Refractive index and extinction coefficient of NH₂CH  =  NH₂PbI₃ perovskite photovoltaic material

Very recently, the NH₂CH  =  NH₂PbI₃ (FAPbI₃) perovskite material has attracted considerable attention in fabricating solar cells (SCs). For a photovoltaic material, its refractive index and extinction coefficient, n(λ) and k(λ), as functions of λ, are important to study its optical properties and to estimate the power conversion efficiency potential for the SCs made of it. As far as we know, to date there has been no reports of n(λ) and k(λ) for FAPbI₃ material. In this article, with spectroscopic ellipsometry (SE) measurements, the n(λ) and k(λ), as well as E _g  =  1.45 eV for FAPbI₃, are acquired. The fast deposition crystallization (FDC) procedure combined with the slowed down annealing (SDA) process is applied to fabricate smooth and uniform FAPbI₃ film on quartz substrate. Several kinds of organic solvents were tried as the second solvent in the FDC procedure, and it is found that when petroleum ether is used, the smallest surface roughness and good FAPbI₃ material purity of the FAPbI₃ film can be acquired. The k(λ) results for FAPbI₃ obtained by SE, calculated from the n(λ) using the Kramers-Kronig relationship, by absorbance, and by first-principles calculations, are compared. The n(λ) and k(λ) for FAPbI₃ are also compared with those for CH₃NH₃PbI₃, GaAs and c-Si.