Origin of the absorption edge fine structure in Pb-based organic-inorganic 1D perovskites

Herein, using UV-vis variable temperature diffuse reflectance (DR) spectroscopy, significant findings were revealed upon the examination of the absorption spectra of three one-dimensional (1D) perovskites (pyridinium lead bromide (PyPbBr₃), piperidinium lead bromide (PipPbBr₃), and piperidinium lead iodide (PipPbI₃)) in the temperature range of 100-300 K. Results were analyzed and combined with our previously reported data on pyridinium lead iodide (PyPbI₃). A distinct fine structure at the edge of the absorption/DR spectra was observed for all four compounds at cryogenic temperatures. The analysis of the temperature-stimulated evolution of the absorption edge fine structure enabled the identification of the apparent fundamental transition in each spectrum. Two low-energy absorption bands, shifted by 0.05-0.085 eV (ab1) and 0.20-0.27 eV (ab2) at 100 K relative to the apparent bandgap, were also identified. The absorption bands were relatively narrow (fwhm ≤ 0.21 eV) and temperature-independent, making them unobservable at ambient temperatures due to overlap with the red-shifted intrinsic absorption edge. The absorption bands showed similarities in properties of all the compounds. They were tentatively assigned to electronic transitions within Pb²⁺ ions in the [PbBr₆] and [PbI₆] units, given that Pb was the single common compositional element in the investigated 1D perovskites.