ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2015

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1294

Exploring Organic Dyes forGrätzel Cells Using Time-ResolvedSpectroscopy

AHMED M. EL-ZOHRY

ISSN 1651-6214ISBN 978-91-554-9349-3urn:nbn:se:uu:diva-263143

Dissertation presented at Uppsala University to be publicly examined in Häggsalen,Ångströmlaboratoriet, Uppsala, Thursday, 19 November 2015 at 10:15 for the degree ofDoctor of Philosophy. The examination will be conducted in English. Faculty examiner: Prof.Eric Vauthey (University of Geneva).

AbstractEl-Zohry, A. M. 2015. Exploring Organic Dyes for Grätzel Cells Using Time-ResolvedSpectroscopy. Digital Comprehensive Summaries of Uppsala Dissertations from theFaculty of Science and Technology 1294. 84 pp. Uppsala: Acta Universitatis Upsaliensis.ISBN 978-91-554-9349-3.

Grätzel cells or Dye-Sensitized Solar Cells (DSSCs) are considered one of the most promisingmethods to convert the sun's energy into electricity due to their low cost and simple technologyof production. The Grätzel cell is based on a photosensitizer adsorbed on a low band gapsemiconductor. The photosensitizer can be a metal complex or an organic dye. Organic dyescan be produced on a large scale resulting in cheaper dyes than complexes based on rareelements. However, the performance of Grätzel cells based on metal-free, organic dyes is nothigh enough yet. The dye's performance depends primarily on the electron dynamics. Theelectron dynamics in Grätzel cells includes electron injection, recombination, and regeneration.Different deactivation processes affect the electron dynamics and the cells’ performance.

In this thesis, the electron dynamics was explored by various time-resolved spectroscopictechniques, namely time-correlated single photon counting, streak camera, and femtosecondtransient absorption. Using these techniques, new deactivation processes for organic dyes usedin DSSCs were uncovered. These processes include photoisomerization, and quenching throughcomplexation with the electrolyte. These deactivation processes affect the performance oforganic dyes in Grätzel cells, and should be avoided. For instance, the photoisomerizationcan compete with the electron injection and produce isomers with unknown performance.Photoisomerization as a general phenomenon in DSSC dyes has not been shown before,but is shown to occur in several organic dyes, among them D149, D102, L0 and L0Br. Inaddition, D149 forms ground state complexes with the standard iodide/triiodide electrolyte,which directly affect the electron dynamics on TiO2. Also, new dyes were designed with theaim of using ferrocene(s) as intramolecular regenerators, and their dynamics was studied bytransient absorption.

This thesis provides deeper insights into some deactivation processes of organic dyes used inDSSCs. New rules for the design of organic dyes, based on these insights, can further improvethe efficiency of DSSCs.

Keywords: Laser spectroscopy, DSSCs, DSC, Electron dynamics, Deactivation processes,Isomerization, Twisting, TICT, Quenching by protons, Semiconductor, Electrolyte, Electroninjection, Regeneration, Recombination

Ahmed M. El-Zohry, Department of Chemistry - Ångström, Box 523, Uppsala University,SE-75120 Uppsala, Sweden.

© Ahmed M. El-Zohry 2015

ISSN 1651-6214ISBN 978-91-554-9349-3urn:nbn:se:uu:diva-263143 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-263143)

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"O my Lord! Increase me in knowledge." (Ta-Ha) (20:114)

The Holy Quran.

DedicationFor my

Father's soul.. Mother.. Family.. Wife and kids..

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Isomerization and Aggregation of the Solar Cell Dye D149

Ahmed El-Zohry, Andreas Orthaber, and Burkhard Zietz Journal of Physical Chemistry C, 2012, 116, 26144 - 26153.

II Photoisomerization of the cyanoacrylic acid acceptor group – a potential problem for organic dyes in solar cells Burkhard Zietz, Erik Gabrielsson, Viktor Johansson, Ahmed M. El-Zohry, Licheng Sun, and Lars Kloo Physical Chemistry Chemical Physics, 2014, 16, 2251.

III Ultrafast Twisting of the Indoline Donor Unit Utilized in So-

lar Cell Dyes: Experimental and Theoretical Studies Ahmed M. El-Zohry, Daniel Roca-Sanjuán, and Burkhard Zietz Journal of Physical Chemistry C, 2015, 119 (5), 2249 - 2259.

IV Fine-Tuning of the Twisted Intramolecular Charge Trans-fer (TICT) Energy Level by Dimerisation An Overlooked Piece of the TICT Puzzle Ahmed M. El-Zohry, Martin Karlsson, and Burkhard Zietz Editing revised manuscript to Journal of Physical Chemistry A.

V Concentration and Solvent Effects on the Excited State Dy-namics of the Solar Cell Dye D149 - The Special Role of Protons Ahmed M. El-Zohry and Burkhard Zietz Journal of Physical Chemistry C, 2013, 117 (13), 6544-6553.

VI Dimer formation for Indoline Dyes in Solutions and Proton Impact on Surface. Ahmed M. El-Zohry, M. Pastore, S. Agrawal, F. De Angelis, and Burkhard Zietz Manuscript in preparation.

VII Interactions with Iodide Electrolyte Affect the Electrons Dynamics in Grätzel Cells Ahmed M. El-Zohry and Burkhard Zietz Manuscript in preparation.

VIII Ferrocene as a Rapid Charge Regenerator in Grätzel Cells Ahmed M. El-Zohry, Jiayan Cong, Martin Karlsson, Lars Kloo, and Burkhard Zietz Manuscript in preparation.

Reprints were made with permission from the respective publishers.

Contributions to Papers

A B C D E F G H I Notes I -NMR II Data on ZrO2 only III -Theoretical Studies IV -Theoretical Studies V VI -Theoretical Studies VII VIII -Solar Cell measurements

A: Paper Number. B: Concept. C: Design. D: Acquisition of Data. E: Analysis of Data. F: Interpretation of Data. G: Drafting the Article. H: Writing the Article. I: Revising and Approving the Article. (-) in the notes means that I have not done these experiments.

Table of Contents

1. Introduction ......................................................................................... 13 1.1 Personal Motivations ....................................................................... 13 1.2 Towards a Better Future .................................................................. 13 1.3 The Thesis Outline .......................................................................... 16

2. Fundamentals ....................................................................................... 17 2.1 Light. ............................................................................................... 17 2.2 Electronic Absorption and Emission. .............................................. 18 2.3 Dye-Sensitized Solar Cells (DSSCs). ............................................. 20

2.3.1 Background. ........................................................................... 20 2.3.2 Electron Dynamics in DSSCs. ............................................... 23

3. Materials and Methods ........................................................................ 28 3.1 Organic Dyes................................................................................... 28 3.2 Steady State Absorption and Emission ........................................... 28 3.3 Ultrafast fs-Transient Absorption ................................................... 30 3.4 Time-Correlated Single Photon Counting (TCSPC) ....................... 32 3.5 Streak-Camera ................................................................................. 33 3.6 Data Analysis .................................................................................. 34

4. Photoinduced Large-scale Motions (Papers I-IV) ............................. 35 4.1 The First Goal ................................................................................. 35 4.2 Photoisomerization .......................................................................... 36

4.2.1 In solution .............................................................................. 37 4.2.2 On Surfaces ............................................................................ 39

4.3 Twisting .......................................................................................... 40 4.4 TICT in Cyanoacrylic Dyes ............................................................ 43 4.5 Conclusions ..................................................................................... 46

5. The Rhodanine (Papers V-VI) ............................................................. 47 5.1 The Second Goal ............................................................................. 47 5.2 Dimer Formation and Solvent Effect .............................................. 47 5.3 The Rhodanine Moiety .................................................................... 49 5.4 On ZrO2 ........................................................................................... 50 5.5 Conclusions ..................................................................................... 52

6. Interactions with the Electrolyte (Paper VII) ....................................... 53 6.1 The Third Goal ................................................................................ 53 6.2 Emission Quenching in Acetonitrile ............................................... 53 6.3 Monitoring electrons inside TiO2 .................................................... 56 6.4 Conclusions ..................................................................................... 57

7. Linked Ferrocene (Paper VIII) ............................................................ 59 7.1 The Final Goal ................................................................................ 59 7.2 Steady State Measurements ............................................................ 60 7.3 Time-Resolved and Solar Cell Measurements ................................ 62

7.3.1 In MeCN ................................................................................ 62 7.3.2 On TiO2 .................................................................................. 63 7.3.3 Solar Cell Performances ........................................................ 65

7.4 Conclusions ..................................................................................... 66

General Conclusions and Outlook ................................................................ 67

Svensk sammanfattning ................................................................................ 69

Arabic Summary ( العربي الملخص ) ................................................................... 71

Acknowledgements ....................................................................................... 73

References ..................................................................................................... 75

Abbreviations and Symbols

A Absorbance or Acceptor AM Air Mass BOA Born-Oppenheimer Approximation CB Conduction Band CI Conical Intersection CV Cyclic Voltammetry CyA Cyano-Acrylic acid D Donor or Donor Unit in the Indoline dyes DABCU organic base "1,8-Diazabicyclo[5.4.0]undec-7-ene" DAS Decay Associated Spectra DFT Density Functional Theory DSSCs Dye Sensitized Solar Cells ESA Excited State Absorption Fc Ferrocene FF Fill Factor fs Femtosecond (10-15s) GSB Ground State Bleach GVD Group Velocity Dispersion HOMO Highest Occupied Molecular Orbital ICT Internal Charge Transfer IR Infrared IVR Intramolecular Vibrational Redistribution Jsc Short Circuit Current LUMO Lowest Occupied Molecular Orbital NHE Normal Hydrogen Electrode NMR Nuclear Magnetic Resonance ns Nano-second (10-9s) PEHs Potential Energy Hyper-surfaces PESs Potential Energy Surfaces PMMA Poly-Methyl Methacrylate ps Pico-second (10-12s) ROD Recombination to Oxidized Dye ROE Recombination to Oxidized Electrolyte SE Stimulated Emission TA Transient Absorption in the UV-Vis region

TA-IR Transient Absorption in the IR region TPA Triphenylamine UV-Vis Ultraviolet-Visible light VC Vibrational Cooling Voc Voltage at Open Circuit

Efficiency

1. Introduction

1.1 Personal Motivations Before being enrolled in the Ph.D. challenge, I was interested in two fields to do my Ph.D.: renewable energy sources and laser spectroscopy. These two areas were where I wished to focus my efforts to earn a Ph.D. At that time, I had no idea about the advanced contents of these two fields and the possible connections between these two exciting areas.

Also, in that context, I would also thank the organizers of the HOPV con-ference that was held in Uppsala May 2012. This conference was one of the main motivations that encouraged me to enter these exciting fields focusing on Grätzel cells using time-resolved spectroscopy. Fortunately enough, I could bridge to some extent what I hoped to learn within these years as a Ph.D. candidate at Uppsala University.

1.2 Towards a Better Future Establishing reasonable and widely applicable solutions for the energy crisis have more benefits than people might think. Many lecturers discuss the "Re-newable Energy Resources" or "The Energy Crisis" based on two broad as-sumptions. The first assumption highlights the limitations of fossil fuels as an energy source, which will not meet the energy needs in the near future. The second assumption connects the global warming problem and the vast amounts of CO2 produced by burning the fossil fuels.I

However, other reasons are also important and should be stressed espe-cially to the politicians and the public. Solving the energy problem will have a direct impact on the global peace, poverty, and economics. Most of the current/modern wars between different nations are connected either directly or indirectly to energy resources. Renewable energy resources will affect the economic situation in most countries especially those still developing.1 In addition, different governments around the world face problems connected

I In 2050, the global emissions need to be reduced by 50% according to an international agreement

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to deficient economic situations. The amount of subsidies given to the energy sector in 2009 in some countries were $65 Billion (Iran), $22 Billion (India), $17 Billion (Egypt), $5 Billion (Ukraine), and $3 Billion (Qatar).2 More numbers are shown in Figure 1.1. For instance, such vast amount of money can be used in an efficient way if applied in renewable energy sec-tors. However, the global investment in the renewable energy sector in-creased from $60 billion in 2000 to $300 billion in 2011.3 Figure 1.2 shows the possible connections between energy and factors affecting our life. Of course, these factors can be more complex, but at least those factors are the minimum goals to be connected in a good shape.

Iran Saudi Arabia

Russia India

ChinaEgypt

VenzuelaIndonesia

UAEUzbekistan

IraqKuwaitPakistanArgntina

UkraineAlgeria

MalaysiaThailand

BangladeshMexicoTurkmenistan South Africa

Libya

0 10 20 30 40 50 60 70Billion dollars

Figure 1.1. The total amount of subsidies to fossil fuels that includes electricity, gas, oil, and coal in 2009.2

Figure 1.2. The possible connections present between energy and other factors in our life according to the author's view.

The principles behind many of the currently known renewable energy sources are quite simple. However, the optimum usage of these sources is hard to be affordable now due to different reasons. Ideally, the "new" energy resource should be cheap, clean, easy to handle, and environmentally friendly. Among different suitable ideas to supply energy, the sun is the most promising source of energy and the largest one.4 The sun provides tremen-dous amounts of energy to our planet. The sun sends roughly in one hour the same quantity of the energy consumed by the people on earth in one year

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(2009).5 Solar cells or photovoltaic cells (PV) are used to convert the sun's energy into electricity. In 1954, silicon was the first element to show the PV outcome.6 Different materials and methods have been reported for the usage of the sun energy (via photons) to produce electricity7 or towards producing fuels, such as H2, via molecular catalysts8.

One of the known PVs is called Grätzel cell. The Grätzel cell was invented in the laboratory of Prof. M. Grätzel at EPFL, Switzerland, and became widely known after his famous Nature paper in 1991 showing an efficiency of 7.5% using an Ru-based dye.9 Different breakthroughs and modifications have occurred to the development of the Grätzel cell leading to an increase in its efficiency. Grätzel cells have been employed recently for water splitting to produce hydrogen using organic dyes as photosensitizers under visible light and neutral pH condition, which is a challangeing pro-cess.10 Grätzel cells are relatively cheap in small scales, easy to prepare, and highly applicable in different fields with different structures in comparison with silicon-based solar cells. However, the wide application of Grätzel cells faces many problems, starting from the poor usage of solar spectrum, producing and splitting of charge carriers in the cells, to the optimum way of storing the energy in batteries or chemical bonds. These technical problems increase the cost of PVs including Grätzel cells in comparison with other processes like biomass burning4, which can hinder the broad application of these cells. However, different companies have started the implementation of this technology in buildings (Figure 1.3). So, hopefully, improving the solar cell performances can help paving the way towards a better future.

Figure 1.3. Part of the semitransparent window at the entrance of the SwissTech Convention Center at the EPFL campus in Lausanne, which was formed by large dye solar cells panels, made by Solaronix (this photo was captured in 2014).

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1.3 The Thesis Outline Herein, different photophysical and photochemical processes are shown for some selected organic dyes that are used as photosensitizers in Grätzel cells. The fundamental goal of studying the organic dyes used in Grätzel cells is to overcome the obstacles towards highest efficiency with the lowest price.

The most common blamed problem for the low efficiency of organic dyes is the aggregation problem, which is a matter of debate.11-15 However, this work focuses on other deactivating processes. These processes include isomerization, twisting, twisted intramolecular charge transfer (TICT), quenching by protons, and quenching the dye's excited state by the iodide electrolyte. These topics have not been discussed much in the literature con-sidering Grätzel cells.

After this brief introduction, the rest of the thesis is divided into six sections. Section two focuses on fundamental aspects of the interaction between light and matter, and some principles in Grätzel cells or dye-sensitized solar cells (DSSCs). In section three, the materials, and instruments are described. Then, the results and discussions are divided into four goals, in which each goal is in one chapter.

Section four shows the summary of papers I-IV focusing on the different mechanical motions of the organic molecules used in DSSCs. The mechanical motions that include isomerization, twisting, and TICT can do several effects such as the following: 1) competing with the electron injection process, 2) forming unknown isomers, 3) reducing/enhancing the recombination process.

Section five summarizes the results of papers V-VI, in which discussing the role of protons, particularly for the D149 dye. It has been found that D149 can be quenched by protons present in solvents and on semiconductor surfaces. Different pieces of evidence have been found from the lifetime measurements of D149 under different condictions.

Section six summarizes the results of paper VII, where illustrating the interactions between D149 and the iodide electrolyte. Different proofs have been found for the complexation between this electrolyte and D149. These complexes have various effects on the electron kinetics on TiO2.

Section seven summarizes the results of paper VIII, where investigating the role of covalently attached ferrocene moiety to L1 on the electron dynamics in DSSCs. The electron transfer processes are discussed for a se-ries of organic dyes connected to ferrocene moieties. These processes in-clude electron injection, internal regeneration, and recombination. These processes have been investigated in solution and on semiconductor surfaces.

Indeed, all these findings can increase the knowledge about organic dyes used in Grätzel cells, and help to approach higher efficiencies with lower costs.

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2. Fundamentals

2.1 Light. Light has been a matter of interest for centuries. Many scientists have con-tributed to the understanding of light such as Ibn Al-Haytham, Huygens, and Faraday (Figure 2.1). However, Maxwell summarized the classical picture of light in the Electromagnetic Theory.16 Light consists of two simultaneously propagating waves of electric and magnetic fields in space (Figure 2.1). The electric part is more relevant to be presented herein. The time-dependent expression for the electric field component in one direction is ( , ) =( ) + , where is the maximum amplitude of the electric field in x-direction, and is the radiation angular frequency.16

Figure 2.1. Images for some scientists who contributed to the understanding of light (left). A simplistic representation of the electromagnetic radiation (light) propagating in space, which is a combination of orthogonal electric and magnetic fields (right).

Depending on the magnitude of , the energy and the type of electromag-netic radiation can be defined. Basically, there are no theoretical limits for the energy that can be carried by the electromagnetic wave; however, the common range extends from radio frequency (~10-9 eV -rays (~107 eV). Due to the tremendous efforts of Max Planck, and Albert Einstein, we know that the propagating energy waves are carried by photons that interact with the matter and induce excitation within it, when the photon's energy matches the energy differences between the energy levels within the matter: ( ) = .

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2.2 Electronic Absorption and Emission. The molecules are distributed in different energy states according to the giv-en temperature following the Boltzmann factor.17 At steady state, assuming that the molecule has a singlet ground electronic state, S0, with a wavefunc-tionII ( ), and a higher energy electronic state, S1, with a wavefunction ( ), the overlap integral is given by | = 0, which means that the electronic transition is forbidden. The molecule has to be perturbed first by interacting with an oscillating electric field ( ) having an energy ( ) that matches the energy difference between S0 and S1. According to the perturbation theory, the total Hamiltonian operatorIII for a molecule in the presence of an electric field is = + , where is the Hamiltonian operator of a molecule, = , where is the dipole moment operator. Briefly, this perturbation effect makes a time-dependent superposition between the wavefunctions of the two states that is the temporary overlap between occupied and non-occupied orbitals. In the meanwhile, the electron can oscillate back and forth between these orbitals like an oscillating dipole, which generates the transi-tion dipole moment. The transition dipole moment determines the strength of the absorption band.IV The molecule in the excited state is not stable forever, and it has to relax eventually going back to S0 giving its energy to the sur-roundings via different photophysical processes (Figure 2.2).16,18,19

According to the Franck-Condon principle, absorption happens via verti-cal transitions, which means that there is no change in the nuclear coordi-nates of the system, process 1 in Figure 2.2. Frequently, there are differences in electronic distribution between the aforementioned states, which lead to different minimum positions of potential energy surfaces (PESs). Consequently, higher vibrational energy states are populated in the S1 state, these kinds of transitions are known as "vibronic transitions". The overall electric dipole transition moment that controls the absorption is | | = ( ) ( )

where and are the vibronic coupling terms in S0 and S1, ly.V is the electric dipole moment, which is independent of the locations of the nuclei. The overall integral between the two vibrational states in S0

and S1 is known as Franck-Condon factor. Different overlaps between vibra-tional states lead to a series of vibrational transitions that can be detected in an electronic spectrum.VI The relative intensities of these transitions are pro-portional to the square of the Franck-Condon factors.18

II The wavefunction contains all the needed information about the system. III The Hamiltonian operator is the operator for the total energy of a system. IV Dipole strength = | | | | . V | | = | | . VI In rigid aromatic molecules such as anthracene.

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Later on, the excited molecule relaxes to the S0 via radiative processes such as fluorescence (S1 S0), and phosphorescence (T1 S0), or non-radiative processes such as internal conversion (S1 S0) or by changing electron spin "intersystem crossing" (T1 S0 or S1 T1).17 Vibrational relaxations occur prior to electronic relaxation towards S0.18 Similarly to absorption, the emission also depends on the Franck-Condon principle. Hence, vibronic structures may also occur in the emission spectra.18

As seen in Figure 2.2, a red shift is typically observed between the ab-sorption and emission maxima for the measured solute that is known as Stokes shift.20 The non-thermalized excited molecules populate the high-frequency vibrational modes, and then the excess energy of these molecules is redistributed to lower frequency thermalized modes via intramolecular vibrational redistribution (IVR). Frequently, the IVR process can be fol-lowed either by the band broadening of the transient signals or by following the red shifts of the characteristic vibrational transient bands in the IR re-gion. IVR happens typically on the femtosecond to sub-picosecond time scale.21 IVR is complicated to be monitored alone as it is coupled/followed by vibrational cooling (VC)VII process. In VC, the vibrational modes of the excited solute and the neighboring solvents molecules are coupled, where the energy is dissipated to the solvent. The time window for VC extends from femtoseconds to picoseconds depending on the solvent used.21,22 VC can be monitored via the narrowing of the transient emission bands.21

In addition, there are solvation dynamics, where solvent molecules adjust their configurations according to the equilibrated excited state of the solute. These dynamics extends from femto- to nanoseconds.22 The solvation dy-namics can be monitored typically by following dynamic red shifts of the time-resolved emission bands. Different Stokes shifts can be observed for the emission maxima on the same excited solute depending on the solvents properties.20,21 Different solvation dynamics components have been detected in various solvents within this thesis, in papers I, III, IV, and VIII.

VII It is known also as intermolecular vibrational relaxation.

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Figure 2.2. A schematic representation of different photophysical processes going on after interaction with light "Jablonski diagram": 1, absorption (S0 S1); 2, fluorescence (S1 S0); 3, internal conversion (S1 S0); 4, intersystem crossing (T1 S0); 5, phosphorescence (T1 S0); 6, intersystem crossing (S1 T1). The wavy arrows are for non-radiative vibrational relaxation processes. For simplicity, the solvation coordinate is not shown.

2.3 Dye-Sensitized Solar Cells (DSSCs). 2.3.1 Background. DSSCs or Grätzel cells have attracted the attention of scientific community since the report by O’Regan and Grätzel in Nature.9 The breakthrough at that time was mainly about the efficiency 7.5%) due to the large surface area of the semiconductor nanoparticles of TiO2, and the cell's low cost in comparison with silicon based PVs when using organometallic dyes. In that work9, the TiO2 acts as an n-type semiconductor, as TiO2 receives an elec-tron from the adsorbed dye. In that case, the hole on the dye moves to the electrolyte and an anodic current is produced.23 This cell is based on a single junction devices, and the calculated thermodynamic efficiency is limited to 31% for a single electron-hole pair created by one photon according to Shockley-Queisser limit.24 This theoretical limit has not been achieved in DSSCs due to various limiting processes.

Inside DSSCs, there are three main components: a dye, a low band gap semiconductor, and a redox couple. These components are located between

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two electrodes to close the circuit and gain the energy from the energetic electrons produced.

Concisely, the semiconductor should have some specific properties such as roughness, porosity, and high surface area. All these properties would contribute to light absorption, light scattering, charge injection, charge recombination, and charge transport.25-27 Different metal oxides can be used, such as ZnO, and SnO2, however, TiO2 still gives the best performance.26,27 Different methods are present to control the shape, size, and desired properties of the TiO2 semiconductor nanoparticles.25 The semiconductor is typically screen printed onto a glass sheet covered by a conducting layer such as fluorine-doped tin oxide (FTO).27

For the redox couple, the iodine/iodide mixture, which was used in earlier work by O’Regan and Grätzel, was standing for a long time as standard elec-trolyte until the discovery of other electrolytes.9,26 For the liquid redox elec-trolytes, different options exist, such as ferrocene28, cobalt complexes29, and copper complexes30. Beside the liquid-based redox couple, there are other types of electrolytes such as gel, polymer, ionic liquids, and solid organic hole conductors.26 Also, various solvents and additives are currently added to electrolytes to enhance the efficiency of the cells.26 Unfortunately, most of these additives are used without known mechanisms.

The most relevant component within the context of the thesis concerns the photosensitizer, the organic dye. The function of the dye is to absorb part of the incident solar spectrum. The solar spectrum is maximized at ~500 nm similar to the spectrum of the blackbody at 5800 K according to Wien displacement law (Figure 2.3).17 The solar spectrum consists mainly of visible and IR photons. Most of the current dyes absorb in the visible region, and one of the current challenges is to make efficient dyes absorbing in the IR region.26 Dyes can be organic or organometallic ones. The well-known N3 dye that is Ru-based complex gave efficiency ( ) of ca. 10%.31 The rec-ord was found recently for a Zn-porphyrin based dye with .32 How-ever, metal-free organic dyes are preferable to replace the organometallic ones, especially the noble metal based. Organic dyes are easy to prepare and modify, made of abundant elements, and strong light absorbers.26 The max-imum reported efficiency for using a metal-free organic dye was about 12.5%.33 There are many strategies to consider in the design of organic dyes; however, the standard procedure is based on the push-pull strategy.34-36 In this strategy, an electron rich moiety (D) is covalently connected to an elec-tron- - -A) system. The electron density is mostly localized on the D part in the ground state. Then, upon light absorption, an intramolecular charge transfer (ICT) is induced,

-bridge and the electron density is mostly localized on the A part.

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Figure 2.3. The electromagnetic spectrum of the sun measured at different atmospheric levels and the matching spectrum of a blackbody at temperature of 5800 K. The differences between the yellow and the red spectra are due absorptions of O2, CO2 and H2O. The original data are found on the NREL.37

The A moiety is connected to an anchoring group to bind the semiconductor surfaces. The anchoring group such as a carboxylic acid aids the charge on A to be transferred to the conduction band of a low band gap semiconductor as TiO2 (Figure 2.4). Depending on the dyes' adsorption modes and the overlap between the dyes' orbitals in the excited state and semiconductor surfaces, the photocurrent, and efficiency can be tuned.38,39 Two D units were used for the dyes studied in this thesis: an indoline unit in papers I-VII, and a triphenylamine (TPA) in papers IV, and VIII. Differ-ent acceptor units are shown in the thesis such as cyano-acrylic, and rhodanine moieties.

Figure 2.4. A schematic representation of the push-pull system including ICT and CT from the acceptor group in the anchored dye to TiO2.

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2.3.2 Electron Dynamics in DSSCs. There are various occurring processes inside DSSCs; some are beneficial to the , while others limit the beneficial ones are electron injection, regeneration, electron diffusion, and electron reduction at the counter elec-trode. The limiting processes include the deactivation of the dye's excited state, and charge recombination processes. All these processes are summarized in Figure 2.5. Marcus theory has been used to describe the electron transfer processes in DSSCs.40 Although Marcus originally treated the electron transfers for homogenous systems, the theory has been extended extensively to the fields of chemistry and biology41 including the electron transfers in the electrolyte-semiconductor interfaces42. Among different pa-rameters in Marcus theory, the donor-acceptor distance and energy differ-ences between them are the main parameters that can be tuned to affect elec-tron transfer rates in DSSCs.43 As mentioned before, the adsorbed dye should absorb a large part of the solar spectrum (process 1 in Figure 2.5) under standard conditions (AM 1.5 G “global”)VIII. The broader the dye ab-sorption spectrum, the higher the obtained short circuit current ( )IX, which

n below: =

where is the open circuit voltageX, FF is the fill factorXI, and is the power of the incident light. In addition, the LUMO (precisely S+/S*) level of the adsorbed dye should be above the conduction band (CB) of the TiO2 for an efficient electron injection process (process 3 in Figure 2.5).

The electron injection process is expected to be non-exponential due to the heterogeneity of the TiO2 surface, different binding modes of dye adsorp-tion, the presence of aggregates and so on.26 The electron injection can take place from singlet44 or triplet states44, or even from both45,46. Another mech-anism has been proposed for a direct formation of TiO2 (e-)/S+ without in-cluding the ligands of metal complexes, as in metal-cyano compounds.45 This mechanism was argued to be present due to the coupling of vibrational modes between the compound and the semiconductor surface.45

VIII The path length of the solar spectrum to the surface of the earth is called air mass (AM).

which gives 1000 Wm-2. IX The maximum theoretical is 26 mA cm-2 at a solar cell absorption onset of 800 nm. X is the energy difference between the Fermi level (the electrochemical potential of elec-trons in solids) in TiO2 and the energy of the redox couple used. can be measured by changing the voltage until zero current, where the energy difference between the two men-tioned energy levels is zero. XI = , which corresponds to the ratio of the area under the J-V curve to the area defined by and .

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Figure 2.5. A simplified diagram for the electron dynamics in a DSSC. The numbers for the electron transfer processes are discussed in the text. Numbers in red indicate the beneficial processes, whereas the blue ones are for the competing loss processes.

The electron injection process has been in a debate for a long time wheth-er an ultrafast process of sub-picoseconds46-49 or also containing slow pro-cesses of sub-nanoseconds50-55. Visible ultrafast transient absorption has been mostly used to follow the electron injection by monitoring the for-mation of the oxidized dye on injecting semiconductors.47-49,56,57 However, recently, it was shown that this range of visible probe is not accurate due to the overlap between signals from the electron absorption, the oxidized dye and the excited state of non-injecting dyes on surfaces. This overlap makes the data analysis quite challenging, in addition to the unknown molar absorptivities of these species. Indeed, using the IR probe is more accurate to follow the electron injection into the CB of TiO2.52-54,58,59 The IR probe has been used before for different systems for monitoring the electron injection.52-54,58,59 The electron's transient absorption extends from 3,333 to 11,111 nm with a broad featureless shape.60 To the best of my knowledge, even with the IR probe, electron injection process differs from dye to dye and there is no general rule to quantify how fast is the electron injection. For example, the electron injection of D149 in paper VII has a fast component of 450 fs and slower component of 30 ps with equal amplitudes, whereas for L1 dye in paper VIII the electron injection has only an ultrafast component of 440 fs. The quantum yield for the electron injection ( ) is defined as follows: = +

where , and are the rate constants for the electron injection and the observed excited state decay of the dye used (processes 3 and 2 in Figure 2.5). Keeping high would increase the 61 and this can be afforded

24

by minimizing by blocking deactivation processes such as isomeriza-tion, and quenching by protons (Papers I-VII).

After electron injection, the electron is supposed to diffuse through the TiO2 reaching the FTO, where the photocurrent is detected26, and the electron gives most of its energy to the user (~0.46 eV/photon)XII (process 4 in Figure 2.5). At the counter electrode, electrons can reduce the oxidized species of the electrolyte (process 8 in Figure 2.5).44 The electron diffusion (process 4 in Figure 2.5) is strongly influenced by surrounding ions in the electrolyte, the incident light intensity, the presence of traps, and grain boundaries.26 Also, the electron transport inside TiO2 is influenced by the recombination processes with the oxidized dyes or the oxidized species in the redox couple (processes 6, and 7 in Figure 2.5). These processes make the lifetime for the electron transport between the two electrodes in the range of milliseconds to seconds.44

The recombination process to an oxidized dye (ROD) (process 7 in Figure 2.5) competes typically with the regeneration process (process 5 in Figure 2.5).26 ROD is a second order reaction as it depends on the number of the oxidized species among the adsorbed dyes on the surface and the number of electrons in the conduction band of the semiconductor ( =[ ]), where is the rate constant, is the electron concentration at the semiconductor surface.62 While a high leads to faster recombination, it is still needed to increase the , the electron lifetime in the CB, and the

.61,63,64 ROD is often studied by nanosecond visible transient absorption or slower setups, where it extends from nano- to milliseconds with non-exponential kinetics.65-68 The ROD is slower than the electron injection pro-cess due to the electron trapping inside the metal oxide69 and the partial mo-bility of the oxidized sensitizer via physical movement or hoping of holes45. The ROD rate decreases by increasing the distance between the hole of the dye and electron inside TiO2.65 The energy difference between the electron in the conduction band and the ground state of the dye is typically on the order of 1.5 eV.26 A matter of debate is whether this value lies in the MarcusXIII inverted70,71 or normal region72. The ROD was monitored for complexes of D149, and L1Fc dyes in paper VII-VIII using an IR probe.

To reduce ROD, and close the circuit, one needs an electrolyte to facilitate the regeneration process by filling the hole of the adsorbed oxidized dye through regeneration (process 5, Figure 2.5).26 Regeneration is a slow process by nature as it is typically a diffusion-limited process, which

A fast regeneration process increases

XII i-ciency of 0.7. XIII Normal region means that the electron transfer rate increases with the driving force, whereas in the inverted region the electron transfer rate decreases with the increase of driving force.

25

and by increasing the number and lifetime of electrons inside the semi-conductor.47,4961 The regeneration quantum yield ( ) is defined as fol-lows: = +

where the and are pseudo first order rate constants. The regenera-tion time relies on a diffusional process, so the efficiency of the regeneration process depends on the diffusion constant ( ) of the material used and its concentration as follows26: = × [ ] A typical regeneration lifetime is ca. nanoseconds taking into account a con-centration of 0.1 M at least, and a of 109-1010 M-1 s-1. Different sub-stances have been tested as electron donors such as I-, Br-, SCN-, and spiro-MeOTAD.26 I- has been used as a standard reductant and has a close to unity with a half lifetime ranges from 100 ns to 10 68,73. The following equations represent the currently accepted mechanism for the regeneration process via I- for various dyes (Dye) used in DSSCs68,74,75:

+ ( … ) ( … ) + + 2 + Regeneration by I- can be also accelerated by the adsorption of small cations such as Li+, and Mg2+ on the TiO2 surface. These cations enhance a large population of I- to be near the surface.76 Also, higher can be obtained by reducing the energy gap between the redox couple and the ground state of the oxidized dye64,77; different redox couples were used to achieve that goal.78 More importantly, the minimum driving forceXIV has been determined to be 0.5 eV for the I- electrolyte to afford simultaneously high regeneration rate and .68,71,74,79 Recently, other redox couples have shown high perfor-mances in DSSCs such as ferrocene electrolytes with different oxidation potentials28. However, it has been stated that the ideal driving force for re-generating the oxidized dye by ferrocene electrolytes is ca. 0.36 eV.67 Paper VIII presents the results of a covalently linked ferrocene to an organic dye, where an ultrafast regeneration step could be monitored on semiconductor surfaces.

The recombination to the oxidized species in the electrolyte (ROE) has been commonly studied by the transient response of (process 6 in Figure 2.5).80 The time scale of this process varies from milliseconds to seconds

XIV The standard sensitizer has a +1.1 V vs. NHE, where the redox potential of I-/ I3- is about

+0.35 V vs. NHE, which gives a driving force of 0.75 eV.

26

depending on the type of electrolyte.26,44 The slowest rate has been observed for iodide/triiodide electrolyte.80 For the iodide/triiodide electrolyte, recom-bination to I2 was faster than to I3

- 81, and the binding of I2 to the adsorbed dyes enhances these ROE losses82.

In addition, all the previous electron kinetic processes are quite sensitive to, the dye's structure, the anchoring group, the pH, the excitation wave-length, ionic strength, temperature, and the solvents used.45 As can been seen, the electron kinetics in DSSCs is quite complex, and it is hard to draw a solid conclusion based only on one process without influences from others. Therefore, understanding the photochemistry and the photophysics of the adsorbed organic dyes is considered just as few steps of a long road towards the full assimilation of DSSCs.

27

3. Materials and Methods

3.1 Organic Dyes The organic dyes used in this thesis were synthesized externally. Some of these dyes belong to the indoline family. The indoline dyes were showing very promising efficiencies among other organic families.83 These dyes (D149, D102, and D131) have shown the best efficiency in DSSCs among tens of other dyes within the same indoline family.84 The high efficiecny was the main motivation behind using these dyes. The indoline family was a generous gift from Masakazu Takata, Mitsubishi Paper Mills, Japan. The structures of these dyes are shown in Figure 3.1. The indoline donor unit (abbreviated as D) was obtained from Chemicrea Inc., Japan. Cyano acrylic dyes including L0, L0Br, L1, L1Fc, L1Fc2, and L1ester were synthesized and analyzed in KTH, Sweden. The structures are shown later in each sec-tion according to the topic discussed. However, the structure of the main two dyes L1 and L0 are shown in (Figure 3.1).

Figure 3.1. Structures of indoline donor unit D (blue), and other dyes (other colors) with different acceptor units (left). Structures of the cyano-acrylic dyes L0 and L1 (right). The colors of the solid-state dyes are also shown.

3.2 Steady State Absorption and Emission The absorbance of a substance depends (l), and concentration (c). By measuring the intensity of the transmitted light (It) to the incident light (I0), transmittance and absorbance can be calculated

28

as follows: = log( ) = log = , that is known as Beer-Lambert law.85 In practice, a reference sample (r) is measured, so = log . The molar absorptivity of a molecule defines the probability to absorb light that defines the oscillator strength ( ), which is directly pro-portional to the integral of absorption band in wavenumber scale ( ) as fol-lows: = | | | | = 4.32 × 10 ( )

where is the refractive index, and is a dimensionless unit.86 After absorption of short laser pulses (emission is vanished between the

two successive pulses), the concentration of excited molecules [ ] decays to the ground state according to the following equations:

[ ] = ( + )[ ] [ ] = [ ] exp ( ), = 1 ( + ) where and are the radiative and non-radiative rate constants, respec-tively. Knowing and the fluorescence quantum yield , one can de-termine as follows: = (1 ). Also, the radiative lifetime (s) for spontaneous emission can be estimated knowing the emission wave-

m) and the oscillator strength as follows87: = 25200 .

The fluorescence intensity at time after pulsed excitation is ( ) = [ ] = [ ] exp ( ) However, under continuous illumination ( represents the intensity of the incident light), i.e. steady state conditions, the rate of concentration change for [ ] equals zero; the following equations express the fluorescence inten-sity under steady state conditions: [ ] = 0 = ( + )[ ] = [ ] = ( + ) =

These simple equations show that the fluorescence emission intensity is con-trolled mainly by the fluorescence quantum yield.86 The specifications of the steady-state absorption and emission instruments were described previously.88

29

3.3 Ultrafast fs-Transient Absorption Two transient absorption setups were mostly used within the thesis to study DSSCs (10-12-10-9 s): the first one has a probe in the UV-Vis range (TA), and the second one has a probe in the IR range (TA-IR). Both systems have simi-lar basics and principles using lasers. LASER stands for Light Amplification by Stimulated Emission of Radiation. Lasers have a wide variety of uses from CD players to military applications. Lasers are characterized by direc-tionality, monochromaticity, brightness, and coherence. According to Ein-stein's approach, the rate of excitation of S1 S2 is shown below: ( ) = ( ) ( ) = 23 | | | | where is the Einstein coefficient for transitions between state 1 and 2, ( ) is the spectral radiant energy density per unit frequency of the inci-dent light (J.m-3.s), and ( ) is the number of molecules in the ground state at time t. When the excited molecules relax again to the ground state, there are two radiative pathways: spontaneous emission and stimulated emission, and their rates are shown below: . : ( ) = ( ) . : ( ) = ( ) ( ) : = where ( ) is the number of molecules in the excited state at time t. At spontaneous emission, the molecule simply relaxes without external induc-tion. However, for stimulated emission, external light with a matched fre-quency is needed to induce the relaxation of excited molecules producing two emitted photons at the same time, which is also called amplification.17

To produce a laser light, population inversion is needed that means N2>N1. Two states are not sufficient to obtain such a condition, and three states at least are required, where N3>N2 can be achieved. State 3, in this case, is a long lived excited state, and state 2 is a short one, where lasing happens between state 3 and 1, as in the ruby laser.85 After producing laser pulses, other requirements are needed such as the energy of the laser pulses. For instance, the efficiency of the frequency doubling needs a specific amount of energy to obtain a reasonable energy per pulse. Therefore, laser amplification is required to obtain this threshold energy condition (Figure 3.2). Generally, the laser setup has three basic components depending on the chirped pulse amplifier scheme89: a seed laser, a continuous high power pump laser, and an amplifier, which at the end ejects short pulses of typically 800 nm as a fundamental beam (Figure 3.2). The stretched mode-locked seed

30

pulse meets the pump laserXV inside the gain medium in the amplifier, and after several round trips, a high intense laser pulse is ejected from the amplifier.89 The ejected 800 nm laser pulses are not suitable to study most of the substances, so other wavelengths including white light can be generated using beta-barium borate (BBO), or CaF2 crystals.

The study of the molecule's excited state kinetics happens by the control of the time differences between the excitation and the probe pulses. The excitation pulse excites the molecule and puts it the desired excited state.XVI Then a later coming weak probe light, with a delay , can measure the ab-sorbance of the excited molecule. By measuring the difference in absorb-ances of the sample before and after excitation (by using a chopper), a dif-ference absorbance spectrum can be generated as described in this equation:

= = ( ) ( ) = log ( ( )( , ))

Figure 3.2. A schematic representation of the amplification step of the chirped pulse amplification setup using Ti:Sapphire (Ti3+/Al2O3) as a gain medium. The gain me-dium is pumped with a high power long duration pulses of ~250 ns. An individual pulse from a train of mode-locked pulses (seed pulse) is selected to hit the crystal at the Brewster’s angle after being stretched. The pulse passes many times (~10–20 times) through the gain medium to get more energy. After reaching the desired pow-er, the amplified pulse is sent out from the amplifier then compressed. Controlling of the number of passages can be achieved using the /4 and the pockels cells (is con-trolled by external voltage to change the polarization also). More details can be found in the literature.89

The difference spectrum contains basically three main features that are wavelength and time dependent: 1) a positive featureXVII represents the

XV The pump keeps the gain medium (Ti:Sapphire) under population inversion condition. XVI Typically, 0.1 to 10 % of the ground state molecules are excited. XVII Also, for a product absorption, such as CT state or isomerized state.

31

excited state absorption (ESA); 2) a negative feature resembles the inverse of the steady state absorption of the molecule, ground state bleach (GSB); 3) a negative feature resembles the inverse of steady state emissionXVIII, stimulated emission (SE).90 The specifications of the instruments used for transient absorption measurements using before.91,92

3.4 Time-Correlated Single Photon Counting (TCSPC) In TCSPC, one can measure the emission decay of a molecule in a fast and an accurate way, thanks to the high repetition rate of the laser (ps or fs lasers). Lifetimes from at least ca. 100 ps could be measured with TCSPC. In TCSPC, one aims to detect one photon per laser pulseXIX at most, where multi-photons per laser pulse are not produced. However, this is not what occurs in practice, and typically on average less than one photon is detected per pulse; the detection rate is ca. 1 photon per 100 laser pulses. The accura-cy of the measurements depends on the arrival of randomly emitted photons to the detector at different time channels. The accuracy of the short lifetimes depends on the electronics being used to define time registry of arriving pho-tons.

During the sample excitation by a laser pulse, a parallel simultaneous electrical signal is sent to the electronics. This arrival time of this signal is measured by a constant function discriminator (CFD) and then a linear in-crease in the voltage starts when the signal passes through time to amplitude converter (TAC). This signal is called the start. At that time, the electrical signal from the emitted photon passes through the CFD, a signal is sent to the TAC to stop the voltage increase. This signal is called the stop. The time difference between the start and stop corresponds to the time delay after examining signal by the rest of the electronics. Repeating these measure-ments many times gives the histogram plot at the end. Figure 3.3 shows the basic parts within TCSPC.20 The specifications of TCSPC have been de-scribed previously in details.88

XVIII It happens due to the interaction with the probe light. However, it resembles the sponta-neous emission in most cases. The spontaneous emission is not likely to be observed within the used setup due to geometrical reasons. XIX The dead times range for the electronics vary from 10 microseconds to 100 nanoseconds.

32

Figure 3.3. A simplified scheme for basic parts of the TCSPC.

3.5 Streak-Camera With the streak camera, one can measure faster events than in TCSPC down to picoseconds. Recent cameras have an instrument response down to hun-dreds of femtoseconds. In addition, the wavelength distribution of the emis-sion is resolved, so additional information such as solvation dynamics, or emission from different species can be resolved.20 Also, multiple photons at different energies can be detected at the same time for each laser pulse. However, photons with the same energy that arrive at the photocathode at the same time may not be counted accurately, and an emission correction curve is needed. When emitted photons hit the photocathode, different elec-trons are produced and pass through a voltage sweep that defines the arrival time of the associated photon on a phosphor screen. The sensitive screen is connected to a charged coupled device (CCD) to image the time-wavelength resolved emission at the end of the measurement.20 Figure 3.4 shows a sim-plified scheme for streak camera. The specifications of the streak camera system were described before.88

Figure 3.4. A simplified representation of emission detection via streak camera.

33

3.6 Data Analysis Different procedures have been used for fitting the data from TCSPC, streak camera and transient absorption. Both global and single wavelength fitting procedures were used to extract lifetimes assuming a Gaussian instrument response function.93 The following equation is the simplest version used for fitting kinetic traces: ( ) = ( )/ . For accurate detection of fast events that occur within the width of the laser pulses, a response function is used to correct for the molecules being excited within the time duration of the pump laser pulses. Typically, the laser pulse is assumed to follow a Gaussian shaped temporal profile with a width and a location of t0, where its equation is ( ) = 12 ( ) , = 2 2 2

To combine these two equations, a convolution process is needed by multiplying the two functions and introducing the complementary error function to convolute both the exponentials and the Gaussian.93 The fitting function in the case of mono-exponential will become: ( ) = 12 (1 + erf( . 12))

For the TA, an additional function for the cross phase modulation artifact at early times, around time zero, is needed to correct the time zero and the spectral shapes.94 This observation happens due to an interaction between the highly intense pump pulse and the weak probe pulse spectrum in a nonlinear medium "Kerr medium", such as glasses. This phenomenon is known as Kerr effect, where a change in the refractive index (n) happens depending on the optical intensity, and the nonlinear refractive index of the medium (n2). In the current case, = ( , , ). Indeed, as the UV-Vis probe light spans over a wide range of frequency, typically from 350-900 nm, and each frequency has its own refractive index, the chirp due to cross phase modulation is not a linear function.89 Also, one should not forget that this modulation is absent in the reference probe spectrum. However, this problem and others were studied before by applying various modifications for the fitting equations.94,95

34

4. Photoinduced Large-scale Motions (Papers I-IV)

The effect of photoinduced large-scale motions on organic dyes used in DSSCs has been ignored for a long time. This chapter shows different pho-toinduced motions in solution and on semiconductor surfaces including pho-toisomerization, twisting and twisted intramolecular charge transfer process-es in various organic dyes. These processes are expected to affect the per-formance of DSSCs. 4.1 The First Goal This chapter summarizes the results of papers I-IV. The original question was relatively simple: “Why does D149 have a short lifetime of ~100 ps in MeOH, not several ns as known for an organic dye?” This question was asked by Dr. Zietz after a given seminar by Dr. Lohse summarizing his pub-lished paper96 about the ultrafast spectroscopic measurements of the well-known organic dye, D149. Also, an earlier theoretical study showed that the calculated radiative decay rate of D149 should be ca. (3.2 ns)-1 in methanol.97 Fortunately, at least for me, no clear answer was known. D149 showed a high efficiency in DSSCs ( 98 and was studied spectroscopically before.96,99 The double bond in the linker unit of D149 was suspected to isomerize and cause a short excited state lifetime.

The concept of photoisomerization implies that upon excitation, one can distinguish between the cis and the trans forms of the same dye. Ideally, the cis and trans forms have different absorption spectra and probably different photophysical properties. The photoisomerization has been monitored in solution and on semiconductor surface (ZrO2) for different dyes (Paper I, and II).88,100 The short lifetimes of these dyes in solution were attributed to the -bond). The measurements on ZrO2 were crucial to show that adsorbed molecules on semiconductor surfaces can isomerize despite the crowded environment. One might think that isomerization would not affect the performance of the cells, as the electron injection is assumed to be an ultrafast process wiping other slow processes. However, another group, par-tially based on our results, showed that the isomerization process in another class of organic dyes on low band gap semiconductor (TiO2) could affect the solar cell performance with irradiation over time.101

35

In paper III, a twisting process was found in the donor unit (D) of the indoline family that includes D149.102 The twisting process happens at the diphenyl moiety, where the two phenyl rings change their dihedral angles around the double bond. The term "twisting" is used instead of "isomerization" since no isomers were formed by definition. The twisting process of D deactivates the excited state non-radiatively via a conical inter-section (CI) with the ground state.102 The twisting process was also detected in other dyes like D149. Twisting is expected to reduce the electron injection quantum yield in DSSCs, as it is a fast process. In paper VI, TICT is pre-sented for cyano-acrylic dyes such as L1, and D131. This TICT process is unique as it does not only depend on the polarity and the medium flexibility, but also on the dimer formation at the anchoring group. This dimerization increases the acceptor strength and changes the excited state from a long-lived ICT state to a short-lived TICT state.

4.2 Photoisomerization The photoinduced cis-trans isomerization process requires simply a C=C double bond and light to change the dihedral angles around the double bond. This process happens in the excited state rather than the ground state that has a high-energy barrier to form an isomer directly (Figure 4.1). During the photoisomerization, the Born-Oppenheimer approximation is violated near the CIs, and non-radiative transitions are allowed.103 The breakdown of Born-Oppenheimer approximation implies rapid motions of nuclei, and in-separable electronic and nuclear wavefunctions that can lead to mixing of vibrational and electronic states.104 At the CI, the ground, and excited elec-tronic states are degenerate, and a mixture of isomers can be formed.

Figure 4.1. A schematic representation of the photoisomerization process via a CI in one dimension (twisting angle as an example).

36

4.2.1 In solution D149 has an excited state lifetime of ca. 700 ps in CHCl3 (for D149's structure see Figure 4.2).105 Embedding D149 in a polymethylmethacrylate (PMMA) matrix extends the emission lifetime to several ns with a main component of ca. ~2.5 ns that is very close to the calculated one97 (Figure 4.3). Two different NMR spectra for non-irradiated and irradiated samples of D149 in DMSO were obtained due to the formation of a new isomer. D102 (only one rhodanine ring) was useful to assign the location of the pho-toisomerization process at the C=C between the donor moiety and the first rhodanine ring in D149 (Figure 4.2). The reversibility of isomerization was proven via the UV-Vis spectra when irradiating at two different wavelengths (387 nm, S2 state and 590 nm, S1 state, (Figure 4.4)).

SN

N

O

R

H

HH COOH

S

N

O

S

S

D149: R=

D102: R=

b

d

c a

Figure 4.2. Chemical structures of D102 and D149. Hydrogens with superscript letters are referred to in the NMR-spectra (left). 1H-NMR spectra of irradiated (λ=400 nm, top) and non-irradiated (bottom) of D149 in DMSO-d6 with labeled protons before (Ha-Hd) and after (Ha'-Hd') irradiation (right). These figures were reused from Figure 1 and Figure 7 in paper I.

Figure 4.3. Fluorescence decay of D149 in CHCl3 (black) and PMMA (red) after 406 nm excitation using TCSPC.

37

0 5 10 15 20 251

10

100

1000

0.7 ns

2.5 nsAs a main component

D149 in: CHCl3 PMMA

Cou

nts

Time (ns)

300 400 500 6000.0

0.2

0.4

0.6

0.8Ab

sorp

tion

Wavelength (nm)

D149 in CH3CNirradiated with = 387 nm:

0 s 10 s 30 s 70 s 120 s 180 s 300 s 480 s 900 s 1800 s

Irradiation

300 400 500 600

0.0

0.2

0.4

0.6

0.8

Abso

rptio

n

Wavelength (nm)

D149 in CH3CN after UV irradiation,irradiated with = 590 nm:

0 s 10 s 20 s 80 s 180 s 600 s 1200 s 2100 s

Irradiation

Figure 4.4. Absorption spectra of D149 in CH3CN after irradiation with UV light (left) and with visible light after UV irradiation (right). Irradiation at the S2 state was used to afford a high quantum yield of the new isomer. As the cis isomer has more absorption to the red than the trans isomer, the 590 nm was selected to irradiate cis more than trans isomers. These figures are reused from Figure 6 in paper I.

The lifetime for the photoisomerization process was calculated to be ca. 1.0 ns in CHCl3 and basic-acetonitrile. This process is considered a slow isomer-ization process in comparison with others like the one in rhodopsin.106 To determine the energy barrier for the isomerization process, temperature de-pendent lifetime measurements were done in basic acetonitrile (to break dimers of D149, paper V) using TCSPC (Figure 4.5). An activation energy of ca. 13 kJ/mol was obtained, using Arrhenius equation = , where k is the experimental rate, A is the pre-exponential factor, Ea is the activation energy, and R and T are the gas constant and temperature, respec-tively.

Figure 4.5. Emission decay of D149 in basic-acetonitrile at different temperatures, ranging from 3 This figure is reused from Figure 8 in paper V.

38

4.2.2 On Surfaces Following up the photoisomerization process of D149 in solution, another dye, L0Br, with another anchoring group was tested in various solvents, PMMA, and more importantly on a high band gap semiconductor (ZrO2). L0Br could give clearer difference between the absorption spectra of the two isomers compared to L0 (structures of the dyes are shown in Figure 4.6). L0Br has shown an isomerization upon irradiation at the red edge (~400 nm) in ethanol and in PMMA plastic matrix. In addition, these isomerized forms could be reconverted to the trans form upon irradiation in the blue region (~325 nm). Figure 4.6 shows the absorption change upon irradiation of ad-sorbed L0Br on ZrO2 surfaces. The back-isomerization on ZrO2 was not detected, probably due to a photo-degradation process on the semiconductor surface.

In a previous study, the performance of organic dyes without double bonds was better than those with double bonds.107 Also, two isomers of an organic dye have shown ca. 20% difference in efficiencies under the same conditions.108 Recently, other organic dyes showed photoisomerization on ZrO2 and TiO2 surfaces upon UV irradiation, and could be controlled by use of cyclodextrin that hinders/allows isomerization on surfaces.109 Briefly, these measurements and others show the importance of such an ignored pro-cess for the dyes being used in DSSCs.

300 350 400 450 500 550

0.0

0.5

1.0

1.5

Abs

orpt

ion

spec

tra

Wavelength(nm)

Irr. time (s) 0 10 20 40 50 90 115 200 300 400 500

335 nm*

Figure 4.6. The structures of L0, and L0Br (Left). The absorption change upon irra-diation of L0Br on ZrO2 at 400 nm under N2 and low light irradiation to avoid pho-to-oxidation (right). An isosbestic point is seen at 335 nm and marked by an aster-isk. The right figure is reused from Figure 6 paper III.

39

4.3 Twisting D was used to synthesize the dyes in the indoline family.84,110 The structure of D is shown in Figure 4.2. The absorption and emission spectra of D are shown in Figure 4.7, in addition to the absorptions of dyes within the same family. The lifetime of D in different solvents is very short, <50 ps (Figure 4.8). In contrast, embedding D in PMMA shows longer-lived excited species with a main lifetime of ca. 2.55 ns (Figure 4.8). The excited state decay of D required multi-exponential fits with short lifetimes that overlap with solva-tion dynamics. Therefore, D was measured in different solvents to investigate the influence of the solvent on the excited dynamics of D and to remove the overlap between the D's dynamics and the solvation dynamics components. These results are summarized in Table 4.2. After excluding the solvation dynamics by comapring them with the reported values22, D exhibits three main kinetic components: a rising component of 90 fs and two decay components of 3.5 ps and 23 ps.102 DFT calculations showed that after excitation the electron density moves from the nitrogen region to the diphenyl moiety where another double bond is exist (Figure 4.2).

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Abs

orpt

ion

Wavelength (nm)

D in: Toluene Acetonitrile Methanol

exc

40 35 30 25 20 15

0.0

0.2

0.4

0.6

0.8

1.0

Norm

alized Em

isison

Wavenumber (103 cm-1)

300 350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Abs

orba

nce

Wavelength(nm)

D D131 D102 D149

Figure 4.7. The normalized absorption and emission spectra (ex. 400 nm) for D in different solvents. The calculated oscillator strengths are also shown as blue bars. The shapes of the fluorescence spectra are valid for the wavelength scale (left). Normalized absorption spectra of the D unit and different D-dyes in the indoline family (D131, D102, and D149) in toluene (right). The left figure is reused from Figure 2 in paper III.

Table 4.1. Summary of lifetimes for D in different solvents. Amplitudes are shown in percentages and correspond to a mixture of relaxation and deactivation processes.

Solvent 1 (ps) [%] 2 (ps) [%] 3 (ps) [%] 4 (ps) [%] Toluene 0.1 [39] 0.6a [12] 2.8a [11] 21 [38] CHCl3 0.08 [43] 0.73a [12.5] 4.70a [13] 21 [31.5]

Acetonitrile 0.06a [61] 0.7a [8.5] 3.4 [16] 28 [14.5] MeOH 0.09 [43.5] 0.9a [9.5] 5.5a [29] 25 [18]

a Solvation dynamics.

40

0 5 10 15 20 25 301E-3

0.01

0.1

1

10kC

ount

s

Time (ns)

IRF PMMA

2.55 ns As a main component

Figure 4.8. Emission decay of D in PMMA matrix. The emission decay of D in solvents resembles the IRF (left). The 2D plot TA of the D unit in toluene. The steady-state emission spectrum is superimposed on the stimulated emission (black line) (right). The right figure is reused from Figure 3 in paper III.

Different mechanical motions can happen in the excited state at this dou-

ble bond leading to an ultrafast deactivation of the excited state such as py-ramidalization ( ), tilting ( ), C=C torsion ( ), and bond rotation ( ) (Figure 4.9).103 The excited molecule can do one-dimensional type of these move-ments or multidimensional depending on the system. In the multidimension-al model, the CI is not an energy point, but rather a hyper-surface.

Figure 4.9. Schematic representation of different mechanical motions the pyrami-dalization ( ), tilting ( ), C=C torsion ( ), and bond rotation ( ) angles.

As these mechanical movements could not be detected experimentally, theoretical calculations were used to investigate the PEHs of the ground and lowest-lying excited states at twisted and pyramidalised geometries. The CI point was found . This CI crossing between the PEHs of the S0 and S1 states might match with the ul-trafast non-radiative decay of D (Figure 4.10).

41

Figure 4.10. CASPT2/ANO-S-VDZP energies of the ionic (11A') and biradical (11A'') states The structures for the CI and (11A')min points are shown. A ghost atom has been added (in cyan) to help visualizing the pyramidaliza-tion of the ethylene bond. Red arrows indicate the decay path. This figure is reused from Figure 7 in paper III.

More importantly, the twisting process in the D unit is still present in the indoline dyes, such as D102 and D149. The TA was used to investigate these dyes in acetonitrile under different conditions, and a fast component of ~20 ps was found. Fluorescence quantum measurements were used to examine the nature of this component in D149 whether it is a "structural" relaxation in the excited state, leading to a slightly modified, but still electronically excited molecule, or it is a depopulation of the excited state, representing a new deactivation channel. Also, the emission quantum yield was used to investigate the nature of the short lifetime component of ca. 3.5 ps in D (Table 4.1). The emission quantum yield of D was measured in MeOH and acetonitrile. The calculated quantum yield was relying on the longest life-time only (~25 ps) and the lifetime of D in PMMA (~2.5 ns), assuming that the latter is very close to the radiative lifetime. The experimental emission quantum yield was done by using steady state emission using coumarine dyes as standard references. Table 4.2 shows that there are differences be-tween the experimental and the calculated quantum yields. These differences show that the fast component in D, ~3.5 ps, contributes to the deactivation of the D's excited state, which means that two different populations of excited D molecules twist in different time scales. Similar calculations and observa-tions are shown for D149, which indicate that the twisting decay channel is also present in D149. Although the short lifetimes of D102 and D149 are slightly modified than in D, in absolute numbers and amplitudes. This modification of lifetimes can be due to the disappearance of one decay channel or to the closeness of these lifetimes, where the discrimination between them is difficult.

42

Table 4.2 Measured and calculated quantum yields of D and D149. This table is reused from Table 5 in paper III.

Compound Solvent exp (a) calc (longest )

D MeOH 0.22% 0.99% Acetonitrile 0.23% 1.12%

D149 MeOH 0.64% 3.9% Acetonitrile 4.64 % 12.8% (dimers)(b)

a) Average value was calculated by using C334 and C343 as references in the steady state measurements. b) D149 forms dimers (paper V).

4.4 TICT in Cyanoacrylic Dyes Cyano-acrylic moiety (CyA) is used extensively as an acceptor group in organic dyes used in Grätzel cells.34 These dyes such as L1 and D131 (Figure 3.1) show interesting behaviors in the absorption and emission spec-tra upon concentration variation in polar solvents. Such behaviors have been reported before for other CyA dyes without deep understanding.111 However, the dimer formation was used an interpretation for such changes in the ab-sorption and emission measurements.111. The variation in absorption and emission spectra could be also monitored by adding bases or acids. There-fore, the role of acid-base equilibrium cannot be ignored. However, for the sake of simplicity, dimers and monomers species will be used in the follow-ing text to define the different species.

Herein, deeper investigation has been done primarily for L1 dye, where changes in the absorption and emission spectra were accompanied by change in lifetimes and the excited-state type of L1. TICT state has been assumed to form with concentration variation of L1 in polar solvents. TICT is the twisting of a single and double bonds through adiabatic photoreactions.112 TICT is mainly present in molecules based on Donor-Acceptor (D-A) strate-gy.112,113 Avoiding TICT happens through two main strategies: (1) reducing the acceptor strength, which increases the minimum energy of TICT; (2) making stiff molecules.113 Understanding TICT in this class of dyes is im-portant for DSSCs applications.

The nature of electronic excitation in L1 is ICT that occurs from the TPA to the CyA moiety.114 The ICT in acetonitrile has an absorption maximum, and an emission maximum of ca. 404 nm, and 575 nm, respectively. Howev-er, by increasing the concentration, a new absorption shoulder at ca. 470 nm, and a new weak red emission at ca. 700 nm starts to appear (Figure 4.11). The red emission has been assigned to the presence of TICT state in the L1 dye when it only dimerises. This red emission band has not been only ob-served in acetonitrile, but also in other polar solvents such as DMSO. In contrast, in non-polar solvents such as CHCl3, no such shifts in absorption or emission were observed by changing the concentration. However, dimer

43

formation was also assumed in non-polar solvents due to two reasons: (1) the absorption maxima of L1 in non-polar solvents were close to the location of dimers in polar solvents (close to ~470 nm); (2) the dimer formation through the anchoring groups has been assumed to be very strong and non-polar sol-vents cannot break these dimer bonds.

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce

Wavelength(nm)

Concentrations of L1in Fresh MeCN in ( M)

0.1 0.2 0.3 0.5 0.7 1.0 1.4 2.3 3.1 4.5 6.2

500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

TICT

Nor

mal

ized

Em

issi

on

Wavelength (nm)

600 nM 6 uM 10 uM 17 uM 60 uM

ICT

Figure 4.11: Normalized absorption spectra of L1 in MeCN at different concentra-tions (0.1- 6.2 left). In the inset, the absolute spectra are shown. Emission spectra from L1 in MeCN at different concentrations at an excitation wavelength of 470 nm (right). These figures are reused from Figure 3 in paper IV.

By using time-resolved emission techniques, the lifetimes for those bands, the blue, and the red ones could be measured at different concentrations. L1 in most of these solvents shows two lifetimes, a short one varies from 50 ps to 200 ps, and a longer one extends from 0.5 to 3.0 ns. The short lifetimes are present at high concentrations of L1 in polar solvents and their ampli-tudes depend on the concentration used. These short lifetimes are assigned to the emission from the TICT state. On the contrary, the long lifetimes domi-nate at low concentration of L1 in polar solvents, which are assigned to the emission from ICT state. The long lifetime in CHCl3 resembles the lifetime of L1 in PMMA in which the TICT was assumed to be blocked. This simi-larity shows that the long-lived species in CHCl3 corresponds to the ICT state.

The emission of L1 in DMSO shows the most striking spectral changes with variation of concentration (Figure 4.12). At low concentration, the main emission, centred at 550 nm, has been assigned to the ICT state of mono-mers. However, the emission spectra are not homogenous due to an emission tail between ca. 650-720 nm. Kinetic traces at 700 nm were fitted and a life-time of ca. 215 ps was obtained. At 550 nm, two lifetimes of ca. 300 ps and 1.4 ns were obtained (the latter from TCSPC). The 200-300 ps component was assigned to the red-shifted emission from TICT state. At high concentra-tion, the ICT emission band has nearly completely vanished, and the TICT emission band is centred at 650 nm with a lifetime of 175 ps.

44

Figure 4.12. Emission of L1 in DMSO at low (~10 μM, top) and high (~100 μM, bottom) concentration. These figures are reused from Figure 4 in paper IV.

According to the current picture, after exciting the dimer of L1 in a polar solvent, the ICT state is populated, and then followed by a formation of TICT state, where the decay occurs. Figure 4.13 shows the TA-2D plot for L1 in MeCN, where a high concentration of L1 was used. Three regions are shown in Figure 4.13, GSB centered at ca. 450 nm, ESA centered at ca. 575 nm, and SE at ca. 700 nm. Three independent kinetic traces at each region were extracted and fitted exponentially. At 550 nm, three lifetimes were needed for a good fit. The first one is a rising component of ca. 160 fs, fol-lowed by two decay lifetimes of 0.7 and 98 ps. At 700 nm, similar results were obtained. However, at 450 nm, where the GSB dominates, only two decay lifetimes were obtained, in which the rise component was missing. This rising component, 160-350 fs, has been assigned to the formation of the TICT state after ICT population; the TICT state was assumed to be without a significant barrier.112 Similar observations were found for D131 in MeCN at high concentrations. After population of ICT state, the lifetime for the TICT state formation is ca. 600 fs, with a significant blue shift of the ESA due to the absorption of TICT state (Figure 4.13). The transformation of ICT to TICT in cyano-acrylic dyes was assumed to happen when increasing the acceptor strength by dimer formation.

45

Figure 4.13: Kinetic traces at different wavelengths and normalized extracted spectra at different times for L1 in MeCN (left). The spectral change of D131 in MeCN over time. The structure of D131 is shown in the inset. Steady state measurements, ab-sorbance, and emission are shown in dotted lines (right).

4.5 Conclusions Photo-isomerization is present in organic dyes used in DSSCs. This process could be detected in different families of organic dyes. Photo-isomerization can affect the order of the dyes present on the semiconductor surfaces, reduce electron injection quantum yield, produce isomers with unknown performances, and open unneeded contacts between the electrolyte and the semiconductor. Also, an ultrafast twisting process could also be identified in the D unit of the indoline family in solutions. The twisting process was also found in other dyes such as D149. As this process is quite fast and deactivate the dye's excited state, it can compete with the electron injection process (see paper VII). Twisting is expected to be present in other donor units used in DSSCs, such as TPA in D35.49 The concept of TICT was also introduced for cyano-acrylic dyes such as L1 and D131. The connections between TICT and applications in solar cells are expected to be crucial (see paper VIII). In addition, the formation of TICT via dimerization of L1 monomers highlights the weak side of sophisticated techniques such as fs-TA that are limited by using a high concentration of the solute. For example, if one relies only on the fs-TA analysis, one would assume that the ICT state of L1 in acetonitrile has a lifetime of ~110 ps. Of course, this will lead to misinterpretation of the photophysics for L1 dye. It seems plausible that other organic dyes used in DSSCs can also form TICT states.

46

5. The Rhodanine (Papers V-VI)

Herein, the effect of protons on the excited state of D149 is presented. These protons come either from carboxylic acids, protic solvents, or D149 itself. The presence of protons quenches the excited state of D149 in both, sol-vents, and on ZrO2. 5.1 The Second Goal A summary is presented here about the results of papers V-VI including the effect of solvent, dimerization, interactions with acids and esterification, on the excited state lifetime of D149. Most of these effects are connected to the presence of the rhodanine moiety in D149 that is used as an acceptor group (D149's structure in Figure 4.2). Most of these phenomena were not detecta-ble by neither simple nor sophisticated techniques such as steady state ab-sorption or ultrafast TA. However, TCSPC was used successfully to monitor these phenomena. Afterwards, this knowledge was used to understand the role of protons to quench the excited adsorbed D149 on a high band gap semiconductor (ZrO2). Two interesting points initiated this project: 1) the difference of emission lifetimes for D149 in acetonitrile and methanol, 2) the different emission lifetimes of D149 in acetonitrile at different concentra-tions.

5.2 Dimer Formation and Solvent Effect D149 has been measured in different solvents showing different lifetimes without a clear reason.96 Despite the similarity of the dielectric constants of

lifetimes, ~ 300 ps, and ~100 ps, respectively.88,105 Deep investigation has revealed that D149 has different lifetimes spanning from 300 to 700 ps in acetonitrile depending on the concentration used (Figure 5.1). This depend-ence was not observed in protic solvnets such as MeOH. The range of the concentration used in Figure 5.1 is between Such a range of concentration cannot be investigated by using streak camera or TA, as high signal/noise needs a high concentration of the solute. This highlights the advantage of TCSPC to investigate such a behavior at very low concen-tration.

47

-200 0 200 400 600 800 10000

5

10

15

20

D149 in MeOH 100 nM: ( = 100 ps) 100 μM: ( = 100 ps) IRF

kCou

nts

Time(ps) Figure 5.1. The emission decay of D149 at different concentrations using TCSPC. The inset shows the integrated emission decay versus D149's concentration, where the complex-quencher model is used (left), reused from Figure 3 paper VI. The fluorescence decay of D149 in methanol at different concentrations (right), reused from Figure 2 paper V.

A dimer formation has been hypothesized between the D149 monomers in acetonitrile, where the dimer has short lifetime (~300 ps) and the mono-mer has ~700 ps. To test this hypothesis, the integrated emission decay is shown against the D149's concentration (inset of Figure 5.1). The quencher model shows a good fit (Figure 5.1). The quencher model has been based on a ground state complex between an excited D149 and an optically silent quencher "another D149" as shown below: 149 + = 149. , = [ 149. ][ 149][ ] = + . [ ]1 + .[ ]

I stands for the emission from the complex; I0 and I are the monomer emis-sion intensity at a very dilute solution and the dimer emission intensity at a concentrated solution, respectively. The extracted formation constant for D149.D149' 6 M-1, which is a high dimerization value for a carboxylic acid.115 Similar dimer formation was found in D102 and D205, but with different equilibrium constants. In addition, theoretical calculations showed the possibility of dimerization for D149 via the carboxylic groups. The dimers were found to be very stable (low relative energies) in compari-son with monomers.

An organic base (DABCU) was added to dimers of D149 in acetonitrile. As expected, the dimers have been broken and the lifetime becomes similar-ly to the monomers lifetime of D149 (Figure 5.2). D149 was measured in different solvents, where dimers exist in aprotic solvents such as acetonitrile and DMSO. On the contrary, dimers are prohibited in protic solvents such as MeOH, and water. The lifetime of the monomers depends on the hydrogen bond strength of the solvent used as shown in Figure 5.2.

48

0 2000 40000

5000

10000

15000

20000

Coun

ts

Time(ps)

D149 in:Acetonitrile onlyAcetonitrile + DABCU ( = 720 ps)Acetonitrile + Water ( = ~200 ps)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

100

200

300

400

500

600

700

800

900

H2OMeOH

EtOH

CH3CN

DMSO

THF CHCl3

Acetone i-PrOH

C6H6

CH2Cl2

CH3CNC6H6Toluene

DMSO

Life

time

(ps)

Hydrogen bond donor strength Figure 5.2. The fluorescence decay of D149 in acetonitrile (black curve), with addition of a base (red), and in water (blue) (left). The plot of D149's lifetime in different solvents vs. the hydrogen bond donor strength ( ) (right). Red values are high concentration values. These figures are reused from Figure 3, and 7 in paper III.

5.3 The Rhodanine Moiety The esterification of D149 (D149Ester) inhibits the dimer formation as the lifetimes at different concentrations in acetonitrile were identical. However, the lifetime of D149Ester is quite short (~350 ps) not 700 ps as the monomer of D149.

For further understandings, different carboxylic acids and protic solvents were added with varied concentrations to "monomers" of D149 and D149Ester. Figure 5.3 shows that the integerated emission of D149 is reduced only when water concentration is above ca. 0.1 M. However, for carboxylic acids, the reduction of the integerated emission is substantial already several orders of magnitude lower, ca. 10-4 M for formic acid. The inflection points are related to the complexation between D149 and the used acids. These complexes have lower lifetime than D149. As water cannot form a complex with D149 through carboxylic acid, so it quenches the excit-ed D149 via dynamic quenching20. For D149Ester, very similar curves are shown for all quenchers, where emission is quenched only at quencher con-centration above ca. 0.1 M. Therefore, dynamic quenching was assumed for the D149Ester. These measurements highlight two points: 1) the rhodanine ring interacts with protons, and causes a dynamic quenching process to the excited dye, 2) the presence of carboxylic acid increases the excited state quenching probability by forming ground state complexes with other carboxylic acids.

49

1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10 100500

1000

1500

2000

2500

H2O HCOOH BrAcOH ClAcOHTo

tal F

luor

. Int

ensi

ty (a

rb. u

.)

Concentration (M)1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10 100

600

800

1000

1200

1400

1600

H2O HCOOH BrAcOH ClAcOH EtOH MeOH PrOOH iPrOH iBuOOHTo

tal F

luor

. Int

ensi

ty (a

rb. u

.)

Concentration (M) Figure 5.3. Total fluorescence intensity of D149 (left), D149Ester (right) as a func-tion of different quencher concentration, reused from Figure 5 in paper VI.

5.4 On ZrO2

According to the theoretical calculations, there are different ways of dye adsorption on the semiconductor surfaces and depending on the binding modes, the electron injection and recombination can be affected.116 For the COOH group, a bridged bidentate (BB) anchoring mode has been considered the most stable adsorption mode in theoretical calculations and for the FT-IR measurements of different dyes (Figure 5.4).116 In the BB mode, the two oxygen of the COOH group adsorb on the Ti atoms of TiO2, and the proton adsorbs on a nearby oxygen site as shown in Figure 5.4. In that paper116, an organic dye, rh-L0, that has rhodanine-3-acetic acid as an acceptor group was examined. Theoretical calculations of rh-L0 showed that rh-L0 lies al-

2 surfaces.116 The distance between the TiO2 surface and the acceptor group in rh-L0 is ca. 4.0 Å116 (Figure 5.4). Similar binding modes have been shown earlier for D149, and a closer distance of 2.0 Å has been expected for D149.11,117

Previously, it has been shown that the electrolyte containing an organic 8%) rather than the

one without a base.118 Later on, all the electrolytes used with D149 in DSSCs were containing organic bases.98 All these information emphasis that D149 is probably quenched by the nearby protons on TiO2. ZrO2 was used to follow this aspect. The measurements on ZrO2 allow monitoring the excited state of adsorbed dyes without electron injection process. D149 and D149Ester were adsorbed separately under the same conditions on ZrO2. TCSPC was used to investigate these dyes on ZrO2 through their emission decays. All the fitted data show bi-exponential decays (Table 5.1). The weighted average lifetime20 is shown to facilitate the comparison between different samples. For D149 in acetonitrile, the average lifetime is ca. 0.6 ns. The addition of DABCU to the soaking path (acetonitrile) increased the average lifetime doubly to be ca. 1.3 ns instead of 0.6 ns in acetonitrile. A higher amount of

50

DABCU was also tested and similar results are shown (Table 5.1). There-fore, one can assign this increase in lifetime to the removal of adsorbed pro-tons by DABCU. For the adsorption of D149Ester, the average lifetimes are similar to D149 with basic acetonitrile, due to the absence of protons in the case of D149Ester (Table 5.1).

On the other side, D131 "cyano-acrylic as an acceptor group" was also adsorbed under similar conditions. The average lifetimes for the adsorbed D131 in acetonitrile and acetonitrile-DABCU are similar (Table 5.1). This similarity in D131 confirms that the changes in lifetimes for D149 is due to the interactivity of the rhodanine moiety with the adsorbed protons on ZrO2.

Figure 5.4. The optimized geometrical structure for the bridged bidentate (BB) an-choring mode of acetic acid on the TiO2 surfaces (left). The optimized structure of the rh-L0 dye on TiO2 using the bridged bidentate anchoring mode. These images are taken from Ref.116 (right). Reproduced from Ref.116 with permission from The Royal Society of Chemistry.

Table 5.1. Lifetimes of different dyes adsorbed on ZrO2 surfaces. Lifetimes and amplitudes are in picosecond and percent, respectively. Reused from Table 3 in paper VI.

Dye Soaking solvent 1 2 avg

D149 acetonitrile 460 ± 6 (80%) 1300 ± 40 (20%) 628 acetonitrile -DABCUa 900 ± 10 (66%) 2230 ± 23 (34%) 1352 acetonitrile -DABCUb 870 ± 5 (76%) 2330 ± 40 (24%) 1220

D149Ester acetonitrile 660 ± 6 (78%) 1900 ±18 (22%) 932

D131 acetonitrile 755 ± 4 (73%) 3050 ± 38 (27%) 1375 acetonitrile -DABCUa 1025 ± 12 (83%) 3550 ± 180 (17%) 1455 = +

a DABCU concentration is 10 times the dye concentration. b DABCU concentration is 100 times the dye concentration.

51

5.5 Conclusions Within these two papers V-VI, few points are concluded: To the best of my knowledge, this is the first time to report that

dimerization of dyes can happen at a very low range of concentration (~nM), where conventional techniques such as steady state absorption, or advanced techniques such as TA cannot detect these dimerization process. However, TCSPC technique could be used to detect these as-pects. As has been shown, dimer can have different lifetimes than mon-omers that might lead to wrong assumptions and conclusions when stud-ying such complexed systems.

The properties of solvents can induce other processes like dimerization or proton-excited state quenching for organic dyes such as D149.

The quenching of excited adsorbed dyes by adjacent adsorbed protons is likely to happen on semiconductor surfaces.

52

6. Interactions with the Electrolyte (Paper VII)

In this chapter, the influence of complexation between individual compo-nents of the standard iodine/iodide electrolyte and D149 is discussed in MeCN and on TiO2. Such formed complexes on semiconductor surfaces would affect the electron injection and recombination processes on TiO2 surfaces. 6.1 The Third Goal These substances, I-, I2, and I3

- form the standard redox electrolyte that has been used for decades in liquid based Grätzel cells.26 The mixture of I2/I- in solar cells is necessary to form I3

- ( + , = 10 . in MeCN).119,120 In DSSCs, the electrolyte is used in a high concentration (e.g. 1.0 M of I-) to afford high regeneration efficiency.121 The iodide electrolyte is characterized by a slow ROE, due to the involvement of two electrons process as shown in the following equations26,75: + = . + . + = 2

The interaction between the electrolyte and the adsorbed dye is a matter of interest. For example, it was hypothesized that the high efficiency of the N3 dye with iodide electrolyte is mainly due to special interaction of I- with N3.68,122 In contrary, complexes with the oxidized species (I2/ I3

-) may increase the ROE. 123 Therefore, unnecessary complexes between the oxi-dized species of the electrolyte and the adsorbed dye should be avoided as suggested before.81,123-125 Therefore, understanding the kinetics of D149 as an organic dye in the presence of different additives is relevant for DSSCs.

6.2 Emission Quenching in Acetonitrile As described before, D149 consists of two units, the D, and the acceptor units. Therefore, the D unit was tested to interact with the components of the redox couple. The emission from the D unit has been quenched differently depending on the nature of the quencher (I-, I2, I3

-) via static quenching. The emission of the D unit was quenched strongly in the presence of I3

- com-pared to other quenchers (Figure 6.1). Linear fits were used to fit the ratio of the emission intensities (I0/I) against the concentration of I- and I2 according

53

to Stern-Volmer equation ( = (1 + [ ]) ) with quenching constants of 1000 M-1 and 44600 M-1, respectively.20 While a bad linear fit was found in the case of I3

- (Figure 6.1 B). However, the quenching sphere of action mod-el was successful to fit the obtained data quite well for I3

- than the linear fit model.20 In the sphere model, there is no real ground-state complex is being formed, instead, an electrostatic interactions is assumed.20 The quenching constant for D with I3

- at 25 C is 260,000 M-1. I3- is expected to be an oxida-

tive quencher for the D excited state, which means extracting an electron from the D excited state (D*):

+ + +

As can be seen, the complexes formed between the D unit and the oxidized species (I2 and I3

- ) are more favourable than with than the reduced species (I-), which is not desired in DSSCs.

450 500 550 600 650 7000

500

1000

Em

issi

on In

t. (k

Cou

nts)

Wavelength(nm)

Increasing triiodide

(A)

1μ 10μ 100μ

1

10

I2 (kq= 44600 M-1) I- (kq= 1090 M-1) I3

- (kq= 550,000 M-1) Fits

I 0/I

Conc. of additive (M)

(B)

Figure 6.1. (A) The emission intensity from D in the absence (Black line) and the presence of different concentrations of I3

- (Gray lines); emission from I3- is the red

line. Concentration of D was -Volmer plots for D moiety with different additives concentrations. The concentration range of D moiety locates between 11- various additives. Excitation was at 400 nm. Reused from Figure 2 in paper VI.

For the D149Ester, similar experiments to D were done. Static quenching mechanism matches the emission quenching of D149Ester in all cases (Figure 6.2). I2 and I3

- quenched the excited state of D149Ester in a similar way with equilibrium constants of 15600 M-1 and 14700 M-1, respectively. Whereas I- quenched the excited state of D149Ester in a weaker fashion with a binding constant of 2,900 M-1.

For D149 in MeCN, the observed quenching behaviors were more com-plicated than in D149Ester due to the presence of D149 dimers.105,126 With all the quenchers, there were losses in the emission intensities and noticeable decrease in the lifetimes of D149. The dynamic quenching process was not expected due to the low concentration used and the short lifetime of D149.

54

Also, the reduction of the lifetime for D149 reaches to certain values with a plateau (Figure 6.2), which indicates formation of new complexes with new lifetimes. The total emission decay represents the lifetime of the monomers and dimers of D149. The binding constants of D149 with different quenchers were calculated using the same equation for calculating the binding constant of chloroacetic acid with D149105 and in paper VI (Table 6.1).

1μ 10μ 100μ0.8

1.2

1.6

2.0

2.4

2.8

I2, ks= 15.6 K M-1

I-, ks= 2.9 K M-1

I3-, ks= 14.7 K M-1

FitsI 0/I

Concentration of additives (M)1E-6 1E-5 1E-4

1.51M

1.60M

1.68M

1.76M

1.85M

1.93M

2.02M

2.10M

I-

I2 I3

-

Inte

gera

ted

emis

sion

Additive concentration (M) Figure 6.2. Stern-Volmer plots for D149Ester with different additives. Excitation was at 400 nm (left). Integration of emission decay for D149 vs. different concentra-tion of additives (I-, I2, and I3

-) (right). Reused from Figure 4 and 5 in paper VI.

Table 6.1: The quenching constants (M-1) for different combinations of dyes and quenchers at room temperature. Reused from Table 1 in paper VI.

Fluorophore/Additive I2 I- I3-

Type Value Type Value Type Value D (SQ) 44700 (SQ) 1090 (SQ) 260000 D149 (BC) 250350 (BC) 201380 (BC) 20100 D149Ester (SQ) 15600 (SQ) 2900 (SQ) 14700

SQ (static quenching), BC (binding constant, lower emissive complex)

Moreover, Figure 6.3 shows a transient absorption measurement for the D149-I2 complex in MeCN. The spectrum of the complex, taken at 50 ns after excitation, resembles the spectrum of D149(+) on ZnO48, probably due to formation of charge transfer complex and the oxidation of D149. The kinetic trace at 620 nm shows that this charge transfer complex lives longer than ca.

Figure 6.3).

55

Figure 6.3. Kinetic trace at 620 nm for D149 (30 μM) with I2 (130 μM) in MeCN. Difference spectrum at 50 ns for the D149/I2 complex is shown in the inset. Excita-tion was 530 nm. Reused from Figure 6 in paper VI.

6.3 Monitoring electrons inside TiO2 In this part, the obtained data are shown for probing the electrons inside the CB of TiO2 after exciting the adsorbed D149 by visible light (~525 nm), and probing by IR light (centered at ca. ~5000 nm). The sensitized films were prepared under the same conditions to ensure the adsorption of D149 complexes on the TiO2. Figure 6.4 shows the kinetic profile for the injected electrons from D149 (black trace). The electron injection has two lifetimes, 450 fs (47%), and 32 ps (53%). The ROD is slow and could not fully followed by our setup. The recombination lifetimes are 920 ps (16%), and a long component (84%). Recently, bi-exponential components (>300 fs and 33 ps, with ca. equal amplitudes) were used to fit the electron injection of D149 on TiO2.53 The obtained date from the IR probes are different from the UV-Vis probe48,127 for following the electron injection of D149 on low band gap semiconductors. In these studies using UV-Vis probe49,134, the electron injection rates were (150 fs)-1 and (250 fs)-1 on ZnO and TiO2, respective-ly.49,134

For the adsorbed D149/I3- complex, the electron injection rate is faster

than the adsorbed D149 (Table 6.2). This observation shows probably that a new competing process occurs, mainly due to the expected electron transfer to I3

-. Also, the ROD is clearly faster, where the amplitudes of the long com-ponent are reduced to 60%, and the first lifetime becomes 400 ps (Table 6.2). For the D149/I2 complex, more effects are shown: 1) one main compo-nent for the electron injection disappears; 2) the electron recombination be-comes faster (Table 6.2). Surprisingly, for D149/I- complex, the electron injection lifetime is <100 fs, and the ROD becomes faster than in the D149+

56

0 200 400 600 800 1000 1200 1400 1600 1800

-0.08

-0.06

-0.04

-0.02

0.00

0.02

Kinetic trace at 620 nm for D149/I2 in MeCN

Time (ns)

400 500 600 700

-0.05

0.00

0.05

D149/I2 in MeCN at 50 ns after excitation

Wavelength (nm)

KT

(Table 6.2). The D149+/I- complex has a faster recombination than D149+, this has been detected before on the scale with some Ru-complexes that interact with I-68. However, this sample was not standing for measuring many scans, so degradation effects could not be excluded. After knowing the kinetic profile of each complex, the electron kinetics of D149 on TiO2 was measured in contact with a full electrolyte prepared as reported previously.98 The lifetimes are summarized in Table 6.2. Indeed, it is hard to assign each lifetime to its complex on the surface.

Table 6.2. Summary of lifetimes in ps probed at 5000 nm for the kinetics of elec-trons inside the CB of TiO2. Amplitudes are shown in % in parentheses.

Dye Electron injection Electron recombination 1 (%) 2 (%) 1 (%) 2 (%) 3 (%)

D149 0.45 (47) 32 (53) 920 (16) ----- D149/I3

- 0.14 (85) 3.5 (15) 400 (40) ) ----- D149/I2 0.22 (100) ----- 35 (50) 290 (20) Long ) D149/I- <0.1 (100) ----- 2.0 (35) 30 (36) 800 (29)

Electrolyte 0.22 (100) ----- 35 (25) 770 (72) )

0 10 100 1000 10000

400 ps 40%+ long 60%

3.5 ps, 15%

140 fs, 85%290 ps, 20 % + long 30 %

920 ps 16%+ long 84%

32 ps, 53%

Inte

nsity

A

Time (ps)

D149 D149/I3

-

D149/I2

450 fs, 47%

220 fs

35 ps, 50 %

0 10 100 1000 10000

2 ps, 35 % 45 ps, 36%800 ps, 29%

<100 fs

Inte

nsity

A

Time (ps)

D149 D149/I-

D149/Full electrolyte

725 ps, 72 %+ long , 3%

920 ps 16%+ long 84%

32 ps, 53%

450 fs, 47%

220 fs

35 ps, 25 %

Figure 6.4. Electron kinetics inside the conduction bands of TiO2 probed at 5000 nm. Different D149 complexes were adsorbed on the TiO2 surfaces. Comparison of electron kinetics of D149, D149/I2, and D149/I3

- (left). The comparison with the full electrolyte, and D149/I- (right). All kinetic traces are normalized at 1.0 ps. Reused from Figure 8 in paper VI.

6.4 Conclusions From the above results, several points are interesting to stress and conclude: Apparently, the IR probe is more accurate than UV-Vis probe to follow

the electron dynamics in Grätzel cells. The electron kinetics can be different depending on the conditions used,

the composite of the electrolyte and the additives used.

57

complexes with I2 and I3- are dangerous to be formed on semiconductor

surfaces. These complexes can affect the electron dynamics in the fs to ps time scales.

Apparently, this is the first time to figure out that such complexes on surfaces can affect the kinetics of the electron injection, not only the electron recombination, in contrast to what has been assumed to the independence of injection kinetics in the presence of I-/I3

- redox electrolyte.50

Even though faster electron injection processes happen with the complexes of D149, one cannot expect a better performance of these complexes due to the fast ROD.

Previous studies have shown that binding of I- to the adsorbed dyes should improve the regeneration step.68,128 Although D149 shows such a binding with I-, the binding towards the oxidative species as I2 can illus-trate the fast recombination for the D149/I- complex on TiO2. These oxi-dative species can adsorb on the adsorbed dyes and accelerate the ROD. This adsorption would also hinder the diffusion of the oxidized species to the counter electrode and closing the circuit in DSSCs.

Other dyes as well can form adsorbed complexes on the semiconductor surfaces, giving different efficiencies with various electrolytes and additives, not only relying on the energy levels of the redox couple. Therefore, putting general rules for electron kinetics inside the DSSCs may not be fully correct all the time, as dye's structure can influence the discussed mechanisms.

58

7. Linked Ferrocene (Paper VIII)

In this chapter, the effect of ferrocene as a strong reductive moiety linked to the L1 dye is discussed. The aim of such a system was to follow the hole transfer from the oxidized dye (L1+) to the ferrocene moiety, aiming to increase the regeneration rate, and deacreas the ROD. This system was studied in acetonitrile and on semiconductor surfaces. 7.1 The Final Goal L1 was tested in DSSCs showing a reasonably good efficiency (~ 5.2%).114,129 As mentioned before, the regeneration process is diffusion lim-ited and usually takes a long -ms) to occur. One of the methods to improve the efficiency of the working cell is to increase the quasi-Fermi level, one can think about increasing the rate of the electron injection or/and decreasing the charge recombination (reduce the dark current).64,77 There is a difference of 200 mV that can be gained from the difference between the CB of the semiconductor and the quasi-Fermi level.77 Since, the electron injec-tion has been assumed to be optimized47, decreasing the electron recombina-tion seems to be more important. Filling the hole on the oxidized dye through a bond was shown to be more efficient than an electron transfer through space in the quinone-porphyrin systems.130

Therefore, two new dyes were synthesized. The first one was connected covalently to one ferrocene (L1Fc) and the second one to two ferrocenes (L1Fc2) (see Figure 7.1 for chemical structures). Ferrocene has been ex-pected to reduce the oxidized dye directly after charge injection that would lead to a longer distance between the opposite charges.

59

Figure 7.1. Chemical structures of L1, L1Fc, and L1Fc2 dyes. 7.2 Steady State Measurements Figure 7.2 shows the absorption and emission measurements for the studied dyes. The red-shifted emission band of L1 at high concentration is due to TICT state as shown before in paper IV. Although the linked dyes with Fc(s) show similar concentration dependence in the absorption spectra (formation of dimers), the emission of L1Fc and L1Fc2 are located at ca. 550 nm similar to monomers of L1 in acetonitrile. Therefore, ICT state is expected for L1Fc, and L1Fc2. This difference emphasizes the reductive power of Fc can hinder the TICT process. Also, the emission from L1Fc2 is very weak probably due to the stronger quenching of L1 by two Fcs.

Figure 7.2. Absorption (solid lines) and normalized emission spectra (dashed lines) of L1, L1Fc, and L1Fc2 in MeCN.

60

300 400 500 600 7000

5000

10000

15000

20000

25000

30000

Mol

ar A

bsor

ptiv

ities

(M-1 c

m-1)

Wavelength(nm)

L1 L1Fc L1Fc2 L1_monomer

Norm

alized Em

ission

The quantification of the oxidation potentials for these dyes was done by cyclic voltammetry (CV) as shown in Figure 7.3. The L1Fc has two oxida-tion peaks that match quite well the oxidation peaks of Fc and L1. This simi-larity shows that the covalently linked Fc has not affected much the electron-ic properties of L1 in the ground state and using L1 can be a good reference system. Moreover, this shows that Fc can reduce the expected hole in the ground state of L1 when injecting electrons to TiO2 (Figure 7.4). As the potential energy of the Fc+/Fc is more negative than the one of the L1 dye by 0.6 volte, the rate of ROD is expected to be lower for the linked dye with Fc. The absorption spectrum for L1+ at oxidation potential of 1.2 V is shown in Figure 7.4. As can be seen for the L1+ absorbance, the S1 band of L1 is depleted, and a strong absorption band appears near to the IR region due to the formation of TPA+.131 This band is crucial to follow the oxidized species of L1 on TiO2 surfaces.

-0.5 0.0 0.5 1.0

Cur

rent

(a.u

.)

E(V) vs. Ag/Ag+

Ferrocene L1 L1Fc

200 400 600 800 10000.0

0.5

1.0

1.5

2.0

2.5

Abs

orba

nce

Wavelength(nm)

L1

L1+

Figure 7.3. The cyclic voltammograms of the working dyes L1, L1Fc, and free fer-rocene as a reference in MeCN, after nitrogen purge for 20 min (left). The absorp-tions for the neutral and mono oxidized forms of L1 in MeCN at 1.2 V (right).

Figure 7.4. A schematic representation of the energy levels of L1 and L1Fc versus normal hydrogen electrode (NHE).

61

7.3 Time-Resolved and Solar Cell Measurements 7.3.1 In MeCN TA was used to study these dyes in the visible range. The obtained spectra for the L1 dyes are shown in Figure 7.5. Figure 7.5 shows also the extracted kinetic traces for L1, L1Fc, and L1Fc2 in MeCN at 600 nm (ESA). For L1Fc and L1Fc2, the traces show considerably short lifetimes. Multi-exponential decays were used for fitting the kinetic traces (Table 7.1). The lifetime of ICT state of L1 is ~1.5 ns (paper IV), while it is reduced drastically to an average timeXX of ca. ~ 1.5 ps in L1Fc and ~ 0.75 ps in L1Fc2. These later lifetimes represent the recombination lifetimes between the reduced L1 (L1-

), and the Fc+, not to the reduction process of L1* by Fc since L1-/Fc+ should live longer than the L1*. The reduction process of L1* by Fc has been as-sumed to be an ultrafast process (<50 fs), which could not be detected. Although these data are quite surprising and not expected, this ultrafast reduction process is reasonable since the driving force between the ground state of Fc and L1 is ca. 0.6 eV. Indeed, the recombination between L1- and Fc+ is an ultrafast process probably due to the instability of the charge sepa-ration state between the closely nearby moieties. Therefore, Fc moiety could quench two processes in L1, TICT and ICT. The recombination process has been enhanced when the positive charge was distributed on two ferrocene moieties as shown in L1Fc2. More experiments are needed to detect the re-duced species of L1 near to the UV region. Figure 7.6 shows a summary of the kinetic processes of L1 dyes in MeCN.

0 1 10 1000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Time (ps)

L1 L1Fc L1FC2

@ 600 nm

(A)

450 500 550 600 650 700

Nor

mal

ized

A

Wavelength (nm)

L1 L1Fc L1Fc2

(B)

Figure 7.5: (A) Kinetic traces of L1, L1Fc, and L1Fc2 at 600 nm extracted from the 2D plots of TA. (B) Normalized spectra at 525 nm for L1, L1Fc, and L1Fc2 dyes at time zero.

XX = 10( . ( ) )

62

Table 7.1. Summary of lifetimes obtained from global fitting procedures using TA for L1, L1Fc, and L1Fc2 in MeCN

Dye 1 (ps) 2 (ps) 3 (ps) L1 0.21 (-47%) 0.76 (18%) 99 (35%) L1Fc --- 0.56 (81%) 59 (19%) L1Fc2 --- 0.75 ---

Figure 7.6: Schematic representation of the kinetic processes of L1 dyes in MeCN followed by TA.

7.3.2 On TiO2 To mimic the conditions in DSSCs, L1 and L1Fc dyes were adsorbed on TiO2. Figure 7.7 shows the 2D plots for the L1 and L1Fc on TiO2 after 400 nm excitation. The extracted kinetic traces for L1 on TiO2 at 530 nm (GSB) and 610 nm show long-lived species that are due to the formation of L1+ after an electron injection of ca. 1.0 ps. More importantly, the behavior was different for L1Fc on TiO2, where the long-lived species were detected only at 550 nm (GSB) after an electron injection of ca. 100 fs. On the contrary, short-lived species were detected at 650 nm with an average lifetime of ca. 11 ps. Figure 7.8 shows the normalized kinetic traces for L1 and L1Fc at the mentioned wavelengths. From the normalized traces, one can see the similarities between the GSBs, but not for the ESAs due to the electron transfer from Fc to L1+, and the formation of TiO2

(-)—L1—Fc(+). In addition to the change of lifetimes, Figure 7.9 displays the similarities

between the shape of transient absorption measurements extracted from TA and the spectroelectrochemistry measurement for L1. The shifts between the two spectra can be attributed to the Stark effect.132 However, these features are absent for the L1Fc on TiO2 at the same selected time, 100 ps (Figure 7.9). Unfortunately, the transient signal from the Fc+ is expected to be extremely weak ( -1 cm-1 at 620 nm).133

63

Figure 7.7. 2D plots for the adsorbed L1 and L1Fc on TiO2.

1 10 100 1000

L1 530 nm 610 nm

L1Fc 550 nm 650 nm

Time (ps)

Figure 7.8. Normalized kinetic traces for L1 (red) and L1Fc (blue) at the GSB (dashed), and transient species (solid) regions.

500 520 540 560 580 600 620 640 660 680 700

-10000

-5000

0

5000

10000

15000

Mol

ar a

bsor

ptiv

ity (M

-1 c

m-1)

Wavelength (nm)

for the L1+ at 100 ps for the L1+ at steady state

500 550 600 650 700

Nor

mal

ized

spe

ctra

Wavelength (nm)

L1 L1Fc

Figure 7.9. A comparison between the L1+ spectra from the spectro-electrochemistry, and from the transient absorption measurement at 100 ps (left). A comparison between the transient normalized spectra of L1 and L1Fc on TiO2 at 100 ps (right).

64

7.3.3 Solar Cell Performances The TA data were very promising towards the achievement of the main idea. Therefore, the solar cell performances of L1, L1Fc, and L1Fc2 were done using standard electrolyte (iodine/iodide). The performance of L1 resembles the reported one114 with efficiency of 5.5 %. However, the efficiencies of L1Fc and L1Fc2 are lower than L1; L1Fc and L1Fc2 gave efficiencies of 1.3%, and 0.75 %, respectively (Table 7.2).

Table 7.2. Summary of the obtained results for the solar cell performances. VOC (mV) JSC (mA/cm2) ff (%) (%) L1 692 ± 2 10.4 ± 0.1 75.8 ± 0.5 5.5 ± 0.1 L1Fc 582 ± 2 3.7 ± 0.2 59.4 ± 0.4 1.3 ± 0.1 L1Fc2 570 ± 3 2.4 ± 0.1 53.4 ± 0.8 0.7 ± 0.1

Exploring these results, one can figure out that the main loss comes from

the produced current (65% loss compared to photocurrent of L1). This loss could be due bad electron injection or fast ROD. To understand these as-pects, TA-IR was used to monitor the electrons in the CB of TiO2 as shown before in paper VII. Figure 7.10 shows the electron kinetics for L1 and L1Fc dyes on TiO2. For L1, the electron injection has occurred in ~ 440 fs, and the ROD has occurred in a slow fashion with two lifetimes, 530 ps (16%), and >12 ns (84%). For L1Fc, the electron injection was faster than in L1 with a lifetime of ~110 fs. However, contrary to L1, the ROD for L1Fc was very fast with an averaged lifetime of ~ 300 ps. With a rough estimation, the loss in the IR signal of L1 is ca. 30% from maximum, however, it is ca. 80% for L1Fc (Figure 7.10). Multiplying the photocurrent produced for L1Fc by 2.6 gives an estimated photocurrent of 9.9 mA/cm2 that is very close to the L1 (Table 7.2).

2 4 10 100 1000

Nor

mal

ized

Kin

etic

s

Time (ps)

L1 L1Fc

Figure 7.10. The kinetic traces at 5000 nm for the electrons in the CB of TiO2 after electron injection process from adsorbed L1 and L1Fc dyes.

65

This fast recombination in L1Fc can illustrate the low performance of L1Fc, despite the detected strong reductive quenching of Fc using TA. Ag-gregation was suspected for L1Fc dye that can cause this fast recombination. However, different additions of CDCA as an anti-aggregation agent were tested and tiny enhancements were observed in the solar cell performances and the recombination process. Other possibilities for such behavior can arise from the nature of dyes alignment on the TiO2 surfaces, assuming that the L1 dye is attached perpendicularly to the surface, whereas the L1Fc attaches horizontally to the surface due to the asymmetric shape of the molecule. The horizontal orientation of L1Fc or L1Fc2 would increase the rate of recombination due to the short distance between the electron (in TiO2) and holes (on the dye). Previously, the orientation between the D "quinone" and the A "porphyrin" units has been shown to affect the electron transfer rates.130 Also, it has been shown in paper IV that L1 does TICT, however, L1Fc and L1Fc2 do not show TICT, which can illustrate the faster recombination for the L1Fc case. The difference in energy levels between ICT and TICT can also illustrate the faster injection of L1Fc (~110 fs) than in L1 (~440 fs). It has been shown recently that dyes doing TICT have slower recombination than others do not.134 The last assumption can be related to the Marcus inverted region, which means that although the energy difference between the electrons in the CB and the holes on the Fc+ is smaller than the difference between the electrons in the CB and the holes on the L1+, the rate of recombination is faster in the former case. However, further studies are needed to clarify these questions.

7.4 Conclusions The impact of the linked Fc moiety to an organic dye (L1) was studied

aiming to afford an ultrafast regeneration process for the holes present on the L1 dye after electron injection to TiO2. The aim was quite successful in ace-tonitrile and on TiO2 as shown by TA and TA-IR. The benefit of the Fc+/Fc moieties as optically inactive substances in the studied regions makes the detection of the L1+ species and the injected electrons more clear to be fol-lowed and interpreted. However, the performance of these modified dyes in DSSCs is not better than the original dye due to a faster recombination process to the Fc+ moiety than to L1+. The fast recombination to the Fc+ can be due to different assumptions which are not clear at the moment which one is more important. In a summary for the kinetic processes of L1Fc on TiO2, the electron injection occurs in ~110 fs (~440 fs for L1), followed by a regeneration form the Fc moiety in ~11 ps, followed by a fast recombination process to the Fc+ in ~300 ps instead of >12 ns to the L1+.

66

General Conclusions and Outlook

The Grätzel cell is a promising technology that might be applied in the near future on a wide scale to convert the sun's energy into electricity or fuels. Metal-free organic dyes are good candidates to be used in Grätzel cells due to cost and environmental aspects. On the other hand, organic dyes suffer from various deactivation processes that hinder the way to obtain high effi-ciencies.

In this thesis, previously unknown deactivation processes have been illustrated for various kinds of well-known organic dyes used in Grätzel cells. These dyes include D149, D131, L0, L0Br, and L1. These dyes have shown relatively high efficiencies in comparison with other organic dyes. These deactivation processes were unravelled with the help of different time-resolved spectroscopic techniques such as time-correlated single photon counting, streak camera, and femtosecond transient absorption measure-ments. The results and discussions within the thesis were distributed in four parts.

The first part of the thesis discusses the mechanical motions that happen in organic dyes under irradiation in solutions and on semiconductor surfaces. These motions can occur at different parts of the dye structure, such as the donor, and the linker units. Blocking these motions would enhance the elec-tron injection quantum yield, and avoid unwanted recombination processes to the oxidized species of the electrolyte. However, other processes such as TICT are expected to be beneficial to reduce the recombination process to the oxidized dyes on surfaces. Therefore, smarter and more careful design principles are highly needed for the synthesis of organic dyes used in DSSCs.

The second part shows the importance of selecting acceptor and how the selection can induce new photophysical properties of the dye used in solu-tions and on semiconductor surfaces. The rhodanine moiety has been found to interact with protons leading to quenching of the excited state of the dye. This quenching happens in protic solvents, either through dimerization, or via complexations with carboxylic acids. This quenching can happen on surfaces leading to a lower electron injection quantum yield.

The third section uncovered the possibility of complex formation on sem-iconductor surfaces between the adsorbed dyes, such as D149, and the stand-ard electrolyte in Grätzel cells. These complexes have direct effects on the

67

electron dynamics on semiconductor surfaces such as electron injection and recombination processes. The exact structures of these complexes are not known yet, and further investigations are needed.

As can be seen, within the first three parts, D149 is considered the primar-ily investigated dye, which has been studied from different angles. Different photophysical and photochemical processes are present in D149, including photoisomerization, twisting, an excited state quenching by protons, and complex formation with iodide/triiodide electrolyte. Although D149 has shown a very good efficiency compared to other organic dyes, the presence of these deactivation processes is hoped to inspire the community for further improvements of dyes' chemical structures.

The final part presentes the possibility of using ferrocene moiety (Fc) as an ultrafast regenerator for organic dyes on semiconductor surfaces. Howev-er, the fast recombination process to the ferrocenium (Fc+) moiety was a significant limiting step, and further investigation are needed for better un-derstanding.

Last but not least, this thesis opens up for new ideas, questions and chal-lenges in the field of dye-sensitized solar cells. It is hoped that the outcomes from this thesis can help for future developments of organic dyes used in Grätzel cells.

68

Svensk sammanfattning

Grätzelceller är en lovande teknik som kan tillämpas i stor skala i en nära framtid för att övervinna bristen på energiresurser. Organiska färgämnen är möjliga kandidater för användning i Grätzelceller på grund av låg kostnad och miljöaspekter. Å andra sidan så lider organiska färgämnen av olika de-aktiveringsprocesser som hindrar att höga verkningsgrader uppnås.

I denna avhandling har en delhittils okända deaktiveringsprocesser visats och diskuterats för olika typer av välkända organiska färgämnen som an-vänds i Grätzelceller. Dessa färgämnen har olika donor- och acceptorenheter. Bland dessa har vi D149, D131, L0, L0Br och L1 som har en relativt hög verkningsgrad i jämförelse med andra organiska färgämnen. Dessa upptäck-ter gjordes med hjälp av olika tidsupplösta spektroskopiska tekniker som bygger på laserspektroskopi såsom tidsupplöst enfotonräkning, streak-kamera och femtosekunds pump-probmätningar med ultravioletta, synliga och infraröda detektorer. Resultat och diskussioner inom avhandlingen för-delades i fyra delar.

Den första delen av avhandlingen diskuterar mekaniska rörelser som sker i organiska färgämnesmolekyler under bestrålning i lösningar och på halv-ledarytor. Dessa rörelser kan ske i olika delar av färgämnesmolekylen, t.ex. donor- eller acceptorenheterna, men önskas blockeras i syfte att öka elektro-ninjektionskvantutbytet och oönskade rekombinationsprocesser till det oxi-derade tillståndet av elektrolyten. Emellertid är andra processer såsom TICT fördelaktiga för att reducera rekombinationsprocessen till oxiderade färgäm-nesmolekyler på ytor. Detta kan i sin tur leda till smartare design och syntes av färgämnet.

Den andra delen visar vikten av att välja bra acceptorenheter i färgämnen såsom en rodaninenhet och hur valet kan framkalla nya fotofysikaliska egen-skaper hos färgämnen som används i lösningar och på halvledarytor. Rodan-inenheten visar sig vara känslig för närvaro av protoner som leder till ut-släckning av det exciterade tillståndet av färgämnet. Denna utsläckning kan inträffa på ytor och leder till ett lägre elektroninjektionskvantutbyte.

Den tredje delen avslöjar möjligheten till komplexbildning mellan adsor-berade färgämnen på halvledarytor, såsom D149, och standardelektrolyter som används i Grätzelcellerna, såsom jodid/trijodid. Dessa komplex har en direkt effekt på elektrondynamiken på halvledarytor såsom elektroninjektion och rekombinationsprocesser. De exakta strukturerna hos dessa komplex är

69

inte kända ännu. Detta är emellertid fortfarande ett mycket intressant ämne öppen för fortsatta undersökningar.

Som kan ses inom de tre första delarna är D149 det färgämne som hu-vudsakligen undersökts och har studerats från olika infallsvinklar. Olika fotofysikaliska och fotokemiska processer i D149 har presenterats, inklusive fotoisomerisering, vridning, utsläckning med protoner och komplexbildning med jod/trijodid som elektrolyt. Även om D149 har visat en mycket god verkningsgrad, kan dessa inaktiveringsprocesser inspirera till ytterligare förbättringar av andra färgämnens kemiska strukturer.

Den sista delen för fram möjligheten att använda en ferrocenenhet som en ultrasnabb regenerator för organiska färgämnen på halvledarytor. Men den snabba rekombinationsprocessen till ferroceniumdelen är ett viktigt begrän-sande steg, och ytterligare undersökningar behövs för bättre förståelse.

Sist men inte minst, öppnar denna avhandling upp för nya idéer, frågor och utmaningar om färgämnessensiterade solceller. Förhoppningen är att resultaten från denna avhandling kan bidra till den framtida utvecklingen av Grätzelcellerna, främst de organiska färgämnen som används.

översatt av Mohammad Mirmohades

70

Arabic Summary ( )

(Grätzel Cell)

.

D149,

D131, L0, L0Br, L1( .

.

(Donor moiety)

(Acceptor moiety) (Electron injection)

(recombination) (TICT)

(recombination) .

(Rhodanine moiety)

.

D149

71

.

.

(Ferrocene) (Ultrafast regeneration)

(Ferrocenium)

.

.

2015 -

72

Acknowledgements

First, praise to Allah, who gave me the power and ability to do this work. I hope this work will be used to severe the human being for the good sake. Several people have contributed directly and indirectly to this work, and I hope to mention them all.

Dr. Burkhard Zietz, my actual practical supervisor, I cannot imagine that four years have passed like that, of course, the good times fly quickly. I have learned a lot from you including, scientific argumentation, writing, discus-sion, and patience.

Dr. Ana Morandeira, my scientific big sister, your kindness, your smile, and your mediterranean characters helped me a lot.

Prof. Leif Hammarström, my official main supervisor, you inspired me a lot through your science, kindness, modesty, support, and your great help all the time.

Prof. Emad Mukhtar, thanks for many things, probably without your care of my email, I would not be here in Uppsala.

Dr. Starla Glover, I bothered you a lot with my papers and this thesis, you have not only revised my English, but also the science via your nice com-ments and questions.

Thanks for the laser team, you were helpful all the time, generous, and smart. Thanks to Dr. Erik Göransson, Dr. Allison Brown, Jens Föhlinger, and Dr. Liisa Antila. Special thanks to Dr. Jonas Petersson, in addition to the laser help, for his assistance with introducing me to the Swedish system.

Thanks for the nice roommates during these years, Dr. Jonas Sandby Lis-sau, Dr. Abhinandan Makhal, Dr.Marc Bourez, and Ben Johanson. Thanks to the Egyptian taste from my brother Dr. Mohamed Qenawy. Special thanks to the kind, extremely helpful, and the best roommate, Mohammad Mirmohades.

Thanks to our helpful group members, the previous, and the current ones. Thanks to Dr. Todd Markle, Dr. Daniel Streich, Pedram Ghamgosar, Dr. Prateek Dongare, Dr. Tianfei Liu, Luca D'Amario, Lei Zhang, Shihuai Wang, Dr. Reiner Lomoth, Dr. Jacinto Sa, Dr. Daniel Fernandes, Mariia Pavliuk, and Dr. Edgar Mijangos,

Thanks to our head of chemistry department, Prof. Jan Davidsson.

73

Thanks to the CAP people, you have widened my view of science a lot. You are also a lot, so I would mention the leaders only, Prof. Stenbjörn Styr-ing, Prof. Peter Lindblad, Prof. Ann Magnuson, Prof. Sascha Ott, and Prof. Villy Sundström.

I would like to express my deep thanks to the co-authors of my papers during my Ph.D., Dr. Andreas Orthaber, Viktor Johansson, Dr. Erik Gabrielsson, Prof. Licheng Sun, Prof. Lars Kloo, Dr. Daniel Roca-Sanjuan, Dr. Martin Karlsson, Dr. Mariachiara Pastore, Dr. S. Agrawal, Prof. Filippo De Angelis, and Dr. Jiayan Cong,.

Many thanks for Dr. Dongqin Bi, Prof. Gerrit Boschloo, and Prof. Anders Hagfeldt for giving me the opportunity to work with perovskite.

Special thanks to other cool and helpful people, Dr. Somnath Maji, Dr. Hemlata Agarwala, Dr. Patrícia Raleiras, Dr. Felix Ho, Dr. Fikret Mamedov, Dr. Anders Thapper, Dr. Giovanny Parada, Sonja Pullen, Dr. Erik Johansson, Dr. Haining Tian, and Hanna Ellis

Thanks for the hidden help from the administrators, wise Susanne, nice Åsa, singer Jessica, and smart Sven.

Thanks for the nice Egyptian and Arab friends, Wael Kamel, Shady Younis, Taha Ahmed, Dr. Assem Abu hatab, Dr. Ehab, Dr. Islam, Farid El-masry, Omar Sarah, and many others….

Thanks to my neighbors, Lotti Fred, Tjana, Phillip Hirsch, and Andrea. Many thanks for the all people I have met and enjoyed my time with them

here in Uppsala. Thanks to my previous supervisors in my previous studies, Prof. Elham

Hashem, and Dr. Fabrizio Messina. Thanks to Dr. Omar Mohammed for all the help, he has done for me.

Thanks to my parents, any good thing I do because of you, you have suf-fered a lot for my life; I hope that Allah may bless you anytime, anywhere. I love you. Thanks to all members of my Family, you give me a lot of power.

Thanks to my kids, Hala, Salma, and Nour for disturbing me all the time . The best thing in the world is to see you laughing.

Thanks to the Unknown Soldier, the most beautiful soldier, who arranges our house, takes care of every single piece relating to me, may Allah bless you my lovely wife, Asmaa.

Please forgive me if I miss to mention some of you, I have a poor memory .

Ahmed Maher El-Zohry Uppsala 2015

74

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(4) Agency, I. E. 2010, p 1-738. (5) Styring, S. Artificial Photosynthesis for Solar Fuels. Faraday

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Acta Universitatis UpsaliensisDigital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1294

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A doctoral dissertation from the Faculty of Science andTechnology, Uppsala University, is usually a summary of anumber of papers. A few copies of the complete dissertationare kept at major Swedish research libraries, while thesummary alone is distributed internationally throughthe series Digital Comprehensive Summaries of UppsalaDissertations from the Faculty of Science and Technology.(Prior to January, 2005, the series was published under thetitle “Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology”.)

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