GOLDEN, Colo., Jan. 10, 2023 — To improve the efficiency and stability of wide-bandgap perovskite solar cells, researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and the University of Toledo used a gas-quenching process to produce perovskite film, along with an inverted architecture for the solar cells. The team said its all-perovskite tandem cell exhibited an efficiency of 27.1%, a high photovoltage of 2.2 V, and excellent operational stability.
"This new growth approach can significantly suppress the phase segregation," NREL senior scientist Kai Zhu said. Zhu is principle investigator on the project and lead author of the study.
Developing highly stable and efficient perovskites based on mixture of bromine and iodine is considered critical for creating tandem solar cells. In a tandem solar cell, a narrow-bandgap layer is deposited on top of a wide-bandgap layer. The difference in bandgaps allows for more of the solar spectrum to be captured and converted into electricity.
However, tandem perovskite solar cells with mixed bromide and iodide anions can be plagued by issues with bromine-iodine (Br-I) phase segregation, which can limit device voltage and stability. A high concentration of bromide, leading to rapid crystallization of the perovskite film, can cause defects that reduce performance. When exposed to light and heat, the two elements tend to separate and thus limit the solar cell’s voltage and stability.
By using a nontraditional method for growing perovskites, the NREL-led group surmounted these issues. Instead of applying a traditional antisolvent to the crystallizing chemicals to create a uniform perovskite film, the researchers used gas quenching, an approach in which a flow of gas — in this case, nitrogen — is blown onto the chemicals.
The researchers combined rapid bromide crystallization with the gentle gas-quench method to prepare highly textured, columnar, 1.75-eV, Br-I mixed, wide-bandgap perovskite films with reduced defect density. When the gas-quenching process was applied to high-bromide-content perovskite chemicals, it forced the crystals to grow together in a tightly packed manner from top to bottom, like a single grain. The top-down growth method formed a gradient structure with a bromide-rich surface layer. The subsequent columnar growth created films with reduced defect density.
The antisolvent approach promotes rapid, uniform crystal growth within the perovskite film, which can cause the crystals to crowd each other, leading to defects where the grain boundaries meet.
By helping to prevent the bromide and iodine from separating, the gas-quench method yielded a perovskite film with improved structural and optoelectronic properties. In addition, the gas-quench method is statistically more reproducible than the antisolvent approach, the researchers said. They also experimented with argon and air as the drying gas and achieved results similar to those achieved using nitrogen.
The newly developed approach builds on previous work by Zhu. In the earlier research, published in 2022, he and his colleagues inverted the architecture of a perovskite cell. The difference between a conventional architecture and an inverted one is defined by how the layers are deposited on the glass substrate. The inverted architecture demonstrates high stability and is easily integrated into tandem solar cells.
With gas-quenching and the inverted architecture, the researchers produced a perovskite solar cell with a power conversion efficiency of >20% and ~1.33-V open-circuit voltage (VOC). The cell demonstrated excellent operational stability, with less than 5% degradation over 1100 hours of operation under 1.2 sun at 65 °C. When further integrated with a 1.25-eV, narrow-bandgap perovskite solar cell, the all-perovskite, two-terminal tandem device demonstrated an efficiency of 27.1% with a high VOC of 2.2 V and good operational stability.
The team’s approach to growing perovskites could increase the potential for high-performance, all-perovskite tandem devices and further the development of other perovskite-based tandem architectures, such as those that incorporate silicon.
The research was published in Science (www.science.org/doi/10.1126/science.adf0194).