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Scholarly Works (30 results)

  • Thesis
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UC Berkeley Electronic Theses and Dissertations (2019)

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.

    Cover page: Phase transition and ion transport in lead halide perovskite nanostructures
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2018)
    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.
      Cover page: Rich Chemistry in Inorganic Halide Perovskite Nanostructures
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2018)
      Pt-based alloys have shown great promise as cathodic catalysts for cost-effective proton-exchange membrane fuel cells. Post-synthesis treatment has been recognized as a critical step to improve the catalytic performance of Pt-based alloys. Here, we present the effects of catalyst processing on the catalytic behavior of Pt-Ni nanoframe electrocatalysts in oxygen reduction reaction. The Pt-Ni nanoframes were made by corroding the Ni-rich phase from solid rhombic dodecahedral particles. A total of three different corrosion procedures were compared. Among them, electrochemical corrosion led to the highest initial specific activity (1.35 mA cm-2 at 0.95 V versus reversible hydrogen electrode) by retaining more Ni in the nanoframes. However, the high activity gradually went down in a subsequent stability test due to continuous Ni loss and concomitant surface reconstruction. On the other hand, the best stability was achieved by a more-aggressive corrosion using oxidative nitric acid. Although the initial activity was compromised, this procedure imparted a less-defective surface, and thus, the specific activity dropped by only 7% over 30 000 potential cycles. These results indicate a delicate trade-off between the activity and stability of Pt-Ni nanoframe electrocatalysts. The obtained understanding of how to balance the activity-stability trade-off via catalyst processing can be generalized to other Pt-based alloys.
        Cover page: Effects of Catalyst Processing on the Activity and Stability of Pt–Ni Nanoframe Electrocatalysts
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2021)
        Abstract: Triiodide perovskites CsPbI 3, CsSnI 3, and FAPbI 3 (where FA is formamidinium) are highly promising materials for a range of optoelectronic applications in energy conversion. However, they are thermodynamically unstable at room temperature, preferring to form low-temperature (low-T) non-perovskite phases with one-dimensional anisotropic crystal structures. While such thermodynamic behavior represents a major obstacle toward realizing high-performance devices based on their high-temperature (high-T) perovskite phases, the underlying phase transition dynamics are still not well understood. Here we use in situ optical micro-spectroscopy to quantitatively study the transition from the low-T to high-T phases in individual CsSnI 3 and FAPbI 3 nanowires. We reveal a large blueshift in the photoluminescence (PL) peak (~38 meV) at the low-T/high-T two-phase interface of partially transitioned FAPbI 3 wire, which may result from the lattice distortion at the phase boundary. Compared to the experimentally derived activation energy of CsSnI3 (~1.93 eV), the activation energy of FAPbI 3 is relatively small (~0.84 eV), indicating a lower kinetic energy barrier when transitioning from a face-sharing octahedral configuration to a corner-sharing one. Further, the phase propagation rate in CsSnI3 is observed to be relatively high, which may be attributed to a high concentration of Sn vacancies. Our results could not only facilitate a deeper understanding of phase transition dynamics in halide perovskites with anisotropic crystal structures, but also enable controllable manipulation of optoelectronic properties via local phase engineering. Impact Statement: Metal halide perovskites are a new class of semiconductors with great promise for a variety of optoelectronic applications. Owing to their soft ionic lattice, halide perovskites often exhibit rich phase transitions between different crystal structures, frequently from the “active” perovskite phases to undesirable “inactive” non-perovskite phases. Understanding and controlling this transition is vital for developing stable, high-performance devices. However, there is limited understanding on how different symmetry, crystal structure, and defect would impact such a phase transition process. In this report, in situ optical micro-spectroscopy is used to systematically investigate the phase transitions from non-perovskite to perovskite phases in individual CsSnI3 and FAPbI3 wires. Compared to the transition from an edge-sharing octahedral non-perovskite structure to a corner-sharing perovskite structure in CsSnI3, the activation energy for the FAPbI3 phase transition is relatively small, indicating a lower energy barrier when starting from a face-sharing octahedral structure in FAPbI3. The high concentration of Sn vacancies is probably responsible for the much higher phase propagation rate in CsSnI3 when compared to a CsPbBrxI3-x system with the same crystal structure but different halide vacancies. Our experimental results expand theknowledge of phase transition in halide perovskites and offer important guidance toward rationally designing more stable and efficient perovskite devices.
          Cover page: Phase transition dynamics in one-dimensional halide perovskite crystals
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Triiodide perovskites CsPbI3, CsSnI3, and FAPbI3(where FA is formamidinium) are highly promising materials for a range of optoelectronic applications in energy conversion. However, they are thermodynamically unstable at room temperature, preferring to form low-temperature (low-T) non-perovskite phases with one-dimensional anisotropic crystal structures. While such thermodynamic behavior represents a major obstacle toward realizing high-performance devices based on their high-temperature (high-T) perovskite phases, the underlying phase transition dynamics are still not well understood. Here we use in situ optical micro-spectroscopy to quantitatively study the transition from the low-T to high-T phases in individual CsSnI3and FAPbI3nanowires. We reveal a large blueshift in the photoluminescence (PL) peak (∼38 meV) at the low-T/high-T two-phase interface of partially transitioned FAPbI3wire, which may result from the lattice distortion at the phase boundary. Compared to the experimentally derived activation energy of CsSnI3(∼1.93 eV), the activation energy of FAPbI3is relatively small (∼0.84 eV), indicating a lower kinetic energy barrier when transitioning from a face-sharing octahedral configuration to a corner-sharing one. Further, the phase propagation rate in CsSnI3is observed to be relatively high, which may be attributed to a high concentration of Sn vacancies. Our results could not only facilitate a deeper understanding of phase transition dynamics in halide perovskites with anisotropic crystal structures, but also enable controllable manipulation of optoelectronic properties via local phase engineering.
            Cover page: Phase transition dynamics in one-dimensional halide perovskite crystals
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            ConspectusA new generation of semiconducting materials based on metal halide perovskites has recently been launched into the scientific spotlight, exhibiting outstanding optoelectronic properties and providing promise for the development of efficient optical devices. As a vivid example, solar cells made from these materials have quickly reached conversion efficiencies exceeding 25%, now on par with well-established technologies, like silicon. Their widespread success is due, in part, to a unique ability to retain high-quality optoelectronic performance while being easily solution-processed into thin films. This feature is what defines them as a brand-new class of optoelectronic materials, with the ability to compete with traditional semiconductors requiring higher processing costs, like the III-Vs or II-IVs. However, the interesting photophysics of metal halide perovskites come with a catch; their soft ionic lattice promotes complex thermal-induced phase transitions and a high capacity for postsynthetic compositional changes, e.g., halide anion exchange. Such dynamic behavior has ultimately made understanding several important structure-property relationships ambiguous and obstructed a clear path toward commercialization due to inherent phase instability.Our aim in this Account is to highlight the fundamental aspects of metal halide perovskites that dictate a stable crystal structure and enable efficient anion exchange, through the lens of thermodynamic preference and phase formation energies. Taking the all-inorganic CsPbI3-xBrx system as a suitable case study, we focus on several ways in which its thermodynamically unstable perovskite structure can be maintained at room temperature and elucidate the restructuring pathways taken during destabilization. In addition, we will discuss the origin and mechanisms of postsynthetic anion exchange in CsPbX3 (X = I, Br, Cl) perovskites, with emphasis made toward direct visualization using in situ optical microspectroscopy and arriving at quantitative results. For several notable features of halide perovskites dealt with in this Account, e.g., strain stabilization, nonperovskite phase restructuring pathway, and lattice anion diffusion, we attempt to rationalize them using state-of-the-art materials modeling techniques.It is within this spirit that we not only modify a broad range of properties existing within metal halide perovskites but also regulate them for enhanced material functionality. For example, controlling partial phase changes and local replacement of halide composition in CsBX3 (B = Pb, Sn and X = I, Br, Cl) nanowires can facilitate the formation of optoelectronic heterojunctions, due to the abrupt change in local crystal structure and the correlated transition in optoelectronic properties. From this combined perspective, metal halide perovskites appear as highly dynamic systems, whereby structural and compositional modifications have a large impact on the underlying phase stability and optoelectronic properties. Thus, we highlight several scientific aspects important to the fundamental understanding of metal halide perovskites, ranging from the underlying mechanism and kinetics through which phase destabilization and anion exchange take place, to tuning the thermodynamic energy landscape using external stimuli. We anticipate that providing a clear perspective for these topics will help deepen our knowledge of the nature of ionic semiconductors and provide the stimulus required to build new research directions toward utilizing halide perovskites within versatile optoelectronic devices.
              Cover page: Phase Transitions and Anion Exchange in All-Inorganic Halide Perovskites
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2020)
              Halide perovskites have attracted increasing research attention with regard to their potential for optoelectronic applications. Because of its low activation energy, ion migration is implicated in the long-term stability and many unusual transport behaviors of halide perovskite devices. However, direct observation and precise control of the ionic transport in halide perovskite crystals remain challenging. Here, we have designed an axial CsPbBr3-CsPbCl3 nanowire heterostructure, in which electric-field-induced halide ion migration was clearly visualized and quantified. We demonstrated that halide ion migration is dependent on the applied electric field and exhibits ionic rectification in this solid-state system, which is due to the nonuniform distribution of the ionic vacancies in the nanowire that results from a competition between electrical screening and their creation/destruction at the electrodes' interfaces. The asymmetric heterostructure characteristics add an additional knob to control the ion movement in the design of advanced ionic circuits with halide perovskites as building blocks.
                Cover page: Solid-State Ionic Rectification in Perovskite Nanowire Heterostructures
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2017)
                Cesium lead iodide (CsPbI3), in its black perovskite phase, has a suitable bandgap and high quantum efficiency for photovoltaic applications. However, CsPbI3 tends to crystalize into a yellow non-perovskite phase, which has poor optoelectronic properties, at room temperature. Therefore, controlling the phase transition in CsPbI3 is critical for practical application of this material. Here we report a systematic study of the phase transition of one-dimensional CsPbI3 nanowires and their corresponding structural, optical, and electrical properties. We show the formation of perovskite black phase CsPbI3 nanowires from the non-perovskite yellow phase through rapid thermal quenching. Post-transformed black phase CsPbI3 nanowires exhibit increased photoluminescence emission intensity with a shrinking of the bandgap from 2.78 to 1.76 eV. The perovskite nanowires were photoconductive and showed a fast photoresponse and excellent stability at room temperature. These promising optical and electrical properties make the perovskite CsPbI3 nanowires attractive for a variety of nanoscale optoelectronic devices. [Figure not available: see fulltext.]
                  Cover page: Structural, optical, and electrical properties of phase-controlled cesium lead iodide nanowires
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2017)
                  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.
                    Cover page: Critical Role of Methylammonium Librational Motion in Methylammonium Lead Iodide (CH3NH3PbI3) Perovskite Photochemistry
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2018)
                    Alloying different semiconductors is a powerful approach to tuning the optical and electronic properties of semiconductor materials. In halide perovskites (ABX3), alloys with different anions have been widely studied, and great band gap tunability in the visible range has been achieved. However, perovskite alloys with different cations at the "B" site are less understood due to the synthetic challenges. Herein, we first have developed the synthesis of single-crystalline CsPb xSn1- xI3 nanowires (NWs). The electronic band gaps of CsPb xSn1- xI3 NWs can be tuned from 1.3 to 1.78 eV by varying the Pb/Sn ratio, which leads to the tunable photoluminescence (PL) in the near-infrared range. More importantly, we found that the electrical conductivity increases as more Sn2+ is alloyed with Pb2+, possibly due to the increase of charge carrier concentration when more Sn2+ is introduced. The wide tunability of the optical and electronic properties makes CsPb xSn1- xI3 alloy NWs promising candidates for future optoelectronic device applications.
                      Cover page: Electrical and Optical Tunability in All-Inorganic Halide Perovskite Alloy Nanowires
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