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Standard photovoltaic cells require extremely pure polysilicon, which is made from quartz - a mineral comprised of silicon and oxygen (SiO2). Many facets of a photovoltaic cell and its various production processes also use glass made from fused quartz.
The stability, transmissivity to light and heat-resistant qualities of quartz have made it indispensable to the creation of semiconductors, and by extension, photovoltaic cells. Quartz used in the production of solar cells is nearly inert, very resilient, and able to withstand the high temperatures found in semiconductor production and evaluation processes.
To be considered microelectronics grade (eg-Si), silicon must be of very high purity, with an impurities level of less than 1 part per billion. However, the purity demands for solar-grade silicon (sog-Si) are substantially lower: less than 1 part per million.
In a standard production process for a photovoltaic cell, metallic silicon is reacted with hydrogen chloride at 650 degrees Celsius to create a liquid called trichlorosilane. This liquid is purified through distillation, evaporated and then deposited as polycrystalline silicon on silicon ingots. The process is relatively energy-intensive, calling for between 100 and 160 kWh to fabricate just 1 kilogram of purified silicon.
Next, in the process, the purified polycrystalline silicon is liquified, doped with boron, and solidified into rectangular or cylindrical ingots weighing around 100 kilograms. These crystal ingots are mechanically processed to form blocks with a standard length of 156 mm.
Silicon disc wafers, approximately 200 µm-thick, are cut with wire saws from the ingots. Silicon wafers are not always shaped like discs, however. They are often made into squares or other shapes based on how tightly the wafers must fit together for density or efficiency purposes.
For photovoltaic cells, cleaned wafers are typically etched to generate a textured exterior. This texturization cuts down on unwanted reflection of sunlight and considerably boosts light absorption, especially in the longer-wavelength end of the spectrum.
A pn-junction on the outside of the wafer, comprised of a boron-doped p-base and a phosphorus-doped n-emitter, converts it into a large-scale semiconductor diode. This feature is fabricated through the use of a highly-concentrated phosphorus dopant, which is diffused into the wafer at a temperature around 900 degrees Celsius until it reaches a depth of 0.3 μm.
Using a plasma-enhanced chemical vapor deposition (PECVD) process, a hydrogen-containing silicon nitride layer is placed on the front side of the wafer. This layer boosts the efficiency of the cell and acts as an anti-reflection layer. It also gives a photovoltaic cell its signature dark blue color.
All semiconductor manufacturers use quartz and fused-quartz products, including in production equipment and labware used in research, development, and evaluation. Specifically, quartz glass can be found in the production furnaces, and reaction chambers used to make photovoltaic cells.
One of the purest materials commercially available, fused quartz can have a nominal purity as high as 99.996 percent. The purity of fused quartz is useful for semiconductor producers since even trace impurities can be passed to silicon wafers, resulting in lower performance. For instance, there is a wide variety of contaminants that can affect the performance of dopants to create an occasion for lag, which disrupts semiconductor performance. Even small quantities of alkali can decrease wafer yields.
The extremely high purity of fused quartz is unparalleled in the glass industry. Fused quartz glass also has a higher quality than widely-used borosilicate products. Many optical laboratories prefer the purity of fused quartz because it allows a relatively wide range of light wavelengths to completely pass through, a crucial feature for many research applications.
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