Sven Södergren

Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO2 semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.

Photoelectrochemical studies of colloidal TiO2 films: The effect of oxygen studied by photocurrent transients

The effect of oxygen in nanocryst. colloidal TiO2 film photoelectrodes were studied by optically induced photocurrent transient measurements. O2 played an important role in the recombination of electrons and holes. Some effects esp. assocd. with the nanoporous morphol. of these photoelectrodes are discussed.

Verification of High Efficiencies for the Grätzel Cell : A 7% Efficient Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films.

Verification of High Efficiencies for the Gratzel Cell : A 7% Efficient Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films.

A new method for manufacturing nanostructured electrodes on glass substrates

The present paper describes a new method for manufacturing a nanostructured porous layer of a semiconductor material on a conducting plastic substrate for use in an electrochemical or photoelectrochemical cell. The method involves the deposition of a layer of semiconductor particles on conducting plastic and the compression of the particle layer to form a mechanically stable, electrically conducting, porous nanostructured film at room temperature. Photoelectrochemical characteristics of the resulting nanostructured films are presented showing, for example, overall solar to electric conversion efficiencies of up to 4.9% (0.1 sun). The potential use of the new manufacturing method in future applications of nanostructured electrodes is discussed.

N 1s x-ray absorption study of the bonding interaction of bi-isonicotinic acid adsorbed on rutile TiO2(110)

N 1s x-ray absorption spectra of bi-isonicotinic acid (2,2′-bipyridine–4,4′-dicarboxylic acid) on rutile TiO2(110) have been studied experimentally and quantum chemically. Differences between multilayer and monolayer spectra are explained by the adsorbate bonding to the substrate. A connection to the electronic coupling in dye-sensitized electrochemical devices is made.

Charge transport properties in the nanostructured ZnO thin film electrode – electrolyte system studied with time resolved photocurrents

The charge transport in nanoporous ZnO was studied by laser flash induced photocurrent transients. The results are discussed using a diffusion model and compared with previous results on TiO2. The charge transport was highly dependent on the potential giving apparent diffusion coeffs. for the electron ranging from 1×10-4 to 1×10-6 cm2/s with an applied bias of +100 and +300 mV vs. Ag/AgCl in ethanol, resp. The electrolyte was 0.5 M LiClO4 in ethanol. The potential dependence was much more pronounced for ZnO than for TiO2. The charge transport was also dependent on the electrolyte giving a linear dependence between the cond. of the electrolyte and the apparent electronic diffusion coeff. The dependence of the light intensity was also studied. Intensity-dependent losses were obsd.

Photocurrent Losses in Nanocrystalline/Nanoporous TiO2 Electrodes Due to Electrochemically Active Species in the Electrolyte

The photoelectrochemical properties of nanostructured highly porous TiO2 electrodes with different thicknesses (2 to 38 mu m) were investigated. The effects of electron accepters, such as oxygen and iodine, in the electrolyte were studied by action spectr

Electronic structure of electrochemically Li-inserted TiO2 studied with synchrotron radiation electron spectroscopies

The electronic properties of TiO2 and electrochemically Li-inserted TiO2 have been studied using synchrotron radiation photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) in conjunction with resonant photoelectron spectroscopy. Core level (Ti 2p) and valence level spectra show the presence of Ti3+ states in LixTiO2. The x values determined from core level peak intensities were found to be directly correlated to the inserted amount of Li+ determined electrochemically. The x-dependent width of the Ti 2p peaks is consistent with a two-phase regime at intermediate x values. Resonant photoelectron spectroscopy at the Ti 2p edge was performed for TiO2 and Li0.5TiO2 to delineate the Ti4+ and Ti3+ contributions to the XAS spectrum.

Electron dynamics within Ru-2,2′-bipyridine complexes—an N1s core level excitation study

The dynamics of electron transfer within an excited ruthenium complex as well as between a ruthenium complex and a nanostructured TiO2 film is addressed on the core hole few femtosecond lifetime scale. The ruthenium complexes studied are Ru(bpy)32+·2Cl− (where bpy is 2,2′-bipyridine) as well as Ru(bpy)2(dcbpyH2)2+·2PF6− (dcbpy is 4,4′-dicarboxy-2,2′-bypyridine) anchored to a nanostructured TiO2 surface, where the latter system constitutes the photoactive part of a dye-sensitized solar cell. The N1s core level of the ruthenium complexes was excited by using synchrotron radiation, and the experimental techniques used were X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). The occupied molecular orbital structure and the N1s partial density of unoccupied states are mapped and compared to calculated orbital structures.

Redox properties of nanoporous TiO2 (anatase) surface modified with phosphotungstic acid

Nanostructured anatase TiO2 electrodes surface modified with inorg. metal oxide clusters, i.e., phosphotungstic acid (PWA), was studied by XPS and spectroelectrochem. The surface modifiers were electroactive and could be addressed reversibly by changing the applied bias. The formation of new states in the electronic structure of TiO2 and the PWA was monitored by XPS using a novel prepn. technique. The coloration efficiency of the deposit was about 20 cm2/C. Prolonged electrochem. cycling degraded the elec. contact between the PWA and the TiO2 surface.