4141 Single Crystal and Polycrystalline Silicon

Single crystal silicon cells are produced by a series of processes: (1) growing crystalline ingots of p-silicon, (2) slicing wafers from the ingots, (3) polishing and cleaning the surface, (4) doping with n material to form the p-n junction, (5) deposition of electrical contacts, (6) application of antireflection coating, and (7) encapsulation. The Czochralski process is the most common method of growing single crystal ingots. A seed crystal is dipped in molten silicon doped with a p-material (boron) and drawn upward under tightly controlled conditions of linear and rotational speed and temperature. This process produces cylindrical ingots of typically 10 cm diameter and 1 m length. Polycrystalline ingots are produced by casting silicon in a mold of preferred shape (rectangular). Molten silicon is cooled slowly in a mold along one direction in order to orient the crystal structures and grain boundaries in a preferred direction. In order to achieve efficiencies of greater than 10%, grain sizes greater than 0.5 mm are needed and the grain boundaries must be oriented perpendicular to the wafer. Ingots as large as 400 cm x 40 cm x 40 cm can be produced by this method. Ingots are sliced into wafers by internal diameter (ID) saws or multiwire saws impregnated with diamond abrasive particles. Both of these methods result in high wastage of valuable crystalline silicon. Alternative methods that reduce wastage are those that grow polycrystalline thin films.

A p-n junction is formed in the cell by diffusing a small amount of n material (phosphorous) into the top layer of a silicon wafer. The most common method is diffusion of phosphorous in the vapor phase. In that case, the back side of the wafer must be covered to prevent the diffusion of vapors from that side. Electrical contacts are attached to the top surface of silicon crystals in a grid pattern to cover no more than 10% of the cell surface, and a solid metallic sheet is attached to the back surface. The front electrode grid is made either by vacuum metal vapor deposition through a mask or by screen printing. Finally, titanium dioxide (TiO2) and tantalum pentoxide (Ta2O5) are deposited on the cell surface to reduce reflection from more than 30% of untreated silicon to less than 3%. Anti-reflective (AR) coatings are deposited by vacuum vapor deposition, sputtering, or chemical spraying. Finally, the cell is encapsulated in a transparent material to protect it from the environment. Encapsulants usually consist of a layer of either polyvinyl butyryl (PVB) or ethylene vinyl acetate (EVA) and a top layer of low iron glass.

4.1.4.2 Amorphous Silicon

Amorphous silicon (a-Si) cells are made as thin films of a-Si:H alloy doped with phosphorous and boron to make n and p layers, respectively. The cells are manufactured by depositing a thin layer of a-Si on a substrate (glass, metal, or plastic) using glow discharge, sputtering, or CVD methods. The most common method is by an RF glow discharge decomposition of silane (SiH4) on a substrate heated to a temperature of 200 to 300°C. To produce p-silicon, diborane (B2H6) vapor is introduced with the silane vapor. Similarly, phosphene (PH3) is used to produce n-silicon. The cell consists of an n-layer, an intermediate undoped a-Si layer, and a p-layer on a substrate. The cell thickness is about 1 |im. The manufacturing process can be automated to produce rolls of solar cells from rolls of substrate.

Figure 4.8 shows an example of roll-to-roll, a-Si cell manufacturing equipment using a plasma CVD method. This machine can be used to make mul-tijunction or tandem cells by introducing the appropriate materials at different points in the machine.

4.2 PV System Efficiency and Design

Figure 4.9 shows a typical current voltage curve for a solar cell. The power output is the product of the load current IL and voltage VL.

The power output exhibits a maximum as shown. To maximize power output, the cell must operate at this condition no matter what the environmental conditions. This so-called maximum power point tracking is readily accomplished by electronic controllers that adjust the operating voltage in real time to the maximum power point voltage.

FIGURE 4.8

Schematic diagram of continuous a-Si manufacturing process.

FIGURE 4.8

Schematic diagram of continuous a-Si manufacturing process.

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FIGURE 4.9

Typical current and power characteristics of a solar cell. Power axis to the right has units of W.

Figure 4.10 indicates the effect of incident solar radiation intensity and the load resistance on the performance of a silicon cell at a fixed temperature. Cell temperature also affects cell performance in such a way that the voltage and, thus, the power output decrease linearly with increasing temperature. Therefore, PV cells operate best when the cells are cool and the solar irradi-ance is high. It is worth noting that the maximum power point is at essentially the same voltage irrespective of the array irradiance. PV panels and arrays have similar performance characteristics.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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