Together with our experimental collaboration Prof. Ni Zhao, Junke Jiang has demonstrate the combination of both small Cs+ and large BA+ cations leads to most efficient passivization for perovskite films. The results is recently published on Journal of Materials Chemistry A.

Abstract: Two-step sequential deposition has been shown as an effective method to improve the quality of perovskite films and performance of perovskite solar cells (PSCs). In this work, we have developed a two-step sequential deposition method, incorporating an alkali cation (e.g., Cs+) and a benzylammonium (BA+) cation in formamidinium (FA+)-based perovskite thin films for highly efficient and stable PSCs. By combining experimental characterizations and theoretical calculations, we demonstrate that not only the co-incorporation of alkali cations and BA+ significantly improves crystallinity and orientational growth of perovskite films, but also Cs+ enhances passivation effect of BA+ ions on perovskite surface via enhanced NH…I hydrogen bonding. In particular, inclusion of Cs+ in BA+-passivated perovskites promotes the preferred orientation of defect-free (1 0 0) facets. As a result, we observed remarkable improvements in both open-circuit voltage and fill factor of formamidinium-benzylammonium-cesium (FABACs)-based PSCs, achieving a champion efficiency of 22.5%. More importantly, the FABACs perovskite films demonstrated superior resistance to humidity and photo-thermal stress, and the perovskite-based devices (without encapsulation) retained over 95% of the initial efficiency after exposure to air for 2 months. Manipulating perovskite composition by combining small alkali and large organic cations in two-step sequential deposition provides a new path to highly efficient and stable perovskite devices.