film microwave devices, on a thin plastic substrate are used. The structure likewise is not a heavy “rigid” unit, but lather of light weight flexible construction. —Phased array antenna. The antenna does not need to be physically “aimed” at the receiver but is steered by controlling each element’s phase. The distributed thin-film SPS applies the integrated circuit approach to the satellite solar power concept. By designing small, relatively complex building blocks (i.e. self powered transmitters), and replicating them many times over, the total system is constructed. This allows the design, development, and testing to be performed at this small block level, and the manufacturing done on a high volume item. Integration and control of these building blocks in a total system will be critical. Two major areas to be considered are physical construction of the satellite and control of the phased arrays. This paper discusses the status of the enabling technologies for an integrated solar cell/transmitter approach, conceptual designs of solar power satellites that use such an approach, possible applications, and development needed. Thin Film Photovoltaics In the 1980’s considerable research was devoted to development and commercialization of thin-film photovoltaics for terrestrial power generation. Thin-film solar cells consist of thin (- 1-5 pm) films of photovoltaic material deposited on a supporting substrate. Efficiencies around ten percent have been achieved with amorphous silicon and copper indium diselenide thin-films, and encouraging results from other thin film technologies such as CdTe and CuInS2. Table 1 shows the historical progress in efficiency of several of the thin-film materials over the last few years [9]. This compares to typical space qualified cell efficiency of about 14% for currently used silicon cells, 19% for GaAs cells, and projections of over 20% efficiency for cells to be used in the 1990’s. (Since the initial presentation of this paper in 1991, further progress has been made in thin-film solar cells, as shown in figure 1. A terrestrial (AM1.5) active-area efficiency of 15.8% has been reported for CdTe cells [10], and 14.8% has been reported for CuInSe2 solar cells [11]. These efficiencies are expected to correspond to approximately 12.5% and 12% efficiency under space (AMO) illumination, essentially identical to the numbers predicted for “1990s” in Table 1). Because of the high optical absorption constant of these materials, the active material may be as thin as one to two microns, inherently yielding extremely lightweight cells. However, very little current research is aimed at depositing thin- film cells on lightweight substrates, since most of the applications being considered are terrestrial, where weight is not as critical. To enable their use in space, technology for deposition on extremely lightweight substrates will need to be developed.
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