In solar cells, light excites electrons out of a semiconductor, leaving behind positively charged "holes". These charge carriers are separated and collected as current. Tandem cells were developed to exploit the full spectrum of sunlight more effectively and boost solar cell efficiency. They are composed of two different semiconductors that absorb various wavelengths of light.
The primary candidates for this technology are silicon, which absorbs mostly red and near-infrared light, and perovskite, which efficiently captures visible light. Monolithic tandem cells are created by layering these semiconductors on a support, typically using silicon wafers produced by the zone melting process with a polished or nanostructured surface. However, these wafers are costly.
Cheaper silicon wafers produced by the Czochralski process feature micrometer-scale pyramidal structural elements that enhance light capture due to their lower reflectivity compared to smooth surfaces. However, coating these wafers with perovskite introduces defects in the crystal lattice, impacting the electronic properties, impeding electron transfer, and increasing electron-hole recombination. This reduces both efficiency and stability.
A team led by Prof. Kai Yao from Nanchang University, in collaboration with Suzhou Maxwell Technologies, the CNPC Tubular Goods Research Institute (Shaanxi), the Hong Kong Polytechnic University, the Wuhan University of Technology, and Fudan University (Shanghai), has developed a surface passivation strategy to smooth out these surface defects. They use a thiophenethylammonium compound with a trifluoromethyl group (CF3-TEA) applied through a dynamic spray coating process. This forms a uniform coat even on microtextured surfaces.
The CF3-TEA coating, due to its high polarity and binding energy, effectively mitigates the effects of surface defects, suppressing nonradiative recombination and optimizing electronic levels to facilitate electron transfer to the solar cell's electron-capturing layer. This surface modification enables perovskite/silicon tandem solar cells, based on common textured Czochralski silicon wafers, to achieve nearly 31% efficiency and maintain long-term stability.
Research Report:Surface Molecular Engineering for Fully Textured Perovskite/Silicon Tandem Solar Cells
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