Abstract:Recently, there has been increasing interest in utilizing solar thermophotovoltaics (STPV) to convert sunlight into electricity, given their potential to exceed the Shockley-Queisser limit. Encouragingly, there have also been several recent demonstrations of improved systemlevel efficiency as high as 6.2%. In this work, we review prior work in the field, with particular emphasis on the role of several key principles in their experimental operation, performance, and reliability. In particular, for the problem of designing selective solar absorbers, we consider the trade-off between solar absorption and thermal losses, particularly radiative and convective mechanisms. For the selective thermal emitters, we consider the tradeoff between emission at critical wavelengths and parasitic losses. Then for the thermophotovoltaic (TPV) diodes, we consider the trade-off between increasing the potential short-circuit current, and maintaining a reasonable opencircuit voltage. This treatment parallels the historic development of the field, but also connects early insights with recent developments in adjacent fields. With these various components connecting in multiple ways, a system-level end-to-end modeling approach is necessary for a comprehensive understanding and appropriate improvement of STPV systems. This approach will ultimately allow researchers to design STPV systems capable of exceeding recently demonstrated efficiency values.

No abstract

In this work, we propose a rare-earth-based ceramic thermal emitter design that can boost thermophotovoltaic (TPV) efficiencies significantly without cold-side filters at a temperature of 1573 K (1300 °C). The proposed emitter enhances a naturally occurring rare earth transition using quality-factor matching, with a quarter-wave stack as a highly reflective back mirror, while suppressing parasitic losses via exponential chirping of a multilayer reflector transmitting only at short wavelengths. This allows the emissivity to approach the blackbody limit for wavelengths overlapping with the absorption peak of the rare-earth material, while effectively reducing the losses associated with undesirable long-wavelength emission. We obtain TPV efficiencies of 34% using this layered design, which only requires modest index contrast, making it particularly amenable to fabrication via a wide variety of techniques, including sputtering, spin-coating, and plasma-enhanced chemical vapor deposition.

No abstract

Deliberate control of thermal emission properties using nanophotonics has improved a number of applications including thermophotovoltaics (TPV), radiative cooling and infrared spectroscopy. In this work, we study the effect of simultaneous control of angular and spectral properties of thermal emitters on the efficiencies of TPV systems. While spectral selectivity reduces sub-bandgap losses, angular selectivity is expected to enhance view factors at larger separation distances and hence to provide flexibilities in cooling the photovoltaic converter. We propose a design of an angular and spectral selective thermal emitter based on waveguide perfect absorption phenomena in epsilon-near-zero thin-films. Aluminum-doped Zinc-Oxide is used as an epsilon-near-zero material with a cross-over frequency in the near-infrared. A high contrast grating is designed to restrict the emission in a range of angles around the normal direction, while an integrated filter ensures spectral selectivity to reduce sub-bandgap losses. Theoretical analysis shows an expected relative enhancement of the TPV system efficiency of at least 32% using selective emitters with ideal angular and spectral selectivity at large separation distances compared to a blackbody. This enhancement factor, however, reduces to 3.9% with non-ideal selective emitters. This big reduction of the efficiency is attributed to sub-bandgap losses, off-angular losses and high-temperature dependence of optical constants.

Selective filtering of spectral and angular optical transmission has recently attracted a great deal of interest. While optical passband and stopband spectral filters are already widely used, angular selective transmission and reflection filtering represents a less than fully explored alternative. Nonetheless, this approach could be promising for several applications, including stray radiation minimization and background emission exclusion. In this work, a concept for angle-selective reflection filtering using guided mode resonance coupling is proposed. Although guided mode resonance structures are already used for spectral filtering, in this work, a novel variation on angle-selective reflection filtering using guided mode resonance coupling is proposed. We investigate angle-dependent properties of such structures for potential use as angularly selective reflection filters. We utilize interference between diffraction modes to provide tunable selectivity with a sufficient angular width. Combining these structures with thermal emitters can exclude selected emission angles for spatially selective thermal emissivity reduction toward sensitive targets, as well as directionally selective emissivity exclusion for suppression of solar heating. We show a very large selective reduction of heat exchange by 99.77% between an engineered emitter and a distant receiver, using just a single groove grating and an emitting substrate in the emitter's side. Also, we show a selective reduction of heat exchange by approximately 77% between an emitter covered by engineered sets of angular selective reflection filters and a nearby sensitive target. The suggested angle-selective structure may have applications in excluding background thermal radiation: in particular, thermal emission reduction for daytime radiative cooling, sensitive IR telescope detectors, and high-fidelity thermoluminescent spectroscopy. I. INTRODUCTION Controlling the angular selectivity of optical transmission is a recently emerging branch of photonics, which has recently attracted a great deal of interest [1-5]. With recent advances in nanophotonics, broadband angular selectivity has recently been achieved in the laboratory. Some examples include microscale compound parabolic concentrators to limit the emission angle for solar cells [1,6], non-resonant Brewster modes in metallic gratings for angle-selective broadband absorption and selective thermal emission [7] and 1D photonic crystal heterostructures [8,9]. This approach can also allow for significant reduction of unwanted optical noise over a wide frequency range [4]. These examples show that selective angular transmission is well-established. However, a tunable angle-selective reflection peak has not been demonstrated yet. In fact, Babinet's principle indicates that it should generally be possible to achieve such a goal, through processes such as inversion [10]. Such an approach could be uniquely useful for elimination of unwanted optical components from a certain direction, for example to mitigate optical noise...

Designing and fabricating nanopatterns are under continuous investigations in a variety of areas such as plasmonics, [1] biosensors, [2] and electronics. [3] Large-area fabrication of such nanostructures with high accuracy, [4][5][6] tunability, [7][8][9] as well as hierarchy, [10] remains as a significant challenge, especially on metals. Although nanoimprint lithography (NIL) [11] has been used to fabricate nanoscale patterns in a wafer-level [12,13] and achieve multiband and omnidirectional optical properties, [14] the patterned polymer resists such as polystyrene suffers from relaxation as the temperature increases, [15] leading to the obstacle for fabricating hierarchical nanostructures by deforming polymers through a consecutive nanoimprinting, [16] due to the thermo-rheological nature of polymer resists beyond the glass transition temperature (T g ). Even if sophisticated hierarchical molds can alleviate the difficulty, sequential spin-coating of polymer resists as well as reactive-ion etching (RIE) of such molds is not cost-effective for most applications. [17][18][19] Direct imprinting metallic structures by deformation has been recognized as a challenge due to issues such as grain size effect [20] and localized dislocation bursts. [21] Gao et al. developed laser shock imprinting (LSI) process to induce a high strain rate deformation to induce nanoscale superplasticity in metals and overcome the aforementioned fabrication barriers, resulting in conformable and high-fidelity nanoimprinting of metals using e-beam fabricated silicon as nanomolds. [22] Recently, Jin et. al developed the LSI process by replacing silicon molds with polymer (e.g., SU-8) molds to achieve biomimic surface nanostructures in a large area. [23] Although polymers are soft in general compared with ceramics and metals, its strain rate sensitivity of polymer is very high, which helps maintain its shape during high strain rate deformation, which enabled LSI of metallic microstructures. [23] One advantage of soft polymer molds is their great potential in industrial-level manufacturing. However, there are several challenges to overcome for polymer molds to be used for NIL in metallic components. First of all, due to the low modulus of polymer mold, the resolution of imprinting is limited to microscale (e.g., ≈3 µm using SU8 mold). [23] It is of great Large-area patterning of metals in nanoscale has always been a challenge. Traditional microfabrication processes involve many highcost steps, including etching and high-vacuum deposit, which limit the development of functional nanostructures, especially multiscale metallic patterns. Here, multiplex laser shock imprinting (MLSI) process is introduced to directly manufacture hierarchical micro/nanopatterns at a high strain rate on metallic surfaces using soft optical disks with 1D periodic trenches as molds. The unique metal/polymer layered structures in inexpensive soft optical disks make them strong candidates of molds for MLSI processes. The feasibility of MLSI on hard metals toward soft molds...

No abstract