Michael F. Toney

Stanford University, Stanford, California, United States

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Publications (469)2748.71 Total impact

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    ABSTRACT: P-type transparent conducting films of nanocrystalline (CuS)x:(ZnS)1-x were synthesized by facile and low-cost chemical bath deposition. Wide angle x ray scattering (WAXS) and high resolution transmission electron microscopy (HRTEM) were used to evaluate the nanocomposite structure, which consists of sub-5-nm crystallites of sphalerite ZnS and covellite CuS. Film transparency can be controlled by tuning the size of the nanocrystallites which is achieved by adjusting the concentration of the complexing agent during growth; optimal films have optical transmission above 70% in the visible range of the spectrum. The hole conductivity increases with the fraction of the covellite phase and can be as high as 1000 S cm(-1), which is higher than most reported p-type transparent materials and approaches that of n type transparent materials such as indium tin oxide (ITO) and aluminum doped zinc oxide (AZO). Heterojunction p-(CuS)x:(ZnS)1-x/n-Si solar cells were fabricated with the nanocomposite film serving as a hole selective contact. Under 1 sun illumination, an open circuit voltage of 535 mV was observed. This value compares favorably to other emerging heterojunction Si solar cells which use a low temperature process to fabricate the contact, such as single-walled carbon nanotube/Si (370-530 mV) and graphene/Si (360-552 mV).
    No preview · Article · Feb 2016 · Nano Letters
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    ABSTRACT: In this study we present the results of an extensive multiscale characterization of the flow properties and structural and capillary heterogeneities of the Heletz sandstone. We performed petrographic, porosity and capillary pressure measurements on several subsamples. We quantified mm-scale heterogeneity in saturation distributions in a rock core during multi-phase flow using conventional X-ray CT scanning. Core-flooding experiments were conducted under reservoirs conditions (9 MPa, 50 °C) to obtain primary drainage and secondary imbibition relative permeabilities and residual trapping was analyzed and quantified. We provide parameters for relative permeability, capillary pressure and trapping models for further modeling studies. A synchrotron-based microtomography study complements our cm- to mm-scale investigation by providing links between the micromorphology and mm-scale saturation heterogeneities.
    No preview · Article · Feb 2016 · International Journal of Greenhouse Gas Control
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    ABSTRACT: A method to produce highly efficient and long range vertical charge transport is demonstrated in an undoped polythiophene thin film, with average mobilities above 3.1 cm(2) V(-1) s(-1) . These record high mobilities are achieved by controlled orientation of the polymer crystallites enabling the most efficient and fastest charge transport along the chain backbones and across multiple chains. The significant increase in mobility shown here may present a new route to producing faster and more efficient optoelectronic devices based on organic materials.
    No preview · Article · Jan 2016 · Advanced Materials
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    ABSTRACT: Organic bulk heterojunction (BHJ) solar cells are a promising alternative for future clean-energy applications. However, to become attractive for consumer applications, such as wearable, flexible, or semitransparent power-generating electronics, they need to be manufactured by high-throughput, low-cost, large-area-capable printing techniques. However, most research reported on BHJ solar cells is conducted using spin coating, a single batch fabrication method, thus limiting the reported results to the research lab. In this work, we investigate the morphology of solution-sheared films for BHJ solar cell applications, using the widely studied model blend P3HT:PCBM. Solution shearing is a coating technique that is upscalable to industrial manufacturing processes and has demonstrated to yield record performance organic field-effect transistors. Using grazing incident small-angle X-ray scattering, grazing incident wide-angle X-ray scattering, and UV-vis spectroscopy, we investigate the influence of solvent, film drying time, and substrate temperature on P3HT aggregation, conjugation length, crystallite orientation, and PCBM domain size. One important finding of this study is that, in contrast to spin-coated films, the P3HT molecular orientation can be controlled by the substrate chemistry, with PEDOT:PSS substrates yielding face-on orientation at the substrate-film interface, an orientation highly favorable for organic solar cells.
    No preview · Article · Jan 2016 · ACS Applied Materials & Interfaces
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    ABSTRACT: Cholesterol self-assembles into weakly ordered aggregates when tethered to a crosslinked hydrogel network of poly(ethylene glycol) (PEG). PEG-diacrylate and cholesterol-PEG-acrylamide (PEG-chol) were co-polymerized in organic solvent and transferred to water for equilibrium swelling. Small-angle x-ray scattering revealed self-assembled cholesterol structures not present during network synthesis. At lower ratios of PEG-tethered cholesterol to PEG (<12% cholesterol based on total solid content), cholesterol aggregates into the dense, weakly ordered crosslink junctions of the PEG network. The hydrogel networks exhibited classic affine behavior during compressive mechanical testing, and cholesterol aggregation enhanced the elastic modulus. At high PEG-chol to PEG ratios (12-20% cholesterol based on total solid content), cholesterol self-assembles into domains with lamellar-like meso-ordering. The structural transition causes network deswelling and significantly reduces material brittleness upon deformation.
    No preview · Article · Jan 2016
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    ABSTRACT: We describe a compact roll-to-roll (R2R) coater that is capable of tracking the crystallization process of semiconducting polymers during solution printing using X-ray scattering at synchrotron beamlines. An improved understanding of the morphology evolution during the solution-processing of organic semiconductor materials during R2R coating processes is necessary to bridge the gap between "lab" and "fab". The instrument consists of a vacuum chuck to hold the flexible plastic substrate uniformly flat for grazing incidence X-ray scattering. The time resolution of the drying process that is achievable can be tuned by controlling two independent motor speeds, namely the speed of the moving flexible substrate and the speed of the printer head moving in the opposite direction. With this novel design, we are able to achieve a wide range of drying time resolutions, from tens of milliseconds to seconds. This allows examination of the crystallization process over either fast or slow drying processes depending on coating conditions. Using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) inks based on two different solvents as a model system, we demonstrate the capability of our in situ R2R printing tool by observing two distinct crystallization processes for inks drying from the solvents with different boiling points (evaporation rates). We also observed delayed on-set point for the crystallization of P3HT polymer in the 1:1 P3HT/PCBM BHJ blend, and the inhibited crystallization of the P3HT during the late stage of the drying process.
    No preview · Article · Dec 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: Doping activity in both beta-phase (β-) and amorphous (a-) Sn-doped gallium oxide (Ga2O3:Sn) is investigated by X-ray absorption spectroscopy (XAS). A single crystal of β-Ga2O3:Sn grown using edge-defined film-fed growth at 1725°C is compared with amorphous Ga2O3:Sn films deposited at low temperature (<300°C). Our XAS analyses indicate that activated Sn dopant atoms in conductive single crystal β-Ga2O3:Sn are present as Sn4+, preferentially substituting for Ga at the octahedral site, as predicted by theoretical calculations. In contrast, inactive Sn atoms in resistive a-Ga2O3:Sn are present in either +2 or +4 charge states depending on growth conditions. These observations suggest the importance of growing Ga2O3:Sn at high temperature to obtain a crystalline phase and controlling the oxidation state of Sn during growth to achieve dopant activation.
    Full-text · Article · Dec 2015 · Applied Physics Letters
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    ABSTRACT: High-boiling-point solvent additives, employed during the solution processing of active-layer formulations, impact the efficiency of bulk heterojunction (BHJ) organic solar cells by influencing the morphological/topological features of the multicomponent thin film. Here, we aim at a better understanding of how these additives change the aggregation landscape in the casting solution prior to film deposition via a multiscale computational study of the aggregation phenomena of phenyl-C61-butyric-acid methyl ester (PCBM) in various solutions. The energetic landscape of PCBM-solvent/solvent-additive intermolecular interactions is evaluated at the electronic-structure level through symmetry-adapted perturbation theory to determine the nature and strength of noncovalent forces important to aggregation. Molecular dynamics simulations highlight how the choice of solvent and solvent additives control the formation of molecular aggregates. Our results indicate that high-boiling point solvent additives change the effective interactions among the PCBM and casting-solvent molecules and alter equilibrium PCBM aggregate size in solution.
    No preview · Article · Nov 2015 · Chemistry of Materials
  • Alexander Sharenko · Michael F. Toney
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    ABSTRACT: Solution-processed lead halide perovskite thin-film solar cells have achieved power conversion efficiencies comparable to those obtained with several commercial photovoltaic technologies in a remarkably short period of time. This rapid rise in device efficiency is largely the result of the development of fabrication protocols capable of producing continuous, smooth perovskite films with micrometer-sized grains. Further developments in film fabrication and morphological control are necessary, however, in order for perovskite solar cells to reliably and reproducibly approach their thermodynamic efficiency limit. This Perspective discusses the fabrication of lead halide perovskite thin films, while highlighting the processing-property-performance relationships that have emerged from the literature, and from this knowledge, suggests future research directions.
    No preview · Article · Nov 2015 · Journal of the American Chemical Society
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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: With consumer electronics transitioning toward flexible products, there is a growing need for high-performance, mechanically robust, and inexpensive transparent conductors (TCs) for optoelectronic device integration. Herein, we report the scalable fabrication of highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films via solution shearing. Specific control over deposition conditions allows for tunable phase separation and preferential PEDOT backbone alignment, resulting in record-high electrical conductivities of 4,600 ± 100 S/cm while maintaining high optical transparency. High-performance solution-sheared TC PEDOT:PSS films were used as patterned electrodes in capacitive touch sensors and organic photovoltaics to demonstrate practical viability in optoelectronic applications.
    No preview · Article · Oct 2015 · Proceedings of the National Academy of Sciences
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    ABSTRACT: A study was conducted to characterize PbCl2-derived CH3NH3PbI3-xClx with X-ray absorption near edge structure (XANES) to verify the amount of Cl remaining in the film after annealing and to determine the chemical state of the Cl with a goal of understanding the role Cl played in the final hybrid-perovskite film. Perovskite films were made from a 1:3 molar mixture of PbCl2 and MAI in DMF spin-coated onto a substrate. The ex situ annealed films were used to quantify the amount of Cl and to get accurate XANES for intermediate annealing times.
    No preview · Article · Oct 2015 · Chemistry of Materials
  • Linda Y. Lim · Shufen Fan · Huey Hoon Hng · Michael F. Toney
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    ABSTRACT: There is limited understanding of the lithiation reaction mechanisms for crystalline Ge and the effects of various battery components, including conductive additive type, on Ge transformations into amorphous and crystalline phases during electrochemical charge and discharge processes. In this work, we study the dependence of the phase transformations of crystalline Ge anodes on two common carbon-based conductive additives used in lithium-ion battery electrodes through operando X-ray diffraction and X-ray absorption spectroscopy. We find that Ge electrodes using carbon nanotubes as conductive additives exhibit higher structural and electrochemical reversibility compared with those with carbon black additives as well as better stability in cycling. On the basis of this, a proposed strategy to prolong the cycle life of crystalline Ge anodes is presented. Our operando XRD and XAS results show how the reaction pathways (phase transformations and local structural changes) of Ge anodes depend on conductive additive and impact the battery cycling performance.
    No preview · Article · Oct 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: Using atomic layer deposition of Al2O3 coating, improved high-voltage cycling stability has been demonstrated for the layered nickel-manganese-cobalt pseudoternary oxide, LiNi0.4Mn0.4Co0.2O2. To understand the effect of the Al2O3 coating, we have utilized electrochemical impedance spectroscopy, operando synchrotron-based X-ray diffraction, and operando X-ray absorption near edge fine structure spectroscopy to characterize the structure and chemistry evolution of the LiNi0.4Mn0.4Co0.2O2 cathode during cycling. Using this combination of techniques, we show that the Al2O3 coating successfully mitigates the strong side reactions of the active material with the electrolyte at higher voltages (>4.4 V), without restricting the uptake and release of Li ions. The impact of the Al2O3 coating is also revealed at beginning of lithium deintercalation, with an observed delay in the evolution of oxidation and coordination environment for the Co and Mn ions in the coated electrode due to protection of the surface. This protection prevents the competing side reactions of the electrolyte with the highly active Ni oxide sites, promoting charge compensation via the oxidation of Ni and enabling high-voltage cycling stability. (Figure Presented).
    Full-text · Article · Sep 2015 · Chemistry of Materials
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    ABSTRACT: Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.
    No preview · Article · Aug 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.
    Full-text · Article · Aug 2015 · Nature Communications
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    ABSTRACT: The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Jul 2015 · Advanced Materials
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    ABSTRACT: The elevated level of atmospheric carbon dioxide (CO2) has caused serious concern of the progression of global warming. Geological sequestration is considered as one of the most promising techniques for mitigating the damaging effect of global climate change. Investigations over wide range of length-scales are important for systematic evaluation of the underground formations from prospective CO2 reservoir. Understanding the relationship between the micro morphology and the observed macro phenomena is even more crucial. Here we show Synchrotron based X-ray micro tomographic study of the morphological buildup of Sandstones. We present a numerical method to extract the pore sizes distribution of the porous structure directly, without approximation or complex calculation. We have also demonstrated its capability in predicting the capillary pressure curve in a mercury intrusion porosimetry (MIP) measurement. The method presented in this work can be directly applied to the morphological studies of heterogeneous systems in various research fields, ranging from Carbon Capture and Storage, and Enhanced Oil Recovery to environmental remediation in the vadose zone.
    Full-text · Article · Jun 2015 · Scientific Reports
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    Linda Y Lim · Shufen Fan · Huey Hoon Hng · Michael F Toney
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    ABSTRACT: Operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies of Ge anodes are carried out to understand the effect of cycling rate on Ge phase transformation during charge/discharge process and to relate that effect to capacity. It is discovered that the formation of crystalline Li15Ge4 (c-Li15Ge4) during lithiation is suppressed beyond a certain cycling rate. A very stable and reversible high capacity of ≈1800 mAh g−1 can be attained up to 100 cycles at a slow C-rate of C/21 when there is complete conversion of Ge anode into c-Li15Ge4. When the C-rate is increased to ≈C/10, the lithiation reaction is more heterogeneous and a relatively high capacity of ≈1000 mAh g−1 is achieved with poorer electrochemical reversibility. An increase in C-rate to C/5 and higher reduces the capacity (≈500 mAh g−1) due to an impeded transformation from amorphous LixGe to c-Li15Ge4, and yet improves the electrochemical reversibility. A proposed mechanism is presented to explain the C-rate dependent phase transformations and the relationship of these to capacity fading. The operando XRD and XAS results provide new insights into the relationship between structural changes in Ge and battery capacity, which are important for guiding better design of high-capacity anodes.
    Full-text · Article · Jun 2015 · Advanced Energy Materials

Publication Stats

20k Citations
2,748.71 Total Impact Points

Institutions

  • 1993-2015
    • Stanford University
      • • Department of Chemical Engineering
      • • Department of Materials Science and Engineering
      Stanford, California, United States
  • 2013
    • Harvard University
      • Department of Chemistry and Chemical Biology
      Cambridge, Massachusetts, United States
    • Diamond Light Source
      XPW, England, United Kingdom
    • University of California, Berkeley
      Berkeley, California, United States
  • 2012
    • National Institute of Standards and Technology
      • Polymers Division
      Maryland, United States
    • King Abdullah University of Science and Technology
      Djidda, Makkah, Saudi Arabia
  • 2011
    • Carnegie Mellon University
      • Department of Materials Science and Engineering
      Pittsburgh, Pennsylvania, United States
  • 2010
    • University of Canterbury
      • Department of Physics and Astronomy
      Christchurch, Canterbury, New Zealand
  • 2007
    • University of Houston
      • Department of Chemical & Biomolecular Engineering
      Houston, Texas, United States
    • Palo Alto Research Center
      Palo Alto, California, United States
  • 2004
    • University of Minnesota Duluth
      • Department of Chemistry and Biochemistry
      Duluth, Minnesota, United States
  • 1999
    • Sogang University
      Sŏul, Seoul, South Korea
  • 1998
    • University of Wales
      Cardiff, Wales, United Kingdom
  • 1995
    • William Penn University
      Worcester, Massachusetts, United States
  • 1980-1994
    • University of Washington Seattle
      • Department of Physics
      Seattle, Washington, United States