Northwestern researchers tackle solar cell instability at its source
A longstanding challenge for next-generation solar cells begins in the moments when their light-absorbing materials first take shape. Northwestern University researchers have developed a new approach to control this early crystallization process, preventing materials from separating into distinct regions that can undermine both the efficiency and stability of perovskite solar cells.
The strategy enabled the researchers to create perovskite-organic tandem solar cells with a certified power conversion efficiency of 26.4 percent. The devices also retained 91 percent of their initial efficiency after 1,000 hours of aging under continuous illumination at 65 degrees Celsius.
The study was published in Nature and supported by the Paula M. Trienens Institute for Sustainability and Energy at Northwestern University.
Rather than finding a new way to repair or stabilize perovskite materials after they form, the Northwestern team focused on preventing instability from developing in the first place.
“Instead of asking ‘how do we stabilize the film after segregation occurs,’ we asked, ‘what if we intervened during crystallization itself, at the moment the mismatch is actually created?’” said Pengju Shi, the study's first author.
Tackling an imbalance
Tandem solar cells are designed to capture more of the sun's energy than conventional single-junction cells. They accomplish this by stacking materials that absorb different portions of the solar spectrum.
Wide-bandgap perovskites are particularly promising for the top layer of these devices. To achieve the desired bandgap, researchers combine iodide and bromide in a single crystal structure.
But the two components do not naturally form at the same pace. Bromide-rich regions crystallize faster than iodide-rich ones, causing the material to separate into microscopic domains instead of forming a uniform film. This process, known as phase segregation, creates defects that reduce voltage and contribute to degradation when solar cells are exposed to heat and light. Shi compares the problem to trying to mix oil and vinegar while simultaneously freezing them. If one component solidifies before the other, the mixture can become locked into separate regions rather than forming a uniform material.
Researchers have developed several strategies to mitigate the effects of phase segregation, including additives, interfacial treatments, and other methods to stabilize the finished material. The Northwestern team instead approached segregation as a problem rooted in the kinetics of crystallization itself.
“Segregation isn't really a compositional flaw; it's a kinetic one,” Shi said. “Iodide-rich and bromide-rich regions crystallize at different rates, and no amount of downstream passivation changes that underlying mismatch. It only manages the consequences.”
Getting two materials in sync
The researchers developed a molecular strategy that selectively interacts with the faster-forming bromide-rich component, slowing its crystallization so that it more closely matches the pace of the iodide-rich material.
In effect, the added molecule acts as a referee, preventing one component from getting too far ahead of the other as the film forms.
By synchronizing the process, the researchers produced more compositionally uniform wide-bandgap perovskite films and reduced the defects associated with phase segregation. The approach worked across perovskites with multiple bandgaps, suggesting the underlying principle is not limited to a single material composition.
The result was not only more stable material but also better-performing solar cells. Single-junction cells (1.88 eV bandgap) incorporating the strategy achieved efficiencies up to 18.9 percent, compared with maximum efficiencies of 17.6 percent for control devices. After approximately 1,500 hours of continuous illumination at 65 degrees Celsius, the treated devices retained at least 90 percent of their initial efficiencies. Control devices degraded substantially more quickly.
For Shi, seeing stability and efficiency improve simultaneously was one of the project's biggest surprises.
“Efficiency and stability usually trade off against each other in this field — you can often get one only by sacrificing the other,” Shi said. “Seeing both move in the same direction from a single, simple molecular intervention was the moment we realized we'd found something closer to a root cause than a patch.”
A collaboration seeded by Generate
The research grew from a collaboration supported by the Trienens Institute's Generate research pillar, which brings together Northwestern researchers working to develop new methods and materials for the generation, conversion and storage of energy.
A Generate seed grant brought together the research groups of Bin Chen and Tobin Marks, combining complementary expertise to explore an early-stage research idea. That collaboration ultimately expanded to include researchers across Northwestern and other institutions and developed into the work reported in Nature.
“The Generate pillar seed grant brought together research groups with complementary expertise,” Chen said. “I'm very proud of what our teams made of it: an early-stage idea is now a Nature paper.”
“This achievement is an excellent of the scientific synergy that the Generate pillar can assemble to attack an important problem in solar energy conversion, clearly demonstrating that the whole is greater than then sum of it’s parts” says key author Tobin Marks.
Mercouri Kanatzidis, a key author on the study and co-chair of the Generate research pillar, said the work demonstrates how fundamental materials chemistry can help address practical barriers facing emerging energy technologies.
“I am especially proud that Northwestern continues to play a leading role, driven by outstanding colleagues who combine fundamental insight with sophisticated device engineering and a powerful culture of collaboration,” Kanatzidis said.
“This is exactly what the Trienens Institute exists to do — pair foundational science with an insistence on scaling and partnerships, so discoveries like this one deliver abundant, affordable, low-carbon-intensity electricity on real rooftops and real grids,” said Lee Zachos, executive director of the Paula M. Trienens Institute for Sustainability and Energy.
Building a better tandem device
The researchers then incorporated their wide-bandgap perovskite into a tandem device, pairing it with an organic solar cell that captures a different portion of the solar spectrum.
The National Photovoltaic Industry Measurement and Testing Center independently measured an efficiency of 26.4 percent.
The tandem cells also retained 91 percent of their initial efficiency after 1,000 hours of continuous illumination at 65 degrees Celsius. The combination of efficiency and stability is particularly important for moving emerging photovoltaic technologies beyond the laboratory. Conventional silicon solar cells are already approaching their theoretical efficiency limit, making tandem architectures an attractive route to extracting more electricity from the same amount of sunlight.
But a higher initial efficiency has limited practical value if a solar panel quickly degrades. “Solar panels are financed and installed on the assumption they'll produce power reliably for 20 to 25 years,” Shi said. “If a tandem loses a large fraction of its efficiency within months under normal heat and light exposure, the higher starting efficiency doesn't translate into lower cost per unit of electricity generated — it just means faster, more expensive replacement.”
Significant challenges remain before perovskite-based tandem cells can reach rooftops. Researchers will need to reproduce the approach in much larger modules and demonstrate that the technology can withstand moisture, ultraviolet light, and other outdoor conditions while remaining stable for the decades expected of commercial solar panels.
Still, Shi said the underlying strategy could have applications beyond the specific tandem cell demonstrated in the study. The same principle could potentially be adapted to perovskite-silicon and all-perovskite tandem cells, as well as other mixed-halide compositions where uneven crystallization limits performance.
The discovery itself also came from an unexpected direction. According to Shi, the team was pursuing a different research question when a side observation suggested the approach could address the longstanding problem of halide segregation.
Now, the researchers hope to test the strategy at larger scales and under real-world conditions.
“Over the next several years, I'd expect this principle to be combined with ion-migration-suppressing chemistries and tested at module scale under real outdoor conditions,” Shi said. “That's the real bridge between a Nature paper and a rooftop.”