efficiency between 10 and 12%, whereas violet cells produced in large batches have an efficiency of 14%. Light reflection is another loss mechanism in solar cells. The index of refraction of a vacuum is one and that of polished silicon is 3.4. This mismatch in reflective indices gives rise to high reflection losses. To minimize this effect, an antireflective transparent coating is usually applied to the front surface of the cell. This reduces the loss to less than 10%, which is acceptable for typical applications. In order to improve the cell further, special surface texturing treatments are required. In the case of silicon, preferential etches are applied which form pyramids approximately 2000-6000 nm in height. Light is reflected off the angular surfaces of these pyramids several times until almost all of it is absorbed (figure IV.B.1.7) hence the name, a "black" cell. The reflection of a black cell is less than 5%, and they have had average efficiencies of approximately 15% in test runs of 1000 units. Pyramid-like structures form in silicon because it has a face-centered cubic structure. When oriented in certain planes, the corners of the crystals appear prominently when the silicon is exposed to certain preferential etches. Deep groove-like structures are also possible with etchants as well as with ion milling techniques. The adaptability of these surface antireflective techniques to nonsilicon materials has yet to be demonstrated. All incident energy which cannot be converted by the solar cell into electrical energy is absorbed as heat, which also constitutes a loss. As the temperature of the semiconductor material increases, its conversion efficiency decreases. The effect of efficiency decrease with temperature for each of the three materials is shown in figure IV.B.l.a.8. As in the previous figure, gallium arsenide exhibits superior characteristics over that of either silicon or cadmium sulfide. The current collection electrode on the top surface of the solar cell represents another loss in efficiency because it masks a finite portion of the conversion area from incident radiation. This electrode typically has a comb or tree-shaped design so that current flow in any one area of the cell is reduced to a minimum. The greater the amount of current that must flow through the semiconductor material, the greater will be the I^R losses (heat). A compromise in electrode area and I2R losses is reached around 5% of the active area of the cell. In addition to loss in the bulk material as a result of lateral current flow, solar cells also have I^R losses associated with the metallic electrodes themselves and with a contact resistance to the semiconductor. Although the resistance is small, it is nevertheless in series with the current flow at all times and causes an I^R loss in the cel 1.
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