Perovskite-silicon tandem cells split the solar spectrum between two absorbers, with the perovskite top cell capturing high-energy blue light while the silicon cell below converts the red portion. By using two materials with different bandgaps, these tandem structures can convert a larger share of incident sunlight into electricity than single-junction silicon cells. The overall performance depends strongly on the interfaces, where photogenerated charges must be efficiently extracted with minimal recombination losses.
A central component in this architecture is the self-assembled monolayer, or SAM, which forms an ultrathin molecular contact only a few nanometers thick. This SAM is designed to facilitate charge transport from the perovskite into the underlying charge collection layers. On pyramidally textured silicon surfaces, however, standard SAM molecules with simple alkyl chains can arrange unevenly, leaving gaps and inhomogeneous coverage that reduce device efficiency.
To overcome this limitation, the researchers synthesized a molecule engineered for textured, rough surfaces, enabling more uniform coverage and stable electronic contact. This tailored molecular structure improves charge transport across the interface and establishes a more robust junction between the perovskite and silicon subcells. During detailed analysis of the interface chemistry, the team noticed that a commercially available SAM precursor contained trace amounts of bromine-bearing impurities.
Those brominated species turned out to be beneficial because they passivated defects at the interface and raised the efficiency of the tandem cells. "That such a small chemical change can have such a large effect surprised even us," explains project leader Aydin. "This discovery shows how decisive the precise interplay of materials at the molecular level is for the energy yield of emerging solar cells." Building on this finding, the researchers deliberately combined brominated and non-brominated molecules to harness the defect-passivating effect without sacrificing overall chemical stability.
The resulting SAM design allows denser packing of molecules on the textured surface and better passivation of electronic defects at the perovskite-silicon interface. This denser layer enhances charge extraction, increases device stability, and supports higher operating efficiencies. By fine-tuning the molecular composition of the contact, the group created conditions where photogenerated carriers move more effectively into the electrical circuit rather than recombining at the interface.
Using this optimized SAM, the tandem devices reached a certified efficiency of 31.4 percent, placing the LMU-led collaboration among the laboratories pushing the performance of perovskite-silicon tandems. The fact that the result was achieved on industrially relevant crystalline silicon bottom cells underscores the potential for transferring the approach toward commercial production. The new SAM also improves long-term stability because the densely packed molecules shield the sensitive interface region from chemical and structural damage over time.
The team now plans to subject the tandem cells to accelerated aging protocols to probe their behavior under conditions that simulate long-term outdoor exposure. "As the next step, we want to show that our tandem cells can prove their worth not just in the lab, but also in accelerated aging tests, which gives insight about real environmental condition behavior," says Aydin. In parallel, the researchers are evaluating how the technology can be adapted for use in space, with a focus on satellites in low Earth orbit where low mass, radiation tolerance, and high power output are essential.
Research Report:Enhanced charge extraction in textured perovskite-silicon tandem solar cells via molecular contact functionalization
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