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Open Access Publications from the University of California

Scholarly Works (18 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2019)

Lead-based halide perovskites ABX3 (A= Cs, CH3NH3, CH(NH2)2; B=Pb; X = Cl, Br, I) have emerged as a promising class of semiconducting materials for optoelectronic applications because of their long carrier diffusion length, high absorption coefficient, bright photoluminescence property and the wide band-gap tunability. Lead halide perovskites are comparable in optoelectronic performance to conventional semiconductor technology, yet, they can be synthesized via solution-based methods, making their fabrication versatile and low-cost. However, due to the intrinsic toxicity of Pb, partially or completely replacing lead with other metal cations has been an important topic in recent years.

This dissertation explores three approaches, substitution, alloying and doping, to replace or decrease the amount of Pb in halide perovskite, not only for reducing the toxicity of halide perovskite, but also for tuning the materials’ properties and enabling new applications. Chapter 1 provides an overview of the composition engineering in semiconductors, followed by an introduction of halide perovskites. In the subsequent chapters, the applications of the three approaches would be specifically discussed. Chapter 2 examines the optical and electrical tunability in all-inorganic halide perovskites by alloying CsPbI3 with CsSnI3. The first synthesis of single crystalline CsPbxSn1-xI3 nanowires (NWs) has been developed. The electronic band gaps of CsPbxSn1-xI3 NWs can be tuned from 1.3 eV to 1.78 eV by varying the Pb/Sn ratio, which leads to the tunable photoluminescence (PL) in the near infrared range. More importantly, 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 CsPbxSn1-xI3 alloy NWs promising candidates for future optoelectronic device applications. The substitution method is discussed in Chapter 3. Due to the toxicity of lead, searching for a lead-free halide perovskite semiconducting material with comparable optical and electronic properties is of great interest. Rare-earth based halide perovskite in which Pb is replaced by rare earth metal represents a promising class of materials for this purpose. The first solution phase synthesis of single-crystalline CsEuCl3 nanocrystals with a uniform size distribution centered around 15 nm is discussed in this chapter. The CsEuCl3 nanocrystals have photoluminescence emission centered at 435 nm, with a full width at half maximum (FWHM) of 19 nm. Furthermore, CsEuCl3 nanocrystals can be embedded in a polymer matrix which provides enhanced stability under continuous laser irradiation. Lead-free rare earth cesium europium halide perovskite nanocrystals represent a promising candidate to replace lead halide perovskites. In Chapter 4, doping, the intentional addition of impurities into semiconductor crystals is discussed. A new colloidal method has been used for the synthesis of non-emissive large bandgap transparent halide perovskite Cs2SnCl6 nanocrystals with a diameter of 15 nm. As the transparent media is doped with bismuth ions, it displays strong blue emission with a photoluminescence quantum yield of 22.7 ± 3.9 %. The Density Functional Theory reveals that doping is influenced by the (111) facets of the transparent nanocrystals, which have the lowest surface energy, and that dopants tend to form on the surface of the nanocrystals, acting as luminescent centers. This study not only offers a new perspective to understand the doping mechanisms at nanoscale, but also offers a promising alternative to lead-based halide perovskites nanocrystals including promising optical performance and decreased toxicity.

    Cover page: Cation Composition Engineering of Halide Perovskite Semiconductor Nanostructures
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Multimetallic nanowires with long-range atomic ordering hold the promise of unique physicochemical properties in many applications. Here we demonstrate the synthesis and study the stability of Cu3Au intermetallic nanowires. The synthesis is achieved by using Cu@Au core-shell nanowires as precursors. With appropriate Cu/Au stoichiometry, the Cu@Au core-shell nanowires are transformed into fully ordered Cu3Au nanowires under thermal annealing. Thermally-driven atom diffusion accounts for this transformation as revealed by X-ray diffraction and electron microscopy studies. The twin boundaries abundant in the Cu@Au core-shell nanowires facilitate the ordering process. The resulting Cu3Au intermetallic nanowires have uniform and accurate atomic positioning in the crystal lattice, which enhances the nobility of Cu. No obvious copper oxides are observed in fully ordered Cu3Au nanowires after annealing in air at 200 °C, a temperature that is much higher than those observed in Cu@Au core-shell and pure Cu nanowires. This work opens up an opportunity for further research into the development and applications of intermetallic nanowires. [Figure not available: see fulltext.].
      Cover page: Morphology-controlled transformation of Cu@Au core-shell nanowires into thermally stable Cu3Au intermetallic nanowires
      • 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 (2023)
          Halide perovskites are promising light-absorbing materials for high-efficiency solar cells, while the crystalline phase of halide perovskites may influence the device's efficiency and stability. In this work, we investigated the thermally driven phase transition of perovskite (CsPbIxBr3-x), which was confirmed by electron diffraction and high-resolution transmission electron microscopy results. CsPbIxBr3-x transitioned from δ phase to α phase when heated, and the γ phase was obtained when the sample was cooled down. The γ phase was stable as long as it was isolated from humidity and air. A template matching-based data analysis method enabled visualization of the thermally driven phase evolution of perovskite during heating. We also proposed a possible atomic movement in the process of phase transition based on our in situ heating experimental data. The results presented here may improve our understanding of the thermally driven phase transition of perovskite as well as provide a protocol for big-data analysis of in situ experiments.
            Cover page: Thermally driven phase transition of halide perovskites revealed by big data-powered in situ electron microscopy
            • 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
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2019)
              Ion exchange, as a postsynthetic transformation strategy, offers more flexibilities in controlling material compositions and structures beyond direct synthetic methodology. Observation of such transformation kinetics on the single-particle level with rich spatial and spectroscopic information has never been achieved. We report the quantitative imaging of anion exchange kinetics in individual single-crystalline halide perovskite nanoplates using confocal photoluminescence microscopy. We have systematically observed a symmetrical anion exchange pathway on the nanoplates with dependence on reaction time and plate thickness, which is governed by the crystal structure and the diffusion-limited transformation mechanism. Based on a reaction-diffusion model, the halide diffusion coefficient was estimated to be on the order of [Formula: see text] This diffusion-controlled mechanism leads to the formation of 2D perovskite heterostructures with spatially resolved coherent interface through the precisely controlled anion exchange reaction, offering a design protocol for tailoring functionalities of semiconductors at the nano-/microscale.
                Cover page: Quantitative imaging of anion exchange kinetics in halide perovskites
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2018)
                Facile ionic transport in lead halide perovskites plays a critical role in device performance. Understanding the microscopic origins of high ionic conductivities has been complicated by indirect measurements and sample microstructural heterogeneities. Here, we report the direct visualization of halide anion interdiffusion in CsPbCl3-CsPbBr3 single crystalline perovskite nanowire heterojunctions using wide-field and confocal photoluminescence measurements. The combination of nanoscale imaging techniques with these single crystalline materials allows us to measure intrinsic anionic lattice diffusivities, free from complications of microscale inhomogeneity. 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. Spatially resolved photoluminescence lifetimes and surface potential measurements provide evidence of the central role of halide vacancies in facilitating ionic diffusion. Vacancy formation free energies computed from molecular simulation are small due to the easily deformable perovskite lattice, accounting for the high equilibrium vacancy concentration. Furthermore, molecular simulations suggest that ionic motion is facilitated by low-frequency lattice modes, resulting in low activation barriers for vacancy-mediated transport. This work elucidates the intrinsic solid-state ion diffusion mechanisms in this class of semisoft materials and offers guidelines for engineering materials with long-term stability in functional devices.
                  Cover page: Intrinsic anion diffusivity in lead halide perovskites is facilitated by a soft lattice
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2020)
                  Because of the toxicity of lead, searching for a lead-free halide perovskite semiconducting material with comparable optical and electronic properties is of great interest. Rare-earth-based halide perovskite represents a promising class of materials for this purpose. In this work, we demonstrate the solution-phase synthesis of single-crystalline CsEuCl3 nanocrystals with a uniform size distribution centered around 15 nm. The CsEuCl3 nanocrystals have photoluminescence emission centered at 435 nm, with a full width at half-maximum of 19 nm. Furthermore, CsEuCl3 nanocrystals can be embedded in a polymer matrix that provides enhanced stability under continuous laser irradiation. Lead-free rare-earth cesium europium halide perovskite nanocrystals represent a promising candidate to replace lead halide perovskites.
                    Cover page: Lead-free Cesium Europium Halide Perovskite Nanocrystals
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2018)
                    Semiconductor p-n junctions are fundamental building blocks for modern optical and electronic devices. The p- and n-type regions are typically created by chemical doping process. Here we show that in the new class of halide perovskite semiconductors, the p-n junctions can be readily induced through a localized thermal-driven phase transition. We demonstrate this p-n junction formation in a single-crystalline halide perovskite CsSnI3 nanowire (NW). This material undergoes a phase transition from a double-chain yellow (Y) phase to an orthorhombic black (B) phase. The formation energies of the cation and anion vacancies in these two phases are significantly different, which leads to n- and p- type electrical characteristics for Y and B phases, respectively. Interface formation between these two phases and directional interface propagation within a single NW are directly observed under cathodoluminescence (CL) microscopy. Current rectification is demonstrated for the p-n junction formed with this localized thermal-driven phase transition.
                      Cover page: Phase-transition–induced p-n junction in single halide perovskite nanowire
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