Phase transition and ion transport in lead halide perovskite nanostructures
Lead-halide perovskites are a family of semiconductor materials with excellent optoelectronic properties ideally suited for next-generation photovoltaic and light-emitting applications. Particularly, inorganic perovskites CsPbX3 are drawing increasing research interests owing to their enhanced stability toward moisture, oxygen, and heat, compared to the organic-inorganic hybrid perovskites (e.g. methylammonium lead iodide). However, the fundamental understandings of intrinsic physical properties in inorganic perovskites are still elusive.
Comparing to traditional semiconductors, the halide perovskites have highly reconfigurable crystal structure with relatively easy structural rearrangements and facile ion migration. In chapter 1, the brief introduction of halide perovskites and their unique soft ionic lattice is discussed. There are rich structural phase transitions in the inorganic perovskites owing to their soft and dynamical ionic lattice. The fundamental understanding of intrinsic phase transition behavior is still elusive as the previous study mostly focus on the inhomogenous polycrystalline thin film. Semiconductor nanowires are considered as a good perform for studying their intrinsic physical properties and potential building blocks for various applications in electronics, optoelectronics and energy harvesting. In chapter 2, I developed a novel synthetic method to grow CsPbI3 nanowires, which serve an excellent platform to the intrinsic phase transition. CsPbI3 nanowires undergo a structural phase transition from a non-perovskite to a perovskite phase with thermal heating with significant differences in optical and electric properties. The transformed perovskite phase exhibits meta-stability in inert atmosphere. In chapter 3, we discuss that moisture could introduce halide vacancy in the crystal lattice and lowers the kinetic barrier from perovskite phase to non-perovskite phase, resulting in a reverse phase transition. Via stable, controllable and reversible phase transition, we further realize robust thermochromic solar cells for smart photovoltaic window applications.
Another feature of the soft ionic lattice is facile ion migration. Anion exchange chemistry was demonstrated in CsPbX3 nanostructures with high photoluminescence efficiency throughout the exchange reaction. Nanostructured perovskite heterojunctions are considered as promising building components to extension in high-density modern optoelectronic applications. In chapter 4, we demonstrate CsPbX3 nanowire halide heterojunctions via developing a novel localized anion exchange. These well-defined heterostructures show high spatially resolved down to about 500 nm with RGB multi-color emission, which represent key building blocks for high-resolution displays. Similarly, CsSnBr3-CsPbBr3 cation heterojunction nanowires can be further realized via localized cation exchange. Beyond potential device applications, Nanostructured perovskite heterojunctions enable rich fundamental study, such as the solid-state ion interdiffusion dynamics. In chapter 5, the intrinsic solid-solid anion exchange dynamics can be spatially resolved in these perovskite hetero-junction nanowires through confocal imaging techniques. The intrinsic anion diffusivity in single-crystalline system can be obtained quantitatively. Halide diffusivities were found to be between 10−13 and ∼10−12 cm2/second at about 100 °C, which are several orders of magnitudes lower than those reported in polycrystalline thin films. Vacancy formation free energies computed from molecular simulation are small due to the easily deformable perovskite lattice, accounting for the high equilibrium vacancy concentration.
In a short summary, during my Ph.D research, I have done pioneering work on inorganic perovskite CsPbX3 (X = Cl, Br, I) nanowires synthesis and the systematical study of phase transition dynamics and anion exchange using these single-crystal nanowires platform. My work not only enriches the fundamental understandings in this new class of semiconductor materials but also offer guidelines for engineering the perovskite materials with novel functional devices.