Hans Siegbahn

Nanostructured ZnO electrodes for dye-sensitized solar cell applications

Dye-sensitized photoelectrochem. solar cells constitute a promising candidate in the search for cost-effective and environment-friendly solar cells. The most extensively studied, and to date the most efficient systems are based on titanium dioxide. In this paper, the possibilities to use nanostructured ZnO electrodes in photoelectrochem. solar cells are investigated. Various exptl. techniques (e.g. IR, photoelectron, femtosecond and nanosecond laser spectroscopies, laser flash induced photocurrent transient measurements, two- and three-electrode photoelectrochem. measurements) show that the thermodn., kinetics and charge transport properties are comparable for ZnO and TiO2. The prepn. techniques of ZnO provide more possibilities of varying the particle size and shape compared to TiO2. However, the dye-sensitization process is more complex in case of ZnO and care needs to be taken to achieve an optimal performance of the solar cell.

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.

Electronic Structure of TiO2/CH3NH3PbI3 Perovskite Solar Cell Interfaces

The electronic structure and chemical composition of efficient CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2 were studied using photoelectron spectroscopy with hard X-rays. With this technique, it is possible to directly measure the occupied energy levels of the perovskite as well as the TiO2 buried beneath and thereby determine the energy level matching of the interface. The measurements of the valence levels were in good agreement with simulated density of states, and the investigation gives information on the character of the valence levels. We also show that two different deposition techniques give results indicating similar electronic structures.

The Auger electron spectrum of water vapour

Abstract The Auger electron spectrum of water vapour has been recorded and analyzed. For the analysis, an approximate formula for calculating the intensities of the Auger electron lines is derived. It is shown, that the calculated intensities along with theoretical energies of the Auger transitions account well for the observed spectrum. In particular, new assignments in terms of transitions to triplet final states are suggested.

Vibrational and lifetime line broadenings in ESCA

Abstract The line profile of the narrow, symmetric 1s line from neon, recorded with the new ESCA instrument with X-ray monochromatization, is analyzed. The natural linewidth of this line is found to be 0.23 ± 0.02 eV, in good agreement with theoretical calculations of the oscillator strengths for Auger transitions and X-ray emission. Spectra from molecules show frequently asymmetric core electron lines under high resolution. This rules out previous explanations based on a chemical influence on the natural lifetime. Contrary to earlier assumptions, vibrational excitations are shown to be important in core electron spectra. For methane, the vibrational energy spacing is large enough to allow the vibrational lines to be partly resolved. Recent results from accurate PNO CI calculations on methane agree well with the experimental findings. The Franck-Condon transitions in the C1s and N1s lines from CO and N2 are shown to be well described in the harmonic approximation and approximating the potential curves of the highly excited core hole states with the potential curve for the ground state of NO+, X1 Σ+. Knowledge of vibrational excitations in core electron spectra is shown to be valuable in the analysis of high resolution X-ray emission spectra of free molecules.

The ESCA Spectra of Benzene and the Iso-electronic Series, Thiophene, Pyrrole and Furan

The ESCA spectra of C6H6, C4H4S, C4H5N and C4H4O have been studied using MgKα radiation, 1253.6 eV. The study included both the core orbitals and the valence orbitals. The C1s chemical shifts have been compared with the shifts predicted by the potential model, using both CNDO and ab initio gross atomic charges. We have obtained the binding energies of the deeper lying valence orbitals which cannot be reached by ultraviolet radiation. Further, the valence orbital spectra have been analysed using line intensities predicted from the atomic population of the molecular orbitals.

ESCA applied to liquids

Abstract The conditions for applying ESCA on liquid samples are discussed. A “liquid beam” technique is developed which meets the various requirements. The first spectrum of a liquid is presented, namely from formamide. It is shown that one can adjust the liquid beam so that a complete separation between the ESCA signals from the liquid and the vapour is achieved. The binding energies of the core levels of the elements in the molecule differ from each other in the liquid and the vapour phase by about 1.6 eV. One can also obtain electron spectra from dissolved chemical species. This is illustrated by potassium iodide dissolved in formamide.

Adsorption of Bi-Isonicotinic Acid on Rutile TiO2 (110)

Bi-isonicotinic acid ~2,28-bipyridine–4,48-dicarboxylic acid! is the ligand of several organometallic dyes, used in photoelectrochemical applications. Therefore the atomic scale understanding of the bonding of this molecule to rutile TiO2(110) should give insight into the crucial dye–surface interaction. High resolution x-ray photoelectron spectroscopy ~XPS!, near edge x-ray absorption fine structure ~NEXAFS!, and periodic intermediate neglect of differential overlap ~INDO! calculations were carried out on submonolayer bi-isonicotinic acid rutile TiO2(110). Data from multilayers is also presented to support the submonolayer results. For a multilayer, XPS shows that the carboxyl groups remain in the ~pristine! protonated form, and NEXAFS show that the molecular plane is tilted by 57° with respect to the surface normal. For the submonolayer, the molecule bonds to the rutile TiO2(110) surface via both deprotonated carboxyl groups, with a tilt angle of 25°, and additionally an azimuthal orientation of 44° with respect to the @001# crystallographic direction. The adsorbant system was also investigated by quantum mechanical calculations using a periodic INDO model. The most stable theoretical adsorption geometry involves a twist around the molecular axis, such that the pyridine rings are tilted in opposite directions. Both oxygen atoms of each carboxyl group are bonded to five-fold coordinated Ti atoms ~2M-bidentate!, in excellent agreement with the experimental results.

ESCA studies of CO2, CS2 and COS

Abstract The core level electron spectra of CO 2 , CS 2 and COS excited by Mg Kα radiation have been studied to identify shake-up satellite lines associated with ionization from these levels. A number of such lines have been seen and possible assignments have been suggested using the excited states of the molecule as a guide. The valence spectra have also been recorded and they too were found to be rich in shake-up structure. The observed variation of the valence line intensities is discussed and compared with predictions made from an intensity model. The validity of distinguishing between π and σ symmetries in linear molecules in applying the intensity model is confirmed.

Calculation of molecular ESCA spectra by the multiple‐scattering‐Xα method

The recently developed multiple‐scattering method has been applied to the solution of the approximate SCF (Xα) equations. Through the concept of the transition state, it is possible with this method to estimate the ionization energies measured in electron spectroscopy (ESCA) experiments. Comparisons with experimental ionization energies for several small molecules (i.e., CF4, NH3, N2, CO, NO, CO2, N2O, C3O2) show in general, better agreement than semiempirical orbital energies and about the same as ab initio MO‐LCAO‐SCF orbital energies.