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  Guerra, J.; Tejada, A.; Korte, L.; Kegelmann, L.; Töfflinger, A.; Albrecht, S.; Rech, B.; Weingärtner, R.: Determination of the complex refractive index and optical bandgap of CH₃NH₃PbI₃ thin films. Journal of Applied Physics 121 (2017), p. 173104/1-6

10.1063/1.4982894
Open Access Version

Abstract:
We report the complex refractive index of methylammonium lead iodide (CH3NH3PbI3) perovskite thin films obtained by means of variable angle spectroscopic ellipsometry and transmittance/reflectance spectrophotometry in the wavelength range of 190 nm to 2500 nm. 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 emiconductors. 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 (αℏω)2-plot method to obtain accurate bandgap values.