Hidetoshi Washio

Future development of InGaP/(In)GaAs based multijunction solar cells

Although technologies for the InGaP/InGaAs/Ge cell have been matured, there is still room for improvement of the InGaP/(In)GaAs/Ge cell in practical level. Band gap of the top cell should be increased a little to get higher V/sub OC/. Thinning the Ge substrate is thought to be effective to increase a power per weight even for rigid panel. For concentrator application, grid pitch, cell size and current matching design should be optimized with taking account of the spectrum of concentrated light. The InGaP/(In)GaAs based solar cells shall be cornerstone in high efficiency multijunction solar cells in future. High efficiency cell consisted of 1 eV lattice-match material such as InGaAsN is strongly desired for high efficiency 4-junction or 6-junction cell. Wafer bonding and layer transfer techniques might be sophisticated to make solar cells. Paper-like InGaP/GaAs solar cells with efficiency of 29.4% on flexible metal film developed by SHARP Corp. are newly reported. Material cost of the cell is basically very low, because it has only very thin layers of III-V compounds and cheap metal film. Thin film technology shall be a hint for future cells.

Application of InGaP/GaAs/InGaAs triple junction solar cells to space use and concentrator photovoltaic

InGaP(1.88eV)/GaAs(1.43eV)/InGaAs(0.98eV) triple junction solar cells with high efficiencies of 37.9% (AM1.5G) and 44.4% (AM1.5D, 302-suns) have been attained by inverted lattice-mismatch fabrication procedure. Cell structure was optimized to improve radiation resistance for space use. Lightweight space solar sheet has been developed by using the film type triple junction cells. Good performance of the solar sheet has been confirmed in reliability test and demonstrated in space flight test. Concentrator module using the triple junction cells have shown over 10% higher efficiency than the present type InGaP/(In)GaAs/Ge triple junction cells. High performance of the concentrator module has been demonstrated in outdoor field testing.

Paper-Thin InGaP/ GaAs Solar Cells

A paper-thin, lightweight InGaP/GaAs solar cell with high efficiency and flexibility has been developed. A high-efficiency thin-film cell can be obtained for cell fabrication both before and after removing the substrate. Introducing a tunnel junction as the contact layer between the cell and metal film improves cell characteristics (Fill Factor (FF) and open-circuit voltage (Voc)). A highly doped n-type layer is necessary for good ohmic contact at the metal film interface. High radiation resistance of a thin-film cell was confirmed for a GaAs cell with a one-micron base layer. The thin-film cell was laminated for better handling. The laminated cell efficiency was about 22%. Anti-reflective coating is necessary on the laminate film to improve cell efficiency. A prototype unit panel using the laminated cells was developed for space application. An output power per weight of over 1W/g is possible for the unit panel. However, development of a bypass diode with thin-film structure is currently a problem, and reliability tests need to be performed for the unit panel

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.

Development of high efficiency thin silicon space solar cells

The improvement of the efficiencies of thin silicon solar cells for space use was studied. The representative 3 types of nonreflective surface structures were evaluated and their characteristics were clarified. Solar cells with these structures showed excellent electrical performance and the best result of 18.0% efficiency for 100 /spl mu/m thick cells was attained. Cells made from 2 /spl Omega/ cm substrates showed higher electrical performance and radiation hardness than those from 10 /spl Omega/ cm substrates. The electron irradiation tests showed that the current degradation of the cells with lower resistivity substrates were reduced and the improved solar cells can be used for future satellites.<<ETX>>

Investigation of Non-Ohmic Properties for Thin Film InGaP/GaAs Solar Cells

An efficient, flexible and lightweight thin-film InGaP/GaAs solar cell has been developed. We discovered a reduction in open-circuit voltage (Voc) of the thin-film InGaP/GaAs cell due to the non-ohmic characteristics in the interface between the metal film and p-type contact layer on the rear of the cell. Additionally, the non-ohmic characteristics influenced the temperature coefficient of Voc. Cells with 2.34 V and 2.08 V Voc were evaluated in this investigation. The difference in Voc was caused by a difference in carrier concentration of the layer which contacts thin metal film on the rear of the cell. These results indicate that the reduction in Voc of the thin-film cell is due to an opposite voltage generated in the interface between the contact layer and the metal film. The characteristics were improved by introducing a tunnel junction into the interface. Good ohmic characteristics in the interface between the rear of the thin-film cell and the metal film is required for improving efficiency

Electrical properties of thin silicon space solar cells

Abstract Thin silicon solar cells with a Non-Reflective front Surface (NRS), a passivated rear surface and locally diffused BSF structure have been considered to be hopeful candidates as space solar cells of the next generation. In this paper, the electrical properties of textured surface cells and flat (non-textured) surface cells are compared and the surface recombination velocity S e of these cells are discussed. Then, the electrical properties of 10 Ω cm substrate cells and 2 Ω cm substrate cells are compared and the carrier recombination at the rear Si/SiO 2 interface is discussed using energy band diagrams.

Development of both-side junction silicon space solar cells

This paper reports the recent results of improving the radiation hardness of silicon solar cells, which is SHARP and NASDAs project since 1998 (Tonomura et al., Second World Conference on Photovoltaic Solar Energy, 1998, pp. 3511-3514). Newly developed 2 x 2 cm 2 Si solar cells with ultrathin substrates and both-side junction (BJ) structure showed 72.0 mW (13.3% efficiency) maximum output power at AMO, 28°C after 1 MeV electron irradiation up to 1 x 10 15 e/cm 2 and the best cell showed 72.5 mW (13.4%) maximum output power. These solar cells have p-n junctions at both front and rear surfaces and showed less radiation degradation and better remaining factor than previous solar cells.

Development of high efficiency silicon space solar cells

The cell design and manufacturing process for the high efficiency thin silicon space solar cells (call NRS/LBSF cell) were finalized and their characteristics were qualified. The 100 /spl mu/n NRS/LBSF cells showed efficiencies of maximum 18.5% and average 18.0%. After electron irradiation of 1E+15e/cm/sup 2/, the NRS/LBSF cell showed about 1.2 times higher output power than the conventional 200 /spl mu/m BSR cell.

New silicon space solar cells with IBF (integrated bypass function)

New silicon space solar cells with IBF (integrated bypass function) which enable prevention of a failure caused by a reverse biasing are proposed. To investigate the feasibility of these cells, some preliminary experiments were performed. From the results of these experiments, it is confirmed that these cells have no problems intrinsically as space solar cells and contribute to obtaining higher reliability of a solar array.