Optoelectronic modulation via isomerism-induced structural effects in low-dimensional bismuth halide perovskites

Abstract

The size, functionality and orientation of organic spacer cations in low dimensional perovskites strongly influence their optoelectronic properties. The effect of spacer cation isomerism leads to the formation of distinct ordered perovskites with well-defined structure–property relationships. In this work, the influence of organic spacer isomers—phenylene diammonium (PDA) cations, namely (o-PDA)₂Bi₂I₁₀, (m-PDA)₂Bi₂I₁₀, and (p-PDA)₂Bi₂I₁₀·6H₂O, on the optoelectronic behavior of the perovskite-like hybrid organic bismuth halide PDA₂Bi₂I₁₀ is systematically investigated. Structural and electronic variations arising from isomerism are examined using X-ray diffraction, UV-visible absorption spectroscopy, and photoelectron spectroscopy, with further validation provided by density functional theory (DFT) calculations. The PDA cation enables the formation of edge sharing [Bi₂I₁₀]⁴⁻ dimers, placed close to each other with the I–I distance between adjacent dimers well within the covalent bond requirements. Octahedral dimers are separated by PDA cations, wherein the ortho position of the substituent in the spacer cation induces higher strain. Both functional groups interact with the same dimer, causing higher angle distortions and density of states resulting in delocalized electron wavefunctions. The extra advantage of the N contribution to the same dimer in OPDA is that it favours higher absorption and conduction, strongly influencing the optoelectronic properties. Revealing such structure property relationships where weak yet cooperative interactions can still yield macroscopic differences in performance can provide a guide for the design of such low dimensional hybrid organic bismuth halides with desirable properties for functional devices.