Researchers at Natcore Technology have created a silicon wafer whose average reflectance is only 0.3% in the region of the solar spectrum – visible and near infrared – which makes the surface of this type of “blackest” ever recorded.
Compared with the silicon wafers used in the manufacture of photovoltaic panels classics, the proportion of incident light reflected from the surface is reduced by a factor of ten on this spectrum, which is the source of about 80% of useful energy which can be recovered from the sunlight.
The color (black) black silicon results from the almost total absence of light reflected off the surface of the wafer (wafer) porous. In the field of solar cells, the “dark” is highly desirable because it indicates that the incident light is absorbed for the conversion of energy rather than being reflected and therefore wasted.
Quantitatively, the reflectance is the proportion of light striking a given surface. And a reflectance of 0.3% means that only 0.3% of the incident light is reflected by the surface of the solar cell, while the remainder (99.7%) becomes available for conversion into electrical energy. A tenfold reduction would mean that the reflectance of light used in most (up to 3%) could “enter” into the cell, increasing of their effectiveness. (Eg A cell efficiency of 18% would increase to 18.5%).
But there are other benefits to be derived from black silicon. A panel composed of black silicon solar cells produce much more energy on a daily basis than will a panel made from cells with an antireflection coating. First, because it reflects less light. Second, because it works better in the morning and afternoon when the sun “hits” at a certain angle. Moreover, this new type of cell also outperforms the conventional panels on cloudy days.
The thought process by Natcore began with an uncoated silicon wafer, whose texture had an average reflectance of approximately 8%, which gave it a mottled gray. Nanoscale pores were etched on the wafer surface by immersing for a few minutes in a liquid solution at room temperature. Then using the process of house liquid phase deposition (LPD), Natcore scientists have completed these pores and are coated with silicon dioxide. This combination, which combines both passive and coating will help to reduce the reflection coefficient. After completion of surface treatment, the slices were taken to the center of photovoltaic research and development at the University of Toledo, where the reflectance was measured.
The last step of Natcore aims to improve the efficiency of solar cells.
The solar cells equipped with antireflection coatings that pass through a process of chemical vapor deposition have a reflectance of approximately 4%. With silicon black, NREL U.S. Department of Energy has lowered the figure below 2%. Natcore technology now allows to reduce this threshold to 0.3%. “The Absolute Black is in the light that absolute zero is heat,” said Dr. Dennis Flood, chief technology officer at Natcore. “And come close to zero with a process of reflection that we can use in a commercial production of solar cells is just incredible.”
Natcore recently obtained an exclusive license from NREL in order to develop and market a range of products based on black silicon.
“We are already working with two manufacturers manufacturers to design a production tool,” said Natcore CEO Chuck Provini. “The tool would be 2,000 wafers per hour black. We will establish other parameters in our laboratory. When the design is complete, we will take control of the tool. We have already started talking with potential customers Italy, China and India.”
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