Hitoshi Sai

100 nm period silicon antireflection structures fabricated using a porous alumina membrane mask

An ordered anodic porous alumina membrane has been used as a lithographic mask of SF6 fast atom beam etching to generate a 100 nm period antireflection structure on a silicon substrate. The antireflection structure consists of a deep hexagonal grating with 100 nm period and aspect ratio of 12, which is a fine two-dimensional antireflection structure. In the wavelength region from 400 to 800 nm, the reflectivity of the silicon surface decreases from around 40% to less than 1.6%. The measured results are explained well with the theoretical results calculated on the basis of rigorous coupled-wave analysis.

High-temperature resistive surface grating for spectral control of thermal radiation

Spectral emittance and thermal stability of two-dimensional W gratings are investigated to obtain high-temperature resistive selective emitters. Numerical calculations based on rigorous coupled-wave analysis are performed to determine the structural profile of gratings with good spectral selectivity. According to the determined parameters, two-dimensional W gratings composed of rectangular microcavities with the period of 1.0 μm are fabricated on single crystalline and polycrystalline W substrates. The grating shows a strong emission peak which can be explained by the confined modes inside the cavities. The grating with 200 nm wall thickness made from a single crystalline W shows very high thermal stability over 1400 K, while the polycrystalline grating is deformed at a high temperature because of the grain growth.

Relationship between the cell thickness and the optimum period of textured back reflectors in thin-film microcrystalline silicon solar cells

Periodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells. When the textures have a moderate aspect ratio, the optimum period for obtaining a high short circuit current density (JSC ) is found to be equal to or slightly larger than the cell thickness. If the cell thickness exceeds the texture period, the cell surface tends to be flattened and texture-induced defects are generated, which constrain the improvement in JSC . Based on these findings, we have fabricated optimized μc-Si:H cells achieving a high efficiency exceeding 10% and a JSC of 30 mA/cm2.

Wide-Angle Antireflection Effect of Subwavelength Structures for Solar Cells

The angle-dependent reflectivity of several surface structures was analyzed and evaluated with the viewpoint of solar cell applications. Numerical analysis showed that a Si subwavelength structure (SWS) maintains a lower reflectivity at large incident angles than conventional light-trapping techniques such as a random pyramid texture, and that it can contribute to increasing the output power of solar cells under oblique irradiation. This wide-angle antireflection effect was demonstrated by fabricating test crystalline Si cells with several surface structures including a SWS and measuring their angle-dependent short-circuit current densities.

Spectral control of thermal emission by periodic microstructured surfaces in the near-infrared region

Thermal emissive properties of microstructured surfaces are measured in the near-infrared region. Two-dimensional periodic microstructured surfaces with metal coatings are fabricated with Si anisotropic etching and laser ablation techniques. The structural periods of the samples are 2.0 and 1.5 microm. Clear selective-emission bands are observed experimentally. This selective emission is attributed to the resonance effect between the emissive field and the surface microstructures. In addition, numerical calculation computed with rigorous coupled-wave analysis (RCWA) is performed on the microstructured samples. The selective-emission peaks measured through experiments can be reproduced well by RCWA, and this result suggests strongly that the thermal radiation from periodic structures may have spatial coherence. It is confirmed that the surface microstructure can be applied to the control of spectral emission from high-temperature materials.

Solar selective absorbers based on two-dimensional W surface gratings with submicron periods for high-temperature photothermal conversion

Spectral properties and thermal stability of two-dimensional (2D) W surface gratings with submicron periodicity are investigated to develop solar selective absorbers for high-temperature applications. Numerical calculations based on the rigorous coupled-wave analysis technique have been performed for simulating the spectral properties of W gratings. The results indicate that the gratings with microcavities have a good spectral selectivity suited for high-temperature applications, while the gratings with micropyramids realize a high solar absorptance over 0.92. 2D surface gratings with submicron holes have been fabricated on W substrates by the fast atom beam etching with highly ordered porous alumina masks. They have shown good spectral selectivity and sufficient thermal stability at 1170 K under a vacuum atmosphere. The observed absorption band is considered to originate from the standing wave resonance between the electromagnetic fields and the standing wave mode generated inside the holes.

Triple-junction thin-film silicon solar cell fabricated on periodically textured substrate with a stabilized efficiency of 13.6%

We report a high-efficiency triple-junction thin-film silicon solar cell fabricated with the so-called substrate configuration. It was verified whether the design criteria for developing single-junction microcrystalline silicon (μc-Si:H) solar cells are applicable to multijunction solar cells. Furthermore, a notably high short-circuit current density of 32.9 mA/cm2 was achieved in a single-junction μc-Si:H cell fabricated on a periodically textured substrate with a high-mobility front transparent contacting layer. These technologies were also combined into a-Si:H/μc-Si:H/μc-Si:H triple-junction cells, and a world record stabilized efficiency of 13.6% was achieved.

Enhanced photocurrent and conversion efficiency in thin-film microcrystalline silicon solar cells using periodically textured back reflectors with hexagonal dimple arrays

Periodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells for enhancing their photon absorption and photovoltaic performance. In a systematic survey of 1 -μm-thick μc-Si:H cells, the best performance is obtained with a period of 1.5 μm and an aspect ratio of 0.20–0.25 with a high current density exceeding 26 mA/cm2 and a marked efficiency of 10.1%. These results demonstrate the high potential of periodic textures or surfacegratings for improving the conversion efficiency of thin-film silicon solar cells.

Selective Emission of Al2O3/Er3Al5O12 Eutectic Composite for Thermophotovoltaic Generation of Electricity

The emissive properties of Al2O3/Er3Al5O12 eutectic composite were measured in the temperature range of 1000 to 1700 K. It was confirmed that the Al2O3/Er3Al5O12 eutectic composite has selective emission bands at a wavelength of 1.5 µm attributable to Er3+ ions. It is found that the intensity obeys the T4-law. The emittance of over 0.8 is observed in the selective region. Since these emission bands match up the sensitive region of the GaSb PV cell spectrally, the Al2O3/Er3Al5O12 eutectic composite is a suitable emitter material for using in thermophotovoltaic generation systems. The effects of temperature and thickness on the selective emission efficiency have been studied and discussed.

SPECTRALLY SELECTIVE THERMAL RADIATORS AND ABSORBERS WITH PERIODIC MICROSTRUCTURED SURFACE FOR HIGH-TEMPERATURE APPLICATIONS

Spectral absorptance and emittance of W surface gratings with short periodicity corresponding to the VIS to NIR wavelengths have been investigated to develop spectrally selective devices for high-temperature applications, such as selective solar absorbers and selective radiators for thermophotovoltaic systems. Numerical calculations based on rigorous coupled-wave analysis have been performed to evaluate the grating parameters. Two kinds of W surface gratings composed by microcavities have been fabricated by fast atom beam etching with two different lithography techniques. These gratings have shown strong absorption or emission due to the surface microstructures in the VIS to NIR region, whereas their reflectance in the IR region is kept at a high level. Their high thermal stability is confirmed from heating tests under vacuum or reduced atmospheres. Physical aspect of the interaction of electromagnetic wave with lossy gratings has also been discussed briefly.