Perovskite-silicon solar cells combine two semiconductors that utilize different regions of the solar spectrum. The upper perovskite layer primarily absorbs high-energy, short-wavelength light, while the underlying silicon cell mainly utilizes longer wavelengths. One challenge is depositing the thin perovskite layer uniformly, rapidly, and over large areas. “For industrial manufacturing, not only efficiency matters, but also whether a process is fast, robust, and scalable,” says Prof. Ulrich Paetzold from KIT’s Institute of Microstructure Technology and the Light Technology Institute (LTI). “We were able to demonstrate that a particularly fast vacuum process not only produces uniform layers, but also enables efficient perovskite-silicon solar cells.”
CSS Process Accelerates Coating
The rapid vacuum process is based on close-space sublimation (CSS). In this process, the precursor materials evaporate and react on the silicon cell to form a perovskite layer. One major advantage of the CSS process is the low consumption of precursor material per coating cycle and the reusability of the sources. “In the experiment, the conversion was completed after 10 minutes — an important advance for a vacuum process,” explains co-author Sofia Chozas-Barrientos from the University of Valencia.
In addition to achieving uniform coating, the upper perovskite layer must also absorb the appropriate portions of light. This property is controlled via the material’s bandgap: in the upper sub-cell, the bandgap must be larger so that, like a filter, it absorbs and transmits the appropriate portions of light, allowing the perovskite and silicon layers to be matched to one another. Since bromine can increase the bandgap, the researchers initially tested a bromine-containing inorganic precursor layer. However, during conversion into perovskite, the desired proportion was not retained in the material.
“The solution was a mixed organic source consisting of methylammonium iodide and methylammonium bromide,” says co-author Dr. Alexander Diercks from the LTI. By adjusting the ratio of these two components, the researchers were able to control the bromine content in the finished material and achieve a bandgap of 1.64 eV.
A Step Toward Industrial Production
For industrial manufacturing, the CSS process must also work on structured surfaces because these extend the path of light within the cell and thereby increase absorption. The researchers therefore tested the CSS process on silicon bottom cells with smooth, nano-structured, and microstructured surfaces. Comparable perovskite layers were produced on all three surfaces without requiring adjustments to the process parameters. Scanning electron microscopy and X-ray analyses showed uniform coverage. The tandem solar cells produced in this way achieved efficiencies of 23.5% on smooth silicon cells, 23.7% on nanostructured cells, and 24.3% on microstructured silicon cells.