Haibo Xue, in collaboration with colleagues from The Chinese University of Hong Kong has proven that large cations like Guanidium can fit into the perovsikte lattice with simultaneous introduction of the small-size ions like Cs+ and Br.

In this research, FAPbI3 was used as a basic framework and Cs+ and Br as substitutional ions to investigate the role of lattice composition in GA+ incorporation. The key finding is that simultaneous introduction of the small-size Cs+ and Br ions in the FAPbI3 lattice is critical for creating sufficient space to accommodate the large GA ions and that the presence of the Cs+ ions prevents the formation of a GA-contained low-dimensional phase, which both assist GA+ incorporation in the three-dimensional FAPbI3 perovskite lattice. Once entered the perovskite lattice, the GA+ ions can stabilize the lattice structure via forming strong hydrogen bonds with their neighboring halide ions. Such structure modification suppresses halide vacancy-formation, thus leading to improved material properties and also solar cell performance. The combined theoretical and experimental work has been published on Advanced Functional Materials.

Reference: Y. Zhou, H. Xue, Y. H. Jia, G. Brocks, S. Tao and Ni Zhao, Enhanced incorporation of Guanidium in Formamidinium-based perovskites for efficient and stable photovoltaics: the role of Cs and Br, Advanced Functional Materials, 2019, 1905739.