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Open Access Publications from the University of California

Scholarly Works (13 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2015)

Thin film solar cells have attracted considerable attentions due to their lightweight, low material consumption, ease of fabrication, and potentially high power-conversion efficiency. However, significant cost reductions as well as large-scale production are necessary to compete with the conventional utility power. To reduce cost, vacuum-free manufacture process for each solar cell component is needed. This dissertation focus on novel and cost-effective methods on solution processes of thin film solar cells, ranged from top transparent electrode, the various absorbing layers, and eventually achieving high performance devices. For the transparent electrode, our focus is on the silver nano-wires, and the nano-junction within the network. For the absorbing layer, our targets are: Cu2ZnSn(S,Se)4 (CZTS), and CH3NH3PbX3 (MAX, X= I, Cl, Br) perovskite materials, respectively. In Chapter 2, a formation of Ag-NW contacts was proven to be an efficient method to reduce the contact resistance using the Joule heating, current crowding, and electromigration effect. Transparent electrodes have been applied to optoelectronic devices. Chapter 3, a metal nanowire composite layer was demonstrated to achieve fully solution-processed as a component of CuIn(S,Se)4 (CISS) solar cells. The high performance of the nanowire composite and the role of such electrodes in high performance solar devices were elucidated. Further improvement of the nanowire network to achieve better stability was also demonstrated. Chapter 4 presented a doping control method to investigate the effect of sodium dopant to the CZTS solar cells in terms of device performance, properties, and film formation. A novel synthesis method of CZTS:Na nanocrystal was demonstrated. Sodium incorporation method successfully improved the efficiency by around 50%. In addition, the role of sodium incorporation was investigated. In Chapter 5, the transformation of solution-processed MAX3 perovskite solar cells was investigated and possible reaction pathways were proposed. Secondary phases from this process were identified and the corresponding physical properties during the transformation were further studied.

    Cover page: Novel Thin Film Solar Cells: Film Formation/Properties and Device Physics
    • Article
    • Peer Reviewed
    LBL Publications (2019)
    Hybrid metal halide perovskites have shown an unprecedented rise as semiconductor building blocks for solar energy conversion and light-emitting applications. Currently, the field moves empirically towards more and more complex chemical compositions, including mixed halide quadruple cation compounds that allow optical properties to be tuned and show promise for better stability. Despite tremendous progress in the field, there is a need for better understanding of mechanisms of efficiency loss and instabilities to facilitate rational optimization of composition. Starting from the device level and then diving into nanoscale properties, we highlight how structural and compositional heterogeneities affect macroscopic optoelectronic characteristics. Furthermore, we provide an overview of some of the advanced spectroscopy and imaging methods that are used to probe disorder and non-uniformities. A unique feature of hybrid halide perovskite compounds is the propensity for these heterogeneities to evolve in space and time under relatively mild illumination and applied electric fields, such as those found within active devices. This introduces an additional challenge for characterization and calls for application of complimentary probes that can aid in correlating the properties of local disorder with macroscopic function, with the ultimate goal of rationally tailoring synthesis towards optimal structures and compositions.
      Cover page: Understanding macroscale functionality of metal halide perovskites in terms of nanoscale heterogeneitiesCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      LBL Publications (2020)
      The antisolvent dripping time during spin-coating of CH3NH3PbI3 (MAPbI3) strongly impacts film morphology as well as possible formation of the intermediate precursor phase and—consequently—device performance. Here, we use in situ photoluminescence (PL) to directly monitor the fast-occurring changes during MAPbI3 synthesis. These measurements reveal how the ideal timing of the antisolvent leads to homogeneous nucleation and pinhole-free films. In addition, these films show significantly reduced nonradiative recombination with 1.5 orders of magnitude difference in absolute PL quantum yield compared to films where no antisolvent is applied. Low-temperature PL measurements confirm that antisolvent treatment reduces the number of trap states presumably in the bulk material. However, if the antisolvent is dripped late, heterogeneous nucleation via the orthorhombic (MA)2(DMF)2Pb3I8 intermediate phase leads to a needle-like morphology that can be correlated to a red-shifted in situ PL signature. We find that the ideal dripping window is very narrow when using dimethylformamide as the solvent, confirmed by device performance metrics. Finally, the use of in situ PL is discussed to gain information about nucleation and growth and ultimately increase reproducibility.
        Cover page: Dynamics of Antisolvent Processed Hybrid Metal Halide Perovskites Studied by In Situ Photoluminescence and Its Influence on Optoelectronic Properties
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2016)
        A vibronic resonance between Au{111} surface states and adsorbed CN vibrations has been predicted, which we target for study. We have formed stable monolayers of cyanide on Au{111} and observe a hexagonal close-packed lattice with a nearest neighbor distance of 3.8 ± 0.5 Å. Cyanide orients normal to the surface attached via a Au-C bond. We show that the substrate-molecule coupling is particularly strong, leading to ultrafast electron transfer from the cyanide molecules to the Au{111} substrate as measured by resonant Auger spectroscopy using the core-hole clock method. The CN/Au{111} system is a simple example of a strongly interacting adsorbate-substrate system and will be the subject of a number of further studies, as discussed.
          Cover page: Surface Structure and Electron Transfer Dynamics of the Self-Assembly of Cyanide on Au{111}
          • Article
          • Peer Reviewed
          LBL Publications (2020)
          The exploration of the synthetic space of halide perovskites hinges on an enormous number of parameters requiring time-consuming experimentation to decouple and optimize. Here, the formation of the prototype material CH3NH3PbI3 (MAPbI3) is investigated at different time and length scales using multimodal in situ measurements to monitor the evolution of crystalline phases, morphology, and photoluminescence as a function of the lead precursors. Kinetically fast formation of crystalline precursor phases already during the spin-coat deposition is observed using lead iodide (PbI2) or lead chloride (PbCl2) routes. These precursor phases most likely template final MAPbI3 film morphology. In particular, the emergence of the “needle-like” structure is shown to appear before film annealing. In situ photoluminescence measurements suggest nanoscale nucleation followed by rapid nuclei densification and growth. Using this multimodal in situ approach, different formation pathways can be identified either via precursor phases in the PbI2 and PbCl2 routes or direct perovskite formation from molecular building blocks as observed in the lead acetate (PbAc2) route. Correlation of in situ results with photovoltaic device performance demonstrates the power of in situ multimodal techniques, paves the way to a fast screening of synthetic parameters, and ultimately leads to controlled synthetic procedures that yield high-efficiency devices.
            Cover page: Revealing the Dynamics of Hybrid Metal Halide Perovskite Formation via Multimodal In Situ Probes
            • Article
            • Peer Reviewed
            LBL Publications (2019)
              Cover page: Probing the in situ dynamics of structure–property evolution in hybrid perovskite thin films spincoated from complex fluids by a custom-designed beamline-compatible multimodal measurement chamber
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2016)
              β-Amyloid aggregates in the brain play critical roles in Alzheimer's disease, a chronic neurodegenerative condition. Amyloid-associated metal ions, particularly zinc and copper ions, have been implicated in disease pathogenesis. Despite the importance of such ions, the binding sites on the β-amyloid peptide remain poorly understood. In this study, we use scanning tunneling microscopy, circular dichroism, and surface-enhanced Raman spectroscopy to probe the interactions between Cu2+ ions and a key β-amyloid peptide fragment, consisting of the first 16 amino acids, and define the copper-peptide binding site. We observe that in the presence of Cu2+, this peptide fragment forms β-sheets, not seen without the metal ion. By imaging with scanning tunneling microscopy, we are able to identify the binding site, which involves two histidine residues, His13 and His14. We conclude that the binding of copper to these residues creates an interstrand histidine brace, which enables the formation of β-sheets.
                Cover page: Copper Ion Binding Site in β‑Amyloid Peptide
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2015)
                Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.
                  Cover page: The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2015)
                  2D molybdenum disulfide (MoS2 ) has distinct optical and electronic properties compared to aggregated MoS2 , enabling wide use of these materials for electronic and biomedical applications. However, the hazard potential of MoS2 has not been studied extensively. Here, a comprehensive analysis of the pulmonary hazard potential of three aqueous suspended forms of MoS2 -aggregated MoS2 (Agg-MoS2 ), MoS2 exfoliated by lithiation (Lit-MoS2 ), and MoS2 dispersed by Pluronic F87 (PF87-MoS2 )-is presented. No cytotoxicity is detected in THP-1 and BEAS-2B cell lines. However, Agg-MoS2 induces strong proinflammatory and profibrogenic responses in vitro. In contrast, Lit- and PF87-MoS2 have little or no effect. In an acute toxicity study in mice, Agg-MoS2 induces acute lung inflammation, while Lit-MoS2 and PF87-MoS2 have little or no effect. In a subchronic study, there is no evidence of pulmonary fibrosis in response to all forms of MoS2 . These data suggest that exfoliation attenuates the toxicity of Agg-MoS2 , which is an important consideration toward the safety evaluation and use of nanoscale MoS2 materials for industrial and biological applications.
                    Cover page: Differences in the Toxicological Potential of 2D versus Aggregated Molybdenum Disulfide in the Lung
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2016)
                    Cyanide monolayers on Au{111} restructure from a hexagonal close-packed lattice to a mixed-orientation "ribbon" structure through thermal annealing. The new surface structure loses most of the observed surface features characterizing the initial as-adsorbed system with "ribbon" domain boundaries isolating rotationally offset surface regions where the orientation is guided by the underlying gold lattice. A blue shift to higher frequencies of the CN vibration to 2235 cm-1 with respect to the as-adsorbed CN/Au{111} vibration at 2146 cm-1 is observed. In addition, a new low-frequency mode is observed at 145 cm-1, suggesting a chemical environment change similar to gold-cyanide crystallization. We discuss this new structure with respect to a mixed cyanide/isocyanide monolayer and propose a bonding scheme consisting of Au-CN and Au-NC bound molecules that are oriented normal to the Au{111} surface.
                      Cover page: Hexagons to Ribbons: Flipping Cyanide on Au{111}
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