Towards fluorinated Ruddlesden–Popper perovskites with enhanced physical properties: a study on (3-FC6H4CH2CH2NH3)2PbI4 single crystals

Abstract

The quasi two-dimensional Ruddlesden–Popper perovskites (RPPs) have drawn unprecedented research attention due to their superior long-term environmental stability, abundant structural diversity, and favorable performance in energy-related application scenarios. Rational molecular design strategies, especially proper selections and modifications on the bulky organic spacer cations, play a vital role in obtaining RPPs with desired properties. Recently, employing fluorinated organic cations for further improving the stability and artificial tuning of the performance of RPP based devices has become a promising strategy. However, the mechanism of how fluorination affects the orientation of the spacer cations and the interactions between the organic slabs and the inorganic frameworks is still controversial and needs further investigations, especially on the structure–property relationship based on well-developed single crystals. Herein, we adopted meta-fluorophenethylamine (3-FPEA) as the spacer cation and successfully synthesized a RPP, (3-FPEA)₂PbI₄. Centimeter-sized single crystals of this material were grown through the bottom-seeded solution growth (BSSD) method by using hydroiodic acid as the self-solvent. (3-FPEA)₂PbI₄ exhibits superior thermal stability (from −175 °C to 281 °C), while a reversible phase transition occurs at 212 °C and it remains stable for at least 90 days when exposed to RH 80% at room temperature. Structural refinement and optical characterization indicate that the meta-substitution of fluorine breaks its original symmetry from monoclinic Cc to triclinic P space group accompanying a certain degree of disorder of the Pb²⁺ ions as well as the equatorial bridging I⁻ ions and leads to a smaller band gap (2.20 eV), blue-shifted photoluminescence peak (516 nm), and longer decay time (τ_ave = 1.36 ns). This work offers further insights into the structure–property relationships of the fluorinated RPPs and provides an effective strategy for designing novel RPPs with superior photoelectric properties.