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.