Yi Xiang Yeng

High-temperature stability and selective thermal emission of polycrystalline tantalum photonic crystals

We present the results of extensive characterization of selective emitters at high temperatures, including thermal emission measurements and thermal stability testing at 1000 °C for 1h and 900 °C for up to 144 h. The selective emitters were fabricated as 2D photonic crystals (PhCs) on polycrystalline tantalum (Ta), targeting large-area applications in solid-state heat-to-electricity conversion. We characterized spectral emission as a function of temperature, observing very good selectivity of the emission as compared to flat Ta, with the emission of the PhC approaching the blackbody limit below the target cut-off wavelength of 2 μm, and a steep cut-off to low emission at longer wavelengths. In addition, we study the use of a thin, conformal layer (20 nm) of HfO(2) deposited by atomic layer deposition (ALD) as a surface protective coating, and confirm experimentally that it acts as a diffusion inhibitor and thermal barrier coating, and prevents the formation of Ta carbide on the surface. Furthermore, we tested the thermal stability of the nanostructured emitters and their optical properties before and after annealing, observing no degradation even after 144 h (6 days) at 900 °C, which demonstrates the suitability of these selective emitters for high-temperature applications.

Recent developments in high-temperature photonic crystals for energy conversion

After decades of intense studies focused on cryogenic and room temperature nanophotonics, scientific interest is also growing in high-temperature nanophotonics aimed at solid-state energy conversion. These latest extensive research efforts are spurred by a renewed interest in high temperature thermal-toelectrical energy conversion schemes including thermophotovoltaics (TPV), solar‐ thermophotovoltaics, solar‐thermal, and solar‐thermochemical energy conversion systems. This field is profiting tremendously from the outstanding degree of control over the thermal emission properties that can be achieved with nanoscale photonic materials. The key to obtaining high efficiency in this class of high temperature energy conversion is the spectral and angular matching of the radiation properties of an emitter to those of an absorber. Together with the achievements in the field of highperformance narrow bandgap photovoltaic cells, the ability to tailor the radiation properties of thermal emitters and absorbers using nanophotonics facilitates a route to achieving the impressive efficiencies predicted by theoretical studies. In this review, we will discuss the possibilities of emission tailoring by nanophotonics in the light of high temperature thermal-to-electrical energy conversion applications, and give a brief introduction to the field of TPV. We will show how a class of large area 2D metallic photonic crystals can be designed and employed to efficiently control and tailor the spectral and angular emission properties, paving the way towards new and highly efficient thermophotovoltaic systems and enabling other energy conversion schemes based on high-performance high-temperature nanoscale photonic materials.

Design of wide-angle selective absorbers/emitters with dielectric filled metallic photonic crystals for energy applications

The design and simulation of a wide angle, spectrally selective absorber/emitter metallic photonic crystal (MPhC) is presented. By using dielectric filled cavities, the angular, spectrally selective absorption/emission of the MPhC is dramatically enhanced over an air filled design by minimizing diffraction losses. Theoretical analysis is performed and verified via rigorous coupled wave analysis (RCWA) based simulations. An efficiency comparison of the dielectric filled designs for solar thermophotovoltaic applications is performed for the absorber and emitter which yields a 7% and 15.7% efficiency improvement, respectively, compared to air filled designs. The converted power output density is also improved by 33.5%.

Performance Analysis of Experimentally Viable Photonic Crystal Enhanced Thermophotovoltaic Systems

One of the keys towards high efficiency thermophotovoltaic (TPV) energy conversion systems lies in spectral control. Here, we present detailed performance predictions of realistic TPV systems incorporating experimentally demonstrated advanced spectral control components. Compared to the blackbody emitter, the optimized two-dimensional (2D) tantalum (Ta) photonic crystal (PhC) selective emitter enables up to 100% improvement in system efficiency. When combined with the well characterized cold side tandem filter and the latest InGaAs TPV cells, a TPV energy conversion system with radiant heat-to-electricity efficiency of 25% and power density of 0.68 W cm(-2) is achievable today even at a relatively low temperature of 1320 K. The efficiency could be increased to ∼ 40% (the theoretical 0.62 eV single bandgap TPV thermodynamic limit at 1320 K is 55%) as future implementation of more optimized TPV cells approach their theoretical thermodynamic limit.

Photonic crystal enhanced silicon cell based thermophotovoltaic systems.

We report the design, optimization, and experimental results of large area commercial silicon solar cell based thermophotovoltaic (TPV) energy conversion systems. Using global non-linear optimization tools, we demonstrate theoretically a maximum radiative heat-to-electricity efficiency of 6.4% and a corresponding output electrical power density of 0.39 W cm(-2) at temperature T = 1660 K when implementing both the optimized two-dimensional (2D) tantalum photonic crystal (PhC) selective emitter, and the optimized 1D tantalum pentoxide - silicon dioxide PhC cold-side selective filter. In addition, we have developed an experimental large area TPV test setup that enables accurate measurement of radiative heat-to-electricity efficiency for any emitter-filter-TPV cell combination of interest. In fact, the experimental results match extremely well with predictions of our numerical models. Our experimental setup achieved a maximum output electrical power density of 0.10W cm(-2) and radiative heat-to-electricity efficiency of 1.18% at T = 1380 K using commercial wafer size back-contacted silicon solar cells.

Broadband angular selectivity of light at the nanoscale: Progress, applications, and outlook

Humankind has long endeavored to control the propagation direction of light. Since time immemorial, shades, lenses, and mirrors have been used to control the flow of light. In modern society, with the rapid development of nanotechnology, the control of light is moving toward devices at micrometer and even nanometer scales. At such scales, traditional devices based on geometrical optics reach their fundamental diffraction limits and cease to work. Nano-photonics, on the other hand, has attracted wide attention from researchers, especially in the last decade, due to its ability to manipulate light at the nanoscale. This review focuses on the nano-photonics systems that aim to select light based on its propagation direction. In the first half of this review, we survey the literature and the current state of the art focused on enabling optical broadband angular selectivity. The mechanisms we review can be classified into three main categories: (i) microscale geometrical optics, (ii) multilayer birefringent ma...

Effects of surface diffusion on high temperature selective emitters

Using morphological and optical simulations of 1D tantalum photonic crystals at 1200K, surface diffusion was determined to gradually reduce the efficiency of selective emitters. This was attributed to shifting resonance peaks and declining emissivity caused by changes to the cavity dimensions and the aperture width. Decreasing the structures curvature through larger periods and smaller cavity widths, as well as generating smoother transitions in curvature through the introduction of rounded cavities, was found to alleviate this degradation. An optimized structure, that shows both high efficiency selective emissivity and resistance to surface diffusion, was presented.

Omnidirectional wavelength selective emitters/absorbers based on dielectric-filled anti-reflection coated two-dimensional metallic photonic crystals

We demonstrate designs of dielectric-filled anti-reflection coated (ARC) two-dimensional (2D) metallic photonic crystals (MPhCs) capable of omnidirectional, polarization insensitive, wavelength selective emission/absorption. Up to 26% improvement in hemispherically averaged emittance/absorptance below the cutoff wavelength is observed for optimized hafnium oxide filled 2D tantalum (Ta) PhCs over the unfilled 2D Ta PhCs. The optimized designs possess high hemispherically averaged emittance/absorptance of 0.86 at wavelengths below the cutoff wavelength and low hemispherically averaged emittance/absorptance of 0.12 at wavelengths above the cutoff wavelength, which is extremely promising for applications such as thermophotovoltaic energy conversion, solar absorption, and infrared spectroscopy.

Photonic Crystals: Shaping the Flow of Thermal Radiation

In this paper we explore theory, design, and fabrication of photonic crystal (PhC) based selective thermal emitters. In particular, we focus on tailoring spectral and spatial properties by means of resonant enhancement in PhCs. Firstly, we explore narrow-band resonant thermal emission in photonic crystals exhibiting strong spectral and directional selectivity. We demonstrate two interesting designs based on resonant Q-matching: a vertical cavity enhanced resonant thermal emitter and 2D silicon PhC slab Fano-resonance based thermal emitter. Secondly, we examine the design of 2D tungsten PhC as a broad-band selective emitter. Indeed, based on the resonant cavity coupled resonant modes we demonstrate a highly selective, highly-spectrally efficient thermal emitter. We show that an emitter with a photonic cut-off anywhere from 1.8 μm to 2.5 μm can be designed.

NUMERICAL STUDY OF A SOLAR THERMOPHOTOVOLTAIC ENERGY CONVERTER WITH HIGH PERFORMANCE 2D PHOTONIC CRYSTALS

Solar thermophotovoltaic (STPV) systems convert solar energy into electricity via thermally radiated photons at tailored wavelength to increase energy conversion efficiency. In this work we report the design and analysis of a STPV using a high- fidelity 2D axisymmetric thermal-electrical hybrid model that includes thermal coupling between the absorber/emitter/PV cell and accounts for non-idealities such as temperature gradients and parasitic thermal losses. The radiative spectra of the absorber and emitter are engineered by using two-dimensional periodic square array of cylindrical holes on a tantalum (Ta) substrate. The optimal solar concentration and resulting temperature are determined by considering the energy losses associated with re-emission at the absorber, low energy (below band gap) emission at the emitter, and carrier thermalization /recombination in the PV cell. The modeling results suggest that the overall efficiency of a realistic planar STPV consisting of Ta PhCs and existing InGaAsSb PV cells with a filter can be as high as ~8%. The use of high performance PhCs allows us to simplify the system layout and operate STPVs at a significantly lower optical concentration level and operating temperature compared with STPVs using metallic cavity receivers. This work shows the importance of photon engineering for the development of high efficiency STPVs and offers design guidelines for both the PhC absorber/emitter and the overall system.