Marie Freebody, Contributing Editor, email@example.com
The quest for high
power-conversion efficiency in most thin-film solar cells has often been hindered
by the “thick and thin” challenge, in which a cell must be thick enough
to collect a sufficient amount of light, yet thin enough to extract current. Now
a novel solar cell devised by physicists at Boston College in Chestnut Hill and
inspired by the coaxial cable resolves this dilemma by being both optically thick
and electrically thin.
This is an electron microscope image of a nanocoax solar cell array comprising 2-μm-tall
metal nanopillars conformally coated with 100 nm of amorphous silicon and 70 nm
of indium tin oxide. The nanocoaxes in the array are spaced 900 nm apart (see scale
bar). Images courtesy of Boston College.
“The challenge has to do with the desire to make a highly
optically absorbing cell that is simultaneously highly efficient at extracting electric
charge as current,” said one of the creators, Michael Naughton, a professor
of physics at the college. “In simple terms, a thick cell achieves the former,
while a thin cell, the latter. Our nanocoax design achieves both, which has intrinsic
advantages in terms of cost, so long as good efficiency can be achieved.”
The so-called nanocoax solar cell boasts greater efficiency than
any previously designed nanotech thin-film (noncrystalline) solar cell. As reported
in the July 2010 issue of Physica Status Solidi, the nanocoax cells, made with
amorphous silicon, yield power-conversion efficiency in excess of 8 percent, and
similar cells yielded efficiencies of more than 9.6 percent.
A nanocoax solar cell array is shown at different magnifications. On the left is a completed
cell array, while the middle image illustrates how a focused-ion beam is used to
mill out a section of the array, exposing the constituent components. On the right
is an expanded section that has been milled by a focused ion beam.
Naughton also reports that, since publication, his team –
in collaboration with Solasta Inc. of Newton, Mass., and the Neuchatel Institute
of Microengineering at the EPFL in Neuchatel, Switzerland – has achieved even
greater initial efficiency of more than 10.5 percent, as certified by the National
Renewable Energy Laboratory.
Many solar cells are based on crystalline semiconductors due to
their superior energy-conversion efficiency compared with noncrystalline thin-film
cells. The problem is that the dominant material (crystalline silicon) must be relatively
thick to collect light. On the other hand, noncrystalline materials such as amorphous
silicon are strongly absorbing but less efficient electrical conductors.
To overcome the “thick and thin” barrier, Naughton
and his colleagues started by separating the optics from the electronics by forming
an array of vertically oriented nanocoaxes. In each nanoscale coaxial wire in the
array, an inner and outer metal surround a dielectric medium, just as in a conventional
coaxial wire. The difference here is that the dielectric is a photovoltaic material
such as silicon. “In such a wire, light collection is governed by the height
(length) of the nanocoax, while charge (electron and hole) extraction is governed
by the thinness of the photovoltaic in the coax annuli, which can be quite thin
and still absorb light,” he said.
The result is a geometry that separates the “photo”
from the “voltaic” and enables high energy-conversion efficiency using
thinner, less expensive thin-film materials.
“The nanocoax configuration achieves efficiency more than
50 percent higher than a conventionally prepared ‘planar’ thin-film
silicon cell,” Naughton said.
Solar cells of this type (amorphous silicon) face a major problem
known as the Staebler-Wronski effect, in which light-induced degradation damages
the conversion efficiency of the cell. The team says the ultrathin nature of the
nanocoax cell reduces this destructive effect because the nanocoax architecture
spatially distributes incident light in such a way that the local intensity per
unit volume is less than in a planar cell.
The nanocoax concepts and the nanocoax solar cell array were
invented by professors Krzysztof Kempa, Michael Naughton and Zhifeng Ren, and by
Drs. Yang Wang and Jakub Rybczynski, all in the department of physics at Boston