Optical constants manipulation of formamidinium lead iodide perovskites: ellipsometric and spectroscopic twigging†

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 Cs_0.1FA_0.9PbI₃ (CsFAPI) thin films deposited from pre-synthesized powder, by 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. The methylammonium free pure CsFAPI-based perovskite solar cell fabricated in p–i–n configuration, measured a competitive efficiency of 16.14%, characterized by a J_SC of 23.995 mA cm⁻², V_OC of 912 mV, and FF of 73.74%.