Optical response of mixed methylammonium lead iodide and formamidinium tin iodide perovskite thin films

Mixed tin (Sn) and lead (Pb) based perovskite thin films have been prepared by solution processing combining methylammonium lead iodide (MAPbI3) and formamidinium tin iodide (FASnI3) precursors. Optical response in the form of complex dielectric function (ε = ε1 + iε2) spectra and absorption coefficient (α) spectra of (FASnI3)1-x(MAPbI3)x based perovskite films have been extracted over a spectral range 0.74 to 5.89 eV using spectroscopic ellipsometry. Absorption band edge energy changes as a function of composition for films including FASnI3, MAPbI3, and mixed x = 0.20, 0.35, 0.40, and 0.6 (FASnI3)1-x(MAPbI3)x perovskites. (FASnI3)0.60(MAPbI3)0.4 is found to have the minimum absorption band edge energy near ∼1.2 eV.