· 1 The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
· 2 The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure "drags" the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.
Making single crystal silicon
3 Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or "boule" of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.
Making silicon wafers
4 From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multi-wire saw. (A diamond saw produces cuts that are as wide as the wafer—. 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
Product Flow
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity, Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants. This first conversion of sunlight to electricity was one percent efficient. That is, one percent of the incoming sunlight was converted into electricity. Willoughby Smith in 1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that selenium, when exposed to light, produced an electrical current. Charles Fritts, in the 1880s, also used gold-coated selenium to make the first solar cell, again only one percent efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free solar energy to be a means of decentralization, predicting that solar cells would replace power plants with individually powered residences.
To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. [To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. At this point, the silicon is still not pure enough to be used for solor cells and requires further purification.] at this point, the silicon is still not pure enough to be used for solar cells and requires further purification.
This illustration shows the makeup of a typical solar cell. The cells are encapsulated in ethylene vinyl acetate and placed in a metal frame that has a mylar back sheet and glass cover.
through a photoresist, silkscreened, or merely deposited on the exposed portion of cells that have been partially covered with wax. All three methods involve a system in which the part of the cell on which a contact is not desired is protected, while the rest of the cell is exposed to the metal.
8 After the contacts are in place, thin strips ("fingers") are placed between cells. The most commonly used strips are tin-coated copper.
The anti-reflective coating
9 Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer. The most commonly used coatings are titanium dioxide and silicon oxide, though others are used. The material used for coating is either heated until its molecules boil off and travel to the silicon and condense, or the material undergoes sputtering. In this process, a high voltage knocks molecules off the material and deposits them onto the silicon at the opposite electrode. Yet another method is to allow the silicon itself to react with oxygen- or nitrogen-containing gases to form silicon dioxide or silicon nitride. Commercial solar cell manufacturers use silicon nitride.
Encapsulating the cell
10 The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar back sheet and a glass or plastic cover.
Quality Control
Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely affect the overall efficiency of the cells. The primary research goal is to find ways to improve the efficiency of each solar cell over a longer lifetime. The Low Cost Solar Array Project (initiated by the United States Department of Energy in the late 1970s) sponsored private research that aimed to lower the cost of solar cells. The silicon itself is tested for purity, crystal orientation, and resistivity. Manufacturers also test for the presence of oxygen (which affects its strength and resistance to warp) and carbon (which causes defects). Finished silicon disks are inspected for any damage, flaking, or bending that might have occurred during sawing, polishing, and etching.
During the entire silicon disk manufacturing process, the temperature, pressure, speed, and quantities of dopants are continuously monitored. Steps are also taken to ensure that impurities in the air and on working surfaces are kept to a minimum.
The completed semiconductors must then undergo electrical tests to see that the current, voltage, and resistance for each meet appropriate standards. An earlier problem with solar cells was a tendency to stop working when partially shaded. This problem has been alleviated by providing shunt diodes that reduce dangerously high voltages to the cell. Shunt resistance must then be tested using partially shaded junctions.
An important test of solar modules involves providing test cells with conditions and intensity of light that they will encounter under normal conditions and then checking to see that they perform well. The cells are also exposed to heat and cold and tested against vibration, twisting, and hail.
The final test for solar modules is field site testing, in which finished modules are placed where they will actually be used. This provides the researcher with the best data for determining the efficiency of a solar cell under ambient conditions and the solar cell's effective lifetime, the most important factors of all.
After the initial purification, the silicon is further refined in a floating zone process. In this process, a silicon rod is passed through a heated zone several times, which serves to 'drag" the impurities toward one end of the rod. The impure end can then be removed. Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is dipped into melted polycrystalline silicon. The seed crystal rotates as it is withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced out of the ingot.
5 The wafers are then polished to remove saw marks. (It has recently been found that rougher cells absorb light more effectively, therefore some manufacturers have chosen not to polish the wafer.)
Doping
6 The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms "burrow" into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth. A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.
Placing electrical contacts
7 Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated
Purifying the silicon
The Manufacturing Process
Photovoltaic Cell Manufacturing Process
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