Shirou Kawakita

Number of Arcs Estimated on Solar Array of a Geostationary Satellite

Plasma parameters of geosynchronous orbit measured by Los Alamos National Laboratory satellites are analyzed statistically. For each set of plasma parameters, charging analysis is carried out, taking a geostationary satellite representing a typical telecommunication satellite as an example. The number of expected trigger arcs on a solar array is calculated based on the charging duration with severe inverted potential gradient conditions expected in orbit. Using the number of trigger arcs, an appropriate duration of electrostatic discharge ground test is proposed to test the insulation strength against sustained arc phenomena.

Annealing Enhancement Effect by Light Illumination on Proton Irradiated Cu(In,Ga)Se2 Thin-Film Solar Cells

In this paper, we investigated the high radiation tolerance of copper indium gallium di-selenide (CIGS) thin-film solar cells by conducting in situ measurements of short circuit current and open circuit voltage of CIGS thin-film solar cells during and after proton irradiation under short circuit condition. We found that the annealing rate of proton-induced defects in CIGS thin-film solar cells under light illumination with an AM0 solar simulator is higher than that under dark conditions. The activation energy of proton-induced defects in the CIGS thin-film solar cells with (without) light illumination is 0.80 eV (0.92 eV), which implies on enhanced defect annealing rate in CIGS thin-film solar cells due to minority-carrier injection.

Si Substrate Suitable for Radiation-Resistant Space Solar Cells

Irradiating group-III (B, Al, Ga)-doped Czochralski (CZ)-grown Si substrates as well as B-doped magnetic Czochralski (MCZ)-grown and floating-zone (FZ)-grown Si substrates with 10 MeV protons or 1 MeV electrons, we investigate both the reduction in the hole concentration and the conversion of p type to n type using Hall-effect measurements. In all the 10 Ω cm CZ-Si, the density of each acceptor species is reduced by irradiation, and finally the conversion occurs with 1017 cm-2 fluence of 1 MeV electrons or with 2.5×1014 cm-2 fluence of 10 MeV protons. In 10 Ω cm MCZ-Si and 10 Ω cm FZ-Si, on the other hand, the conversion does not occur under the same conditions. Moreover, the reduction in the hole concentration for the FZ-Si is much less than the others. From these results, it is elucidated that the conversion as well as the reduction in the hole concentration is strongly dependent on the concentration of oxygen in Si, not on the type of acceptor species in Si. Therefore, the p-type FZ-Si substrate is appropriate for radiation-resistant space solar cells such as n+/p/p+ Si solar cells and upcoming III–V tandem solar cells on n+/p Si substrates.

Investigation of Power System Failure of a LEO Satellite

The Solar Array Paddle generation power of ADEOS-II satellite dropped suddenly. We have investigated the cause of the anomaly. The telemetry indicated the generation power dropped by 2 A-step. This corresponded in the generation power of each array circuit. No anomalous telemetry was found except for attitude when the anomaly occurred. FTA was carried out and the possible locations for the anomaly were identified; Solar Array Paddle or Solar Array Paddle Harness. The laboratory tests showed the possibility that sustained arcs occurred between harnesses and the heat produced by arcs destroyed the bundled harnesses.

ESD Ground Test of Solar Array Coupons for a Greenhouse Gases Observing Satellite in PEO

An electrostatic discharge test was carried out on a solar array paddle (SAP) for the Greenhouse gases Observing SATellite (GOSAT) on a simulated polar Earth orbit (PEO) environment. To simulate the spacecraft charging conditions in a PEO with an auroral band, when electrons with higher energy than LEO plasma flow to the Earth, three conditions of dielectric charging can be considered on each side of the SAP. At first, the threshold voltage differences of discharge inception were measured experimentally using solar array coupons simulating both the beginning and the end of life under six charging conditions. Next, the Multiutility Spacecraft Charging Analysis Tool was employed to analyze the spacecraft surface potential and the time needed for charging. From the threshold value and the charging analysis, the probability and the number of discharges were estimated in each charging condition during a lifetime of the GOSAT. Finally, the performance of the coupons against discharges was evaluated for each charging condition. All coupons had no sustained arc during these tests; however, there was some electrical degradation of the solar cell. The power degradation during the lifetime of the GOSAT was estimated from these results, and the design of the solar array coupon was confirmed to satisfy the power demand even if the estimated number of discharges occurs in the lifetime.

Sustained Arc Between Primary Power Cables of a Satellite

We investigated the power loss due to the sustained arc between primary satellite power cables. If the multi layer insulator (MLI) film on a satellite is electrically floating, energetic electrons in space will charge this film. We carried out an ESD test on cables with cracks and wrapped with this film. When the negative voltage on the MLI exceeded 600 V, a trigger arc discharge occurred between the MLI and the cables. Subsequently, a secondary arc electric discharge occurred between the cables themselves. After several discharges, this secondary arc caused sustained arc which burned out the cables. The heat caused by arc tracking between the hot and return cables made them burn out. If this phenomenon had occurred in space, the satellite would have suffered great damage.

Photoluminescence Analysis of Proton Irradiation Effects in Cu(In,Ga)Se2 Solar Cells

We investigated proton irradiation effects on Cu(In,Ga)Se2 (CIGS) solar cells with a structure of ZnO/CdS/CIGS using photoluminescence (PL) spectroscopy. After proton irradiation, near band-edge emission from the CIGS layer was considerably reduced, while deep-level emission emerged at 0.8 eV. Subsequent annealing restored the intensity of the near band-edge emission slightly, and reduced the 0.8 eV band substantially. An anomalous spectral variation caused by the annealing suggests that the 0.8 eV band does not originate from a single irradiation-induced defect. After the irradiation, the 1.35 eV emission from the CdS layer was moderately reduced, and the band-edge emission from the ZnO layer had disappeared. These bands recovered to the initial state after the annealing. Qualitative correlation was observed between the PL properties and the electrical properties of the cell.

In-situ measurement of degradation of Cu(In,Ga)Se/sub 2/ thin film solar cells during electron and proton irradiations

Observation of illumination-enhanced annealing of radiation damage to CIGS thin-film solar cells has been reported. In-situ measurements of short circuit current and open circuit voltage of CIGS cells during and after electron and proton irradiations under the short-circuit condition have been carried out in this study. The annealing rates of proton-induced defects in the solar cells under the AM0 solar simulator illumination and short-circuit condition is found to be enhanced compared to that under dark condition. The activation energy of proton-induced defects in CIGS solar cells with and without the AM0 illumination is 0.8 eV and 0.92 eV, respectively, which implies enhancement of defect annealing rates in CIGS thin-film solar cells for minority-carrier injection. Very high annealing rates of electron-induced defects are also obtained. These results suggest that CIGS cells have desirable potential for space applications.

Analysis of Anomalous Degradation of Cu(In,Ga)Se2 Thin-Film Solar Cells Irradiated with Protons

We analyzed the radiation response of Cu(In,Ga)Se2 (CIGS) solar cells to high-fluence protons. An in situ measurement system was constructed to measure the electrical performance of the CIGS solar cells immediately after proton irradiation. Using this system, abrupt degradation of Isc in the cells irradiated with 0.38 MeV protons at room temperature was observed, which was caused by a decrease in the effective acceptor density of the CIGS absorbing layer according to its capacitance–voltage characteristics. Admittance spectra of proton irradiated CIGS solar cells indicate that the proton-induced defect might be an InCu antisite defect. This defect is a donor-like defect. Therefore, it compensates the acceptor density. The reduction in the density caused by majority carrier trapping by the defect caused an increase in the resistivity of the CIGS absorbing layer. Consequently, a type-conversion of the absorbing layer in the cells occurred because of high-fluence protons.

Proton-beam-induced defect levels in CuInSe2 thin-film absorbers: An investigation on nonradiative electron transitions

Electron nonradiative relaxation through the proton-irradiation-induced defects in CuInSe2 solar cell material were investigated by using a piezoelectric photothermal spectroscopy (PPTS). Among the observed three peaks at 1.01, 0.93, and 0.84eV, it was concluded that the peak at 0.84eV was due to the proton-irradiation-induced defect. This is because this peak appeared after irradiation with the proton energy of 0.38MeV and the fluence of 1×1014cm−2. The peaks at 1.01 and 0.93eV were attributed to free band-edge exciton and intrinsic defect level, respectively. The intensities for the latter two peaks were not affected by the irradiation. Since the irradiation defect was clearly observed at room temperature, we concluded that the PPTS technique was a very sensitive tool to study the defect level in the irradiated semiconductor thin-film solar cell structures.