Exploring Halide Perovskite Structural Tunability to Design Materials for Dynamic Photovoltaic Windows

Halide perovskites offer exciting potential as photovoltaic materials and simply as semiconductors. Specifically, their structural tunability has become of greater interest as researchers begin to search for novel ways to tune the materials to achieve improved solar cell stability or to target new applications. One potential technology which halide perovskites could enable is dynamically switchable photovoltaic windows: windows which a user can transition between photovoltaically active (dark) and non-photovoltaic (transparent). We build toward this goal in this work by investigating the intercalation and deintercalation of methylamine gas into 2-dimensional Ruddlesden-Popper phase halide perovskites, as well as into mixed 2-D/3-D perovskite films. As has been shown with 3D methylammonium lead iodide films, the intercalation of methylamine into the halide perovskite lattice results in a color change to a clear crystal phase. We find that in some 2-D perovskite systems, deintercalation of the methylamine gas is incomplete, resulting in the formation of some CH₃NH₃PbI₃; however, other 2-D perovskite phases show reversible intercalation/deintercalation with methylamine, indicating stronger binding between the long-chain ligand and the lead halide octahedra of the 2-D perovskite sheet. When integrated into a hybrid 2-D/3-D structure of the type A'₂(CH₃NH₃)_nPb_nI_3n+1, where A' is a strongly binding R-NH₃⁺ moiety such as phenethyl ammonium (PEA), these materials show promising reversibility for methylamine intercalation/deintercalation. This work reveals the relative affinity of various R-NH₃⁺ molecules for the halide perovskite lattice, showing that many of these are not replaced by methylamine, and indicates that templating the 3-D CH₃NH₃PbI₃ structure with long-chain ammonium cations could lead to better reversibility in dynamic photovoltaic windows. On the other hand, some of the same features which could lead to better intercalation/deintercalation of methylamine should result in improved thermal stability in mixed 2D/3D materials. Characterization by x-ray diffraction revealed significant variation in degradation rates for mixed 2D/3D materials upon exposure to dry heat. Work continues to connect thermal stability to the methylamine intercalation and deintercalation reversibility, to develop improved guidelines for the design of 2D/3D halide perovskite materials for an array of applications.