In this paper, a one-dimensional multilayer is optimized for potential applications as thermophotovoltaic (TPV) selective emitter. The proposed TPV emitter was fabricated through a magnetron sputtering process by using the radio frequency (RF) magnetron sputtering system. The spectral emittance of the proposed TPV emitter is measured by using spectral transmittance and reflectance measurement system at wavelength from 0.3 μm to 2.5 μm at near-normal incident 8˚. The bidirectional reflectance distribution function BRDF is measured by three axis automated scatterometer (TAAS). The effect of the diffraction orders and plane of incidence on the spectral emittance of the proposed TPV emitter is calculated numerically by using the rigorous coupled-wave analysis (RCWA). The emittance spectrum of the proposed TPV selective emitter shows three close to unity emission peaks which are explained by the surface plasmon polariton (SPP), gap plasmon polariton (GPP) and magnetic polariton (MP) excitation. The results show that the proposed emitter has high emittance value in the spectral range of 0.69 < λ <1.97 μm and low reflectance distribution BRDF·cosθ value. It also has multi-reflection points and it works as a mirror. The measured results are in good agreement with the simulation results. The results show that the proposed TPV emitter, if used as a selective emitter with a low band gap photovoltaic cell (GaSb), would lead to high TPV overall efficiency and high electrical output power.

Thermophotovoltaic; Selective Emitter; Rigorous Coupled Wave Analysis (RCWA); A Magnetron Sputtering Process; Spectral Emittance

The depleting fossil fuel reserves encourage us to look for solutions such as new sources of energy, high energy conversion efficiency and recycling of waste heat by using of alternative energy sources. Thermophotovoltaic (TPV) systems provide a viable solution for fulfilling these requirements [1] [2] . Thermophotovoltaic (TPV) systems are capable of converting thermal infrared radiation directly into electricity by using photovoltaic effect [1] -[5] . The concept of TPV dates back to 1960s. It was only in recent years that technological improvements in the field of low band gap (0.50 - 0.75 eV) photovoltaic cells such as GaSb, GaInAs and GaInAsSb cells and high temperature selective emitting materials, have evoked a renewed interest in TPV generation of electricity [1] [5] [6] . The TPV system consists of heat source and optical cavity which comprises of a thermal radiator (emitter), photovoltaic PV cell and the spectral control components. The spectral control components are often classified in two types, cold-side and hot-side components. The cold-side components operate near room temperature and are installed on the cold side of the gap in the TPV system near or on the TPV cell, such as filters or reflectors or anti-reflective coatings on the TPV cell. The hot-side components operate at elevated temperatures (at or close to emitter temperature). They are installed upon or integrated into the emitter and are often called selective emitters [7] . The TPV system promises to be a very clean efficient and quiet source of electrical power. It is portable, with no moving parts (so low maintenance), with high power density output and it uses a wide variety of energy sources. As a result, TPV system can be more commonly used in many applications such as remote electrical and thermal energy supplies and aerospace power applications [1] [2] [8] -[12] . It has a very good potential to be an alternative to traditional batteries. TPV can also be used in the solar energy field, by using solar radiation to heat the emitter [13] [14] . However, the most obvious drawbacks of TPV systems are their low throughput and poor conversion efficiency, due to a large amount of unusable radiation [1] [15] . A highly efficient TPV system demands the optimization of the output power and throughput. Recently near-field thermal radiation has been proposed to enhance the throughput and conversion efficiency by bringing the emitter and TPV cell in close proximity, [3] . The conversion efficiency can be improved by controlling the emission spectrum and directions. One possible solution to increase the conversion efficiency and output power is to apply microscale radiation principles in TPV systems. Micro/nanostructures emitter (a selective emitter) can enhance TPV conversion efficiency by reducing the losses due to spectral mismatch between the source and the cell (low bandgap photons and carrier thermalisation). The output power can be increased by using micro/nanostructures in the emitter and filter. This reduces the amount of unusable radiation [16] . The throughput can also be increased by using micro/nanostructures, because it reduces the distance between the emitter and the TPV cell to sub-wavelength dimensions [17] . There are basically two different types of emitters, namely broadband emitters such as blackbody (BB) and selective emitters which are characterized by strong emission at certain wavelengths. A TPV system uses an emitter, which is heated up by various energy sources to high temperatures, as a source of radiation for photovoltaic energy conversion [2] [18] . That means that a large amount of unusable electromagnetic radiation impinges on the photovoltaic cell (PV).

Recently, the periodic micro/nanostructures surfaces in one-two or three (1D-2D-3D) dimensional have been investigated as promising potential to selective TPV emitters. Wang and Zhang [2] used 1D tungsten trilayer grating/thin-film nanostructure as a selective TPV emitter. Chen and Zhang [17] proposed and demonstrated the potential of 1D complex grating as a selective TPV emitter whose emittance is enhanced by exciting surface polsmon polaritons (SPPs). Narayanaswamy and Chen [20] also studied 1D photonic crystal (PhC) as a selective TPV emitter. Heinzel et al. [21] fabricated 2D wavelength-selective emitters for the near-infrared spectral range. Pralle et al. [22] manufactured 2D gold gratings that can emit over selected wavelengths in the mid-infrared. Sai et al. [23] experimentally demonstrated 2D tungsten grating with a strong emission peak and high thermal stability over 1400 K. A 3D tungsten photonic crystal [24] and 3D metallic woodpile as a TPV emitter was recently fabricated with an efficiency which exceeds 32% [25] .

An ideal TPV emitter should have high emissivity, close to unity, corresponding to the spectral region where a TPV cell has high quantum efficiency and as low as possible emissivity outside that region. It also needs to be polarization-insensitive so that high emissivity for both transverse electric wave and transverse magnetic wave can be achieved.

In this paper, a 1D 5-layer microstructure made of tungsten and silicon dioxide SiO₂ in the form of (W/SiO₂/ W/SiO₂/W) is proposed and fabricated to use as a wavelength-selective and polarization-insensitive TPV emitter. The radiative properties were calculated and measured by the rigorous coupled-wave analysis RCWA and the spectral transmittance and reflectance measurement system. The bidirectional distribution function BRDF was also measured by using three axis automated scatterometer system TAAS at room temperature. The effects of the diffraction order and plane of incidence on the spectral emittance were investigated numerically.

Rigorous coupled-wave analysis RCWA method was used for analyzing the diffraction of electromagnetic waves by periodic gratings. It analyzes the diffraction problem by solving Maxwell’s equations accurately in each of the three regions (input, multilayer and output), based on Fourier expansion [26] . The diffraction efficiency for each diffraction order is calculated with incident wave properties, feature dimensions, and dielectric function of the materials. The dielectric function of the materials is expressed as, where n is the refractive index and k is the extinction coefficient. The accuracy of the solution computed depends solely upon the number of terms retained in space harmonic expansion of electromagnetic fields, which corresponds to the diffraction order. Figure 1 is a schematic of the proposed TPV emitter considered in this study. The proposed TPV emitter consisted of alternating 5-layer made of tungsten W and silicon dioxide SiO₂ in form of (W/SiO₂/ W/SiO₂/W) and the entire structure was deposited on quartz substrate. The thickness of the layers were:, , ,. Those layers were deposited on a quartz substrate. Tungsten was chosen as the emitter material because it has high melting point and it withstands high temperatures. The wavelength-dependent dielectric optical constants of tungsten and silica were obtained from tabulated data in Ref [27] . Angle is the corresponding emission angle. The emittance was calculated indirectly from the reflectance, based on Kirchhoff’s law. The space along the z-axis was divided into seven regions, as illustrated in Figure 1. The incident radiation is assumed to be linearly polarized and can be characterized by the incident wavevector as

where being the wavelength in vacuum is the free-space wavevector. The wavevector in the region M and region VII, respectively can be expressed as

The normalized electric field of incidence can be expressed as:

The normalized magnetic field in region I can be expressed as

R is the reflected amplitude of the electric field.

The electric and magnetic field components in region M can be expressed as a Fourier series:

where and are the normalized amplitude of the jth space-harmonic fields that satisfy Maxwell’s equation in each divided layer. The magnetic field components in region VII can be expressed as

T is the transmitted amplitude of the electric fields.

The proposed TPV emitter 1D 5-layer W/SiO₂ is prepared through a magnetron sputtering process. The radio frequency (RF) magnetron sputtering system (JGP450, Shenyang Co. Ltd.) was used to deposit a thin film from sputtering targets onto a substrate and to fabricate the emitter. The tungsten (W) and silicon dioxide (SiO₂) were used as sputtering targets and quartz as the substrate. The sputtering power and pressure were kept at 100 W and 150 W for tungsten and silicon dioxide, respectively, and operation pressure. The base vacuum level was. The Ar flow rate was kept at 30 SCCM and controlled by a mass flow meter. A single W layer was deposited on quartz substrate for 1200 s, and a single SiO₂ layer was deposited on it for 3600 s. Figure 2 illustrates the W and SiO₂ sputtering targets after deposition onto a substrate.

The thickness of the sputtering targets which were deposited onto a substrate per unit time can be calculated by using the following equations:

W: (9)

SiO₂: (10)

where y is the thickness of the layer per (nm) and x is the time of deposition per minute, time for SiO₂ per hour.

The layers thicknesses are 60, 60, 20,400 and 1000 nm, yielding the deposited rates to be 0.2205 nm/s, 0.0588 nm/s, for both of the W and SiO₂, respectively. Scanning electron microscope SEM was used for characterization of the surface topography and morphology of the selective TPV emitter. The sample, before SEM, was coated by a thin layer of conductive material (gold), as shown in Figure 3. Figure 4 shows SEM of the selective TPV emitter sample. The spectral transmittance and reflectance measurement system was employed to measure the spectral emittance indirectly from the spectral reflectance at wavelength from 0.3 μm to 2.5 μm. The bidirectional reflectance distribution function BRDF is defined as the ratio of the reflected power to the incident power [28] . Three-axis automated scatterometer TAAS was used to measure BRDF of the TPV emitter sample

at normal incident and for both of the TE and TM polarization. It was also used to measure the BRFD at different incidence angles for TM polarization. The measured results were compared with the simulation results those obtained by RCWA method.

In order to enhance TPV overall efficiency and electrical output power, we proposed 1D 5-layer nanostructure TPV selective emitter. The spectral emittance and bidirectional reflectance distribution function BRDF of the proposed TPV emitter are experimentally measured. Three close-to-unity peaks on the emittance exist in the spectral emittance of the proposed TPV emitter in our spectral region of interest. They are explained by the

mechanisms of the SPPs, GPPs and MPs excitation. The measured results show that the proposed emitter has high reflectance distribution value at zenith angle, while it has low value at. The maximum measured emittance value of the emitter is about 94% at wavelength 1.76 μm. The measured results are in good agreement with the simulation results. The performance of the proposed TPV emitter recommends it to be used in TPV applications, with GaSb cell (low band gap cell), as an efficient power generator.

This work is supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 51276049) and the Fundamental Research Funds for the Central Universities (No. HIT. BRETIII.201227). A very special acknowledgement is made to the editors and referees whose constructive criticism has improved this paper.