4. Choice of Materials Researchers7-8 have investigated several different thin-film photovoltaic materials. Amorphous silicon (a-Si) compares favorably with other materials, such as CdS, Cu2S, and CuInSe2 in terms of efficiency. All of these materials are radiation-tolerant, thereby eliminating the need for a protective cover glass. They are also low in mass, and therefore, inexpensive for the quantities that will be needed. Amorphous silicon is at a slight disadvantage in terms of light degradation (10 to 15% after two years, compared to, say, where h and 4> are given in Equations 4 and 3, respectively. Thus, for a given altitude, rectenna latitude, and power beam frequency, the allowable peak beam intensity determines the SPS diameter.
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