The prototype. Credit: University
College London
Semi-transparent solar cells that
could be added to windows to efficiently harvest energy from indoor light as
well as the sun have been developed by an international team led by UCL
researchers.
The technology, described in a paper in Advanced Energy Materials,
could help turn buildings into power generators at night and on cloudy days, as
well as in sunshine.
The researchers engineered solar
windows that let in 30% of sunlight—ordinary glass might let in 80% or
90%—while generating a record amount of energy from indoor light and
efficiently harvesting energy from sunshine.
Senior author Dr. Mojtaba
Abdi-Jalebi, at the UCL Institute for Materials Discovery, said, "Rooftops
are commonly fitted with solar panels but the vast window areas of many modern
buildings remain largely untapped as an energy resource.
"In our study, we showed it is
possible to keep the window transparent so it can let light through while
maintaining the solar cells' efficiency.
"The next step is to engineer
flexible solar cells that can be applied to curved structures like windows on
the Shard or on cars as well as non-rigid surfaces such as clothes or
backpacks. We would also like to build solar cells over larger scales than we
achieved in this study.
"The longer-term vision is to
make semi-transparent photovoltaics as easy to integrate as a window film. As
the technology matures, these devices could potentially be developed into
flexible films that can be applied directly onto vehicle glass, sunroofs and
other transparent surfaces to generate clean electricity without major
structural changes."
Lead author Siming Huang, a Ph.D.
student at UCL's Institute for Materials Discovery, said, "Another
advantage of this technology is that by obscuring some of the sunlight it acts
in the same way as tinted windows, saving a portion of the energy required to
keep the building cool. This is especially important in hotter areas of the
world that use a high proportion of energy on air conditioning."
Designing for light and transparency
The team used a material called perovskite, which is increasingly used in outdoor solar panels
and, unlike traditional silicon-based solar panels, has the potential to
generate energy from indoor light because its composition can be adjusted to
better absorb indoor light's specific wavelengths.
They used computer modeling to
determine the best arrangement and thickness of layers in the solar cell for
preserving both transparency and efficiency.
Based on this modeling, they
engineered a perovskite layer—the layer that absorbs light—that was 185
nanometers thick (about 500 times thinner than a human hair). In typical solar
cells, the layer of perovskite is three or four times thicker.
They added a molecule (3-trifluoromethyl-1H-1,2,4-triazole) that reduced defects in the perovskite known as
"traps," which can cause electrons to get stuck before their energy
can be harnessed. This molecule also helped stabilize the perovskite crystal
structure, preventing degradation over time.
A transparent electrode breakthrough
In addition, the researchers
engineered a transparent electrode. In perovskite solar cells, the electrode
(i.e. the part that conducts electricity out of the cell) is typically made of
gold and blocks light. To make their electrode more transparent, the team
sandwiched a thin layer of gold between two transparent layers of molybdenum
oxide, which helped light pass through the gold by reducing reflection.
The team fabricated a 30cm by 30cm
panel and found the solar cells could convert 22% of bright indoor light (1,000
lux) into electricity, as well as 14% of sunlight. They found the bare device
retained 80% of its efficiency over 300 hours of continuous exposure to light
under a standard accelerated durability test, showing it was robust over time.
Provided by University College London
Source: Solar windows could harvest indoor light and sunlight while staying semi-transparent
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