by | Oct 5, 2026
Researchers have uncovered a way to suppress halide segregation in tandem solar cells, improving both efficiency and stability. Their strategy works across multiple mixed-halide perovskite compositions, suggesting the approach may be broadly applicable to this important class of photovoltaic materials.
Tandem solar cells stack two different light-absorbing materials on top of one another, allowing them to convert more sunlight into electricity than either material alone. Mixed-halide perovskites—crystals containing both iodide and bromide ions—are commonly used as the top layer because their composition can be tuned to absorb high-energy light while permitting longer wavelengths to pass through to the lower cell. However, because iodide and bromide crystallize at different rates, the resulting films tend to be uneven in composition, with bromide-rich regions forming near the surface.
Imagine baking a cake in which one ingredient solidifies before the others. Instead of a smooth, even cake, dense pockets would form throughout the batter.
Something similar happens in mixed-halide perovskite films.
“This uneven crystallization induces defects in the film, causing charge and energy losses, and ultimately limits the efficiency and stability of the solar cell,” explains Baomin Xu, a professor in the Department of Materials Science and Engineering at Southern University of Science and Technology in Shenzhen, China.
Additives are widely used to reduce defects, but previous research has focused on the effect of the additive. “What makes our approach different is that we focus not only on what the additive does, but also on where it sits within the film,” says Xingzhu Wang, study co-lead and professor at Southern University of Science and Technology and the University of South China.
Shifting the focus to where the additive settles inside the film offers a way to control the crystallization process from the very beginning of film formation.
While preparing the perovskite film, the researchers added a fluorinated molecule called PFDOA. At first, they viewed it as just another crystallization additive. Only after studying the final films did they notice something unexpected.
When they analyzed the elemental composition of the film surface and the bottom interface exposed after peeling off the film, they found a clear fluorine signal at the top of the film and almost no signal at the bottom.
The additive formed a vertical concentration gradient, placing its highest concentration at the film surface—the same region where bromide tends to crystallize first.
“The position where the additive concentration is highest happens to be exactly where the bromide-containing component is most likely to crystallize too early,” said Wang.
This means crystallization can be controlled precisely where it is most likely to go awry.
The researchers describe their strategy as “gradient-controlled crystallization”: rather than slowing crystallization throughout the film, the additive selectively moderates the regions most prone to defects.
Because crystallization begins at the surface, the higher concentration of PFDOA in this region acts as a “brake” for rapidly crystallizing bromide ions. Deeper inside the film, where crystallization proceeds more slowly, the additive has less influence.
The result is a more uniform microstructure throughout the film thickness and higher-performing solar cells. The PFDOA-treated perovskite cells reached a power conversion efficiency of 17.78%, compared with 16.73% for untreated devices. When PFDOA was added to perovskite/organic tandem solar cells, the best-performing device achieved an efficiency of 25.91%.
What surprised the researchers most was that their strategy worked across several different mixed-halide compositions. According to Xu, this finding suggests the concept may extend to other mixed-halide perovskite formulations.
“As long as there is a problem of the bromide-containing and iodide-containing components not crystallizing in step with each other, it may be possible to use an additive concentration gradient to control the crystallization rate at different positions in the film,” says Wang.
How the strategy translates to commercial manufacturing, however, remains to be seen.
In this study, the devices were fabricated using spin coating and an antisolvent process on films with small areas. Commercial solar modules require continuous, large-area manufacturing methods, and it remains unclear whether the same concentration gradients can be reliably reproduced during industrial production.
Likewise, the stability tests were performed on unencapsulated devices in a nitrogen atmosphere rather than under realistic outdoor conditions.
“Our device retained more than 80% of its efficiency after 600 hours of continuous operation, but this was measured under unencapsulated conditions, in a nitrogen atmosphere, and at close to room temperature,” says Wang. “It cannot yet tell us how a module would behave outdoors, after heat, humidity, and long-term climate variation.”
The next challenge for Wang, Xu, and their team is not simply to drive up efficiencies, but to understand the principles underlying the formation of these gradients. “The next step is to further understand why this concentration gradient forms, and how it can be actively designed and controlled,” says Xu. This understanding would allow for a more general design principle that can be deliberately applied to solar cell fabrication.
Reference: Jiangfeng Wang et al., Enhancing Perovskite/Organic Tandem Solar Cell Efficiencies via Gradient-Regulated Crystallization of Mixed-Halide Perovskites. Advanced Energy Materials (2026), DOI: 10.1002/aenm.71115
Featured Image: Figure 5B from 10.1002/aenm.71115
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