A collaborative team of researchers has developed a novel molecular passivation strategy that significantly improves the performance and stability of perovskite solar cells, marking a pivotal advancement for the next-generation photovoltaic technology. The new method, detailed in the journal Science, addresses critical surface defects that hinder efficiency, resulting in a certified steady-state efficiency of 26.15 percent for devices measured at 0.05 square centimeters.
This breakthrough tackles one of the primary obstacles preventing perovskite solar cells from reaching their theoretical efficiency limits. Perovskites, a class of materials with a specific crystal structure, are highly promising for solar energy due to their exceptional ability to convert sunlight into electricity. However, the ionic nature of perovskite crystals makes them prone to dissociation and the formation of defects on their surface, which leads to energy loss. The international team, led by researchers from ShanghaiTech University and Northwestern University, designed a specialized molecule that binds more effectively to the perovskite surface, preventing these defects from forming and improving the extraction of electrical charge.
Dual-Site Binding Mechanism
The core of the innovation lies in a passivation molecule with two distinct binding sites. Traditional organic passivators typically bind to only a single active site on the perovskite surface, resulting in a relatively weak connection. These single-site molecules can also stack vertically, creating a barrier that impedes the movement of electrons from the perovskite layer to the electron transport layer.
Designing a Superior Passivator
To overcome these limitations, the research team utilized 4-chlorobenzenesulfonic acid sodium, or 4Cl-BZS. This molecule was engineered to interact with two adjacent lead ions on the perovskite’s surface simultaneously through its chlorine atom and its sulfonic acid group. This dual-site binding creates a much stronger and more stable bond, effectively locking the surface structure in place and suppressing the formation of performance-degrading defects. This enhanced binding energy is critical for preventing carrier recombination, a process where electrons lose their energy before they can be harvested as electricity.
Improved Charge Extraction
Beyond simply fortifying the surface, the dual-site binding mechanism also dictates the orientation of the 4Cl-BZS molecules. They align parallel to the perovskite surface, creating a smooth, uniform layer. This orientation is crucial because it minimizes the distance between the perovskite and the electron transport layer, in this case, C60. By closing this gap, the passivator facilitates more efficient electron transfer, ensuring that more of the energy captured from sunlight is converted into usable current.
Pushing Performance Benchmarks
The application of the 4Cl-BZS passivator resulted in a significant leap in photovoltaic conversion efficiency. In a laboratory setting, the team recorded a peak efficiency of 26.9 percent. For official validation, devices were sent to the authoritative testing institution Newport, which certified a steady-state efficiency of 26.15 percent for a 0.05 cm² cell and 24.74 percent for a larger 1 cm² cell. These certified results have been included in the prestigious Best Research-Cell Efficiency Chart published by the U.S. National Renewable Energy Laboratory (NREL), establishing new world benchmarks for single-junction perovskite solar cells.
Exceptional Operational Stability
A major hurdle for the commercialization of perovskite solar cells has been their long-term stability, especially under real-world operating conditions like high temperatures. The new passivation strategy demonstrated remarkable improvements in this area as well. Devices treated with the dual-site binding molecule retained over 95 percent of their initial efficiency after 1,000 hours of continuous operation at a high temperature of 65°C while under constant illumination. This exceptional durability showcases the passivator’s ability to protect the perovskite layer from degradation, a critical step toward practical applications.
Collaborative Research and Future Impact
The research was a joint effort involving multiple institutions and corresponding authors, including Associate Professor Ning Zhijun at ShanghaiTech University, Professor Edward H. Sargent, and Research Assistant Professor Chen Bin at Northwestern University. The findings are expected to have a significant impact on the field, providing a new chemical blueprint for designing passivators that can simultaneously boost efficiency and stability. As researchers continue to refine these molecular interfaces, the gap between the performance of perovskite cells and their theoretical limits narrows, bringing this transformative solar technology closer to widespread industrial adoption and a key role in the global transition to renewable energy sources.