Showing papers by "Michael Grätzel published in 1999"

Acid-Base Equilibria of (2,2'-Bipyridyl-4,4'-dicarboxylic acid)ruthenium(II) Complexes and the Effect of Protonation on Charge-Transfer Sensitization of Nanocrystalline Titania.

Abstract: The ruthenium complexes [Ru(dcbpyH2)2(Cl)2] (1), [Ru(dcbpyH2)2(NCS)2] (2), (Bu4N)4[Ru(dcbpy)2(NCS)2] (3), and (Bu4N)2[Ru(dcbpyH)2(NCS)2] (4) were synthesized and characterized by cyclic voltammetry, UV−vis absorption, and emission, IR, Raman, and NMR spectroscopy. The absorption and emission maxima of these complexes red shifted with decreasing pH, and showed pH-dependent excited-state lifetimes. The ground-state pKa values were determined by spectrophotometeric methods, and the dissociation of protons was found to occur in two steps (pKa = 3 and 1.5). The Ru(II)/(III) couple in the complex (Bu4N)4[Ru(dcbpy)2(NCS)2] is shifted ca. 290 mV negatively with regard to that of the complex [Ru(dcbpyH2)2(NCS)2] due to the replacement of H+ by tetrabutylammonium cation. The negative shift for the dcbpy-based reduction potential is even larger, i.e., about 600 mV compared to that of the complex [Ru(dcbpyH2)2(NCS)2]. The effect of deprotonation on the performance of these complexes as photosensitizers for nanocrysta...

Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms.

Abstract: Reference LPI-ARTICLE-1999-017View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Nanocrystalline Mesoporous Strontium Titanate as Photoelectrode Material for Photosensitized Solar Devices: Increasing Photovoltage through Flatband Potential Engineering

Abstract: Nanocrystalline SrTiO3 is synthesized by hydrothermal treatment of nanocrystalline titanium dioxide in the presence of strontium hydroxide. Working photoelectrochemical solar cells are produced using these nanometer-sized semiconductor particles as photoelectrode materials. At AM 1.5, measured open circuit voltages were roughly 100 mV higher than in solar cells produced using nanocrystalline titanium dioxide (anatase), in agreement with a simple relation between semiconductor conduction band edge and open circuit voltage for these cells. Photocurrents measured in the SrTiO3 cells were roughly 1/3 those measured with TiO2 (anatase) -based cells. On the basis of flash laser photolysis and absorptance studies, we suggest that low dye loading and possibly suboptimal dye−oxide interactions can be the cause for the relatively low photocurrents in the SrTiO3 system.

Long-Lived Photoinduced Charge Separation and Redox-Type Photochromism on Mesoporous Oxide Films Sensitized by Molecular Dyads

Abstract: The photoinduced charge separation in three different assemblies composed of an electron donor D and a chromophore sensitizer S adsorbed on nanocrystalline TiO2 films (D−S|TiO2) was investigated. In all of the systems, the sensitizer was a ruthenium(II) bis-terpyridine complex anchored to the semiconductor surface by a phosphonate group. In two of the assemblies, the donor was a 4-(N,N-di-p-anisylamino) phenyl group linked to the 4‘ position of the terpyridine, either directly (dyad D1−S) or via a benzyl ether interlocking group (dyad D2−S). In the third system, the sensitizer and the donor (3-(4-(N,N-di-p-anisylamino)phenoxy)-propyl-1-phosphonate) were coadsorbed on the surface ((D3+S)|TiO2). Laser flash photolysis showed that the photoinduced charge separation process follows the sequence D−S*|TiO2 D−S+|(e-)TiO2 D+−S|(e-)TiO2 D−S|TiO2 Resonance Raman spectroscopy indicates that in the excited assemblies D2−S*|TiO2 and (D3+S*)|TiO2, one electron is promoted from the metal center to the terpyridine ligand...

Efficient near-IR sensitization of nanocrystalline TiO2 films by zinc and aluminum phthalocyanines

Abstract: Several zinc(II) and aluminum(III) phthalocyanines substituted by carboxylic acid and sulfonic acid groups were anchored to nanocrystalline TiO2 films. By irradiation with visible light the photovoltaic behavior of the electrodes containing LiI/LiI3/propylene carbonate electrolyte was measured. Most efficient results were found using zinc(II) 2,9,16,23-tetracarboxyphthalocyanine, with a current conversion efficiency at 700 nm approaching 45%. It is shown that electron injection into TiO2 occurs from the excited singlet state of the phthalocyanine derivatives. High stability of the cell performance under continuous irradiation was found.

Mesoporous oxide junctions and nanostructured solar cells

Abstract: Mesoporous films of wide-band gap semiconductor oxides are an important new class of electronic materials. They are constituted by a network of nanocrystalline particles of oxides, such as titania, niobia or zinc oxide, sintered together to allow for charge carrier transport to take place. The pores between the nanoparticles are filled with an electrolyte or a solid state organic hole conductor forming an interpenetrating heterojunction of very large contact area. These junctions exhibit extraordinary opto-electronic properties due to their large surface area to volume ratio leading to applications in different domains, such as photovoltaics, intercalation batteries, electrochromic and electroluminescent displays, photocatalysis and chemical sensors. Of particular interest are dye-sensitized heterojunctions, where photo-induced charge separation occurs at the interface between the mesoporous oxide and the hole conductor or the electrolyte. Photovoltaic cells based on this concept form a viable alternative to conventional silicon cells. Solar to electric power conversion efficiencies exceeding 10% have been reached with mesoporous titania films derivatized with molecular charge transfer sensitizers and used in conjunction with organic iodide/triiodide-based redox electrolytes. Long-term accelerated light-soaking tests have shown the system to be intrinsically stable. This article summarized recent developments in this field including a discussion of solid state dye-sensitized heterojunctions employing spirobifluorene-connected arylamines as hole transport materials.

Self-Assembled Monolayers as Interfaces for Organic Opto-electronic Devices

Abstract: Charge injection into an organic semiconductor can be improved by using a self-assembled monolayer of functionalized molecules grafted on the electrode. This new interface can be designed in order to reduce the Schottky barrier between the conductive electrode and the organic semiconductor. The polarizability of the molecules involved can also be chosen in order to increase the adhesion of the molecular semiconductor onto the electrode. We present Kelvin Probe experiments and saturated photovoltage measurements performed on a number of such derivatized electrodes. They permit a quantitative description of the potential shifts due to the self-assembled monolayers which are related to the electrical dipoles of the individual molecules constituting them. When conjugated sites contributing to the band states of the organic semiconductor are placed too close to the electrode in the negative part of the image-force potential, two new effects unfavorable to charge injection can appear. We demonstrate that it is convenient to separate the attachment group of the molecule from the conjugated core by a spacer of non-conjugated sites in order to reduce these undesirable effects.

Mediator Transport in Multilayer Nanocrystalline Photoelectrochemical Cell Configurations

Abstract: Reference LPI-ARTICLE-1999-019doi:10.1149/1.1391698View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Photoinduced charge injection from vibronically hot excited molecules of a dye sensitizer into acceptor states of wide-bandgap oxide semiconductors

Abstract: Reference LPI-ARTICLE-1999-011View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

The excitation wavelength and solvent dependance of the kinetics of electron injection in ru(dcbpy)2(ncs)2 sensitized nanocrystalline tio2 films

Abstract: Reference LPI-ARTICLE-1999-025View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution

Abstract: Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction.

Surface Modification of a Hydrogen-Bonded Pigment: A Fluorescence Spectroscopy Study

Abstract: The effect of hydrogen-bonding additives on the surface modification of a hydrogen-bonded organic pigment is studied using absorption and fluorescence spectroscopy. Evidence for surface dissolution that occurs during the mixing of solids is presented. The spectral signatures of monomers and hydrogen-bonded aggregates of the pigment that are present on the surface of crystallites of the pigment are quite distinct, as revealed by fluorescence excitation and emission spectra.