Scientists
at Natcore Technology Inc., using simple liquid bath processes, have
created a black surface on a silicon wafer with an average reflectance
in the visible and near-infrared region of the solar spectrum of 0.3%,
making it the “blackest” silicon solar cell surface ever recorded.
Compared with standard production cells now available, this represents a
tenfold reduction in reflectance over that portion of the spectrum,
which is the source of about 80% of the usable power that can be drawn
from sunlight.
The
black color of black silicon results from the near-total absence of
reflected light from the porous wafer surface. With solar cells,
“blackness” is highly desirable because it indicates that incident light
is being absorbed for conversion to energy rather than being reflected
and thus wasted.
Quantitatively,
reflectance is the proportion of light striking a surface that is
reflected from it. Thus a reflectance of 0.3% means that 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 for
conversion into electrical energy.
A
tenfold reduction in reflectance would mean that up to 3% more usable
light would get into the cell, effectively increasing the cell
efficiency by that amount. (An 18% efficient cell becomes an 18.5% cell,
for example.)
But
there are additional benefits to be derived from black silicon. A panel
made from black silicon solar cells will produce significantly more
energy on a daily basis than will a panel made from cells using the
industry standard antireflective coating. First, because it reflects
less light. Second, because it performs better during the morning and
afternoon hours when the sun hits at an angle. (It also outperforms
standard cell panels on cloudy days.) Its higher energy output, combined
with a lower cost using Natcore’s patented process, could quickly make
black silicon the global solar technology of choice.
Natcore’s
process began with an uncoated, textured silicon wafer that had an
average reflectance of approximately 8%, giving it a mottled gray
appearance. First, nanoscale pores were etched into the wafer surface by
submerging it for a few minutes in a liquid solution at room
temperature. Next, using the company’s liquid phase deposition (LPD)
process, Natcore scientists filled the pores and then over-coated them
with silicon dioxide. This combination step both coated and passivated,
thereby allowing lower reflectance. After the surface treatments were
completed, the wafers were taken to the State of Ohio’s Photovoltaic
Research and Development Center at the University of Toledo, where the
reflectance was measured.
This
is the latest milestone in Natcore’s drive to improve the performance
of solar cells. Conventional cells, with antireflective coatings made
via a chemical vapor deposition process that requires a high-temperature
vacuum furnace and hazardous gases, have a reflectance of about 4%.
With black silicon, the U.S. 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 zero is to heat.” says Dr.
Dennis Flood, Natcore’s Chief Technology Officer. “And getting close to
zero reflectance with a process that we can use for the production of
commercial solar cells is simply astounding.”
Natcore
was recently granted an exclusive license by NREL to develop and
commercialize a line of black silicon products based on NREL patents.
Natcore’s reflectance accomplishment came about as a natural part of its
work associated with that license.
“We
are already working with two equipment manufacturers to design a
production tool,” says Natcore President and CEO Chuck Provini. “The
tool would make 2,000 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 potential
customers in Italy, China and India.”`
“NREL
has a long history of working with companies to move renewable energy
technologies to the market. We’re encouraged and gratified at Natcore’s
success as it relates to our commercial license agreement for NREL’s
black silicon technology,” says William Farris, NREL’s Vice President of
Commercialization & Technology Transfer.
Source: Natcore Technology Inc.
