Large‐Area Ultrabroadband Absorber for Solar Thermophotovoltaics Based on 3D Titanium Nitride Nanopillars

Abstract

Refractorymetal-based broadband absorber/narrowband emitters is a flourishing field in energy harvesting where the physical and chemical stability of the metals at high temperatures provide efficient absorption/emission of solar/ heat energy. [1][2][3] Advancements in solar/ thermophotovoltaics (S/TPV) must be accompanied by thermally stable devices in order to withstand extreme operating conditions. The fundamental limiting factor [Shockley Queisser (SQ) efficiency limit] in traditional single-junction solar cells is its inability in converting the broad solar spectrum into a narrow range of wavelengths defined by the PV cells. [4] Solar photons with energies below the bandgap of the PV cell are not converted, and the photons with energies higher than the bandgap lose their additional energy through a process known as thermalization. In solar thermophotovoltaics (STPV), an intermediate system composed of absorber and emitter is used to overcome the SQ limit by harvesting the solar energy followed by the emission of narrowband radiation. [5] The absorber should provide unitary absorption in the entire solar spectral range over a range of incidence angles with polarization insensitive nature, and minimum thermal reradiation to avoid near-infrared heat radiation at elevated temperatures. Through thermal conduction, the absorbed heat energy is transferred to the emitter, which is tailored to emit a narrowband radiation defined by the bandgap energy of a PV cell. Thus, the absorber needs to withstand high temperatures to transfer a large amount of heat energy to the emitter. The low-loss noble metals are successfully used in unitary absorbers so far, particularly Au and Ag. [6][7][8] However, noble metals are not compatible with hightemperature photovoltaic applications and standard silicon manufacturing processes (complementary metal oxide semiconductor, CMOS, technology), owing to low melting point and diffusion of noble metals into silicon. Annealing the substrates at high temperatures induce oxidation, surface diffusion, corrosion, cracking, and delamination of thin films from the substrate. [7] The situation is even worse in the case of the Broadband absorbers, with the simultaneous advantages of thermal stability, insensitivity to light polarization and angle, robustness against harsh environmental conditions, and large area fabrication by scalable methods, are essential elements in (solar) thermophotovoltaics. Compared to the noble metal and multilayered broadband absorbers, high-temperature refractory metal-based nanostructures with low-Q resonators are reported less. In this work, 3D titanium nitride (TiN) nanopillars are investigated for ultrabroadband absorption in the visible and near-infrared spectral regions with average absorptivities of 0.94, over a wide range of oblique angles between 0° and 75°. The effect of geometrical parameters of the TiN nanopillars on broadband absorption is investigated. By combining the flexibility of nanopillar design and lossy TiN films, ultrabroadband absorption in the vi...

Ultra-thin titanium nitride films for refractory spectral selectivity [Invited]

“…Ti, Zr and Hf, provide a promising platform for the development of thin films with excellent thermal stability, due to their metallic optical response and ceramic-like physical and chemical properties (high strength, hardness, chemical inertness and corrosion resistance). Titanium nitride (TiN), a refractory, CMOS compatible ceramic with an optical appearance resembling gold [39,40], allows for the application of thin-film emitters incorporating low-loss metallic films at significantly higher temperatures, when compared to the optically similar gold [41]. Since the optical properties of TiN films deposited onto substrates depend strongly on the substrate and the conditions under which it is deposited, we characterize the dielectric function, and compare with literature values for gold [ fig.…”

Hybrid Solar Absorber–Emitter by Coherence‐Enhanced Absorption for Improved Solar Thermophotovoltaic Conversion

“…An ideal STPV absorber must simultaneously absorb input power and suppress radiation loss, while an STPV emitter should possess high emissivity only at the target infrared wavelength matching well with the base PV cell . Extensive efforts have been devoted to design and fabricate such absorbers and emitters, including intrinsically spectrally selective materials, multilayer structures, cermets, plasmonic metamaterials, and photonic crystals . Up to now, most of the reported absorbers and emitters are demonstrated by two isolated structures, such as the compound metallic structures patterned on both sides of a substrate in a recently developed STPV system .…”

Large‐Area, Ultrathin Metasurface Exhibiting Strong Unpolarized Ultrabroadband Absorption

“…However, the bandwidth of these devices is limited in the visible range. For ultrabroadband light absorption, 3D‐truncated TiN nanopillars have been demonstrated for the visible and NIR spectral regions with an average absorptivity of 0.94 . This TiN nanopillar‐based device, which comprises silicon nanopillars with a large thickness (>1 µm), typically requires expensive, time‐consuming fabrication processes, and its thermal emissivity is not verified.…”