A new generation of solar panels could emerge by using a special coating of organic molecules on solar cells, according to a research team in the journal Chemistry in question This coating increases the efficiency of monolithic tandem cells made of silicon and perovskite and also reduces their cost, as they are made from standard silicon wafers with industrial microstructures.

In solar cells, light “knocks” electrons out of a semiconductor, leaving positively charged “holes.” These two charge carriers can be separated and collected into a current. Tandem cells are designed to better utilize the full spectrum of sunlight and increase the efficiency of solar cells.

Tandem elements consist of two different semiconductors that absorb different wavelengths of light. The main candidates for this technology are silicon, which absorbs mainly red and near-infrared light, and perovskite, which uses visible light very efficiently. Monolithic tandem cells are made by coating a substrate with two types of semiconductors, one on top of the other. For perovskite/silicon systems, this is usually achieved using silicon wafers produced by local melting and having a polished or nanostructured surface. However, they are very expensive.

Silicon wafers made using the Czochralsky method, with micrometer pyramidal elements on their surface, are much cheaper. These microtextures contribute to better light capture, as they reflect less than a smooth surface. However, the process of coating these wafers with perovskite introduces many defects in the crystal lattice that affect the electronic properties. The transfer of free electrons becomes more difficult, and the recombination of electrons and holes occurs more and more through non-light-emitting processes. Both the efficiency and stability of the perovskite layer decrease.

Groundbreaking development in surface passivation

A Chinese team led by Professor Kai Yao from Nanchang University, Suzhou Maxwell Technologies, CNPC (Shaanxi) Tubular Products Research Institute, The Hong Kong Polytechnic University, Wuhan University of Technology and Fudan University (Shanghai) has developed a surface passivation strategy that smooths out the surface defects of the perovskite layer. Trifluoromethyl thiophenethylammonium compound (CF3-TEA) is applied using a dynamic sputtering process. This creates a very smooth layer even on micro-textured surfaces.

The CF3-TEA coating is highly effective in reducing the effects of surface defects due to its high polarity and binding energy. Nonradiative recombination is suppressed and electron levels are adjusted such that electrons at the interface can be more easily transferred to the electron-capturing layer of the solar cell. Surface modification with CF3-TEA enables perovskite-silicon tandem solar cells based on conventional Czochralski-textured silicon wafers to achieve very high efficiencies of approximately 31% and maintain long-term stability.