Temporary Liquid Trick Grows Near-Defect-Free Perovskite Crystals for Solar Cells and LEDs

Perovskite solar cells have long promised to shake up the energy world: they are cheap to make, can be printed from solution like ink, and in the laboratory have reached efficiencies that rival the silicon panels blanketing rooftops today. Yet one stubborn problem has kept them from fulfilling that promise at scale. The thin films at the heart of these devices are mosaics of countless tiny crystals, and the seams between those crystals are riddled with defects that bleed away precious energy. Now a team co-led by researchers at the University of California San Diego, working with collaborators at Princeton University and Sungkyunkwan University in the Republic of Korea, has unveiled an elegantly simple fix: a temporary liquid that melts its way through the crystallization process and then vanishes, leaving behind larger, cleaner, more orderly crystals. The work was published on October 8 in the journal Science.

The core insight of the new study is deceptively straightforward. Rather than fighting the defects that form as perovskite crystals jostle into place during fabrication, the researchers gave the crystals room to fix themselves. They accomplished this by mixing two additional salts, zinc bromide and methylammonium chloride, into the solution used to make the perovskite film. When the material was heated during processing, these two additives combined to form what chemists call a eutectic: a mixture that melts at a lower temperature than either of its individual components. This low-melting mixture turned liquid precisely where it was needed most, at the boundaries between the growing crystals.

That temporary liquid proved transformative. In a conventional perovskite film, crystals nucleate and grow rapidly, locking in a chaotic arrangement full of grain boundaries, the interfaces where misaligned crystals meet. These boundaries harbor structural and electronic defects that cause some of the energy captured from sunlight to be lost before it can be harvested as electricity. With the eutectic present, the boundaries between growing crystals became liquid-like, giving the crystals space to rearrange and grow into a more orderly structure. As heating continued, one of the additive ingredients, methylammonium chloride, evaporated away, while the zinc component remained concentrated around the crystal boundaries, where it helped neutralize the electronic defects that remained.

Study co-first author Connor Dolan, a chemical and nano engineering PhD alumnus from the lab of 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, described the approach as a new way of steering crystal growth. “We introduce a new way of tuning the growth of halide perovskites by developing an additive mixture that actively evolves during crystallization,” Dolan said. “What’s even cooler is that the end product removes performance-limiting defects at the edges of the crystalline grains.” The key word is actively: unlike static additives that simply sit in the film, this mixture changes state and composition as the material crystallizes, playing a different role at each stage of the process.

To understand exactly what was happening inside the film, the researchers turned to advanced X-ray imaging techniques capable of watching the process unfold at extremely small scales. What they saw confirmed the mechanism. The material around the crystal boundaries became disordered and liquid-like during heating, then re-ordered at the same temperature as the extra salt evaporated and crystal growth finished. The entire solid-liquid-solid transformation occurred at a single temperature, which the researchers find particularly intriguing. The treatment produced larger, better-ordered crystals with fewer structural defects and a more uniform distribution of the material’s chemical properties across the film, precisely the qualities that device engineers prize.

The improvements carried directly through to device performance. Solar cells and light-emitting diodes built with these eutectic-treated perovskites achieved efficiencies ranging from about 24 percent to 26 percent, placing them at the high end for perovskite-based devices. Just as importantly for real-world deployment, the solar cells retained most of their original efficiency after continuously operating for eight weeks under simulated sunlight, a durability result that addresses one of the most persistent criticisms of perovskite technology.

The physics behind those numbers is telling. In a solar cell, energy from sunlight excites electrons and leaves behind holes in the material, creating paired charge carriers known as electron-hole pairs. How long these pairs survive before recombining and releasing their energy as waste heat is a key indicator of how well a perovskite performs. “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,” Dolan explained. That claim is remarkable because it spans not just perovskites but an entire class of semiconductors, suggesting the material produced here has an unusually clean electronic landscape.

Perhaps the most encouraging sign for the field is that the method appears to be chemistry-agnostic. The researchers found that the approach worked across the different perovskite compositions they tested, rather than being a one-off trick for a single formulation. “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.” That generality matters because perovskite research is a sprawling enterprise, with hundreds of labs tweaking compositions of halide perovskites, a family of materials defined by their crystal structure rather than any single recipe. A growth method that improves all of them could become a shared foundation.

The broader context makes the advance especially timely. Unlike conventional silicon solar cells, which require energy-intensive, high-temperature processing of rigid wafers, perovskites can be processed from solutions and deposited as thin films, opening the door to simpler, lower-energy manufacturing and potentially flexible, lightweight panels. But scaling perovskite devices from small laboratory cells to full-size modules has been notoriously difficult, because efficiency tends to drop as the device gets bigger and the odds of encountering a defect multiply. Films with fewer grain boundaries and cleaner interfaces should, in principle, suffer less from this scaling penalty, which is exactly the problem the eutectic method targets.

The team is now exploring additional eutectic chemistries to better understand and control the crystallization process, hoping to generalize the curious solid-liquid-solid growth mechanism further. The work, detailed in the study “Volatile eutectics to tailor crystallization for perovskite optoelectronics,” 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. If the strategy proves as broadly applicable as the early results suggest, the humble trick of adding a pinch of salt that melts and then disappears could become a standard tool for manufacturing the next generation of solar cells and LEDs, turning one of perovskite technology’s oldest weaknesses into a solved problem.

Subject of Research: Volatile eutectic additive engineering for defect reduction in halide perovskite crystallization for solar cells and LEDs

Article Title: A temporary melting liquid could help build better perovskite solar cells and LEDs

Article References: A temporary melting liquid could help build better perovskite solar cells and LEDs. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: perovskite, solar cells, halide perovskites, eutectic, crystallization, grain boundaries, defects, LEDs, thin films, photovoltaics, UC San Diego, Science journal