This text dates from 1992, and will be updated soon...
Virtually all spacecraft use solar cells to provide power by using the Sun's light. Early solar cells were small in area, inefficient, heavy and costly. Much of the cost came from interconnecting hundreds of the cells to provide even a single watt of electricity to the spacecraft. Over the past 25 years, OAST has sponsored research in this area to more efficiently convert light to electricity, increase the area of solar cells so that fewer were required to produce the needed power, and find low cost means of making large area lightweight solar arrays that could be used on missions like the Space Station Freedom.
By the 1970s, OAST determined that the efficiency of the silicon solar cell could be increased nearly twofold and started a research program with industry. Furthermore, the semiconductor industry had been successful in producing large area single crystal silicon ingots suitable for solar cell use. As a parallel research activity, a flight project was undertaken to demonstrate a large area solar array in space that would be one-third the weight of the existing arrays. NASA needed such a solar array to extend the stay time of the Space Shuttle (STS) on orbit. The union of the high performance, large area cell and the lightweight array would offer a new national capability.
As a result of OAST's research, large area silicon solar cells (5.9cm x 5.9cm) with efficiencies over 14% were demonstrated. This represented a 30% increase in the power output of cells of this size, significantly reducing the number of cells that would be required for the SSF solar arrays. They were also interconnected successfully by a new solderless welding technique that could be fully automated. Tests on modules of 4 cells showed good resistance to the temperature cycles that the array would be exposed to in low earth orbit. Further, a large area lightweight solar array, OAST-1, was successfully flown on STS 41D (September1984), demonstrating the viability of lightweight solar arrays (60 W/kg). Subsequently the Space Station Freedom selected this array design, and initiated a program to further increase the size of the silicon solar cells. This effort produced 8cm x 8cm cells with efficiencies of 14.8% and welded interconnections that have withstood more than 7 years of low earth orbit temperature cycles.
Nearly 200,000 of these advanced cells will be used as the primary supply for the permanently manned Space Station Freedom, providing a peak power of 187.5 kilowatts. Because of the sun/shade cycles in the space station orbit, much of this power will be used to charge batteries for the dark phase, leaving 56.25 kilowatts to be continuously supplied for life support and experiments.
This technology was developed over a 25 year period by the sustained efforts of OAST, the NASA Centers, and commercial solar cell and array suppliers. For additional information contact Dennis Flood, Photovoltaic Branch, MS 301-1, NASA Lewis Research Center, 21000 Brookpark Road, Cleveland OH 44135. Telephone (216) 433-2303.