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

Abstract

Low-dimensional layered perovskites (LDLPs) have emerged as a promising candidate 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 a photoactive material in photovoltaic devices. One of the effective methods to overcome this bottleneck is to introduce an inherent dipole moment through fluorination of the aromatic alkylammonium organic cation in the perovskite crystal structure. In this work, the structure-property correlation in LDLP based on the position of the fluorine atom (ortho-, meta-, and para-) on the monofluorinated N-methyl-1-phenylmethanaminium (NMPM) as A-site cation is investigated. Positional isomerism at the A-site cation results in inter- and intramolecular hydrogen bonding-induced variations in the crystal packing, including changes in the interlayer spacing and distortion of the inorganic layers. Of the three LDLP derivatives, the ortho-fluorinated derivative showed a triclinic structure with the highest optical band gap. The meta-and para-fluorinated derivatives are isostructural, both adopting the orthorhombic symmetry and having a similar band gap. Theoretical calculations also support the variation in band gap energy. Photoluminescence analysis reveals that ortho-substituted emission was governed by self-trapping of excitons, whereas band-to-band emissions dominated in isostructural meta-and para-derivatives. Positron annihilation spectroscopy (PAS) study envisages that the varied percentage of intrinsic defects in as-synthesised materials results in ferroelectric responses from an otherwise centrosymmetric crystal structure. Further, the photo-ferroelectric behaviour of all three materials is probed, using chronoamperometry and polarisation-electric field (P-E) loop measurements.