FY2010 - Oak Ridge National Laboratory

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<strong>Laboratory</strong>-Wide Fellowships— Weinberg Fellowship Mission Relevance Rechargeable lithium ion batteries represent a highly desirable class of electrical energy storage technology that has applications geared towards portable electronic devices and hybrid electric vehicles due to their high-energy density storage capacity. The accelerated design and development of the nextgeneration electrodes and electrolytes for electrical energy storage devices necessitate the development of unique in situ experimental methodologies that are aimed at providing a mechanistic insight into the nanometer-scaled, microstructural, and chemical changes that occur during the electrochemical energy conversion process. This research aligns well with the grand scientific challenge as outlined by the Basic Research Needs for Electrical Energy Storage. Although the prime focus of this research program has been on EES materials, it is recognized that the development and implementation of these versatile in situ TEM devices can be utilized in a variety of other research activities. Results and Accomplishments In this project an in situ electrochemical cell TEM holder was developed specifically for the characterization of electrochemical processes in EES systems. The holder was designed in collaboration with Hummingbird Scientific (a leading developer of in situ TEM devices). The electrochemical cell is comprised of biasing microchips (silicon chips with electron transparent SiN viewing windows and integrated biasing contacts), microfluidic delivery system, and electrical connections for interfacing the electrochemical cell to a potentiostat (for electrochemistry testing). A focused ion beam/scanning electron microscope (FIB-SEM) instrument is currently being used to fabricate and attach micrometer-scale electrodes onto the biasing contacts, which are imprinted on the biasing microchips. Once the electrodes are attached to the chips, the microchip assembly is placed into the tip of the custom-built liquid flow, electrochemical cell TEM holder. Preliminary results have shown that it is feasible to create on-chip microbatteries using this in situ platform and successfully perform in situ electrochemistry experiments. In situ dynamic observations during the electrochemical charging processes showed the formation and growth of the nanometer-scale SEI on the graphite anode within an electrolyte consisting of 1 M LiCLO 4 in EC:DEC. To gain a fundamental understanding of the dynamically evolving nature of the SEI, the in situ electrochemical flow-cell TEM holder developed in this program will be utilized in future studies to dynamically monitor the kinetics and growth mechanisms of the SEI during electrochemical exposure, study the influence of electrolyte/electrolyte additives on SEI formation and stability, and determine SEI breakdown degradation mechanisms. 05918 Extended Defect Chemistry and Optoelectronic Activity in Solar Photovoltaic CdTe Thin Films Chad M. Parish Project Description Solar photovoltaics (PVs) will only be competitive to grid electricity if their cost can be significantly reduced and their efficiency improved. To achieve these goals, CdTe thin-film absorber layers deposited onto low-cost substrates such as metal foils or glass are an alternative to expensive silicon or GaAs-based solar cells. CdTe is a strong candidate because its bandgap of 1.5 eV is optimal for high-efficiency singlejunction devices under solar irradiation, and CdTe can be grown by multiple low-cost methods. The current understanding of dopant behavior and interface activity in CdTe is incomplete and controversial. Understanding the relationship between doping and interface character will allow science-based design of 264
<strong>Laboratory</strong>-Wide Fellowships— Weinberg Fellowship future materials with improved photovoltaic efficiency. Complementary electron microscopy and atom probe techniques were explored to characterize the structures in CdTe thin films and other energy-related materials in order to provide a more fundamental understanding of the doping effects and multidopant synergies at interfaces in CdTe absorber layers. By improving the basic understanding of how to characterize CdTe layers, future lower-cost, higher-efficiency solar cells can be enabled. Mission Relevance Solar energy is a domestic, non-carbon renewable energy source, and the development of low-cost, highefficiency PV technologies will have a large impact on U.S. manufacturing. Improved efficiencies of PV cells and component absorber layers have relevance to the DOE Energy Efficiency and Renewable Energy (EERE) mission and improved manufacturability to the DOE Industrial Technologies mission. The primary thrust of this project is to develop a deeper understanding of defect behavior in compound semiconductor PV thin films, which is relevant to the Basic Energy Sciences (BES) Materials Science and Engineering (MSE) mission. The most immediate impact of this project was the development of experimental characterization techniques, relevant to both CdTe PVs and energy-related materials in general, and these techniques have been made available through the SHaRE BES-Scientific User Facilities Division (SUFD) program. Results and Accomplishments Studies on CdTe thin films allowed the proper experimental methodologies to be developed, yielding valuable information to enable future studies to produce science-based understandings of solar PV systems. First, CdTe is found to be highly sensitive to damage from electron or ion beams; therefore, focused ion beam (FIB) sample preparation methodologies were refined to allow preparation of beamsensitive material on a highly insulating substrate and to prepare site-specific specimens for electron backscatter diffraction. Second, transmission electron microscopy (TEM) techniques were refined to allow high-resolution elemental mapping via X-ray spectrum imaging and energy-filtered TEM with minimal perturbation of the sample structure due to fast electron damage. These techniques were then applied to the CdTe/CdS active interface in PV device structures, where S-Te interdiffusion was characterized for relation to device performance. Te-S interdiffusion was found to be less than 10 nm, and small defective areas at the CdS/CdTe and CdS/substrate boundaries were found by MVSA despite being missed by standard analysis techniques. In addition to experimental methodology refinements, mathematical techniques were developed. Experiments such as the X-ray spectrum imaging applied to the CdTe/CdS interface produce prodigious quantities of data, and unbiased statistically derived methods are needed to draw proper materials science and engineering conclusions from such experimental data. Mathematical methods and computer codes were developed to (1) use multivariate statistical analysis to produce an unbiased, low-noise description of atom probe tomography reconstructions and (2) resolve the contributions of embedded particles from the surrounding matrix in TEM X-ray spectrum imaging of thin samples using multivariate curve resolution mathematical techniques. These mathematical methods were tested by application to a diverse variety of different energy materials. Information Shared Miller, M. K., C. M. Parish, and C. Capdevila. 2010. “A MVSA Approach to Mine Information from APT Data.” International Field Emission Symposium 2010. Parish, C. M. 2010. “Multivariate Curve Resolution to Determine Fully-Embedded-Particle Compositions in STEM-EDS Spectrum Images.” Microscopy and Microanalysis 2010 Conference Proceedings 16(Suppl. 2), 276–277. 265
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FY2010 - Oak Ridge National Laboratory

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