Studies of the crystal structure, defect chemistry, and transport properties of halide perovskite semiconductors
Keywords: Engineering.
Chemical Engineering.

Semiconductors are a class of materials that exhibit a gap between occupied and unoccupied electronic states. This gap is tunable and gives rise to unique optical and electronic properties; as a result, semiconductors form the basis of established technologies including microprocessors, lasers, and photovoltaics. Crystalline semiconductors are a subset of this broad materials class with characteristic long-range order, where the electronic structure is well defined by molecular orbitals within the repeat unit. The present work describes investigations of structure-property relations, defect-forming reactions, crystallography, and magnetic properties of the halide perovskites, an emergent and broad class of crystalline semiconductors with applications in optoelectronic technologies including photovoltaic cells and light-emitting devices. The successful incorporation of lead-halide perovskites and lead-free halide double perovskites in optoelectronic devices has spurred remarkable discoveries and innovation in recent years. Despite impressive research-scale device performance, scale-up and long-term stability continue to lag and present outstanding questions. Indeed, many of the recognized limitations are intrinsic to the halide perovskite crystal, including thermodynamic phase instability, mobile ions, and an abundance of point defects. These characteristics manifest in devices as light, moisture, and temperature sensitivity and current-voltage hysteresis, underscoring the necessity for investigations of the perovskite structure and defect chemistry. Chapter 1 introduces the crystal chemistry and electronic structure of the halide perovskites, as well as properties of interest for technological applications, synthesis techniques, and measurement methodologies relevant to this work. Chapters 2 and 3 establish composition-structure-property relationships and self-assembly effects in rapidly deposited lead-iodide perovskite nanocrystal solids. Halide-perovskite nanocrystals form the basis of quantum dot solar cells and light-emitting devices with color tunability. Specific to light emission, the crystallographic phase (space group) of the nanocrystals influence luminescence efficiency through excited-state symmetry. Using synchrotron grazing incidence wide-angle X-ray scattering (GIWAXS) measurements, we found that the average crystallographic phase of lead-iodide perovskite nanocrystals could be tuned by A-site cation composition in (FA)xCs1--xPbI3 (FA = formamidinium) nanocrystals (Chapter 2). Signatures of orientational ordering were observed in these measurements of (FA)xCs1--xPbI3 nanocrystal solids after spin-coating, an effect typically seen from nanocrystal superlattices formed by slow evaporation of the supporting solvent. We investigated orientational order using crystallographic texture analysis and further optimized spin-coating parameters and colloidal solution chemistry to achieve superlattice-like orientational ordering with this rapid fabrication method (Chapter 3). Chapter 4 focuses on the predominant role of halogen vacancy defects through our characterization of spontaneous iodine exchange in an iodide double perovskite, Cs2SnI6. This unique type of external defect reaction is mostly appreciated for its role in oxide crystals at high temperature; we show that the analogous exchange between iodine vacancies and gaseous iodine is spontaneous at room temperature and directly influences electronic properties. We provide evidence that the iodine vacancy is a shallow electron donor, leading to striking n-type self-doping through manipulation of external pressure of iodine. Single-crystal measurements allow for analysis of the thermodynamics of the exchange equilibrium and the diffusion-limited kinetics associated with transport of vacancies and iodide anions in the bulk crystal. The chemical origin, generality, and implications of this defect reaction across the family of halide perovskite semiconductors are discussed. Chapter 5 presents the synthesis and characterization of a novel magnetic layered chloride perovskite formed through mechanochemical alloying of single and double layered perovskites. The alloy incorporates three unique metal ions through "mosaic" tiling: Ag(I), Cr(III), and Cr(II), which is notably distorted through a Jahn-Teller elongation with the long axis in the perovskite sheet. The disordered solid exhibits a large magnetic moment owing to the combination of two paramagnetic ions—Cr(III) and high-spin Cr(II)—and we investigate the superexchange pathway and magnetic ground state for various [Ag(I)Cr(III)]:Cr(II) ratios. These magnetic property measurements are complemented by lattice-adaptive shrinking cell simulations, which suggest a complex interplay between the [Ag(I)Cr(III)]:Cr(II) ratio, the Jahn-Teller elongation of Cr(II), and properties of the disordered solid including miscibility and percolation thresholds. These novel alloys exemplify a generalized approach to easily synthesize layered magnets with control over both intraplane magnetic exchange interactions and interplanar distance. Chapter 6 introduces a series of complementary single-crystal X-ray scattering studies to reveal the nature of crystallographic disorder in Cs2SnI6. Analysis of energy-resolved diffuse scattering allows us to differentiate between correlated ionic disorder, point-defect-based disorder, and thermal disorder. Manipulating the defect chemistry of Cs2SnI6 via external iodine pressure (introduced in Chapter 4) produced no marked change in energy-integrated reciprocal space maps, total X-ray scattering measurements used to investigate diffuse scattering in momentum space. Inelastic X-ray scattering measurements indicate that the origin of the diffuse scattering intensity is phonon-based, with a strong contribution from low-lying acoustic modes. Significant mode broadening is observed near the zone boundaries, evidencing acoustic-optical coupling that confirms predictions of rattler-like lattice dynamics in Cs2SnI6. The temperature dependence of a low-lying optical mode (octahedral tilting eigenvector) is further discussed in the context of other halide perovskites, particularly given that no cubic-tetragonal phase transition is observed upon cooling from room temperature to 10 K. Finally, Chapter 7 outlines the advances in our understanding of the halide perovskites through this research, connecting the conclusions of Chapters 2-6 to provide perspective on future studies in the focus areas of crystal chemistry, defect chemistry, and disorder.