Structural Features and Optical Properties of CH3NH3Pb(1−x)SnxCl3 Thin-Film Perovskites for Photovoltaic Applications

In alkyl ammonium lead halide based perovskites, the replacement of toxic Pb +2 with a suitable nontoxic divalent metal cation without losing the photovoltaic performance is one of the prime challenges to the researchers. The understanding of the effect of replacing Pb +2 on the structural and optical properties of alkyl ammonium lead halide based perovskites, and thereafter correlating their photovoltaic performances, comprise a fundamental study which is important towards developing efficient and non-toxic solar cells. In the present work, we used a wet chemical process to substitute Pb +2 with Sn +2 in different proportions into CH 3 NH 3 Pb x Sn (1− x ) Cl 3 . The value of the Goldschmidt tolerance factor, which is a measure of structural stability of the perovskite lattice, was estimated theoretically. The theoretical calculations were correlated further with the experimentally obtained x-ray diffraction patterns of the original and substituted perovskites. The optical properties of CH 3 NH 3 Pb (1− x ) Sn x Cl 3 (0 ≤ x ≤ 1) perovskite thin-films were investigated by the ultraviolet–visible (UV–vis) absorption spectroscopy. The bandgap energy ( E g ) for CH 3 NH 3 Pb (1− x ) Sn x Cl 3 (0 ≤ x ≤ 1) were estimated from the optical absorption spectra. The Urbach energy ( E U ) which predicts defects, disorder and crystalline imperfections within semiconducting thin-films were estimated for the prepared perovskite thin films. The steepness parameter which apprises about strength of electron–phonon ( E e–p ) interaction within perovskites were also estimated from the optical absorbance spectra to understand the effect of replacing Pb +2 with Sn +2 . In addition, the variations in the surface morphologies of the prepared perovskites were studied using scanning electron microscopy. The I–V characteristics of the different cells were analysed and, finally, we attempted to correlate their photovoltaic performances with the opto-structural properties.