Power/Performance Bits: March 9

Healing perovskites
Researchers at Brown University found that while perovskite solar cells can crack easily, they are also capable of healing those cracks.

“The efficiency of perovskite solar cells has grown very quickly and now rivals silicon in laboratory cells,” said Nitin Padture, a professor in Brown’s School of Engineering and director of Brown’s Institute for Molecular and Nanoscale Innovation. “Everybody’s chasing high efficiency, which is important, but we also need to be thinking about things like long-term durability and mechanical reliability if we’re going to bring this solar cell technology to the market. That’s what this research was about.”

While perovskite solar cells have shown promising efficiency gains and can be manufactured more cheaply than traditional silicon cells, they are also brittle. This poses a difficultly for one of perovskite cells’ primary attractions, that it can be manufactured as a thin film that is flexible and semi-transparent.

“In material science, things that are easy to make also tend to be easy to break,” said Padture. “That’s certainly true of perovskites, which are quite brittle. But here we show they’re also quite easy to fix — cracks in perovskite films can be healed by compressing them or with moderate heat.”

A cracked perovskite film (left) can be fully healed (right) with some compression or a little heat. Credit: Padture Lab / Brown University

The researchers deposited perovskite films on plastic substrates. They then bent the substrate to put tensile stress on the perovskite film while using a scanning electron microscope (SEM) to detect cracks. Once the film was cracked, the researchers then bent the substrate in the opposite direction to see if compressive stress might heal those cracks.

SEM imaging showed that the cracks had disappeared. To make sure the cracks were fully healed and not merely hidden, the researchers used X-ray diffraction. By measuring the size of a material’s atomic lattice, the technique can reveal whether a formerly cracked area is now able to carry a mechanical load. Those tests also indicated fully healed cracks.

The researchers found that heat was just as effective in healing cracks. Temperatures around 100 degrees Celsius were enough to completely heal cracks in perovskite films.

Padture says that the research was aimed at better understanding the basic properties of perovskite materials, and more work needs be done to develop methods of applying this information in a commercial setting. But knowing that perovskite films are easily healed could be useful as these kinds of solar cells move toward commercialization.

Generating electricity from droplets
Researchers from Nagoya University and Kyushu University built a nanogenerator capable of supplying up to 5 volts of electricity from the movement of liquid across a molybdenum disulfide (MoS2) film.

The device generates electricity when drops of water slide down on its upper surface. This technology is expected to be applied to self-powered devices used in liquids, including sensors monitoring the quality of wastewater from factories.

“To use MoS2 for the generator, it was necessary to form a large-area single-layer MoS2 film on a plastic film. With conventional methods, however, it was difficult to grow MoS2 uniformly on a large-area substrate,” said Yutaka Ohno, a professor at the Institute of Materials and Systems for Sustainability at Nagoya University. “In our study, we succeeded in fabricating this form of MoS2 film by means of chemical vapor deposition using a sapphire substrate with molybdenum oxide (MoO3) and sulphur powders. We also used a polystyrene film as a bearing material for the MoS2 film, so that we were able to transfer the synthesized MoS2 film to the surface of the plastic film quite easily.”

The new generator is flexible enough to be installed on the curved inner surface of plumbing, and the team said it could be used to power IoT devices used in liquids, such as self-powered rain gauges and acid rain monitors, as well as water quality sensors that can generate power from industrial wastewater while monitoring it.

“Our MoS2 nanogenerator is able to harvest energy from multiple forms of liquid motion, including droplets, spraying, and sea waves,” said Ohno. “From a broader perspective, this device could also be used in applications involving hydrodynamics, such as generating electricity from rainwater and waterfalls.”