Development of spectrally selective infrared emitter forthermophotovoltaic power generation
波長選択機能を有する熱光起電力発電用赤外線源の開発
Motofumi Suzuki, Department of Micro Engineering, Kyoto University

1. Introduction
Thermophotovoltaic (TPV) power generation has recently attracted considerable attention as ameans to use waste heat [1-4]. In order to improve the efficiency of TPV power generation, it isnecessary to develop thermal emitters that selectively emit infrared (IR) photons havingenergies that match those of the bandgaps of photovoltaic (PV) devices; these devices thenconvert the IR photons to electricity.
Makino and Wakabayashi [5] have successfully developed simple selective emitters based on acombination of NiO thin films and a Ni substrate; the emittance of the developed emitters in theIR region reaches up to 0.7. Although this value is much larger than the emittance of uncoatedNi, there is scope for obtaining an increase of approximately 0.3. In this study, we havedeveloped efficient emitters having an emittance of nearly 1 at the designed wavelength byusing a β-FeSi2 thin film/stainless steel substrate system.
2. Antireflection (AR) coatings for metals
According to Kirchhoff’s law on thermal radiation, the spectral emittance of a surface is equal
to its spectral absorbance at thermal equilibrium. For a thin film system fabricated on an opaque
substrate, emittance ε is described as

ε = 1− R ,(1)where R is the reflectance. An ideal emitter emits only those photons whose energies equal thatof the bandgap of the PV device, which is usually in the IR region. Thus, a highly efficient
selective emitter has a reflectance spectrum with the minimum at the wavelength corresponding
to the bandgap of the PV device, and it exhibits higher reflectance at other wavelengths. Suchreflectance spectrum can be achieved by depositing interference AR coatings on highlyreflective metal substrates. However, the application of AR coatings to metals has not been
investigated intensively. In this section, we discuss the AR conditions for a simple system
consisting of dielectric thin film/metal substrate.

－ 147 －Fig. 1 Calculated RNN spectra of thin films fabricated on SUS304 substrates for n = 2–6.
Fig. 1 indicates the reflectance spectra at normal incidence RNN for dielectric thin films
deposited on a metal substrate. In the spectral calculations, the refractive index n of the thin film
was varied between 2 and 6, and the optical constants of the substrate were assumed to beidentical to those of the SUS304 stainless steel substrate [6]. The thickness of the dielectricfilms was tuned such that the reflectance attains a minimum at a wavelength of λ = 2 μm.
Clearly, the reflectance for n = 5 at λ = 2 μm is the lowest of the four calculated spectra. Inaddition, the reflectance at shorter or longer wavelength region is kept at high values. Although
the refractive indices of most of the conventional optical materials are lower than 2.5, a few
semiconductors have higher refractive indices in the IR region. The refractive index of β-FeSi2,
which is approximately 5 in the IR region, is particularly high [7]. High refractive index
materials such as β-FeSi2 are considered to be appropriate for the AR coating of metals.
However, thus far, there have been no reports on AR β-FeSi2 coatings. Thus, we attempt todevelop interference AR coatings made of β-FeSi2 in order to realize spectrally selective
infrared emitters with high efficiency.

3. ExperimentThin films of FeSi2 were deposited by DC magnetron sputtering onto a polished SUS304
substrate. The base pressure in the deposition chamber was about 5 × 10-4 Pa. The films weredeposited in an Ar pressure of 6 × 10-1 Pa at a discharge voltage of 500 V and a discharge
current of 20–40 mA. The substrate temperature during deposition was kept constant between
room temperature (RT) and 700 K. The crystal structure of the films was analyzed by X-ray－ 148 －ID

Table 1 List of samples.
Thickness (nm) Substrate temperature

A60

RTB

99

RTC

155

RTD229RTE

48700 KF

101700 KG146700 K

H

225700 K

Fig. 2 XRD patterns of samples H (fabricated at 700K), D (fabricated at RT), and SUS304
substrate.
diffraction. The fabrication conditions of samples are listed in Table 1. The reflectance spectraof the specular reflection at an angle of incidence of 15°, R15°(-15°), were measured at RT and 700
K in air to evaluate the performance of the emitters.

4. Results and discussion
Fig. 2 shows the XRD patterns of samples D, H, and the SUS304 substrate. In the XRD patternof sample H, we observe two peaks at 2θ = 29.12° and 49.40°, which correspond to the(202)/(220) and (422) diffractions of β-FeSi2 [8], respectively, whereas no peaks other thanthose observed in the XRD pattern of the SUS304 substrate are observed in case of sample D.

－ 149 －Fig. 3 Reflectance R15°(-15°) spectra of samples E–G (fabricated at 700 K) and samples A–C(fabricated at RT) measured at RT in air.
We confirmed that the diffraction peaks due to β-FeSi2 are observed for the samples whose
substrate temperatures exceeded 700 K.
The reflectance spectra of samples A–G are shown in Fig. 3. Although all the samples show
minimum reflectance in the IR region, the reflectance of the samples fabricated at RT (samplesA–C) are always higher than 0.3. This is due to the fact that the crystallographic structures ofsamples A–C are not β-FeSi2 but amorphous, and therefore, the refractive index of the films islikely to be much different from that of β-FeSi2. On the other hand, the reflectance spectra ofsamples fabricated at 700 K (samples E–G) contain thin films of β-FeSi2 and show a deep
minimum (0.142 for E, 0.077 for F, and 0.026 for G). Thus, the AR coating for metals is
successfully realized by using β-FeSi2 thin films.
In order to utilize for IR emitters for TPV generation, the AR characteristics in the IR region arerequired to be maintained at high temperatures in air. Fig. 4 indicates the reflectance spectra ofsample G measured at RT and 700 K. No significant difference is found between these spectra.
In addition, we confirmed that the reflectance spectra at 700 K are quite stable for more than 15min. Therefore, metals with AR coatings of β-FeSi2 can be effectively used in IR emitters
employed for TPV power generation.

－ 150 －Fig. 4 Reflectance R15°(-15°) spectra of sample G (fabricated at 700 K) measured at RT and 700
K in air.
5. Conclusion
We investigated spectrally selective IR emitters that can be utilized for TPV power generationby using a high refractive index material such as β-FeSi2. The interference AR coating forSUS304 was successfully realized by using a β-FeSi2 thin film. In addition, samples with
β-FeSi2 AR coatings were stable at 700 K. Therefore, we concluded that the β-FeSi2/SUS304
system is effective for IR emitters that follow Kirchhoff’s law on thermal radiation.

Acknowledgments
We acknowledge Yasuyuki Kaneko and Professor Yoshihito Maeda of Kyoto University for
useful discussions. This work was supported by JFE 21st Century Foundation.

References[1]

H. Sai, H. Yugami, K. Nakamura, N. Nakagawa, H. Ohtsubo, and S. Maruyama,"Selective emission of Al2O3/Er3Al5O12 eutectic composite for thermophotovoltaic
generation of electricity," Jpn. J. Appl. Phys. 39 (4A), 1957-1961 (2000).

[2]

R. G. Mahorter, B. Wernsman, R. M. Thomas, and R. R. Siergiej, "Thermophotovoltaic
system testing," Semicond. Sci. Technol. 18 (5), S232-S238 (2003).

－ 151 －[3]A. Licciulli, D. Diso, G. Torsello, S. Tundo, A. Maffezzoli, M. Lomascolo, and M.
Mazzer, "The challenge of high-performance selective emitters for thermophotovoltaic
applications," Semicond. Sci. Technol. 18 (5), S174-S183 (2003).

[4]

S. Basu, Y. B. Chen, and Z. M. Zhang, "Microscale radiation in thermophotovoltaic
devices - A review," Int. J. Energy Res. 31 (6-7), 689-716 (2007).

[5]

T. Makino and H. Wakabayashi, "Development of a Functional Surface for Emitting
Spectrally-Selective Thermal Radiation[in Japanese]," Trans. Jpn. Soc. Mech. Eng. B

72 (723), 2741-2746 (2006).
[6]

T. Makino, "Present research on thermal radiation properties and characteristics ofmaterials," Int. J. Thermophysics 11 (2), 339-352 (1990).

[7]

H. Udono, I. Kikuma, H. Tajima, and K. Takarabe, "Polarized optical reflection study
on single crystalline β-FeSi2," J. Mater. Sci.: Mater. Electron. 18 (0), 65-69 (2007).

[8]

Y. Dusausoy, J. Protas, R. Wandji, and B. Roques, "Structure Cristalline du Disiliciurede fer, FeSi2," Acta Cryst. B 27, 1209-1218 (1971).

－ 152 －