Perovskites, the star photovoltaic materials that efficiently convert light to electricity, have another impressive energy-producing capability. Scientists have discovered that light also boosts the material’s ability to convert vibrations into electric currents, an effect called photoflexoelectricity, by more than 10,000% (Nat. Mater. 2020, DOI: 10.1038/s41563-020-0659-y).
“This is the first time that the photoflexoelectric effect has been measured in any semiconductor,” says Gustau Catalán, a physicist at the Catalán Institution for Research and Advanced Studies. The property might not be exclusive to perovskites, he says, and might be found in other photovoltaic materials. This capability could someday yield new types of energy-harvesting devices that produce electricity from light and motion, such as body movements or the wasted mechanical vibrations of a motor.
Flexoelectricity, the property by which materials produce electricity when bent, is typically seen in materials called dielectrics, and the amount of electricity generated is tiny. Researchers have observed light enhancing this property in lipid membranes and liquid crystals, Catalán says, but the enhancement is small, less than 10%.
Catalán, Longlong Shu, a materials scientist at Nanchang University, and their colleagues wanted to look for flexoelectricity in perovskites. They tested two halide perovskites commonly used in solar cells: methylammonium lead bromide and methylammonium lead chloride. The researchers sandwiched the perovskite crystals between two electrodes—the top electrode was transparent—on one end so that they formed a cantilever. The team could vibrate the free end with an actuator and then measure the electric current generated. When they hit the sample with light, the electric current produced jumped by 10,000%.
The current is still small, tens of nanoamperes, but even that is a million times more than the flexoelectric current found in conventional dielectrics. Catalán also points out that the current depends on the frequency of oscillation, which was slow in this experiment, about 10 Hz. But the current should go up with higher frequency oscillations and by increasing the area of the device.
Pradeep Sharma, a mechanical engineer at the University of Houston, says that this advance “is likely to initiate a new field of research.” But several materials-engineering advances are needed to make the technology practical. For starters, it would be important to know how light and mechanical fatigue from the vibrations can degrade perovskites.
Catalán also emphasizes that this is a first-of-its-kind exploration of a physics phenomenon. Translating the findings into useful technology is still a long way off. He and his colleagues are now trying to determine whether other materials might be better photoflexoelectrics. “We hope engineers will take a shot at making actual devices out of this,” he says.