2. Survey of the Current State of the Art Current Generation Technology The current generation of space solar cells consists of silicon and gallium arsenide cells [1]. The vast majority of all solar cells flown in space are silicon cells. Conventional silicon solar cells have a thickness on the order of 250 microns; these can be chemically and mechanically thinned to a thickness of 62 microns. Production cells (e.g. for the space station ‘Freedom' solar array) are typically 14% efficient [2], The best silicon cells measured in the laboratory are about 18.1% efficient [3]. Silicon solar cell technology has recently had major gains in performance, and the previous estimates for the ‘limits' to performance have had to be revised upwards. New estimates taking into account new technologies such as light trapping and surface passivation suggest achievable efficiencies of up to 22%, with values of 20% likely in the near term. Higher efficiencies can be achieved with gallium arsenide, which has a bandgap closer to the optimum for the space solar spectrum. The best verified efficiency measured on a GaAs cell to date is 21.4% AMO [4]. A higher efficiency has been reported (22.5% AMO [5]), but not yet independently verified. Cells manufactured using current production technology have a somewhat lower efficiency. In a manufacturing technology program, Applied Solar Energy Corporation (ASEC) delivered 6000 2X4 cm GaAs cells with an average performance of 18% AMO to demonstrate production readiness, and 115 000 cells of an earlier design with performance averaging 16.6% [6], LSI in Japan has demonstrated production runs of 120 cells with an average efficiency of 20% AMO [7]. GaAs on Ge. Cascade solar cells made of GaAs on germanium have shown the potential for higher efficiencies than conventional GaAs cells; 21.7% efficiency has been measured under the simulated AMO spectrum [8,9]. Unfortunately, high altitude tests have shown that the actual space solar spectrum does not have enough long wavelength irradiance to fully bias the germanium subcell on, and the actual efficiency is lower than the tested values [10], This problem can potentially be eliminated either by improving the Ge subcell or by adding Al to the GaAs to let through more light. Tobin et al. calculate a limit efficiency for this cell design of 35.7%, compared to 27.5% for GaAs alone [10], It is also possible to make GaAs cells on inactive germanium substrates. This allows the substrate to be thinned, reducing the cell weight. GaAs on germanium cells have been produced with 78 micron thickness [11], for a considerable weight reduction compared to conventional GaAs cells. Next Generation Technology Ultrathin Silicon. With light trapping and surface passivation, the optimum thickness of a single-crystal silicon cell decreases and the efficiency increases. For highest end- of-life efficiency, the optimum thickness may be as low as 2 microns, leading to potentially very high specific power. The radiation tolerance of such ultrathin cells may be extremely good, since the thickness is less than the diffusion length even after radiation damage. Calculations predict that such ultrathin cells have efficiency and radiation tolerance as good as that of III-V solar cells [12]. Indium Phosphide. Considerable interest has recently been focused on indium phos-
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