desired holographic exposure, however no contact is required with the copy hologram. The object and reference beams for the holographic exposure are generated by discrete “coupling elements” on the master hologram and coupled through a prism to form an interference pattern on the copy hologram. The system can be implemented using relatively inexpensive lab equipment, but also has potential for large-scale production of holographic elements. A prototype system was developed and used to fabricate an experimental holographic lens array with a large aperture (9.6cm X 6cm) and high median diffraction efficiency (95.6%).

An optoelectrical model has been developed to simulate thin-film photovoltaic solar cells with periodically corrugated metallic backreflectors. The rigorous coupled-wave approach (RCWA) is used to calculate the absorption across the solar spectrum. This enables the calculation of the generation rate that drives a drift-diffusion model for the electrons and holes. The drift-diffusion equations are discretized using a hybridizable-discontinuous- Galerkin (HDG) scheme. The Newton{Raphson method is used to solve the resulting nonlinear system, with upwinding and homotopy used for stabilization. Numerical results concerning the convergence of HDG indicate that the HDG model is efficient and can be used to assess and improve solar cell designs.

Single threshold solar cells absorb photons only above a certain energy threshold, and the photon energy in excess of this threshold is generally lost as heat. One strategy to make better use of high energy photons, near or above twice the band gap, is to split excitons in two. In organic materials, the process by which this is performed is singlet fission. We have shown that the Shockley-Queisser limit is lifted to 45.9% for an ideal singlet fission device. Endothermic singlet fission is desired for maximum energy conversion efficiency, and such systems have been considered to form an excimer-like state with multi-excitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. In this talk we show, using TIPS-tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states, to the detriment of singlet fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet fission material must be designed that avoids excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion. The talk will summarize worldwide efforts to date in singlet fission photovoltics.

The highest efficiency solar cells are also excellent optical emitters. Band edge luminescence results when no other loss mechanisms compete with power conversion, allowing an ideal maximum efficiency of 33.7%. Constraining the angular range of emitted light, in order to promote light trapping and photon recycling within the semiconductor, increases this maximum to 45.1%. By analogy, here we show how a strategy for integrating highly luminescent aligned semiconductor rod-shaped nanocrystals (nanorods) into luminescent solar concentrators (LSCs) can also improve light trapping in the design and similarly enhance performance in comparison with conventional LSCs. This efficiency improvement relies on an asymmetry in the angular dependence of emission versus absorption that can be provided by the stokes shift of the radiation remitted by the nanorod. Our analysis predicts efficiency increases even when non-radiative loss is comparable to current GaAs cells and nanorod optical performance is consistent with state-of-the-art synthetic preparations.

Lead based colloidal quantum dots (CQDs) are a promising optoelectronic material system for solar harvesting, as the excitonic peak can be tuned from the visible to the near infra-red. These materials can be manufactured in the solution phase and spin-cast to form dense, cross-linked, semiconductor films on a variety of substrates, reducing the cost of device fabrication. However, like other non-crystalline based semiconductors, these films exhibit short carrier lifetimes and diffusion lengths. Active layers must be kept very thin to ensure efficient extraction of the photogenerated carriers, and device efficiencies will ultimately be limited by absorption. In this work we present a conceptual model of light trapping by resonant mode coupling in thin, mode limited devices. We show that targeting certain guided modes results in larger overall absorption, resulting in two key insights: 1) that a large spatial overlap of the mode profile with the grating is important; and 2) CQD materials have favourable material constants to benefit from absorption in the near-field of plasmonic resonances. Photocurrent enhancements of up to 250% at the exciton peak are achieved by coupling to optimal guided modes, compared with an increase of 25% in the same wavelength region for coupling to non-optimal modes. This result demonstrates that insights gained from an understanding of the mode profile of low mode density optoelectronics can be leveraged to provide tuneable light trapping and dramatically increase the absorption enhancement.

The efficiency of a crystalline silicon solar module decreases as its operating temperature rises. Module cooling is possible via selective reflection of sub-bandgap photons so that they are not parasitically absorbed. Selecting from a library of common dielectrics, we numerically optimize the design of two-layer mirrors at the outer glass surface of a crystalline Si solar cell module. The mirrors are designed to maximize the annual energy yield of a module by both reflecting light below the bandgap and enhancing the transmission of light above the bandgap. Combined ray-tracing and finite element simulations determine the power output and temperature of the module over time. Since any two-layer mirror would replace a conventional single-layer glass anti-reflection coating on the module glass, we study the ability of a two-layer structure to improve on a single-layer coating. The best two-layer designs improve the annual energy yield over a module with a glass anti-reflection coating and reduce the module operating temperature.

III-V semiconductor nanowires (NWs) are promising building blocks for next generation solar energy conversion at low cost. NW ensembles constitute a new class of metamaterial, where the optical properties of the array are tuned by the individual NW type, geometry and collective arrangement. Arising from the refractive index mismatch between NW and air and the sub-wavelength features, light propagation and distribution inside the nanostructure is strongly dependent on wavelength-geometry relation. While the photonic properties of small dielectric structures have been widely studied within the framework of Mie scattering, those of vertically standing nanowires cannot be explained with the same mathematical framework. In particular, coupling to poorly confined waveguided modes drives the absorption spectrum in NWs. The difficulty in obtaining III-V NWs on transparent substrates or self-standing, hampers the obtaining of their absorption properties experimentally. This work investigates how NWs interact with light from both theoretical and experimental methods. We experimentally illustrate the 3D extinction cross-section of GaAs NWs at different wavelengths by using fluorescence confocal microscopy. We demonstrate that by probing ordered arrays with fluorescence confocal microscopy, the effective absorption coefficient and cross-section can be obtained without the need of a transparent substrate. From these properties, we discuss a new variety of solar cell designs, that are not possible in the bulk form. Finally, we will also show that scanning probe microscopy is an emerging tool for the characterization of nanoscale solar cells, as well as a new fabrication approach to create 3D nanostructures on demand.

Urban areas currently account for 67-76% of global final energy consumption. With urban living expected to increase from roughly half of the world's population to 70% by the middle of this century, energy stewardship in the urban built environment is critical to a sustainable energy future. The rapid decrease in the cost of photovoltaics (PV) in the past decade makes building-integrated photovoltaics an appealing direction for next-generation buildings. In particular, PV windows are particularly attractive because they leverage the most architecturally popular, yet energy inefficient, element in the building façade. There are two popular approaches to PV energy conversion in windows: (1) Transparent designs leverage conversion of non-visible light5. They have the potential for high visible light transmittance (VLT) but are limited by low solar-to-electrical power conversion efficiencies (PCE). (2) Semitransparent designs provide higher PCE by harnessing visible light, but this is achieved with a direct tradeoff for VLT. The advent of switchable photovoltaics offers a new avenue to energy conversion in windows by combining high PCE PV with the energy-saving benefits of dynamic glazing. This talk will cover the theoretical benefit of switchable PV windows and the current status of two distinct experimental approaches.

Light-absorption enhancement is one of the key research areas related to the development of high-efficiency crystalline silicon (c-Si) solar cells. Surface structuring which can reduce not only the surface reflection but also increase the optical path length is an efficient way to increase light absorption in a broadband wavelength range. Among the surface structures, vertically aligned silicon microwires (MWs) have been extensively investigated as a means for developing highly efficient c-Si solar cells because of the outstanding broadband antireflection and radial junction effect which enables efficient charge collection. The incident light is absorbed along the long MW axis, while photo-induced carriers can be collected along the short radial direction. To realize the highly efficient radial junction c-Si solar cells, we have developed novel technologies such as fabrication process of MWs, high conductive and transmittance top electrode, shape-controlled MWs, and high purity doping process. The high-aspect-ratio MWs (> 10:1) were successfully fabricated through both optimized metal-assisted chemical etching (MACE) and deep reactive ion etching (DRIE) processes. To achieve the high efficiency of MW radial junction solar cells, we developed the high conductive and transparent top electrode to replace the conventional bus-finger electrode which has a significant shading loss. We devised a novel micro-grid top electrode which shows superior transmittance (over 97%) and low sheet resistance (less than 30 Ω/□). By applying the micro-grid electrode on the top surface, the MWs solar cells showed outstanding fill factor (81.2%) and improved efficiency (16.5%). Although our MWs radial junction solar cell showed improved efficiency with the micro-grid electrode, it needs to increase the light absorption capability to maximize the efficiency. As an efficient way to decrease the flat-top-surface reflection of the MWs and increase the light absorption property of the radial junction solar cells, a tapered-MW structure was employed using a simple wet-etching process. When a c-Si wafer with MWs is dipped in a silicon etchant (RSE-100, transene), the top part of the MWs that has a shorter diffusion path compared to the bottom part is etched more quickly because of the different chemical diffusion path lengths leading to the formation of tapered MWs. Since the diameter of the tapered MWs gradually increased from the top to the bottom, the tapered MWs can act as a buffer layer to compensate for the mismatch between the refractive indexes of air (1) and the silicon substrate (4). Thus, the surface reflection of the tapered MWs was observed to be less than 2% at a wavelength of 550 nm. The tapered MW based radial junction solar cells exhibit improved efficiency up to 18.9% thanks to the enhanced light absorption property. As the last step for optimizing the device structure of the MWs solar cells, we developed high purity doping process using acid dopant sources that showed improved minority carrier lifetime (from 79.29 µs to 272.24 µs). Accordingly, we achieved high efficiency (20.2%) MWs radial junction solar cell by applying all of the developed our technologies such as the micro-grid electrode, tapered MWs, and high purity doping process. At present, we are aiming at developing an ideal passivation layer to achieve the more than 25% efficiency of the radial junction solar cells. Therefore, we believe the MW structures with the suggested technologies become a foundational technology for the highly efficient radial junction solar cells.

This paper presents the development of three image processing tools to analyze defects and predict performance of the photovoltaic modules using infrared thermography, electroluminescence and ultraviolet induced fluorescent images of the modules. The MATLAB processing tool uses an algorithm aimed at detecting defects and quantifying them in terms of area affected and intensity of the defect. Each image was studied for visual defects, processed and the results from the three techniques were compared. The algorithms lead to detection of defect location with high accuracy. The size and intensity of the defect was computed based on pixel information that was correlated with performance parameters like short circuit current, fill factor, and series resistance depending on the image processing technique used. The infrared image processing technique aided in hotspot detection and removing outliers with elevated cell temperatures for a correlative study with electroluminescence imaging. Electroluminescence image processing demonstrated linear correlation between the inactive cell area and performance parameters like fill factor and series resistance. Ultraviolet induced fluorescence image processing resulted in precise segmentation of browned area and showed a linear correlation with the short-circuit current drop. Ultraviolet induced fluorescence images indicated at the presence of cracks in cells with non-uniform browning based on the corresponding electroluminescence images. The modules in the study were from three different manufacturers to show that the processing tool can work for the different modules.

This article studies stability control of laser wireless energy transfer, and experiment is presented. Based on transfer optical experiment, the experiment of broadband gallium arsenide (GaAs) cells and monocrystalline silicon (Si) cells photoelectric conversion ratio was tested. We comprehensively studied the factors that affect the conversion efficiency of photovoltaic cells. Different wave band, incident power, uniformity of power density, incident angle, and temperature were listed as main factors in our experiment. And uniformity of power density and temperature are found to be important for the efficiency of photocell conversion. With the relatively uniform power density at 808 nm, the highest photoelectric conversion ratio for GaAs photocell and Si photocell was measured to be 55% and 25% respectively, Besides, photoelectric conversion ratio was insensitive to angle that range ±10°. This work may provide experimental data accumulation to improve the conversion efficiency for long-distance wireless laser energy transmission and even solar power station in the near future.