Perovskite*

The biggest challenge of the 3D high-efficiency perovskite solar cells, such as CH₃NH₃PbI₃ and CH(NH₂)₂PbI₃, is their device instability. 2D perovskite compounds such as butylammonium methylammonium lead iodide perovskite, [CH₃(CH₂)₃NH₃]₂(CH₃NH₃)_n-1Pb_nI_3n+1, present a solution to this problem. Here we present our recent quasi-elastic neutron scattering on a powder sample of the 2D 1-layer system ([CH₃(CH₂)₇NH₃]₂PbI₄) as a function of temperature. Upon cooling, this system undergoes an orthorhombic phase(Acam) to another orthorhombic phase(Pbca) and further to a monoclinic phase(P2₁/a). Our group theory and density functional theory analysis show that the rotational dynamics is dominated by C₃ rotation of NH₃ and CH₃ groups. The relaxation rate of the C₃ modes decreases with decreasing temperature through the orthorhombic-to-orthorhombic transition. On the other hand, in the low-temperature monoclinic phase, the C₃ rotation becomes very slow to detect. The relevance of this finding to electronic properties will also be discussed.