Role of positional isomerism in A-site organic cation: structural variation driven photophysical and ferroelectric responses in centrosymmetric layered perovskites

Abstract

Low-dimensional layered perovskites (LDLPs) have emerged as promising candidates in photovoltaics owing to their enhanced environmental stability. However, the high exciton binding energy, relatively poor conductivity and low charge carrier concentration in this class of materials limit their application as photoactive materials in photovoltaic devices. One effective method to overcome this bottleneck is to introduce an inherent dipole moment through the fluorination of the organic cation in the perovskite crystal structure. In this work, the structure–property correlation in LDLPs based on the position of the fluorine atom (ortho-, meta-, and para-) in monofluorinated N-methyl-1-phenylmethanaminium (NMPM⁺) as an A-site cation is investigated. Positional isomerism in the A-site cation results in inter- and intramolecular hydrogen bonding-induced variations in crystal packing, including changes in the interlayer spacing and distortion of the inorganic layers. Among the three LDLP derivatives, the ortho-fluorinated derivative shows a triclinic structure with the highest optical band gap. The meta- and para-fluorinated derivatives are isostructural, both adopting orthorhombic symmetry and having a similar optical band gap. Theoretical calculations also support the variation in optical band gap. Photoluminescence analysis reveals that emission of ortho-substituted derivative is governed by self-trapping of excitons, whereas band-to-band emissions dominate in isostructural meta- and para-derivatives. The positron annihilation spectroscopy (PAS) study suggests that the variation in the concentration of intrinsic defects in the as-synthesised materials results in ferroelectric responses from an otherwise centrosymmetric crystal structure. Furthermore, the photo-ferroelectric behaviour of all three materials is investigated using chronoamperometry and polarisation–electric field (P–E) loop measurements.