Hiroyuki Toyota

Deep-level photoluminescence due to dislocations and oxygen precipitates in multicrystalline Si

We have demonstrated the presence of a dislocation-related component and a component due to oxygen precipitates in a broad deep-level photoluminescence (PL) band in multicrystalline Si at room temperature. In PL intensity mapping, the lower-energy side of the deep-level PL band at about 0.79 eV appeared as a dark line along a small-angle grain boundary (SA-GB) surrounded by a bright band on either side, while the higher-energy side at about 0.87 eV as a bright line along the SA-GB. These intensity variations agree with the low-temperature PL intensity patterns for well-established dislocation-related lines of D1/D2 and those for oxygen precipitates, respectively. These patterns were observed around SA-GBs with a misorientation angle of 1–2°, and were assumed to be due to the distribution of secondary defects or impurities trapped by the strain field around dislocation clusters forming SA-GBs and that of preferential oxygen precipitation on the dislocations. A spectral component associated with the D3/D4 l...

Donor−acceptor pair luminescence in compensated Si for solar cells

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

Gaseous electrical discharge characteristics in air and nitrogen at cryogenic temperature

In designing superconducting electrical power apparatus, the knowledge of cryogenic gas and liquid insulation characteristics is essential. The authors have studied the discharge characteristics of relatively long-gap configurations in air and nitrogen at a cryogenic temperature. A sphere-to-sphere electrode with a gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics basically obey Paschens law at cryogenic temperature for 50 Hz, AC, DC and lightning impulse voltage applications. A rod-to-plane electrode with a gap length of 20 to 330 mm is used for measurements in nonuniform electric field. In air at cryogenic temperature and nitrogen gas at both room and cryogenic temperature, streamer-like corona discharge appears near the tip of the rod electrode before the breakdown, and the breakdown voltage increases linearly with gap length. In air at room temperature thin film-like corona discharge, however, appears near the tip of the rod electrode before breakdown, and the breakdown voltage becomes higher than the other case. In order to examine the variation of corona discharge characteristics, some additional experiments are conducted. As a result, it becomes clear that thin film-like glow corona discharge appears when electronegative gas is contained and sufficient electrons are supplied from the cathode.

Polarized photoluminescence imaging analysis around small-angle grain boundaries in multicrystalline silicon wafers for solar cells

We have shown the effectiveness of polarized photoluminescence imaging for analyzing the structural and spectroscopic properties of small-angle grain boundaries (SA-GBs) in multicrystalline Si. The dislocation-related deep-level emission band at approximately 0.79 eV at room temperature was found to be polarized, whereas the band-edge emission did not show the polarization effect. The anisotropy of the 0.79 eV band was classified into two groups depending on the tilt and twist characteristics of SA-GBs determined by the electron backscatter diffraction measurement.

Durability evaluation of InGaP/GaAs/Ge triple-junction solar cells in HIHT environments for Mercury exploration mission

The Japan Aerospace Exploration Agency has been developing the Mercury Magnetospheric Orbiter (MMO), which is Japanese part of the BepiColombo mission. During its mission around Mercury, the spacecraft will be exposed to high solar irradiance of up to 11 suns, with an estimated maximum solar panel temperature of 230°C. In such an environment, solar cells are required to operate under high intensity and high temperature (HIHT) conditions. Therefore, it is necessary to evaluate the durability of solar cells to meet the power requirements throughout the mission life. We conducted a continuous operation test under HIHT conditions to examine the validity of the solar array configuration, using the interior planetary thermal vacuum chamber. Our HIHT tests clarified the following facts: (i) Transparency of the coverglass and the performance of the solar cells do not degrade and (ii) transparency of the DC93-500 adhesive in the top cell response region degrades mainly due to ultraviolet exposure at high temperatures. We decided to use AR0213 coverglass (from JDSU) with a thickness of 300 µm, which have a longer cut-on wavelength in ultraviolet region. With this configuration, the predicted decrease in Pmax due to the HIHT environment is 17.3% and that due to radiation effects is 11.0% Our new design will offer the available power at EOL of 394.2 W, which is 46.7 W greater than the required power.

Fine Structure Due to Donor–Acceptor Pair Luminescence in Compensated Si

A fine structure on the higher energy side of donor–acceptor (DA) pair luminescence at 4.2 K has been analyzed in compensated Si involving P donors and B acceptors. We calculated the density distribution of DA pairs against photon energy from the number of pairs as a function of the transition energy of respective pairs. A close agreement was obtained between the density curve and the observed spectral structure using the generally accepted values of energy gap and P donor and B acceptor ionization energies. This allows us to conclude that the structure is due to discrete DA pair recombination.

Donor-acceptor pair luminescence in B and P compensated Si co-doped with Ga

The electrical activity of Ga impurities in a high concentration range in B and P highly compensated Si co-doped with Ga for photovoltaic applications has been investigated through the analysis of donor-acceptor pair luminescence. We have identified the fine structure due to the pair luminescence between P-donors and Ga-acceptors based on a comparison with a theoretical spectrum using their generally accepted ionization energies in low concentration ranges in uncompensated Si. The fine structure showed no dependence on dopant concentrations in the P and Ga concentration ranges from 7 × 1016 to 4 × 1017 cm−3 and from 5 × 1016 to 3 × 1017 cm−3, respectively, which leads us to suggest that both P and Ga impurities act as isolated donors and acceptors without noticeable reduction of ionization energies due to high doping.

First flight demonstration of glass-type space solar sheet

The electrical performance of a glass-type space solar sheet (G-SSS) was demonstrated in space. G-SSS comprises InGaP/GaAs dual-junction and InGaP/GaAs/InGaAs triple-junction solar cells. It is lightweight solar generation sheet, less than 0.5mm thick. It is mounted on the “HISAKI” (SPRINT-A) small scientific satellite, which was launched on September 14, 2013. The initial flight data were successfully acquired and this flight demonstration was a world-first experiment for G-SSS using III-V multi-junction thin-film solar cells. The cells demonstrated superior performance and the electrical outputs matched the flight prediction.

High-Speed Deep-Level Luminescence Imaging in Multicrystalline Si Solar Cells

We demonstrated high-speed imaging of photoluminescence (PL) and electroluminescence (EL) for not only band-to-band but also multiple deep-level emissions in a multicrystalline Si solar cell. We used a cooled InGaAs camera with a photosensitive range of 900 - 1700 nm equipped with band-pass filters for the selective detection of various deep-level emissions. The exposure time for imaging was only 1 - 10 seconds. Comparisons of the present PL images with the microscopic PL mappings confirmed for us that essentially the same luminescence patterns were obtained.

Degradation analysis of InGaP/GaAs/Ge triple-junction solar cells in high-temperature and high-light-intensity environments by luminescence techniques

This paper demonstrates results of durability tests and photoluminescence spectroscopy of InGaP/GaAs/Ge triple-junction solar cells for Japans inner planetary missions. Degradation of the solar cells under high-light intensity and high temperature environments was evaluated by a forward current injection test and a continuous operation test. These tests revealed that the forward current application possibly causes more degradation than the actual solar cell operation. We diagnosed the solar cells in more detail by photoluminescence spectroscopy. The intensity of the band-edge emission decreased excessively after the forward current injection test, but did not decrease after the continuous operation test. This result agreed well with the change in the electrical property. No new deep level emissions were detected after the tests.