A Temporary Melting Liquid Could Help Build Better Perovskite Solar Cells and LEDs

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• Liezel Labios - [email protected]

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• Perovskites
• Solar Cells

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A team co-led by researchers at the University of California San Diego has developed a new way to make perovskite solar cells - a promising next-generation solar technology - more efficient and durable. By creating a temporary liquid while the material crystallizes, researchers were able to grow perovskite crystals that were larger, more uniform and contained fewer defects.

Solar cells, the building block of large solar panels, and light-emitting diodes made with these perovskites achieved efficiencies ranging from about 24% to 26%, which are on the high end for perovskite-based devices. The solar cells also retained most of their original efficiency after continuously operating for eight weeks under simulated sunlight.

The findings were published on Oct. 8 in Science.

Perovskites are a class of materials that have the potential to enable low-cost, high-efficiency solar cells. Unlike conventional silicon solar cells, perovskites can be processed from solutions and deposited as thin films, which could also enable simpler, lower-energy manufacturing.

But making a high-quality perovskite solar cell is challenging. Perovskite films are typically made up of many tiny crystals packed together. The interfaces between these crystals, known as grain boundaries, can contain defects that cause some of the energy captured from sunlight to be lost. These defects can become an even bigger problem when trying to make larger solar panels, where efficiency drops as the device gets bigger.

The team developed a way to grow perovskites with fewer of these defects. UC San Diego researchers led by David Fenning, professor and the Francine Berman Endowed Chair in the Aiiso Yufeng Li Family Department of Chemical and Nano Engineering at the UC San Diego Jacobs School of Engineering, collaborated with researchers at Princeton University and Sungkyunkwan University, Republic of Korea.

Their study focused on a type of material known as a halide perovskite. They mixed two additional salts - zinc bromide and methylammonium chloride - into the solution used to make the perovskite. As the material was heated, the two additives formed a low-melting mixture called a eutectic.

The eutectic served as a temporary liquid around the boundaries between growing crystals, which gave the crystals room to rearrange and grow into a more orderly structure. As the material was continually heated, one of the additive ingredients, methylammonium chloride, evaporated. Zinc remained concentrated around the crystal boundaries, where it helped neutralize electronic defects.

"We introduce a new way of tuning the growth of halide perovskites by developing an additive mixture that actively evolves during crystallization," said study co-first author Connor Dolan, a chemical and nano engineering PhD alumnus from Fenning's lab. "What's even cooler is that the end product removes performance-limiting defects at the edges of the crystalline grains."

The researchers used advanced X-ray imaging techniques to watch this process unfold at extremely small scales. They directly observed evidence that the material around crystal boundaries becomes disordered and liquid-like during heating, then becomes ordered again at the same temperature as the extra salt evaporates and crystal growth finishes. They also found that the treatment produces larger, better-ordered crystals with fewer structural defects and a more uniform distribution of the material's chemical properties. The researchers hope to generalize this curious solid-liquid-solid growth mechanism since it all occurs at the same temperature.

These improvements translated into better-performing devices.

In a solar cell, energy from sunlight excites electrons and leaves behind "holes" in the material. How long these "electron-hole" pairs last before recombining is a key indicator of how well the perovskite performs, Dolan explained. "Perovskites made with the eutectic-forming additives had the longest lasting electron-hole pairs ever reported for any direct-bandgap semiconductor, which speaks to the promise of this approach," he said.

The researchers also found that this approach worked across different perovskite chemistries they tested.

"It seemed like everything we tried worked!" Dolan said. "I'm hopeful that this mechanism has the chance to be really generalizable and provide a platform for further innovation in the perovskite community."

The researchers are now exploring additional eutectic chemistries to better understand and control the crystallization process. The work could ultimately provide a new strategy for producing higher-quality perovskite materials and help advance the development of efficient, scalable solar technologies.

Full study: "Volatile eutectics to tailor crystallization for perovskite optoelectronics"

This work was supported by the U.S. Department of Energy; the U.S. National Science Foundation; startup funds from Princeton University; the National Research Foundation of Korea; and the Korea Research Institute of Chemical Technology.