Making perovskite solar cells more durable
A team co-led by researchers at the University of California San Diego has developed a new way to make perovskite solar cells more efficient and durable.
By creating a temporary liquid while the material crystallises, researchers were able to grow perovskite crystals that were larger, more uniform and contained fewer defects. The findings 'Volatile eutectics to tailor crystallisation for perovskite optoelectronics' were published on Oct. 8 in Science.
The grain boudnaries between the many tiny crystals in peroviskits 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 at the UC San Diego Jacobs School of Engineering, collaborated with researchers at Princeton University and Sungkyunkwan University, Korea.
Their study focused on 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 neutralise electronic defects.
“We introduce a new way of tuning the growth of halide perovskites by developing an additive mixture that actively evolves during crystallisation,” 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 generalise this curious solid-liquid-solid growth mechanism since it all occurs at the same temperature.
These improvements translated into better-performing devices. 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.
The researchers are now exploring additional eutectic chemistries to better understand and control the crystallisation 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.
