Crystal Structures of a Photovoltaic Two-Dimensional Perovskite

Arguably the biggest challenge of the high-efficiency perovskite solar cells, such as CH$_3$NH$_3$PbI$_3$ and CH(NH$_2$)$_2$PbI$_3$, is their device instability. A recent study\footnote{Tsai, Hsinhan, et al. "High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells." \textit{Nature} 536.7616 (2016): 312-316.} of two dimensional (2D) perovskite compounds, (CH$_3$(CH$_2$)$_3$NH$_3$)$_2$(CH$_3$NH$_3$)$_{n-1}$Pb$_{n}$I$_{3n+1}$, proposed a solution to this problem. This class of materials shows a maximum photovoltaic efficiency of 12.52\%, without any degradation over 2250 hrs of standard light illumination, or 650 hrs of 65\% relative humidity test. In this talk, we present our neutron scattering experiments to study the crystal structure of the 2D 3-layer perovskite (n = 3) as a function of temperature. We have observed two phase transitions between 12 K and 370 K. Rietveld refinements as well as the first principle calculations were used to determine the structures of all three phases.