NREL's Black Silicon Increases Solar Cell Efficiency by Reducing Reflected Sunlight (Fact Sheet) Page: 1 of 2
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An inexpensive, one-step nanocatalytic wet-chemical
etch makes high-efficiency solar cells2010o
-
Photo of the entire eye of a moth, overlaying a scanning elec-
tron micrograph of a portion of the lens of a moth's eye, where
each bump is only several hundred nanometers in diameter.
NREL's black silicon etch creates nanostructures akin to those
of the moth's eye to drastically reduce light reflection.If you want an efficient solar cell, you want the cell to
absorb as much of the light shining on it as possible.
Less reflection means more light absorption, and
hence, higher efficiency and more electricity.
The makers of solar cells have devised ways to boost
the amount of light absorbed-including a micro-
meter-scale textured surface and a thin layer of a
material with optical properties different than the
underlying solar energy converter. Unfortunately,
the equipment and processes for applying these
conventional antireflection layers add cost to the
solar cell. But even when both techniques are used,
cells still only absorb between 93% and 97% of the
sunlight.
To further reduce reflected sunlight and increase cell
efficiency at lower cost, NREL scientists invented the
Black Silicon Nanocatalytic Wet-Chemical Etch. This
antireflection etch process turns silicon wafers-the
most common solar cell material-black because they
absorb more than 98% of the light shining on them.
And NREL has validated its black silicon cells at 16.8%
conversion efficiency.
So How Does It Work?
The NREL team created an inexpensive liquid etch
that creates gold nanoparticles that immediately
begin to catalyze the etch to produce a nanometer-
scale porous surface on the cell wafer's surface. The
porous features must be much smaller than the
wavelength of the incident light; therefore, nanoscale
features are needed to suppress reflection of the
entire spectrum of sunlight.,Ah~
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Schematic of a graded-density surface zone of thickness "d"
that suppresses light reflection. The expansion shows that
the wavelength of light in the non-porous silicon at depth is
about one-third that in air. The graded zone shows the transi-
tion of wavelength between these two extremes without
any reflection because of the gradual change in density and
refractive index ("n") with depth. The wafer appears black
because all light is being absorbed.This nanoporosity causes a gradual change in silicon
density with depth, with a related change in its
refractive index-a measure of how much the velocity
of a light wave is reduced within the material. The
important result is that light entering the cell does
not encounter any sharp interfaces that would reflect
the ray out of the cell and waste energy that could
have produced electricity.
The well-known "frog in a kettle" story provides a
simple analogy to the response of the incoming light
to the nanoporosity created by NREL's black silicon
etch. The frog starts out in a kettle of cold water,
but does not perceive that the water is gradually
being heated until it begins to boil and it's too late.
In the happier black silicon story, light starts out
travelling through low-refractive-index air, but does
not "perceive" that the index is gradually increasing
until it finally reaches that of the non-porous silicon at
depth. Just as the frog would have jumped out of the
kettle if it had felt the water getting too hot, so also
the light would have been reflected out of the wafer if
it had encountered a significant contrast in refractive
index. The change in index with depth was so small as
to be imperceptible to the light-yet the light went
from one medium to another with a much different
property without detecting an interface and being
reflected out of the silicon.
Black Silicon (graded density)
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NREL's Black Silicon Increases Solar Cell Efficiency by Reducing Reflected Sunlight (Fact Sheet), report, November 1, 2010; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc835326/m1/1/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.