Researchers have developed a new method to improve the efficiency and durability of perovskite solar cells by controlling how the materials crystallize. The study shows how a temporary liquid phase can help produce larger, more uniform crystals with fewer defects.
The team, co-led by researchers at the University of California San Diego (UC San Diego), used this approach to improve perovskite-based solar cells and light-emitting diodes. The devices achieved efficiencies of around 24 percent to 26 percent, among the higher values reported for perovskite devices.
Solar cells made using the technique also retained most of their initial efficiency after eight weeks of continuous operation under simulated sunlight.
A liquid phase improves crystal growth
Perovskites are materials that could help make solar technology more affordable while delivering high efficiency. They can be processed from liquid solutions and deposited as thin films, potentially reducing the energy and complexity required for manufacturing compared with conventional silicon solar cells.
However, producing high-quality perovskite films remains challenging. These films typically contain many small crystals, with boundaries between neighboring crystals known as grain boundaries. Defects in these regions can cause energy losses and reduce device performance.
The problem becomes more significant when manufacturers attempt to produce larger solar panels, where maintaining efficiency across a greater area is difficult.
To address this issue, researchers led by David Fenning, professor of chemical and nano engineering at UC San Diego, worked with colleagues from Princeton University and Sungkyunkwan University in South Korea.
The team added zinc bromide and methylammonium chloride to the solution used to produce a type of material called a halide perovskite. When heated, the additives formed a low-melting mixture known as a eutectic. This mixture created a temporary liquid around growing crystal boundaries, allowing the crystals to rearrange and form a more orderly structure.
Additives help remove performance-limiting defects
As heating continued, methylammonium chloride evaporated, leaving zinc concentrated around the crystal boundaries. The remaining zinc helped neutralize electronic defects that could otherwise interfere with the movement of electrical charges.
“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.”
Using advanced X-ray imaging, the researchers observed changes in the material at extremely small scales. Their measurements showed that the regions around crystal boundaries became disordered and liquid-like during heating before returning to an ordered state as the extra salt evaporated and crystal growth ended.
The treatment produced larger crystals with fewer structural defects and a more uniform distribution of chemical properties. The researchers are also interested in understanding why this solid-liquid-solid transformation occurs at the same temperature.
Solar cells retain efficiency during testing
The improved crystal structure translated into better device performance. In solar cells, sunlight excites electrons and leaves behind positively charged holes. The time these electron-hole pairs survive before recombining helps indicate how effectively a material can support solar energy conversion.
“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 noted.
The researchers also tested the method across different perovskite compositions and found that it worked with the chemistries they examined.
“It seemed like everything we tried worked!” Dolan added. “I’m hopeful that this mechanism has the chance to be really generalizable and provide a platform for further innovation in the perovskite community.”
Researchers explore wider applications
The team is now investigating other eutectic mixtures to better control crystallization.
The researchers hope the approach can provide a broader strategy for producing high-quality perovskite materials and support the development of efficient solar technologies that can be manufactured at larger scales.
The study was published in the journal Science.