Spectroscopic ellipsometry study of FA_xMA_1−xPbI₃ hybrid perovskite single crystals

Organic-inorganic hybrid perovskites have the generic formula ABX$3$⁠, where X denotes a halide anion and A and B stand for an organic and a metal cation, respectively. These hybrid compounds are semiconductors with highly interesting and tunable properties. They are the subject of intense research for several applications, mainly in solar cells but also as light-emitting diodes and lasers. Tunability of the optical properties may be accomplished by site-substituted solid solutions in A, B, or X sites. In particular, some mixed-cation systems show increased stability for these applications. In this work, the authors report on the variation of the optical constants in mixed A-site cation methylammonium/formamidinium (MA/FA) lead iodide perovskites FA$x$MA$1−x$PbI$3$ evaluated by spectroscopic ellipsometry in single crystal samples. The two main polymorphs of FAPbI$3$ are measured. The study is complemented by photoluminescence measurements to gain a better insight into observed spectroscopic features related to the electronic interband transitions of the crystals and their structural integrity. The main result is a continuous variation of the pseudocubic perovskite band structure of the solid solutions between the two end compounds MAPbI$3$ and $α$-FAPbI$3$⁠.