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A technology that would enable low-cost, high-efficiency
solar cells to be made from virtually any semiconductor material has been
developed by researchers with the U.S. Department of Energy (DOE)’s Lawrence
Berkeley National Laboratory (Berkeley Lab) and the University of California
(UC) Berkeley. This technology opens the door to the use of plentiful,
relatively inexpensive semiconductors, such as the promising metal oxides,
sulfides, and phosphides, that have been considered unsuitable for solar cells
because it is so difficult to tailor their properties by chemical means.
“It’s time we put bad materials to good use,” says
physicist Alex Zettl, who led this research along with colleague Feng Wang. “Our
technology allows us to sidestep the difficulty in chemically tailoring many
earth abundant, non-toxic semiconductors and instead tailor these materials
simply by applying an electric field.”
Zettl, who holds joint appointments with Berkeley
Lab’s Materials Sciences Division and UC Berkeley’s Physics Department where he
directs the Center of Integrated Nanomechanical Systems (COINS), is the
corresponding author of a paper describing this work in Nano Letters. The
paper is titled “Screening-Engineered Field-Effect Solar Cells.” Co-authoring
it were William Regan, Steven Byrnes, Will Gannett, Onur Ergen, Oscar
Vazquez-Mena, and Feng Wang.
Solar cells convert sunlight into electricity using
semiconductor materials that exhibit the photovoltaic effect—meaning they
absorb photons and release electrons that can be channeled into an electrical
current. Photovoltaics are the ultimate source of clean, green and renewable
energy but today’s technologies utilize relatively scarce and expensive
semiconductors, such as large crystals of silicon, or thin films of cadmium
telluride or copper indium gallium selenide, that are tricky or expensive to
fabricate into devices.
“Solar technologies today face a cost-to-efficiency
trade-off that has slowed widespread implementation,” Zettl says. “Our
technology reduces the cost and complexity of fabricating solar cells and
thereby provides what could be an important cost-effective and environmentally
friendly alternative that would accelerate the usage of solar energy.”
This new technology is called “screening-engineered
field-effect photovoltaics,” or SFPV, because it uses the electric field
effect, a well understood phenomenon by which the concentration of charge-carriers
in a semiconductor is altered by the application of an electric field. With the
SFPV technology, a carefully designed partially screening top electrode lets
the gate electric field sufficiently penetrate the electrode and more uniformly
modulate the semiconductor carrier concentration and type to induce a p-n
junction. This enables the creation of high quality p-n junctions in
semiconductors that are difficult if not impossible to dope by conventional
chemical methods.
“Our technology requires only electrode and gate
deposition, without the need for high-temperature chemical doping, ion
implantation, or other expensive or damaging processes,” says lead author
William Regan. “The key to our success is the minimal screening of the gate
field which is achieved through geometric structuring of the top electrode.
This makes it possible for electrical contact to and carrier modulation of the
semiconductor to be performed simultaneously.”
Under the SFPV system, the architecture of the top
electrode is structured so that at least one of the electrode’s dimensions is
confined. In one configuration, working with copper oxide, the Berkeley
researchers shaped the electrode contact into narrow fingers; in another
configuration, working with silicon, they made the top contact ultra-thin
(single layer graphene) across the surface. With sufficiently narrow fingers,
the gate field creates a low electrical resistance inversion layer between the
fingers and a potential barrier beneath them. A uniformly thin top contact
allows gate fields to penetrate and deplete/invert the underlying
semiconductor. The results in both configurations are high quality p-n
junctions.
Says co-author Feng Wang, “Our demonstrations show
that a stable, electrically contacted p-n junction can be achieved with nearly
any semiconductor and any electrode material through the application of a gate
field provided that the electrode is appropriately geometrically
structured.”
The researchers also demonstrated the SFPV effect
in a self-gating configuration, in which the gate was powered internally by the
electrical activity of the cell itself.
“The self-gating configuration eliminates the need
for an external gate power source, which will simplify the practical
implementation of SFPV devices,” Regan says. “Additionally, the gate can serve
a dual role as an antireflection coating, a feature already common and
necessary for high efficiency photovoltaics.”
