ACTAUNIVERSITATIS
UPSALIENSISUPPSALA
2018
Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1645
Shining Light on Molecules
Electron Transfer Processes in Model Systems forSolar Energy Conversion Investigated by TransientAbsorption Spectroscopy
JENS FÖHLINGER
ISSN 1651-6214ISBN 978-91-513-0273-7urn:nbn:se:uu:diva-343443
Dissertation presented at Uppsala University to be publicly examined in Siegbahnsalen,Lägerhyddsvägen 1, Uppsala, Friday, 4 May 2018 at 09:15 for the degree of Doctor ofPhilosophy. The examination will be conducted in English. Faculty examiner: ProfessorBenjamin Dietzek (Jena University).
AbstractFöhlinger, J. 2018. Shining Light on Molecules. Electron Transfer Processes in ModelSystems for Solar Energy Conversion Investigated by Transient Absorption Spectroscopy.Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science andTechnology 1645. 74 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0273-7.
In the recent years, solar energy conversion has attracted a huge research interest due to thepotential application for limiting the greenhouse effect. In many solar cells and solar fuel cells,understanding of charge transfer (CT) and recombination is important for future improvementof the overall efficiency. One important tool for that is transient absorption spectroscopy (TAS).
Mesoporous nickel oxide films were investigated due to their potential application in p-type dye-sensitized solar cells (DSSCs), tandem DSSCs and dye sensitized solar fuel cells(DSSFC:s). Firstly, it was found that the hole generated by band gap excitation is trapped onan ultrafast time scale by Ni3+ states. It was possible to observe a direct signal from the holesby transient mid-IR absorption spectroscopy allowing for direct detection of hole injection andtrapping. On a ns time scale, the trapped holes relaxed to much less reactive holes which favoredlong lived NiO-dye charge separation (CS).
A series of perylene monoimide (PMI) dyes with different anchoring groups was studied.Differences in binding affinity and stability were found. Nevertheless, all PMIs showedultrafast charge separation and similar recombination kinetics. Furthermore, the effect of MLCTlocalization of ruthenium polypyridyl complexes was investigated. All those dyes showed slowor no hole injection. At the same time, a self-quenching process was found for all compoundsthat limited the photoconversion efficiency.
Furthermore, a new core-shell structure of p-type DSSCs was proposed and investigated.Here, the liquid electrolyte was replaced by a layer of TiO2. That system was found to undergoboth injection and regeneration of the dye on an ultrafast time scale (below 1 ps). Furthermore,the CS state did not show any decay within 2 ns making this structure interesting for applicationin DSSCs.
A pentad consisting of a known Ru-based (electro)chemical water oxidation catalyst (WOC)linked to two zinc-porphyrin-fullerene dyads (ZnP-C60) was investigated. The charge transferprocesses leading to the first oxidation of the WOC were understood. Low levels of wateroxidation were detected in presence of a sacrificial electron acceptor.
The gained understanding of the CT dynamics and recombination processes thus allows newstrategies to improve the efficiency in molecular systems for solar energy conversion.
Keywords: photophysics, photoinduced electron transfer, transient absorption spectroscopy,laser spectroscopy, solar energy conversion, p-type DSSCs, Charge separation, recombination,mesoporous NiO
Jens Föhlinger, Department of Chemistry - Ångström, Physical Chemistry, Box 523, UppsalaUniversity, SE-75120 Uppsala, Sweden.
© Jens Föhlinger 2018
ISSN 1651-6214ISBN 978-91-513-0273-7urn:nbn:se:uu:diva-343443 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-343443)
To my family
List of papers
This thesis is based on the following papers, which are referred to in the textby their Roman numerals.
I Unveiling hole trapping and surface dynamics of NiO
nanoparticles
L. D’Amario∗, J. Föhlinger∗, G. Boschloo, and L. HammarströmChem. Sci., 2018, 9, 223
II Direct Spectroscopic Observation of Hole Trapping in
Dye-Sensitized NiO Films by Transient mid-IR Absorption
Spectroscopy
J. Föhlinger, L. Antila, D. Narouzi, S. Ott, and L. HammarströmManuscript in preparation
III A Comparative Investigation about the Role of Anchoring group
on Perylene Monoimide Dyes in NiO Based Dye-Sensitized Solar
Cells
Y. Farré, J. Föhlinger, A. Planchat, Y. Pellegrin, E. Blart,L. Hammarström, and F. OdobelManuscript in preparation
IV Self-quenching and Slow Hole Injection May Limit the Efficiency
in NiO-based Dye-Sensitized Solar Cells
J. Föhlinger, S Maji, A. Brown, E. Mijangos, S. Ott, andL. HammarströmManuscript submitted
V Ultrafast dye regeneration in a core–shell NiO–dye–TiO2mesoporous film
L. Tian∗, J. Föhlinger∗, P. B. Pati, Z. Zhang, J. Lin, W. Yang,M. Johansson, T. Kubart, J. Sun, G. Boschloo, L. Hammarström, andH. TianPhys. Chem. Chem. Phys., 2018, 20, 36
VI A Ruthenium Complex–Porphyrin–Fullerene–Linked Molecular
Pentad as an Integrative Photosynthetic Model
M. Yamamoto, J. Föhlinger, J. Petersson, L. Hammarström, andH. ImahoriAngew. Chem. Int. Ed. 2017, 56, 3329.
Papers not included in this thesis
During my PhD studies, I also contributed to the following paperswhich are not included in the thesis.
VII Ultrafast Interligand Electron Transfer in [Ru(4,4’-dicarboxylate
-2,2’-bipyridine)2cis-(NCS)2]4- and Implications for Electron
Injection Limitations in Dye-Sensitized Solar Cells
B. Pettersson Rimgard, J. Föhlinger, J. Petersson, M.Lundberg, B.Zietz, A. M. Woys, S. A. Miller, M. R. Wasielewski, andL. HammarströmManuscript submitted
VIII Light driven electron transfer in methyl-bipyridine phenol
complexes is not proton coupled
R. Tyburski, J. Föhlinger, and L. HammarströmManuscript submitted
IX Ultra long-lived electron-hole separation within water-soluble
colloidal ZnO nanocrystals: Prospective Applications For Solar
Energy Production
A. M.Cieslak et. al.Nano Energy 2016, 30, 187
X Soret fluorescence involved in Caryophyllales plants ultraviolet
protection
J. Sá, M. V. Pavliuk, A. M. El-Zohry, D. L. A. Fernandes, J. Föhlinger,and E. MukhtarSci. Lett. J. 2016, 5, 226
XI Solid State p-Type Dye-Sensitized NiO-dye-TiO2 Core-Shell Solar
Cells
L. Tian, J. Föhlinger, Z. Zhang, P. B. Pati, J. Lin, T. Kubart, Y. Hua,J. Sun, L. Kloo, G. Boschloo, L. Hammarström, and H. TianChem. Commun., accepted manuscript DOI: 10.1039/C8CC00505B
Reprints were made with permission from the publishers.
* Shared first authorship
Contribution to the papers
My contributions to the papers included in this thesis are stated below:
I. Performed the fs-transient absorption experiments,their data analysis,and participated in the discussion. Approved the manuscript
II. Participated actively in the design of the study, performed the UV/Vistransient absorption experiments, and participated in the data analysisand discussion. I was lead for writing the manuscript.
III. Performed the transient absorption measurements, contributed to the dis-cussion and wrote that section in the paper.
IV. Performed all measurements (except for spectroelectrochemistry) of thebis-tridentate compounds, main responsible for data analysis, discus-sion, and writing of the manuscript.
V. Performed all transient absorption spectroscopy measurements and theirdata analysis. I participated actively in discussion and writing of themanuscript and approved the final version.
VI. Participated in the fs-transient absorption measurements and their dataanalysis, performed the nanosecond timescale experiments and their dataanalysis, participated in writing the manuscript, and approved the finalversion.
Contents
1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.1 Climate Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2 World Energy Production Sustainability Problem . . . . . . . . . . . . . . . . . . . . . . . 131.3 Solar Energy Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.1 Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.3.2 Artificial Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.3 Importance of Electron-Transfer Processes in Solar
Energy Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.1 Photophysical Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Electron-Transfer Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1 Marcus Theory for Electron-Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.2.2 Photoinduced Electron-Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Charge-Transfer Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.4 Dye-sensitized Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.1 Interfacial Electron Transfer in DSSCs . . . . . . . . . . . . . . . . . . . . . . . . . . 232.4.2 Solid-State DSSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5 Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.1 Steady-State Absorption and Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.2 Time-Resolved Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.3 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4 Hole Trapping in Nickel Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.1 Aim of the study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.4 Implications for NiO-based p-type DSSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5 Perylene Monoimides with Different Anchoring Groups . . . . . . . . . . . . . . . . . . . . . . . 385.1 The different Anchoring Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6 Self-quenching limits the efficiency of NiO based DSSCs . . . . . . . . . . . . . . . . . . . . . 436.1 Aim of the Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
7 Ultrafast regeneration in Core-Shell Structure DSSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.1 Design of the Core-Shell Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
8 Macromolecular Pentad for Water Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518.1 Expected processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
9 Conclusions and Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
10 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
11 Populärvetenskaplig sammanfattning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
List of Abbreviations
ALD Atomic layer deposition
BET Back electron transfer
C60 Fullerene
CB Conduction band
CS Charge separated state
CSh Charge shift
CT Charge transfer
DAS Decay associated spectra
DSSC Dye-sensitized solar cell
DSSFC Dye-sensitized solar fuel cell
ETM Electron transporting material
GSB Ground state bleach
HER Hydrogen-evolving reaction
HOMO Highest occupied molecular orbital
HTM Hole transporting material
IA Induced Absorption
IPCE Incident photon-to-current efficiency
LC Ligand centered
LUMO Lowest unocupied molecular orbital
MLCT Metal-to-ligand charge transfer
NDI naphthalene diimide
OECD Organization for Economic Co-operation and Development
OER Oxygen-evolving reaction
PEC Photoelectrochemical cell
PET Photoinduced electron transfer
PMI Perylene monoimide
PMI perylene monoimide
PSI Photosystem I
PSII Photosystem II
RS Reactant state
SC Semiconductor
SE Stimulated Emission
ss-DSSC Solid-state dye-sensitized solar cell
TAS transient absorption spectroscopy
VB Valence band
WOC Water oxidation catalyst
ZnP Zinc porphyrin
1. Motivation
1.1 Climate ChangeIn December 2015, the Paris Agreement was signed by 195 countries. In thattreaty, the governments of these countries have agreed to the following (article2 from reference 1):
a) Holding the increase in the global average temperature to well below2 ◦C above pre-industrial levels and to pursue efforts to limit the tem-perature increase to 1.5 ◦C above pre-industrial levels, recognizingthat this would significantly reduce the risks and impacts of climatechange;
b) Increasing the ability to adapt to the adverse impacts of climate changeand foster climate resilience and low greenhouse gas emissions de-velopment, in a manner that does not threaten food production;
c) Making finance flows consistent with a pathway towards low green-house gas emissions and climate-resilient development.
As can be seen from this text, there is a consensus on the relation betweengreenhouse gases and temperature increase. One of the most "famous" green-house gases is carbon dioxide (CO2). Its concentration in the atmosphere hasincreased significantly within the last decades with significant contributionfrom mankind. To meet the temperature increase, green house gas emissionshave to be decreased. To reach this goal, CO2 emissions have to either bestored, reduced, or a combination of both.
1.2 World Energy Production Sustainability ProblemA major source for anthropogenic CO2 is the combustion of fossil fuels. Thisincludes both electricity production and usage for transport. Today, the mainglobal energy source are fossil fuels which contribute with more than 75 % tothe global energy production[2].
Globally, energy consumption has increased exponentially and is expectedto continue[3]. The largest contribution to future increases in energy consump-tion is expected to originate from developing countries, evolving towards lifestandards comparable to those in the Organization for Economic Co-operationand Development (OECD) countries. This implies a need for new energy
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sources to meet present and future demands. One advantage of many devel-oping countries is the absence of (modern) infrastructure, making it easier toemploy new concepts such as decentralized grids. Furthermore, there is notyet a commitment to any energy source, allowing for choice in the adoption ofoptimal and sustainable energy alternatives.
Employing renewable, greenhouse gas emission free technologies to meetadditional energy demands in the future will at least help to not increase emis-sions. Simultaneously, there has to be a transition to renewable energies in theOECD. In contrast, in these countries, there is a higher need for implemen-tation of renewable energy technologies that are compatible with the alreadyexisting infrastructure, which is to a large extend based on fossil fuels.
1.3 Solar Energy ConversionOne of promising renewable and sustainable energy sources is the sun, whichdelivers 120000 TW to Earth[4]. Compared to the total energy consumptionof 30 TW to 35 TW[2], one can calculate that the solar irradiation of a littlebit more than two hours would be able to cover mankind’s annual energy con-sumption. To replace, for example, Sweden’s annual fossil fuel consumption(150 TWh [5]), it would be sufficient to cover 0.4 % of the Swedish land areawith solar cells with 15 % efficiency and the average annual solar irradiation of1000 kWhm−2 [6, 7]. This corresponds roughly to four times the land takenup by golf courses and ski slopes or ≈ 15% of the built up land only[8]. Wheneven aiming for the replacement of nuclear energy, this percentage of neededland increases to 0.7 %. Therefore, solar energy is an appealing source asfuture renewable and sustainable energy source.
1.3.1 Solar CellsOne very common and mature solar energy conversion technology is the gen-eration of biomass from plants. This approach, however, faces low efficiencyaround 0.15 % solar energy to fuel[9]. This leads to a necessity of large landuse. Another drawback is that the plants for biomass production compete withfood crops and thus potentially increase prices for food. Hence, biomass alone,is not an (optimal) solution for meeting the future energy demand.
Another well known alternative for solar energy conversion are solar cells.Their common general working principle is:
1. Absorption of sun light2. Separation of charges3. Extraction of charges.
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To date, most solar cells available are based on the concept of a p-n junc-tion. Such solar cells consist of doped crystalline n-type (excess electrons)and p-type (depletion of electrons) semiconductors brought in contact witheach other. This leads to the formation of an interface layer where positiveand negative charges cancel each other. As a consequence, a smooth gradientof the bands is formed resulting in an intrinsic electric field within this layer.When absorbing light, an electron-hole pair is generated in the semiconductor.If this happens within the interface layer, the above mentioned electric fieldpulls electrons and holes in different directions, thus apart from each other.The separated charges are finally extracted from the solar cell by electricalcontacts.
The theoretical efficiency limit for a single junction solar cell is 33 %, cal-culated by Shockley and Queisser [10]. One way to circumvent this limit is theapplication of multi-junction solar cells[11–13]. The efficiency record to dateis a solar cell with four junctions giving a total conversion efficiency of 46 %under illumination with concentrated sunlight[14, 15]. The most commoncommercial solar cells, however, show efficiencies of approximately 15 %[16].
Another emerging technology are the so called third generation solar cells.These include organic solar cells, perovskite solar cells, quantum dot solarcells, and dye-sensitized solar cells (DSSCs, also known as Grätzel cells).The main focus of this thesis will be on the latter and therefore, only those aredescribed in more detail. DSSCs consist of a transparent mesoporous semi-conductor sensitized by a dye. Especially the DSSCs have several possibleadvantages: by changing the type of dye and dye concentration, these solarcells can have basically any color and be transparent. This makes them attrac-tive for possible build-in architecture. Furthermore, they can have a higherpower conversion efficiency at low light levels such as ambient or stray lightcompared to p-n junction based cells[17]. A detailed description of devicestructure and working principle will be presented in chapter 2.4.
1.3.2 Artificial PhotosynthesisThe production of solar electricity has the disadvantage of discontinuity. Es-pecially in winter and during night, when the energy demand is high, there islittle to no solar electricity generation, a fact that makes power storage neces-sary. One option to store electricity is the use of batteries. Their typical energydensity ranges from 10 Whm−3 to 200 Whm−3[18]. A more appealing en-ergy storage alternative are fuels because of their even larger energy densities.Therefore, the conversion of solar energy directly into chemical energy in theform of fuels is an appealing approach for long term energy storage. Anotherpositive effect is the potential usage in the already existing fuel based infras-tructure. A well known example for this conversion is natural photosynthesis
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in plants, which splits water into oxygen and utilizes the formed electrons andprotons together with CO2 to form sugars as energy storage.
In this process, plants utilize sunlight in a two step reaction (so called Z-scheme). In Photosystem II (PS II), the energy of absorbed photons is used forcharge separation, and subsequent splitting of water into molecular oxygen,protons and electrons. The electrons are then transported to Photosystem I byan electron transport chain. In a second step, when photons are absorbed inPS I, it gains sufficient energy to produce a reductant (NADPH) which later isused for carbon fixation to form biomass. The protons form a concentrationgradient which then is used to produce ATP.
This Z-scheme is mimicked in artificial photosynthesis. As there is no needfor vital processes as in plants (e.g. respiration, metabolism), the theoreticalmaximum solar to bond conversion efficiency is 40 %[12], which is equal tothat of a double junction solar cell. Even here, water is used as a cheap sub-strate and electron source. The reducing power at the second step is aimed tobe used for either proton reduction to molecular hydrogen or for CO2 reduc-tion to form CO or other higher value hydrocarbons such as e.g. formic acid.More details about these processes can be found in chapter 2.5.
1.3.3 Importance of Electron-Transfer Processes in Solar EnergyConversion
As discussed above, light absorption and charge separation processes are im-portant for the functioning of solar energy conversion devices. Therefore, adeeper understanding and finding of unwanted and unproductive side reac-tions, especially of the initial steps after light absorption, can be crucial forfurther improvements of the devices. Hence, the main focus of this thesis ison the first charge-transfer processes following light absorption in differentsolar energy conversion systems which will be described more detailed in thefollowing chapters.
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2. Theory
2.1 Photophysical ProcessesIn the following sections, the interaction between molecules and light will bediscussed. Light can be described as photons having both particle and wavecharacter[19, 20]. Due to the wave character, they can be assigned a frequencyand wavelength. Due to the particle character, every photon is attributed anenergy which is proportional to the frequency E = hν .
Figure 2.1. Schematic of steadystate absorption and emission.Electronic and vibrational states(left) and the corresponding occu-pation of molecular orbitals (right)For simplicity, the S2 state is il-lustrated as a HOMO to LUMO+1transition.
According to quantum mechanics, pho-tons can only interact with a molecule if thephoton energy corresponds to the energy dif-ference between two states in the moleculeas depicted by the horizontal bars in fig-ure 2.1. Depending on the character of thestates, there is different energy spacing, e.g.electronic and vibrational states. The elec-tronic states are formed by different occupa-tions of the molecular orbitals as exempli-fied in the right hand part of figure 2.1. Inthe state with lowest energy, the ground state(GS), often denoted S0, normally the lowestmolecular orbitals are occupied. Light ex-cites an electron from one of those occupiedorbitals into an unoccupied orbital to forman excited state (ES). For many dyes, theelectronic transition with the lowest energypossible can be described as promotion of anelectron from the highest occupied molec-ular orbital (HOMO) to the lowest unoccu-pied molecular orbital (LUMO) to form a S1state. For other dyes, this transition has tobe described by excitation of multiple elec-trons. Despite the lowest energy difference,the electron can also be excited to higher un-occupied molecular orbitals such as e.g. S0– S2 transition. Higher transitions can also originate from energy levels lowerthan the HOMO (e.g. HOMO-1) to a LUMO orbital. The light inducing elec-tronic transitions are mostly in the visible to UV region.
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The electronic states often have a vibrational substructure as shown in figure2.1. This substructure is due to different vibrational states of the molecule. Atransition between those energy levels changes the amplitude of the molecularvibration. The corresponding light is in the infrared region. In the case ofmolecules, the IR signal consists of distinguished peaks that correspond tovibrational modes in the molecule. In the case of semi-conductors, there isespecially a broad featureless IR signal if there are free charge carriers, e.g.electrons in the conduction band (CB) [21, 22].
After excitation to higher states than the S1-excited state, most moleculesin solution quickly dissipate the excess energy to the surrounding solvent andrelax to the lowest vibrational state of the S1-excited state[23]. Therefore,most transitions and reactions from the excited state happen from the S1,υ = 0state where υ describes the vibrational quantum number of the correspondingelectronic state (i.e. S1).
Besides absorption, many molecules also can emit light. When going backinto the ground state, the molecule can emit light as depicted by the right handarrow in the left part of figure 2.1. This light can provide information aboutrelaxation processes and the vibrational structure of the ground state. Possibledecreases in its intensity upon interaction with other molecules or materials,i.e. quenching, can give information about the efficiency of those quenchingmechanisms.
Except for the singlet states as described above, there are also triplet states(not shown in figure 2.1) which differ by a spin flip of one electron. Becauseof the spin flip, singlet-triplet transitions are spin forbidden and suffer fromlow probabilities.
2.2 Electron-Transfer Theory2.2.1 Marcus Theory for Electron-TransferThe theory of electron-transfer was mainly developed by Marcus[24–26]. Itpredicts the rate at which an electron can be transferred from an electron donor(D) to an electron acceptor (A). For both reactant state (RS, D + A) and chargeseparated state (CS, D+ + A-), the free energy surface is approximated as aparabola with respect to the reaction coordinate as shown in figure 2.2. Thereaction coordinate is a projection of three dimensional system on a one di-mensional axis indicating the ratio of RS and CS character. At the equilibriumconfiguration of the RS (req,D+A), the ground state energy is lower than that ofthe CS. At the crossing of the parabolas on the other hand, the electronic con-figurations of both (distorted) reactant state and (distorted) CS have the samefree energy, making it possible for the electron to tunnel from one state to theother. This nuclei-configuration is also called the transition state (TS). Theenergy difference between RS and TS is the activation energy ΔG‡. The reor-ganization energy λ is the energy that would be released if the electron would
18
Figure 2.2. Schematic of Marcus parabolas. Left: Parabolas indicating reactant state(RS, D+A), charge separated state (CS, D++A-), reorganization energy (λ ), drivingforce (ΔG0) and activation energy ΔG‡ according to Marcus theory. [25–27]. Right:Three different CS state equilibrium free energies result in different activation ener-gies, if the driving force is larger than the reorganization energy, it is called invertedregion due to the decrease in rate upon increase in driving force.
relax to the equilibrium configuration of the CS (req,D++A−) after a transferdirectly into the CS from the equilibrium conformation of the GS (req,D+A).Using geometric considerations, the activation energy can be calculated to beΔG‡
ET = (λ +ΔG0)2/(4λ ).In the case of weak electronic coupling (non-adiabatic) rate expression for
electron transfer takes a Arrhenius type form and is:
kET =2πh
H2rp(4πλkBT )−1/2 exp
[−(λ +ΔG0)2
4λkBT
](2.1)
where Hrp =⟨ψP|Hrp|ψR
⟩is the energy of the electronic coupling between
reactant and product states, h the reduced Planck constant, kB the Boltzmanconstant, and T the temperature.
As can be seen from equation (2.1), there is an increase in ET rate withincreasing driving force that reaches a maximum when −ΔG0 = λ . At evenlarger driving forces, the ET rate diminishes again. This can be rationalized bythe increase of activation energy in the right part of figure 2.2. This counter-intuitive decrease in rate is the inverted region which was predicted by Marcusin 1956 [24]. However, it took until 30 years for the first clear and generallyaccepted experimental evidence obtained by Closs and Miller [28, 29]. Areason for the experimental difficulties were excited states of the CS. As thosehave less driving force, they also result in a lowered activation energy, whichconsequently leads to a constant observed rate in the predicted inverted region.
19
Distance Dependence of ET
If there is weak electronic coupling between the donor and the acceptor andthe driving force is constant, the rate depends on the square of the energy ofthe coupling HRP. The coupling includes the overlap integral of the electronicwave functions of donor and acceptor. As those decay exponentially in space,also the coupling is assumed to decay exponentially by
HRP = H0 exp[−β
R−R0
2
](2.2)
where β is a constant determining the decrease in rate with increasing dis-tance, R the donor-acceptor distance and R0 the van-der Waals contact radiusand H0 the electronic coupling at van-der Waals contact. Hence, short dis-tances and large electronic coupling between D and A are necessary for fastand efficient electron transfer. A common strategy to slow down (unwanted)electron transfer processes is the enlargement of the distance of the reactantsas well as diminishing their electronic coupling. Both strategies are employedwithin this thesis (see below).
Estimation of the Driving Force
The driving force for ET can be estimated by the standard electrode potentialof the donor (E◦(D+•/D)) and the acceptor (E◦(A/A−•)), and the electrostaticwork terms that account for the Coulomb interaction of products and reactants,respectively (w(D+•A−•) and w(DA)) by using
ΔG◦ET = NA
{e[E◦(D+•/D)−E◦(A/A−•)
]+w(D+•A−•)−w(DA)
}(2.3)
where NA is the Avogadro constant and e the elementary charge[30, 31]. Ascan be seen, it strongly depends on the redox potentials of the molecules in-volved. However, there is a correction term including the change in Coulombinteraction upon charge separation. The magnitude of this term is harder tomeasure, especially if there are interfaces involved (see below). Hence, thevalues for driving forces derived from equation (2.3) only can give a orderof magnitude of the driving force. This means especially that a seeminglyslightly uphill process (estimated from the electrochemical standard electrodepotentials) may still take place.
2.2.2 Photoinduced Electron-TransferInstead from the ground state of the molecules, the electron transfer can alsobe initiated from an excited state of one of the ET partners. This photoinducedelectron transfer (PET) process can be divided into oxidative and reductivequenching. The difference between these two processes is whether the electrondonor or acceptor is excited, respectively (see figure 2.3).
20
Figure 2.3. Scheme of photoinduced electron transfer (PET) including oxidativeand reductive quenching. For simplification, the excited state is approximated as aHOMO-LUMO transition.
The driving force for PET is based on equation (2.3) with the exception thatthe electrochemical standard potential of the excited donor/acceptor is used.The formula for PET, ΔPET G◦, thus transforms to
ΔG◦PET = ΔG0
ET −E00 (2.4)
where ΔE00 is the vibrational zero electronic energy of the excited partner.[31,32] The use of the vibrational zero energy implies the assumption that theelectron transfer takes place from the vibrationally relaxed excited state. Ifthe PET takes place before thermalization, there can be a higher driving forcemaking PET possible from "hot states". In that case, the driving force is in-creased by the access energy above the vibrationally relaxed S1-state.
Despite this change in driving force, the Marcus theory for electron trans-fer remains unchanged. This includes especially the distance dependence forelectron transfer. This implies that larger donor acceptor distances slow downelectron transfer reactions. This is especially used in natural photosynthesiswhere back electron transfer (BET) is suppressed by increasing the distance ofthe electron-hole pair. A similar approach is often applied in artificial systems.
2.3 Charge-Transfer TransitionsTransitions do not necessarily take place within one (part of a) molecule.When an electron donor and acceptor are in close proximity to each other(either by covalent binding, coordination, or by electrostatic interactions), anelectron can be transferred from the HOMO of the donor to the LUMO of theacceptor[19, 33]. Hence, the electron tunnels from the donor to the acceptorduring the excitation process. In contrast to PET, there is no excited donor or
21
acceptor state involved in such a charge transfer (CT) transition. This addi-tional optical process results in a new, absorption band, red shifted comparedto those of the moieties/molecules alone. As a consequence, CT transitionsare commonly utilized to improve the light harvesting properties of the pho-tosensitizers. Many metal complexes show a metal-to-ligand charge-transfer(MLCT) band. Here, an electron is transferred from the metal center to the lig-and that is the easiest to be reduced, which results in an oxidized metal centerand a reduced ligand. By that, the localization of the electron on the ligandscan be influenced by their reduction potential.
The properties of these CT-bands are strongly solvent dependent (solvato-chromic)[34]. Polar solvents generally stabilize charge separation and lowerthus the transition energy. Hence, the CT bands are more red shifted the morepolar the solvent is. This effect can be used to distinguish CT bands fromπ −π∗ transitions of the ligands (ligand centered (LC) states).
2.4 Dye-sensitized Solar CellsOne solar energy converting system applying the principle of PET are dye-sensitized solar cells (DSSCs). The concept of an n-type DSSC based onmesoporous titanium dioxide (TiO2) was invented by O’Reagan and Grätzelin 1991 [35]. The working principle of a tandem dye-sensitized solar cell con-sisting of a n-type and a p-type DSSC respectively is shown in figure 2.4. Forindividual n- or p-type DSSCs the respective counter electrode is not designedto be photoactive. Both are very similar with only minor differences. Afterexcitation of the dye 1 , the photo-excited dye injects an electron into the n-type TiO2 or accepts an electron from p-type NiO (hole injection) 2 to formthe oxidized/reduced dye respectively. The so oxidized/reduced photosensi-tizer is regenerated by the redox mediator M in the electrolyte, 3 resettingthe dye back to the ground state. For this process to take place, the mediatorhas to diffuse close to the oxidized/reduced dye to perform the regeneration; itis therefore a diffusion limited process in most cases. To close the circuit, theelectrons travel from the (photo)anode to the (photo)cathode through a loadwhere they can perform work while the mediator shuttles between the twoelectrodes.
However, there are also several unwanted side reactions that diminish theefficiency of the DSSCs. These processes are the deactivation of the excitedstate, 4 where the dye goes back into the ground state without an electronbeing transferred into/from the semiconductor. Another process is recombina-tion where the oxidized/reduced dye recombines with the electron/hole in therespective semiconductor 5 . Especially in p-type DSSCs this process of-ten takes place at the order of hundreds of picoseconds, thus competing withthe regeneration of the photosensitizer [13, 36–40]. This is in contrast to n-
22
Figure 2.4. Scheme of a tandem dye-sensitized solar cell. For individual p- or n-type DSSCs the respective counter electrode is not designed to be photoactive. Left:Structure and schematic electron flow through the cell. Right: Energy diagram of thesemiconductor-dye interfaces and wanted forward reactions (blue) and unwanted sidereactions (red). For more details, see text.
type DSSCs where recombination is taking place at the micro- to millisecondtime-scale and is therefore suppressed by regeneration [41–43]. Additionalto the electron/hole-dye recombination, recombination can also take place be-tween the electron/hole in the TiO2 and the NiO respectively and the redoxmediator 6 . All of these processes 1 to 6 follow the theory of PET asdiscussed above. The kinetics are, however, complicated to understand as theexact nature of the states in the semiconductors are not known. The conceptof DSSCs is employed in papers II to V.
Despite their possible application in tandem cells and in dye-sensitized so-lar fuel cells, p-type DSSCs are not as widely investigated as their n-typecounterparts. The until now most common p-type semiconductor is nickeloxide (NiO). The efficiency of p-type DSSCs (2.5 %)[44] is much lower thanthat of n-type (14 %)[45]. For both p-type and n-type DSSCs, charge separa-tion mostly takes place on the sub picosecond time scale. The main differencebetween these two different types of DSSCs is believed to be recombinationkinetics, see below.
2.4.1 Interfacial Electron Transfer in DSSCsIn most cases, DSSCs employ ultrafast, i.e. at a sub-picosecond time scale,electron or hole injection upon excitation[36, 46–48]. The reason for this isthe high density of accepting states due to the band structure of the semicon-ductors. Consequently there are many acceptor states, allowing for barrierlesselectron transfer. However, there have been reports about a slower injectionphase (at picosecond timescale) which mostly have been attributed to local-ization of charges with respect to the semiconductor[49–52]. As seen from
23
equation 2.2, a localization of the MLCT state on the ligand close to a NiO sur-face potentially can slow down ultrafast hole injection. Similarly, an electronfurther away from the TiO2 surface would hinder injection in n-type DSSCs.
The major difference between n-type and p-type DSSCs are the time scalesof recombination processes. For n-type DSSCs, recombination is excitationintensity dependent[41]. As soon as there is more than one electron in oneTiO2 particle, recombination is accelerated due to higher probability of chargesto meet and recombine. In addition to the intensity dependence, charge re-combination in TiO2 also is very sensitive to applied voltage and electrolytecomposition. This is attributed to different occupation ratios of electrons inthe conduction band and in trap states within the bandgap.
The recombination kinetics of NiO-based p-type DSSCs differs signifi-cantly from those of their counterparts. Firstly, it was found that the recom-bination is not light intensity-dependent [53, 54]. Furthermore, investigationof the dependence on an applied voltage showed a bi-phasic decay with con-stant time scale for both phases but an increasing ratio of the fast componentat positive potentials[55]. The different recombination phases were attributedto different trapping states. A deeper understanding of hole trapping in NiOwould thus allow for development of new strategies to slow down recombina-tion and consequently increase the overall efficiency of p-type DSSCs.
2.4.2 Solid-State DSSCsOriginally, DSSCs employed solution phase electrolytes based on organic sol-vents and a redox mediator[35, 43]. Potential leaking of the electrolyte posesa threat to the environment, which makes it important to seal the cells tightly.Furthermore, the regeneration of the oxidized/reduced dye relies on diffusionof the redox mediator through the electrolyte, which limits the regenerationtime to the diffusion limit from the nano- to micro second time scale[56, 57].
For both n-type and p-type DSSCs, there have been recent advances re-placing the electrolyte by a hole/electron conducting material (HTM/ETM)such as spiro-OMe-TAD [58, 59] and PCBM [60]. Due to the proximity ofthe HTM/ETM and the rigidity of the system, regeneration is not diffusionlimited any more. In the case of p-type DSSCs, the application of PCBM asETM shortened the regeneration time to ≈ 50ps, thus much shorter than formost electrolyte based DSSCs [60]. An extension of this concept was appliedin paper V and will be explained in more detail in chapter 7.1
2.5 PhotocatalysisExcept for generating electricity from sun light, its energy can also be con-verted to fuels by photoelectrochemical cells (PEC). A common strategy is tosplit water into molecular oxygen and hydrogen, a reaction that stores 1.23 eV
24
per oxygen molecule. In a PEC, the principle of natural photosynthesis is ap-plied. Water splitting takes place in two half reactions to avoid the necessity ofhigh energy photons for water splitting: water oxidation (oxygen-evolving re-action, OER) and proton reduction (hydrogen-evolving reaction, HER, whichcorresponds to carbon fixation in plants). [4, 61–64] Except for hydrogen, alsoother fuels with higher energy densities per weight, such as formic acid fromCO2 reduction are aimed for.
The general working principle on the example of water splitting is shownin figure 2.5. Both reactions are initiated independently by the absorption of aphoton, in most cases by a photosensitizer. At the photoanode, the catalyst isoxidized by the photosensitizer. After uptake of in total four oxidation equiva-lents by the catalyst and release of four protons, molecular oxygen is evolved.The protons diffuse through a membrane towards the photocathode while thereduced photosensitizer is re-oxidized by the anode. At the cathode, the ca-talyst is reduced by the photosensitizer. After the uptake of two reductionequivalents and two protons by the catalyst, molecular hydrogen is released.There are many different architectures for photoelectrochemical cells. In thehomogeneous case, catalyst and photosensitizer are in solution.
Figure 2.5. Principle of a photoelectro-chemical cell. At the photoanode (left),water is oxidized to protons and molecu-lar oxygen. The protons are later reducedat the photocathode to form molecular hy-drogen. The electrodes can either be ho-mogeneous (i.e. in solution phase) as de-picted in this figure or heterogeneous (i.e.the active material is part of the electrode).
In most cases, they are linkedchemically to each other to facili-tate the first charge separation. Thisprinciple is also applied in paper VI.An alternative strategy is the utiliza-tion of heterogeneous catalysis, e.g.by linking sensitizer and catalyst ona semiconductor surface which re-sults in similar charge-transfer pro-cesses to DSSCs[64]. In these dye-sensitized solar fuel cells (DSSFCs)the reduction/oxidation of the pho-tosensitizer is done by the semicon-ductor followed by a charge shift orelectron hopping between photosen-sitizer and catalyst [65, 66]. As theinitial CT dynamics in DSSCs andDSSFCs are similar, improvementsof DSSCs also gain DSSFCs.
In other cases, the electrode ma-terial acts simultaneously as photonabsorber and catalyst. For thoseelectrodes, metal oxides are oftenused[67]. To date, it is not clearwhich design is the most promisingfor solar energy conversion to fuels.
25
Recently, a life cycle analysisfor large scale hydrogen productionbased on nanorod silicon solar cells was made[68]. There, it was found thatthere is a positive energy balance with an energy payback time of 8 years. Themain part of the energy for their production was, however, used for produc-tion of the nano structured silicon cells. Here, DSSFCs potentially have theadvantage of using cheap mesoporous TiO2 and NiO semiconductors that donot require a lot of energy to be produced. The usage of molecules also hasthe advantage of tunability. In natural photosynthesis, the structure of the cat-alytically active sites are well defined by the protein backbone. In principle itis possible to fine tune the structure of a molecular catalysts in a similar way.
As complete PECs and DSSFCs are rather complicated and difficult to in-vestigate, it is common to use sacrificial electron donors or acceptors and onlyinvestigate and optimize one of the two half reactions[69]. This was also donein paper VI where only the OER was investigated. As there are four electronsinvolved in this process, it is more complicated that the other half reactionwhich often only is a two electron process.
Under solar light illumination, a single dye in DSSCs absorbs light witha rate of approximately 1 s−1[70]. Because of this, the CS lifetime in dye-catalyst systems has to be on the same order of magnitude to avoid recombi-nation losses. The strategy to circumvent this problem is to increase the ratioof dyes to catalysts to match charge separation with catalytic activity.
26
3. Methods
3.1 Steady-State Absorption and EmissionAs described in chapter 2.1, dyes have distinct energy levels and consequentlydifferent light absorption properties making it possible to identify molecularspecies. To measure absorption spectra of molecules, spectrophotometers areused. A typical setup is shown in the left part of figure 3.1. The light of alamp is directed through a monochromator to allow measurements at singlewavelengths. The light is then split into two beams. One is directed throughthe sample to measure the intensity of light it transmitted (I1) while the otherbeam is a reference beam to access the light intensity in absence of the ab-sorbing materials (I0). Knowing these two intensities, the absorption is thencalculated as
A = log10
(I0
I1
)= ελ · c · l
where ελ is the wavelength dependent, dye specific molar extinction coeffi-cient of the dye at wavelength λ , c the dye concentration, and l the path lengthof the light through the sample. The shape of the absorption spectrum alsogives information about the electronic properties of the molecule and aggre-gation or degradation.
Fluorescence and phosphorescence are measured with fluorometers whosegeneral working principle is described on the right part of figure 3.1[71]. Thesample is excited by monochromatic light from a xenon lamp where the wave-length is selected with help of a monochromator. To account for possiblefluctuations in the excitation intensity, a small fraction of the excitation lightis reflected on a photodiode. To diminish the influence of stray light hitting
Figure 3.1. Scheme of the working principle of an absorption spectrometer (left)and a fluorescence spectrometer (right). BS: beam splitter, PD: photodiode, PMT:photomultiplier tube.
27
the detector, the emission from the sample is generally detected at right anglefrom the excitation. Additionally long pass filters are used in the emissionpath to suppress the second order of the monochromator and scattering fromthe excitation light.
3.2 Time-Resolved AbsorptionTo investigate the kinetic processes after excitation of molecules, transient ab-sorption spectroscopy (TAS) is used. The basic principle is described in figure3.2.
t=0
t=0t>0
Detector
pump
probe
Mono-chromator
Figure 3.2. General scheme of apump-probe experiment.
The sample is excited by a monochromaticlaser pulse (pump). After a certain timedelay, a polychromatic laser pulse (probe)is used to measure the transmission of thesample. By varying the time delay betweenpump and probe, kinetics of the processes in-volved can be measured.
The pump pulse (often hundreds of nJ)excites typically only a small fraction of themolecules. Therefore, the absorption of the
sample does not change a lot upon excitation (see figure 3.3). The (few) ex-cited molecules, however, have a different absorption spectrum compared tothe molecules in the ground state. In order to eliminate the contribution ofunexcited molecules to the absorption, the difference between pumped andunpumped sample is calculated to result in a difference spectrum (right part offigure 3.3).
A typical TA spectrum shows in most cases three features:
• Ground-state bleach (GSB)• Stimulated emission (SE)• Induced absorption (IA)
The GSB arises because the number of molecules in the ground state is di-minished after excitation as those in the excited state do not contribute to theground state absorption any more. Hence light is less absorbed and results ina negative absorption change upon excitation. The GSB has the same shapeas the negative GS absorption spectrum as indicated by the gray dotted linein figure 3.3. The other process leading to a negative TA signal is stimulatedemission. In that case a photon from the probe pulse, which has the same en-ergy as the S1 state can make a molecule emit one photon in the same directionwhile relaxing to the ground state. This process results in more photons thanin the unpumped sample leading to a an apparent decrease in absorption andhence a negative feature as well. It can be noted that this process only applies
28
Figure 3.3. General scheme of transient absorption spectroscopy. The TA spectrumresults from subtracting the absorption spectrum of the unpumped sample from thatone of the pumped sample. The green spectrum is an excited state difference spectrumand the yellow one is that of the reduced state. The gray curves are negative steadystate absorption (dotted line) and emission (dashed line) for illustration of the originof the bleach.
for fluorescence, hence it is an indicator for the presence of excited singletstates. The corresponding feature resembles the negative emission spectrumof the compound (dashed gray line figure 3.3).
The most important TA feature is the IA, which is the only one that is pos-itive. Here, the absorption that arises from molecules that were excited bythe pump is measured. Hence it contains information about excited statesand possible reactions occurring in excited states. Because of that, it will becalled induced absorption (IA), even though the name excited-state absorptionis commonly used in the literature. As most dyes show spectral changes uponoxidation or reduction, IA allows one to follow the processes after excitation.In the example of figure 3.3, the TA spectra of the excited (green) and the re-duced (yellow) state of a molecule are shown. As expected, both spectra showsimilar GSB features while the reduced species has a pronounced peak wherethe excited state shows SE.
One special case of induced absorption is the Stark effect. Here, the dyeis in a changed electric field. If this field has a component parallel to thetransition dipole moment of the molecule, the wavelength of this transition isred or blue shifted depending on whether the components are parallel or anti-parallel respectively. Due to the small shift, the TA signal is proportional to thederivative of the steady-state absorption of the molecule as given in equation(3.1)
ΔA =dAdν
· Δ�μΔ�Eh
(3.1)
29
Figure 3.4. Scheme over the TA-setup used for measurements in this thesis. BS: Beamsplitter, WL: White light generation, Ch: Chopper, PD: Photodiode, DS: Delay state
where ΔA is the transient absorption signal, dA/dν , the first derivative of theabsorption, Δ�μ and Δ�E the change in dipole moment and electric field uponexcitation, respectively.
To obtain information about kinetics of the sample, TA spectra at differenttimes after excitation were recorded by changing the delay between probe andpump pulse. This is typically done by directing one of the beams through anoptical delay line which changes the path length without affecting the positionand direction of the beam.
The laser setup for the UV/Vis TA measurements in this thesis is illustratedin figure 3.4[72]. Ultrashort 800 nm pulses are generated in the seed laser.With the help of a pump laser, these are amplified in a Ti:Sapphire amplifier.Its output beam is split into two fractions by a beam splitter. One fraction isconverted to the desired excitation wavelength in a optical parametric ampli-fier (TOPAS). Before being directed on the sample, it passes through a chopperblocking every second pump pulse, which allows for shot-to-shot comparisonof pumped and unpumped sample. The other fraction of the 800 nm light isfirst directed through a delay line to allow for adjustment of the time delaybetween pump and probe. Afterwards, it is converted to a super continuum(white light) by focussing into a CaF2 or sapphire crystal. The white light issplit by a beam splitter into a probe and a reference beam accounting for fluc-tuations in probe light intensity. Both pump and reference beams are directedthrough a grating to the detector consisting of diode arrays.
For the measurements in the mid-IR, a similar setup is used[73]. The onlydifferences are that the infrared probe is generated by a TOPAS with sub-sequent difference generation. Furthermore, the pump is delayed to avoidchanges in absorption of the mid-IR probe light by air due to changes in thepath length.
30
Figure 3.5. Schematic of the calculation of decay associated spectra (DAS). Left:Spectra at different time delays evolving from purple to dark red. Middle: Kinetictraces, at different wavelengths indicated in the spectra by the colored lines. Right:Decay associated spectra, i.e. amplitude of the components at the fitted wavelengths
3.3 Data AnalysisThe transient absorption data recorded according to the procedure describedabove were analyzed with a home-written matlab routine. From the recordedTA spectra at different time delays, kinetic traces were extracted by plottingthe TA signal at certain wavelengths against time (see figure 3.5, middle). Theso obtained kinetic traces were fitted to a sum of exponential decays and anoffset according to equation (3.2)
ΔA(t)λ = ∑i
ci(λ ) · e−t−t0(λ )
τi (3.2)
Because of the refractive index, the speed of light in media is wavelengthdependent. Therefore, light with different wavelengths reaches the sample atdifferent times, hence the maximal overlap between pump and probe (timezero, t0(λ )) is wavelength dependent as well. This phenomenon is referredto as chirp. To account for this, t0(λ ) is used as a floating parameter duringthe fitting procedure for all traces. To correct the spectra from the chirp, thefitted t0(λ ) are interpolated and subtracted from the measured time for everywavelength accordingly.
The rise of the components was calculated by convoluting it with a Gaussianshaped excitation pulse whose full width at half maximum (FWHM) was alsoa free fitting parameter. Except for the rise of the signal around time zero,there is also an artifact[74–76]. This originates from cross phase modulationwhich is the non-linear interaction between pump- and probe pulse and can bedescribed according to equation (3.3)[74].
ca · e−(t−t0)
2
2σ2 · sin(δ −β ∗ (t − t0 −Δt)2) (3.3)
The kinetic traces are then fitted according to equation (3.2) convoluted witha Gaussian response and the artifact according to equation (3.3). Hereby, the
31
lifetimes of the components are fitted globally, i.e. wavelength independent.Decay associated spectra (DAS) are the wavelength dependent coefficients ofthe different components. Figure 3.5 shows the schematic calculation of thedecay associated spectra from the global fitting. These contain informationabout the spectral changes associated with the corresponding lifetime. Fromthis, conclusions about the ongoing processes can be drawn by comparisonbetween the DAS and those expected for these processes if reference spectra(e.g. of the reduced state) are known.
A point that does not include any spectral change, like 532 nm in figure3.5, is called isobestic point. Its position and shifts are indicators for presentspecies and the number of different processes. Compared to differences inpeak positions, changes of the isobestic points are easier to distinguish andfacilitate therefore data analysis.
32
4. Hole Trapping in Nickel Oxide
4.1 Aim of the studyThe fast recombination in p-type DSSCs is believed to be the main reason forthe difference in solar conversion efficiency compared to their n-type counter-parts[36–40]. As described in section 2.4, recombination in sensitized NiOfilms was found to occur in a biphasic fashion, which was attributed to twodifferent kinds of holes[55]. Understanding of the nature of the holes is im-portant for designing cells with slower recombination. To date, a commonlyused strategy is the application of dyads. These are, however, syntheticallydemanding[77–79].
To investigate hole kinetics in NiO, UV/Vis transient absorption upon band-gap excitation (paper I) of bare NiO and transient mid-IR of sensitized films(paper II) were performed. After bandgap excitation, which leads to the cre-ation of electron-hole pairs, the spectra obtained in the UV/Vis region werecompared to reference spectra of different nickel states that were recordedpreviously[80].
Transient IR spectrosopy of (dye-sensitized) TiO2 films revealed a feature-less broad band signal which was attributed to free charge carriers (electronsin the conduction band)[21, 22]. For NiO, similar signals might arise fromfree holes in the valence band (VB) as those also are free charge carriers.Therefore, these are two complementary measurements for investigation ofthe hole trapping process in NiO. To be able to attribute the possible mid-IRsignal to holes, transient mid-IR absorption was performed on NiO films withtwo different dyes: a perylene monoimide (PMI), which exhibits typical re-combination within 10 ps to 100 ps and a perylene monoimide - naphthalenediimide dyad (PMI-NDI) having a much longer excited state lifetime due thecharge shift from the PMI to the NDI moiety resulting in a longer electron-hole distance[77]. Hole kinetics are expected to follow the same trend, unlesshole trapping leads to a decreased or changed transient signal. However, sucha disappearance of the free charge carrier signal is not observed for electronsin TiO2[21, 81] upon electron trapping[42, 46, 82].
4.2 Results and DiscussionFirstly, an unsensitized NiO-film was excited at 355 nm. The subsequent TAspectrum in the fs-setup was globally fitted by a biexponential decay and an
33
Figure 4.1. Decay associated spectra of an unsensitized NiO film upon bandgap ex-citation at 355 nm. The TA data were fitted from 1 ps with a biexponential decay andan offset. For comparison, the 126 ps DAS is also shown enlarged by a factor of 10.Figure reprinted from paper I
offset. The corresponding decay associated spectra (DAS) are shown in figure4.1. The 2.9 ps component and the long offset component (τ3 >> 2ns) showsimilar spectral shapes with IA at wavelengths longer 500 nm. The 126 pscomponent has a DAS displaying a broad band with a maximum peak around470 nm.
The DAS could be compared to previously obtained reference spectra[80](see figure 4.2). The infinite component shows large similarity with Ni4+-states, thus trapped holes. As there is no pronounced rise in the DAS of the twofaster components, this spectrum contributes to the TA from the beginning.Because of this, hole trapping at Ni3+ states to form Ni4+h is concluded to takeplace before the start of the fitting, thus 1 ps.
The DAS of the 2.9 ps component exhibits a broad absorption absorption inthe below 500 nm. This feature often is assigned to electrons in the conductionband of TiO2[83, 84]. Assuming that the electrons in the valence band of NiOgive similar signals, this process can be assigned to electron trapping. The126 ps component shows good agreement with the steady state absorption ofNi3+-states. Due to the similarity of DAS, this process can be attributed toa change from Ni3+-states to a non absorbing state, i.e (Ni2+). Hence, thisprocess can be attributed to electron trapping in deep trap states.
The hole trapped in a NiO4+ can perform hole relaxation while undergoingthe following reaction
Ni4+h +Ni2+ → Ni3+h +Ni3+
The relaxed hole in a Ni3+h state has a lower reactivity than its counterpart ina Ni4+h state. This is an explanation for the above mentioned biphasic recombi-
34
Figure 4.2. Comparison of the 126 ps and the infinite component with previouslypublished reference spectra of the nickel states[80]. Figures reprinted from paper I
nation kinetics as reported in reference 55. This process could be observed fora NiO film sensitized with a Ru-NMI dye (for molecular structure, see figure6.1). 20 ns after excitation, the TAS showed contribution from both Ni4+ statesand the Ru-NMI dye (see figure 9 in paper I). At 5 μs, the Ni4+ features havedisappeared while the Ru-NMI features still remain. Due to that, the holerelaxation time can be assigned to take place on a time scale of tens of nanoseconds, even though this depends on the concentration of Ni2+ impurities.
Summarizing the findings above, Ni3+-states can act as efficient electronand hole traps in NiO. In the case of hole trapping, a reactive Ni4+h -state isformed. This state relaxes within tens of nano seconds to a less reactive Ni3+hhole. This process is the explanation for the biphasic recombination dynamicsreported previously. This is also likely the explanation for the large differencein CS lifetime of dyads compared to single dyes only as e.g. reported byreference 77.
The observed TA spectra upon band gap excitation have, however, onlysmall signals (around 1 mOD). This is in many cases smaller than the TAsignal from the dyes obtained with our setup (order of 10 mOD see below).Thus, hole kinetics are often shaded by those of the dye, even though they arepossible to detect as discussed above. Mid-IR is a potential alternative han-dle for direct spectroscopic observation of holes. Performing transient mid-IRabsorption spectroscopy on sensitized NiO films, there is indeed a broad bandsignal (see paper II, figure 3). Because of the absence of spectral features, thekinetics were constructed by taking the average over the whole spectral rangein order to diminish noise. For the kinetics of the sensitized NiO films, thereis a clear signal within the first 100 ps (see paper II, figure 4). For the unsensi-tized NiO and sensitized ZrO2, there is only a very short-lived signal (within0.5 ps) which is attributed to an artifact due to cross phase modulation. In an-other study performed in our group, a NiO film sensitized with a dye and acatalyst was investigated by transient mid-IR absorption spectroscopy. There,only the short-lived signal was observed, which was attributed to the absenceof hole injection due to energy transfer from the dye to the catalyst[85]. There-
35
�
�
�
�
Figure 4.3. Comparison of the kinetic traces of PMI and PMI-NDI in the UV/Vis andmid-IR upon excitation at 540 nm with an intensity of 125 nJ/pulse.
fore, the signal on the sensitized films can be attributed to the hole in the nickeloxide.
When comparing the decay kinetics of the hole signal with that of the PMI-anion, there is a good agreement between the two (see figure 4.3). This is,however, not true for the PMI-NDI dyad, which exhibits a much longer life-time of the CS compared to PMI. The decay of the hole signal is unchangedfor PMI and PMI-NDI, thus indicating that this is due to hole trapping. Thisis at a slightly slower time scale compared to the results from bandgap ex-citation in paper I. This difference in hole trapping lifetime could either beexplained by different nature of the holes generated by bandgap excitation andinjection, respectively. Furthermore, the sensitization dyes could also influ-ence the nature of the hole traps, thus changing trapping kinetics. Despitethe slight difference in timescale, hole trapping was found to take place at thesub-ns time scale. This is potentially an explanation of the usually found fastrecombination in NiO based DSSCs.
4.3 Future workTo investigate whether hole trapping takes also place at a timescale longerthan 1 ps upon bandgap excitation, transient mid-IR absorption might be con-clusive. To be able to distinguish between electron and hole dynamics, anelectron or hole scavenger might be useful. Unfortunately, such an experi-ment is not possible to date with the setup used, as it does not allow for UVexcitation. However, an upgrade of that measurement setup is about to come,which will allow such experiments to be performed.
36
4.4 Implications for NiO-based p-type DSSCsThe understanding of hole trapping in dye sensitized NiO films leads to thepossibility to develop strategies to improve the performance of NiO basedDSSCs. Ni3+-states were found to be trapping sites that form reactive holes.Therefore, their reduction by physical or chemical means might slow down therecombination kinetics[80]. Another possible strategy is the prolongation ofthe CS lifetime (often 100s of ps[40, 86–90]) to exceed hole relaxation (tensof ns as discussed above). After the hole relaxation to form a less reactive hole(Ni3+h ), the CS lifetime might be on the ms to s time scale as seen for manydyads[40, 77, 79, 91]. Hence, it is only a formal increase in lifetime of onethe order of magnitude to obtain a prolonged charge separation by 2 to 3 or-ders. The resulting recombination time scale would then be at the same orderof magnitude as that for dye-sensitized TiO2-films. By that, the most efficientloss process in p-type DSSCs would be suppressed.
37
5. Perylene Monoimides with DifferentAnchoring Groups
5.1 The different Anchoring GroupsIn paper III, the influence of the binding group of a dye on the performanceof NiO-based p-type DSSCs was reported. To date, the most commonly usedanchoring groups are carboxylate and phosphonate groups[92]. The basic ideaof this study was to change the binding group of a perylene monoimide (PMI)dye and investigate influence of the binding group on the photovoltaic perfor-mance. The structure of the dye and the anchoring groups used are shown infigure 5.1. The binding properties of the different groups such as dye load-ing and desorption were investigated. The photovoltaic performance in NiObased DSSCs were measured and related to charge separation, recombinationand regeneration investigated by TAS.
NO Ot-Bu
t-Bu
O O
t-Bu t-Bu
O OH
N
PO3H2
NOH
O O
NOH
O
OH
O
NH2
=PMI-CO2H PMI-Py PMI-HQ PMI-NH2
PMI-acacPMI-PO3H2 PMI-DPA
Figure 5.1. Molecular structure of the perylene monoimides with different anchoringgroups investigated in paper III.
5.2 Results and discussionAll PMIs of the series show similar steady-state absorption and emission spec-tra: The absorption has a maximum peak around 540 nm and a weaker vibronicshoulder at ca. 500 nm, which is typical for a π −π∗ transition of PMI-baseddyes. Both PMI-NH2 and PMI-HQ show a slightly broadened and red shiftedabsorption compared to the other dyes (see figure 1 in paper III). This red shiftis attributed to the electron donating character of these anchoring groups.
38
Figure 5.2. Steady-state absorption and photographs of the PMIs coloring the NiOfilms (left) and dye loading obtained from desorption experiments (right) Reprintedfrom paper III
Dye loading studies were performed by soaking NiO films in DMF solu-tion of these dyes. PMI-Py and PMI-NH2 did not attach to the film in adetectable amount. In contrast, dye soaking with the other dyes led to coloredfilms (see figure 5.2). Investigating their absorption spectra, a change in in-tensity of the two absorption bands is observed, namely the 500 nm band hasa higher absorption than the 540 nm one. (A comparison of the absorption ofPMI-CO2H in solution and on NiO can be found int the supporting informa-tion of paper III). Similar changes in intensity have previously been attributedto aggregation[93, 94]. Hence, all investigated PMIs show signs of aggre-gation independently from the anchoring group. Dye loading was measuredthrough desorption experiments by soaking the sensitized NiO films in a DMFsolution of phenyl phosphonic acid. Both absorption of the sensitized film andthe dye loading measurements suggest the following order of affinity for theanchoring groups PMI-PO3H2 > PMI-DPA > PMI-CO2H > PMI-HQ >PMI-acac.
Figure 5.3. Transient absorption spectra of PMI-CO2H (left) and PMI-HQ (right)in THF solution after excitation at 540 nm. For comparison the negative steady stateabsorption (dotted lines) and emission spectra (dashed lines) are displayed as well.
39
Figure 5.4. Transient absorption spectra of PMI-CO2H (left) and PMI-HQ (right)sensitized on NiO in propylene solution after excitation at 540 nm. For comparisonthe negative steady state absorption (dotted lines) and emission spectra (dashed lines)are displayed as well.
To investigate the influence of the anchoring group on the electron transferprocesses, TA measurements were performed of the PMIs in THF solution andgrafted on NiO in presence of propylene carbonate or the electrolyte includingthe redox mediator. The TA spectra of PMI-CO2H and PMI-HQ in THFsolution are shown in figure 5.3. PMI-CO2H shows negative TA featuresfrom 480 nm to 650 nm and positive features in the remaining spectral region.The bleach corresponds well to the negative steady state absorption (GSB) andemission (SE), as expected for a singlet excited state. For PMI-HQ, there aresimilar features at later times (longer than 10 ps). At shorter times, there arespectral changes in the region of SE due to a Stokes shift of the emission.
After sensitizing NiO with the dyes, the shape of the TA changes (see fig-ure 5.4). There is still the GSB but the SE has disappeared. Instead, there is abroad and strong IA at wavelengths below 580 nm which can be attributed tothe reduced state of the PMI unit as observed the spectroelectrochemical re-duction experiments of PMI-CO2H, (see figure 5 in paper III). Hence, it canbe concluded that hole injection takes place. For the PMI-HQ, the early spec-trum (0.1 ps) has a negative feature in the region of the SE which potentiallycan be attributed to some remaining SE. This feature, however, disappears atlater times to leave a positive IA feature similar to PMI-CO2H. Hence, holeinjection can be concluded to take place for PMI-HQ as well but at a slightlyslower time scale.
The TA kinetics of the PMIs on NiO are shown in figure 5.5. In propy-lene carbonate solution, PMI-CO2H, PMI-PO3H2, PMI-acac and PMI-DPA
show basically the same kinetics. Only PMI-HQ shows a fast component atthe sub-ps time scale which is rationalized by the slower hole injection than forthe other complexes as discussed above. The recombination kinetics, howeverare unchanged for all PMIs.
To be able to observe possible differences in time scale of regeneration, theTA of the PMI-sensitized films in the T2/T--electrolyte (T- is 1-methyl-1H-
40
Figure 5.5. Kinetics traces of the TA spectra of PMI-sensitized NiO films in propy-lene carbonate (left) and in presence of the T2/T- electrolyte (right). The excitationwavelength was 540 nm.
Table 5.1. Photovoltaic performance and dye loading of the PMIs
DyesJsc Voc [mV]
FF [%] η [%] Dye loading
[mAcm−2] [nmolcm−2]
PMI-CO2H 1.52 ± 0.06 161 ± 7 25.4 ± 0.3 0.062 ± 0.001 22.3PMI-HQ 2.21 ± 0.09 164 ± 4 23.8 ± 0.6 0.086 ± 0.003 9.3PMI-DPA 1.33 ± 0.09 168 ± 6 24.6 ± 0.6 0.055 ± 0.005 23.7PMI-acac 2.08 ± 0.08 169 ± 8 27.9 ± 0.2 0.098 ± 0.002 7.7
PMI-PO3H2 1.27 ± 0.12 181 ± 7 17.7 ± 0.4 0.041 ± 0.004 38.2
tetrazole-5-thiolate and T2 its dimer, for molecular structures, see supportinginformation of paper III) were investigated as well. There is the same trendas in the propylene carbonate only. PMI-CO2H, PMI-acac and PMI-DPA
show basically the same kinetics. Again, PMI-HQ shows slightly slower holeinjection compared to the other complexes. The decay of the signal in presenceof the T2/T- electrolyte is slower compared propylene carbonate only, an effectthat was observed earlier[95]. Thus, it is not possible to quantify regeneration.However, there is no obvious change in recombination/regeneration kineticsof the PMI sensitized NiO films in the presence of the electrolyte.
This result is encouraging as the anchoring group does not seem to affect thekinetics significantly. Hence, no large changes in photovoltaic performanceare expected. Upon comparison of the photovoltaic performance of the dif-ferent PMIs (see table 5.1), it can be noted that there are differences amongthem. Surprisingly, the highest efficiencies are obtained with the dyes withlowest dye loading and vice versa. The open circuit voltage and fill factor aremostly unchanged. The largest difference is in the short circuit current wherePMI-acac and PMI-HQ show approximately twice the JSC compared to theother PMIs.
To understand the difference in short circuit current and resulting photo-voltaic efficiency, incident photon-to-current efficiencies (IPCE) of the cellswere measured (Figure 5.6). For PMI-HQ, the IPCE spectrum is broader than
41
0%5%10%15%20%25%30%35%
400 450 500 550 600 650
IPCE(%)
Wavelength (nm)
PMI-acacPMI-CO2HPMI-HQPMI-PO3H2
Figure 5.6. Incident photon to current efficiencies (IPCE) of p-type DSSCs based onthe PMIs utilizing the T2/T- redox mediator.
that of the other PMIs, which is the reason for the higher photocurrents andphotovoltaic efficiency. The spectral shape of the IPCE curves of PMI-CO2H,PMI-DPA, PMI-acac and PMI-PO3H2 do not show deviations among eachother. Interestingly, their spectral shape is more similar to that of the aggre-gates than that of the monomeric (solution) ones. Hence, the aggregates alsocontribute to the photovoltaic performance to some extend.
The most striking result is the much higher IPCE values for PMI-acac (ca.30 %) compared to the other ones with IPCE of ca 20 %. This 30 % IPCE ismore than the remaining TA signal for PMI-acac (below 20 %). Therefore,it can be concluded that (some) regeneration takes place on the sub-ns timescale.
In summary, there are no significant changes in charge separation and re-combination dynamics of those PMIs that bind to the NiO surface. This allowsfor free choice of anchoring group depending on binding properties withoutconsidering potential changes in charge carrier dynamics. However, aggrega-tion was found to have the major impact on NiO-based DSSCs produced withthe present series of PMIs.
42
6. Self-quenching limits the efficiency of NiObased DSSCs
6.1 Aim of the StudyIn the following chapter, I will describe the potential influence of a self-quenching process of Ru-based dyes on the photovoltaic efficiency of NiObased p-type DSSCs which was found in paper IV. Originally, the dyes -NRuN,-NRuC and -CRuN (for structures, see figure 6.1) were designed to featuredifferent localization of the excited state. All three compounds are rutheniumcomplexes based on bis-tridentate diquinolinylpyridine (dqp) ligands. -NRuC
and -CRuN have one cylcometalated ligand (dqb), thus one ligand having acoordinating nitrogen atom exchanged by a carbon. As the cyclometalationleads to more negative reduction potential, the MLCT state is expected to lo-cate the electron on the non-cyclometalated ligand[88, 96–99]. In addition,the cyclometalation leads to extension of the HOMO on the cyclometalatedligand[100–103]. Different orientation of the MLCT transition might influ-ence the hole injection and recombination properties of the dyes. The dif-ference between -NRuC and -CRuN is the relative orientation towards thecarboxylic acid anchoring group leading to the cyclometalated ligand furtheraway or closer to the NiO surface for -NRuC and -CRuN, respectively, hencebeing a barrier for HI in the case of -NRuC and -CRuN facilitating this pro-cess in the case of -CRuN.
To obtain the reference spectra of the reduced state of the spectra, spectro-electrochemistry was performed. Mesoporous NiO and ZrO2 films were sen-sitized with the compounds by immersing them in a concentrated dye solutionin acetone over night. Subsequently, TAS was performed in order to investi-gate whether the hole injection properties follow the expectations as describedabove.
NHOOC
N
N
N
N
N
Ru
-NRuN
NHOOC
N
N N
N
Ru
-NRuC
HOOC
N
N
N
N
N
Ru
-CRuN
Ru
NN
OHO
OH
O
NN
O
OH
OHO
N
NN
O2N
O
O
Ru-NMI
Figure 6.1. Structures of the investigated compounds. Reprint from paper IV
43
Furthermore, the time scale of hole injection of a Ru-based dye (Ru-NMI)known to show photovoltaic performance[104] was investigated. The direc-tion of the MLCT state is not favoring hole injection as the electron is expectedto be localized on one of the dicarboxybipyrine (dcbpy) ligands. The IPCE ofthis dye was, however, reported to be 20 %. The hole injection was assumedto be ultrafast but not proven. Hence, its timescale was investigated by meansof TAS as well.
6.2 Results and DiscussionUpon excitation in solution, the three dyes show similar photophysical prop-erties to those that do not have an anchoring group[96, 97]. The lifetime for thecyclometalated complexes is in the order of 10 ns while the non-cyclometalated-NRuN has a longer lifetime of 220 ns in air saturated solution.
After sensitization of the dyes on the semiconductor surfaces, there are noobvious spectral changes (see figure 3 in paper IV). In contrast to the PMIs inpaper III, there is no obvious sign of aggregation of the ru-complexes on theNiO surface. However, the absorption band of the cyclometalated complexesis broader and less structured compared to the ones of the PMIs. Therefore,aggregation cannot be excluded completely.
The spectral features of the TA do not change when sensitizing the com-plexes on the wide bandgap semiconductor ZrO2 (see figure 6.2). When sensi-tizing the complexes on NiO, only -NRuN shows spectral changes of the TA.This could originate from (inefficient) hole injection. For the cyclometalatedcompounds -NRuC and -CRuN, spectral changes are negligible. This couldeither be explained by the absence of hole injection, or by HI followed by aneven faster recombination.
When investigating the kinetics of the signal, it can be noted that the de-cay of the signal is accelerated when sensitized on a semiconductor surfaceas compared to in solution (see figure 6.2). For -NRuN, the kinetics on NiOis slightly accelerated on NiO compared to ZrO2. This is attributed to holeinjection at a timescale of approximately 100 ps to 1000 ps. The yield andexact time scale of this process can, however, not be determined as the sig-nal of the reduced dye is overlaid by the TA of the excited state. For -NRuC
and -CRuN, there is no significant change in kinetics between sensitized ZrO2and NiO. Therefore, hole injection can be excluded for the two cyclometalatedcompounds -NRuC and -CRuN. Hence, the major deactivation pathway forthese compounds is attributed to self quenching. Concentration quenching isa phenomenon that has been known for a long time in solution[105, 106], inliposomes[107] and on surfaces[108]. It can be noted that the quenching timein our system is on the same order of magnitude as the common recombina-tion time in NiO based DSSCs[40, 86–90]. Therefore, a similar unproductivequenching process of the excited state might easily be mistaken for recombina-
44
Figure 6.2. Selected decay associated spectra (left) and kinetic traces of the IA andthe GSB normalized at 1 ps (right) of the three Ru-complexes -NRuN, -NRuC and-CRuN. The decay associated spectra of the components of the cyclometalated Rucomplexes -NRuC and -CRuN did only show minor deviations and no sign for holeinjection.
45
Figure 6.3. TA spectra of a Ru-NMI sensitized NiO film after excitation at 400 nm.
tion of the reduced state as HI normally is taking place within the instrumentalresponse function of the transient absorption setup. This applies especially fordyes with similar excited state and reduced state TA spectra.
When exciting a Ru-NMI-sensitized NiO film at 400 nm (see figure 6.3),the initial TA spectrum resembles that of the excited state as reported previ-ously [104]. At later times (mostly above 1 ns (figure 6.3, right), the GSB ofthe Ru-MLCT-state disappears and a peak around 510 nm appears. The finalspectrum can be assigned to the reduced NMI unit of Ru-NMI by comparingto the reference spectrum[104]. Hence, the hole injection takes place on aunprecedentedly slow time scale of approximately 5 ns.
By comparing the initial amplitude of the GSB with that of the NMI•−radical, and assuming similar extinction coefficients[109, 110], the yield ofRu-NMI-reduction on NiO can be estimated to be around 20 %. This is com-parable with the already reported absorbed photon to current conversion effi-ciency (APCE)[104].
Hence, the excited state quenching of Ru-NMI seems to be the main lossprocess in Ru-NMI-based p-type DSSCs. Hence, the quenching observed inpaper IV is a potential bottle neck for p-type DSSCs employing slow injectingdyes. This has especially be kept in mind because one strategy for prolonga-tion of the CS lifetime is the decoupling of dye and semiconductor, either bynon-conjugated linkers[111] or insulating layers deposited by ALD[112, 113].
46
7. Ultrafast regeneration in Core-ShellStructure DSSCs
7.1 Design of the Core-Shell StructureThe competition between regeneration and recombination between dye andsemiconductor is a crucial factor for the overall efficiency of DSSCs. A com-monly used strategy to change to favor the regeneration is the prolongation ofthe lifetime of the CS state[40, 77, 79, 91]. In many cases, dye-acceptor dyadsare used to slow down recombination by increasing the charge separation dis-tance. The same effect would also be obtained by accelerating regeneration.In electrolyte based DSSCs, regeneration often is limited by (slow) diffusionof the mediator.
In solid-state DSSCs (ss-DSSC) , the liquid electrolyte is replaced by ahole- or electron conducting material (HTM, ETM). This diminishes the riskof leaking and drying out of the cells. Furthermore, it was shown that theregeneration time is shortened significantly due to the elimination of moleculardiffusion processes[51, 52, 60]. However, in p-type ss-DSSCs, regenerationwas still not much faster than recombination, leading to low efficiencies.
One efficient ETM is TiO2 which is frequently applied in n-type DSSCs.There have been few studies trying to put together p-type NiO and n-type TiO2to create a dye sensitized p-n junction[114–116]. In those studies, however,mesoporous NiO and TiO2 were prepared independently and mixed subse-quently or sensitized after preparation. Therefore, only few molecules wereoriented between the two semiconductors and could contribute to hole- andelectron injection. In paper V, we propose the application of atomic layer de-position (ALD) to bury a mesoporous dye sensitized NiO film in TiO2 to allowfor good contact between the dye and TiO2 to facilitate electron injection. Thestepwise process to prepare the suggested core-shell structure is described infigure 7.1.
As ALD requires relatively high temperatures, a thermally stable CT dye(PB6, structure shown in figure 7.1) was designed. As observed by EDX,there was good penetration of the TiO2 into the mesoporous PB6-sensitizedNiO film (see figure 2 in paper V). Upon excitation, the electron is expectedto be located at the PMI unit, thus in close proximity to the TiO2 while thepositive charge is located on the TPA unit, thus close to the NiO facilitatingboth hole and electron injection. Both processes often are observed to bepredominantly ultrafast. The TiO2, however, is in contrast to common n-typeDSSCs not completely crystalline. In addition, there is only physical contact
47
Figure 7.1. Left: scheme of the processes leading to the proposed core-shell meso-porous film allowing the orientation of hole injection into NiO and electron transferinto TiO2 (regeneration). Reprinted from paper V, SI. Right: Molecular structure andexpected orientation of PB6 in the Core-shell system.
between PB6 and TiO2 instead of covalent linkage and conjugation of thelinker in common sensitized TiO2 films. Therefore, the question was whetherelectron injection can take place and which process is the faster one.
In order to differentiate the electron and hole injection as well as to obtainreference spectra, transient absorption of PB6 on NiO, TiO2, ZrO2, and in onNiO and ZrO2 with ALD TiO2 respectively, was measured.
7.2 Results and discussionThe TA spectra upon excitation of NiO-PB6-TiO2 are shown in figure 7.2left. The initial TA spectrum shows IA with a peak around 440 nm and below630 nm while, there is a GSB from 480 nm to 630 nm.
�
�
�
�
Figure 7.2. TA spectra of NiO-PB6-TiO2 upon excitation at 550 nm at different timedelays (left), and TA spectra averaged from 390 ps to 1900 ps compared with the firstderivative of the ground state absorption of the film to predict the Stark-effect signal(right)
At later times, the spectrum changes and features a red shift of the IA from400 nm to 550 nm and a ground state bleach at longer wavelengths up to ca.
48
�
Figure 7.3. Kinetics of NiO-PB6-TiO2 (left) and comparison of the TA spectra ofdifferent PB6-sensitized films 0.6 ps after excitation. Reprint from paper V
700 nm. This late spectrum follows well the first derivative of the groundstate absorption spectrum with respect to wavelength (see figure 7.2, right).Therefore, this signal can be attributed to a Stark effect originating from theelectric field between injected electrons and holes close to the molecules.
The kinetic traces at 530 nm and 670 nm show a change in sign upon for-mation of the Stark-shifted spectrum with a half time below 1 ps. Therefore, itcan be concluded that both hole injection into NiO and electron injection intoTiO2 take place at ultrafast fast timescales.
Comparing the spectra at 0.6 ps after excitation, there is a shoulder around630 nm for NiO-PB6-TiO2, which is not present in ZrO2-PB6-TiO2. Thisshoulder can be attributed to the reduced PMI unit of PB6•−, which resultsin similar spectra of the reduced form as the PMIs in paper III. Thus, holeinjection into NiO takes place first, namely within the response of the TAsetup (≈ 200fs). Observing the kinetics at 530 nm (from GSB to the positiveStark feature) and 670 nm (from IA to the negative Stark feature) shows thatthe electron injection is fast as well (t1/2 < 1ps). As the Stark effect requiresstrong electric fields, and there are no clear signs of oxidized or reduced PB6,it can be concluded that both electron and hole injection take place from thesame molecules. Hence, appearance of the Stark effect gives the time scaleof dye regeneration. To our knowledge, this is unprecedentedly fast, with asub-ps half time.
It can also be noted that there is no significant recombination between thehole in NiO and the electron in TiO2 within the 1.9 ns time window of the TAsetup. For most NiO sensitized films, including NiO-PB6, the CS lifetime is inthe order of tens to hundreds of picoseconds. The absence of any significantdecay of signal of NiO-PB6-TiO2 within the first 1.9 ns is therefore a quitelarge prolongation, making this concept interesting for potential application indye sensitized solar cells. The recombination rate, however, cannot be esti-mated by the disappearance of the Stark-signal as charge migration within thesemiconductors also would lead to a decreased electric field and therefore asignal decay.
49
One potential problem for the application in DSSCs is the structure of ALDTiO2. Due to the relatively low temperature during the ALD process, the TiO2was found to be amorphous instead of being crystalline. This might have anegative effect on its electron transport properties. Another potential drawbackis the direct contact between NiO and TiO2 which might open up for chargerecombination between the hole in NiO and the electron in TiO2. A possibleway to diminish this recombination is the introduction of an insulating layer,such as Al2O3, in between the two semiconductors.
50
8. Macromolecular Pentad for Water Splitting
8.1 Expected processesIn the following chapter, I will discuss the findings of paper VI, investigating amacromolecular pentad which was designed to photochemically oxidize waterto molecular oxygen, the anodic reaction of a photoelectrochemical cell. Asdiscussed in section 2.5, water oxidation is a four electron - four proton processwhich is therefore catalytically demanding. In this paper, however, only thefirst oxidation step of the catalyst was investigated spectroscopically.
Figure 8.1. Chemical structure of the molecular Ru-(ZnP-C60)2-pentad.
The pentad, whose chemical structure is shown in figure 8.1, was designedas follows: The center is a well known molecular Ru based (electro)chemicalwater oxidation catalyst showing high turn over frequencies and turn overnumbers[117–119]. At each axial pyridine ligand, there is a zinc porphyrin(ZnP) substituted. This provides good light harvesting properties due to highextinction coefficients and a broad range of visible light absorption[120]. Thetetraethylene glycol groups on the ZnP units improve the water solubility.Connected to each ZnP, there is a Buckminster fullerene (C60) which actsas electron acceptor with small reorganization energy[121]. Many ZnP-C60dyads have been investigated and show generally electron transfer from theexcited ZnP to the C60 moiety[122–125].
The expected processes upon excitation of the ZnP moiety (the only onewith appreciable absorption) of the Ru-(ZnP-C60)2-pentad are shown in fig-ure 8.2. Upon excitation, PET between the ZnP* and the C60 might take place.This process has a decent driving force. The subsequent charge shift (CSh)of the positive charge from the ZnP to the Ru catalyst is ≈ −0.53eV for thefirst oxidation of the catalyst. Subsequent oxidations of the WOC, however,
51
Ru (ZnP C60)2
GCS = 0.56 eV
GCSH > 0.05 eV
ZnP* (S1)
ZnP•+ C60•
exc.= 560 nm
Ru•+ C60•
Figure 8.2. Overview of the possible charge-transfer processes after excitation of theRu-(ZnP-C60)2 pentad. The blue arrows indicate the wanted forward processes andthe red ones the unwanted back reactions. For clarity, the unchanged units of theRu-(ZnP-C60)2 pentad are omitted in the excited and charge separated states.
become less favored with driving forces down to −0.05 eV for the fourth ox-idation step for water oxidation. This is a general problem for accumulativecharge transfer[69].
Despite the desired forward reactions (as indicated by the blue arrows infigure 8.2), there are deactivation processes such as excited state deactivationand charge recombination (red arrows). In order to investigate the first oxi-dation of the catalyst, transient absorption spectroscopy was performed. Onecomplication is that the ZnP is the only moiety that gives spectral signaturesexcept the C•−
60 radical that has a pronounced absorption around 1000 nm, awavelength range that is out of the detecting range of the femtosecond TASsetup. Thus, the only detectable moiety is the ZnP. To be able to obtain rateconstants for all processes and quantify the presence of possible quenching, aZnP-C60-dyad (ZnP-C60) and a Ru-ZnP2-triad (Ru-ZnP2) were investigatedas well.
All measurements were performed in a 30/70 THF/water (v:v) mixture.This solvent was found to be best for solubility and for promoting charge sep-aration and out competing energy transfer to the C60. To exclude the influenceof excess energy, the ZnP was excited in the Q-band at 560 nm for the fs-TASmeasurements.
To test the potential water oxidation capability, Ru-(ZnP-C60)2 was dis-solved in a solution of 15 mM methyl viologen as electron relay, and 15 mMsodium persulfate as electron acceptor in phosphate buffer (pH 8.0, 0.1 M)with 40 % THF. For the photocatalysis experiment, 6.2 mM and 12.3 mM so-lution of Ru-(ZnP-C60)2were exposed to a white light source (λ > 400nm)with an intensity of 100 mWcm−2. Indeed, photochemical O2 evolution wasdetected, even though with low yield.
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Figure 8.3. TA spectra of ZnP-C60(upper left), Ru-ZnP2(upper right), Ru-(ZnP-
C60)2(lower left), and overview of the found processes and the corresponding ratesupon excitation of the Ru-(ZnP-C60)2 pentad at 560 nm (lower right). For clarity, theparts that are unchanged with respect to the ground state are omitted.
8.2 Results and DiscussionThe TA of the ZnP alone, could not be investigated as it showed rapid ag-gregation in the solvent mixture (on the minute time scale), thus much fasterthan the time for performing one TA experiment. In contrast to that, ZnP-C60,Ru-ZnP2 and Ru-(ZnP-C60)2 did not show any sign of aggregation as provenby unchanged absorption spectra before and after the TA measurement and nochange in TA upon 12 consecutive scans. This might be rationalized by sterichindrance for π-stacking of the ZnP when being linked to other molecules.
When exciting ZnP-C60, (figure 8.3 upper left), the initial spectra (blue)feature a GSB around 430 nm and at 560 nm. Additionally, there is SE around600 nm and 660 nm accompanied by IA with peaks at 390 nm and 470 nm.Due to the SE, this spectrum can be attributed to the 1ZnP*-state. Especially inthe UV, the spectral form of the IA changes with simultaneous disappearanceof the SE. The remaining spectrum forms with a time constant of τ2 ≈ 20psand shows a biphasic recombination with time constants of 600 ps (from fs-TAS) and 1.7 μs (from ns-TAS, see figure 3 in paper VI). This spectrum canbe attributed to the ZnP•+-radical, thus the CS state of the dyad. The tworecombination components of the CS state can most likely be attributed tosinglet and triplet CS states where the latter has the longer lifetime due to thespin forbidden back electron transfer. Unfortunately, singlet and triplet states
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have basically identical spectral features in the visible making it impossible todistinguish between these two.
After excitation of Ru-ZnP2, there is again the TA spectrum of the ZnP*-state. In contrast to ZnP-C60, this signal decays with two time constants of4 ps and 30 ps without spectral changes. This excited state deactivation is at amuch faster time scale than the recombination in ZnP-C60 and even the sameorder of magnitude as the charge separation. As porphyrins mostly exhibit anexcited state lifetime in the nanosecond time scale, this fast deactivation mustoriginate from quenching by the Ru catalyst. This process could either takeplace via energy transfer (followed by fast excited state deactivation of the Ru-catalyst as reported earlier[126]) or by electron transfer followed by fast BET.As both cases do not accumulate any species, the nature of this deactivationpath cannot be further investigated.
When exciting Ru-(ZnP-C60)2, the TA shows initially the spectrum ofZnP* which is again followed by that of the ZnP•+ radical. However, theyield of reduction is much smaller compared to ZnP-C60. This can be ex-plained by the quenching process of the ZnP∗ by the Ru-catalyst found in Ru-
ZnP2. When measuring in the nanosecond regime, the lifetime of the reducedZnP•+ radical decays with a lifetime of 40 ns which is much faster comparedto the 1.7 μs of ZnP-C60. At the microsecond time scale, there is a signal ofthe C•−
60 radical absorbing at 1000 nm only decaying with a time constant of18 μs. Due to that, it can be concluded that the disappearance of the ZnP•+radical can be attributed to the expected CSh to the Ru-catalyst, hence arrivalof the first oxidation equivalent. By this, we were able to observe the completefirst electron cycle of Ru-(ZnP-C60)2.
The overall photocatalytic activity was measured and found to be very low.The Ru-catalyst is known to have its high activity due to di-radical couplingforming a peroxo species[119] needing two oxidation steps per catalyst (fromRuIII-OH2 to RuV=O). Due to that, the biradical coupling pathway avoids thelow driving force fourth oxidation of the Ru-catalyst (−ΔG = 0.05eV). How-ever, it is unlikely that this process is possible for Ru-(ZnP-C60)2 as there is ahigh probability of steric hindrance by the axially substituted ZnP-C60-dyads.One way to circumvent this steric hindrance is the synthesis of catalyst dyadsor triads which were found to improve the catalytic activity for chemical andphotochemical water oxidation[127].
In order to complete the catalytic cycle, subsequent oxidation steps of theRu-catalyst have to take place before recombination processes can take place.As there are only two photosensitizers per catalyst, the approximate photon ab-sorption rate is still in the order of 1 s−1 opening up for unproductive quench-ing processes.
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9. Conclusions and Future work
Photoinduced electron transfer processes play an important role in solar en-ergy conversion systems. In the present thesis, p-type nickel oxide films forpotential application in DSSCs (papers I to V) and a supramolecular pentadfor water oxidation (paper VI) were described.
Firstly, the hole trapping properties and kinetics of the p-type semiconduc-tor NiO were reported in papers (I and II). It was found that Ni3+-states couldact as electron and hole traps, respectively, upon bandgap excitation. Electrontrapping was found to take place with a 126 ps time scale to form Ni2+e whilehole trapping was assigned to occur at the sub-ps timescale forming reactiveNi4+h -states. Hole injection in sensitized films was furthermore observed byvisible pump mid-IR probe transient absorption spectroscopy featuring a broadband signal assigned to free charge carriers. The hole trapping in this systemwas found to take place at a 10 ps to 100 ps time scale. This difference in holetrapping might originate from different character of the holes produced uponband gap excitation and hole injection from a dye, respectively. The trappedhole (Ni4+h ) was found to relax on a time scale of tens of nano seconds result-ing in less reactive holes (Ni3+h -states). This explains the previously reportedbiphasic recombination kinetics[55] and the large difference in CR time scalefor dyads compared to the dyes only[77].
The identification of the hole trap states thus provides the possibility todevelop new strategies to affect the hole trapping and relaxation kinetics. Asa consequence, this can slow down recombination and favor thus regenerationof the dye. One direction is to reduce the number of Ni3+ states to avoidformation of the reactive Ni4+ holes, or to accelerate the hole relaxation tothe less reactive Ni3+h . Furthermore, this finding lowers the requirements forthe CS lifetime of photosensitizers: Prolonging the recombination time scale(with Ni4+h ) to more than 100 ns, thus one order of magnitude, the overall CSlifetime will likely expand to the ms to s time scale due to the slow phase ofrecombination[55].
The broad band mid-IR signal of the holes also provides a tool to observehole injection and trapping in NiO. These processes are very difficult to ob-serve with UV/VIS TA in dye-sensitized films because these processes oftenare overlaid by the stronger TA of the dyes.
In papers III and IV, the role of the dyes on injection and recombinationkinetics was investigated. For the perylene monoimide dyes with differentanchoring group from paper III, the binding affinity of the anchoring groupswas found to provide the largest difference among the dyes. There were no
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detectable changes in CS and recombination kinetics. Hence, it is possible tochoose the anchoring group according to binding properties without sacrifyinginjection efficiency. However, aggregation on the surface was found to occurfor all PMIs and resulted in diminished photovoltaic performance.
For the ruthenium based polypyridyl complexes that have different MLCTlocalization and were reported in paper IV, hole injection was found to beunusually slow or to not occur at all. Simultaneously, a self-quenching mech-anism was found for all those dyes that competes with the slow hole injection.This finding gives a warning for the design of p-type DSSCs with weakerelectronic coupling between dyes and a semiconductor, as this might result ina lower hole injection yield due to competition with a similar process.
A mesoporous NiO-dye-TiO2 core-shell structure was designed, producedand investigated in paper V. Hole injection took place on a <200 fs time scale.An unprecedentedly fast dye regeneration (i.e. subsequent electron injectioninto TiO2) on the sub-ps time scale was detected by a signal caused by theStark effect. This signal did not decay notably during the 1.9 ns time windowof the TA measurement. Hence, the recombination slowed down significantlycompared to the corresponding dye-sensitized NiO film. This structure mighttherefore be interesting for solar cell applications. However, the TiO2 wasfound to be amorphous, thus a potential bottle -neck for electron diffusionand extraction. Furthermore, the direct contact between NiO and TiO2 mightresult in relatively fast recombination (but still slower than 2 ns). A study ofthe performance of this core-shell structure in a DSSC was performed but notincluded in this thesis. (paper XI)
Finally, a macromolecular pentad consisting of a ruthenium based wateroxidation catalyst (WOC) linked to two zinc prophyrin-fullerene dyads (ZnP-
C60) was reported. The CT processes leading to the first oxidation of the WOCwere understood. It was found that the excited state of the porphyrin wasquenched by the ruthenium catalyst. This quenching was competing with theinitial charge separation within on of the ZnP-C60-moieties, thus lowering theconversion efficiency. The initial charge separation within ZnP-C60, yieldedin a charge shift to form the oxidized WOC on a timescale of 40 ns. The pentadshowed water oxidation under illumination in presence of sacrificial electronacceptors, albeit with low quantum yield.
The understanding of CT and recombination kinetics opens up for the de-velopment of new strategies for improvements of NiO based p-type DSSCsand DSSFCs. It would be easy to suggest to abandon NiO as p-type SC. How-ever, other p-type SCs like, CuCrO2, CuGaO2, KxZnO, etc., have not turnedout to be more efficient than NiO-based photocathodes[128, 129]. Therefore,NiO-based DSSCs and DSSFCs still are an important research field for solarenergy conversion.
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10. Zusammenfassung
Bei der Pariser Klimakonferenz wurde beschlossen, die globale Temperatur-erhöhung aufgrund des Treibhauseffektes auf zwei Grad zu beschränken. Umdieses Ziel zu erreichen, müssen die Treibhausgasemissionen stark gesenktwerden. Der Hauptgrund für diese Emissionen ist die Verbrennung von fos-silen Brennstoffen. Eine mögliche Strategie für deren Minderung ist die Ver-wendung von erneuerbaren Energien, wie z.B. Wind-, Wasser-, und Sonnen-energie. Dabei stellt die Sonneneinstralung die mit Abstand gößte Energie-quelle dar.
Die am weitesten verbreitete Technologie zur Verwendung von Solarener-gie sind Solarzellen, die heutzutage hauptsächlich aus Silizium bestehen. Si-liziumsolarzellen haben jedoch die Nachteile einer verringerten Effizienz beigeringerer Sonneneinstralung und eines großen Energiebedarfs bei der Her-stellung. Neue Solarzellstechnologien, die noch im Forschungsstadium sind,vermeiden diese Nachteile und haben darüber hinaus den Vorteil, dass siedurchsichtig und farbig sein können. Dadurch können sie potentiell in die Ar-chitektur eingearbeitet werden. Eine solche Technologie, deren Entwicklungschon weit fortgeschritten ist, sind Grätzelsolarzellen, die nach ihrem ErfinderMichael Grätzel benannt sind.
Abbildung 10.1. Funktionsprinzip undAufbau einer Grätzelsolarzelle.
Deren prinzipieller Aufbau undFunktionsweise sind in Abbildung10.1 dargestellt. Sie bestehen auszwei Elektroden, einer Kathode(negativer Pol) und einer Anode(positiver Pol). Meistens ist ei-ne der beiden Elektroden photoak-tiv, d.h. sie kann nach Absorptionvon Licht Elektronenübertragungs-prozesse starten, und die anderehingegen nur passiv. Photoanodeund Photokathode sind aus porösenHalbleitermaterialien aufgebaut. Inden meisten Fällen wird Titanium-dioxid (TiO2) für Photoanoden undNickeloxid (NiO) für Photokatho-den verwendet. Auf den Halbleiternsind Farbstoffmoleküle, die sicht-bares Licht absorbieren, aufgebracht. Nachdem ein Farbstoffmolekül auf der
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Photoanode Licht absorbiert hat, hat es genügend Energie um ein Elektron anden Halbleiter abzugeben. Dieses wird dann zu einem leitenden Material undanschließend durch einen externen Stromkreis zur Kathode geleitet. Nachdemein Farbstoffmolekül in der Photokathode Licht absorbiert hat, kann es einElektron vom Nickeloxid aufnehmen. Um den Stromkreis zu schließen, wirdein Elektrolyt (eine Ionen beinhaltende Lösung) hinzugefügt, der das Elektronvom Farbstoff zurück zur Anode transportiert. Ein Vorteil dieser Art von So-larzellen ist die relativ einfache Austauschbarkeit der Farbstoffe. Dadurch istes einfacher, die Effizienz zu optimieren, oder die Farbe nach belieben auszut-auschen.
Für diese Doktorarbeit wurden hauptsächlich die ersten Elektronentransfer-prozesse nach Lichtabsorption in Photokathoden untersucht. Mit Hilfe vonzeitaufgelöster Spektroskopie konnten diese Prozesse verfolgt werden. Da-zu wurde genutzt, dass die meisten Farbstoffmoleküle ihre Farbe verändern,wenn sie ein Elektron aufgenommen oder abgegeben haben. Dadurch konntenRückschlüsse gezogen werden, wo sich die Elektronen befinden. Neben denoben beschriebenen erwünschten Prozessen gibt es noch viele mögliche un-erwünschte Nebenprozesse. Oft können positive und negative Ladungen sichgegenseitig neutralisieren, wodurch die ursprüngliche Sonnenenergie in Formvon Wärme verloren geht. Daher ist es wichtig, diese Nebenprozesse zu ver-stehen, um Strategien für deren Vermeidung entwickeln zu können.
In den ersten beiden Artikeln, auf denen meine Arbeit aufbaut, wurden dieEigenschaften des Nickeloxids untersucht. Es war bereits bekannt, dass die po-sitiven Ladungen im Nickeloxid aufgehalten werden, wodurch diese oft nichtzum Strom beitragen können. Mit zeitaufgelöster Spektroskopie war es mög-lich herauszufinden, wo im Halbleiter dies geschieht. Durch diese Erkenntnis-se ist es jetzt möglich, neue Strategien zur Verbesserung der Photokatoden zuentwickeln.
Neben dem Halbleitermaterial wurden unterschiedliche Farbstoffmolekü-le untersucht. Zuerst wurde der Einfluss des Teils, der die Moleküle an dieHalbleiteroberfläche bindet, auf die Effizienz der Photokathode untersucht.Die Gruppen unterschieden sich in ihrer Bindungsaffinität. Die Moleküle, diegut an die Oberfläche banden, kamen so nah aneinander, dass sie anfingen zuinteragieren. Diese Wechselwirkung erschwerte die Elektronenprozesse, waszu dem unerwarteten Ergebnis führte, dass die Solarzellen, welche das meisteLicht absorbierten die kleinsten Ströme gaben.
In einer weiteren Molekülserie wurde untersucht, ob die Geschwindigkeitdes Elektronentransfers durch Design der Moleküle beeinflusst werden kann.Überraschenderweise wurde dieser für zwei von drei Molekülen durch einenbislang unbekannten Nebenprozess komplett unterdrückt, auch für das, vondem der schnellste Prozess erwartet wurde. Bei einem anderen Farbstoff, vondem schon seine Eignung für Grätzelzellen bekannt war, wurde festgestellt,dass ein ähnlicher Nebenprozess der Hauptgrund für die Begrenzung der Effi-zienz war, weil der Elektronentransfer langsamer als gedacht war. Daher sollte
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dieser neu gefundene Nebenprozess bei der weiteren Enwicklung von Grätzel-zellen beachtet werden.
Es ist relativ einfach, Sonnenenergie in Strom umzuwandeln. Da die Son-neneinstralung nicht konstant ist, ist es nötig, die Überschussenergie von Son-nenschein für dunklere Perioden zu lagern. Dazu gehören sowohl kurzzeiti-ge (Tag-Nacht) als auch langfristige (Sommer-Winter) Speicher. Eine Mög-liche Lösung ist die Verwendung von Akkumulatoren, welche jedoch eherfür kurzzeitige Speicherung geeignet sind. Eine bessere Alternative wäre es,Brennstoff direkt mit der Sonneneinstralung zu produzieren. Ein solcher sogenannter Solarbrennstoff kan z.B. durch Wasserspaltung in Sauerstoff undWasserstoff produziert werden. Dies geschieht in zwei Teilreaktionen, wie inAbbildung 10.2 dargestellt.
Abbildung 10.2. Grundprinzip fürdie Wasserspaltung mit Hilfe vonSonnenlicht.
An der Phototoanode nimmt ein Farbstoff(FS) nach Lichtanregung ein Elektron voneinem Katalysator (KAT) auf. Nachdem derKatalysator insgesamt vier Elektronen abge-geben hat, kann dieser Wasser zu elementa-rem Sauerstoff und vier Protonen oxidieren.Die Protonen diffundieren durch eine Mem-bran zur Photokathode, während die Elek-tronen durch einen externen kurzgeschlosse-nen Stromkreis dorthin geleitet werden. Ander Photokathode nimmt der Farbstoff einElektron auf und überführt es weiter an denKatalysator. Nachdem dieser zwei Elektro-nen erhalten hat, kann er zwei Protonen zueinem Wasserstoffmolekül umwandeln. DerWasserstoff kann dann als Solarbrennstoffverwendet werden und z.B in einer Brenn-stoffzelle zu Strom umgewandelt werden.Es wird auch an anderen Katalysatoren ge-forscht, welche CO2 zu kohlenstoffbasiertenBrennstoffen umwandeln. Diese können leichter in unserer bestehenden Infra-struktur verwendet werden.
In dem letzten Teil meiner Doktorarbeit wurde ein Katalysator für die Was-seroxidation (Sauerstoffproduktion) mit Farbstoffmolekülen und Elektronen-akzeptoren verknüpft. Diese Pentade zeigte die erwartete Sauerstoffprodukti-on. Dabei war es möglich, die Elektronentransferprozesse, bis zur ersten Oxi-dation des Katalysators zu verfolgen.
Im Zuge dieser Doktorarbeit wurden mehrere Systeme zur Umwandlungvon Sonnenenergie untersucht. Dabei wurde die Natur von mehreren unpro-duktiven Nebenprozessen aufgedeckt, was es ermöglicht, zukünftige Strate-gien für deren Vermeidung zu entwickeln, wodurch die Effizienz gesteigertwird.
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11. Populärvetenskaplig sammanfattning
För att nå FN:s mål att begränsa den globala uppvärmningen till mindre äntvå grader behövs en stark minskning av utsläppen av växthusgaser. Dessa ut-släpp uppstar till största del genom förbränning av fossila bränslen. En möjlig-het att minska utsläppen är att ställa om till förnybara energikällor såsom sol,vind och vatten. Solinstrålningen utgör den största energitillgången på jorden.I Sverige skulle endast 0.4 % av markytan behöva täckas med solceller med15 % effektivitet för att ersätta landets användning av fossila bränslen. Dettamotsvarar ungefär fyra gånger så stor yta som alla golfbanor och skidbackartillsammans eller en tiondel av all bebyggd mark.
Den vanligaste tekniken för att ta till vara på solenergi är solceller. Dagenssolceller är mestadels baserade på kisel vilka tyvärr har nackdelar som mins-kad verkningsgrad vid lägre solintensitet samt en energikrävande produktions-process. Det forskas på nya typer av solceller utan dessa nackdelar och somdessutom kan vara transparenta och färgade, vilket också gör dem attraktivaför implementering inom arkitekturen och användning i inomhusmiljöer. Envanlig sådan solcellstyp är Grätzelceller, uppkallade efter sin uppfinnare Mi-chael Grätzel.
Figur 11.1. Schema över funktionsprinci-pen av en färgämnessensitiserad solcell.
Grätzelcellens principiella upp-byggnad och verkningsprincip är be-skriven i figur 11.1. De består avtvå elektroder, en katod (negativ pol)och en anod (positiv pol). Oftast ärenbart av dessa fotoaktiv, dvs. flyt-tar elektroner med hjälp av ljuset.En sådan får prefixet foto-. Bådefotokatoder och -anoder byggs uppav porösa halvledare, i de flesta falltitandioxid (TiO2) för fotokatoder el-ler nickeloxid (NiO) för fotoanoder.På halvledarytorna finns färgämnensom absorberar synligt ljus. Efter attfärgämnet på katoden har absorbe-rat ljus har det tillräckligt med ener-gi för att ge ifrån sig en elektrontill titandioxiden. Elektronen trans-
porteras sedan till ett ledande material och vidare genom en extern strömkretstill fotokatoden. När färgämnet i katoden absorberar ljus kan det ta emot en
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elektron från nickeloxiden. För att sluta kretsen används en elektrolyt (en lös-ning med joner) som kan ta upp elektronerna från färgämnet på nickeloxidenoch transportera det tillbaka till molekylerna på titandioxiden. En fördel medGrätzelsolceller är att det är möjligt att byta ut färgämnet för att optimera ef-fektiviteten eller för att få till olika färger på solcellen. Detta gör systemetmodulärt och någorlunda enkelt att förändra och optimera.
I denna avhandling har det huvudsakligen undersökts elektronöverförnings-processerna i fotoanoder efter att färgämnena har tagit upp ljus. För detta an-vändes det tidsupplöst spektroskopi. Genom att färgämnena ändrar färg ef-ter att ha tagit upp eller gett ifrån sig elektroner kunde det dras slutsater omelektronernas position. Förutom de ovan beskrivna önskade processerna finnsdessvärre många oönskade sidoprocesser. I många fall kan positiva och ne-gativa laddningar ta ut varandra (rekombinera) vilket gör att den absorberadesolenergin går förlorad i form av värme. Av den anledningen har det varit vik-tigt att identifiera sidoprocesserna för att sedan kunna utveckla strategier föratt förhindra dessa.
Först undersöktes egenskaperna av fotokatodens nickeloxid. Det var redankänt att positiva laddningar i nickeloxiden kan fångas av någon oönskad pro-cess, något som leder till att den inte kan bidra till ström. Med hjälp av tidsupp-löst spektroskopi kunde vi identifiera var i halvledaren detta sker och på grundav det är det nu möjligt att utveckla nya strategier för att undvika denna fångst-process i nickeloxiden.
Som nämnt ovan är det möjligt att byta ut färgämnet. Det undersökteshuruvida olika kemiska grupper som förankrar ett färgämne på halvledarenpåverkar elektronöverföringen. De olika grupperna visade olika förmåga attbinda moleylerna till nickeloxiden vilket yttrade sig i att flera molekyler fannspå samma yta. För vissa grupper ledde det till att molekylerna kom för näravarandra, vilket medförde en interaktion mellan grannmolekyler. Detta försvå-rade de önskade elektronöverföringsprocesserna och ledde till den ovanligaeffekten att solcellerna som absorberade mer ljus gav lägre effekt.
Molekylerna i en annan serie förväntades vara olika effektiva på att ta emotelektronen från nickeloxiden. Istället för en elektronöverföring observeradesen oproduktiv släckningsprocess där färgämnen ger ifrån sig solenergin i formav värme utan ett elektronen överfördes som önskat.
I de flesta Grätzelceller används en elektrolyt baserad på organiska lös-ningsmedel. Detta innebär en risk för läckage och uttorkning med följd attsolcellen blir förstörd. Det finns redan flera försök att ersätta elektrolyten meden elektronledare som är i fast form. I artikel V har det föreslagits en ny pro-duktionsprocess av fotokatoden så att elektronöverföringen underlättas. Resul-taten visar på en mycket snabbare elektronöverföring från färgämnet jämförtmed vid användandet av en flytande elektrolyt. En annan positiv effekt varatt en vanlig oönskad process samtidigt blev mycket långsammare än vanligt.Både dessa effekter gör detta system intressant för användning i Grätzelceller.
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Det är förhållandevis enkelt att producera el utifrån solenergi, men eftersomsolskenet inte är konstant krävs en möjlighet att lagra energin från mer tillmindre soliga tillfällen, dvs. från dag till natt och från sommar till vinter. Enmöjlig lösning är användning av batterier, vilket dock enbart är lämpligt förkortare tidsskalor, dvs. på dygnsbasis. Ett bättre alternativ vore att kunna pro-ducera en form av bränsle med hjälp av solen. Ett möjligt sådant solbränse kanproduceras genom spjälkning av vatten till vätgas och syrgas. Detta sker i tvådelreaktioner som beskrivs i figur 11.2. Vid fotokatoden tar ett färgämne (FÄ)
Figur 11.2. Schema över funktionsprinci-pen av ett system för vattenspjälkning.
upp en elektron från en kataly-satormolekyl (KAT). Efter att fyraelektroner har tagits upp kan kata-lysatorn spjälka två vattenmolekylertill en syrgasmolekyl och fyra pro-toner. Protonerna diffunderar genomett membran till fotokatoden medanelektronerna transporteras dit genomen extern kortsluten strömkrets. Vidfotanoden tar färgämnet (FÄ) uppljus varpå en elektron kan överförasfrån kretsen till katalysatorn genomfärgämnet. När katalysatorn har ta-git emot två elektroner kan den om-vandla två protoner till en vätgas-molekyl. Vätgasen kan sedan använ-das som solbränsle, t.ex med hjälp avbränsleceller i en vätgasbil. Det finnsockså forskning kring andra kataly-satorer som kan omvandla koldioxidtill flytande kolbaserade solbränslenvilka lätt skulle kunna implemente-ras i dagens infrastruktur.
I den sista artikeln i denna avhandling undersöktes ett system för vattenoxi-dation (syrgasproduktion) där färgämnet, katalysatorn och en elektronacceptorär ihoplänkade. Länken mellan katalysatorn och färgämned ledde till att fär-gämnet släcks ut med följd att elektronöverföringen blev ineffektiv. Systemetkunde trots allt oxidera vatten, om än med låg verkningsgrad.
I denna avhandling identifierades flera oönskade sidoprocesser i molekyl-baserade system för solenergiomvandling. Genom denna ökade förståelse ärdet nu möjligt att utveckla ytterligare strategier för förbättrad effektivitet.
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12. Acknowledgements
This work would not have been possible without the help of many people: col-leagues, collaborators, administrators and friends. It is certainly not possibleto mention everyone within only two pages.
First of all, I want to thank my supervisors Leif Hammarström and Jan
Davidsson for giving me the opportunity and trust for my PhD studies. Theunique environment you created in fotomol and the department, that makes itto much more than only a lab, has certainly had a big impact on my work andscientific development. Thank you also for all the scientific discussions thathelped me to develop and for sharing your knowledge with me.
I also want to thank Ana Morandeira and Jonas Sandy Lissau for takingme as a project student during my exchange studies. Without you, I mightnever have got to know this wonderful scientific climate I was allowed to ex-perience every day during my PhD studies.
To the fs-guys Jonas Petersson, Allison Brown and Erik Göransson whopatiently answered all my easy questions while learning how operate the lasersystem and who I learned a lot from; to Burkhard Zietz for all help when thelaser system caused trouble, for all the nice discussions, and for the nice timeboth inside and outside the lab; to my colleagues and collaborators from thelab: Edgar Mijangos, Somnath Maji, Robin Tybursky, Liisa Antila, Lei
Tian, Haining Tian and Luca D’Amario. It was a pleasure working togetherwith you and I really enjoyed all these scientific and non-scientific discussionsthat made every day of work to something special.
To Vincent Wang, Mélina Gilbert Gatty and Wesley Swords for proof-reading my thesis and finding all those typos and small mistakes, and of courseall the fun time in the office; Mariia Pavliuk for all the positive energy youspread, for being an amazing photographer and for taking the cover photo;Starla Glover for the help with the nanosecond measurements and for beingsuch motivating and solely positive person; all the students, PhD students andpost-docs along the way: Anna, Alexander, Moreno, Daniel, Mohammad,
Sonja, Hanna, Prateek, Ahmed, Lei Z, Tianfei, Shihuai, Mohamed, Nico-
las, Michele, Shameem, Juri, Belinda and Astrid
To my collaborators, Fabrice Odobel and his group for the interestingstudy on the PMIs, Prof. Imahori and Masanori Yamamoto for designingand synthesizing the really interesting pentad for water splitting, and Michael
Wasielewski and Ann Woys for hosting Jonas and me in Evanston while weperformed our first transient IR measurements. I am also grateful for the travelgrants from Liljewalchs Stiftelse, that allowed me attending interesting con-ferences in the USA and France.
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As teaching also was a large part of my PhD studies, I want to thank Nes-
sima, Reiner, Christer, Felix, Viktor, Natasha, Kari for all discussions aboutteaching, students and a lot more; to the lab technicians Johanna and Joseph
for all the support in the teaching lab.To the administration: Susanne Söderberg, Åsa Furberg, Jessica Stål-
berg, Anna Fahlén and Eva Larsson for all the help with different problemsfrom my very first day at the department, answering all my questions abouteverything concerning my PhD studies and also about ordering, invoices, andother things. A big thank you also to Sven Johansson for always being helpfulwhenever something needs to be fix.
Livet under de senaste fem åren bestod inte bara av plugg och arbete. Enväldig stor del av min fridid spenderade jag med Philochoros och med philo-
choristerna. Stort tack till er för alla avbrott i vardagen, dansövningar ochtrevliga danskvällar i Stornoret, i Uppsala och på stämmorna. Ett stort tackspeciellt till familjen Sala för all gemensam tid och för att ni är sadan bravänner.
Vielen Dank auch an meine Eltern und meine Schwester, die mich immerunterstützt haben, auch als ich in das weit entfernte Uppsala ziehen wollte.Danke für das Korrekturlesen der deutschen Zusammenfassung und vor allemdafür, dass ihr immer für mich da seid.
Till slut vill jag tacka min fru Sofia för att ha korrekturläst den svenskasammanfattningen, för att hon alltid stödjer mig, för att hon fyller ut varje dagoch för att hon helt enkelt är bäst.
64
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