Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 639

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Dye/Semiconductor Interface

An Electron Spectroscopic Study of Systems forSolar Cell and Display Applications

BY

KARIN WESTERMARK

ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2001

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Dissertation for the Degree of Doctor of Philosophy in Physics presented at UppsalaUniversity in 2001

AbstractWestermark, K., 2001. Dye/Semiconductor Interfaces: An Electron Spectroscopic Studyof Systems for Solar Cell and Display Applications. Acta. Univ. Ups., ComprehensiveSummaries of Uppsala Dissertations from the Faculty of Science and Technology 639,61 pp. Uppsala. ISBN 91-554-5055-5.

The properties relevant for electron transfer processes between dye molecules andsemiconductor substrates, titanium dioxide (TiO2) and zinc oxide (ZnO), have beenstudied by means of photoelectron spectroscopy, PES, near edge X-ray absorptionspectroscopy, NEXAFS, and resonant photoemission, RPES.

For dye-sensitized solar cells, the currently used dyes are rutheniumpolypyridine complexes adsorbed to the semiconductor via carboxyl linker groups. Aseries of such complexes has been investigated, and the most efficient dye so far, cis-bis(4,4’-dicarboxy-2,2’-bipyridine)-bis(isothiocyanato)ruthenium(II), RuL’2(NCS)2,was studied in more detail. The results revealed a high content of thiocyanate orbitals inthe highest occupied molecular orbital, HOMO, of this complex, which partly explainsits efficiency in the solar cell. The thiocyanate ligands were found to be highlyinfluenced by the substrate when the dye is adsorbed onto ZnO, which is not the casefor the corresponding TiO2 system.

A bridge bonding between TiO2 and the L’ ligand was proposed, where thecarboxyl groups are deprotonated and all oxygens interact with surface titanium ions.For ZnO, the results indicate a different bonding geometry, involving protonatedcarboxyl groups.

For the display system a dye molecule, which shifts color upon electrochemicaltreatment, was adsorbed on TiO2 and studied in its reduced and oxidized states. Themajor electronic difference between the two states was shown to occur on the nitrogenatom. In addition, a reversible photoreduction process during the measurements wasobserved.

Key words: Dye-sensitized, nanostructured, electron spectroscopy.

Karin Westermark, Department of Physics, Uppsala University, Box 530, SE-751 21Uppsala, Sweden

© Karin Westermark 2001

ISSN 1104-232XISBN 91-554-5055-5Printed in Sweden by Eklundshofs Grafiska AB, Uppsala 2001

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To My Parents

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List of Papers

This thesis is based on a collection of papers, which will be referred to inthe text by Roman numerals (I-VIII).

I. The electronic structure of the cis-bis(4,4’-dicarboxy-2,2’-bipyridine)-bis(isothiocyanato)ruthenium(II) complex and itsligand 2,2’-bipyridyl-4,4’-dicarboxylic acid studied with electronspectroscopyH. Rensmo, S. Södergren, L. Patthey, K. Westermark, L. Vayssières,O. Kohle, P. A. Brühwiler, A. Hagfeldt, and H. SiegbahnChem. Phys. Lett. 1997, 274, 51

II. Adsorption of bi-isonicotinic acid on rutile TiO2 (110)L. Patthey, H. Rensmo, P. Persson, K. Westermark, L. Vayssières,A. Stashans, Å. Petersson, P. A. Brühwiler, H. Siegbahn, S. Lunell,and N. MårtenssonJ. Chem. Phys. 1999, 110, 5913

III. XPS studies of Ru-polypyridine complexes for solar cellapplicationsH. Rensmo, K. Westermark, S. Södergren, O. Kohle, P. Persson,S. Lunell, and H. SiegbahnJ. Chem. Phys. 1999, 111, 2744

IV. Triarylamine on nanocrystalline TiO2 studied in its reduced andoxidized state by photoelectron spectroscopyK. Westermark, S. Tingry, P. Persson, H. Rensmo, S. Lunell,A. Hagfeldt, and H. SiegbahnAccepted for publication in J. Phys. Chem. B.

V. PES studies of Ru(dcbpyH2)2(NCS)2 adsorption onnanostructured ZnO for solar cell applicationsK. Westermark, H. Rensmo, H. Siegbahn, K. Keis, A. Hagfeldt,L. Ojamäe, and P. PerssonIn manuscript.

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VI. Electron spectroscopic studies of bis-(2,2'-bipyridine)-(4,4'-dicarboxy-2,2'-bipyridine)-ruthenium(II) and bis-(2,2'-bipyridine)-(4,4'-dicarboxy-2,2'-bipyridine)-osmium(II)adsorbed on nanostructured TiO2 and ZnO surfacesK. Westermark, H. Rensmo, A. C. Lees, and H. SiegbahnIn manuscript.

VII. N1s X-ray absorption and resonant photoelectron spectroscopystudy of Ru-2,2’-bipyridine complexes for solar cell applicationsK. Westermark, H. Rensmo, J. Schnadt, P. Persson, S. Södergren,P.A. Brühwiler, S. Lunell, and H. SiegbahnIn manuscript.

VIII. Determination of the electronic density of states at ananostructured TiO2/Ru-dye/electrolyte interface by means ofphotoelectron spectroscopyK. Westermark, A. Henningsson, H. Rensmo, S. Södergren,H. Siegbahn, and A. HagfeldtIn manuscript.

Comments on My Participation

The results here are largely the product of a teamwork, involving the effortof many persons. I have taken part in all experimental work presented inthis thesis, as well as in the data analysis and discussions. I had the mainresponsibility for the experiments and experimental analysis in Paper IV-VIII.

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Contents

1. Introduction 9

2. Electron Spectroscopic Methods 122.1 Photoelectron Spectroscopy 12

2.1.1 The PES process 142.1.2 Surface Sensitivity 162.1.3 Satellites 162.1.4 Intensity 172.1.5 Line Widths and Shapes 172.1.6 Decay Processes 18

2.2 X–ray Absorption Spectroscopy 182.2.1 Probing of Atom-Specific Unoccupied Levels 192.2.2 Geometry Determination by NEXAFS 19

2.3 Resonant Photoemission Spectroscopy 202.4 Experimental Details 21

2.4.1 Synchrotron Radiation 212.4.2 Experimental Setups 222.4.3 The PES Setup 222.4.4 Beamline 22, 51 and I411 at MAX-lab 232.4.5 Combining PES with Electrochemistry 24

3. The Systems 263.1 Dye-Sensitized Solar Cells 26

3.1.1 The Dye 263.1.2 The Metal Oxide 303.1.3 Anchoring of the Dye to the Surface 323.1.4 The Counter Electrode and the Electrolyte 32

3.2 Displays based on Organic Sensitizers 33

4. Dyes Adsorbed onto TiO2 and ZnO 354.1 Bonding Geometry 35

4.1.1 dcbpyH2 Adsorbed onto Single Crystal TiO2 354.1.2 Dye Complexes Adsorbed onto Nanostructured TiO2 374.1.3 Dye Complexes Adsorbed onto ZnO 39

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4.2 Molecular Orbital Compositions 424.3 Energy Level Matching 45

4.3.1 TiO2 Bandgap States and Influence of Cations 454.3.2 Dye Complexes Adsorbed onto TiO2 474.3.4 Dye Complexes Adsorbed onto ZnO 48

4.4 Excited State Dynamics 49

5. Triarylamine Adsorbed onto TiO2 51

Acknowledgements 54

Bibliography 56

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1. Introduction

In recent years concerns about energy resource depletion and the globalenvironmental impacts of energy production have heightened public,industrial and political interest in renewable energy forms. As the nameimplies, renewable energy is sustainable, i.e. it can be used today withoutdepleting future resources. This is in contrast to e.g. fossil fuels, whichhave a finite economic lifetime as an energy resource, even if this mightspan several centuries. Solar energy is a renewable energy source providingearth vast amounts of clean energy. Solar energy can be converted tochemical energy, e.g. by photosynthesis in the plants, to heat by solarcollectors, or to electricity by solar cells.

A solar cell uses light to excite electrons from lower to higher energylevels, from which they are collected and brought out to an external circuit.At present, solar cells based on single and multi-crystalline silicon providethe best compromise between performance and production costs. In thesecells, the indirect bandgap of silicon makes it necessary to use a thicknessof at least 50 µm (taking back surface reflection into account) because withthinner devices not all sunlight can be absorbed. The charge carriers musttherefore travel over a relatively long distance, which is only possible whentheir lifetime is large enough. High quality material is demanded to meetthis requirement, which in turn necessitates costly synthesis routes withadvanced equipment. Much attention is therefore given to alternative solarcell materials showing similar performance as silicon cells, but with muchlarger cost reduction potential in possible production technologies.

Dye-sensitized solar cells have recently become a promisingalternative in the search for cost-effective and environment-friendly cells.Figure 1.1 shows a schematic picture of the cross section of such a solarcell. The active electrode film consists of dye molecules adsorbed on ananostructured semiconductor film (A). Such a film consists of nano-crystals in electrical contact with each other, forming a highly porousstructure. The film is attached to a conducting, transparent substrate (B),constituting one of the electrodes in the system. This electrode is connectedto a counter electrode (C) via an external circuit. Between the electrodes an

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electrolyte penetrates the pores in the nanostructured film. When the dye isilluminated, an electron is excited to a higher energy level in the dye andinjected into the conduction band of the semiconductor. Since the injectionprocess can be made several orders of magnitude faster than the lifetime ofthe excited state (up to about 106 times faster for the most efficient dyes),the probability for recombination within the excited dye is low. Afterinjection, the electron is transported through the semiconductor to the back-contact to perform electrical work in an outer circuit. It is subsequentlyreturned to the cell via the counter electrode. The dye molecule is left in itsoxidized state, but it is regenerated by a reducing agent in the electrolytesolution between the two electrodes. The oxidized solution species diffusestowards the counter electrode where it is reduced by the electrode, and thecycle is completed. Thus no net chemical process takes place in the system.

Figure 1.1. A schematic picture of the cross section of a dye-sensitized, nanostructuredsolar cell. The electron transfer processes in the cell are also indicated.

Nanostructured semiconductor films have also been found useful forother applications. One example is displays in which an applied electricpotential is used to oxidize or reduce molecules adsorbed on thesemiconductor surface. The adsorbed molecules are dyes which shift colorupon oxidation or reduction. Triarylamine is such a molecule, beingcolorless in its neutral (reduced) state and blue when it is oxidized.

In this work, dye/semiconductor interfaces have been investigated bymeans of electron spectroscopy. The studies have mainly focused on theproperties relevant for electron transfer processes between adsorbed dye

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molecules and semiconducting substrates. The dyes used for the solar cellapplications were mainly ruthenium bipyridine complexes adsorbed viacarboxyl linker groups to the semiconductors, titanium dioxide (TiO2) andzinc oxide (ZnO). The molecular orbitals involved in the optical excitationsleading to electron transfer were studied, as well as the bonding betweenadsorbate and substrate. For the display system, electrochemically reducedand oxidized triarylamine adsorbed on TiO2 was studied in order to revealthe influence of the electrochemical treatment in the electronic andmolecular structure of the triarylamine layer.

The thesis is divided into five chapters: Following this initialchapter, the experimental methods used in the thesis are described inChapter 2. In Chapter 3, an overview is given of the systems studied,focusing on the dye molecules. Finally, Chapter 4 and 5 summarize theresults obtained in the papers included in this thesis.

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2. Electron Spectroscopic Methods

In this chapter, electron spectroscopic methods are described. Thesemethods are used to study the electronic structure of a system. Informationis obtained by letting light of a well-known energy interact with a gas,liquid or solid. In this thesis only solid samples were studied, and thereforeonly such samples will be discussed here.

Figure 2.1. An overview PES spectrum of Ru(dcbpyH2)2(NCS)2 adsorbed on a nano-structured TiO2 film. The inset shows a closeup of the C1s and Ru3d region.

2.1 Photoelectron Spectroscopy

Photoelectron spectroscopy, PES (also X–ray PES, XPS), is a surfacesensitive technique providing information on the occupied electronicenergy levels in a system. In a photoelectron spectroscopic measurementthe number of electrons is recorded as a function of binding energy. Thisspectrum provides information on what elements are present in the sampleand their chemical environment. The electrons in an atom can be broadlydivided into two categories: core and valence electrons. Core electrons aretightly bound to the nucleus (high binding energies) and do not participatedirectly in chemical bonding between atoms. Valence electrons, on the

600 500 400 300 200 100 0Binding Energy [eV]

O1s

N1s

C1s Ru3d

Ti2p

S2pS2s

290.0 285.0 280.0

Ru3d5/2

C (C-C)

C (COOH)

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other hand, are more loosely bound (low binding energies) and are thus theactive agents in the formation of chemical bonds.

Due to their high binding energies, the orbitals of the core electrons havenearly atomic character also for molecules and solids. Thus, all atoms havetheir own set of core electron binding energies and they can therefore beused for qualitative analysis of the sample (see Figure 2.1). The detailedpositions of the core levels, however, depend on the chemical state of theprobed atoms (chemical shifts). The shifts are of the order of a few electronvolts, and may to a first approximation be interpreted in terms of potentialmodels [1]. In such models, the core electron binding energy shifts aregiven by the electrostatic potential energy changes close to the nucleusinduced by redistributions occurring among the valence electrons. Forexample, valence electron transfer from the environment to the atomconsidered will give a negative contribution to the electrostatic potentialclose to the nucleus and thus a lowering of the core electron bindingenergy. Chemical shifts can therefore be used for determination of thechemical environment of an atom; an example of this is given in Figure 2.1.

Since the valence electrons are directly involved in the formation of chemi-cal bonds they are more direct probes of the bonding process than the coreelectrons. The interpretation of valence spectra is, however, more difficultsince in this binding energy region the atomic orbitals overlap and formmolecular orbitals or bands, and comparison with quantum chemicalcalculations is generally required. In such calculations the Hartree-Fockequation [2] is solved by using ab initio, semiempirical or densityfunctional theory (DFT) methods. Molecular orbitals, φi

MO, are usuallytaken to be linear combinations of atomic orbitals, LCAO:

φiMO=Σiciψi

AO (2.1)

where ci are expansion coefficients and ψi are atomic orbitals, constitutingthe basis set for the calculation. The calculations provide a set of molecularorbitals having eigenvalues εi. These are related to the ionization energiesof the electrons via Koopmanns’ theorem:

EB=-εi (2.2)

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Figure 2.2. The principal processes for a) photoelectron spectroscopy, PES, and b) X–ray absorption spectroscopy, XAS.

an approximation which neglects changes in the molecular orbitals uponionization (electronic relaxation). When analyzing experimental valencespectra, the atomic contributions to the different features may be estimatedon the basis of the expansion coefficients in Equation 2.1.

2.1.1 The PES ProcessIn a PES experiment the sample is exposed to monochromatic light with awell-defined photon energy hν. The photoelectric effect is utilized tomeasure the binding energies of electrons, see Figure 2.2a. According tothe photoelectric law [3], the binding energy EB is given by

EB = hν - E’k (2.3)

where the binding energy, EB (also referred to as the ionization potential),is referred to the vacuum level and E’k is the kinetic energy of thephotoelectron just outside the sample surface (see Figure 2.3a). Forcondensed-phase samples, binding energies are commonly referred to theFermi level. The two types of binding energy are related via the samplework function (φs):

EB=EBF+φs (2.4)

The kinetic energy Ek is measured in the spectrometer and is related to EBF

via:

EBF = hν - Ek - φsp (2.5)

a) PES b) XAS

hν hνe-

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Figure 2.3. a) An energy level diagram for a sample in electrical equilibrium with thespectrometer. b) The corresponding PES spectrum for a gold sample (cut-off andvalence levels).

where Ek and φsp are defined in Figure 2.3. In view of (2.5), for conductingcondensed phase samples, spectra can be referred to the Fermi level. Thus,the spectra can be calibrated using lines from simultaneously presentcalibration substances (e.g. a noble metal surface) whose binding energiesversus the Fermi level have been established from other measurements.Other calibration possibilities are ambient hydrocarbon C1s (alwayspresent in non-UHV conditions) or other lines internal to the sample, whichmay be considered constant in binding energy for the purposes at hand.

It is, however, also possible to obtain vacuum-level referenced bindingenergies (EB) via an alternative procedure. By measuring the energy of theelectrons having zero kinetic energy (the “cut-off” of the secondaryelectron distribution), the position of the vacuum level of the sample can bedetermined since the photon energy is known (or may be accuratelydetermined in synchrotron radiation measurements), see Figure 2.3b. Thespectra can therefore be calibrated versus the vacuum level without resortto calibration substances.

Energy/ eV hν

b)

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2.1.2 Surface SensitivitySurface sensitivity is not related to the optical excitation process, since thelight penetrates several hundreds of Ångströms. The surface sensitivity isinstead a consequence of the very short escape depth of the PES peak(elastic) electrons in matter. For the photon energies normally used theescape depth is only 5-25 Å, depending on the kinetic energy of theelectron. This makes PES a very surface sensitive technique. The minimumescape depth, and thus the highest surface sensitivity, is obtained atelectron kinetic energies of about 50 eV. The surface sensitivity can bemoderated by changing hν or by changing the exit angle of the electronsfrom the sample surface plane (grazing exit angles give higher surfacesensitivity for flat surface samples). Electrons from the deeper lying partsof the sample loose energy due to inelastic scattering processes, and mostof them will remain inside the solid. Some will, however, reach the surfacewith enough energy to leave the solid, leading to a background in thespectrum. Such photoelectrons are normally affected by several inelasticscattering processes, and the background will therefore have a continuousenergy distribution.

2.1.3 SatellitesApart from the main (elastic) photoemission described above, otherprocesses may occur and give rise to additional lines in the photoelectronspectrum. For example, shake-up satellites may appear at higher bindingenergies (i.e. lower kinetic energies) than the main line. The satellitespectrum is a consequence of the strong perturbation of the electronicstructure caused by the sudden removal of an electron. In a simple picture,the relaxation process leads to additional valence excitations which giverise to new spectral features at lower kinetic energies of the outgoingphotoelectrons. For example, in unsaturated systems such as benzene, π toπ* transitions are strong and require low (less than 10 eV) excitationenergy. In these cases shake up lines will occur within 10 eV from the mainline towards higher binding energies in a spectrum.

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2.1.4 IntensityThe intensity, Ia, of a peak in a photoelectron, PE, spectrum does not onlydepend on the surface density ρa of the corresponding element, but also onthe differential cross section σa for ejection of electrons from the relevantorbital as well as the mean free path Λ of the electrons in the sample. Thedifferential cross section in turn depends on the photon energy hν and theangle θ between the photon polarization vector and the direction of thephotoelectron, and the mean free path is a function of the kinetic energy ofthe photoelectron. Also, there is an intensity dependence due to the elec-tron-optical transport from the sample to the detector, given by the kineticenergy of the electrons by means of a spectrometer function, S(EKin). Theintensity of a PES peak can thus be written as

Ia∝σa(hν,θ)⋅ρa⋅Λ(Ekin)⋅S(Ekin) (2.6)

The dependence of the differential cross section on hν and θ varies fordifferent elements and orbitals. The relation above can thus be used todetermine the orbital parentage of valence levels. The atomic origin of themolecular orbitals may be identified by changing hν or θ and comparingthe variation with tabulated differential photoionization cross sectionvariations for atomic orbitals.

As implied by (2.6), when using photoelectron intensities to estimate therelative surface concentration of an element, it is important to keep thevariation of differential cross section, mean free path and spectrometerfunction between different elements in mind.

2.1.5 Line Widths and ShapesThe resolution of the lines in a PE spectrum depends on several differentparameters. Due to the finite lifetime of the core hole, the lines will have aninherent linewidth. A short lifetime gives a broad line and vice versa. Theinherent line shape is generally a Lorenzian. Experimental factors, such aswidth and shape of the exciting radiation and the resolution and imagingproperties of the analyzer, contribute to the total line width with a Gaussiandistribution. The resolution can be changed by changing the pass energy ofthe electrons in the analyzer, the slit widths in the monochromator, theelectron lens or in the analyzer. The total line shape resulting from theabove factors is to a good approximation given by a convolution of Loren

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zian and Gaussian distributions, a so-called Voigt profile. In addition, PElines may be broadened by excitations during the PES process, such asvibrational excitations.

2.1.6 Decay ProcessesThe creation of a core hole in the PES process leaves the atom in a veryunstable state and the hole is quickly filled by an electron from an upperlevel. The corresponding excess energy is either released as a photon(radiative decay) or transferred to another electron, which then obtainsenough energy to leave the sample (Auger decay). The energy of thephoton or the Auger electron is characteristic for the atomic speciesindependent of the excitation photon energy, and it can therefore be used asan atomic fingerprint tool.

2.2 X–ray Absorption Spectroscopy

In X–ray absorption spectroscopy, XAS, electrons from a core level areexcited to unoccupied levels or to the continuum by absorption of photons(see Figure 2.2b). Thus, a spectrum of the number of excited electrons as afunction of photon energy is recorded. The absorption probability of X–rays in matter varies smoothly with the photon energy except at certainenergies, absorption edges, corresponding to excitation into unoccupiedbound states. These resonant features are usually referred to as Near EdgeX–ray Absorption Fine Structure (NEXAFS). This method is thus used tostudy the unoccupied electronic energy levels of a system, lying betweenthe occupied valence levels and the energy of free electrons (vacuumenergy). At higher photon energies slow oscillations in the continuousspectrum may be observed, from which information on the bondingdistance and coordination number can be extracted. XAS in this region isalso called Extended X–ray Absorption Fine Structures (EXAFS). In thischapter only NEXAFS will be described.

For the core levels studied here Auger decay dominates the deexcitation ofthe core hole. The number of absorbed photons is proportional to thenumber of core holes and thus to the number of emitted Auger electrons.The absorption spectrum can therefore be recorded by measuring thecurrent produced by the Auger decay. An advantage with this detection

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mode is that it makes the technique surface sensitive, due to the finiteescape depth of the electrons in the material.

2.2.1 Probing of Atom-Specific Unoccupied LevelsWhen exciting electrons from a core level, the unoccupied levels located onthe core hole site are primarily probed. It is therefore possible to distin-guish contributions from different species in the unoccupied levels. Thetransitions obey dipole selection rules, which for example means thatp orbitals are probed by excitation from an s core orbital. It should be keptin mind when analyzing the NEXAFS spectra that the core hole will affectthe electronic energy levels leading to a lowering in the energy of theelectronic levels due to relaxation effects.

2.2.2 Geometry Determination by NEXAFSIn certain cases, NEXAFS measurements can be utilized to determine theorientation of molecules on a surface. This requires polarized light, a well-defined surface and, generally, a small molecule. The excitation to the un-occupied states is determined by the dipole selection rule and the intensityis given by Fermi's golden rule. The latter states that the intensity for a 1selectron that is excited by linearly polarized light into a σ or π state is pro-portional to sin2θ or cos2θ, respectively, where θ is the angle between theE-vector of the photons and the direction of the maximum orbital ampli-tude. The orientation of an orbital, and thus of a molecule, can therefore bederived by recording the intensity of the σ and π resonances as a functionof the photon incidence angle if the orientation of the sample and thedirection of the polarization vector are known. For example, the excitationof C1s electrons to π∗ in a benzene ring, where the π-orbitals are perpendi-cular to the molecular plane, will reach a maximum when also the E-vectoris perpendicular to the molecular plane.

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2.3 Resonant Photoemission Spectroscopy

Resonant Photoemission, RPES, is photoelectron spectroscopy using aphoton energy that coincides with an excitation energy (resonance) of acore orbital in the system (i.e. with a NEXAFS peak), where an electron isexcited to a bound state. If the energy of this neutral excited state liesabove the minimum ionization threshold, it decays via an Auger processleading to electron emission, so-called autoionization.

Figure 2.4. Scheme of a,b) the decay processes in resonant photoemission (followingan excitation corresponding to XAS, see Fig. 2.2b), and c) Auger decay.

In an autoionization spectrum, features originating from two differentdecay processes are present: participator and spectator decay (see Figure2.4). In participator decay, the excited electron takes part in the decayprocess in filling the core hole. The final states are in this case the same asfor a normal PE valence band spectrum, having a hole in one of the valencelevels. The intensity of the valence levels which contain contributions fromthe element where the core excitation took place are, however, enhanced.Spectator decay, on the other hand, is an autoionization process where theexcited electron remains in its excited state during the decay process, thecore hole is filled by a valence electron, and another valence electron isemitted. This leads to the same final states as for PE shake-up satellitesdiscussed above.

RPES can be used to extract kinetic information on electronic processesthat occur within the lifetime of the core hole, typically a few femtoseconds[4-7]. If the electron in the excited state of a molecule has a possibility tointeract with empty orbitals, e.g. in the substrate, electron transfer mayoccur within a timescale substantially shorter than the autoionization decayprocesses discussed above. In such a case, the resonant spectrum will

a) Participatordecay

b) Spectatordecay

c) Augerdecay

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instead be dominated by an Auger spectrum identical to that recorded farabove threshold.

2.4 Experimental Details

2.4.1 Synchrotron RadiationSeveral types of light sources can be utilized for PES measurements:characteristic X–rays (e.g. AlKα, MgKα), UV (e.g. HeIα), and synchrotronradiation. Synchrotron radiation, SR, is emitted when charged particles areaccelerated while they travel at very high, relativistic, speed.

In a synchrotron, electrons are traveling at a speed close to the speed oflight. They are kept in an ultra high vacuum storage ring, which normallyconsists of straight sections and "corners" where magnets bend the orbit ofthe electrons. The electrons will emit light only when their orbits aredeflected, i.e. at the bending magnets. To get more light, insertion devicessuch as wigglers and undulators can be put in the straight parts. Theseconsist of arrays of magnets that cause oscillations of the otherwise straightelectron trajectories, and produce radiation with other characteristics thanthe light from a bending magnet. In an undulator, interference effectsproduce radiation of high brightness concentrated at specific photon ener-gies, instead of the broad continuous light distribution emitted from abending magnet. The photon energies are (odd) harmonics of afundamental frequency and can be altered by changing the magnetic field(pole gap) of the undulator.

SR has several advantages compared to characteristic X–ray radiation. Ithas a continuous wavelength distribution spanning over a wide energyrange, in contrast to the discrete levels of characteristic X–rays. This makesit possible to choose the photon energy best suited for the experiment. ForXAS studies, where the photon energy is scanned during the measurement,synchrotron radiation is required. The intensity (or rather the spectralbrightness; number of photons in a 0.1% fractioned bandwidth at hν/s·solidangle·source area·ring current) of SR is much higher than for a conven-tional X–ray source. In addition, the light is polarized: it is plane polarizedin the electron orbit plane where the E-field only has a horizontal

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component, and elliptically polarized out of the orbit plane where the E-field also has a vertical component.

The disadvantage with SR is the size and the cost of the equipment: a syn-chrotron radiation facility is a very large and expensive construction. Thereis one in Sweden, the Swedish National Laboratory MAX-lab in Lund. Allof the measurements in this thesis except parts of the measurements forPaper III and IV were performed at MAX-lab, while the remainingmeasurements for Paper III and IV were made in Uppsala, using monochro-matized AlKα radiation (hν 1487 eV).

2.4.2 Experimental SetupsThe experimental setups are located in connection to the bending magnetsor insertion devices. The light from the storage ring is directed to theexperimental set-up (end station) via a beamline, which essentially consistsof an evacuated tube equipped with a monochromator and photon optics.The end station, which contains all equipment necessary for the measure-ment, is normally divided into three parts: an introduction chamber, apreparation chamber and an analysis chamber. The introduction chamber isused for moving samples in and out from the vacuum system. The prepa-ration chamber is used for sample preparation such as sample cleaning andadsorption of molecules onto single crystal surfaces. The analysis chamberis where the actual measurements take place and may contain several typesof detectors, in particular for PES an electron energy analyzer fordetermining the kinetic energy of the photoelectrons. The sample istransferred between the chambers via a translational manipulator rod. InFigure 2.5, a schematic picture of the system used for the non-UHVmeasurements in this thesis is shown. Here the preparation chamber alsoserves as an introduction chamber.

2.4.3 The PES SetupIn a PES experiment using SR, the monochromator is used to select thedesired photon energy for the measurement. The sample should beelectrically conducting and have a good electrical contact with its holder.Otherwise the sample surface will become charged during the measure-ment, since electrons are emitted from the sample, which will cause thepeaks in the spectrum to drift.

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Figure 2.5. Cross section of a PES setup.

The emitted electrons enter the analyzer, which in the present work consistsof two hemisphereres with an electrostatic field inbetween. An electronwith high kinetic energy will have an orbit with a larger radius than anelectron with low kinetic energy. Thus, only electrons within a certainrange of energies will reach the multichannel detector of the analyzer. Theelectrons are retarded or accelerated by an electron lens before they enterthe analyzer, which is set to transmit electrons of a certain range around amean (pass) energy. The retardation/acceleration potential is varied, and thenumber of electrons transmitted through the analyzer is recorded. Thespectrum thus obtained is subsequently calibrated in binding energyaccording to Section 2.1.1.

2.4.4 Beamline 22, 51 and I411 at MAX-LabThe measurements for Paper II and parts of the measurements for Paper Iwere performed at the former beamline 22 at MAX I, using light frombending magnets [8]. This beamline had an end station specialized forsurface analysis and operating in ultra high vacuum (UHV, 10-10 mbarrange). It is now relocated to the new storage ring MAX II (beamlineD1011). The preparation chamber is generally equipped with devices suchas a sputtering gun for cleaning of the sample, gas inlets for surface adsorp-tion, and a LEED (Low Energy Electron Diffraction) instrument for studiesof the surface crystal structure. In our case it was also equipped with a

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specially built doser for evaporation of bi-isonicotinic acid onto the samplesurface. The analysis chamber contains both a PES analyzer and amultichannel plate detector for X–ray absorption spectroscopy.

For Paper II and parts of the measurements for Paper I and VIII, the endstation of the former beamline 51 at MAX I was used [9]. This end stationis now moved to beamline I411 at MAX II [10], where the measurementsfor Paper V-VIII and parts of the measurements for Paper IV were made.At beamline 51 and I411 the light is provided from an undulator, and theend station operates in the 10-6-10-7 mbar range. The analyzer can berotated for this end station, which gives possibilities to measure the numberof emitted photoelectrons as a function of the angle between the photonpolarization direction and the direction of the detected photoelectrons. Thiswas utilized in Paper I and II. The end station can be used for solid, gas aswell as liquid phase samples. In our case it was rebuilt to combine PES andelectrochemical measurements. The setup used for this was the same asused in our lab in Uppsala.

2.4.5 Combining PES with ElectrochemistryWhen electrochemically modified samples are studied, these should betransferred from the electrochemistry preparation to the photoelectronspectrometer analysis chamber without being exposed to air. Oxygen andwater in the air could otherwise react with the oxidized or reduced sample.Therefore the electrochemistry is performed in an argon-filled drybox afterwhich the sample is transferred into the PES setup in an air-tight protectiontube. The sample holder for PES also constitutes the lid of the protectiontube (stainless steel), which is sealed by an o-ring. This sample holder ismounted on the manipulator rod, when the preparation chamber is closed.When a pressure of 5×10-4 mbar is reached, the protection tube is removedand the sample can be transferred into the analysis chamber. Thisarrangement was used for the measurements in Paper IV.

Cyclic VoltammetryCyclic voltammetry, CV, is an electrochemical method commonly used togive a “fingerprint” of a chemical substance, and also for detailed studies ofreaction kinetics [11]. In CV a linearly swept potential is applied to thesample, which is immersed in an electrolyte solution. A cyclic voltammo-gram, showing the current as a function of the potential, is recorded. Thisgives a picture of the potentials for which charge transfer reactions take

25

place, and also indicates if these are reversible or not. For the cyclicvoltammetry measurements performed on the system studied in Paper IV, athree-electrode setup was used, see Figure 2.6. This consists of threeelectrodes; the working electrode, which is the sample of interest, thereference electrode, and the counter electrode. The electrodes are connectedto a potentiostat, which controls the potential of the working electrode.

Figure 2.6. A schematic picture of a three-electrode setup [11].

Three-electrode setup

Workingelectrode

Counterelectrode

ReferenceelectrodeV

i

Powersupply

EW.E vs ref

3. The Systems

3.1 Dye-Sensitized Solar Cells

Figure 3.1 shows a simple picture of the energy levels involved in theelectron transfer processes in the dye-sensitized solar cell depicted inFigure 1.1. The dye sensitization concept was invented in order to find aphotoelectrochemical system based on a semiconductor which is stableagainst photocorrosion and yet absorbs light in the visible region. Manymetal oxides fulfill the former requirement, however they only absorb UV-light. A way to extend their spectral response is to adsorb dye moleculesthat absorb visible light on the semiconductor surface: dye sensitization. Asseen from Figure 3.1, the function of a dye-sensitized solar cell relies onthe interaction and energy matching between the different componentsinvolved in the photoelectrochemical cycle. In the cycle an electron isinjected from the dye to the metal oxide and following a series ofsubsequent steps regenerates the dye via a redox couple in the surroundingelectrolyte. In this section the different parts and processes of the systemsare described, with an emphasis on the dye and metal oxide which are inthe main focus in this thesis.

3.1.1 The DyeThe dye should absorb visible light and, upon excitation, inject an electroninto the conduction band, CB, of the semiconductor (see Figure 3.1). Inorder to design a dye for efficient solar cells, there are several requirementsthat have to be met. The excited states of the dye should match theconduction band of the semiconductor to favor electron injection. Inaddition, the dye should be located close to the semiconductor, otherwiseluminescence or nonradiative decay takes place instead of electroninjection from the excited molecule. In order to absorb as much sunlight aspossible per unit area, the dye molecules should be small and have a broadand intense absorption in the visible region. Finally, the dye has to endurerepeated oxidation and regeneration cycles for a long-term stability.

27

Figure 3.1. A schematic picture of the electronic energy levels in a dye-sensitized solarcell.

It is difficult to find a dye which satisfies all of the above require-ments. Ru(bpy)3

2+, where bpy is 2,2’-bipyridine (see Figure 3.2a), is one ofthe most extensively studied transition metal complexes within photo-chemistry due to its combination of chemical stability, redox properties,excited state reactivity, luminescence emission, and long excited statelifetime [12]. Due to the large number of studies on these complexes, it ispossible to tailor complexes with certain properties by choosing properligands [13-17]. By variation of the ligands the electronic structure of acomplex is modified, and thus chemical and physical properties such asredox potentials and optical absorption are altered. The closely relatedosmium bipyridine derivatives have similar properties. To date, rutheniumand osmium polypyridine complexes are the most promising sensitizers fordye-sensitized solar cell applications.

Complex Chemistry TerminologyAs an introduction in the following discussion of ruthenium and osmiumcomplexes as sensitizers, a brief outline is given here of the currentterminology. For a more complete description, see for example refs. [18,19].

Ruthenium and osmium are d-block elements. Their oxidation stateis normally +2. The oxidation state is, however, a formalism used forkeeping track of the electrons, rather than the actual charge of the metal. Ruand Os compounds are often referred to as complexes, which are defined asa metal atom or ion bound to a set of ligands that donates lone electronpairs to the metal. The valence level (Ru4d and Os5d, respectively) for the

28

neutral metal contains eight electrons, and therefore Ru2+ and Os2+ have sixd-electrons, a d6 configuration. The metal ion usually has a coordinationnumber of six. Ruthenium and osmium compounds are often treated withinan octahedral symmetry; this statement is, however, not generally true, ande.g. Ru(bpy)3

2+ has D3 symmetry.

To a first approximation, d-block complexes can be regarded aseither ionic or covalent. In reality, though, most compounds lie somewhereinbetween these two extremes. Crystal field theory, CFT, is an ionic modelrepresenting the metal atom and the ligands as point charges. It assumesthat the properties of metal and ligand are retained in a complex. In mole-cular orbital theory, MO, on the other hand the interaction between metaland ligands is described by linear combinations of atomic orbitals (seeSection 2.1), which allows for charge transfer between the central metalatom and its ligands.

For Ru2+ or Os2+ bipyridine complexes, the highest occupied molecu-lar orbital, HOMO, mainly has d character. The energy of this level may beinfluenced by other coordinating ligands, e.g. Cl-, I-, CN- and NCS-. Itcontains six electrons, normally referred to as the t2g set of orbitals, whichare degenerate for the octahedral complexes but split when the symmetry isdistorted. The lowest unoccupied orbital, LUMO, almost exclusivelycontains ligand bipyridine π∗ orbitals, which implies that such ligandslargely control the properties of the LUMO.

Excitations in which electrons are excited from metal centeredorbitals to orbitals localized on a ligand are called MLCT (metal to ligandcharge transfer) transitions. Other types of optical transitions are the metalcentered, MC, and the ligand centered, LC, transitions. The strong colors ofRu and Os complexes are due to MLCT transitions where an electronmainly localized on the metal atom is transferred to a ligand centered π∗orbital. These are the excitations lowest in energy for Ru and Oscomplexes. Complexes with bipyridine ligands also absorb strongly in theUV region due to π−π∗ transitions (LC).

29

Figure 3.2. The molecular structure of a) Ru(bpy)32+, b) Ru(dcbpyH2)2(NCS)2, and

c) dcbpyH2.

Tailoring a Dye Sensitizer The absorption and redox properties of a complex can be changed invarious ways [20]. Some of these are:(1) π∗-level tuning, e.g. using a ligand with a lower π∗ level in order toincrease the absorption in the red region. (2) Ground state tuning (t2g tuning). This energy level, the HOMO level, iscontrolled by the donor strength of the ligands. A strong donor, such as ahalide, will shift the level to higher energies, yielding a red-shifted absorp-tion spectrum. (3) Increase of the MLCT absorption coefficient, for example by introdu-cing phenyl or methyl groups on the bipyridine ligands.

So far, the most efficient dye/semiconductor system for solar cellapplications is cis-bis(4,4’-dicarboxy-2,2’-bipyridine)-bis(isothiocyana-to)ruthenium(II), Ru(dcbpyH2)2(NCS)2, (see Figure 3.2b) used in combina-tion with TiO2 and the I-/I3

- redox couple. It seems that this dye, also knownas “N3”, has a good balance of the properties mentioned above [14]. If e.g.halide ligands are used instead of -NCS- in order to increase the t2g energylevel (lower the redox potential) [13], the HOMO level seems to be shiftedtoo much for efficient regeneration of the oxidized dye by iodide (thedriving force becomes too small). The absorption coefficients are relativelyhigh due to the charge transfer properties of the complex. In addition, thecomplex is relatively small and adsorbs strongly to the surface, which givesa dense loading of molecules per surface area.

N

NCSNCS

COOH

COOH

NN

HOOC

HOOC

NRu

NN

N

N

N

N

Ru

N N

COOHHOOC

a) b) c)

30

Electron InjectionThe injection rate of a dye electron into the semiconductor has been thesubject of many studies using laser transient measurements [21-25]. There isgeneral consensus that electron transfer occurs from the excited state beforevibrational relaxation. Due to instrumental limitations only upper limits forthe charge transfer time have been given, the lowest one being 25 fs forRu(dcbpyH2)2(NCS)2 [22].

By resonant photoemission (RPES, see Section 2.3), dynamics in thelow and sub-femtosecond region can be probed [4,7,26]. A recent RPESstudy of a model system consisting of dcbpyH2 adsorbed on a single crystalrutile TiO2 surface (110) gives an upper limit for electron transfer fromdcbpyH2 to the substrate of 2.5 fs [27].

Dye DegradationThe dye has to be stable on a long term basis. A commercial solar cellshould have a lifetime of at least 20 years, corresponding to around 108

redox cycles for the dye. Thus, even very small fractions of molecules thatare lost in side reactions will become a large problem in a solar cell. Inaddition, for outdoor applications, the dye has to sustain high temperatures.The stability of the system Ru(dcbpyH2)2(NCS)2 on nanocrystalline TiO2with I-/I3

- used as a redox couple has been studied [28]. It was shown thatafter exposure to full simulator light, corresponding to six years of outdoorillumination, the performance was practically unchanged. However, if a toolow iodide concentration was used, the dye was converted toRu(dcbpyH2)2(CN)2. This substitution process takes 0.1-1 s, while theregeneration of the dye only takes a few nanoseconds if a sufficiently highiodide concentration is used. Moreover, if Ru(dcbpy2)2(NCS)2 in a solutioncontaining iodide is heated, substitution of NCS- by I- can take place [28].This reaction is, however, suppressed when the dye is bound to TiO2, dueto the fast injection of the excited electron into the conduction band of thesemiconductor.

3.1.2 The Metal OxideThe use of dye sensitization to convert light to electricity was initiallystudied during the 70’s, starting with the work of Tributsch and coworkers[29] using single crystal substrates. A main problem in these studies wasthat with only one monolayer of dye absorbing light, most light simplypassed through the single crystal-based solar cell. Additional dye layers did

31

not improve the efficiency of the solar cell since only the dye adsorbeddirectly on the surface injects electrons efficiently to the semiconductor. In1985, the dye sensitization technique made an unexpected breakthroughwhen it was discovered that it is possible to use rough polycrystalline metaloxide electrodes instead of single crystals [30]. The high internal area ofsuch electrodes increases the amount of surface-bound dye, and thus theefficiency of the cell. The internal surface area of the electrodes was thenincreased even further, by using nanocrystals having a diameter of about 10nm [31]. So-called nanostructured1 metal oxide films are made of suchnanocrystals, sintered to form a mechanically stable, sponge-like structurein which the nanocrystals are in electrical contact with each other.

The high efficiency of dye-sensitized solar cells based on nanostructuredmetal oxide films was against expectations from semiconductor physics,since the structure contains a large number of defects (e.g. surface statesand grain boundaries) which may act as recombination centers for electron-hole pairs. The film is also exposed directly to the electrolyte, increasingthe recombination possibilities even further. In addition, there is no obviouselectrical driving force for the electron to travel towards the back contact ina nanostructured film, since there is no internal electric field (band-bending) as in systems based on large crystals. Band-bending occurs whena single crystal semiconductor is in contact with an electrolyte and theFermi level of the semiconductor adjusts to the redox potential of theelectrolyte. However, the dimensions of a nanocrystalline semiconductorparticle are much smaller than the width of the band-bending zone (thedepletion layer), and therefore band-bending does not occur.

The electron transport mechanism in the nanocrystalline network hasinstead been described in terms of a diffusion model [32]. Thus, theelectrons diffuse in the network due to the concentration gradient present inthe film, and the conducting glass acts as a sink where the electrons leavethe film. The rate of the electron transport is of the same order ofmagnitude as the diffusion of cations in the electrolyte. This was explainedby a screening of the electron by cations in the electrolyte and by polarsolvents [33,34]. The nanostructure is therefore not an obstruction for, butinstead aids electron transport [35].

1 Other names are nanoporous, mesoporous, nanocrystalline or colloidal.

32

TiO2 and ZnOIn this thesis, two different metal oxides, titanium dioxide,TiO2, and zincoxide, ZnO, were studied and their properties are briefly described here. Athorough survey is given in ref. [36]. TiO2 is a transition-metal oxide (d0),with the valence band and the conduction band mainly having O2p andTi3d character, respectively. Point defects such as oxygen vacancies giverise to bandgap states corresponding to partial population of Ti3d orbitalsin the upper half of the bandgap energy region. Titanium has several stableoxidation states, Ti4+,3+,2+, and the bandgap state is interpreted as one or twoelectrons occupying a metal site and therefore reducing the initial Ti4+ state.ZnO, on the other hand, is a post-transition-metal oxide (d10), with avalence band of O2p and Zn4s character, and a conduction band of Zn4scharacter. Zinc does not have any other stable oxidation states than Zn2+.The creation of defects (predominantly oxygen vacancies) does not produceany new filled states in the bandgap region.

3.1.3 Anchoring of the Dye to the SurfaceDye molecules are commonly linked to metal oxide surfaces via carboxylgroups. Phosphonate groups constitute an interesting alternative due totheir stronger adhesion to the TiO2 surface [37], but the efficiency of solarcells based on phosphonated dyes is not, at least not for the moment, asefficient as those using carboxyl linker groups.

For the injection it is not completely clear whether the mainimportance of the interlocking group lies in keeping the dye close to thesurface or to create a mixed ligand/semiconductor orbital [38]. According toFermi’s golden rule, the large density of accepting states in the TiO2conduction band can give an ultrafast injection even if the overlap is weakor intermediate. It is thus not certain that a mixed state of π∗ orbitals of thecomplex and semiconductor orbitals (Ti3d in the case of TiO2) is requiredfor efficient charge injection from a sensitizer to a broad conduction band[38,39].

3.1.4 The Counter Electrode and the ElectrolyteThe counter electrode should be efficient in transferring electrons back tothe redox couple in the electrolyte. For high efficiency cells a platinizedconducting glass surface (glass coated with fluorine-doped tin oxide, FTO)is used, where the platinum acts as a catalyst.

33

The redox couple in the electrolyte is used to regenerate the oxidizeddye. It should have a high diffusion coefficient and be able to penetrate thenanostructured film. These requirements are best matched by a liquidelectrolyte. A common electrolyte is the iodide/iodine couple in an organicsolvent. Organic solvents are used instead of water-based ones, since thedye usually is unstable in water. The problem with liquid electrolytes isgenerally that they are very difficult to encapsulate for long time. Forcommercial use it is necessary to overcome this problem in an efficientmanner. Recently, several groups have succeeded in using solid or quasi-solid electrolytes instead [40-42]. Although these cells have one or twomagnitudes lower efficiency than those using liquid electrolytes, they arepromising for low-power applications. Another approach is to useconducting plastic substrates for the nanostructured film and the counterelectrode instead of the commonly used glass substrates [43].

3.2 Displays Based on Organic Sensitizers

Electrochromism is defined as a reversible color change in a materialcaused by an applied electric field or current. For example, TiO2 shiftscolor from colorless (for nanostructured films; white for larger, light-scattering crystals) to deep blue if a negative potential is applied when theelectrode is immersed in a solution containing small cations. The cationsare inserted into the crystal lattice in order to charge compensate theelectrons that are forced into the TiO2 film. This process, which is used forbattery applications, can also be utilized for making displays.

There is also another possible route of making displays of nano-crystalline systems. A certain type of dye molecules, which shift color uponoxidation or reduction, can be attached to the semiconductor surface. Theperhaps most well-known molecules with these properties are theviologens. They shift from colorless to colored upon reduction and areoften used to "detect electrons" in laser spectroscopy experiments. In thisthesis another dye, 3-Ethyl(p-N,N-dimethylamino)phenyl)amino)-propyl-1-phosphonic acid, triarylamine (Figure 3.3) was studied.

34

Figure 3.3. Molecular structure of the triarylamine molecule.

The triarylamine molecule is colorless in its neutral state and bluewhen it is oxidized. The electrochemical properties of this moleculeadsorbed on a nanostructured TiO2 surface have been examined in previousstudies [44,45]. The electron is not energetically allowed to enter the TiO2,since the conduction band edge is higher in energy than the oxidationpotential of the triarylamine. It therefore appeared impossible to oxidize thetriarylamine on the TiO2 surface, but surprisingly oxidation of themolecules does occur. The electron transport in this system was found toproceed via lateral electron hopping inside the triarylamine monolayer [44].Hence, the TiO2 substrate mainly serves as an inert supporting material inthis case. The same electron transport mechanism has later been shown totake place in other systems such as Os(bpy)2(dcbpyH2)

2+⋅2PF6- [46,47], or

Ru(bpy)2(dcbpyH2)2+⋅2PF6

- [47] adsorbed on TiO2.

35

4. Dyes Adsorbed onto TiO2 and ZnO

4.1 Bonding Geometry

Carboxyl groups are commonly used for anchoring of dyes on metal oxidesurfaces. However, the bonding mechanism is in many cases not obvious.For complexes adsorbed via a commonly used carboxylated ligand,dcbpyH2, to nanostructured TiO2 surfaces, initial Raman and IR measure-ments suggested that bonding mainly occurs via ester linkage [48,49].However, other IR studies instead propose a carboxylate link [50], in abridge coordination or bidentate chelating bonding mode [51,52]. Forformic acid adsorbed on a single crystal rutile TiO2 (110) surface, UHV-scanning tunnelling microscopy [53] and NEXAFS [54] measurementssuggest adsorption along rows of fivefold coordinated titanium ions.Formic acid on rutile (110) has also been examined by photoelectrondiffraction measurements [55]. The results in combination with Hartree-Fock calculations suggest bridge bonding for this molecule [56].

Figure 4.1. Different adsorption geometries of a carboxyl group on a metal oxidesurface: a) monodentate bonding, b) monodentate, ester, c) bridge, and d) bidentatechelate. "M" denotes the surface metal ion (e.g. Zn or Ti).

4.1.1 dcbpyH2 Adsorbed onto Single Crystal TiO2

In Paper II, dcbpyH2 (biisonicotinic acid), see Figure 3.2c, adsorbed on asingle crystal rutile TiO2 (110) surface was examined by PES, NEXAFSand periodic INDO calculations. The reason for choosing thedcbpyH2/rutile system was the possibility of constructing well-definedadsorbate-substrate structures under UHV conditions for this system. Theidea was to use this as a model system for the solar cell case.

O O

R

MO O

R

MMO

R

O

MO O

R

MMH

a) b) c) d)

36

Figure 4.2. O1s spectra of dcbpyH2: a) multilayer on TiO2, and b) adsorbed moleculeson a rutile TiO2 (110) surface.

The PES measurements showed that while the O1s peak from non-bonding dcbpyH2 molecules (multilayer on TiO2) consisted of threedifferent components, the O1s peak for the adsorbed molecules (monolayeron TiO2) contained only two components (see Figure 4.2). The threecomponents for the multilayer were assigned to C-OH oxygen, C=Ooxygen and TiO2 oxygen, respectively, going from higher to lower bindingenergies. In the monolayer case, the C-OH component had vanished, andonly a component at the same binding energy as the C=O oxygen and onefrom the TiO2 substrate remained. Thus, both carboxyl groups in thesurface-adsorbed molecule were deprotonated, and the spectra closelyresembled that of formic acid adsorbed on rutile TiO2 (110) [53,57].

The tilt and azimuthal angles of the adsorbed molecule were determi-ned experimentally by NEXAFS. They were compared to the results fromperiodic INDO calculations, which were performed for five differentgeometries: two monodentate (ester) and three bridge linkages. Two ofthese structures, one monodentate and one bridge adsorption bondingmode, were found to yield azimuthal angles in accordance with theNEXAFS results. From the measurements it was not possible to directlydeduce the correct one of these, since the C=O and the C-O-Ti signalsappear at the same binding energy in the O1s spectra. The bridge linkage,shown in Figure 4.3, was, however, also shown to give the lowest totalenergy in the calculations, and it was therefore concluded to be the mostprobable bonding mode of the molecule.

37

Figure 4.3. The adsorption geometry of dcbpyH2 on rutile (110).

The twist angle between the bipyridine rings found for this bondingmode was found to be sufficiently small to allow both nitrogen atoms tochelate to for example a ruthenium atom as in a stable Ru complex. Itwould therefore be possible for the dcbpyH2 molecule to retain the samebonding geometry when used as a ligand in a complex, indicating that thesame bonding mode would be possible also for an entire Ru complex.

4.1.2 Dye Complexes Adsorbed onto Nanostructured TiO2

In Paper III and VI a series of ruthenium complexes, as well as one osmiumcomplex, were studied by PES. These complexes were adsorbed on nano-structured anatase TiO2 films by a wet chemistry process. A few complexeswere also studied as multilayers (non-bonding molecules) on a goldsubstrate.

For these systems, the interpretation of the results is more complicated thanfor the model system. The surface geometry and preparation of thesesamples are not as well-characterized as in the case of the dcbpyH2/TiO2system prepared in UHV discussed above. In addition, hydrocarboncontamination is expected in non-UHV measurements. However, bycomparing the results obtained from the model system with those of theadsorbed and non-bonding complexes, a picture of the binding of thecomplexes to the surface could be obtained.

38

Figure 4.4. O1s spectra of a) Ru(dcbpyH2)2(NCS)2 multilayer on Au,b) Ru(dcbpyH2)2(NCS)2 (dotted) and Ru(bpy)2(dcbpyH2)

2+⋅2PF6- adsorbed on TiO2,

c) Ru(bpy)2(dcbpyH2)2+⋅2PF6

- adsorbed on ZnO. The Ru(dcbpyH2)2(NCS)2 spectra werecalibrated with respect to the Ru3d5/2 signal at 281 eV, and theRu(bpy)2(dcbpyH2)

2+⋅2PF6- spectra were aligned to those of adsorbed

Ru(dcbpyH2)2(NCS)2 via substrate signals.

In Figure 4.4, the O1s spectra of Ru(dcbpyH2)2(NCS)2 andRu(bpy)2(dcbpyH2)

2+⋅2PF6- adsorbed on TiO2 and non-bonding

Ru(dcbpyH2)2(NCS)2, are shown. The O1s signal of the multilayer can beresolved into two peaks with equal intensity: an –OH peak at 533.4 eV andan =O peak at 531.9 eV, similar to the case of biisonicotinic acid multi-layer. For the adsorbed dyes, the –OH region for the dye containing onlytwo carboxyl groups is seen to be suppressed compared to the case of thedye having four carboxyls groups. This trend is representative for theresults obtained in Paper III and VI, and it is supported by carboxyl C1sspectra. The results indicate a bonding mode where two carboxyl groupsare deprotonated.

It was not possible to determine whether the molecule binds via both

carboxyl groups on one ligand or via one carboxyl group from eachdcbpyH2 ligand. However, theoretical calculations (see Paper III and [58])

536 534 532 530 528Binding energy/ eV

O1s

b

a

c

39

Figure 4.5. A possible adsorption geometry of dcbpyH2 on anatase TiO2 (101).

have shown that dcbpyH2 may bind to anatase (101) 2 in a similar mode asthat obtained for the same molecule on rutile (110), see Figure 4.5. Due tothe disordered surface and the strong interaction between a carboxyl groupand TiO2, it is not unlikely that both bonding geometries occur. Probablyseveral different bonding modes of the dye complexes on thenanostructured surface are possible. The bonding mode may be sensitive tothe sensitizing conditions, such as reflux treatment [49] and sensitizing time[59].

4.1.3 Dye Complexes Adsorbed onto ZnOIn Paper V and VI, bipyridine complexes absorbed on nanostructured ZnOwere studied. ZnO has a similar bandgap (3.2 eV) to that of anatase TiO2and is regarded as a promising alternative to TiO2 for solar cellapplications. However, while for TiO2, the strong adsorption of thecarboxyl groups favors monolayer growth [13,60], for ZnO the dyesensitizing process has been shown to be more complex and depends onparameters such as sensitizing time and dye concentration [61]. Unlessthese parameters are controlled, dye aggregates may form and create amultilayer on the surface, reducing the solar-to-electric energy conversionefficiencies. 2 This surface is known to border the anatase nanocrystals [73].

40

Ru(dcbpyH2)2(NCS)2 adsorbed on ZnO system for differentsensitizing times was studied by PES in Paper V. For this system, dyeaggregation was observed with increasing sensitization time. Theaggregation process involved the carboxyl groups, as indicated by changesin the O1s spectra with time. The results also gave evidence for thepresence of Zn2+ ions in the multilayers as well as inRu(dcbpyH2)2(NCS)2/Zn2+ aggregates formed as a precipitate in a solutioncontaining dissolved dye and Zn2+ ions. In addition, substantial changes inthe thiocyanate nitrogen and sulphur signals were found, indicating thatalso the –NCS ligands are involved in or influenced by the aggregationprocess.

In order to find a preparation with a high dye loading having aminimum of dye aggregates, the O1s spectra for different sensitizing timeswere compared to that of Ru(dcbpyH2)2(NCS)2 adsorbed on TiO2. Thispreparation should have a high dye loading having a minimum of dyeaggregates, corresponding to the ideal case with respect to solar cell per-formance. Interestingly, the thiocyanate ligands were found to besignificantly influenced by the ZnO substrate also for this preparation.While the N1s and S2p signals were similar for the non-bonded dye and thedye adsorbed on TiO2, the N1s signal originating from the –NCS group wasbroadened for the dye adsorbed on ZnO, and the S2p signal contained atleast two sulphur components; one at higher and one at a similar bindingenergy to that measured for Ru(dcbpyH2)2(NCS)2 on TiO2.

In Paper VI, Ru(bpy)2(dcbpyH2) and Os(bpy)2(dcbpyH2) adsorbed on ZnOand TiO2 were studied. These dyes did not appear to have the sametendency to form aggregates as the Ru(dcbpyH2)2(NCS)2 dye. The resultsindicate a different bonding geometry for these dyes adsorbed on ZnOcompared to adsorption on TiO2. For ZnO a large proportion of theoxygens remain protonated when the molecules are adsorbed on the surface(see Figure 4.4). These results suggest that the molecules are adsorbed viathe monodentate bonding mode shown in Figure 4.1a. Theoreticalcalculations of formic acid on ZnO (1010) show that at high dye coverageson the surface this bonding mode is favored, although a bridge mode ispreferred for individual molecules [62].

41

Also Ru(dcbpyH2)2(NCS)2 may be adsorbed by a monodentate mode, butfor this molecule different O1s contributions are difficult to resolve sincethe molecule probably contains both binding and free carboxyl groups.

42

4.2 Molecular Orbital Compositions

The molecular orbital composition of dcbpyH2, Ru(bpy)32+ and

Ru(dcbpyH2)2(NCS)2 were studied in Paper I and VII. Knowledge of thecomposition of the molecular orbitals of the dye complexes may contributeto the understanding of what properties are important for efficient solar celldyes. It may therefore be a route to the design of new, more efficient dyes.

As seen in Figure 4.6, the outermost valence level spectrum is a three-peakstructure for the dcbpyH2 molecule. These levels are mainly of 2pcharacter, derived from the participating elements C, N and O. The photo-electric cross sections for these atomic orbitals [63] vary uniformly andretain essentially their relative magnitudes over a wide photon energyrange, and thus the structure of the valence spectrum remains largelyunchanged. The valence spectrum obtained from an INDO/S calculationsimulates the experimental spectrum well, in particular in the bindingenergy region below 15 eV where multielectron effects are less important.By subdividing the calculated spectrum into partial contributionsoriginating from the different atoms, the lowest binding energy peak isfound to have mostly N2pπ and lone pair character.

When the dcpyH2 molecule is used as a ligand in theRu(dcbpyH2)2(NCS)2 complex, the nitrogen contribution from thismolecule is shifted towards higher binding energies and is stronglybroadened. A similar behavior of the nitrogen contribution is deduced forthe Ru(bpy)3

2+ complex. This is in line with coordination of the ligands tothe ruthenium ion via the nitrogen atoms involving substantial covalentcharacter. By introducing a doubly charged central ion complexing to theligand nitrogens, the nitrogen levels will generally be pulled down inenergy and there will be a general shift of electron density towards thenitrogen atom region.

For Ru(bpy)32+, the contribution from nitrogen orbitals to the valence

spectrum was singled out by using resonant photoemission. In a resonantspectrum the valence electron structure is investigated at photon energiescorresponding to core excitations from a core level to the unoccupiedvalence levels of the molecule. The valence band features containingnitrogen are then enhanced with respect to other parts of the

Figure 4.6. Experimental and calculated valence band spectra of a) dcbpyH2,b) Ru(bpy)3

2+ and c) Ru(dcbpyH2)2(NCS)2. The experimental spectra (upper curves) wererecorded for multilayers of the molecules: the dcbpyH2 multilayer was prepared byevaporation in UVH while the Ru complex

multilayers were prepared by smearing the dye

crystallites onto roughened gold samples. The photon energies were 150 eV for dcbpyH2

and Ru(dcbpyH2)2(NCS)2, and 396 eV for Ru(bpy) 32+·2Cl-.

valence band. The results showed that for Ru(bpy)32+ the main nitrogen

contribution is located 3.3 eV from the HOMO level. This finding is in goodaccordance with quantum chemical calculations of the Ru(bpy)3

2+ complex,see Figure 4.6 and [64].

For the ruthenium complexes two smaller peaks, which are notpresent in the dcbpyH2 valence spectra, appear at lower binding energies.For Ru(bpy)3

2+ the lowest level mainly contains Ru4d orbitals whereas thesecond lowest level mainly has ligand π character (C2p). ForRu(dcbpyH2)2(NCS)2, on the other hand, both of these levels contain a largefraction of Ru4d orbitals according to the calculations (Paper I and [64]).This finding was also confirmed experimentally by varying the photonenergy. While the p orbitals are expected to vary uniformly with photonenergy for the energies used, as observed for the dcbpyH2 molecule, theRu4d orbitals are expected to increase with photon energy. This behaviorwas observed for the two peaks at lowest binding energies.

Ru

N

25 20 15 10 5 0Binding Energy / eV

Experiment

INDO/S calculation

....... Total

(b)

25 20 15 10 5 0

N

O

Binding Energy / eV

Experiment

Inte

nsity

/ ar

b.un

itsD

OS

, PD

OS

/ ar

b. u

nits

INDO/S calculation

....... Total

(a)

RuSON

25 20 15 10 5 0Binding Energy / eV

Experiment

INDO/S calculation

....... Total

(c)

43

44

Figure 4.7. The frontier orbital structure of Os(bpy)2(dcbpyH2) (upper) andRu(bpy)2(dcbpyH2) (lower) adsorbed on nanostructured ZnO.

The fact that both of the outermost levels for theRu(dcbpyH2)2(NCS)2 molecule contain Ru4d orbitals implies that alsoother atomic orbital symmetries are involved in these levels. According tothe calculations, these contributions mainly arise from the –NCS ligands,the HOMO orbital predominantly having S3p character. This feature distin-guishes this ruthenium complex from the Ru(bpy)3

2+ complex discussedabove, and may have an important consequence for a solar cell. When thedye is excited, a dye electron is injected into the semiconductor and the dyeis regenerated by an electron from a redox couple in the surrounding solu-tion. For Ru(dcbpyH2)2(NCS)2, the injected electron originates from a Ru-NCS orbital. The regeneration of the dye can thus be expected to be moreefficient for Ru(dcbpyH2)2(NCS)2 than for the Ru(bpy)3

2+ complex, sincethe orbital which has lost an electron points out towards the electrolyte.

In Paper VI, the Ru(bpy)2(dcbpyH2)2+ and Os(bpy)2(dcbpyH2)

2+

complexes adsorbed on TiO2 and ZnO were investigated by PES. The N1slevels of these complexes were found to have identical binding energies,indicating that the donation of electrons to the metal center is very similarfor these two complexes. The HOMO levels of the complexes were foundto appear at closely the same binding energies for both dye moleculesadsorbed on both substrates. The Os complex has a lower redox potentialthan the Ru complex, and the HOMO level of the Os complex is thereforeexpected to appear at a lower binding energy. A detailed analysis of thespectra shows that the HOMO level is broader for Os(bpy)2(dcbpyH2)

2+

than for Ru(bpy)2(dcbpyH2)2+. As shown in Figure 4.7 the HOMO level of

the Ru complex can be fitted with a single Gaussian, while the HOMOlevel of the Os complex is clearly assymmetric. The latter signal can bedeconvoluted into two Gaussians, having an intensity ratio of 2:1.

3.0 2.0 1.0Binding energy/ eV

45

The difference between the Ru and Os complex may be interpretedin the following manner. The frontier orbital structure of Ru(bpy)3

2+ andOs(bpy)3

2+ is often described within an octahedral symmetry in which theHOMO contains three degenerate orbitals (the t2g set). The true symmetryof the complexes is, however, lower (D3), which will split the three HOMOlevels into one doubly and one singly degenerate level. The magnitude ofthis splitting depends on the geometry and difference in energy between theinteracting orbitals. From the experimental results the splitting appears tobe very small for the Ru complex, but large enough to be resolved in thespectra for the Os complex. The intensity relation of 2:1 is in accordancewith that expected from theory. The lowest binding energy peak (the singlelevel) is shifted 0.6 eV towards lower binding energy compared to theHOMO level of the Ru complex, a value comparable to the difference inredox potential of 0.42 V measured for the two complexes [65].

4.3 Energy Level Matching

4.3.1 TiO2 Bandgap States and Influence of CationsIn Paper VIII the bandgap region was examined in detail by PES. Trappingof electrons at energy levels in this region will affect the charge transferprocesses at the oxide/dye/electrolyte interface as well as the electrontransport process through the nanostructured network to the externalcircuit. The identity and location of the traps are not clear. Some resultsimply an exponential distribution of traps below the conduction band edge[34,74,75], and an exponential distribution function is normally used in themodeling of the charge transport in the film.

In Figure 4.8 the valence band of a plain, nanostructured TiO2 film isshown together with that of a TiO2 film where lithium ions are electro-chemically inserted into the crystal lattice. The latter sample was used tomodel the TiO2/electrolyte interface. The inset shows a closeup of thebandgap region. To obtain the position of the band edges we used the factthat for a heavily reduced (in our case by insertion of Li+ ions) TiO2 film,

46

Figure 4.8. The valence levels of a plain TiO2 film (solid line) and a TiO2 film insertedwith Li+ ions (dashed).

the Fermi level is pinned to the conduction band edge (CBE) [36]. TheFermi level, and thus the CBE, is placed where the intensity of theintercalated film drops to zero (the step at 3.75 eV). The position of thevalence band edge (VBE) is then calculated from the value of the CBE andthe anatase band gap of 3.2 eV. Also for the as prepared, plainnanostructured TiO2 film the Fermi level is found to coincide with CBE.

For the intercalated film, the most striking effect is that the intensityhas increased with a pronounced peak at about 1 eV below the CBE and forthe levels close to CBE. This increase is due to the presence of the Li+

cations. No new states were observed by simply dipping the TiO2 in 0.1 MLiClO4 (the presence of lithium on the oxide surface was confirmed by theappearance of a Li1s signal at 60 eV), and we therefore conclude that it isthe combination of electrons and Li+, or other cations, which creates thestates in the band gap. Following this observation a dynamic nature of thetraps may be envisaged, namely that trap states are formed upon electroninjection and concomitant screening by cations. The chemical nature of thetraps can be deduced by the appearance of a Ti3+ state in the Ti2p spectrumfollowing Li-insertion.

4.3.2 Dye Complexes Adsorbed onto TiO2

The energy matching between the dye and the semiconductor is crucial in asolar cell. Efficient electron injection from the dye to the conduction band

12 10 8 6 4Binding energy/ eV

7 6 5 4 3

of the semiconductor requires that the excited states of the dye match theconduction band of the semiconductor. By means of PES it is possible tomap the positions of the occupied and unoccupied levels of the dye relativeto the semiconductor, making use also of the visible absorption spectrum ofthe dye and the bandgap of the semiconductor. In Paper III, the position ofthe HOMO level of different ruthenium complexes relative to the valenceband of TiO2 was studied. The binding energy of the HOMO level relativeto the TiO2 valence band was found to shift towards lower binding energieswith an increasing number of negative ligands on the dye.

The positions of the excited states of the Ru(dcbpyH2)2(NCS)2 dyewere estimated by placing the absorption maximum of the dye at 535 nmcorresponding to 2.32 eV above the maximum of the HOMO level, seeFigure 4.9. The results indicate that photons absorbed in the red part of thedye absorption spectrum are excited to states below the conduction bandedge. Electron injection from these states may be possible if a locallowering of the conduction band edge occurs due to hybridization betweenTiO2 and COOH dye orbitals. In such a case injection into the semi-conductor may be feasible also for the red part of the dye spectrum.

The deprotonation of the carboxyl group observed when acarboxylated dye binds to a TiO2 surface (see Section 4.1) should lead tothe formation of a dipole between the protons, which adsorb on the surface,and the carboxylate ion [35]. In addition, the negatively chargedthiocyanate ligands of the dye may contribute to the dipole, depending onthe bonding geometry of the dye on the surface. According to the vacuumlevel alignment model [66], the dipole is expected to give rise to a shift inthe ionization potentials of the TiO2 substrate signals for a dye-sensitizedfilm compared to a plain one. In this model it is assumed that the screeningof the hole, which is left after photoemission of an electron, is equal for thesubstrate and adsorbate. This was found to be the case for a related system,namely chloroindium phtalocyanine adsorbed on a highly orientedpyrolytic graphite, HOPG, substrate [67], and we find it reasonable toassume that the present system has similar properties.

48

Figure 4.9. Valence PES spectra of Ru(dcbpyH2)2(NCS)2 adsorbed on TiO2 (solidlines) and ZnO (dotted lines), calibrated with respect to the bipyridine N1s level. TheHOMO level of the dye is clearly distinguished above the valence band edge. Theexcited states of the dye (dashed lines) are positioned relative to the HOMO maximumfor the dye adsorbed on TiO2 (cf text).

The Ti3p level was found to shift 0.2 eV towards higher ionizationpotential when the dye was adsorbed. The direction of the shift implies apositive charge on the semiconductor side and a negative charge on the dyeside, as expected for a bonding involving deprotonated carboxyl groups. Itmay also indicate an interaction between the surface and thiocyanateligands directed towards the surface.

Since the dipoles formed at the dye/semiconductor interface shift theenergy levels of the dye relative to those of the substrate, they should betaken into account in the energy matching between the dye andsemiconductor for a solar cell. This is a non-trivial task, depending on thedye structure and cations, as well as on the cations in the electrolyte [68].

4.3.3 Dye Complexes Adsorbed on ZnOThe energy level matching for Ru(dcbpyH2)2(NCS)2 adsorbed on ZnO wascompared with that of the same complex adsorbed on TiO2 in Paper V. Theposition of the dye HOMO level, energy calibrated with respect to thebipyridine N1s peak, was found to be closely similar for the two substrates,see Figure 4.9. This indicates that the matching between the occupiedlevels of the dye and oxide is similar for the two substrates. Since thematerials have comparable band gaps this is in accordance with efficient

47

Valence levels

3.0 2.0 1.0 0.0

ZnO TiO2

49

electron injection from the excited dye into the conduction band of theoxides.

A difference in the shape of the HOMO signal of the dye on thedifferent substrates is observed. For TiO2 this signal appears as a pronoun-ced peak, whereas for ZnO the peak is broadened. This broadening mayreflect the influence of the ZnO on the thiocyanate ligands discussed inSection 4.1.3, since the HOMO level contains a significant contributionfrom these ligands (see Paper I and ref. [64]). This interpretation is suppor-ted by a blue-shift in the UV-vis spectrum of the dye when it is adsorbed onZnO compared to when adsorbed on TiO2 [69].

4.4 Excited State Dynamics

In Paper VII the dynamics of excited states of Ru(bpy)32+·2Cl- was studied

by means of resonant photoemission spectroscopy, RPES, and constantinitial state spectroscopy, CIS. A CIS spectrum is derived by integrating thephotoemission contributions for a certain binding energy range whilescanning the photon energy. Comparing the N1s NEXAFS and CIS spectrafor a multilayer of the ruthenium complex with corresponding spectra of adcbpyH2 multilayer, information on the kinetics of the excited electron onthe timescale of the N1s core hole (6 fs) was obtained. The NEXAFSspectra were obtained by recording a broad range of electrons emitted upondecay of the N1s core hole. The CIS spectra, on the other hand, wereintegrated over the participator region only, comprising the two outermostvalence orbitals for dcbpyH2, and the binding energy region around 10 eVfor Ru(bpy) 3

2+·2Cl-.

Participator decay was observed for the first resonance both for theRu(bpy)3

2+·2Cl- and dcbpyH2 multilayer. In contrast, participator decay wassuppressed for the third resonance of the Ru(bpy)3

2+·2Cl- compared to thecase of dcbpyH2, indicating delocalization of the excited electron on theformer molecule on a time scale shorter than the core hole lifetime.According to calculated NEXAFS and total DOS such a delocalization isenergetically possible for the higher resonances of the Ru(bpy)3

2+ molecule,however not for the first resonance. For the smaller dcbpyH2, on the otherhand, the electron in either of the excited orbitals is more confined.

50

Electron transfer to a neighboring molecule during the core hole lifetime isnot likely due to the weak interactions between the molecules.

Further, the properties of the Ru complex when adsorbed on TiO2were studied in order to study whether electron injection into the substrateconduction band occurs within the timescale of the N1s core hole. A car-boxylated form of the complex, Ru(bpy)2(dcbpyH2)

2+·2PF6-, was used to

form a monolayer on nanostructured anatase. The first and second resonan-ces in the NEXAFS spectrum were found to lie below the conduction bandedge, whereas the third and forth lie above (see Figure 4.10). This impliesthat injection from the dye complex into the substrate is energeticallypossible via excitation to the latter resonances. As mentioned in Section2.3, RPES can be used to investigate whether such injection processesoccur on a timescale of the core hole. The resonant spectra of the dyemonolayer were compared with those of the multilayer and with a nitrogenAuger spectrum recorded with a photon energy far above threshold. Asdiscussed in Paper VII, the analysis of the results leads to the conclusionthat injection does not seem to occur within a 6 fs-timescale for this Rucomplex adsorbed on nanostructured TiO2. The difference between theruthenium complex and the dcbpyH2 molecule, for which an upper limit of2.5 fs for electron injection is found, may partly be explained by the initialdelocalization of the electron upon photoexcitation of the Ru complex.

51

5. Triarylamine Adsorbed onto TiO2

The phosphonated triarylamine molecule described in Section 3.2 wasstudied in its reduced and oxidized state by PES measurements and DFTcalculations (Paper IV). The triarylamine molecule (Figure 3.3) adsorbedon nanostructured TiO2 was treated electrochemically in an argon-filleddrybox, after which the sample was brought into the PES setup, asdescribed in Section 2.3.5. The aim of the study was primarily to investi-gate which of the atoms in the molecule are affected in the oxidation/reduc-tion process. The major electronic difference between the reduced andoxidized state was found to be located on the nitrogen atom, which wasshifted 1.5 eV towards higher binding energies in the PES spectrum upontriarylamine oxidation (see Figure 5.1). The calculations support thisfinding: although the calculated HOMO level of the reduced species andthe singly occupied molecular orbital, SOMO, of the oxidized species areboth delocalized π orbitals, the difference in total electron density for thereduced and oxidized state was found to be centered around the nitrogenatom.

Figure 5.1. N1s for triarylamine reduced (top curve) and oxidized in LiClO4. Thebinding energies and peak intensities are calibrated versus Ti2p.

The energy level matching between the triarylamine molecule andTiO2 was found to be a crucial parameter for this system. It was observedthat the blue color of the oxidized molecules was bleached after some hours

402 400 398 396Binding energy/ eV

N1s

N

NN•+

N•+

52

in the X–ray beam during the PES measurement. The bleached spot had thesame shape as the beam but was more widely spread. Since it was possibleto reoxidize the bleached molecules, the process could not be attributed toradiation damage. It was instead interpreted as a reversible light-inducedreduction due to electron transfer from the TiO2 conduction band to theLUMO of the molecule. In the PES spectra this was manifested by theincrease of the N1s peak at lower binding energy at the expense of that athigher binding energy. This explains the presence of a N1s signaloriginating from reduced triarylamine also in the N1s spectra of theoxidized molecule (see Figure 5.1). This reduction process was suggestedto occur via electron transfer from the conduction band of the TiO2 to theSOMO level of the triarylamine molecule, see Figure 5.2. In a PESmeasurement a large number of electron-hole pairs are created in the decayprocess of the core holes. During constant illumination electrons willpopulate the lower parts of the conduction band, where the overlap with thelowest unoccupied state of the triarylamine is large. Electrons will thus betransferred to this level, reversibly reducing the triarylamine molecule.

Figure 5.2. Schematic picture of the energy levels of oxidized triarylamine on TiO2showing the processes occurring during illumination with light of a higher energy thanthe bandgap of the semiconductor. The initial population of the conduction band occursin the decay process of the core holes produced in the PES measurement due to the X–ray illumination.

Triarylamine in its oxidized state is accompanied by a negative ion,which charge compensates the positively charged molecule. Differences inthe N1s spectrum were observed depending on the identity of this counter-ion. Two different counterions, perchlorate, ClO4

-, and triflate, CF3SO3-,

were used. It was noted that the photoreduced spot where the oxidizedtriarylamine had lost its blue color was significantly smaller with a per-chlorate counterion than with a triflate counterion. As discussed in PaperIV, these observed counterion influences can be explained in terms ofdifferences in the organization of the triarylamine/counterion surface layer.Previous studies have shown that perchlorate ions form contact ion pair

53

complexes with structures such as tetrabutylammonium ions [70]. Thetriarylamine cation molecules are suggested to follow the same behavior,which would allow a tightly packed monolayer, with the counterionsinserted between the triarylamine molecules on the surface. Triflate ions,on the other hand, are larger than perchlorate ions [71] and are expected notto fit well into the triarylamine layer. They would thus be expected torather stay outside the monolayer. This ordering of the surface layer is alsoin accordance with electrochemical results [45], showing that the apparentdiffusion coefficient for triarylamine adsorbed on TiO2 is about 1.5 timeshigher in 1 M LiCF3SO3 than in 1 M LiClO4 in acetonitrile. A similar anioninfluence on the arrangement of adsorbed dye layers has recently beencharacterized in detail by electrochemical measurements [72].

54

Acknowledgments

This thesis is the result of several great collaborations, which have beenboth very fun and developing to participate in. I have made many newfriends during my years as a Ph. D. student, and I would like to thank all ofthem, as well as friends from the Before. In particular, I would like toexpress my sincere gratitude to

My supervisor Hans Siegbahn for his trust and support, giving me freehands and at the same time being very interested in the work and alwaystaking time (from nowhere) to discuss results and to share his vastknowledge in physics.

My co-supervisor Anders Hagfeldt for his encouragement and help, and forstanding with one foot in the scientific and one in the applied field,bringing hope that what we do really can be useful.

My “on the floor”-supervisor Håkan Rensmo, for all help, animateddiscussions and hard work, and for being a very nice companion who alsounderstands the importance of “turkisk peppar”. Sven Södergren, foralways being interested, for all ideas and coffees, and for his strange way ofknowing when something is wrong. My fellow-Ph. D. student AndersHenningsson for cheering up all days, no matter how bad or good they are,and for the discussions of everything.

All other co-authors, including Petter Persson, for all “small and quick”calculations and for taking time explaining what the results mean. PaulBrühwiler for all help, patiently sharing his knowledge in the surfacephysics field. Joachim Schnadt for shared “hair-tearing” regarding thebiiso-and-company systems. Karin Keis for good collaboration, help andvaluable advice. Sophie Tingry for a nice collaboration in a tough projectand for being such a friendly person.

The Department of Physics, especially group members, Ylvi, Anders andGreger, for being such nice companions, the Physics V group for letting mesit in their corner in the coffee room and for being very kind and helpful

55

persons, and all fellow lab assistants at “kurslab”. The persons who keepthe wheel running: Loa, Gunnel, Anne, Åsa, Birgit and Karl-Einar, andeveryone in the workshop for help and skilful constructions.

The Department of Physical Chemistry, where I have had great fun. TheSolar cell group: Anders, Sten-Eric et al – we are now too many to becounted! All other present and former Ph. D. students at “Fysikalen” for alot of fun, especially in all the “spex” we have performed, and all othersworking at the department, for making it such a nice place and for lettingme be a part of it, and of course for all great parties! The staff at MAX-lab, for kind help with absolutely everything, spanningfrom “lifting the tower” to borrowing a heat gun when ours was too new.

My friends since long ago: Anna-Mia, Anna, Lina, and Miriam. Thank youfor all support and discussions about life, helping me keeping the distance,and for being true friends.

My Mat-Nat friends and horsewomen friends for a lot of fun.

My parents Kerstin and Bengt, for all pep-talks, encouragement and love,and my brother Anders, for being the best brother in the world.

Mikael, for love and never-ending support, and for “bringing hope towomenkind”!

56

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