A team of researchers from Helmholtz-Zentrum Berlin (HZB) announced that they have set a new conversion efficiency record of 29.15% in a tandem solar cell made of perovskite and silicon.  The result was certified by Fraunhofer ISE.

The researchers explained that solar cells made of two semiconductors with different bandgaps could achieve higher efficiency compared to each sub-cell on its own as tandem cells utilize the solar spectrum more efficiently. Silicon solar cells transform the infrared component of sunlight into electrical energy, whereas perovskite compounds can utilize visible components of sunlight.

In their report, the researchers stated that the newly developed tandem solar cell could retain 95% of its initial conversion efficiency of 29% even after 300 hours of operations under constant air exposure without any protection by encapsulation. They used a perovskite absorber with a bandgap of 1.68 electron volts and concentrated on enhancing the substrate interface.

According to the researchers, a novel carbazole-based molecule with methyl groups was utilized to develop an intermediate layer of organic molecules that arrange themselves to create a self-assembled monolayer (SAM). They applied the SAM to the electrode to boost the flow of charge carriers.

“We first set up the perfect bed, so to speak, on which the perovskite layer lies,” said Amran Al-Ashouri, researcher and first author of the study.

The researchers examined different processes at interfaces between the perovskite, SAM, and electrode using a series of complementary investigation methods. They specifically optimized the fill-factor that is influenced by how many charge carriers are lost on their way out of the perovskite sub-cell.

They said electrons should move towards sunlight through the C₆₀ layer, whereas the holes must travel in the opposite direction and flow through SAM into the electrode.

“However, we saw that holes are extracted much more slowly than electrons, which limited the fill-factor,” said Al-Ashouri. But the SAM layer applied to the electrode assists with the extraction of holes and provides better stability of the perovskite layer.

The researchers used a combination of modeling, photoluminescence spectroscopy, electrical characterization, and terahertz conductivity measurement to differentiate between several processes at perovskite material’s interface and to determine the origin of main losses. They also claimed that the maximum possible efficiency was within reach as they analyzed both sub-cells individually and calculated the maximum possible efficiency of 32.4%.

Researchers from the Kaunas University of Technology, Lithuania; University of Ljublana, Slovenia; University of Sheffield, United Kingdom; Berlin University of Applied Sciences and the Technical University of Berlin, Germany was also involved in the research project.

In August 2020, researchers at Fraunhofer Institute for Solar Energy Systems ISE had announced a new record efficiency of 25.9% for the III-V/Si tandem solar cell grown directly on silicon.

Mercom had earlier reported that scientists at Helmholtz Zentrum Berlin developed tandem cells that combine two different semiconductors and convert different light spectrum parts into electrical energy. That perovskite CIGS tandem cell had achieved a record efficiency of 24.16%.

Image credit: By Own work, CC BY 3.0