Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L12: Focus Session: Photocatalysis and photovoltaic: Excitation, Trapping, and Transport of Charge Carriers at Surfaces and Interfaces |
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Sponsoring Units: DMP DCMP Chair: Wladyslaw Walukiewicz, Lawrence Berkeley National Laboratory Room: 308 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L12.00001: Polynuclear Metal Oxide Photocatalysts in Nanoporous Silica Scaffolds for Artificial Photosynthesis Invited Speaker: |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L12.00002: Understanding and Controlling Photovoltaic Effects in Complex Oxide Thin Films Steven Byrnes, Thomas Conry, Sourav Roger Basu, Lane Martin, Drew Paran, Varada Bal, Joel W. Ager, R. Ramesh Thin-film oxide heterostructures are a promising material system for large-scale photovoltaic energy conversion, as oxides can be cheap, abundant, stable, and highly light-absorbing. As a model system, we have investigated the room-temperature ferroelectric BiFeO$_3$ (BFO). Heteroepitaxial BFO films are grown by both metal-organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD), allowing for a wide range of control over thickness, composition, and ferroelectric domain structure. BFO has been measured to have a direct bandgap at 2.6~eV; moreover its bandgap and other material properties can be controlled by alloying and by modification of stoichiometry. In this work, we will demonstrate the photovoltaic properties of BFO thin films (100--1000~nm) grown heteroepitaxially on oxide bottom electrodes with transparent ITO top contacts. Electrical and external quantum efficiency measurements prove that the photovoltaic effect comes from a Schottky barrier between ITO and p-type BFO, but time-dependent and capacitance-voltage measurements show that ferroelectricity, ion motion, and/or trap states also play an important role in the electrostatics of the device. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L12.00003: Ultrafast Time-Resolved Spectroscopy of Photoinduced Electron Transfer in Novel Photovoltaic Devices L.M. Mier, A.R. Carter, T.L. Gustafson, A.J. Epstein We present work toward an understanding of the fundamental photophysics of photoinduced electon transfer between 9-anthracenecarboxylic acid (9-AC) and TiO$_2$ nanoparticles in order to apply the techniques to a novel photovoltaic device. The active layers of a proposed device consist of a broad-spectrum, metallo-organic absorber\footnote{M.H.Chisholm, et al., Inorg.Chem.\textbf{47}, 3415 (2008).} covalently bound through a carboxylic acid to a nano-porous TiO$_2$ structure. To study the electron transfer, a model compound, 9-AC, is covalently bound to TiO$_2$ nanoparticles. Ultrafast electron transfer from the excited 9-AC to the TiO$_2$ is observed within 50 fs using ultrafast broadband spectroscopy. Further evidence of this transfer is shown from quenching of the fluorescence of the 9-AC with increasing concentrations of TiO$_2$ with no effects on the lifetime of the fluorescence. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L12.00004: Photocatalytic Activity in Nitrogen-doped TiO$_{2}$ Nanowires De Nyago Tafen, Jin Wang, Nianqiang Wu, James P. Lewis We present a comprehensive experimental and theoretical study of the electronic properties and photocatalytic activity of nitrogen-doped anatase TiO$_{2}$ nanowires. UV-Vis spectra showed enhanced absorption in the visible light range for nitrogen doped nanowires compared to the plain sample. The nitrogen doped nanowires exhibit improved photocatalytic activity compared to the plain sample upon visible light irradiation while under exposure to UVA light the photocatalytic activity decreased after nitrogen doping. Furthermore, the incorporation of nitrogen introduces localized states in the band gap. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L12.00005: Interfacial charge dynamics studied by ultrafast electron diffraction Ryan Murdick, Ramanikalyan Raman, Yoshie Murooka, Richard Worhatch, Chong-Yu Ruan Of central importance to nanoscale device technology is the role of charge transfer at interfaces. Using ultrafast electron diffraction, which has recently emerged as a new technique in determining transient surface photovoltages with nanometer sensitivity (Murdick et al., PRB 77, 245329, 2008), we investigate the surface charge and space-charge dynamics at the Si/SiO2 interface. By varying the excitation wavelength, fluence, and pulse duration, we explore various pathways inducing electron tunneling through an insulating barrier to reach the surface states. We show that the surface states have relatively long lifetimes ($\sim $100 ps), but are rechargeable, thus ideal for serving as a charge pump for interfacial devices. Using the Si/SiO2 platform, we extend this diffractive potentiometry approach to study nanoparticle charging and molecular transport. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L12.00006: TiO2 nanowire sensitized by organic dyes for photovoltaic applications: influence of binding groups and molecular dimension Sheng Meng, Efthimios Kaxiras We investigate the electronic couplings including charge separation, injection, and recombination processes between a TiO2 nanowire and a set of organic dye sensitizers, based on the full time-dependent density functional theory treatment of electron excitation and atomic vibrations. For all the cases the highest occupied molecular orbital (HOMO) of dye molecules are found being located in the middle of the TiO2 bandgap and the lowest-unoccupied molecular orbital (LUMO) close to the TiO2 conduction band minimum, leading to enhanced visible light absorption and ultrafast electron injection into the TiO2 conduction band. The influences of the anchoring groups and molecular dimensions to the dye injection dynamics and electron-hole recombination process are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L12.00007: Analysis of Quasiparticle Energy Band Shifts Resulting from Introduction of Nitrogen into Titanates Wei Kang, Mark S. Hybertsen Titanium oxides and many titanates, such as rutile (TiO$_{2})$ and SrTiO$_{3}$, are promising in photo-catalysis for water splitting and photo-degradation of hazardous materials in the environment, although their large band gaps limit utilization of the solar spectrum to the UV region. Experiments show that introduction of nitrogen by various means can significantly affect the band gap. However, catalytic action also depends on individual conduction and valence band alignments. We address these issues by performing theoretical calculations of the energy spectrum for titanium oxides, titanates and various structures introducing nitrogen into the crystals using the GW method. In contrast to density functional theory approaches, the GW method generally leads to energy levels and band gaps that agree well with experiments. We here use this approach to illustrate the mechanism of band shifting in titanates due to the introduction of nitrogen, in particular the differences in correlation effects for nitrogen related energy bands in these materials. This work is supported by the DOE. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L12.00008: Nanocrystal-based Dyads for Solar to Electric Energy Conversion. Lei Wang, Mingyan Wu, David Waldeck We describe a new project which aims to develop a systematic and modular approach to creating a new generation of Gratzel-inspired solar energy conversion devices with the following novel advantages: the ability to capture the entire available range of solar irradiance by employing sets of linked nanoparticles, fabrication by self-assembly, enhanced robustness, and lowered cost through use of nanostructured, rather than molecular, charge transfer elements. The project team is designing, creating, and characterizing linked-nanoparticle dyads, which will act as the charge separation ``engine'' in new generation solar cells. By employing a mixture of dyads it should be possible to efficiently capture the entire solar spectrum. The proposed device architecture has two important advantages over existing solar conversion devices: It can be produced by a self-assembly process. Because of its modularity, each of its components (nanoparticles or organic linker) can be optimized separately. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L12.00009: Energy level alignment of zinc tetraphenylporphyrins derivatives adsorbed on wide band gap semiconductor oxides. Sylvie Rangan, Robert Allen Bartynski, Elena Galoppini Metalloporphyrins play an essential role in photosynthetic mechanisms and therefore are natural candidates for electron transfer mediator in dye sensitized solar cells (DSSCs). Among the possible metalloprophyrins, the zinc tetraphenylporphyrins (ZnTPP) derivatives have been found to have similar electron injection and charge recombination properties as the important standard ruthenium dye N3 for DSSCs, as well as reasonable performances using TiO$_{2}$ or ZnO as substrates. We have investigated the electronic structure, energy level alignment, and their changes with altered surface bonding geometries, using a selective functionalization with carboxylic anchoring groups of the meso-phenyl, of functionalized ZnTPP on single crystal TiO$_{2}$ and ZnO surfaces. Occupied and unoccupied electronic states were determined using direct (ultra-violet and x-ray) photoemission and inverse photoemission in the same ultra-high vacuum analysis chamber. Energy level alignment of the ZnTPP molecular orbitals with respect to the substrates band edges will be compared to the available literature. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L12.00010: Effects of Diffusion on Photocurrent Generation in Single-Walled Carbon Nanotube Films Christopher Merchant, Nina Markovic We have studied photocurrent generation in large carbon nanotube (CNT) films using electrodes with different spacings. We observe that the photocurrent depends strongly on the position of illumination, with maximum observed response occurring upon illumination at the electrode edges. The rate of change of the response decays exponentially, with the fastest response occurring for samples with the smallest electrode spacing. We show that the time response is due to charge carrier diffusion in low-mobility CNT films. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L12.00011: Efficient photocatalysis at pH7: in-situ formation of a water-splitting cobalt catalyst at electrode interfaces Invited Speaker: |
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