Absorption Edge Fine Structure in the PyPbI3 Perovskite Revealed by Variable-Temperature UV–Vis Diffuse Reflectance Spectroscopy

The goal of this Article is to elucidate the manifestation of charge-transfer transitions in the absorption spectra of 1D perovskites based on a single six-membered ring molecule as the "A"-site cation. The temperature behavior of the spectral features of pyridinium lead triiodide (PyPbI3) was examined in the range 100–300 K using variable temperature UV–vis diffuse reflectance (DR) spectroscopy. At room temperature, the absorption/DR spectrum displayed typical features of 1D lead iodide perovskites: a resonance at 3.13 eV and an absorption edge at 2.73 eV. Cooling the sample to cryogenic temperatures (100 K) resulted in the appearance of an absorption edge fine structure that consisted of three features at closely spaced energies of 3.05, 2.92, and 2.78 eV, and a critically different temperature behavior. Moreover, the high-energy feature acquired a clear excitonic resonance shape that shifted to 3.20 eV at 100 K. Analysis of the absorption spectral changes caused by temperature and dilution in nonabsorbing BaSO4 allowed for distinguishing the absorption edge at 3.05 eV at 100 K, which is attributed to the apparent fundamental transition in the DR spectra, along with two absorption bands occurring at 2.96 and 2.83 eV. The possible relation of these bands to charge-transfer transitions associated with the pyridinium cations is discussed.