Long-Lived Coherent Acoustic Phonons in Epitaxially Grown III-V Adiabatic Cavities

We provide evidence of strongly confined coherent acoustic phonons inside high quality factor phononic cavities that exhibit tailoredphonon potentials. Using GaAs/AlAs quasiperiodic superlattices, these functional phonon potentials are realized by adiabatically changing the layer thicknesses along the growth direction. Room temperature ultrafast vibrational spectroscopy reveals discrete phonon levels in the range of $\approx 96-101$ GHz. Additionally, we confirm that phononic cavities significantly retard the energy loss rate of the photoexcited carriers as evidenced by time-resolved photoluminescence measurements. These results highlight the potential of opto-phononic devices that can bridge the divide between phononics and optoelectronics by concurrently engineering electronic and phononic properties.Comment: In this version, we have incorporated a new section addressing the temporal dynamics of cavity phonons. Additionally, the analysis and discussion of time-resolved photoluminescence (TRPL) results have been enhance

Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behaviour. In the higher temperature, higher symmetry phases of these materials, several complex structural features have been observed. The local structure can differ greatly from the average structure and there is evidence that dynamic two-dimensional structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI$_3$ is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, we reveal that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and we report the length and timescales of the motion. Finally, we investigate and visualise the spatial arrangement of these features and their interactions, providing a comprehensive picture of local structure in the higher symmetry phases

Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases

Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping

In recent years, halide perovskite materials have been used to make high performance solar cell and light-emitting devices. However, material defects still limit device performance and stability. Here, we use synchrotron-based Bragg Coherent Diffraction Imaging to visualise nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. We find significant strain heterogeneity within MAPbBr$_{3}$ (MA = CH$_{3}$NH$_{3}^{+}$) crystals in spite of their high optoelectronic quality, and identify both $\langle$100$\rangle$ and $\langle$110$\rangle$ edge dislocations through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, we uncover dramatic light-induced dislocation migration across hundreds of nanometres. Further, by selectively studying crystals that are damaged by the X-ray beam, we correlate large dislocation densities and increased nanoscale strains with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. Our results demonstrate the dynamic nature of extended defects and strain in halide perovskites and their direct impact on device performance and operational stability.Comment: Main text and Supplementary Information. Main text: 15 pages, 4 figures. Supplementary Information: 16 pages, 27 figures, 1 tabl

Anomalous structural evolution and glassy lattice in mixed-halide hybrid perovskites

Hybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed-halide hybrid perovskite single crystals of MAPbI3-xBrx (MA = CH3NH3+ and x = 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed-halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed-halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties.Peer ReviewedPostprint (published version

Understanding the structure and dynamics of hybrid lead halide perovskites for photovoltaics

The rapid progress in creating high-efficiency photovoltaic devices from hybrid and all-inorganic halide perovskites in the last decade has seen increasing interest in the physical properties of these materials. Major differences are observed with respect to conventional semiconductors in terms of the dynamical lattice properties, with these novel processes thought to underlie some of the remarkable optoelectronic properties reported. Thermal lattice fluctuations, known as phonons, are soft and diverge from the classical harmonic picture, and thus effectively act as a novel source of structural fluctuations, commonly absent in classical semiconductors. The organic cation with finite dipole moment, an integral part of the hybrid perovskite lattice, exhibits stochastic reorientation introducing additional perturbations that change the potential energy landscape experienced by charge carriers. To allow for the engineering of novel halide perovskite materials in the quest for further performance improvements, one of the requirements is a high level understanding of their lattice dynamics. Its complex nature arises from the overlapped contributions from cation reorientations and anharmonic fluctuations which this thesis disentangles. This was achieved by inducing structural phase transitions by lowering the lattice temperature and by varying the composition at both A and X site in the perovskite ABX3 lattice. Thus, this thesis combines X-ray, Brillouin and inelastic neutron scattering techniques to probe static and dynamic lattice properties across a wide dynamical range in energy and reciprocal space. It is demonstrated that while formamidinium based perovskites have the acoustic phonon properties attributed to classical semiconductors, methylammonium based perovskites exhibit strong coupling between the cations and the inorganic lattice, leading to acoustic phonon softening near the zone centre. The combination of methylammonium at the A-site and bromine at X site results in local centro-symmetry breaking, as detected through the observation of the piezoelectric coupling between methylammonium and acoustic phonons near the zone centre. Diffuse scattering rods stretching across the Brillouin zone edges are observed at low temperatures through single crystal X-ray diffraction in the MAPbBr1.5Cl1.5 mixed halide perovskite. The elastic neutron scattering confirmed the presence of diffuse scattering signal along the R-M Brillouin zone edge line in MAPbBr3 and FAPbBr3 perovskites in high temperature cubic and tetragonal phases. This is direct evidence for the presence of short range correlated nano-domains, a manifestation of the dynamically disordered halide perovskite lattice. Based on the experimental findings this thesis discusses how the anion and cation composition influences whether domains may possess ferroelastic or ferroelectric character. The insights gained can be exploited for further improvements in perovskite based solar cells, either through ferroelectric photovoltaics or strain engineering

Long‐Lived Acoustic Phonon and Carrier Dynamics in III–V Adiabatic Cavities

Publication status: PublishedFunder: University of New South Wales; doi: http://dx.doi.org/10.13039/501100001773Funder: Australian Institute of Nuclear Science and Engineering; doi: http://dx.doi.org/10.13039/501100001023AbstractEvidence of strongly confined coherent acoustic phonons inside high quality factor phononic cavities that exhibit tailored phonon potentials is provided. Using GaAs/AlAs quasiperiodic superlattices, functional phonon potentials are realized by adiabatically changing the layer thicknesses along the growth direction. Room temperature ultrafast vibrational spectroscopy reveals discrete phonon modes with frequencies in the range of ≈96–101 GHz. Additionally, it is confirmed that phononic cavities impact the energy loss rate of the photoexcited carriers, as evidenced by time‐resolved photoluminescence measurements. These results highlight the potential of concurrently engineering optoelectronic and phononic properties for a range of novel applications.</jats:p

Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains.

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases

Research data supporting "The impact of interfacial quality and nanoscale performance disorder on the stability of alloyed perovskite solar cells"

This repository contains the data required to reproduce the figures from the associated manuscript. The dataset contains optical and electrical microscopic and macroscopic characterisation of a wide array of perovskite solar cells. The dataset is split into 4 sections each corresponding to a figure in the manuscript the data is from. Each folder contains a jupyter notebook used to open, analyse and plot the final figure from the text