Material Scientists Unearth the Reasons Behind Higher Efficiency of Perovskites

Recently, there has been a major revolution in the solar cell industry due to the use of perovskites, which are not only easy to synthesize and inexpensive but also highly flexible to use.

When compared to other materials, there has been a rapid increase in the efficacy of perovskites—from less than 4% in 2009 to more than 20% in 2017—in converting light into electricity. Also, a few industrial specialists consider that perovskites can ultimately surpass the efficiency of silicon, the most prevalent material used in solar cells. However, in spite of the attractiveness of perovskites, Scientists are clueless of the reasons behind their higher efficient.

At present, experiments performed at the Department of Energy’s SLAC National Accelerator Laboratory by using a powerful “electron camera” have demonstrated that when irradiated by light, the atoms in perovskites start whirling around. This finding positively elucidates the reason behind the higher efficacy of these futuristic solar cell materials and provides leads for synthesizing more superior materials.

We’ve taken a step toward solving the mystery. We recorded movies that show that certain atoms in a perovskite respond to light within trillionths of a second in a very unusual manner. This may facilitate the transport of electric charges through the material and boost its efficiency.

,” stated Aaron Lindenberg from the Stanford Institute for Materials and Energy Sciences (SIMES) and the Stanford PULSE Institute for ultrafast science

The outcomes of the research have been reported in the journal Science Advances on 26^th July 2017.

Light Sets Atomic Structure in Motion

If a solar cell material is irradiated by sunlight, the energy of light replaces a few negatively charged electrons in the material, leaving “electron holes” that have positive charge in the place in which the electrons were initially positioned. The holes and electrons get relocated to opposite sides of the material, thus generating a voltage that can be used for powering up electrical devices.

The efficiency of a solar cell is largely reliant on the extent to which the holes and electrons can freely move inside the material. Consequently, the ability of the electrons and holes to move is dependent on the atomic structure of the material. For instance, considering solar cells formed of silicon, silicon atoms get ordered in a very well ordered style inside the crystals and even the slightest imperfections in structure can considerably minimize the potential of material to effectively tap solar energy.

Consequently, it is obligatory to develop silicon crystals through expensive, multistage processes under exceptionally hygienic conditions. On the other hand, “Perovskites are readily produced by mixing chemicals into a solvent, which evaporates to leave a very thin film of perovskite material,” stated Xiaoxi Wu, the lead author of study from SIMES at SLAC. “Simpler processing means lower costs. Unlike silicon solar cells, perovskite thin films are also lightweight and flexible and can be easily applied to virtually any surface.”

However, the question remains, what is the reason behind the efficacy of perovskites in efficiently tapping solar energy? According to Researchers, one important reason may be the movements of the atoms in perovskites in response to light.

In order to discover more about them, Wu and her team analyzed these movements in a prototype material formed of lead, iodine and an organic molecule known as methylammonium. The iodine atoms are arranged in octohedral shape, resembling two pyramids united at their bases. While lead atoms are positioned inside each octohedral structure, the methylammonium molecules are located between the octohedral structures. Such an arrangement was observed to be common in the case of various perovskites tested for use in solar cells.

Previous studies have mostly explored the role of the methylammonium ions and their motions in transporting electric charge through the material. However, we’ve discovered that light causes large deformations in the network of lead and iodine atoms that could be crucial for the efficiency of perovskites.

Xiaoxi Wu, the Lead Author of study from SIMES at SLAC

Unusual Distortions May Enhance Efficiency

The Scientists at SLAC’s Accelerator Structure Test Area (ASTA) initially irradiated a perovskite film with a thickness of below two millionths of one inch by using a 40-femtosecond laser pulse, where 1 femtosecond is equal to one-millionth of one-billionth of 1 second. In order to ascertain the atomic reaction, a 300-femtosecond pulse of highly energized electrons was delivered to the material to detect the manner in which the electrons in the film were deviated. Termed ultrafast electron diffraction (UED), this method enabled the Researchers to recreate the atomic structure.

By repeating the experiment with different time delays between the two pulses, we obtained a stop-motion movie of the lead and iodine atoms’ motions after the light hit. The method is similar to taking a series of ultrafast X-ray snapshots, but electrons give us much stronger signals for thin samples and are less destructive.

Xijie Wang, Co-Author of the study as well as SLAC’s Lead Scientist for UED

The Researchers anticipated that the atoms will be evenly impacted by light pulse in all directions, leading them to agitate around their original locations.

“But that’s not what happened,” stated Lindenberg. “Within 10 trillionths of a second after the laser pulse, the iodine atoms rotated around each lead atom as if they were moving on the surface of a sphere with the lead atom at the center, switching each octahedron from a regular shape to a distorted one.”

The fascinating deformations sustained for a long period of time and were startlingly large, identical to deformations detected in melting crystals.

“This motion could alter the way charges move,” stated Wu. “This response to light could enhance efficiency, for instance by allowing electric charges to migrate through defects and protecting them from being trapped in the material.”

Felix Deschler, a specialist in the area of light-induced physics of innovative materials as well as a Researcher at Cambridge University’s Cavendish Lab, who did not take part in the research, stated that “The results from the Lindenberg group provide fascinating first-time insights into the properties of hybrid perovskites using ultrafast electron diffraction as a unique tool.”

Deschler added, “Knowledge about the detailed atomic motion after photoexcitation yields new information about their performance and can provide new guidelines for material development.”

The DOE Office of Science funded the study through SIMES. Other Researchers who contributed toward this study belonged to the University of Pennsylvania, Columbia University, as well as the Weizmann Institute of Science in Israel.