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This PDF file contains the front matter associated with SPIE Proceedings Volume 7034, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
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The heterojunctions formed between different organic dyes (O/O' heterojunctions), organic dyes with contacting oxide or
metal electrodes (O/I heterojunctions), and semiconductor nanoparticles with organic host polymers and ligands (SC-NP/
O heterojunctions) must be understood and optimized in order to enhance the energy conversion efficiencies of
photovoltaics using these materials as their active components. We have used combinations of UV-photoelectron
spectroscopy, and X-ray photoelectron spectroscopy (UPS/XPS) in the characterization of representative
heterojunctions, and extrapolate these studies to the optimization of new photovoltaic and photoelectrochemical energy
conversion devices.
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In a recent study, it has been shown that organic photovoltaic (OPV) solar cells consisting of polymers with certain
stoichiometric ratios of alkyl thiophene:thieno[3,4-b]thiophene monomeric units in random sequences, when combined
with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), may have potentials for creating more efficient devices. Such a
potential enhancement is mainly due to the light harvesting in most of the visible and near infrared region by these low
band-gap polymers. However, very little is known about the photoinduced energy/electron transfer and transport within
these copolymers. It is important to understand both the ultrafast interactions between these two monomeric units when
they are linked in the copolymers and their interactions with the electron acceptor PCBM in order to determine the
transport mechanisms in these systems, and then to create the architectures that optimize electronic transport properties.
Therefore, three oligomer molecules have been synthesized to model the local interactions in the copolymers, each of
which consists of a thieno[3,4-b] thiophene derivative at its center linked with two alkyl oligothiophene side units. The
alkyl oligothiophene units for the three molecules are 2, 4, or 8 units in length. By performing transient absorption and
fluorescence upconversion measurements, the nature of the early exciton diffusion and energy transfer between these
different units is elucidated.
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The dynamics of photoinduced charge separation, recombination and trapping are examined in a polymer blend
photovoltaic material with ultrafast visible pump - infrared probe and time-resolved infrared spectroscopy. The carbonyl
(C=O) stretch of methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is probed as a local vibrational
reporter of the dynamics in a blend with a poly(p-phenylenevinylene) (PPV) -based conjugated polymer, CN-MEH-PPV.
Following interfacial electron transfer, geminate electron - hole pair dissociation occurs on ultrafast timescales.
Subsequent to this charge separation process, charge carriers become trapped on the microsecond timescale resulting in
the formation of a distinct peak in the vibrational spectra corresponding to the anion of PCBM. The charge trapping
dynamics correspond to the carrier lifetime of similar PPV-based polymer blends as reported in photocurrent transients
from the literature.
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With appropriately selected optical frequencies, pulses of radiation propagating through a system of chemically distinct
and organized components can produce areas of spatially selective excitation. This paper focuses on a system in which
there are two absorptive components, each one represented by surface adsorbates arrayed on a pair of juxtaposed
interfaces. The adsorbates are chosen to be chemically distinct from the material of the underlying surface. On
promotion of any adsorbate molecule to an electronic excited state, its local electronic environment is duly modified, and
its London interaction with nearest neighbor molecules becomes accommodated to the new potential energy landscape.
If the absorbed energy then transfers to a neighboring adsorbate of another species, so that the latter acquires the
excitation, the local electronic environment changes and compensating motion can be expected to occur. Physically, this
is achieved through a mechanism of photon absorption and emission by molecular pairs, and by the engagement of
resonance transfer of energy between them. This paper presents a detailed analysis of the possibility of optically
effecting such modifications to the London force between neutral adsorbates, based on quantum electrodynamics (QED).
Thus, a precise link is established between the transfer of excitation and ensuing mechanical effects.
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Using confocal fluorescence microscopy under ultrahigh vacuum conditions, we investigate the heterogeneous
interactions between a perylene bisimide fluorophore and single crystalline Al2O3 (0001) at the single molecule level.
We find that the dye molecules undergo reversible transitions to long-lived dark states, with bright and dark periods
lasting from several hundred milliseconds to many tens of seconds. These periods are power-law distributed and point
towards charge tunneling processes from the molecule to the substrate. The fluorescence intensity levels show a bimodal
distribution, indicating different classes of adsorption sites on the sapphire surface. This study is aimed at obtaining a
better understanding of interfacial structure and dynamics in order to address ultimately both the growth of organic
semiconductor films on inorganic surfaces and the heterogeneous nature of charge transfer in excitonic solar cells.
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Nobuhito Iguchi, Clyde Cady, Robert C. Snoeberger III, Bryan M. Hunter, Eduardo M. Sproviero, Charles A. Schmuttenmaer, Robert H. Crabtree, Gary W. Brudvig, Victor S. Batista
Siloxanes with the general formula R-(CH2)n-Si-(OR')3 form durable bonds with inorganic materials upon hydrolysis of
labile -OR' groups, and serve as robust coupling agents between organic and inorganic materials. In the field of dye-sensitized
solar cells, functionalization of TiO2 thin-films with siloxane adsorbates has been shown to be useful as a
surface-passivation technique that hinders recombination processes and improves the overall efficiency of light-to-electricity
conversion. However, the attachment of siloxane adsorbates on TiO2 surfaces still remains poorly understood
at the molecular level. In this paper, we report the characterization of 3-(triethoxysilyl) propionitrile (TPS) adsorbates,
covalently attached onto TiO2 surfaces. We combine synthetic methods based on chemical vapor deposition, Fourier
transform (FT) infrared (IR) spectroscopy and electronic structure calculations based on density functional theory (DFT).
We predict that trifunctional siloxanes form only 2 covalent bonds, in a 'bridge' mode with adjacent Ti4+ ions on the
TiO2 surface, leaving 'dangling' alkoxy groups on the surface adsorbates. Our findings are supported by the observation
of a prominent fingerprint band at 1000-1100 cm-1, assigned to Si-O-C stretching modes, and by calculations of binding
enthalpies at the DFT B3LYP/(LACVP/6-31G**) level of theory indicating that the 'bridge' binding (ΔHb= -55 kcal
mol-1) is more stable than 'tripod' motifs (ΔHb= -45 kcal mol-1) where siloxanes form 3 covalent bonds with the TiO2
surface. The alkoxysiloxane groups are robust under heat and water treatment and are expected to be particularly
relevant for analytical methods since they could be exploited for immobilizing other functionalities onto the TiO2 surfaces.
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Reactors driven by highly concentrated sunlight can create conditions well suited to the synthesis of inorganic
nanomaterials. We report the experimental realization of a broad range of closed-cage (fullerene-like) nanostructures,
nanotubes and/or nanowires for MoS2, SiO2 and Si, achieved via solar ablation. The solar technique generates the strong
temperature and radiative gradients - in addition to the extensive high-temperature annealing environment - conducive
to producing such nanostructures. The identity of the nanostructures was established with TEM, HRTEM and EDS. The
fullerene-like and nanotube MoS2 configurations achieved fundamentally minimum sizes predicted by molecular
structural theory. Furthermore, our experiments represent the first time SiO2 nanofibers and nanospheres have been
produced purely from quartz. The solar route is far less energy intensive than laser ablation and other high-temperature
chemical reactors, simpler and less costly.
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We present magnetoresistance measurements on metallic n-type InP sample with a carrier density n=1.241023m-3, far from the metal-insulator transition (MIT). The experiments were carried out at low
temperature in the range 4.2-0.6 K and in magnetic fields up to 1 T. We have observed negative magnetoresistance (NMR) behaviour, and the experimental data are interpreted in terms of the weak localization
and the effect of electron-electron interactions. Experimental data are compared with available theoretical
models using a non-linear regression method with adjustable parameters τε and F. τε is the inelastic scattering time and F is the Hartree-Fock constant.
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This work is a short review of our publications which are devoted to theoretical investigations of electronic structure
of the silver chlorine nanosystems with atomically rough surfaces, edge dislocations, and iodine isoelectronic
substitutional impurities. In particular, we have detected that the iodine hole traps are deeper if substitutional
iodine impurities are displaced near surface steps and kinks, or near line of dislocation.
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