Researchers at the National Renewable Energy Laboratory (NREL) and University of Toledo have developed a new approach to manufacturing perovskite solar cells.
Developing highly stable and efficient perovskites based on a rich mixture of bromine and iodine is considered critical for the creation of tandem solar cells. However, issues with the two elements separating under solar cell operational conditions, such as light and heat, limit the device voltage and operational stability. This challenge is often made worse by the ready defect formation associated with the rapid crystallization of bromine-rich perovskite chemistry with antisolvent processes.
“This new growth approach can significantly suppress the phase segregation,” said Kai Zhu, a senior scientist at NREL, principal investigator on the project, and lead author of the new paper. The new approach addressed the problem of the two elements separating, and it produced a wide-bandgap solar cell with an efficiency of over 20% and 1.33-volt photovoltage.
The efficiency remained intact for over 1,100 hours of continuous operation at a high temperature. Ultimately the researchers achieved an all-perovskite tandem cell obtained an efficiency of 27.1% with a high photovoltage of 2.2 volts and good operational stability.
In the tandem cell, the narrow-bandgap layer is deposited on top of the wide-bandgap layer. The difference in bandgaps allows for more of the solar spectrum to be captured and converted into electricity.
The newly developed approach builds upon work Zhu and his colleagues published earlier this year that flipped the typical perovskite cell. Using this inverted architectural structure allowed the researchers to increase both efficiency and stability and to easily integrate tandem solar cells. They ultimately achieved a perovskite solar cell with 24% efficiency that retains 87% of output after 100 days.
In the recent study, the NREL-led group used that same architecture in that they used an antisolvent applied to the crystalizing chemicals to create a uniform perovskite film. Then they went a step further, and the new approach relied on what is known as gas quenching, in which a flow of nitrogen was blown onto the chemicals. The result addressed the problem of the bromine and iodine separating, resulting in a perovskite film with improved structural and optoelectronic properties.
The previous antisolvent approach allows the crystals to grow rapidly and uniformly within the perovskite film, crowding each other and leading to defects where the grain boundaries meet. The new gas-quenching process, when applied to high-bromine-content perovskite chemicals, forces the crystals to grow together, tightly packed from top to bottom. The researchers found that this significantly decreases the defects. The top-down growth method forms a gradient structure, with more bromine near the top and less in the bulk of the cell. The researchers report that the gas-quenching method was also statistically more reproducible than the antisolvent approach.
The researchers also tried argon and air as drying gas with similar results, indicating that the gas-quench method is a general way for improving the performance of wide-bandgap perovskite solar cells.
The new approach demonstrated the potential of high-performance all-perovskite tandem devices and advanced the development of other perovskite-based tandem architectures such as those that incorporate silicon.