There are two types of technologies that
can significantly reduce the energy consumption of buildings and vehicles: smart
glass and semi-transparent solar cells. The former is a glass that can allow or
block light transmission depending on voltage, light, or temperature being
able to make the indoor climate warmer in the winter and cooler in the summer.
The latter is a special kind of photovoltaic cell that maximizes the transmission
of visible light, while converting the invisible ultraviolet (UV) and
near-infrared (NIR) light spectrum into electricity.
Traditionally, they are two completely
different technologies which can both, but not at the same time, be used to
make windows. But Letian Dou found a way to combine the merits of the two and
created a next-generation energy product: the "smart semi-transparent solar
cell".
In this revolutionary device, the solar
cell is not only semi-transparent, but also "smart" in that it changes color
between a transparent state and the dark state and reversibly under certain
external stimuli. In the transparent state, the device is similar to conventional
glass, allowing light to pass through and heat up the room; when in need, the
device can be switched into the dark state, where unwanted light spectrums are
not only blocked but also converted into power.
With traditional semiconducting materials,
this is almost impossible. Atoms in conventional semiconductors are held
together by strong covalent bonds. Once the bonds break, they cannot reverse
back to the original form without introducing new defects. In smart
semi-transparent solar cells, however, transitions between the transparent state and the
dark state should be completely reversible without degrading the material's
electronic properties.
The breakthrough was achieved by
incorporating a new material called "halide perovskite" into the
semi-transparent solar cell design. Unlike conventional semiconductors, atoms
in halide perovskites are held together by ionic bonds, which make the
crystal lattice "soft" and "dynamic". Reversible switching of the device state
is achieved by the structural phase transition of halide perovskites without breaking any "irreversible" covalent bonds. As the first product of the
kind, the power conversion efficiency of this new type of solar cell is around
5 ~ 7%. This work has attracted tremendous attention from both academia and
industry.
