Proceedings of Asia-Pacific Conference on Perovskite, Organic Photovoltaics&Optoelectronics (IPEROP25)
Publication date: 17th October 2024
Tin-based perovskites, with their environmental friendliness and broader spectral response range, offer an excellent complement to lead perovskite solar cells. In recent years, all-perovskite tandem solar cells featuring a tin-lead mixed structure have rapidly evolved, becoming a significant category of thin-film solar cells. The efficiency of tin-based perovskite solar cells has rapidly developed, however, the oxidation of Sn2+ ions to Sn4+ ions leads to a high defect density in the tin-based perovskite films. This oxidation is one of the key factors limiting the efficiency improvement of tin-based perovskite cells.
In earlier work, our research group discovered that low-dimensional structures could reduce the oxidation of tin-based perovskites[1]. First-principles calculations showed that low-dimensional structures significantly decrease the enthalpy change of the oxidation reaction. Based on this, we deduced that the oxidation of tin-based perovskites is closely related to their energy level orbitals, with a higher HOMO level possibly being a critical factor leading to the easy oxidation of Sn2+ ions.
Building on this understanding, in this work, we introduced electron-withdrawing ligands to suppress the oxidation of tin-based perovskites[2]. The article compares the passivation effects of methylphosphonic acid (MP) and chloromethylphosphonic acid (CMP) on the surface of tin-based perovskites. Based on first-principles calculations, we found that molecules substituted with chlorine atoms and combined with the surface of tin-based perovskites result in a lower HOMO level of SnI2. This confirms the regulatory effect of electron-withdrawing molecules on the electronic structure of the perovskite surface.
Compared to MP molecules, CMP can more effectively suppress the oxidation of Sn2+ in the film. Consequently, the defect density in the tin-based perovskite films was significantly reduced. The efficiency of tin-based perovskite devices prepared using this method showed a notable improvement, with the efficiency of tandem cell devices reaching up to 27.3% (certified efficiency of 26.9%).
We gratefully acknowledge financial support from the National Natural Science Foundation of China (grant nos. 61935016, 92056119, 22175118), National Key Research and Development Programme of China (under grant no. 2021YFA0715502), the Double First-Class Initiative Fund of ShanghaiTech University and the Science and Technology Commission of Shanghai Municipality (grant nos.20XD1402500, 20JC1415800 and 21ZR1442100). We appreciate the Instrument Analysis Center and Centre for High resolution Electron Microscopy (CħEM) of ShanghaiTech University. We thank W. Liu and Y. Huang for helping with the cyclic voltammetry measurement and analysis. The computational support is provided by the high performance computing facility in ShanghaiTech University.
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