developed, which substantially reduced the distributed series resistance and increased efficiencies to 12% AMO. Over the following years (see Table I), a series of developments improved silicon cell AMO efficiencies incrementally to the present level of about 15%. Base resistivities were increased to 10 ohm cm to improve radiation resistance; junction depth was decreased to increase short wavelength response, thus increasing efficiency and also increasing radiation resistance; multilayer antireflection coatings and textured surfaces were developed to increase efficiency; back surface fields were added to reduce the rear surface minority carrier recombination rate thus increasing efficiency (unfortunately this gain was lost after extensive radiation exposure for 200 //m thick cells); cells were made much thinner (~62 /zm thick) which decreased weight and gained back the increased efficiency provided by the back surface field even after typical orbital radiation exposure levels; larger area cells were designed and manufactured using mechanized processes that reduced costs substantially; and back surface reflective contacts were developed that rejected the unusable long wavelength light, thus increasing efficiency by reducing cell operating temperatures in orbit. Gallium arsenide solar cells have also been developed over the years but have only recently been manufactured in the USA for space use. Single junction GaAs devices typically have AMO efficiencies of about 19% while dual junction GaAs on germanium devices have shown efficiencies as high as 22%.
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