Optical constants manipulation of Formamidinium lead iodide perovskites: Ellipsometric and Spectroscopic Twigging

Abstract

Unraveling the knowledge of the complex refractive index and photophysical properties of the perovskite layer is paramount to uncovering the physical process that occurs in a perovskite solar cell under illumination. Herein, we probed the optical and photophysical properties of FAPbI3 (FAPI) and Cs0.1FA0.9PbI3 (CsFAPI) thin films deposited from pre-synthesized powder precursors through the spectroscopic ellipsometer and time-resolved fluorescence spectra. We determined the complex refractive index of perovskite films by fitting the measured spectroscopic ellipsometer data with the three-oscillator Tauc-Lorentz (T-L) model. We deduced that the CsFAPI thin film had a slightly lower absorption coefficient than the FAPI, but a higher refractive index and dielectric constant than the FAPI. The peak photoluminescence (PL) emission of FAPI and CsFAPI thin film on glass substrates was observed around 803 nm and 799 nm, respectively, while on ITO substrates, both FAPI and CsFAPI thin film was quenched and red-shifted to 816 nm. Owing to the CsFAPI thin film's improvement in carrier mobility, decreased electrical resistivity, and longer charge carrier lifetime, we expect it to be more suitable for device fabrication.