Quantum dot solar cells could offer a cheaper, more
efficient alternative to conventional photovoltaic materials
Developing
solar cells that are cheaper to produce and can harness the
sun's energy more efficiently are both important factors in
ensuring the widespread use of solar energy to provide a clean
alternative to fossil fuels in the future.
Stanford researchers have found that adding a single layer
of organic molecules can achieve both these goals by increasing
three-fold the efficiency of quantum dot solar cells, which are
cheaper to produce than traditional solar cells.
Quantum dot solar cells
Quantum dot solar cells use tiny particles of semiconductors
– the "quantum dots" – as the photovoltaic material instead of
bulk materials such as silicon, copper indium gallium selenide
or CdTe. While they are cheaper to produce as they can be made
using simple chemical reactions, they have lagged well behind
traditional solar cells in terms of efficiency.
"I wondered if we could use our knowledge of chemistry to
improve their efficiency," said Stacey Bent, a professor of
chemical engineering at Stanford. She realized that is she was
successful, the reduced cost of these solar cells could lead to
mass adoption of the technology.
Bent says that, in principle, quantum dot solar cells should
be able to reach much higher efficiency due to a fundamental
limitation of traditional solar cells.
Solar cells work by using energy from the sun (photons) to
excite electrons, which jump from a lower energy level to a
higher one, leaving a "hole" where the electron used to be.
Solar cells use a semiconductor to pull an electron in one
direction, and another material to pull the hole in the other
direction. It is this flow of electrons that leads to an
electric current.
The amount of minimum energy required to fully separate the
electron and the hole is specific to different materials and
affects what color, or wavelength of light the material best
absorbs. Because the energy required to excite its electrons
corresponds closely to the wavelength of visible light, silicon
is commonly used to make solar cells. Although higher
efficiencies have been achieved with
multi-junction solar cells, those made of a single material
have a maximum efficiency of about 31 percent – a limitation of
the fixed energy level they can absorb.
Size does matter
Because quantum dots don't share this limitation, they can
theoretically be far more efficient. Instead, the energy levels
of electrons in quantum dot semiconductors depends on their size
– the smaller the quantum dot, the larger the amount of energy
needed to excite electrons to the next level.
This allows quantum dots to be tuned to absorb a certain of
wavelength of light just by changing their size. By building
more complex solar cells that have more than one size of quantum
dot, the solar cells can absorb multiple wavelengths of light.
Organic molecule layer
In an attempt to take advantage of these properties, the
Stanford researchers coated a titanium dioxide semiconductor in
their quantum dot solar cell with a very thin single layer of
organic molecules. These self-assembling molecules packed
together in an ordered way, with the quantum dots present at the
interface of this organic layer and the semiconductor.
The researchers tried several different organic molecules in
an attempt to learn which ones would most increase the
efficiency of the solar cells, but found that the exact molecule
didn't matter. Just having a simple organic layer less than a
nanometer thick was enough to triple the efficiency of the solar
cells.
"We were surprised, we thought it would be very sensitive to
what we put down," said Bent.
New theory
But in hindsight, Bent said the result made sense, and the
researchers came up with a new model with the length of the
molecule, and not its exact nature that mattered. Molecules that
are too long don't allow the quantum dots to interact well with
the semiconductor.
Bent's theory is that once the sun's energy creates an
electron and a hole, the thin organic layer helps keep them
apart, preventing them from recombining and being wasted.
Although the group hasn't yet optimized the solar cells,
currently achieving an efficiency of 0.4 percent at most, they
can tune several aspects of the cell and believe, once they do,
the three-fold increase caused by the organic layer would be
even more significant.
Bent says the cadmium sulfide quantum dots she is currently
using are not ideal for solar cells, and the group plans to try
different materials. The group will also try other molecules for
the organic layer, and could change the design of the solar cell
to try and absorb more light and produce more electrical charge.
Once the
Stanford team has found a way to increase the efficiency of
quantum dot solar cells, Bent hopes their lower cost will lead
to wider acceptance of solar energy.
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