Graduate student Miles Barr holds a flexible and foldable array of solar cells that have been printed on a sheet of paper. Photo: Patrick Gillooly |
The
sheet of paper looks like any other document that might have just come spitting
out of an office printer, with an array of colored rectangles printed over much
of its surface. But then a researcher picks it up, clips a couple of wires to
one end, and shines a light on the paper. Instantly an LCD clock display at the
other end of the wires starts to display the time.
Almost
as cheaply and easily as printing a photo on your inkjet, an inexpensive,
simple solar cell has been created on that flimsy sheet, formed from special “inks” deposited on the paper. You can even fold it up to slip into a pocket,
then unfold it and watch it generating electricity again in the sunlight.
The
new technology, developed by a team of researchers at Massachusetts Institute
of Technology (MIT), is reported in a paper in the journal Advanced Materials.
The paper is co-authored by Karen Gleason, Vladimir Bulovi?, Miles Barr; and six
other students and postdocs.
The
technique represents a major departure from the systems used until now to
create most solar cells, which require exposing the substrates to potentially
damaging conditions, either in the form of liquids or high temperatures. The
new printing process uses vapors, not liquids, and temperatures less than 120 C.
These “gentle” conditions make it possible to use ordinary untreated paper,
cloth, or plastic as the substrate on which the solar cells can be printed.
It
is, to be sure, a bit more complex than just printing out a term paper. In
order to create an array of photovoltaic cells on the paper, five layers of
material need to be deposited onto the same sheet of paper in successive
passes, using a mask (also made of paper) to form the patterns of cells on the
surface. And the process has to take place in a vacuum chamber.
Barr places a sheet of paper with a mask on it into the vapor printing chamber. Photo: Patrick Gillooly |
The
basic process is essentially the same as the one used to make the silvery
lining in your bag of potato chips: a vapor deposition process that can be
carried out inexpensively on a vast commercial scale.
The
resilient solar cells still function even when folded up into a paper airplane.
In their paper, the MIT researchers also describe printing a solar cell on a
sheet of PET plastic and then folding and unfolding it 1,000 times, with no
significant loss of performance. By contrast, a commercially produced solar
cell on the same material failed after a single folding.
“We
have demonstrated quite thoroughly the robustness of this technology,” Bulovi?
says. In addition, because of the low weight of the paper or plastic substrate
compared to conventional glass or other materials, “we think we can fabricate
scalable solar cells that can reach record-high watts-per-kilogram performance.
For solar cells with such properties, a number of technological applications
open up,” he says. For example, in remote developing-world locations, weight
makes a big difference in how many cells could be delivered in a given load.
Gleason
adds, “Often people talk about deposition on a flexible device—but then they
don’t flex it, to actually demonstrate” that it can survive the stress. In this
case, in addition to the folding tests, the MIT team tried other tests of the
device’s robustness. For example, she says, they took a finished paper solar
cell and ran it through a laser printer—printing on top of the photovoltaic
surface, subjecting it to the high temperature of the toner-fusing step—and
demonstrated that it still worked. Test cells the group produced last year
still work, demonstrating their long shelf life.
In
today’s conventional solar cells, the costs of the inactive components are
typically greater than the cost of the active films of the cells themselves,
sometimes twice as much. Being able to print solar cells directly onto
inexpensive, easily available materials such as paper or cloth, and then easily
fasten that paper to a wall for support, could ultimately make it possible to
drastically reduce the costs of solar installations. For example, paper solar
cells could be made into window shades or wallpaper—and paper costs
one-thousandth as much as glass for a given area, the researchers say.
For
outdoor uses, the researchers demonstrated that the paper could be coated with
standard lamination materials, to protect it from the elements.
Others
have tried to produce solar cells and other electronic components on paper, but
the big stumbling block has been paper’s rough, fibrous surface at a
microscopic scale. To counter that, past attempts have relied on coating the
paper first with some smooth material. But in this research, ordinary, uncoated
paper was used—including printer paper, tissue, tracing paper, and even
newsprint with the printing still on it. All of these worked just fine.
The
researchers continue to work on improving the devices. At present, the
paper-printed solar cells have an efficiency of about 1%, but the team believes
this can be increased significantly with further fine-tuning of the materials.
But even at the present level, “it’s good enough to power a small electric
gizmo,” Bulovi? says.
