covers are placed over each cell to protect it from electron and proton bombarc ment in space. The efficiency of solar cell conversion o. sunlight to electricity was only 6% in 1954, when t le first cells were built, but is 14-15% today. Figure 4 summarizes the trends in cell development. Maximum theoretical conversion efficiency is 23% and by 1990 we expect production cells to reach 18%. Cells are now available in larger sizes with larger ingots being grown, and new processes under development by the Department of Energy (DoE) could result in the manufacture of cells from large sheets of silicon. Cell thickness is also being reduced without sacrificing cell performance; cells with thicknesses of 50 gm (2 mils vs standard 10 mils) have been Fig. 3. Solar cell performance characteristics. Fig. 4. Silicon solar cell improvements.
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