Solar materials capable of repairing themselves

Halide perovskites - the great promise of the solar energy field - are able to heal themselves

Microcrystals formed through self-healing repaired a completely destroyed area within a halide perovskite crystal. Courtesy of the Weizmann Institute.

Smart materials can move humanity into an era of renewable energy, and hence into a self-sustaining world. And what is smarter than materials that repair themselves? Weizmann Institute scientists have shown that the so-called substances The halide perovskites - the great promise of the solar energy field - they know, no less, how to heal themselves. The findings, which were published recently in the scientific journal Advanced materials, may not only promote the commercial use of halide perovskites, but also help identify additional materials that heal themselves, and may be used to manufacture various electronic devices.

Scientists have so far struggled to understand the extraordinary performance of halide perovskites - soft semiconductors that can be easily produced at room temperature by mixing and drying two salt solutions. The production process is so simple and fast that it is also expected to give rise to many defects. However, the hallide perovskites convert solar energy into electricity so efficiently, it's almost as if they have no defects. One of the possible explanations for the phenomenon is that the material knows how to repair the defects by itself.

Self-healing in a group of solar materials from the previous generation was discovered about two decades ago by Prof. David Kahn From the Department of Materials and Surfaces together with Lior Kronik, then a post-doctoral researcher, and now a professor and head of the same department, and other colleagues. The discovery led to the hypothesis that the property also exists in other materials. "When we tried to explain the high efficiency of the halide perovskites, we returned to this hypothesis and assumed that while the material was formed and afterwards, it continues to repair the defects within it," explains Prof. Kahn.

>> Accelerated documentation showing that a defect (dark spot) inside a halide perovskite nanocrystal disappeared after the material healed itself

In order to test the hypothesis, the scientists had to deal with a particularly challenging task: to isolate the properties of the material from environmental disturbances by creating a defect that is only inside the material, i.e. not on its surface. The postdoctoral researcher, Dr. David Raphael Sarati, who works with Kahn and Prof. Gerry Hoods from the same department, proposed an original solution: to choose a halide perovskite which absorbs blue photons (light particles) but red light can pass through it without being absorbed. In the experiment, the scientists focused a red laser on a point with a diameter of one micron and a depth of about 100 microns inside a crystal of the material; For this, they used a method called two-photon absorption: if two infrared photons are simultaneously absorbed by the material, this causes the material to behave as if it had swallowed one photon with double energy, i.e. a blue photon. Dr. Sarati explains: "We directed the laser so that we created a blue light at the focus point, the intensity of which is approximately that of sunlight. Because blue light is absorbed by the material, it can cause damage inside the crystal, around its focus point, but not outside that point."

After that, the scientists illuminated the crystal - which emits light after absorbing light - and tracked the defects created in it with the help of a particularly sensitive optical detector. The crystal emitted light as is typical of this material, except in the damaged area which remained darker. But after a while - several minutes to several hours - the dark spot disappeared, and the entire crystal returned to light. The scientists examined the crystal under an electron microscope and discovered that its structure was identical to its original state.

"It is possible that solar cells made of hallide perovskites will heal themselves when it gets dark, thus repairing the damage done to them during the day"

1. A crystal of the halide perovskite under a microscope before and after the creation of the defect: A - normal state, B - damaged state, C - healing, D - after self-healing. Measuring bar: 10 microns. Courtesy of the Weizmann Institute. — 1. A crystal of the halide perovskite under a microscope before and after the creation of the defect: A – normal state, B – damaged state, C – healing, D – after self-healing. Measuring bar: 10 microns. Courtesy of the Weizmann Institute.

Surprisingly, even when the defects were created using blue light with an intensity hundreds of times greater than that of sunlight, and an entire area within the crystal was destroyed, the material still healed itself - and this time with the help of creating a new layer of micro-crystals in place of the single-crystal area that was destroyed. The scientists assumed that the laser beams caused some of the damaged material to break down into its components, which reacted with each other to form the original material. That is, it occurred in the area of recycling, or self-healing. Laboratory chemical experiments supported this explanation.

"Crystals of the hallide perovskites are formed through relatively weak atomic bonds, so it is easy to break them down, but it is also relatively easy to restore them," explains Prof. Kahn.

From the right: Prof. Dan Oron, Ron Tana, Dr. Vyacheslav Kalchenko, Dr. David Raphael Sarati, Yevgeny Rakita, Prof. Michael Albaum and Prof. David Kahn. emit light. Courtesy of the Weizmann Institute.

Self-healing may be particularly beneficial in solar cells, which gradually wear out with use. "It is possible that solar cells made of hallide perovskites will heal themselves when it gets dark, and thus repair the damage caused to them during the day," suggests Prof. Kahn. The research findings may help make these promising materials more durable for the production of solar cells or essential components in optical and electronic devices. An equally important thing: the findings may advance the search for additional materials that heal themselves for the production of sustainable energy and for other uses.

The research group also included Prof. Michael Albaum from the Department of Chemical and Biological Physics and Prof. Dan OronFrom the Department of Physics of Complex Systems, the research students Yevgeny Rakita and Ron Tana and the faculty scientist Dr. Vyacheslav Kalchenko, as well as scientists from the University of Milan in Italy: Lorenz Carmonzi and Prof. Marco Potenza.

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A solar cell made of halide perovskites produces about 10¹⁷Electrons per second for every square centimeter directed to the sun (20 milliamperes). For comparison, an iPhone device needs an electric current 50 times stronger (1 ampere) during charging.

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