Determining Atomic-Scale Structure and Composition of Organo-Lead Halide Perovskites by Combining High-Resolution X‑ray Absorption Spectroscopy and First-Principles Calculations

We combine high-energy
resolution fluorescence detection (HERFD)
X-ray absorption spectroscopy (XAS) measurements with first-principles
density functional theory (DFT) calculations to provide a molecular-scale
understanding of local structure, and its role in defining optoelectronic
properties, in CH3NH3Pb­(I1–xBrx)3 perovskites.
The spectra probe a ligand field splitting in the unoccupied d states
of the material, which lie well above the conduction band minimum
and display high sensitivity to halide identity, Pb-halide bond length,
and Pb-halide octahedral tilting, especially for apical halide sites.
The spectra are also sensitive to the organic cation. We find that
the halides in these mixed compositions are randomly distributed,
rather than having preferred octahedral sites, and that thermal tilting
motions dominate over any preferred structural distortions as a function
of halide composition. These findings demonstrate the utility of the
combined HERFD XAS and DFT approach for determining structural details
in these materials and connecting them to optoelectronic properties
observed by other characterization methods.