1-1. Photovoltaic Space Power History and Perspective E. L. RALPH Summary Photovoltaic space power systems have been the preferred technology for most spacecraft missions. This is due to a combination offactors. Of primary importance is the high reliability afforded by the large redundant matrix of solid state photovoltaic devices that make up a solar array. Of course, the low cost of a solar power system relative to the complete spacecraft is also an important factor. Progress over the past 30 years has continually improved the conversion efficiences, decreased system weights, and reduced costs, so that alternate technologies always have a moving target to surpass if they are to be competitive. This paper describes the progress that has been made and some advances that are likely to be achieved in the future. Background and History Since its invention in 1954, the solar cell has played an important part in the development of space power systems. The first applications for solar cells were in toys and portable radios, but in 1958 space exploration changed everything. Solar power systems using photovoltaic devices and associated battery storage provided an excellent source for spacecraft power needs. These systems utilized a solid state device which was inherently reliable, readily manufacturable, easily qualifiable, scaleable to the desirable power levels, and relatively inexpensive. These attributes made photovoltaic power systems the work horse of the spacecraft industry. Research and development efforts over the past 30 years have been aimed primarily at increasing solar cell conversion efficiency, increasing battery specific power, decreasing array weight, decreasing costs and improving radiation resistance. In addition, much work has been accomplished in improving reliability, increasing the power levels, and developing innovative solutions to specific mission requirements. Solar cell efficiency has consistently increased over the years as shown in Fig. 1. The first solar cells used on the Vanguard Satellite in 1958 were silicon cells of only about 5% air mass zero (AMO) efficiency and were P on N (N doped base) type devices which were very sensitive to high energy particle radiation. After the Van Allen radiation belt was discovered by the Explorer 1 satellite a more radiation resistant solar cell was desired. Switching to a N on P (P doped base) type cell design provided significant improvements in radiation resistance, so most satellites after 1962 utilized silicon devices of this type. About this same time gridded front contacts were E. L. Ralph, Hughes Aircraft Company, PO Box 92919, Los Angeles, CA 90009, USA. Paper number IAF-ICOSP89-1-1.
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