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.

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.

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.

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.

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.

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.

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 (ΔH_b= -55 kcal mol^-1) is more stable than 'tripod' motifs (ΔH_b= -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.

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.

We present magnetoresistance measurements on metallic n-type InP sample with a carrier density n=1.2410²³ m^-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 τ_ε and F. τ_ε is the inelastic scattering time and F is the Hartree-Fock constant.

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.