into its upper surface from 150 nm to 300 nm deep. The process involves the use of special furnaces which operate around 1273K and take about one hour. (a) ELECTRICAL CONNECTIONS/ANTIREFLECTION COATINGS: Electrical connections are formed on a solar cell by depositing various metals on the front and backside. A comb or tree pattern is placed on the surface (Figure IV.B.l.a.24), and the backside is completely covered unless wraparound electrodes are used. The metal coatings are sintered onto the silicon to minimize the contact resistance and improve adhesion. A common metallization system uses a titanium, platinum, and silver combination for the grid electrodes. The back surface is coated with the same metallization or another metal such as aluminum or a combination of nickel and copper. Antireflective coatings of Ta205 or SiOo are applied by vacuum deposition and are necessary to maintain the efficiency of the cell. Polished silicon normally reflects as much as 30 percent of the light which strikes its surface, and the coating reduces this to about 5-10 percent. More effective light energy capture methods have been developed in the laboratory and can reduce the reflection to almost zero. However, these techniques involve wet chemical techniques. A high-quality cover glass is usually cemented to the (Figure IV.B.l.a.25) top surface of the cell to provide protection from proton and electron radiation. The violet cell is very similar to the conventional cell and differs mainly by having a shallower N-P junction (150 nm versus 300 nm) and an antireflective coating of Ta^Os (Figure IV.B.l.a.26). The antireflective coating of a violet cell is thinner than that of the conventional cell so that it will be more responsive in the ultraviolet or blue region of the solar spectrum. The solar cell design used on the SPS will probably correspond to the violet cell. Its efficiency at present is approximately 14 percent and is projected to be 16 percent or better in the future. This cell is also being space qualified, whereas many of the other special cells are still in the laboratory stage of development.
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