Manifestation of excitonic resonance in diffuse reflectance spectra of halide perovskites

The application of variable-temperature UV-vis diffuse reflectance (DR) spectroscopy within the range of 100-300 K has facilitated a detailed investigation of the resonance-type feature observed near the absorption edge of several halide perovskites. This study focused on two 3D perovskites, CsPbBr₃ and MAPbBr₃ (where MA represents methylammonium), as well as two 1D perovskites, piperidinium lead iodide (PipPbI₃) and pyridinium lead iodide (PyPbI₃). The resemblance in shape, spectral positions, and temperature-dependent behavior of the resonance-type features in the DR spectra to those found in the well-documented reflection (specular) spectra of single crystals of the same compounds enabled us to attribute these features in the DR spectra to excitonic characteristics in all the studied perovskites. This finding suggests that specular reflection predominates in the DR spectra within the spectral region of strong excitonic absorption. Therefore, the application of the Kubelka-Munk (KM) transformation to the DR spectra-yielding pseudo-absorption spectra-should be restricted to regions where sub-excitonic absorption is relatively weak, as the KM approach only accounts for isotropically scattered light. Furthermore, our analysis revealed that the DR spectra, with excitonic resonances transforming into pseudo-absorption spectra characterized by two regularly spaced band structures, also encompass a lower energy "excess" absorption band. This structure is typical of the pseudo-absorption spectra observed in various low-dimensional halide perovskites at room temperature and contributes to the discrepancies observed in spectral interpretations.