Speaker: Dibyajyoti Ghosh (Department of Physics & Department of Chemistry, University of Bath, Bath BA2 7AY, U.K.).

Abstract: Metal halide perovskite solar cells have rapidly emerged as leading contenders in photovoltaic technology. However, the poor structural stability and toxicity related to lead seriously hinder the advancement of this field. Compositional engineering is found to be the promising solution to these challenges. In this talk, atomic-scale insights into the local structures and dynamics of mixed A-cation compositions based on formamidimium lead iodide (CH(NH2)2PbI3 ) doped with Cs+, Rb+ and MA+ will be first discussed These compositions show the best performance and have higher stability in the family of hybrid perovskite materials.[1] Our specific findings include the first indication that substitution of low concentrations of smaller cations on the A-site in CH(NH2)2PbI3 results in a global ‘locking’ of the PbI6 octahedra tilting and reduced lattice dynamics. A key impact of this feature is that the rotational or tumbling motion of the CH(NH2)2+ molecular ion in a locked cage is severely restricted. We discuss the impact of locking on the photovoltaic performance and stability. The second part of the talk will be focused on the use of tin and germanium as possible replacements of lead. The tin and germanium-based perovskites are thermodynamically stable and exhibit promising optical properties.[2] These studies help the community to gain a fundamental understanding of composition engineering of hybrid perovskites.

1. Ghosh et al. ACS Energy Lett. 2017, 2, 2424

2. Nagane et al. J. Phys. Chem. C, 2018, 122, 5940