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

Abstract

Abstract: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 CH 3 NH 3 Pb(I 1-x Br x ) 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, Pb-halide octahedral tilting, as well as 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.

The electronic structure and band interface of cesium bismuth iodide on a titania heterostructure using hard X-ray spectroscopy

“…However, there are reports using HERFD-XAS and HAXPES on lead-based perovskite halides. 32,33 By means of complementary X-ray spectroscopy techniques, the obtained data can be used to gain a detailed understanding of the electronic structure of our bismuth-halide based materials. The data are compared against density functional theory (DFT) calculations to reveal information on photovoltaic optical transitions and device design strategies.…”

Understanding macroscale functionality of metal halide perovskites in terms of nanoscale heterogeneities

“…Exploiting the capabilities of advanced characterization techniques to resolve structural, optical and electrical properties at the nanoscale, it becomes evident that there are substantial heterogeneities in polycrystalline metal halide perovskite thin films that determine macroscale device performance metrics, transient characteristics, and synthetic reproducibility. In recent years, imaging or mapping techniques, including PL imaging [14,15,18,62,77], cathodoluminescence (CL) imaging [16,78], atomic force microscopy (AFM) [17,79], scanning micro x-ray diffraction (XRD) [14], electron beam-induced current [80], scanning near-field optical microscopy [77], Kelvin probe force microscopy (KPFM) [81][82][83], transmission electron microscopy (TEM) [14,84], as well as non-laterally resolved techniques based on x-ray absorption spectroscopy [85][86][87], x-ray photoelectron spectroscopy [88], and solid-state nuclear magnetic resonance (NMR) spectroscopy [89][90][91] have been adopted to characterize local properties of metal halide perovskites.…”

“…On the other hand, the distribution of Cs and Rb in the quadruple cation perovskite material is rather homogeneous [88]. For molecular-scale compositional and structural information, the combination of synchrotron based x-ray absorption spectroscopy with first-principles DFT calculations has proven to be very powerful [85]. It is sensitive to the Pbhalide bond lengths, halide chemistry, and octahedral tilting [85].…”

“…For molecular-scale compositional and structural information, the combination of synchrotron based x-ray absorption spectroscopy with first-principles DFT calculations has proven to be very powerful [85]. It is sensitive to the Pbhalide bond lengths, halide chemistry, and octahedral tilting [85]. Finally, solid-state NMR spectroscopy is a suitable technique to shed light on the microscopic composition of MAPb(I 1-x Br x ) 3 [89] and mixed cation halide perovskites [90,91].…”

Efficient simulation of near-edge x-ray absorption fine structure (NEXAFS) in density-functional theory: Comparison of core-level constraining approaches

“…In this situation, simulated spectra from independent first-principles calculations become invaluable for a reliable assignment. 18,[22][23][24][25][26][27][28][29][30] However, especially for large systems such as frequently encountered in supramolecular or surface-adsorption contexts exceeding computational costs largely restrict the types of methodology that can be employed. While in principle highly accurate techniques such as time-dependent density-functional theory (DFT) [31][32][33][34][35][36] , the Bethe-Salpeter approximation 37,38 , coupled-cluster approaches [39][40][41] , or multi-reference calculations [42][43][44] are available for an often quantitative simulation of NEX-AFS spectra, in practice it is presently often only effective core-level occupation constraining approaches in ground-state DFT that are numerically feasible.…”