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Open Access Publications from the University of California

Scholarly Works (24 results)

  • Article
LBL Publications (2012)
    Cover page: Efficient scalable algorithms for hierarchically semiseparable matrices
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2018)

    The perovskite solar cell (PSC) is a photovoltaic device. With the tunable bandgap, long carrier diffusion length, and low exciton binding energy of various perovskite materials, power conversion efficiency (PCE) of 22.7% has been achieved. Though PSCs have high energy conversion efficiencies and low manufacturing cost, low device stability limits its further application and commercialization.

    One of the major bottlenecks hindering the improvement of device stability is the interaction of components in the hole transport layer: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP). LiTFSI is hygroscopic and accelerates perovskite decomposition. tBP, which evaporates easily, is corrosive to perovskite materials. Previous research has assumed that tBP and LiTFSI have similar working mechanisms in perovskite solar cells as what they performed in solid-state dye-sensitized solar cells (ss-DSSCs). However, due to the different device configurations and functional layers, it is necessary to understand the actual functions of these hole transport layer components in PSCs in order to further improve device stability and efficiency.

    In our study, a spectrum-dependent mechanism for the oxidation of hole transport material 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) with LiTFSI in PSCs was proposed. The perovskite layer plays different roles in the Spiro-OMeTAD oxidization for various spectral ranges. Regarding tBP, we confirmed its role as a HTL morphology controller in PSCs for the first time. Our observations suggest that tBP significantly improves the uniformity of the HTL and avoids accumulation of LiTFSI. We also observed the formation of tBP-LiTFSI complexes in PSCs. These complexes in PSCs can alleviate the negative effects of tBP and LiTFSI while maintaining their positive effects on perovskite materials. Consequently, a minor change of tBP:LiTFSI ratio displays huge influences on the stability of perovskite. As a result, the PSCs fabricated with our suggested tBP:LiTFSI ratio have decreased efficiency variation with enhanced device stability. Our understanding of the functions of these hole transport layer components and perovskite on a molecular level paves the way for further improvements to PSCs performance.

      Cover page: Function of Hole Transport Layer Components in Perovskite Solar Cells
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      We report anomalous physical properties of high-quality single-crystalline FeSi over a wide temperature range of 1.8-400 K. The electrical resistivity ρ(T) can be described by activated behavior with an energy gap Δ = 57 meV between 150 and 67 K, below which the estimated energy gap is significantly smaller. The magneto-resistivity and Hall coefficient change sign in the vicinity of 67 K, suggesting a change of dominant charge carriers. At ∼19 K, ρ(T) undergoes a cross-over from semiconducting to metallic behavior which is very robust against external magnetic fields. The low-temperature metallic conductivity depends strongly on the width/thickness of the sample. In addition, no indication of a bulk-phase transition or onset of magnetic order is found down to 2 K from specific heat and magnetic susceptibility measurements. The measurements are consistent with one another and point to complex electronic transport behavior that apparently involves a conducting surface state in FeSi at low temperatures, suggesting the possibility that FeSi is a 3D topological insulator.
        Cover page: Evidence for a conducting surface ground state in high-quality single crystalline FeSi
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2017)
        Atomic layer deposition (ALD) is a commonly used coating technique for lithium ion battery electrodes. Recently, it has been applied to sodium ion battery anode materials. ALD is known to improve the cycling performance, Coulombic efficiency of batteries, and maintain electrode integrity. Here, the electrochemical performance of uncoated P2-Na2/3Ni1/3Mn2/3O2 electrodes is compared to that of ALD-coated Al2O3 P2-Na2/3Ni1/3Mn2/3O2 electrodes. Given that ALD coatings are in the early stage of development for NIB cathode materials, little is known about how ALD coatings, in particular aluminum oxide (Al2O3), affect the electrode-electrolyte interface. Therefore, full characterizations of its effects are presented in this work. For the first time, X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD-coated electrodes. It contains less carbonate species and more inorganic species, which allows for fast Na kinetics, resulting in significant increase in Coulombic efficiency and decrease in cathode impedance. The effectiveness of Al2O3 ALD coating is also surprisingly reflected in the enhanced mechanical stability of the particle which prevents particle exfoliation.
          Cover page: Improvement of the Cathode Electrolyte Interphase on P2-Na2/3Ni1/3Mn2/3O2 by Atomic Layer Deposition
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2022)
          This work reports a method of producing flexible cobalt nanowires (NWs) directly from the chemical conversion of bulk precursors at room temperature. Chemical reduction of Li6CoCl8 produces a nanocomposite of Co and LiCl, of which the salt is subsequently removed. The dilute concentration of Co in the precursor combined with the anisotropic crystal structure of the hcp phase leads to 1D growth in the absence of any templates or additives. The Co NWs are shown to have high saturation magnetization (130.6 emu g-1). Our understanding of the NW formation mechanism points to new directions of scalable nanostructure generation.
            Cover page: Synthesis of flexible Co nanowires from bulk precursors
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2019)
            For commercialization of perovskite solar cells (PSCs), it is important to substitute the alternative electrode for Au to decrease the unit cost. From the early stage, Ag exhibits a potential to be a good counter electrode in PSCs; however, there is an abnormal s-shaped J-V curve with the Ag electrode, and it is recovered as time passes. The perception of the aging-induced recovery process and refutation of the raised stability issues are required for commercial application of Ag electrodes. Herein, we compared the aging effect of PSCs with Ag and Au electrodes and found that only devices with Ag electrodes have a dramatical aging-induced recovery process. We observed the change of photoelectronic properties only in the devices with Ag electrodes as time passes, which mainly contributes to recovery of the s-shaped J-V curve. We verified the work function change of an aged Ag electrode and its mechanism by photoelectron spectroscopy analysis. By comparing the light stability under 1 sun intensity illumination, we can assure the practical stability of Ag electrodes in case of being encapsulated. This work suggests the profound understanding of the aging-induced recovery process of PSCs and the possibility of commercial application of Ag electrodes.
              Cover page: Effect of Metal Electrodes on Aging-Induced Performance Recovery in Perovskite Solar Cells
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2018)
              Lead halide-based perovskite materials have been applied as an intrinsic layer for next-generation photovoltaic devices. However, the stability and performance reproducibility of perovskite solar cells (PSCs) needs to be further improved to match that of silicon photovoltaic devices before they can be commercialized. One of the major bottlenecks that hinders the improvement of device stability/reproducibility is the additives in the hole-transport layer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4- tert-butylpyridine (tBP). Despite the positive effects of these hole-transport layer additives, LiTFSI is hygroscopic and can adsorb moisture to accelerate the perovskite decomposition. On the other hand, tBP, the only liquid component in PSCs, which evaporates easily, is corrosive to perovskite materials. Since 2012, the empirical molar ratio 6:1 tBP:LiTFSI has been wildly applied in PSCs without further concerns. In this study, the formation of tBP-LiTFSI complexes at various molar ratios has been discovered and investigated thoroughly. These complexes in PSCs can alleviate the negative effects (decomposition and corrosion) of individual components tBP and LiTFSI while maintaining their positive effects on perovskite materials. Consequently, a minor change in tBP:LiTFSI ratio results in huge influences on the stability of perovskite. Due to the existence of uncomplexed tBP in the 6:1 tBP:LiTFSI mixture, this empirical tBP-LiTFSI molar ratio has been demonstrated not as the ideal ratio in PSCs. Instead, the 4:1 tBP:LiTFSI mixture, in which all components are complexed, shows all positive effects of the hole-transport layer components with dramatically reduced negative effects. It minimizes the hygroscopicity of LiTFSI, while lowering the evaporation speed and corrosive effect of tBP. As a result, the PSCs fabricated with this tBP:LiTFSI ratio have the highest average device efficiency and obviously decreased efficiency variation with enhanced device stability, which is proposed as the golden ratio in PSCs. Our understanding of interactions between hole-transport layer additives and perovskite on a molecular level shows the pathway to further improve the PSCs' stability and performance reproducibility to make them a step closer to large-scale manufacturing.
                Cover page: Unveiling the Role of tBP-LiTFSI Complexes in Perovskite Solar Cells.
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2018)
                We report the application of a nonvolatile ionic gel as a soft, conductive interface for electrotactile stimulation. Materials characterization reveals that, compared to a conventional ionic hydrogel, a glycerol-containing ionic gel does not dry out in air, has better adhesion to skin, and exhibits a similar impedance spectrum in the range of physiological frequencies. Moreover, psychophysical experiments reveal that the nonvolatile gel also exhibits a wider window of comfortable electrotactile stimulation. Finally, a simple pixelated device is fabricated to demonstrate spatial resolution of the haptic signal.
                  Cover page: Ionotactile Stimulation: Nonvolatile Ionic Gels for Human–Machine Interfaces
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2020)
                    Cover page: Impacts of the Hole Transport Layer Deposition Process on Buried Interfaces in Perovskite Solar Cells
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2017)
                    Lithium metal has been considered the "holy grail" anode material for rechargeable batteries despite the fact that its dendritic growth and low Coulombic efficiency (CE) have crippled its practical use for decades. Its high chemical reactivity and low stability make it difficult to explore the intrinsic chemical and physical properties of the electrochemically deposited lithium (EDLi) and its accompanying solid electrolyte interphase (SEI). To prevent the dendritic growth and enhance the electrochemical reversibility, it is crucial to understand the nano- and mesostructures of EDLi. However, Li metal is very sensitive to beam damage and has low contrast for commonly used characterization techniques such as electron microscopy. Inspired by biological imaging techniques, this work demonstrates the power of cryogenic (cryo)-electron microscopy to reveal the detailed structure of EDLi and the SEI composition at the nanoscale while minimizing beam damage during imaging. Surprisingly, the results show that the nucleation-dominated EDLi (5 min at 0.5 mA cm-2) is amorphous, while there is some crystalline LiF present in the SEI. The EDLi grown from various electrolytes with different additives exhibits distinctive surface properties. Consequently, these results highlight the importance of the SEI and its relationship with the CE. Our findings not only illustrate the capabilities of cryogenic microscopy for beam (thermal)-sensitive materials but also yield crucial structural information on the EDLi evolution with and without electrolyte additives.
                      Cover page: New Insights on the Structure of Electrochemically Deposited Lithium Metal and Its Solid Electrolyte Interphases via Cryogenic TEM
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