• A team of scientists from the University of Valencia and the Karlsruhe Institute of Technology (Germany) has succeeded in producing uniform layers on textured silicon cells
• The method is a vacuum-based, solvent-free, high-speed process for tandem solar cells with efficiencies of up to 24.3%
• The study has been published in the prestigious journal Nature Energy

A research team from the Institute of Molecular Science (ICMol) at the University of Valencia and the Karlsruhe Institute of Technology (KIT) in Germany has developed a solvent-free, vacuum-based fabrication process capable of depositing perovskite layers uniformly even on textured silicon surfaces, at high speed. The findings have been published in Nature Energy.

Solar energy is one of the cornerstones of the energy transition. Perovskite–silicon tandem solar cells offer the possibility of achieving efficiencies higher than those of conventional silicon cells. However, manufacturing them on an industrial scale remains one of the major challenges.

Perovskite–silicon tandem solar cells combine two semiconductors that absorb different regions of sunlight. The upper perovskite layer primarily absorbs high-energy blue light – that is, light with a short wavelength – while the silicon cell beneath mainly uses the longer-wavelength fractions. In this way, tandem solar cells can convert more of the sun’s energy into electricity than conventional silicon-only cells. However, one of the main challenges of this emerging technology is depositing the perovskite layer in a reproducible, uniform and rapid manner.

“For industrial manufacturing, it is not only efficiency that matters, but also whether a process is fast, robust and scalable”, says professor Ulrich Paetzold, from the Institute of Microstructure Technology and the Institute of Light Technology at KIT. “We have demonstrated that a particularly fast vacuum process not only produces uniform layers but also enables the production of efficient perovskite–silicon tandem solar cells”, he reports.

The CSS process accelerates thin-film deposition

The high-speed vacuum process is based on close-space sublimation (CSS). In this process, the source materials are sublimated and transported to the silicon cell, which is positioned only a few millimetres from the material source. There, they react directly to form a perovskite layer. An important feature of the CSS process is its low consumption of raw material for each conversion. This makes it possible to use the same material source for a large number of depositions.

“With this technique, we are able to deposit organic materials at high speed and without solvents, which is difficult to achieve with conventional methods due to the instability of these materials at high temperatures. By reducing the distance between the source material and the substrate, sublimation can be carried out at lower temperatures and deposition occurs much faster”, explains co-author Sofía Chozas-Barrientos, from the University of Valencia. “In our work, the conversion was completed in just ten minutes; for a vacuum process, this is a significant advance”, the researcher adds.

The perovskite bandgap

In addition to achieving uniform coating, the top perovskite layer must also absorb the appropriate fractions of light. This property is controlled by the material’s bandgap: the upper perovskite subcell must have a higher bandgap to absorb higher-energy photons while allowing the rest of the light to pass through to the silicon subcell, thus achieving efficient coupling between the two.

Since bromine can increase the bandgap, the research team initially tested an inorganic precursor layer containing bromine. However, during conversion into perovskite via CSS, the desired proportion was not maintained in the material.

“The solution was to use a mixed organic source composed of methylammonium iodide and methylammonium bromide”, explains co-author Alexander Diercks, who spent six months working on his doctoral thesis in professor Bolink’s group in Valencia, as part of the Horizon Europe Nexus project. “By adjusting the ratio between the two components, we were able to control the bromine content in the final material and achieve a bandgap of 1.64 electronvolts”, adds Diercks.

A step towards industrial production

For industrial manufacturing, the CSS process must be compatible with different types of silicon surfaces, including textured ones commonly used in industrial and commercial silicon. These surfaces feature pyramidal structures between 1 and 2 microns in size on which light is reflected, thereby increasing its path length within the cell and improving absorption.

The research team therefore tested the CSS process on silicon cells with flat and nanotextured surfaces, more common in small-scale laboratory studies, and microtextured surfaces, which are more representative of industrial silicon. Perovskite layers with almost identical properties were formed on all three surfaces, without the need to adjust the process parameters.

Scanning electron microscopy and X-ray analyses revealed uniform coverage. Tandem solar cells manufactured using this process achieved efficiencies of 23.5% on flat silicon cells, 23.7% on nanotextured cells and 24.3% on microtextured cells.

“This is very important for scalability”, says professor Hendrik Bolink of the University of Valencia. “A process that only worked on perfectly smooth surfaces would have limited usefulness for industrial applications. The fact that close-space sublimation produces uniform layers even on textured silicon cells makes this approach highly relevant for industry”, he emphasises.
The study is the result of close collaboration between research groups at KIT and the University of Valencia. Researchers from CONICET-UNL in Argentina and Université Grenoble Alpes/CEA-LITEN in France also participated.

Article reference: Alexander Diercks, Sofía Chozas-Barrientos et al. “Close Space Sublimation as a Versatile Deposition Process for Efficient Perovskite Silicon Tandem Solar Cells”. Nature Energy, 2026. https://doi.org/10.1038/s41560-026-02068-9