Organic cation dynamics and property relationships in layered perovskite derivatives
Date
2022
Authors
Koegel, Alexandra A., author
Neilson, James R., advisor
Ackerson, Christopher, committee member
Kennan, Alan, committee member
Sites, James, committee member
Journal Title
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Volume Title
Abstract
Layered hybrid halide perovskites are materials with applications in solid-state lighting due to their intrinsic white light emission. Layered hybrid perovskite derivates typically have the composition, (A')2 (A)n − 1BnX3n + 1, where A' = R−NH + 3 containing organic cation, A = methylammonium (CH3NH + 3, MA), B = Sn, Pb, X = Cl, Br, I, and n = number of inorganic octahedral layers. They are called "hybrid" materials because of the inclusion of both organic and inorganic moeities in the material. Studies on the three-dimensional perovskite family have shown correlations between restricted rotational motion of the organic cation and structural phase transitions, and electronic properties. However, several questions remain about the coupling between structure, optoelectronic properties, and organic cation dynamics in layered perovskites. Here, I show that the restriction of the organic cation dynamics influences the static inorganic structure. The relevant excited states that produce the observed white light emission are also impacted by the cation dynamics. Chapter One is an overview of layered perovskites and how their structural diversity influences myriad properties. The excited state dynamics proposed in the literature are examined with respect to broad emission. Chapter Two goes in depth and describes the interplay between organic cation dynamics and broadband emission. Quasi-elastic neutron scattering elucidates the dynamic radii of the organic cation ammonium head groups and their role in tilting the inorganic octahedral structure. The smaller crystallographic ii volumes resulting from restricted cation dynamics induces further out-of-plane octahedral tilting. This tilting gives rise to the observed white light emission by the formation of self-trapped excitons. The ammonium headgroup rotations happen on a time-scale that is faster than the recombination of the self-trapped excitons, providing multiple environments for the excited state to sample, leading to inhomogeneous broadening of the white light. In perovskite derivatives, chemical substitution provides an opportunity to change the physical structure. Chapter Three demonstrates how changing the number of inorganic layers influences the cation dynamics. The methylammonium residence times, determined from quasi-elastic neutron scattering, are shorter in the layered perovskite with more inorganic layers. The dielectric screening provided by the increased number of methylammonium cations in the material with thicker inorganic enables the faster molecular motions to occupy larger crystallographic volumes. The inorganic layer hosts the relevant frontier electronic states necessary for broad emission. The population of these frontier states is influenced by a number of factors, namely the out-of-plane tilt angle. Chemical substitution of the inorganic layer affects the out-of-plane tilting; therefore, it is necessary to control the tilt angle as a variable in order to determine a more direct correlation between cation dynamics and white light. Chapter Four discusses the effect of isotopic substitution of the organic cation as a way to understand the influence of dynamics independent of tilt angle. Calculations using a harmonic oscillator approximation show the deuteration of the ammonium headgroup is iii closely coupled to the inorganic lattice, does not have much effect on the residence times of hydrogen motion. Halide substitution in the three-dimensional perovskites leads to reduced organic cation rotation residence times and further correlates to changes in electronic properties. Neutron spectroscopy presented in Chapter Five demonstrates how substitution of the halide site influences the cation dynamics and broadband emission in layered perovskites. Materials with broad emission have a lesser extent of hydrogen rotational motion, which follows previous trends in the literature. Chapter Six further demonstrates the effect of chemical substitution on broad emission and cation dynamics. The formation of solid solutions in the three-dimensional materials influence cation dynamics and phase transitions. White light emission at room temperature is achievable with solid solutions of layered perovskite derivatives. The extent of hydrogen motion determined from neutron scattering does not follow what is previously discussed in Chapter Two. Cation dynamics modify the static inorganic structure and optoelectronic properties in complex, excited state-mediated pathways. The identity of the organic cation dictates the overall perovskite structure and influences the tilting of the octahedra. The cation dynamics influence the broad emission in layered perovskite derivatives. Characterization of these coupled behaviors enable design principles for solid-state lighting applications.
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Subject
emission
hydrogen
scattering
exciton
cations
perovskite