Researchers use vacuum process to produce efficient tandem solar cells

A research team from the Karlsruhe Institute of Technology (KIT) and the University of Valencia has succeeded in applying thin perovskite layers for the production of perovskite-silicon tandem solar cells over a large area, uniformly and quickly using a vacuum process. The manufacturing process for such solar cells, which achieve an efficiency of up to 24.3 percent, can thus be scaled almost arbitrarily for industrial production.

In tandem solar cells, the semiconductors perovskite and silicon are used to utilize different wavelength ranges of sunlight in photovoltaics. The upper perovskite layer absorbs short-wavelength light, while the lower silicon layer absorbs long-wavelength light. This combination allows tandem solar cells to achieve higher efficiency. However, production is not entirely simple. The perovskite layer must be applied very thinly, uniformly, and over a large area during production. At the same time, this must work quickly and reliably to be able to scale the manufacturing process.

The researchers from KIT and the University of Valencia use a fast vacuum process based on Close-Space Sublimation (CSS) to achieve this. In summary, they have published the research results in the study “Close-space sublimation as a versatile deposition process for efficient perovskite silicon tandem solar cells“, which appeared in Nature Energy. In this process, starting materials are evaporated and impinged on the silicon cell at a distance of only a few millimeters from the material source. They react there to form a perovskite layer. Only small amounts of the starting material are needed per coating, the scientists emphasize. The source material can also be reused.

“With close-space sublimation, we were also able to apply the demanding organic starting materials without solvents and in a short time to silicon,” explains Sofia Chozas-Barrientos, one of the participating scientists from the University of Valencia. “In the experiment, the conversion was completed after ten minutes – for a vacuum process, this is an important step forward.”

For the perovskite layer to absorb the desired wavelengths, the band gap in the upper subcell must be larger to absorb and transmit the correct light components. This allows the perovskite and silicon layers to be optimally matched. To achieve this, the researchers used methylammonium iodide and methylammonium bromide as the organic source. The ratio of these two substances can be used to control the bromine content in the finished material, which guarantees the achievement of the required band gap of 1.64 electron volts. Previously, attempts with a bromine-containing inorganic precursor layer had failed because the necessary proportion in the material could not be achieved during the conversion to perovskite.

Adhesion on differently structured silicon layers

The scientists tested the CSS process on silicon surfaces with different structures, from smooth to nano- and microstructured surfaces. This is because, in an industrial manufacturing process, the perovskite layer must be able to adhere to differently structured surfaces. The researchers found that it worked uniformly on all three types of structures without requiring any adjustments. The perovskite and silicon tandem solar cells produced in this way achieved efficiencies of 23.5 percent on smooth surfaces, 23.7 percent on nano- and 24.3 percent on microstructured silicon cells.

Uniform adhesion on nano- and microstructured silicon cells is important for the industrial process. Only then is an industrial manufacturing process possible in practice.

(olb)