This PDF file contains the front matter associated with SPIE Proceedings Volume 11824, including the Title Page, Copyright Information, and Table of Contents.

Increasing crop yields is the most sustainable approach toward the escalating food demand. Promoting crop production using genetic and ecological engineering such as reducing photorespirations, and accelerating recovery from photoprotections, is emerging yet debatable for its wide public acceptance. Alternatively, managing light quantity (intensity) and quality (spectrum) provides a promising and secure venue for improving crop yield, but comes with costs. For example, increasingly adopted supplementary electric lighting, consumed nearly 6 TWh of electricity in 2017 in the United States alone. Meanwhile, not every spectral component of sunlight contributes equally to photosynthesis. Recently have spectral-shifting materials been introduced to convert impinging sunlight into an optimized spectrum for photosynthesis. However, a fundamental optics challenge remains unaddressed: the majority of the internally generated photons are trapped inside the material, leading to seemingly encouraging but inconsistent results. Exploiting a photonic microstructure that is simple to manufacture, we demonstrate a spectral-shifting and unidirectional light extracting film that converts the least effective photosynthetic components of sunlight (green light) into the most effective (red) light with an internal quantum efficiency of 90% and a total external quantum efficiency of 43.8%. More importantly, this film breaks the propagation symmetry of light and extracts most of the otherwise trapped light unidirectionally into free space, rendering an easily adaptable greenhouse envelope material. Using leafy green lettuce as model crops, the micro-photonic film allows us to harvest > 20% more aboveground biomass of lettuce both indoor and outdoor. As we demonstrate experimentally, the photonic thin film can serve as greenhouse envelopes to provide more effective photosynthetic light than that of direct sunlight, opening the door for “red-colored” greenhouses and other protected environments with substantially augmented crop yields.

Due to the intermittent nature of solar energy availability, often an energy storage element such as a battery or supercapacitor is required to store the energy from a solar cell. The combination of a separate solar cell and an energy storage device is not usually suitable for small and compact electronic circuits with small footprints. Although various hybrid solar cell-supercapacitor devices have been studied before, the majority of them are two cells in one package with one electrode being shared between the two cells. We have designed and studied a new class of two-terminal hybrid electrochemical cells made of conducting polymer (e.g., polyaniline) composites with porous electrodes. In one design, a polymer film with embedded dye molecules has been used as a photoactive electrode in a supercapacitor presenting open circuit voltage changes up to 430 mV under illumination. In another design, the conducting polymer was employed in the electrolyte of the cell making a supercapacitor with the capability of harvesting light through the electrolyte. Voltages as high as 138 mV were achieved in the new device. Due to the storage property, the voltage drop 10 min after the secession of light was ~15 mV. While the fabrication of devices with the photoactive gel is easier, the higher efficiency in thin-film devices is more promising. Further development of two-terminal hybrid cells can open doors for designing compact and self-powered wireless sensors for various applications, including wearable electronics.

There is a great demand for high performance materials and technology in space photovoltaics (PV) to meet the power needs of satellites. Transition Metal Di-Chalcogenides (TMDCs) are strong candidates for such applications, as they are very lightweight and resilient to high energy radiation, compared to most PV semiconductors. We have modeled an ultra-thin photovoltaic system based on tungsten disulfide (WS2) and demonstrated performance enhancement by addition of light trapping and anti-reflection coating. Our photovoltaic model consists of a 100 nm thick WS2-based heterojunction solar cell, similar to the Hetero Junction Intrinsic Thin Layer (HIT) solar cell structure. A 1-D grating light trapping structure has been implemented using silver as the reflector material, with the grating period and thickness optimized for highest absorption enhancement. An antireflection coating layer was added to further enhance absorption, with the thickness optimized to minimize surface reflection. We have simulated our model under AM0 solar spectrum over the temperature range of geostationary satellite orbits (313-343K). The baseline photovoltaic model design was calculated to have an efficiency around 12%. The absorption enhancement from light trapping increased the short-circuit current (JSC) by 25%, which gave an efficiency around 16%. The additional absorption due to anti-reflection coating increased the JSC by a further 15%, leading to efficiency around 19%. In addition, TMDC-based solar cells have lower temperature coefficient for efficiency degradation compared to low bandgap semiconductor solar cells. These results show that our TMDC-based photovoltaic system with light trapping and anti-reflection coating is a strong candidate for space photovoltaic applications.

SnO₂ is an attractive semiconducting material suitable for application as the photoanode in dye sensitized solar cells (DSSCs) due to its favourable properties such as wide band gap and notable photo stability. However, improved solar cell performances can be achieved by using composites of SnO2 with several other materials like Al2O3 or MgO. In this study photovoltaic performances of DSSCs with pristine SnO₂ and SnO₂ coated MgO composite photoanodes were investigated with two different dyes, namely Ruthenium N719 and metal free Indoline D149. Significant difference in the efficiencies was observed in DSSCs made with liquid electrolyte having I^-/I3^- redox couple and Pt counter electrode. DSSC fabricated with D149 sensitized pristine SnO₂ showed higher efficiency of 2.07% compared to the efficiency of 1.07% for the DSSC sensitized with N719. Higher energy level of the, lowest unoccupied molecular orbital (LUMO) of D149 than that of N719 and the higher insulating nature of the metal free D149 could be possible reasons which could help rapid electron transfer from the excited dye molecules to the conduction band (CB) of the semiconductor and reduction of recombination of the electrons in the device. It was also observed that DSSC fabricated with N719 sensitized SnO₂/MgO composite photoanode exhibited higher efficiency of 3.43% than the efficiency of 0.67 % for the DSSC having D149 sensitized photoanode. Higher insulating nature of the metal free D149 layer and the SnO₂/Dye interface due to the insertion of MgO would have reduced the electron transfer from the excited dye molecules to the CB of the semiconductor. Therefore, metal free dyes could be used effectively to sensitize pristine SnO₂ photoanode.

Quantum-dot solar cells are a promising high-efficiency concept, but suffer from low absorption. Resonant light trapping can enable to absorb most of the incident light while maintaining good device quality. In this paradigm, the absorption depends critically on the vertical position of the quantum dot layers, but this has been largely ignored so far (this also applies to quantum wells). Here, we show the importance of the position of 10 InAs layers in a GaAs Fabry-Perot cavity. We then extend this approach to multi-resonant absorption, showing the potential absorption gain from optimizing the position of quantum dots in full devices.

In recent years, the interest in photovoltaic supercapacitors has been increasing in order to develop self-powered sensors for a sustainable system. Hence, significant research efforts are needed to enhance the photoelectric and electrochemical performances of hybrid devices. Herein, we have studied the effect of the porosity of different counter electrodes on the performance of the hybrid photovoltaic supercapacitors. The photovoltaic supercapacitors were fabricated in one package with a simple structure including a titanium dioxide (TiO2) coated on fluorine-doped tin oxide (FTO) glass as a working electrode and polyaniline (PANI)-based gel electrolyte. The performance of the hybrid device was studied with four different counter electrodes: a multi-walled carbon nanotube (MWCNT) porous electrode, PEDOT:PSS coated on FTO glass, carbon monolithic electrode, and a carbon-based conductive fabric. The specific capacitance of the device with PEDOT:PSS coated FTO electrode was 255 mF/g in the dark and increased to be 274 mF/g under the light based on the mass of the gel. The hybrid device can be charged when the working electrode is illuminated. The variation in the open circuit voltage (DV) was reached 256 mV in 400 s under illumination, and the voltage drop was 4 mV (−4%) in 600 s of the dark. The current results of the hybrid photovoltaic supercapacitor, with a simple fabrication process and basic structure, are boosting the study for the electrode materials selection to enhance the performance of the hybrid device.

The past several years have seen new developments in tracking-integrated solar concentrators, including new concepts in optics design and tracking methods, increasing clarity regarding high-value target applications, improved performance modeling methods and the development of pre-commercial tracking-integrated concentrator photovoltaic modules. We survey these recent developments and also present preliminary measurements of a tracking-integrated concentrator photovoltaic module under outdoor conditions in Abu Dhabi, United Arab Emirates. We incorporate the data into a semi-empirical model to estimate annual energy yields and assess its technical and economic potential relative to competing technologies.

The continued decline in solar energy prices to below-market levels has made it an enabling technology for energy-intensive "secondary decarbonization" efforts, such as carbon capture and hydrogen production, for which energy prices play an outsized role in their overall economic viability. In this study we implement a techno-economic model to evaluate the impact of falling solar energy costs, both for electricity and heat, on the overall economics of both hydrogen electrolysis and industrial carbon capture, to illustrate how falling solar energy prices can enable the expansion of these technologies. We additionally consider some of the operational challenges involved in the collection and transmission of solar heat, and note the areas in which the emergence of nonimaging collectors as an alternative solar thermal technology with wide applicability has the potential to alleviate some of these challenges.

Solar thermal absorbers lose an increasing amount of heat at higher operating temperatures, limiting their efficiencies. An infrared mirror characterized by a high transmittance in the visible region of the spectrum while having high reflectance in the mid-IR could be crucial for a partial recovery of heat radiated by absorbers operating at high temperatures. Thanks to the cold-side external photon recycling mechanism, an IR mirror applied to an evacuated collector could lead to increase in efficiency of up to 60%. Here, we demonstrate the feasibility of such a mirror, which in principle is developed for high-vacuum flat solar thermal collectors, but is easily adaptable to other applications. The mirror presented has a very simple design, based on a discrete rugate filter scheme, and does not require multi-cathode deposition tools, which further simplifies the manufacturing process.

Adaptive radiative heat management can switch between heating and cooling based on the cover material's mid-IR property. However, such modulation is dependent on the underlying object's emissivity, which limits the applicability of the modulation device. For instance, to achieve the best cooling performance, the human body requires a transmissive cover, and metallic motor vehicles require an emissive one. In this talk, I will introduce a mechanical switchable radiative heat modulator that can change among three different modes - transmission, reflection, and emission - to adapt for the underlying object property. This modulation principle can improve the design flexibility of future adaptive radiative heat management.

Frost formation and snow coverage is troublesome for a wide range of applications including air conditioning, refrigeration, wind turbines, and photovoltaic solar cells. In solar cell applications, snow coverage in winter times and dust coverage lead to a huge power loss. To maintain energy efficiency and operational fidelity, snow, frost, and ice need to be removed in a highly efficient and rapid manner. Even the state-of-the-art de-snowing, defrosting, and de-icing methods suffer from high energy consumption (10 J cm^-2) and taking long times (> 1 min.). Here, we integrated a pulse electrothermal heating method with a transparent and self-cleaning nanoengineered coating capable of achieving ultraefficient and rapid (~ 1 s.) interfacial de-snowing, defrosting, and de-icing. Specifically, the transparency and selfcleaning properties of the coating are designed to remove both snow and dust from solar cells while minimizing the interference to light absorption. We experimentally demonstrate the defrosting performance of our multi-functional coating and show the enhanced efficiency in energy and time for ice and snow removal. This method is applicable to other applications such as heat exchangers, wind turbines, and power transmission lines.

This Conference Presentation, “Materials for thermophotovoltaics,” was recorded for Optics + Photonics 2021 held in San Diego, California.

A new selective solar absorbers (SSA) optimized to work at 250°C without concentration is presented. The new SSA, based on Cr2O3/Cr multilayers, allows to obtain very high thermal efficiency up to 250°C in evacuated flat solar thermal panels without concentration. Multilayers have been deposited by magnetron sputtering on different copper substrates and characterized by reflectivity measurement as well as calorimetric measurements. Stagnation temperatures close to 400 °C have been obtained with a nominal irradiance of 1000W/m2. These results pave the way to the use of evacuated flat thermal panels for green industrial steam generation.

We demonstrate remarkable full-daytime subambient cooling performance with CaCO3 and BaSO4 nanoparticle paints. These fillers have a high electron band gap for low solar absorptance and phonon resonance or polymer absorption in the sky window for high sky window emissivity. With an appropriate particle size and a broad particle size distribution, the BaSO4 nanoparticle film reaches an ultrahigh solar reflectance of 97.6% and a high sky window emissivity of 0.96. During field tests, the BaSO4 film stays more than 4.5 °C below ambient temperature or achieves an average cooling power of 117 W/m2. The BaSO4-acrylic paint is developed with a 60% volume concentration to enhance the reliability in outdoor applications, achieving a solar reflectance of 98.1% and a sky window emissivity of 0.95. Field tests indicate similar cooling performance to the BaSO4 films. Overall, our BaSO4-acrylic paint shows a standard figure of merit of 0.77.

A thermophotovoltaic (TPV) cell is specially designed to minimize the absorption of radiation below its lowest bandgap. This ensures that the unused power is returned to the hot thermal emitter, which keeps it from being wasted. This approach is termed photon recycling because the energy is recycled until it is emitted at a high enough frequency to be efficiently converted. To facilitate this process, we recently created a cell architecture that has a thin air layer behind the light-absorbing semiconductor. The resulting air-bridge cell (ABC) reflects back almost all of the low-energy photons. In this talk, I will discuss the development of an InGaAs ABC that achieved a record-high peak conversion efficiency of 32% and our recent efforts to improve performance.