The primary cause of solar cell efficiency degradation in space is proton and electron radiation. Incident protons and electrons produce crystal lattice defects in the semiconductor material, which in turn, act as recombination centers for holes and electrons (figure IV.B.l.a.9). Thin cells are less susceptible to radiation damage than are thick cells, but there is a limitation on how thin a cell can be. The absorption of light in silicon, for example, is less than that of other semiconductors. As a result, its thickness must be greater than 0.100 mm or the efficiency will be too low to be of practical value. Figure IV.B.l.a.10 shows efficiency degradation as a function of thickness for two types of silicon material. Other materials, such as gallium arsenide, have a much higher absorption coefficient and require only a 0.005 mm thickness. In order to protect the solar cell from radiation, a 0.150- 0.300 mm cover glass is usually mounted on top of the cell. This adds some absorption losses, and of course, reduces cell efficiency. More importantly, however, it generally doubles the cell's weight. Since weight is a critical parameter in the SPS's solar cell arrays, a thin film of durable translucent plastic is being proposed instead of the cover glass. It would provide the same protection as the glass, have slightly greater absorption loss, and would be much lighter. (4) ALTERNATE SOLAR CELL MATERIALS The problems of marginal efficiency, efficiency degradation with temperature, thickness limitations, susceptibility to radiation damage, and high cost have prompted researchers to have interests in photovoltaic materials other than silicon. Alternative materials are listed in figure IV.B.l.a.7, but of this group, only two, cadmium sulfide and gallium arsenide, have been studied extensively. The balance have various physical problems which limit their usefulness. Cadmium sulfide has been known as a photosensitive material for several decades. It was actively studied by the NASA/Lewis Research Center as a photovoltaic converter during the 1960's. Their interest stemmed from a desire to have a flexible solar cell array that could be rolled into a dense form for storage. It was found that polycrystalline cells could be fabricated on both thin metal foils and metallized plastic films. However, the best efficiency that could be obtained was about 8%, and this tended to degrade to a somewhat lower value with time. The cells were also very susceptible to the effects of moisture which required them to be hermetically sealed. The low efficiency and moisture degradation problem eventually caused the effort to be abandoned in favor of silicon. It is interesting to note that there is a greater potential for space manufacturing of cadmium sulfide solar cells than either silicon or gallium arsenide. The high vacuum environment is a convenient advantage because the cells are typically manufactured by a vacuum deposition process. This is not necessarily a simple technique to perform
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