PARIS -- Scientists at Natcore Technology using simpleliquid bath processes said they have created a black surface on a silicon waferwith an average reflectance in the visible and near-infrared region of thesolar spectrum of 0.3%.
This makes it the "blackest" silicon solar cellsurface ever recorded. Compared with standard production cells now available,this represents a tenfold reduction in reflectance over that portion of thespectrum, which is the source of about 80% of the usable power that can bedrawn from sunlight.
The black color of black silicon results from the near-totalabsence of reflected light from the porous wafer surface. With solar cells,"blackness" is highly desirable because it indicates that incident lightis being absorbed for conversion to energy rather than being reflected and thuswasted.
Quantitatively, reflectance is the proportion of lightstriking a surface that is reflected from it. Thus a reflectance of 0.3% meansthat only 0.3% of incident light is reflected from the solar cell's surface,while 99.7% of incident light is absorbed by the cell and is available forconversion into electrical energy. A tenfold reduction in reflectance wouldmean that up to 3% more usable light would get into the cell, effectivelyincreasing the cell efficiency by that amount.
Uncoated silicon wafer (left), an "absolute black" siliconwafer (middle), and a wafer with a standard antireflective coating.But there are additional benefits to be derived from blacksilicon. A panel made from black silicon solar cells will produce significantlymore energy on a daily basis than will a panel made from cells using theindustry standard antireflective coating. First, because it reflects less light.Second, because it performs better during the morning and afternoon hours whenthe sun hits at an angle. (It also outperforms standard cell panels on cloudydays.) Its higher energy output, combined with a lower cost using Natcore'spatented process, could quickly make black silicon the global solar technologyof choice.
Natcore's process began with an uncoated, textured siliconwafer that had an average reflectance of approximately 8%, giving it a mottledgray appearance. First, nanoscale pores were etched into the wafer surface bysubmerging it for a few minutes in a liquid solution at room temperature. Next,using the company's liquid phase deposition (LPD) process, Natcore scientistsfilled the pores and then over-coated them with silicon dioxide. Thiscombination step both coated and passivated, thereby allowing lowerreflectance. After the surface treatments were completed, the wafers were takento the State of Ohio's Photovoltaic Research and Development Center at theUniversity of Toledo, where the reflectance was measured.
Conventional cells, with antireflective coatings made via achemical vapor deposition process that requires a high-temperature vacuumfurnace and hazardous gases, have a reflectance of about 4%. With black silicon,the US Department of Energy's National Renewable Energy Laboratory (NREL)lowered the number below 2%. Now Natcore's technology has reduced it to 0.3%,or virtually zero.
"Absolute black is to reflected light as absolute zerois to heat," says Dr. Dennis Flood, Natcore's Chief Technology Officer."And getting close to zero reflectance with a process that we can use forthe production of commercial solar cells is simply astounding."
Natcore was recently granted an exclusive license by NREL todevelop and commercialize a line of black silicon products based on NRELpatents. Natcore's reflectance accomplishment came about as a natural part ofits work associated with that license.
"We are already working with two equipmentmanufacturers to design a production tool," says Natcore President and CEOChuck Provini. "The tool would make 2000 black silicon wafers per hour.We'll establish other parameters in our lab. When the design is completed,we'll take orders for the tool. We have already begun talking with potentialcustomers in Italy, China and India."
Visit Natcore Technology here.
Courtesy of EETimes and EETimes Europe
