Phosphonium-Based Metal Halide Semiconductors: When a Single Atom Dictates Water and Thermal Stability

Phosphonium-based (R-P(R')₃⁺) metal halide semiconductors (MHS) emerged recently as a promising family of materials offering enhanced water and thermal stability over conventional ammonium-based (R-NH₃⁺) MHS. Despite this performance, there is a lack of systematic studies that elucidate the origin of these features as well as the underlying optoelectronic properties. We report here the synthesis of nine new 1D materials (L)PbX₃ (L = C4-P, C6-P, C6-N, P3-P, P4-P, X = I, Br), using custom-made organic ligands, namely, butyl(triethyl)phosphonium bromide (C4-P), hexyl(triethyl)phosphonium bromide (C6-P), benzyl(triethyl)phosphonium bromide (P3-P), benzyl(trimethyl)phosphonium bromide (P4-P), and hexyl(triethyl)ammonium bromide (C6-N). Some of the materials have been water-stable for 2 years so far, while the assembly of the isostructural (C6-N)PbBr₃ and (C6-P)PbBr₃ allowed us to directly identify the impact of a single atom (phosphorus versus nitrogen) on water stability. Notably, the C6-P analog remains water-stable for 2 years, whereas the C6-N analog dissolves in water. It was found that neither crystal packing nor the lack of hydrogen bond interactions with water is responsible for this record stability performance, challenging currently established claims on the origin of water stability in MHS. All materials feature broad light emission at RT, with (P3-P)PbBr₃ exhibiting a PLQY of 22.0% ascribed to permanent traps, while electrochemical studies uncovered for the first time the redox properties and their strong potential for the sensing of per- and polyfluoroalkyl substances (PFAS) in water samples. Our work showcases the potential of these compounds for applications where thermal and water stabilities are interwoven, potentially rendering their (R-NH₃⁺)-based counterparts obsolete.