|Gallium-Arsenide-Based Solar Cells
Spire Corporation developed a high efficiency (25 percent at 170 suns) gallium arsenide (GaAs)-based solar cell for use in a solar concentrator array under a BMDO SBIR contract. The company recently sold two related GaAs/GaAs concentrator cell arrays with 21 percent efficiency at 3 suns that will fly on the NASA Wide Field Infrared Explorer and the USAF MightySat missions. By using laser light instead of sunlight, Spire's solar cell technology has been adapted to convert laser light to electrical power. Spire has sold over 2,000 laser power converters for short-haul fiber-optic links and makes three types of the converters.
To construct a tandem concentrator solar cell of gallium arsenide (GaAs) on germanium (Ge) for BMDO, Spire Corporation adjusted an epitaxial growth process that has been used on a range of GaAs-based cells. Also, Spire refined a microfabrication technique using mesa etching and high-temperature metallization to manufacture the GaAs/Ge cells. The company started with a precursor cell of GaAs on a GaAs wafer (GaAs/GaAs) before growing the same epilayers on a Ge wafer.
A solar cell is a semiconductor electrical junction device that absorbs the radiant energy of sunlight and converts it into electrical energy, using the sun as a source of electrical power for systems.
From 1988 through 1991, BMDO funded Spire through Wright-Paterson AFB to make tandem two-junction monolithic GaAs/Ge solar cells for concentrator array power sources for space systems. BMDO was interested in developing a higher efficiency, lower mass solar cell to generate electricity on a spacecraft. Higher efficiency means that less cell area is required to generate a specified amount of electrical power, thus reducing the size (and weight) of the array. This translates into lower cost of manufacture and reduced launch costs. Initial interest was in a high concentration cell (about 100 suns). However, the larger lens on higher concentration arrays have more stringent pointing requirements, and BMDO decided to reduce the concentration requirement to 10 suns. Spire made a GaAs/Ge cell with 23 percent efficiency in space (air mass zero) at 10 suns concentration.
The research has spun off into two main product areas: concentrator solar arrays and laser power converters.
Concentrator Solar Arrays: Concentrator solar arrays use optical concentrators such as mirrors or Fresnel lenses to focus sunlight onto the smaller-area solar cells. Concentrating the light on the cell improves the efficiency and reduces the size of the array needed, which lowers the cost. Spire made two different arrays.
GaAs/Ge: The two-junction tandem GaAs/Ge concentrator cell that Spire made was designed for use in a linear array for an Air Force program. It is useful for aerospace applications requiring low-weight substrates since, as a substrate, the germanium can be made thinner and lighter than gallium arsenide.
GaAs/GaAs: As part of the GaAs/Ge development, Spire created a single-junction GaAs/GaAs concentrator solar cell with 25 percent efficiency in space (air mass zero) at 170 suns concentration, providing a power output of 0.74 Watts per cell. (The amount of concentration is measured in “suns,” one sun being the intensity of sunlight falling on Earth.) The cell was designed to produce electricity for powering systems on satellites in space. The cells were tested at Sandia National Laboratory, which still lists them as the most efficient GaAs cells (according to “Progress in Photovoltaics,” Solar Cell Efficiency Table, Ver. 9, Vol.5, 1997, pp. 51-54). However, a limitation of concentrator cells is that, because of more stringent alignment requirements of the lens, they are more sensitive to spacecraft misalignments than are flat solar array panels.
A version of these cells, a 1- by 3-cm linear array, operating at a concentration of 3 suns, will fly on the NASA Wide Field Infrared Explorer and the USAF MightySat missions. Even though the shape of the cells are different, the basic GaAs growth process and manufacturing technology is that developed under the BMDO program.
Laser Power Converters: Spire is using its GaAs concentrator technology to make laser power converters (LPCs) to convert laser light into electricity. Laser light from optical fiber or transmitted across free space is directed to the cell at a wavelength the cell responds to and is converted to electricity and transmitted to a component or system as power. The company makes three different types of LPCs. The GaAs/GaAs LPCs are the most popular. However, Spire has also added some indium to the epitaxial growth process to lower the bandgap from GaAs for additional applications.
GaAs/GaAs LPCs have a cutoff wavelength of 870 nanometers and efficiencies up to 60 percent. GaAs LPCs are mainly used at 800 to 850 nanometers over short fiber links with large-core multimode fibers so that higher power can be launched easily into the fiber (about 1 to 2 Watts). Such laser power converters can be used in transmitting power to sensor instruments in electromagnetically noisy environments. LPCs could be used to power controls and isolate them from high voltage machinery in industrial environments or to limit susceptability to radio frequency noise on a power line to a sensitive sensor. For fusing of explosives, for example, using LPCs on fiber could avoid the danger of a damaged or improperly shielded detonator wire accidentally picking up a radio signal and causing detonation. LPCs on fiber can be used to transmit electricity in grain silos or mines filled with potentially explosive dust.
These LPCs can also transmit power for optical networks and devices. For example, integrating the LPC with an optical transceiver enables the fiber that transmits data to and from the transceiver to also be the source of power for its components—CMOS electronics, LED transmitters, and photodiodes. One fiber can carry all the optical signals needed to send a modulated data query, transmit modulated data replies on a carrier light wave, and power the device.
In(53%)Ga(47%)As LPCs have a cutoff wavelength of 1670 nanometers and efficiencies up to 35 percent. They work at the 1300- to 1550-nanometer wavelength favored for long fiber links in similar applications to the GaAs LPCs.
In(70)%Ga(30%)As LPCs have a cutoff wavelength of 2250 nanometers and 22 percent efficiency. They are available for power conversion from longer wavelength free-space transmission by 2,100-nanometer “eyesafe” lasers. They may be useful for possible future fluoride-based fiber systems that lower attenuation by an order-of-magnitude over current silica fibers by using a wavelength beyond 2,000 nanometers.
Spire holds many patents on solar cells, which they have been making for 12 years, but none specifically on the BMDO technologies.
In January 1998, Spire sold GaAs/GaAs concentrator cells to Composite Optics to populate two panels for use in two satellite programs: the NASA Wide Field Infrared Explorer and the USAF MightySat missions.
The Solar Energy Research Institute (SERI), now the National Renewable Energy Laboratory (NREL), funded Spire to examine GaAs cells for terrestrial applications. On Earth, the GaAs/GaAs concentrator solar cell had 28 percent efficiency, but the GaAs made it too expensive for Spire to compete in the terrestrial power generation arena against silicon concentrators that have somewhat less efficiency, but are less expensive.
The company's most popular product turned out to be the GaAs/GaAs laser power converters. It has sold over 2,000 converters to instrumentation and sensor companies to use on fiber-optic lines. Spire delivered 20 prototype optical transceiver modules with LPCs to a Japanese instrument company.
Spire's research on semiconductors began shortly after the company was started in 1969. The company originally focused on silicon-based materials, particularly for solar cells, and soon after turned its attention to III-V semiconductors. Spire has developed a broad technical expertise in thin film technology for photovoltaics and optoelectronics. Spire has seven metal-organic chemical vapor deposition (MOCVD) reactors including several for processing wafers up to 4 inches. Building on its photovoltaic and solar cell research and MOCVD experience, Spire provides epitaxial compound semiconductor wafers and devices for a broad range of optoelectronic applications including solar cells for satellites, laser power converters, and semiconductor lasers.
Spire trades publicly under the symbol SPIR and has 150 employees.
Steve Wojtczuk, Manager of III-V Photovoltaics
One Patriots Park
Bedford MA 01730-2396