Determination of the complex refractive index and optical bandgap of CH_3NH_3Pbl_3 thin films

摘要

We report the complex refractive index of methylammonium lead iodide (CH_3NH_3PbI_3) perovskite thin films obtained by means of variable angle spectroscopic ellipsometry and transmittance/ reflectance spectrophotometry in the wavelength range of 190nm to 2500nm. The film thickness and roughness layer thickness are determined by minimizing a global unbiased estimator in the region where the spectrophotometry and ellipsometry spectra overlap. We then determine the optical bandgap and Urbach energy from the absorption coefficient, by means of a fundamental absorption model based on band fluctuations in direct semiconductors. This model merges both the Urbach tail and the absorption edge regions in a single equation. In this way, we increase the fitting region and extend the conventional (αhω)~2-plot method to obtain accurate bandgap values.