Cyclic versus Linear Alkylammonium Cations: Preventing Phase Transitions at Operational Temperatures in 2D Perovskites

Abstract

Two-dimensional lead halide perovskites offer numerous attractive features for optoelectronics owing to their soft, deformable lattices and high degree of chemical tunability. While alteration of the metal and halide ions gives rise to significant modification of the bandgap energy, the organic spacer cations offer in-roads to tuning phase behavior and more subtle functionalities in ways that remain to be understood. Here, we study six variations of 2D perovskites changing only the organic spacer cations and demonstrate that these components intrinsically impact material response in important ways such as altering crystallographic structure, temperature-induced phase transitions, and photoluminescence emission. Two-dimensional perovskites containing commonly utilized aliphatic linear spacers, such as butylammonium, undergo phase transitions near room temperature. These transitions and temperature changes induce spacer-dependent variations in the emission spectra. Conversely, 2D perovskites comprising cyclic aliphatic spacers, such as cyclobutylammonium, are found to lack first-order phase transitions. These cyclic molecules are more sterically hindered within the crystal lattice, leading to temperature-induced contraction or expansion along certain crystallographic planes but no other significant thermal effects; additionally, they undergo changes in their emission spectra that cannot be explained by simple thermal expansion. Given the similarities in the dielectric and chemical makeup of this set of six alkylammonium molecules, these results are unexpected and suggest a large structural and thermal phase space via spacer manipulation that could lead to improved 2D perovskite functionalization.

Dopant effect on the optical and thermal properties of the 2D organic–inorganic hybrid perovskite (HDA)2PbBr4

“…Otherwise, the phase transition temperature is critical for solvent-free device fabrication in 2D HOIPs. Though the organic cation effect on the phase transition of 2D HOIPs is well studied, 24,25 the dopant ion effect is rarely studied.…”

Modulating the Dipole Moment of Secondary Ammonium Spacers for Efficient 2D Ruddlesden‐Popper Perovskite Solar Cells

“…The general formula of typical 2D RP perovskites is L 2 A n‐1 B n X 3n+1 , where L is a monovalent spacer cation, A is a monovalent cation such as CH 3 NH 3 + (MA), HC(NH 2 ) 2 + (FA), and Cs + , B is a divalent metal cation (Sn 2+ , Pb 2+ ), X is a halide anion, and n represents the number of inorganic octahedral layers between two adjacent organic slabs [6] . The configuration and functional groups of bulky organic spacers can significantly affect the bond length and angle of the [PbX 6 ] 4− octahedra, thus tuning the carrier transport process and photoelectric properties of the 2D RP PSCs [7,8] …”

Variable Mechanical Properties in Layered Copper Bromide Hybrid Solids Based on Configuration of Organic Cations

“…Incorporated organic cations can also directly influence material properties exhibited by halide-based hybrid materials. These changes are often attributed to intermolecular interactions and are most apparent in layered structures, where small molecules directly interact in the gallery space between inorganic slabs. , These species interact with the inorganic lattice through hydrogen bonds, electrostatic interactions, and/or Van der Waals forces, as well as discrete intermolecular interactions within the gallery space. , Introducing new intermolecular interactions by changing the gallery space occupant has been observed to organicsuppress thermally induced phase transitions, and facilitate out-of-plane charge transport through the organic components. , Importantly, layering in these compounds (induced by the substitution of larger molecules) often enhances the hydrolytic stability relative to the 3D structures, offering a chemical handle for improving longer-term device stability under ambient conditions (a long-standing challenge for this material class) …”