Structural Features and Optical Properties of CH3NH3Pb(1-x)SnxCl3Thin-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 CH3NH3PbxSn(1-x)Cl3. 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 CH3NH3Pb(1-x)SnxCl3 (0 <= x <= 1) perovskite thin-films were investigated by the ultraviolet-visible (UV-vis) absorption spectroscopy. The bandgap energy (E-g) for CH3NH3Pb(1-x)SnxCl3(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 (Ee-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.