Nicholas Berry

University of California, Irvine, Irvine, CA, United States

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Publications (4)31.4 Total impact

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    ABSTRACT: Through density functional calculations, we investigated the segregation of a sulfur vacancy from interior sites outward to the FeS2(100) surfaces in different surface conditions in order to provide guidance for the development of iron pyrite in photovoltaics applications. We found that the surfaces with interior S-vacancy are energetically unstable and bulk S-vacancies tend to hop toward the surface, in particular when the surface composition is in the stoichiometric or S-rich side. The segregation process is accompanied by redox reaction near the vacancy site, Fe(2+) + S(1-) -> Fe(3+) + S(2-), and the activation energy decreases near the surface region. We compare the calculated structural, energetic and electronic properties to experimental data, and provide insights for reduction of vacancy density in optimal annealing conditions.
    03/2013;
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    ABSTRACT: Iron pyrite (cubic FeS2) is a promising candidate absorber material for earth-abundant thin-film solar cells. Here we report on phase-pure, large-grain, and uniform polycrystalline pyrite films that are fabricated by solution-phase deposition of an iron(III) acetylacetonate molecular ink followed by sequential annealing in air, H2S, and sulfur gas at temperatures up to 550°C. Phase and elemental composition of the films is characterized by conventional and synchrotron X-ray diffraction (XRD), Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). These solution-deposited films have more oxygen and alkalis, less carbon and hydrogen, and smaller optical band gaps (Eg = 0.87 ± 0.05 eV) than similar films made by chemical vapor deposition. XPS is used to assess the chemical composition of the film surface before and after exposure to air and immersion in water to remove surface contaminants. Optical measurements of films rich in marcasite (orthorhombic FeS2) show that marcasite has a band gap at least as large as pyrite and that the two polymorphs share similar absorptivity spectra, in excellent agreement with density functional theory (DFT) models. Regardless of the marcasite and elemental impurity content, all films show p-type, weakly-activated transport with curved Arrhenius plots, a room-temperature resistivity of ~1 Ω cm, and a hole mobility that is too small to measure by Hall effect. This universal electrical behavior strongly suggests that a common defect or a hole-rich surface layer governs the electrical properties of most FeS2 thin films.
    Journal of the American Chemical Society 02/2013; · 10.68 Impact Factor
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    ABSTRACT: Iron pyrite (cubic FeS{sub 2}) is a promising candidate absorber material for earth-abundant thin-film solar cells. In this report, single-phase, large-grain, and uniform polycrystalline pyrite thin films are fabricated on glass and molybdenum-coated glass substrates by atmospheric-pressure chemical vapor deposition (AP-CVD) using the reaction of iron(III) acetylacetonate and tert-butyl disulfide in argon at 300 C, followed by sulfur annealing at 500--550 C to convert marcasite impurities to pyrite. The pyrite-marcasite phase composition depends strongly on the concentration of sodium in the growth substrate and the sulfur partial pressure during annealing. Phase and elemental composition of the films are characterized by X-ray diffraction, Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. The in-plane electrical properties are surprisingly insensitive to phase and elemental impurities, with all films showing p-type, thermally activated transport with a small activation energy ({approx}30 meV), a room- temperature resistivity of {approx}1 {Omega} cm, and low mobility. These ubiquitous electrical properties may result from robust surface effects. These CVD pyrite thin films are well suited to fundamental electrical studies and the fabrication of pyrite photovoltaic device stacks.
    Laser Physics Review 10/2012; 2(9). · 10.04 Impact Factor
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    ABSTRACT: Iron pyrite (FeS2) is a promising earth-abundant semiconductor for thin-film solar cells. In this work, phase-pure, single-crystalline, and well-dispersed colloidal FeS2 nanocrystals (NCs) were synthesized in high yield by a simple hot-injection route in octadecylamine and then were subjected to partial ligand exchange with octadecylxanthate to yield stable pyrite NC inks. Polycrystalline pyrite thin films were fabricated by sintering layers of these NCs at 500−600 °C under a sulfur atmosphere.
    Journal of the American Chemical Society 02/2011; 133(4):716-9. · 10.68 Impact Factor