The effect of organic cation dynamics on the optical properties in (PEA)2(MA)[Pb2I7] perovskite dimorphs

Abstract

Two-dimensional (2D) phenylethyl ammonium (PEA⁺)-methyl ammonium (MA⁺) lead iodide ((PEA)₂(MA)[Pb₂I₇]) hybrid perovskite exists as temperature-dependent dimorphs exhibiting an ill-defined phase transition occurring over the 150–200 K range. Raman scattering, photoluminescence, optical absorbance and solid state MAS NMR spectroscopic methods are employed to investigate the structural complexity, disorder and structure/function of this system. The Raman and ¹H MAS NMR data indicate that the lower bounds of the phase transition at ∼150 K are characterised by attenuated rotational modes and slower motional dynamics throughout the disordered MA⁺ organic sublattice, inducing a strengthening of the MA⁺⋯I hydrogen bonds and a 5 meV increase in the excitonic and photoluminescence energies. It is evident that different recombination mechanisms are dominant for the room temperature and low temperature phases reflecting the importance of MA⁺ dynamics in the optical properties of the material. Single crystal X-ray studies are unable to position the organic cations within the Pb₂I₇ framework; however, ¹H/¹³C MAS NMR measurements describe elements of local structural disorder based on conformational isomerism within the PEA⁺ sublattice. Concomitant I⁻ and Pb²⁺ migration and [Pb₂I₇]³⁻ framework defects create stacking disorder, dislocations and dispersed octahedral tilting leading to additional disorder and distributed MA⁺ and PEA⁺ cation dynamics.