Kazunori Shimazaki

Development status of “Space Solar Sheet”

The prototype solar sheet using the paper-like thin film InGaP/GaAs solar cells and thin bypass diodes we newly developed was fabricated. The output power of the prototype solar sheet is 11.3W (Voc=11.88V, Isc=1.12A, F.F.=0.854), and the weight is about 16.7g. So output power per weight is about 0.68W/g. The preliminary reliability tests (thermal shock test, humidity test, high temperature vacuum test) are carried out in the solar sheet. Good reliability of the solar sheet has been confirmed in these tests.

Analysis of Radiation Response and Recovery Characteristics of Amorphous Silicon Solar Cells

The radiation response and recovery characteristics of amorphous silicon (a-Si) thin-film solar cells were investigated. The solar cells studied were a-Si/a-Si dual-junction cell and a-Si/a-SiGe cell. The cells were irradiated with monoenergetic protons from 0.05 to 10 MeV to several fluences. For the cells, the displacement damage effects could model the proton-induced degradation much better than the ionizing radiation effects could. Thus, the changes in photovoltaic parameters could be plotted by a single characteristic curve against the displacement damage dose (Dd) even if the proton energy was low enough to create local damage in the active layers. The results indicated that the proton-induced degradations of the a-Si solar cell correlated with the displacement damage effects. In addition, we have reported the recovery characteristics by thermal annealing and light illumination. The electrical outputs of the cells significantly recovered at 70 and 130 degC but were not restored by light illumination

Electron and proton irradiation effects on substrate-type amorphous silicon solar cells

The electrical performances of single-junction hydrogenated amorphous silicon solar cells are investigated following high energy electron and proton irradiation under controlled temperature conditions of 298 and 331 K. Thermal recoveries after irradiation are also studied. As a result, superior radiation tolerance of a-Si:H solar cells compared to crystalline silicon space solar cells is demonstrated. Also, it is clearly shown that the radiation degradation of all the cell parameters; short-circuit current, open-circuit voltage and fill factor are strongly dependent on temperature during irradiation. The thermal recovery after irradiation is observed even at room temperature. These results indicate that the temperature and the elapsed time after irradiation strongly affect the measured performance degradation of a-Si:H solar cells. Therefore, these features must be taken into account and ground radiation tests for making end-of-life predictions should be carried out under the conditions expected on a mission.

Development status of ultra-lightweight solar panel using space solar sheet

The development of an ultra-lightweight solar panel using space solar sheets is currently being carried out. This panel is utilizing space solar sheets consisting of InGaP/GaAs dual-junction high-efficiency thin-film solar cells. The panel employs a simple frame-type structure. A breadboard model was manufactured. The dimensions of the 1.0-cm-thick unit panel were about 55 cm × 70 cm. Each panel weighs approximately 0.68 kg. In addition to three basic panels, two lateral panels were recently developed and added to the basic panel. This breadboard model achieved high specific power of about 91 W/kg. Moreover, five panels were stacked, resulting in a total thickness of 6 cm. Acoustic and sine vibration tests were performed on the model. The results demonstrated the high durability of the ultra-lightweight solar panel in an acoustic and vibration environment.

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.

Temporal Donor Generation in Undoped Hydrogenated Amorphous Silicon Induced by Swift Proton Bombardment

Seebeck coefficient variations of undoped hydrogenated amorphous silicon (a-Si:H) semiconductors due to swift proton irradiation were investigated using an in-situ thermoelectric power measurement system. Undoped a-Si:H irradiated with 3.0 MeV protons at a fluence regime of 3.1×1011–5.0×1012/cm2 showed a negative Seebeck coefficient although the Seebeck effect was not observed at fluences above 5.3×1013/cm2. These results suggest that donor like centers are generated by low fluence proton irradiation, whereas the donor centers are compensated by radiation-induced defects or themselves disappear after high fluence proton irradiation. These effects decay with time, giving the donor centers a temporal nature.

Progress in development of ultra-lightweight solar panel using space solar sheet

Development of an ultra-lightweight solar panel using space solar sheets is currently being carried out. The panel utilizes space solar sheets consisting of InGaP/GaAs dual-junction high-efficiency thin-film solar cells. The panel employs a simple frame-type structure. The initial breadboard model consisted of three main panels and two lateral panels. The dimensions of the 1.0-cm-thick unit panel were about 55 cm × 70 cm. In this study, the initial breadboard model was improved. The lateral panels were trimmed of weight while maintaining adequate stiffness. Additionally, unique deployment mechanisms for the lateral panels were adopted. The new system is much lighter than the conventional spring-driven mechanism, and as a result, the improved model finally achieved the high specific power of 100 W/kg.

Decomposition of dioctyl phthalate (DOP) using titanium dioxide photocatalyst in a vacuum

We have focused on photocatalytic materials to solve contamination problem for spacecraft. We have fabricated TiO2 thin films and measured decomposition rates of methyl orange (MO) and dioctyl phthalate (DOP) in vacuum by TiO2 thin films as a photocatalyst. From XRD results, fabricated TiO2 thin films have anatase-type crystal structure, which is known to have stronger decomposition activities than rutile-type TiO2. The TiO2 thin films we made were shown to decompose methylene blue (MB) solution, which means that the TiO2 thin films have general photocatalystic activity in atmosphere. In decomposition of MO in atmosphere and vacuum, TiO2 shows photocatalytic activity even in vacuum although the decomposition rate in vacuum is slower than that in atmosphere. In decomposition of DOP in vacuum, DOP was effused from an effusion cell in vacuum chamber and was deposited on a TiO2 thin film using the in-situ measurement apparatus at Tsukuba Space Center, JAXA. Transmission spectra of DOP on TiO2 thin films after UV irradiation were measured to estimate decomposition rate from absorbance of DOP. The results show that TiO2 thin films can decompose DOP even in vacuum. Moreover, H2O can promote the decomposition of DOP. In order to use photocatalyst materials in vacuum for long time, the studies on the durability of photocatalystic activity of TiO2 in vacuum and the effects of O2 and H2O are necessary in the future.

Development of Space Solar Sheet with inverted triple-junction cells

The Space Solar Sheet of two configuration types using InGaP/GaAs/InGaAs triple junction solar cells has been developed. Two types of shielding material are utilized for the Space Solar Sheet. One type uses transparent resin films (Film type), and the other one uses cover-glass and back-sheet (Glass type). This solar sheet has possibility of the realization of the lightweight and high efficiency solar panel. Detailed reliability tests of the two types of solar sheet were performed to confirm their tolerance in a space environment. The test results indicate that they have high reliability in a space environment. Depending on the space environment and the application, two types of solar sheets can be properly used.

Photo- and dark conductivity variations of solar cell quality a-Si:H thin films irradiated with protons

We investigated conductivity variations of hydrogenated amorphous silicon thin films during 10 MeV proton irradiations at fluences of 2.0×1013 or 4.0×1014 /cm2 and for certain duration after stopping the irradiations. During the irradiation the conductivity initially increases and after the certain period of irradiation, it decreases. We also investigated behaviors of light-induced degradations before and after the irradiations. The light-induced degradation results indicate that the radiation-induced changes in the electrical properties do not completely return to their initial values after thermal annealing. Furthermore, our study indicates that the conventional mechanism governing electrical conduction of semiconductor materials under proton irradiation could be applied to high fluence regime. On the other hand, this model could not be extended to lower fluence regime, and thus another conduction mechanism does exist.