54 research outputs found
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Rich Chemistry in Inorganic Halide Perovskite Nanostructures.
Halide perovskites have emerged as a class of promising semiconductor materials owing to their remarkable optoelectronic properties exhibiting in solar cells, light-emitting diodes, semiconductor lasers, etc. Inorganic halide perovskites are attracting increasing attention because of the higher stability toward moisture, light, and heat as compared with their organic-inorganic hybrid counterparts. In particular, inorganic halide perovskite nanomaterials provide controllable morphology, tunable optoelectronic properties, and improved quantum efficiency. Here, the development controlled synthesis of desired inorganic halide perovskite nanostructures by various chemical approaches is described. Utilizing these nanostructures as platforms, anion exchange chemistry for wide compositional and optical tunabilities is described, and the rich structural phase transition phenomenon and mechanism investigated systematically. Furthermore, these nanostructures and extracted knowledge are applied to design photonic, photovoltaic, and thermoelectric devices. Finally, future directions and challenges toward improvement of the optical, electrical, and optoelectronic properties, exploration of the anion and cation exchange kinetics, and alleviation of the stability and toxicity issues in inorganic lead based halide perovskites are discussed to provide an outlook on this promising field
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Critical Role of Methylammonium Librational Motion in Methylammonium Lead Iodide (CH3NH3PbI3) Perovskite Photochemistry.
Raman and photoluminescence (PL) spectroscopy are used to investigate dynamic structure-function relationships in methylammonium lead iodide (MAPbI3) perovskite. The intensity of the 150 cm-1 methylammonium (MA) librational Raman mode is found to be correlated with PL intensities in microstructures of MAPbI3. Because of the strong hydrogen bond between hydrogens in MA and iodine in the PbI6 perovskite octahedra, the Raman activity of MA is very sensitive to structural distortions of the inorganic framework. The structural distortions directly influence PL intensities, which in turn have been correlated with microstructure quality. Our measurements, supported with first-principles calculations, indicate how excited-state MA librational displacements mechanistically control PL efficiency and lifetime in MAPbI3-material parameters that are likely important for efficient photovoltaic devices
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Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation.
Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption
Structural and spectral dynamics of single-crystalline Ruddlesden-Popper phase halide perovskite blue light-emitting diodes.
Achieving perovskite-based high-color purity blue-emitting light-emitting diodes (LEDs) is still challenging. Here, we report successful synthesis of a series of blue-emissive two-dimensional Ruddlesden-Popper phase single crystals and their high-color purity blue-emitting LED demonstrations. Although this approach successfully achieves a series of bandgap emissions based on the different layer thicknesses, it still suffers from a conventional temperature-induced device degradation mechanism during high-voltage operations. To understand the underlying mechanism, we further elucidate temperature-induced device degradation by investigating the crystal structural and spectral evolution dynamics via in situ temperature-dependent single-crystal x-ray diffraction, photoluminescence (PL) characterization, and density functional theory calculation. The PL peak becomes asymmetrically broadened with a marked intensity decay, as temperature increases owing to [PbBr6]4- octahedra tilting and the organic chain disordering, which results in bandgap decrease. This study indicates that careful heat management under LED operation is a key factor to maintain the sharp and intense emission
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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
Recommended from our members
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
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