Indium Phosphide Solar Cells—Recent Developments and Estimated Performance in Space IRVING WEINBERG & DAVID J. BRINKER Summary The current status of indium phosphide solar cell research is reviewed. In the US program, mainly under the aegis of the NASA Lewis Research Center, efficiencies of 18.8% were achieved for standard n/p homojunction InP cells while 17% was achieved for ITO/InP cells processed by sputtering n-type indium tin oxide onto p-type indium phosphide. The latter represents a cheaper, simpler processing alternative. Computer modeling calculations indicate that efficiencies of over 21% are feasible. Initial efforts to produce cheaper, lighter weight and stronger cells are focused toward epitaxial deposition of InP on cheaper, more durable, substrates such as Si. InP solar cells on board the LIPS III satellite show no degradation after more than a year in orbit. Calculated array specific powers are used to estimate the relative performance of arrays containing InP, GaAs and Si, in polar and geosynchronous orbits. Relatively large area cells are produced in Japan with a maximum efficiency of 16.6%. Over 1000 of these latter cells have been manufactured to power a small lunar orbiter on board the Japanese MUSES A satellite. Introduction Most of the satellites currently in space receive their electrical power from silicon solar cells. On the other hand, several satellites have been launched using power obtained from gallium arsenide solar cells [1]. Although these latter cells are more expensive and heavier than the silicon type, their demonstrated higher efficiencies and superior radiation resistance makes their use attractive in specific orbits. Most recently, indium phosphide solar cells have emerged as candidates for use in the space radiation environment. This follows from their significantly increased radiation resistance when compared with gallium arsenide and silicon [2, 3]. In addition, InP cells have been observed to anneal at room temperature under dark conditions and under the influence of incident light [3, 4]. Furthermore, computer modeling calculations show that InP solar cells have theoretical efficiencies above that of Si and slightly below that of GaAs [5, 6]. Thus InP solar cells, when fully developed, have a strong potential to outperform the cells currently used in space. For this reason, in 1985, the NASA Lewis Research Center initiated a program aimed at developing high efficiency, radiation resistant indium phosphide solar cells. In the present case, we review recent significant developments in this program including the experience gained with InP Irving Weinberg & David J. Brinker, NASA Lewis Research Center, Cleveland, OH 44135, USA.
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