Developing EnvironmentallyFriendly Dye-sensitized Solar Cells

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(1)Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1351. Developing Environmentally Friendly Dye-sensitized Solar Cells HANNA ELLIS. ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2016. ISSN 1651-6214 ISBN 978-91-554-9506-0 urn:nbn:se:uu:diva-280291.

(2) Dissertation presented at Uppsala University to be publicly examined in Polhemsalen; Ång/10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 29 April 2016 at 10:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Aldo di Carlo (University of Rome "Tor Vergata"). Abstract Ellis, H. 2016. Developing Environmentally Friendly Dye-sensitized Solar Cells. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1351. 82 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9506-0. Due to climate change and its effects, alternative renewable energy sources are needed in the future human society. In the work of this thesis, the Dye-sensitized Solar Cell (DSC) has been investigated and characterized. DSCs are appealing as energy conversion devices, since they have high potential to provide low cost solar light to electricity conversion. The DSC is built up by a working electrode consisting of a conductive glass substrate with a dye-sensitized mesoporous TiO2 film, a counter electrode with a catalyst and, in between, the electrolyte which performs the charge transport by means of a redox mediator. The aim of this thesis was to develop and evaluate cheap and environmentally friendly materials for the DSC. An alternative polymer-based counter electrode catalyst was fabricated and evaluated, showing that the PEDOT catalyst counter electrode outperformed the platinum catalyst counter electrode. Different organic dyes were evaluated and it was found that the dye architecture affected the performance of the assembled DSCs. A partly hydrophilic organic triphenylamine dye was developed and applied in water-based electrolyte DSCs. The partly hydrophilic dye outperformed the reference hydrophobic dye. Small changes in dye architecture were evaluated for two similar dyes, both by spectroscopic and electrochemical techniques. A change in the length of the dialkoxyphenyl units on a triphenylamine dye, affected the recombination and the regeneration electron transfer kinetics in the DSC system. Finally, three water soluble cobalt redox couples were developed and applied in water-based electrolyte DSCs. An average efficiency of 5.5% (record efficiency of 5.7%) for a 100% water-based electrolyte DSC was achieved with the polymer-based catalyst counter electrode and an organic dye with short dimethoxyphenyl units, improving the wetting and the regeneration process. Keywords: dye-sensitized solar cells, dye, cobalt, triphenylamine, titanium dioxide, aqueous, PEDOT Hanna Ellis, Department of Chemistry - Ångström, Physical Chemistry, Box 523, Uppsala University, SE-75120 Uppsala, Sweden. © Hanna Ellis 2016 ISSN 1651-6214 ISBN 978-91-554-9506-0 urn:nbn:se:uu:diva-280291 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-280291).

(3) List of papers. This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I. Hanna Ellis, Nick Vlachopoulos, Leif Häggman, Christian Perruchot, Mohamed Jouini, Gerrit Boschloo, Anders Hagfeldt PEDOT Counter Electrodes for Dye-sensitized Solar Cells Prepared by Aqueous Micellar Electrodeposition Electrochimica Acta 107:45-51, 2013. II. Hanna Ellis, Susanna Kaufmann Eriksson, Sandra Feldt, Erik Gabrielsson, Peter W. Lohse, Rebecka Lindblad, Licheng Sun, Håkan Rensmo, Gerrit Boschloo, Anders Hagfeldt Linker Unit Modification of Triphenylamine-based Organic Dyes for Efficient Cobalt Mediated Dye-sensitized Solar Cells The Journal of Physical Chemistry C 117(41):21029-21036, 2013. III. Valentina Leandri, Hanna Ellis, Erik Gabrielsson, Licheng Sun, Gerrit Boschloo, Anders Hagfeldt An Organic Hydrophilic Dye for Water-based Dye-sensitized Solar Cells Physical Chemistry Chemical Physics 16:19964-19971, 2014. IV. Hanna Ellis,∗ Ina Schmidt,∗ Anders Hagfeldt, Gunther Wittstock, Gerrit Boschloo Influence of Dye Architecture of Triphenylamine Based Organic Dyes on the Kinetics in Dye-sensitized Solar Cells The Journal of Physical Chemistry C 119(38):21775-21783, 2015. V. Hanna Ellis, Roger Jiang, Sofie Ye, Anders Hagfeldt, Gerrit Boschloo Development of High Efficiency 100% Aqueous Cobalt Electrolyte Dye-sensitised Solar Cells Physical Chemistry Chemical Physics 18:8419-8427, 2016. Reprints were made with permission from the publishers. ∗. Shared first authorship..

(4) Comments on my own contribution I was the main responsible person for paper I, II and V. For these papers, I planned the experimental work, performed most of the experimental work and analyzed the experimental data. I was also the main responsible person for writing the manuscripts. For paper III, I planned and performed a great part of the experimental work and analyzed the experimental data but I was not the main responsible person for the manuscript. For paper IV, I. Schmidt and I shared the first authorship and also the responsibility of writing the manuscript. I did not perform any synthesis of dyes, redox couples, PES measurements, quantum chemical calculations or SEM measurements. I did participate during the SECM measurements but I was not the main responsible person.. I am a co-author of the following papers which are not included in this thesis. • Erik Gabrielsson, Hanna Ellis, Sandra Feldt, Haining Tian, Gerrit Boschloo, Anders Hagfeldt, Licheng Sun Convergent/Divergent Synthesis of a Linker-varied Series of Dyes for Dye-sensitized Solar Cells Based on the D35 Donor Advanced Energy Materials 3(12):1647-1656, 2013 • Hanna Ellis, Valentina Leandri, Gerrit Boschloo, Anders Hagfeldt, Jonas Bergquist, Denys Shevchenko Laser Desorption/Ionization Mass Spectrometry of Dye-sensitized Solar Cells: Identification of the Dye-electrolyte Interaction Journal of Mass Spectrometry 50: 734-739, 2015 • Jinbao Zhang, Hanna Ellis, Lei Yang, Erik M. J. Johansson, Gerrit Boschloo, Nick Vlachopoulos, Anders Hagfeldt, Jonas Bergquist, Denys Shevchenko Matrix-assisted Laser Desorption/Ionization Mass Spectrometric Analysis of Poly(3,4-ethylenedioxythiophene) in Solid-state Dyesensitized Solar Cells: Comparison of in situ Photoelectrochemical Polymerization in Aqueous Micellar and Organic Media Analytical Chemistry 87(7):3942-3948, 2015 • Susanna Kaufmann Eriksson, Ida Josefosson, Hanna Ellis, Anna Amat, Mariachiara Pastore, Johan Oscarsson, Rebecka Lindblad, Anna I. K. Eriksson, Erik M. J. Johansson, Gerrit Boschloo, Anders Hagfeldt, Simona Fantacci, Michael Odelius, Håkan Rensmo Geometrical and Energetical Structural Changes in Organic Dyes for Dye-sensitized Solar Cells Probed using Photoelectron Spectroscopy and DFT Physical Chemistry Chemical Physics 18:252-260, 2016.

(5) Contents. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Aim and Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Energy and Energy Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Energy from the Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Solar Cells – a Spectrum of Possibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 The Dye-sensitized Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 The Different Components of the DSC . . . . . . . . . . . . . . . . . . . . . . . . . .. 11 11 11 13 14 18 21. 2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Characterization of Complete Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Current-Voltage Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Incident Photon to Current Conversion Efficiency . . . . . . . . 2.1.3 Toolbox Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 Impedance Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Characterization of Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Photo-induced Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Transient Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 UV-visible Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Fluorescence Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.7 Electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.8 Scanning Electrochemical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . .. 28 28 28 30 31 32 36 36 38 40 42 43 43 43 48. 3. Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Counter Electrode – Electrolyte Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Working Electrode – Electrolyte Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 The Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 50 50 53 60. 4. Conclusions and Future Outlook. 63. 5. Populärvetenskaplig sammanfattning på svenska. 6. Acknowledgments. References. ............................................................... .................................... 66. ...................................................................................... 73. ......................................................................................................... 74.

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(7) Abbreviations and Symbols. Γ α β ε ηcoll ηreg κ ν ωd φ φin j τ τc. Gamma function Transfer coefficient Distribution constant Molar extinction coefficient Collection efficiency of the photo-generated charge carriers Regeneration efficiency Heterogeneous rate constant Frequency Maximum angular frequency Work function Quantum yield of injected electrons Average lifetime/apparant average lifetime Characteristic stretched exponential lifetime. A AC AM a. Absorbance Alternating current Air mass density area. BIPV. Building-integrated photovoltaics. C CE CH3 NH3 PbI3 cheno CO2. Concentration Counter electrode Methylammonium lead iodine Chenodeoxycholic acid Carbon dioxide. D DSC DTT. Diffusion coefficient Dye-sensitized solar cell 3,3’–dithiobis[4-methyl–(1,2,4)–triazole]. E E0 E0−0. Potential Formal reduction potential Gap between ground state level and first excited level Binding energy Energy of conduction band edge. Ebin ECB. 7.

(8) 8. EDOT EF Ekin kobs Eox krec Ered kreg EV B. 3,4-ethylenedioxythiophene Fermi-level Kinetic energy of photoelectrons Observed regeneration rate constant Oxidation peak potential Recombination rate constant Reduction peak potential Regeneration rate constant Energy of valence band edge. F Fc FF FTO. Faraday constant Ferrocene Fill factor Fluorine doped tin oxide. h HOMO. Planck’s constant Highest occupied molecular orbital. I I I0 I− /I− 3 ilim IPCC IPCE IR. Intensity Current Reference intensity Iodide/triiodide Limiting current Intergovernmental panel on climate change Incident photon to current conversion efficiency Infrared. J Jmax JSC. Current density Current density at maximum power point Current density at short circuit conditions. k◦ K3 [Fe(CN)6 ] kB. Standard rate constants Potassium ferricyanide Boltzman constant. L LHE LUMO. Length Light harvesting efficiency Lowest unoccupied molecular orbital.

(9) MALDI-MS MBI MPN. Matrix assisted laser desorption ionization mass spectroscopy 1-methylbenzimidazole 3-methoxypropionitrile. n n NHE. Dynamic viscosity Number of electrons Normal hydrogen electrode. Pmax PCE PEDOT PES PIA Pin. Maximum power point Power conversion efficiency Poly(3,4-ethylenedioxythiophene) Photoelectron spectroscopy Photo-induced absorption spectroscopy Power of incident light. QD. Quantum dots. R r RCE RD Rrec Rs Rtot. Gas constant Radius Charge transfer resistance at the counter electrode Diffusion resistance Recombination resistance Series resistance Total resistance in the solar cell. S0 S1 SDS SECM SEM. Ground state level First excited state level Sodium dodecyl sulfate Scanning electrochemical microscopy Scanning electron microscopy. t 12 TAS TBP T TEMPO. Half-time value Transient absorption spectroscopy 4-tert-Butylpyridine Temperature 2,2,6,6-Tetramethyl-1-piperidinyloxy. 9.

(10) TiO2 T t1 2 inert t1 2 redox TT− EMI+. Titanium dioxide Transmittance Recombination half-time value Regeneration half-time value 1-ethyl-3-methyl-imidazolium 4-methyl-1,2,4triazole-3-thiolate. UV. Ultraviolet. Vmax VOC V. Voltage at maximum power point Open circuit voltage Voltage. WE. Working electrode. Z ZnO. Impedance Zinc oxide. 10.

(11) 1. Introduction. 1.1 Aim and Outline of the Thesis The aim of this thesis was to develop and evaluate cheap and environmentally friendly materials for the dye-sensitized solar cell (DSC). When the work of this thesis was initiated, the cobalt redox couple had started to compete with the earlier applied standard iodide/triiodide (I− /I− 3 ) redox couple. [1] During this period of time, the different components of the DSC were to be reevaluated. The organic dyes had, for the past few years, started to compete with the traditional ruthenium-based dyes. [2, 3] The platinized counter electrode and the acetonitrile-based electrolyte, were frequently used components. This opened up for possibilities to develop and investigate new and environmentally friendly components for the DSC. The thesis is divided into five chapters. Chapter 1 answers the question "Why?" What is the motivation of this thesis? Chapter 2 answers the question "How?" and provides a brief background of the techniques used, for deeper description of the techniques the reader is advised to consult the references. Chapter 3 answers the question "What?" presenting the results of paper I-V. Chapter 3 is divided into three sections: i) counter electrode – electrolyte interactions ii) working electrode – electrolyte interactions, iii) the electrolyte. The results of paper I-V are discussed under the three mentioned sections. Chapter 4 provides conclusions and a future outlook of the field trying to answer the question of where the field is heading. Finally, Chapter 5 provides this thesis with a popular science summary in Swedish.. 1.2 Energy and Energy Consumption Energy is what drives humans, plants, cars, airplanes, electrical equipment, what heats/cools our houses et cetera. Energy can be measured in different units depending on what one refers to. The SI unit for energy is joule (J), defined as one newton meter (N m). Household energy consumption is given in kWh (1 kWh = 3.6×106 J), while human world energy consumption is given in Mtoe (millions of tonnes oil equivalents). In 2013, the total world final energy consumption was about 9 301 Mtoe [4], which can be translated into 3.9×1020 J or 12 TW (average value). The human population consumes more and more energy every year. [4] The increase in energy consumption is due to both growing population and improving living standards. The latter is most 11.

(12) often referred to poor people increasing their living standard. However, even in the wealthiest countries consumers have shown a robust and sustained desire to enhance their lifestyle in ways that require more energy. [5] The total global energy consumption can be sorted according to different sources used to produce the energy, see Figure 1.1. About 80% of the energy consumed in 2013 came from fossil fuels. [6] Burning fossil fuels generates greenhouse gases, mostly carbon dioxide (CO2 ), which affect the climate on Earth. In the report Climate Change 2014 Synthesis Report Summary for Policymakers, IPCC concludes: Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems. [7] A century ago, Svante Arrhenius presented the idea that CO2 emissions from burning fossil fuels, could raise the infrared opacity of the atmosphere enough to warm the Earth. [8] Based on the consumption rates of 1998 there are 40-80 years of proven oil reserves existing globally, 60-160 years of natural gas, 1000-2000 years of coal, shales and tar sands available. This implies that the estimated fossil energy resources could support a 25 to 30 TW energy consumption rate globally for at least several centuries. [9] Since there are no natural destruction mechanisms of CO2 in the atmosphere, in the absence of geoengineering or active intervention, the effects of atmospheric CO2 accumulation over the next 40-50 years will persist globally for the next 500-2000 years. [9] The fact that the world energy consumption is increasing is not only bad, since it is a consequence of the increased living standards in developing countries. Less people are extremely poor today than three decades ago. [10] However, economic growth needs to be decoupled from fossil-based energy use and CO2 emissions in order to create a sustainable future for the whole population of Earth. [5] A global temperature increase of approximately 4 ◦ C or more above the late 20th century levels, combined with an increasing food demand, would pose large risks to food security globally. Climate change is also projected to reduce renewable surface water and groundwater resources in most dry subtropical regions, intensifying the competition for water. [7] Since many of the extremely poor people live in dry subtropical regions, such as the Sub Saharan Africa, Latin America and the Caribbean, [10] combating climate changes is a way of solving future poverty, inequality and famine.. 12.

(13) Figure 1.1. Division of the global final energy consumption for 2013 divided according to sources from which the energy was derived. [6]. 1.3 Energy from the Sun A sentence often used to motivate photovoltaics states: the Sun irradiates the Earth with more energy in one hour than the world human population consumes in a year. [11] This phrase indicates the huge amount of energy provided by the Sun. If just a part of it could be captured and used in order to replace the fossil fuels, the global energy issue would be solved. Today, modern renewable energy sources account for approximately 10.1% of global energy consumption, whereof 1.3% is produced by photovoltaics together with wind and other energy sources, see Figure 1.1. The Sun is our nearest star and it has been the center of the our solar system for 4.6 billion years. The Sun is a middle aged star, [12] it has been stable for about 4 billion years, and will remain so for another 4 billion years. The Sun is a sphere of gas heated by nuclear fusion reactions at its center. [13] Hot bodies emit electromagnetic radiation with a specific spectrum and intensity determined by the body temperature. The temperatures near the center of the Sun are estimated to reach 20 000 000 K, this is however not the temperature determining the characteristic electromagnetic radiation emission of the Sun. Since the radiation from the interior of the Sun is absorbed near the surface, it is instead the so called photoshere, with its temperature of about 6000 K, which provides the continuous spectrum of electromagnetic radiation reaching the Earth. The radiation is at this temperature close to black body irradiation. A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of its frequency or incident angle. The spectral distribution of the emitted radiation for a perfect black body is specified by Planck’s radiation law. Planck’s law gives the relationship between the irra13.

(14) diation of a body and its temperature; as the temperature of a body increases, its irradiation increases and the peak emission wavelength shifts towards blue wavelengths. [13] This is observed, for example, when a metal is heated and goes from glowing red into shining yellow. Another example is observing a candle flame, at the very bottom closest to the candle the flame is blue, while the top is yellow-red. Light, being photons, has dual particle and wave characteristics. The energy of a photon (E) is determined by its wavelength λ , according to Equation 1.1 where h is Planck’s constant, ν the frequency and c the speed of light. The energy of photons is sometimes given in electron volt (eV), and conversion from nm into eV is done according to Equation 1.2 where e is the value of an eV. E = h×ν = h× E(eV ) =. c λ. h×c 1240 = e×λ λ (nm). (1.1). (1.2). It is the Sun that has provided nature with energy for millions of years. In nature, the photosynthesis is converting the light together with CO2 and water into chemical energy, sugars and oxygen. The sugars can later on be used to fuel activities of the organism. Fossil fuels can be created from organic and botanical matter under certain conditions. A holy grail within research is artificial photosynthesis, to convert the irradiation from the Sun into fuel. In the future, so called solar fuels (fuels produced by artificial photosynthesis using the energy of the Sun) combined with electricity produced by photovoltaics, could provide the human population with environmentally friendly alternatives to the energy sources used today – a step towards a sustainable society.. 1.4 Solar Cells – a Spectrum of Possibilities Solar cells need to be price competitive in order to be able to replace fossil fuels. This means that the solar cells need to be more effective and/or cheaper. [14] There has been a drop in the price of the silicon used for siliconbased solar cells within the recent years, but also a drop in global oil price. Since the energy issue is very complex and extensive, it can probably not be solved in one way with one solution. Instead, it should be solved by combining various solutions depending on different possibilities and opportunities. This means that one solution will not be the best all over the world, at every place. Different regions of the world have different rates of wind, sun, tide, waves and water, which means that a whole spectrum of renewable energy sources should be used to solve the energy problem. Not even one type of 14.

(15) solar cells will probably be enough. Photovoltaics is a collective name for devices that convert photons into electricity, generally called solar cells. There are a number of different types of photovoltaics existing today. [15] One important physical principle within the field of photovoltaics is the photovoltaic effect. The photovoltaic effect refers to when a semiconductor is irradiated by a light source (photons) and generating electron-hole pairs. The electron and the hole can be directed to two different contacts, connected by a circuit and an electric potential difference will be established. The photovoltaic effect was discovered by the physicist Edmond Becquerel in 1839. Becquerel was experimenting with a silver chloride electrode and a platinum electrode immersed in acid, when he observed current upon illumination. [16] Thus, the set-up from the original experiment for discovering the photovoltaic effect resembles the dye-sensitized solar cell, which is the subject of this thesis. As earlier mentioned, there are a number of different photovoltaic technologies; a brief introduction follows. Silicon Solar Cells Silicon is the second most abundant element in the crust of Earth. However, it does not exist as a pure metal in nature, instead it appears as an oxide, silicon dioxide (silica, SiO2 ). Silicon-based solar cells are the most widely used photovoltaic technology today. They have the advantage of silica being available in large quantities in nature. The disadvantage is the high energy consumption for producing the silicon-based solar cell modules, elevated temperatures up to about 1900◦ C are needed during the process. [17] There are single crystal, multicrystalline and amorphous silicon-based solar cells. The most efficient are the single crystal silicon solar cells with efficiencies about 25%. [15] The silicon-based solar cell is produced by doping pure silicon with boron (pdoped) and phosphorus (n-doped), and bringing the p-type and n-type doped silicon in physical contact with each other. When in contact with each other, the electrons from the n-side (one extra electron per phosphorous atom) rush over to the p-type (one hole per boron atom), i.e. an electron migration is taking place over the p-n junction. This makes the p-doped side close to the p-n junction negatively charged, while the n-doped side close to the p-n junction gets positively charged, creating an electric field. Photocurrent is generated when light is absorbed by the silicon. Electron – hole pairs are formed, that are separated by the electric field. The carriers are collected at the selective contacts at opposite side of the cell. [17] The silicon solar cell mechanism is illustrated in Figure 1.2. Thin Film Technologies Thin film technology devices include amorphous silicon, cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and others. [18] Thin film technologies are advantageous compared to silicon-based solar cells since they 15.

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(22) . . Figure 1.2. When illuminating the silicon solar cell electron – hole pairs are formed which are separated by the electric field in the depletion region. The carriers are collected at the selective contacts at opposite side of the cell.. are thinner, can be made flexible and cheaper to produce. Efficiencies slightly above 20% have been established. [15] Organic Solar Cells Organic solar cells are built on similar principles as semiconductor-based solar cells. A junction is established due to conductive organic molecules with different HOMO-LUMO levels being brought in contact with each other. Organic solar cells have the advantage of being very cheap to produce, and the disadvantage of showing stability issues. [19] Efficiencies of about 11% have been achieved with organic solar cells. [15] Quantum Dot Solar Cells There are different types of quantum dot solar cells and they more or less resemble the dye-sensitized solar cell. [20, 21] The quantum dots can be used as sensitizer, and the quantum dot solar cells can be both liquid- and solidelectrolyte based. An example of a material for the quantum dots is leadsulfide. Quantum dot solar cells have attained an efficiency of slightly above 10%. [15] Perovskite Solar Cells Perovskite solar cells are relatively new as photovoltaic technology. Perovskites encompass a broad class of crystalline minerals, discovered during the 19th century in the Ural mountains of Russia. Perovskite is the name for any material that adopts the same crystal structure as calcium titanate. The research area of photovoltaics based on perovskites has grown rapidly since 2012. Since this thesis is not focusing on the perovskite solar cells, the subject is only touched upon in this paragraph and the reader is recommended to read literature [22] for further information, see for example the Journal of Material Chemistry A volume 3, issue 17, 2015, which is a special issue about perovskite solar cells. The advantage of the perovskite photovoltaic technology is that the cells can be cheap and easily fabricated. A disadvantage that could 16.

(23) prevent future commercialisation is that the record-breaking cells contain lead and that the perovskites used for solar cells, being salt-like minerals, dissolve in water or even humid air. There is research going on to replace the lead with for example tin and efficiencies of 6.4% have been reported. [23] For the lead perovskite solar cell the record efficiency of today is 21%. [15].

(24) . The Shockley-Queisser Limit There is a limit to the maximum theoretical efficiency for a solar cell built upon the principle of a single p-n junction, the Shockley-Queisser limit. [24] The maximum efficiency of a single p-n junction solar cell is limited by a number of causes such as recombination, resistances for electron transfer et cetera. However, the most important limitation is the absorption of photons, ultimately limited by the band gap, see Figure 1.3. If one takes the example of silicon solar cells, they possess a band gap of 1.1 eV, generating a maximum efficiency of about 30%. [24] In Figure 1.3 the solar irradiance spectrum for AM 1.5 is shown, where the absorption of ozon (O3 ) in the UV-region below 250 nm can be observed, as well as the absorption of oxygen at about 750 nm and by water (H2 O) at about 900 nm and 1100 nm. CO2 absorbs in the IR-region at about 2000 nm, not covered by Figure 1.3. The band gap of crystalline silicon namely 1.1 eV, equals to 1127 nm; photons with lower energy will not contribute to the efficiency and photons possessing energy above 1.1 eV will only contribute with 1.1 eV. The energy above this level is therefore wasted, see illustration in Figure 1.3.. . .

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(37) . Figure 1.3. Solar irradiance on the surface of the Earth, the spectrum illustrates the AM 1.5 irradiance. The band gap of silicon (1.1 eV) is marked and photons possessing less energy are not absorbed, while the excess energy of the photons with more energy than 1.1 eV will be wasted.. 17.

(38) Generations of Photovoltaics There are three different generations of photovoltaics. The first is limited by the Shockley-Queisser limit and has a high cost per generated power. The commercially available silicon solar cells are examples of the first generation photovoltaics since they have a single p-n junction and a great energy demand for the production process. [17] The second generation is still limited by the Shockley-Queisser limit, but provides a lower cost per generated power due to cheaper material and fabrication process. The thin film technologies belong to the second generation of photovoltaics. The third generation can exceed the Shockley-Queisser limit. Multi-junction (tandem) solar cells and other emerging technologies being able to use electrons that are not thermally relaxed and up-conversion (combining two lower energy photons to a higher one) provide possibilities to exceed the limit stated by Shockley-Queisser. The third generation also builds on cheap materials such as nanomaterials and cheap manufacturing processes, where extreme temperatures are not necessary. The dye-sensitized solar cell, which is the subject of this thesis, is a second generation solar cell since it can be made out of cheap materials.. 1.5 The Dye-sensitized Solar Cell The system Becquerel used when he discovered the photovoltaic effect has many similarities with the dye-sensitized solar cell (DSC). His experiment with electrodes in liquid acid solution resembles the DSC more than a semiconductorbased solar cell. The early experiments toward what today is considered being the DSC, were performed by using dye-sensitized semiconductor electrodes immersed in liquid solutions for generating current and voltage, during the 1970-80. [25–27] These systems attained modest power conversion efficiencies (PCEs). In 1991, Grätzel and O’Regan published a PCE of 7.9%, [28] which is by many considered a breakthrough in the DSC history. After the breakthrough of 7.9%, the research field of DSCs emerged. Today there are relating photovoltaic techniques similar to the original liquid-based DSC. Instead of the liquid electrolyte a solid state hole conductor has been introduced. [29] Quantum dots have been applied as sensitizers both in liquid and solid state devices. [20, 21, 30, 31] P-type systems have been designed [32] and alternative electrolyte mixtures consisting of gel electrolytes, ionic liquids and in-situ polymerized hole conductors have been published. [33–38] The perovskite solar cell was developed from the DSC structure containing mesoporous titanium dioxide (TiO2 ), [39] but has today also shown good results with thin film device architectures, [40–43] as well as with meso-structured device architecture containing different types of scaffolds such as alumina and zirconia. [44–47] A standard liquid DSC is built up by a working electrode consisting of a conductive glass substrate with a dye-sensitized mesoporous TiO2 film, a 18.

(39) counter electrode with a catalyst and, in between, the electrolyte which performs the charge transport by means of a redox mediator (see Figure 1.4). The published world record for a DSC is at present 14%. [48] However, this is not a certified value, i e. measured in certification lab, such as the National Renewable Energy Laboratory (NREL). [15] Instead, there is a world record of 11.9% certified by the Japanese Advanced Industrial Science and Technology Institute (AIST). [49]. Figure 1.4. Schematic illustration of a DSC and working mechanism. Figure 1.4 illustrates a general DSC and a brief summary of the working mechanism follows: 1. The photon is absorbed by the dye and the dye is excited. 2. The electron is injected into the semiconductor. 3. The electron is extracted on the backside of the electrode and moves through the circuit where it can perform electrical work. 4. At the counter electrode the electron reduces the oxidized species of the redox couple in the electrolyte. 5. The reduced species in the electrolyte diffuses to the oxidized dye and regenerates it by reducing it. Figure 1.4 is a generalized schematic figure of the DSC. A more detailed scheme including energy levels and kinetic processes is given in Figure 1.5 and the description of Figure 1.5 is listed. 19.

(40) 1. When a photon hits the dye, the electron is excited from the HOMOlevel to the LUMO-level, instantaneously. 2. The electron is injected into the TiO2 . This process takes place within 100 fs-100 ps, depending on experimental conditions. A detailed discussion about the injection kinetics can be found elsewhere. [50, 51] 3. Regeneration of the oxidized dye occurs within the μs-scale. The regeneration kinetic process has been studied with Marcus theory, see elsewhere. [52] 4. The electron returns to the ground state, both by radiative and nonradiative processes. 5. Recombination of photo-injected electrons in the TiO2 to the oxidized species in the electrolyte. 6. Recombination of photo-injected electrons in the TiO2 to the oxidization level of the oxidized dye.. Figure 1.5. Schematic picture showing the different forward processes (solid black lines) and reverse processes (dashed red lines) and their time scales in the DSC.. It should however be noted that Figure 1.5 does not give the full picture of the kinetics in the DSC. A more detailed description of the kinetics in the DSC can be found elsewhere. [50, 53]. 20.

(41) 1.5.1 The Different Components of the DSC The Working Electrode Early studies of photoelectrochemical systems used a monolayer of dye molecules on a flat semiconductor. This led to an intrinsic limitation in the system, since a flat surface can only be sensitized with a limited amount of dye and thus, the absorption of photons is limited. It was not until the mesoporous structure was introduced, with its 1000-fold enhancement in surface area, that the PCE leaped. [28] An analogy can be found in nature, where the surface area is increased by stacking the chlorophyll containing thylakoid membranes into grana structures. In the early history of the development of the DSC, many different semiconductors were used for the working electrode. In Figure 1.6, a collection of different semiconductors are illustrated. # $! %&. . . . . . . . . . . . .

(42) . . . . . . . . . . . . . . !"!. !". . . . Figure 1.6. Band positions of several semiconductors in contact with aqueous electrolyte at pH 1. The conduction band edge is represented by red color and the valence band edge by green color. On the right side, the reduction potential for water oxidation and the reduction potential of hydrogen are shown. [54]. TiO2 is a versatile compound not only used as a semiconductor in photochemical applications. It is used in white paint, toothpaste, sunscreen and food (E171) et cetera. TiO2 is nontoxic, stable and cheap. It was the semiconductor used in the famous Electrotechnical Photolysis of Water at a Semiconductor Electrode paper by Fujishima and Honda. [55] TiO2 has band positions suitable for hydrogen production from water with a valence band edge more positive than the reduction potential for H2 O/O2 and the conduction band edge more negative than the reduction potential of H2 /H2 O (see Figure 1.6). [54] TiO2 has several crystal forms occurring naturally; rutile, anatase and brookite. Rutile is the thermodynamically most stable form but anatase is the one preferred in DSCs. The reason is the band gap of anatase being 3.2 eV, while it is 3.0 eV for rutile. The higher conduction band edge of anatase leads to higher Fermi-level and higher open circuit voltage (Voc ) in DSC application. 21.

(43) In the work of this thesis only anatase TiO2 has been used. Zinc oxide (ZnO) is very similar to anatase TiO2 considering the valence and conduction band edges, see Figure 1.6. Historically ZnO was used in the research founding the DSC field. [56–58] ZnO has higher electron mobility than TiO2 , which should favor electron transport. The problem with ZnO is its stability, it has been shown that ZnO dissolves to Zn2+ in the solar cell. [59, 60] Nevertheless, there has been extensive research within the DSC field on ZnO. [61] The Dye The task of the dye is to absorb photons, as many as possible, and inject electrons into the semiconductor. The reason why a DSC needs to be dyesensitized, is because the band gap of the semiconductor is so wide that it only absorbs light in the UV-region. Figure 1.6 shows that the band gap of TiO2 is about 3-3.2 eV. This corresponds to an absorption threshold below 400 nm. In this way a great deal of photons are lost and the efficiency will never be high. If a sensitizer is introduced, two more tunable energy levels are added and higher light harvesting is achieved. There are many different kinds of sensitizers such as metal complexes, porphyrins, phtalocyanines and metal free organic dyes. For an extensive review on different kinds of sensitizers for the DSC see elsewhere. [61] Most metal-based dyes for DSC application are ruthenium-based, they have favorable properties such as broad light absorption, suitable energy levels, relatively long lived excited state and good electrochemical stability. There is extensive literature regarding ruthenium complexes used as sensitizers in DSCs. [61] Two famous ruthenium-based dyes within the DSC field are the N719 and the so-called Black dye. N719 was published by Grätzel and coworkers in 1997 [62] and the Black dye in 2001 by the same group [63]. Both the N719 and the Black dye are illustrated in Figure 1.7. O. OH. OH. O. OH. OH. O. TBAOOC. N N N. HO. N. Ru NCS SCN. O. N. NCS. Ru. SCN. N N. SCN COOTBA. O. Black dye. N719. Figure 1.7. The Black dye and the N719 dye, both ruthenium-based. By introducing the Black dye, Grätzel and co-workers extended the incident photon conversion efficiency (IPCE) into the near IR-region up to 920 nm, yielding a PCE of 10.4%. [63] In the work of this thesis organic dyes were 22.

(44) used. Organic dyes have a number of advantages, see the following list. • They can be easy to synthesize. • Since the metal-based dyes often contain rare earth metals (such as ruthenium), the organic dyes can be cheaper and also more environmentally friendly. • The molar extinction coefficient of organic dyes is often higher compared to metal-based. This makes them suitable for solid state solar cells where the TiO2 thickness is limited. • Due to the donor-linker-acceptor concept of the organic dyes, it is easy to design new dyes. The donor-linker-acceptor dyes are, as the name suggests, molecularly designed having three different parts with distinct functions. The names donor and acceptor refer to the ability of the different parts in the molecule to donate or withdraw electron density, respectively. The donor and the acceptor are linked by the π-linker. The linker influences the absorption of the dye, which is dependent on the length, degree of conjugation and intrinsic electron withdrawing and donating ability of the linker. The HOMO-LUMO gap of the donor-linker-acceptor dye is largely depending on the HOMO by the donor and the LUMO on the acceptor, however this is not true since the linker unit inflicts on the HOMO-LUMO gap as well. The acceptor is the part attaching to the semiconductor. D35 is an organic dye with donor-linker-acceptor architecture, see Figure 1.8. The donor is the triphenylamine unit, the linker is the thiophene unit and the acceptor is the cyanoacrylic acid unit. An alternative to dye-type sensitizers are quantum dots (QD). QD are interesting due to their intrinsic properties. The band gap varies with the size of the QD and thus, absorption and redox properties can be tuned in the synthesis of the QDs. [30, 31]. O O O O. N. S NC COOH. Figure 1.8. The organic donor-linker-acceptor dye D35.. 23.

(45) The Redox Couple A number of different redox couples have been used in the DSC throughout the years. [61] Iodide/triiodide (I− /I− 3 ) was the redox couple used in the breakthrough article of 1991. [28] Thereafter, I− /I− 3 was the outstanding redox couple giving the highest PCEs. The success of the I− /I− 3 redox couple can mostly be explained by its slow recombination with electrons in the TiO2 . [64] The redox activity of the I− /I− 3 redox shuttle is a many step process and this is probably the reason why it prevents back reactions. [61, 65] However, the I− /I− 3 redox couple does possess some disadvantages; it has a relatively negative redox potential (0.35 V versus NHE) limiting the attainable VOC and hence also the PCE. [66] Another disadvantage is the high molar extinction coefficient, thereby the redox couple competes with the dye in absorbing photons, limiting the light harvesting efficiency (LHE) of the system. A further disadvantage is that iodide can promote breakdown of passive oxide layers and corrode most metals. [67–69] The certified world record for DSCs is still today with an iodide-based electrolyte. [49] Due to the disadvantages of the I− /I− 3 redox couple, research was conducted in order to explore other redox couples. Cobalt redox couples were identified as promising but had problems with mass transport and recombination. [70–72] It was not until a combination of dye design and cobalt redox couple was found, during 2010 and 2011, that the cobalt redox couples outperformed the I− /I− 3 . [1, 73] This led to the noncertified published world record of 14.3% at the time of writing this thesis. [48] Compared to the I− /I− 3 electrolyte the cobalt redox couple electrolyte exhibits weaker absorption of visible light, [74] which increases the IPCE. Furthermore, unlike I− /I− 3 , cobalt complexes have chemically customizable structures which allows the optimal tuning of reduction potential, and this greatly improves compatibility with a wider range of dye architectures. Several different redox shuttle systems have been tried in the liquid DSC, as mentioned earlier. The bromide/tribromide (Br− /Br− 3 ) achieved 2.6%, [75] the organic 2,2,6,6-Tetramethyl-1-piperidinyloxy radical (TEMPO) rendered 5.4%, [76] ferricyanide/ferrocyanide 4%, [77] copper-based redox couples have yielded around 7%, [78] and thiolate/disulfide has shown about 5%. [79] Recently, the first manganese-based redox couple was published with a PCE of about 4.4%. [80] However, none of these redox shuttles beats the cobalt record stated above, of around 14%. [48, 81] In the work of this thesis cobalt-based redox shuttles have been investigated, I− /I− 3 -based electrolytes have merely been used for comparison. The Electrolyte Solvent The requirements on the solvent for the liquid electrolyte in the DSC are; chemical stability, low viscosity and to provide good solubility for the redox mediator and other additives in the electrolyte. It is also important that the solvent does not cause desorption of the dye, semiconductor or dissolves the 24.

(46) sealing material. In the early studies of the DSC, water was used as solvent for the electrolyte. [26] However, these systems achieved modest efficiencies of 1-2%, with illumination of less than 1 sun. [82, 83] In 1991, when the breakthrough PCE of 7.9% was published, this was however with an organic electrolyte solvent (ethylene carbonate/acetonitrile). [28] The field then turned to organic electrolyte solvents, and several studies in which water was reported having a negative effect on DSC performance were published. [84–86] The electrolyte solvent in record DSCs is today, in most of the cases, acetonitrile. [48, 73, 81] However, acetonitrile is highly flammable, toxic and harmful both to human health and the environment, and therefore impractical for commercialisation. Extensive research for finding less volatile solvents has been performed. Ionic liquids possess low vapor pressures and have shown to give stable systems. [35, 36, 61] Electrolytes can also be based on both organic solvents and ionic liquids, these electrolyte mixtures can be gelated, polymerized, or dispersed with polymeric materials. This gives something called a quasisolid electrolyte. The advantage of the ionic liquid and the quasi-solid based electrolytes is the lower vapor pressure and thereby the stability. The disadvantage is the mass transport limitations. More extensive information about electrolyte solvents in DSCs can be found elsewhere. [35, 36, 61, 87–89] Water would be an excellent electrolyte solvent in DSCs as it is nontoxic, non-flammable and compared to the most used solvent today, acetonitrile, it has lower vapor pressure. Another consideration is the stability. Water will most probably permeate into the DSC modules with time, if not extensive permeation barriers are added to the design. With water as electrolyte solvent permeation of water would not be a problem. Another advantage of water would be the cost, as water is cheaper than organic solvents. Furthermore, water gives the possibility to tune the conduction band edge (ECB ) by the pH. The position of the ECB in colloidal anatase TiO2 particles depends on the pH by the following relationship (Equation 1.3). [90] ECB = −0.26 − 0.06 × pH (V vs NHE at 298K). (1.3). After the breakthrough with organic solvents [28], water was put aside. Until 2010 there were only few articles examining water as electrolyte solvent for the DSC. After 2010, water as electrolyte solvent in the DSC had a revival, this probably due to the studies by O’Regan, where he showed that water content in organic iodide electrolyte DSCs was not necessarily bad for DSC performance. The low JSC was attributed to reduced current carrying capability of the electrolyte in the pores, not to fundamental problems with electron transfer kinetics at the TiO2 /dye/electrolyte interface. [91] In a follow-up study, it was shown that high efficiencies could be attained by improving the wetting of the electrolyte inside the pores, by addition of chenodeoxycholic acid. This led to a PCE of 4%, which was, at that time, ~5 times larger than the previous record for an aqueous electrolyte DSC. [92] 25.

(47) Since then many groups have published water-based electrolyte studies, Spicca and co-workers showed 4% with ferricyanide/ferrocyanide, [77] Sun and co-workers attained 4% with the organic redox couple TT− EMI+ /DTT, [93] the already mentioned TEMPO redox shuttle has been applied in water and rendered 4%. [94] However, just as in organic solvents, cobalt redox shuttles have shown the highest PCE, Spiccia and co-workers have published 5.6% for a cobalt bipyridine redox couple in water. [95] The earlier problems with water as electrolyte solvent were probably much depending on the combination of using I− /I− 3 as redox shuttle with water, and adopting the same parameters as in organic solvent-based DSCs. The main pathway for recombination in systems using I− /I− 3 in 3-methoxypropionitrile (MPN) and propylene carbonate has been shown to be the reduction of I2 . [96] The equilibrium constant of I− and I2 to form I− 3 (Equation 1.4) is in water 6 KM = ∼1000, compared to KM = ∼ 4 ×10 in acetonitrile. [92] The concentration of I2 thus being unavoidably higher in water-based I− /I− 3 electrolytes, compared to equivalent acetonitrile electrolytes. I − + I2 I3−. (1.4). As the equilibrium constant changes, so does the reduction potential, which affects the PCE of water-based I− /I− 3 electrolyte DSCs. [65] In the work of this thesis both organic solvent electrolyte, acetonitrile-based, and water-based electrolyte using cobalt redox couples have been investigated.. The Counter Electrode The task of the counter electrode is to reduce the oxidized species of the redox couple. This should be done with as low resistance as possible in order to attain an efficient DSC. The most common counter electrode material used for the I− /I− 3 redox system has been thermally deposited platinum clusters. The thermally deposited platinum counter electrode is very efficient for the I− /I− 3 system. However, after the introduction of cobalt complexes as redox couple, it was shown that platinum was not the most efficient in the cobalt-based redox system. [97] Instead, conductive polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) have been shown to be more efficient for cobalt-based electrolyte DSCs [97] (paper I), as well as for sulfur-based redox couples. [93] There are a number of publications on different counter electrode catalysts such as carbon materials, conductive polymers, cobalt sulfide and functionalized graphene. [61,98] The recent published world record cells with PCE values of about 13-14% have been produced using a combination of platinum treated fluorine doped tin oxide (FTO)-coated glass, graphene nanoplatelets and gold treated FTO-coated glass. [48, 81] In this thesis, the PEDOT counter electrode has been investigated both in organic 26.

(48) solvent electrolyte (paper I, II and IV) and in water-based electrolyte (paper III and paper V).. 27.

(49) 2. Methods. 2.1 Characterization of Complete Device 2.1.1 Current-Voltage Characteristics One of the most essential measurements of a solar cell is the current-voltage (J-V) measurement. By analyzing the J-V curve, parameters such as the power conversion efficiency PCE, the open circuit voltage VOC , the short circuit current JSC , the fill factor FF and the maximum power point MPP can be obtained. It is important to apply standardized methods when measuring J-V curves, otherwise the results cannot be compared to other published results. For measuring in a correct way the J-V curve should be measured under illumination of a lamp with a spectrum similar to the AM1.5G illumination, where AM stands for air mass, 1.5 is the air mass coefficient defining the solar spectrum after the passage through the atmosphere and G stands for global, see Figure 2.1.. .

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(59) . Figure 2.1. The Sun irradiates the Earth, before the irradiation enters the atmosphere the spectrum equals AM0. After the passage through the atmosphere at zenith, the spectrum of the irradiation is AM1. The spectrum used for characterizing solar cells is AM1.5, which equals a zenith angle of 48◦ .. The air mass coefficient 1.5 defines that the solar irradiation has traveled 1.5 times the length of the atmosphere. Before the passage through the atmosphere, the spectrum is AM0, zero atmosphere. At the surface of Earth, at zenith the spectrum equals AM1. The air mass coefficient is defined as the 28.

(60) ratio of the passage through the atmosphere and the passage at zenith. AM1.5, 1.5 atmosphere thickness, corresponds to a solar zenith angle of 48◦ . An illustration of the different irradiations is shown in Figure 2.1. The intensity of the illumination at AM1.5 is 1000 W m−2 , which equals to 1 sun. The J-V characteristics are monitored under illumination by applying an external potential between the working electrode and counter electrode and measuring the current. The external potential is altered from 0 V to VOC conditions of the solar cell, or opposite depending on scanning direction. The J-V curve can also be measured under dark conditions. This will give information about electron recombination to the oxidized redox species. Since no oxidized dye is present in dark, the dark current is a measure of electrons going in the reverse way, from the TiO2 to the oxidized species of the redox couple. In Figure 2.2 J-V curves measured at 1 sun and in dark are illustrated.. D51 + Co(phen)3 1 sun D51 + Co(phen)3 Dark. J / mA cm-2. 6 4 2 0 0.0. 0.2. 0.4 0.6 E/V. 0.8. 1.0. Figure 2.2. J-V curves of a solar cell with the D51 dye and a [Co(phen)3 ]Cl2/3 waterbased electrolyte (paper V) measured under 1 sun illumination (solid) and in dark (dashed).. The J-V measurements should be carried out with slow enough scan rate so the solar cell has time to adjust and no hysteresis effects appear. [99] Hysteresis has since the entrance of the perovskite solar cells emerged as a problem, since the reported efficiencies of the perovskite solar cells can due to measuring conditions be over, or under estimated. [100] The MPP is given by the Pmax values, defined as the point where the solar cell produces the most power. By this the FF is introduced, which is a measure of the ratio of the Pmax and the JSC and VOC , see Figure 2.3. The PCE of a solar cell is calculated as the ratio between the maximum output power (Pmax ) and the input energy (Pin ): PCE =. Pmax JSC ×VOC × FF = Pin Pin. (2.1) 29.

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(62) . . . . . . . . . Figure 2.3. J-V curve of a solar cell with the LEG4 dye and a [Co(phen)3 ]Cl2/3 waterbased electrolyte (paper V) under 1 sun illumination. By taking the area of the shaded square and dividing it by the area of the one covered by the VOC and the JSC the FF is obtained.. where the FF is defined by Equation 2.2. FF =. Jmax ×Vmax JSC ×VOC. (2.2). The inclination of the J-V curve is related to the resistances in the solar cell. In paper I, the FF was used as a measure of how different kinds of counter electrodes affected the resistance in the solar cells.. 2.1.2 Incident Photon to Current Conversion Efficiency The incident photon to current conversion efficiency (IPCE) is a measure of the efficiency of the solar cell to convert the incoming photons to photocurrent at different wavelengths. This is done by measuring the resulting photocurrent of the solar cell when illuminating with monochromatic light. The IPCE is a measure of the product of different efficiencies such as Light Harvesting Efficiency (LHE), the quantum yield of the electron injection from the excited dye into the TiO2 φin j , the efficiency of the regeneration process ηreg and the collection efficiency of the photo-generated charge carriers ηcoll , see Equation 2.3. IPCE = LHE × φin j × ηreg × ηcoll. (2.3). In Figure 2.4 the IPCE spectra of two solar cells with the organic dyes 30.

(63) D35 and LEG4, in combination with [Co(bpy)3 ](PF6 )2/3 acetonitrile-based electrolyte are shown.. IPCE/ %. 80. LEG4 D35. 60 40 20 0. 400. 500 600 700 Wavelength/ nm. 800. Figure 2.4. IPCE spectra of two solar cells with the organic dyes D35 and LEG4, in combination with [Co(bpy)3 ](PF6 )2/3 acetonitrile-based electrolyte (paper II).. In paper II, Equation 2.3 was used for deriving the deficiency of some of the LEG-series dyes. IPCE spectra have been measured in the work of this thesis to evaluate solar cells.. 2.1.3 Toolbox Techniques Toolbox techniques are a generic way of addressing measuring techniques that measure: electron lifetime, transport time and extracted charge. A more comprehensive description of the toolbox system, used in this thesis, can be found elsewhere. [101] In short, the measurements were performed using a white LED as light source, voltage and current traces were recorded with a 16-bit resolution digital acquisition board, in combination with a current amplifier and a custom made system using electromagnetic switches. Electron Lifetime Measurements In the electron lifetime measurements the lifetime of the electrons in the TiO2 at different VOC s is measured, before it recombines with the oxidized dye or to the oxidized redox species. The experiment is carried out by applying a small modulation with a certain frequency to the bias voltage applied to the LED light source. As a result of the modulation in light intensity, a resulting rise and fall of a new VOC value is measured. Rise and fall time constants are calculated by fitting exponential function to the signal. An average time constant is calculated, corresponding to the electron lifetime τ. 31.

(64) The electron lifetime τ is an important parameter when evaluating different dyes and redox shuttles. Before Hagfeldt and co-workers published a study where the dye design prevented recombination, [1] the problem of the cobalt redox shuttles was the fast recombination, the short τ. [72] In the work of this thesis, the electron lifetime measurement has for example been used for evaluating recombination and low VOC values for different dyes (paper II), and different redox couples (paper V). Electron Transport Measurements In the transport time measurement, the time it takes for the electrons to travel to the back-contact of the working electrode is measured. The experiment is carried out by applying a small modulation with a certain frequency, to the bias voltage applied to the light source. The solar cell is kept at short circuit conditions. The resulting rise and fall to a new JSC as a response to the modulation of the bias voltage of the light source is measured. The corresponding rise and fall times are calculated by exponential fitting to signal. The transport time is obtained by averaging the rise and fall times. Transport time measurements can be used to evaluate the conduction of electrons in the semiconductor. Extracted Charge Measurements Extracted charge measurements can be carried out both at open circuit and at short circuit conditions. The solar cell is illuminated at open circuit conditions or short circuit conditions, after a certain time the light is turned off and simultaneously the solar cell is switched to short circuit conditions while the current is monitored. The extracted charge during the short circuit conditions is obtained by integrating the current with time. At short circuit conditions, the solar cell is illuminated and the current is monitored through a current amplifier. After a certain time, the illumination is turned off and the software integrates the current with time.. 2.1.4 Impedance Spectroscopy The purpose of a solar cell is to generate current and voltage. Thus, in order to reach a high PCE, there should be as little resistance for the flow of current in the solar cell, as possible. In the DSC there are different resistances such as charge transfer resistance at the counter electrode, resistance of electron transport in the TiO2 , recombination resistance of electrons in the TiO2 to the oxidized redox species or to the oxidized dye molecules and diffusion resistance in the electrolyte. By measuring electrochemical impedance spectroscopy (EIS) one can obtain information about different components’ resistance and capacitance. More specifically, there are resistances at the FTO glass, the wires and the external contacts giving rise to the series resistance Rs , at the TiO2 /dye surface and electrolyte interface there is the recombination resistance, Rrec . A 32.

(65) high Rrec is good for the solar cell, since this implies that the resistance for recombination of injected electrons in the TiO2 is high. There is also resistance in the electrolyte due to diffusion of the redox shuttles, RD , given by the Warburg element. At the counter electrode, there is the resistance for the charge transfer process of the reduction of the oxidized redox species in the electrolyte, RCE . In the DSC there is double layer capacitance at the charged electrode surfaces; the working electrode and the counter electrode. At the working electrode, the injected electrons charge the surface negatively, positively charged cations in the electrolyte are attracted and a Helmholtz layer is created generating double layer capacitance. The counter electrode experiences the same phenomena, and also here a double layer capacitance is built up. Resistance is the ability of an electric circuit to resist the flow of electrical current, electrons. Ohm’s law (Equation 2.4) gives the relationship between current I, voltage V and resistance R. V = I ×R. (2.4). Impedance (Z) is introduced when working with AC signals. Under such conditions, current can flow through a capacitor, which is dependent on the AC frequency. By applying a sinusoidal alternating potential (VAC ) to the electrochemical system and measuring the output, a sinusoidal current (IAC ), one obtains Z (Equation 2.5). Z=. VAC IAC. (2.5). Since the input VAC , and the output IAC will be phase shifted, it is convenient to describe impedance by complex numbers (Equation 2.6 and Equation 2.7). z = x + jy j=. √. −1. (2.6) (2.7). Impedance is represented by a real and an imaginary part (Equation 2.8 and Equation 2.9). Z = |Z| × e jθ. (2.8). Z = |Z|cos(θ ) + |Z| jsin(θ ). (2.9). 33.

(66) When measuring impedance of a DSC and plotting the imaginary part on the y-axis and the real part on the x-axis, a Nyquist plot is obtained. When measuring a standard DSC, half circles will appear in the spectrum. A standard Nyquist plot of a DSC is shown in Figure 2.5.. . . . . Figure 2.5. Resistances in a DSC. The total resistance Rtot , is dependent on the series resistance Rs , the charge transfer resistance at the counter electrode RCE , the recombination resistance of electrons in the TiO2 Rrec and the diffusion resistance for the redox shuttle in the electrolyte RD .. In the Nyquist plot, the x-axis gives the real part of the impedance and the y-axis the imaginary part. One normally scans from high frequencies to low frequencies. When measuring a full assembled DSC, the first resistance appearing is the Rs , the resistances at the FTO glass, the wires and the external contacts. This is real and is where the measurement points start in the spectrum. After this a first semicircle appears, this associates to the RCE . The impedance contains both the double layer capacitance of the counter electrode and the resistance for the electron transfer. This gives both an imaginary and a real part. The second semicircle appearing is the TiO2 /dye surface – electrolyte interface. Here the real part gives the recombination resistance Rrec and the electron lifetime τ in the TiO2 can be calculated, see Equation 2.11. The third semicircle gives the diffusion resistance RD , of the redox species in the electrolyte. Analysis of EIS for DSC For interpretation of impedance measurements, the data are fitted to an equivalent circuit. An equivalent circuit for a complete DSC is shown in Figure 2.6. As already described, there is a resistor for the Rs , whereas for the interface TiO2 /dye surface – electrolyte there is a constant phase element (CPE) and a resistor in parallel. The CPE describes the chemical capacitance and is a non 34.

(67) Rs. CPE_TiO2. W. R_TiO2. R_CE CPE_CE. Figure 2.6. An equivalent circuit for a DSC. ideal capacitor. There is a Warburg element for describing the RD . For the interface counter electrode - electrolyte there is once more a CPE and a resistor in parallel. By using this model values of the different elements can be obtained. For fundamental description and profound work of EIS with DSC systems see elsewhere. [102–105] The diffusion coefficient for the redox species can be calculated by Equation 2.10: RD =. RT L z2 F 2 aCD. (2.10). where RD is the diffusion resistance, R the gas constant, T the temperature, L the diffusion length, z the number of electrons transfered by the diffusing species, F the Faraday constant, a the electrode area, C the concentration of the diffusing species and D the diffusion coefficient of the diffusing species. This equation is though only valid for ideal solutions. [106] In paper I, impedance measurements were performed and merely the RCE and the capacitance at the counter electrode were derived. This was performed with symmetric cells in order to simplify the analysis. The diffusion coefficient of the [Co(bpy)3 ]3+ species was calculated by Equation 2.10. In paper IV, impedance measurements were used to derive Rrec and electron lifetime τ for the two different dyes D35 and D45 in full assembled DSCs. The solar cells assembled with the two different dyes D35 and D45 possessed different VOC s, since the same electrolyte was used for both systems the Fermilevel governs the VOC . In order to measure accurate τ values and Rrec , where the difference in driving force for recombination has been ruled out, the measurements in paper IV were performed at the same applied potential for both systems. The applied potential was chosen as an average of 95% of both systems VOC s. In this way, both systems had the same driving force for the recombination process, and the architecture of the dyes should be responsible for the difference in measured Rrec and τ values. At the same time, choosing a value close to Vmax represents the working conditions of the solar cell. Nyquist plots were measured both at one sun and in dark. In dark, the recombination of the electrons in the TiO2 only goes to the oxidized redox species, since no dye molecules are excited. Under 1 sun conditions, the recombination can go both to oxidized redox species and to oxidized dye molecules. In paper IV, the difference in Rrec for 1 sun and in dark measurements were approximated as 35.

(68) the Rrec to the oxidized dye. The τ values were calculated from the Nyquist plots according to Equation 2.11 [107]: τ=. 1 2π fmax. (2.11). where fmax is the frequency at the maximum of the the semi-circle representing the interface TiO2 /dye surface – electrolyte.. 2.2 Characterization of Components 2.2.1 Photo-induced Absorption Spectroscopy Photo-induced absorption (PIA) spectroscopy is a pump probe technique, where a square-wave modulated light (530 nm or 460 nm in this thesis) is superimposed on a probe light (20 W tungsten-halogen lamp). The square-wave modulated light excites the dye molecules. When excitation takes place, the injection of electrons into the TiO2 occurs within femtoseconds to picoseconds. The oxidized dye is quickly regenerated, if a redox couple is present. If no redox couple is present, or the redox couple is unable to regenerate the dye, the oxidized dye will be probed. The transmitted signal from the sample is focused onto a monochromator and detected using a UV-enhanced silicon photo-diode, connected to a lock-in amplifier, via a current amplifier. A more comprehensive description of the PIA-setup and theory can be found elsewhere. [108, 109] A general illustration of the PIA-setup is shown in Figure 2.7..

(69) . . . . . . . . Figure 2.7. Schematic drawing of the PIA-setup used for measurements in this thesis.. In Figure 2.8, the PIA spectra of the organic dyes D35 and D45 sensitized on TiO2 films are shown. No redox couple is present and the spectra are therefore representing the oxidized dyes. There is a negative delta absorption peak at about 525 nm, corresponding to the ground state bleach. The ground state bleach resembles the inverse absorption peak. In the region of the ground 36.

(70) Delta abs./ Arb. units. state bleach can also the Stark effect be observed. The Stark effect is due to the fact that the oxidized dye molecules on the TiO2 and the injected electrons in the TiO2 impose a change in the local electric field. The Stark effect is described in detail elsewhere. [109] In short, the absorbance spectrum of the dye molecules shifts due to the local electric field. In Figure 2.8, a positive signal is observed as the wavelength increases, this is due to the absorbance of the oxidized dye molecules. From Figure 2.8, information necessary to perform transient absorption measurements can be obtained (see section 2.2.2). In paper II, IV and V, PIA spectra of dyes without redox couple present were measured in order to find the absorbance of the oxidized dye.. D35 D45. 8.0E-4 4.0E-4 0.0. -4.0E-4 -8.0E-4 -1.2E-3 400. 500. 600. 700. 800. 900. 1000. Wavelength/ nm. Figure 2.8. PIA spectra of the organic dyes D35 and D45 sensitized on TiO2 films, without redox couple present.. Delta abs./ Arb. units. 3.0E-3 2.0E-3 1.0E-3 0.0. -1.0E-3 D51 + H2O D51 + Co(phen)3. -2.0E-3. LEG4 + H2O LEG4 + Co(phen)3. -3.0E-3. 600. 700 800 Wavelength/ nm. 900. Figure 2.9. PIA spectra of the organic dyes D51 and LEG4 sensitized on TiO2 films with and without redox species present. The black curves correspond to samples with pure water as electrolyte (Dye + H2 O), while the green graphs represent samples with 0.13 M [Co(phen)3 ]Cl2 in water as electrolyte (Dye + Co(phen)3 ) (paper V).. 37.

(71) The PIA spectra of the organic dyes D51 and LEG4 sensitized on TiO2 films, both in presence and absence of redox species, are shown in Figure 2.9. The spectra give information about regeneration. The electrolyte used in Figure 2.9 regenerates the dyes since the delta absorption is zero when electrolyte is added compared to when redox couple is not present. In this thesis, PIA was used to study regeneration of the oxidized dye by the redox couple, as seen in Figure 2.9 (paper III and paper V) and to find the wavelength where the oxidized dye absorbs, as seen in Figure 2.8 (paper II, IV and paper V).. 2.2.2 Transient Absorption Spectroscopy Transient absorption spectroscopy (TAS) is a pump probe technique. In the TAS measurements of this thesis, a nano-second laser was applied and the resolution in time was down to ns-μs. This made it possible to measure the regeneration (∼1-10 μs) and the recombination (μs-ms) electron transfer kinetics in the DSC system. When measuring PIA, information whether the regeneration is taking place or not is obtained, but not at what time-scale (not with the standard apparatus used in this thesis). A schematic drawing of a TAS apparatus is illustrated in Figure 2.10. . . . .

(72) . Figure 2.10. Schematic drawing of a TAS-setup. When TAS is measured, signals as those observed in Figure 2.11 are obtained. In order to retain one time constant for the kinetic process measured, a fit to the signal is needed. The kinetics of the recombination follow a multiexponential time law. [61] The same is valid for the regeneration kinetics, see Figure 2.11b, one reason for this is that recombination happens at the same time-scale as the regeneration. The multi-exponential behavior is due to many reasons, among which trapping/detrapping of electrons in the TiO2 and Gaussian distribution of the reduction potential of the dye contribute. [61] 38.

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