Spectroscopic ellipsometry study of FAxMA1−xPbI3 hybrid perovskite single crystals

Abstract

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, we 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 pseudo-cubic perovskite band structure of the solid solutions between the two end compounds MAPbI 3 and α-FAPbI 3 .

Phase Diagram of Methylammonium/Formamidinium Lead Iodide Perovskite Solid Solutions from Temperature-Dependent Photoluminescence and Raman Spectroscopies

“…Concerning the case of the FA content of 70%, a recent study that combines the temperature-dependent PL measurements with spectroscopic ellipsometry 21 indicates that this sample consists of FA-and MA-rich segregated regions separated by regions with high concentration of vacancies (predominantly of MA 60,61 ), although maintaining an average stoichiometry of x = 0.7. Such a spatial segregation, which might be caused by a higher temperature heating of the solutions when growing this particular sample, 17,37 turns out to determine the near bandgap PL and absorption properties of the perovskite mixed crystal.…”

Photoluminescence of Bound‐Exciton Complexes and Assignment to Shallow Defects in Methylammonium/Formamidinium Lead Iodide Mixed Crystals

“…From ellipsometry measurements, we can only infer that the pure electronic contribution is of the same order of magnitude for both materials, being ε slightly smaller for FAPbI 3 . [ 45 ]…”

Photoelectrochemical Water Splitting Reaction System Based on Metal-Organic Halide Perovskites

“…Tolerance factors of cubic structures are generally between 0.89 and 1, where a tolerance factor lower than 0.89 could induce a tetragonal (β phase) or orthorhombic (γ phase) structure, whereas higher tolerance factors could induce a two-dimensional (2D) layer structure due to the unstable three-dimensional (3D) B-X bonding [7,30,31]. Actually, the transverse phonon easily displaces the X anion from the B-B intermediate location of the cubic structure [32,33]. The non-perovskite δ phase appears in perovskite materials such as HC(NH 2 ) 2 PbI 3 , FAPbI 3 , CsPbI 3 , and CsSnI 3 , which, unlike the β and γ phases, is caused not by B-X-B angle distortion in the α phase, but by the breaking of the B-X bond [34][35][36].…”