The most common approaches to generating power from sunlight are either photovoltaic, in which sunlight directly excites electron-hole pairs in a semiconductor, or solar-thermal, in which sunlight drives a mechanical heat engine. Photovoltaic power generation is intermittent and typically only exploits a portion of the solar spectrum efficiently, whereas the intrinsic irreversibilities of small heat engines make the solar-thermal approach best suited for utility-scale power plants. There is, therefore, an increasing need for hybrid technologies for solar power generation. By converting sunlight into thermal emission tuned to energies directly above the photovoltaic bandgap using a hot absorber-emitter, solar thermophotovoltaics promise to leverage the benefits of both approaches: high efficiency, by harnessing the entire solar spectrum; scalability and compactness, because of their solid-state nature; and dispatchablility, owing to the ability to store energy using thermal or chemical means. However, efficient collection of sunlight in the absorber and spectral control in the emitter are particularly challenging at high operating temperatures. This drawback has limited previous experimental demonstrations of this approach to conversion efficiencies around or below 1% (refs 9, 10, 11). Here, we report on a full solar thermophotovoltaic device, which, thanks to the nanophotonic properties of the absorber-emitter surface, reaches experimental efficiencies of 3.2%. The device integrates a multiwalled carbon nanotube absorber and a one-dimensional Si/SiO2 photonic-crystal emitter on the same substrate, with the absorber-emitter areas optimized to tune the energy balance of the device. Our device is planar and compact and could become a viable option for high-performance solar thermophotovoltaic energy conversion.

thermochemical, and solar thermophotovoltaics. There exist a range of solutions with high absorptivity for low and intermediate temperatures. [1][2][3] However, for many applications, high operating temperatures (>700 K) are advantageous to achieve higher system effi ciencies. Conventional absorbers are unsuitable at these high operating temperatures since there are more considerations to be taken into account. [ 4 ] Firstly, the materials and structures need to be thermally stable and to maintain their optical properties at these high temperatures. Refractory metals are most advantageous due to their high melting point and low vapor pressure. Secondly, it is crucial that the absorber exhibits spectrally selective absorptance; namely high absorptivity in the shorter wavelength range to absorb most of the solar spectrum and low absorptivity (i.e., emissivity) in the longer wavelength range to minimize losses due to re-emission. Furthermore, this selectivity, i.e., the spectral range of high and low absorptivity, has to be tailored for the specifi c system operating conditions to achieve maximum system effi ciency.It is therefore advantageous to use PhCs which offer the possibility to tailor the spectral absorptance [ 5,6 ] and thus optimize system effi ciency. Several absorbers based on 1D multilayer stacks, [ 7,8 ] 2.5D structures such as pyramids, [9][10][11] 3D PhCs in refractory metals, [ 12,13 ] as well as metamaterials [14][15][16] have been proposed. Here, we demonstrate the suitability of a 2D PhC comprising a square lattice of cylindrical cavities etched into a Ta substrate as a highly effi cient and selective absorber at high temperatures, i.e., above 1000 K. While all of the above approaches achieve good spectral selectivity, the 2D PhC design is a compact and thermally robust structure, minimizing the number of interfaces as compared to multilayer or 3D PhC approaches which is crucial for high temperature stability. At the same time, fabrication is simple and scalable and can be achieved by standard semiconductor processes. In this 2D PhC design, the absorptivity of the material is selectively enhanced by the introduction of cavity modes and the spectral range of enhancement, i.e., high absorptivity, can be tuned A high-temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance α α = = 0.86 and thermal emittance ε ε = 0.26 at 1000 K, using high-temperature material properties, is achieved resulting in a thermal transfer effi ciency more than 50% higher than that of a blackbody absorber. Furthermore, an integrated double-sided 2D PhC absorber/ emitter pair is demonstrated for a high-performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double-side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a fl at Ta absorber...

Despite their great promise, small experimental thermophotovoltaic (TPV) systems at 1000 K generally exhibit extremely low power conversion efficiencies (approximately 1%), due to heat losses such as thermal emission of undesirable mid-wavelength infrared radiation. Photonic crystals (PhC) have the potential to strongly suppress such losses. However, PhC-based designs present a set of non-convex optimization problems requiring efficient objective function evaluation and global optimization algorithms. Both are applied to two example systems: improved micro-TPV generators and solar thermal TPV systems. Micro-TPV reactors experience up to a 27-fold increase in their efficiency and power output; solar thermal TPV systems see an even greater 45-fold increase in their efficiency (exceeding the Shockley-Quiesser limit for a single-junction photovoltaic cell).

The nascent field of high-temperature nanophotonics could potentially enable many important solid-state energy conversion applications, such as thermophotovoltaic energy generation, selective solar absorption, and selective emission of light. However, special challenges arise when trying to design nanophotonic materials with precisely tailored optical properties that can operate at hightemperatures (>1,100 K). These include proper material selection and purity to prevent melting, evaporation, or chemical reactions; severe minimization of any material interfaces to prevent thermomechanical problems such as delamination; robust performance in the presence of surface diffusion; and long-range geometric precision over large areas with severe minimization of very small feature sizes to maintain structural stability. Here we report an approach for high-temperature nanophotonics that surmounts all of these difficulties. It consists of an analytical and computationally guided design involving high-purity tungsten in a precisely fabricated photonic crystal slab geometry (specifically chosen to eliminate interfaces arising from layer-by-layer fabrication) optimized for high performance and robustness in the presence of roughness, fabrication errors, and surface diffusion. It offers nearultimate short-wavelength emittance and low, ultra-broadband long-wavelength emittance, along with a sharp cutoff offering 4∶1 emittance contrast over 10% wavelength separation. This is achieved via Q-matching, whereby the absorptive and radiative rates of the photonic crystal's cavity resonances are matched. Strong angular emission selectivity is also observed, with shortwavelength emission suppressed by 50% at 75°compared to normal incidence. Finally, a precise high-temperature measurement technique is developed to confirm that emission at 1,225 K can be primarily confined to wavelengths shorter than the cutoff wavelength. E ver since photonic bandgaps were predicted to exist in appropriately designed periodic subwavelength structures (1-3)-i.e., photonic crystals (PhCs), significant interest has garnered in recent years to exploit this property. Coupled with the recent advancements in nanofabrication techniques, many applications have been made possible, ranging from room-and cryogenic-temperature optoelectronic devices for development of all-optical integrated circuits (4), to highly sensitive sensors (5), low-threshold lasers (6), and highly efficient light emitting diodes (7). PhCs also enable us to accurately control spontaneous emission by virtue of controlling the photonic bandgap (1, 8). In particular, metallic PhCs have been shown to possess a large bandgap (9-12) and consequently superior modification of the intrinsic thermal emission spectra is readily achievable. This is extremely promising for many unique applications, especially high-efficiency energy conversion systems encompassing hydrocarbon and radioisotope fueled thermophotovoltaic (TPV) energy conversion (13,14) as well as solar selective absorbers and emitters for the...