Relative water stability of powder vs. thin films of organic–inorganic halide perovskites including durability of a thin film bis(thiophenyl)-pyrrole lead iodide

Abstract

Organic–inorganic halide perovskites are promising semiconductors for energy-conversion applications, but these materials face an inherent limitation due to poor intrinsic stability, particularly via decomposition upon exposure to water. The dimensionality and stability of perovskite materials are highly dependent on the organic cation in the perovskite. The incorporation of bulky organic ligands in the structure induces 2D layered perovskites where organic ligands are perpendicular to the inorganic layers and can enhance the perovskite moisture stability through the ligand hydrophobicity. Herein, the relative water stabilities of reportedly water-stable organic–inorganic halide perovskites were compared between microparticulate powders and compact thin films. In particular, the 3D perovskite DMASnBr₃ (DMA = dimethylammonium) and 2D perovskite (PEA)₂SnBr₄ (PEA = phenylethylammonium) were synthesized in both forms and characterized for water stability. The microparticulate crystals of both materials exhibited significant stability in water while analogous thin films were highly unstable, which was attributed to the presence of a soluble intermixed phase. A new bulky organic ligand, 8-(2,5-bis(thiophen-2-yl)-1H-pyrrol-1-yl)-octylammonium (2TPO), with hydrophobic wing-like groups to help impede water ingress was synthesized and incorporated into a 2D perovskite structure. A thin film of (2TPO)₂PbI₄ exhibited strongly enhanced durability when fully immersed in water for several minutes or exposed to 85% relative humidity for several days. Material stability was characterized using techniques such as X-ray diffraction (XRD), UV-visible spectroscopy, and X-ray photoelectron spectroscopy (XPS).