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Figure 16.19 Collector area optimization curves for a typical solar heating system. Ref. 10.

diodes. Figure 16.20a. A p-n junction has electronic asymmetry. The n-type regions have large electron densities but small hole densities. Electrons flow readily through the material but holes find it very difficult. P-type material has the opposite characteristic. Excess electron-hole pairs are generated throughout the p-type material when it is illuminated. Electrons flow from the p-type region to the n-type and a flow of holes occurs in the opposite direction. If the illuminated p-n junction is electrically short circuited a current will flow in the short-circuiting lead. The normal rectifying current-voltage characteristic of the diode is shown in Figure 16.20b. When illuminated (insulated) the current generated by the illumination is superimposed to give a characteristic where power can be extracted.

The characteristic voltage and current parameters

Ideally FF is a function only of the open-circuit voltage and in cells of reasonable efficiency has a value in the range of 0.7 to 0.85.

Most solar cells are made by doping silicon, the second most abundant element in the earth's crust. Sand is reduced to metallurgical-grade silicon, which is then further purified and converted to single-crystal silicon wafers. The wafers are processed into solar cells which are then encapsulated into weatherproof modules. Boron is used to produce p-type wafers and phosphorus is the most common material used for the n-type impurity. Other types of solar cells include CdS, CdTe, and GaAs. Theoretical maximum conversion efficiencies of these simple cells are around 21 to 23 percent.

Major factors which, when present in real solar cells, prohibit the attainment of theoretical efficiencies include reflection losses, incomplete absorption, only partial utilization of the energy, incomplete collection of electron-hole pairs, a voltage factor, a curve factor and internal series resistance.

Thin-film solar cells have shown promise in reducing the cost of manufacturing and vertical junction

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