Emilie Genin

Yohann Nicolas

Luc Vellutini

Anne Thienpont

Karine Heuze

Marie-Anne Dourges

Svitlana Poix-Shinkaruk

Laurent Thomas

Imane Barbara

Camille Bertrand

Yannick Hermans

Shun Kashiwaya

Susanne Koch

Szu-Hsuan Lee

Yannick Mousli

Martin Palazzolo

Juan Angel

Moreno Balderrama

Qian Li

Changlong Wang

Jianguo Bai

Shilin Yu

for their friendly help in

encouragement I can never succeed in this endeavor.

iii

SYNTHÈSE EN FRANÇAIS

L'objectif principal de ce travail était i) la conception et la synthèse de nouveaux colorants basés sur des complexes organométalliques présentant une architecture originale D- [M]-A, où [M] est le centre métallique riche en électrons [Ru(dppe)2], pour les dispositifs DSSC de type n et de type p, ii) la caractérisation de leurs propriétés optiques et électroniques ainsi que l'évaluation de leurs performances photovoltaïques dans les dispositifs de type n à base de TiO2 et les dispositifs de type p à base de NiO. Suite à ce travail, notre objectif a été d'élargir le champ d'application de ces photosensibilisateurs organométalliques à des DS-PEC (cellules photoélectrochimiques à colorants) à base de NiO pour la production photoinduite de dihydrogène. Performances photovoltaïques mesurées sous AM 1.5G, 1sun, 100 mW•cm -2 : Jsc = densité de courant de court-circuit, Voc = tension de circuit-ouvert, ff = facteur de forme, = rendement de conversion photovoltaïque. Epaisseur du film de TiO2 = 10 µm (couche transparente) + 5 µm (couche diffusante).

Surface active = 0.159 cm 2 (masque). Q = quantité de colorant greffé sur une électrode de TiO2 de 1 cm 2 .

Cette ingénierie moléculaire a permis d'obtenir des photosensibilisateurs efficaces avec Comme prévu, une plus grande densité de greffage de colorant sur NiO a été observée avec le colorant SL5, comparé aux colorants SL3 et SL4. En dispositifs p-DSSC, en présence du médiateur redox standard iodure/iodure, le colorant SL5 présente des performances photovoltaïques légèrement améliorées ( = 0,046%) par rapport à SL3 ( = 0,039%) et SL4 viii ( = 0,033%) mais une efficacité inférieure à celle obtenue avec SL1 ( = 0,096%).

Cependant, le colorant SL5 a également été testé avec un électrolyte à base de disulfure/thiolate, fournissant ainsi une photo-tension supérieure et la meilleure performance de la série,  = 0,125%.

Sur la base des résultats encourageants obtenus avec SL5 et un électrolyte disulfure/thiolate, nous avons finalement conçu de nouveaux complexes à base de x Ag/AgCl).

SL8-Co

SL8

xi Une électrolyse prolongée en tampon acétate (0, 1 M, pH 4,5) et sous irradiation continue a conduit à une génération photo-électrochimique de H2 avec un rendement faradique de 27%.

Cette performance, comparable à d'autres rapportées dans la littérature, [3][4][5] a pleinement validé notre approche consistant à utiliser des complexes de ruthénium organométalliques comme espèces photoactives pour construire des DS-PEC efficaces à base de NiO.

Ⅰ.1. General Introduction

With the rapid development of the global economy, the growth of population, the acceleration of the urbanization process and the intensification of the environmental pollution, the pressing need for energy, especially clean, safe, and renewable energy, has inevitably become the most critical scientific and technological challenge confronting the entire mankind in 21st century. Despite the emerging world's astounding economic gains over in the past few decades, for a broad swath of mankind, access to sufficient electricity remains a remote dream. Sixty nations were analyzed, representing 90% of Earth's population. 1 Fifteen terawatts is the energy of 150 billion 100-watt light bulbs burning 24 hours a day for a year, which is also how much energy humanity now uses annually. This is equivalent to consume 220 million barrels of oil per day to keep the world spinning. 2 Richard E. Smalley, a professor of Physics and Astronomy at Rice University who is the 1996 chemistry Nobel Prize winner, found by calculating that the global population will reach ten billion by 2050.

And by then, we would need to consume 60 terawatts on a global scale -the equivalent of 900 million barrels of oil per day. This most serious challenge facing mankind in this century was called "Terawatt Challenge" by Professor Smalley. Based on such research background, Professor Smalley generalized the major issues facing mankind into 10 issues (Energy, Water, Food, Environment, Poverty, Terrorism and war, Disease, Education, Democracy, Population), with energy at the top. Most notably, energy can be the key to solve all the other issues. 3 For now, more than 85 percent of the energy consumed by humans comes from the incomplete combustion of the fossil fuels such as oil, coal, and natural gas. Through burning fossil fuels, carbon dioxide and sulfur dioxide have been emitted into the atmosphere which contribute to the global warming and cause the environmental pollution. 4 It is learned that by the end of this century, a mere 82 years from now, we will need to have more infrastructures that run exclusively on renewable generated energy. 5 Solar energy, as a renewable energy source, is inexhaustible in supply and always available for use. It is worth mentioning that solar energy has many unique advantages such as no transportation problems, can be stored in place, green pollution free and so on. Broadly speaking, there are three opportunities involving the conversion and storage of the sunlight in energy: solar-to-heat conversion, solar-to-chemistry conversion and solar-to-electricity conversion. 6 Among them, the solar-to-electricity conversion is mainly realized through solar cells.

Up to now, solar cell production has increased at about 30% per year over decades. 7 Conventional solar cells of today are based on crystalline silicon which are considered as the first generation of solar cells. The second generation of solar cells such as amorphous silicon, CIGS (Copper Indium Gallium Selenide) and CdTe (Cadmium Telluride) are based on thin film technologies. Single junction devices are the foundation of the first and the second generation solar cells. It should be noted that the calculated thermodynamic efficiency limit for single junction devices, i.e. 31%, suppose that a single electron-hole pair is formed when an individual photon is absorbed and all photon energy in excess of the energy gap is lost as heat. To overcome the so-called Shockley-Queisser limit, new concepts are developed which corresponds to third generation solar cells. 7,[START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF] In this context, dye-sensitized solar cells (DSCs) can be regarded as a technology in the process of transition from the second generation to the third generation of solar cells. [START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF] In the present stage, DSCs are attractive photovoltaic devices because their low production cost compared with conventional photovoltaic technologies, design opportunities such as transparency and multi-color options, flexibility, lightweight, short energy payback time (< 1 year), low dependence both on the angle of incoming light and solar irradiance regarding power conversion efficiency (PCE) and so on. [9][10][11][12][13][14] DSCs can be primitively categorized into n-type DSCs (n-DSCs), p-type DSCs (p-DSCs) and tandem DSCs on the basis of the working principles involved through the use of various dyes and nanostructured metal oxide electrodes. 7,[START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF]10,15 Furthermore, research efforts have been also devoted to dye-sensitized photoelectrochemical cells (DSPECs) which enable either proton reduction into hydrogen via water splitting or carbon dioxide reduction into carbon monoxide. In analogy to p-type DSCs, NiO-based photocathodes can be obtained by sensitization of nano-structured thin films with organic or metallo-organic dyes and display photoelectrochemical activity. 12,[16][17][18] As is clear from the above descriptions, dye-sensitized solar cells (DSCs) is a promising technology in the field of solar electricity production since the seminal paper published by O'Regan and Grätzel in 1991 19 which underpins the development of critical renewable energy and environmental technology. In this context, high efficiency/low cost solar cells and visible-light driven hydrogen production from photoactive cathodes based on molecular catalysts and dye-sensitized metal oxide semiconductors are of great interest. Following these earlier significant studies, in this thesis, we focused on the design, synthesis and properties of asymmetric ruthenium-diacetylide organometallic complexes as photosensitizers for a new series of n-type DSCs, p-type DSCs and DSPECs.

Ⅰ.2. Overview of Solar Energy and Solar Cells

Natural society is composed of three elements: matter, energy and information. Since the beginning of human civilization, energy has been an indispensable element of survival and development which can undertake the necessary support to nature through energy transformation, such as mineral energy, nuclear energy, atmospheric circulation energy, terrestrial rational energy and so on. However, a series of environmental issues arising from fossil fuel consumption lead to global warming and ecological environmental deterioration. 20,21 Within the global and national implications for poverty, economic development, and overall social progress, no country can afford to ignore the research on energy, especially on renewable energy. It is obvious that access to economically implemented renewable energy sources is promoted by the increasing public awareness that the Earth's fossil reserves will run out during this century, simultaneously the energy demand will be twofold within the next 50 years. [22][START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF][24][25][26] In the field of renewable energies, solar energy appears as one of the most advantageous resources to fulfill the rising worldwide demand for inexhaustible, clean, abundant and sustainable power sources. [START_REF]TfOas a light brown powder in 93% yield[END_REF]28 In this context, the attractive approaches that can facilitate the harness of the solar energy and mimic natural photosynthesis are very significant and valuable. Following this concept, solar cells have attracted worldwide interest which appear to be the large scale answer to solar energy utilization. [7][START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF][9]29

Ⅰ.2.1. Solar Energy

The sun is a hot star with a surface temperature of 5777K (Figure I-2), which emits a generic form of radiation because of consistently nuclear fusion reactions inside. The light emitted outward from the sun has a range of wavelengths from the ultraviolet and visible to the infrared. A very small fraction of the sun's light is emitted and after passing unhindered through interplanetary space it hits the earth. According to estimates, the total energy of the sun's omnidirectional radiation into the universe is 4×10 14 TW while the radiation to the surface of the earth's atmosphere can be estimated by solar constant. 7,30,31 The solar constant is a flux density measuring mean solar electromagnetic radiation (solar irradiance) per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit (AU) from the Sun (roughly the distance from the Sun to the Earth). The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite as being 1.361 kilowatts per square meter (kW/m 2 ) at solar minimum and approximately 0.1% greater (roughly 1.362 kW/m 2 ) at solar maximum. 32 The solar "constant" is not a physical constant in the modern CODATA scientific sense; that is, it is not like the Planck constant or the speed of light which are absolutely constant in physics. The solar constant is an average of a varying value. In the past 400 years it has varied less than 0.2 percent. 33,34 Hereby, the energy radiated from the sun to the surface of the earth can be estimated as approximate 1.74×10 5 TW -the equivalent of the heat given off by burning 1.86×10 14 tons of standard coal per year. Moreover the energy of 1.74×10 5 TW goes so far as to be higher than the world's total reserves of fossil fuels (equivalent to about 1.00×10 13 tons of standard coal). 35 Due to the progress of social civilization, the social level of industrialization will also continuously deepen accompanied by the problems of worldwide environmental pollution and energy shortage. Energy and environmental issues becoming more and more important, the development and utilization of solar energy are tuning into a key point as it possesses the advantages of being clean, safe and reliable.

Ⅰ.2.2. Solar Radiation and Solar Spectrum

As mentioned above, solar constant is measured by artificial satellite in outer space, which is going to be equal to the sum of light energy versus all wavelengths of sunlight (within the range of 150-4000 nm) under the circumstance of no atmospheric absorption 30,31,36,37 :

Pin =   d P  4000 150 in ) ( =    d ( N hc p ) 4000 150   (1)
The Pin in the formula refers to the luminous energy striked on solar cells in one minute offered by the light with a wavelength of λ while h stands for Plank constant and Np represents photon flux (the amount of radiant energy received from the sun per unit area per unit time). In this way, the measured radiation under the circumstance of no atmospheric absorption is the solar constant. As a matter of fact, the absorption, reflection and scattering of the sunlight by the earth atmosphere have influence on the solar radiation intensity and the distribution of the solar spectrum while the irradiance varies depending on the position of the sun, orientation of the earth and air quality of community. In order to measure this impact, the concept of air mass (AM) was introduced and can be approximated by AM = P/P0•cosθz, where θz, P and P0 represent the zenith angle, the local atmospheric pressure and the standard atmospheric pressure, respectively. The relationship between air mass and θz is shown in The experimental results show that the efficiency of a solar cell is highly sensitive to change of solar spectrum. It is significant to identify standard solar spectrum to accurately compare the performance of solar cells measured at different times in different places. The air mass coefficient is commonly used to characterize the performance of solar cells under standardized conditions, and is often referred to using the syntax "AM" followed by a number.

The standard solar spectrum adopted outer space is called AM 0 corresponding to the situation that solar radiation is equal to solar constant, which is usually used to test performances of solar cells on artificial satellites and spaceships. Whereas the standard solar spectra served as efficiency measurements of solar cells are AM 1.5G (within the diffusion part of sunlight) and AM 1.5D (without the diffusion part of sunlight). The former spectrum is normalized in order that the integrated irradiance (the amount of radiant energy received from the sun per unit area per unit time) is 1000 W•m -2 . The latter spectrum is commonly used to test the performance of concentrator solar cells. [36][37][START_REF]TfOas a red powder in 88% yield[END_REF]

Ⅰ.2.3. Solar Cells

Since the French scientist Dr. E. Becquerel discovered "photoelectric effect" in 1839, the research on solar cells has never been stopped. 12,41 In 1883, Dr. Charles Fritts successfully fabricated the first solar cell based on a gold/selenium rectifier. [START_REF]Ru-diacetylide/Cobaloxime Photocatalytic System for H 2 Evolution from Dye-Sensitized Photoelectrochemical Cells In the sections Ⅲ.3. and Ⅳ.3. we have demonstrated the relevance of using [Ru(dppe)2] organometallic complexes as photosensitizers in NiO-based photocathodes for p-type dye-sensitized solar cells and photo-electrochemical cells. 41,57 On the other hand, in a recent study, our collaborators in the group of Dr. Vincent Artero (CEA Grenoble), reported the preparation of an innovative mono-molecular photocatalyst through covalent linkage of an organic dye and a cobalt diimine-dioxime catalyst. 30 Therefore we designed and prepared a new mono-molecular photocatalytic system via axial coordination of a cobalt-based H2-evolving catalyst, i.e. a cobaloxime complex, to a pyridyl-functionalized Ru-diacetylide photosensitizer coded as SL8. Figure Ⅳ-11 shows the representation of the Ru-diacetylide/Cobaloxime photocatalytic system anchored onto NiO nanoparticulate thin-film. The choice of the cobalt-based catalyst was motivated by the fact that, among cobaloximes, [Co(dmgBF2)2(OH2)is known as[END_REF]43 In 1954, the first monocrystalline silicon (mono-Si) solar cell with eventually conversion efficiency of 6.0% fabricated in American Bell Labs opened a new era of solar energy utilization. 44 In 1963, 242 W photovoltaic module array solar cell and its work system were designed and installed in Japan which was the global largest solar power station at that time. Starting from the year of 1973, solar cells have been used for civilian purpose along with their widely application. 43 Subsequently, the first amorphous silicon (a-Si) solar cell in the world was invented by D. E. Carlson and C. R. Wronski in 1976. 45 M. Grätzel and co-workers introduced the concept of dye-sensitized fractal type TiO2 electrodes with high surface area in 1985 and the efficiency of solar cells based on nanostructured TiO2 electrodes attained 7% in 1991. 19,[46][47][48][49] In general, solar cells are electrical devices that convert the energy of light directly into electricity by the photovoltaic effects, which go through nearly 150 years. A solar cell includes a photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. 50,51 As shown in Since the dawn of silicon based solar cells, the photoelectric conversion efficiency of mono-Si solar cells has been unceasingly improved. 62 Lab efficiency of 26.6 percent for (heterojunction back-contact type) mono-Si cells is the highest in the commercial photovoltaic market while solar module efficiencies -which are always lower than those of their corresponding cells -crossed the 20 percent mark for mono-Si in 2012. These high efficiencies can be combined with other technologies, such as multi-layer solar cells in applications where space and weight become an issue such as powering satellites. 61,63 However, the premise of high conversion efficiency is to use highly purified mono-

Ⅰ.2.3.1.2. Polycrystalline Silicon (poly-Si) Solar Cells

Polycrystalline silicon (poly-Si) is the key feedstock in the crystalline silicon based photovoltaic industry and used for the production of conventional solar cells. For the first time, in 2006, over half of the world's supply of polysilicon was being used by PV manufacturers. 64 The solar industry was severely hindered by a shortage in supply of polysilicon feedstock and was forced to idle about a quarter of its cell and module manufacturing capacity in 2007. 65 Only twelve factories were known to produce solar-grade polysilicon in 2008, however, by 2013 the number increased to over 100 manufacturers. [START_REF]UV-visible absorbance spectra of the sensitized films were recorded on an Agilent Cary 60 UV-Vis spectrometer equipped with a solid sample holder. Film sensitization. NiO electrodes were soaked in a 0.5 mM solution of SL6 or SL7 in MeCN for 24 h on an orbital stirring table. The electrodes were rinsed with MeCN and dried in air. Film sensitization. NiO electrodes were soaked in a 0.5 mM solution of SL6 or SL7 in MeCN for 24 h on an orbital stirring table. The electrodes were rinsed with MeCN and dried in air. Photoelectrochemical measurements. Chronoamperometric and linear sweep voltammograms were recorded with a Biologic SP 300 potentiostat under nitrogen at room temperature using a previously described specific cell in a three-electrode configuration. 22 The NiO electrode is[END_REF] The product manufacturing process of poly-Si solar cells is basically the same as mono-Si solar cells while the conversion efficiencies of both present differences. The conversion efficiency of poly-Si solar cells is around 12%-15% which is lower than mono-Si solar cells. Despite of this, poly-Si solar cells represent good prospects for development due to low dependence on silicon wafers and low production costs. In recent years, with the constant increase of photoelectric conversion efficiencies of the solar cell devices, poly-Si solar cells have been current mainstream products, whose market share has reached over 50% in silicon-based solar cells industry.

Compared with mono-Si solar cells, the reasons for low conversion efficiency of poly-Si solar cells are mainly as follows: 67 i. When produced for the electronics industry, polysilicon contains impurity levels of less than one part per billion (ppb), while polycrystalline solar grade silicon (SoG-Si)

is generally less pure. This leads to reduction of free electrons when the light is absorbed, and furthermore, reduction of the photocurrent and the conversion efficiency.

ii. The binding bonds between silicon atoms are weak and unstable which will be destroyed by the ultraviolet radiation of the sun as time goes on. Therefore the conversion efficiency of poly-Si solar cells will decrease with time.

iii. The manufacturing technology of poly-Si is relatively simple which means the poly-Si will have more impurities and defects. The doping of polycrystalline silicon does have an effect on the resistivity which leads to the low efficiency of poly-Si solar cells.

Ⅰ.2.3.1.3. Amorphous Silicon (a-Si) Solar Cells

Amorphous silicon (a-Si) which seriously limits its development scale and speed. 68,69,72,73 If the critical issues mentioned above can be addressed, we can have faith in the increasing broad application of a-Si thin film solar cells significantly beyond today's level.

Ⅰ.2.3.2. Ⅲ-Ⅴ Solar Cells

Ⅲ-Ⅴ compounds are new semiconductors after the development of germanium (Ge) and silicon (Si). The elements in groups Ⅲ and V of the periodic table can be put together in various way so that there is a wide range of Ⅲ-Ⅴ compounds. In recent years Ⅲ-Ⅴ compound semiconductor materials, including gallium arsenide (GaAs), gallium phosphide (GaP), and indium phosphide (InP), are widely applied in electronic devices such as light-emitting diodes, 74 laser diodes, 75 photocatalysis 76,77 and solar cells [78][79][80][81][82] , because of their high saturated electron velocity, high electron mobility, and appropriate band gap. Among them, GaAs is identified to the optimal light absorption layer material, which band gap is 1.43 eV. The value is close to the band gap for maximum power conversion which is approximately 1.5 eV. 83 GaAs solar cells are promising due to their high efficiency and their resistance to the ionizing radiation in outer space.

To date, GaAs solar cells can reach the photoelectric conversion efficiencies of around 30% with the help of light concentrators and 28.8 ± 0.9 % under standard AM 1.5 sunlight, 100 mW•cm -2 irradiation. 84 In 1954, H. Welker first found photovoltaic effect of GaAs. 85 Although the new concentrator system is expected to open a door to a new age of high efficiency Ⅲ-Ⅴ based solar cells, there are still many issues which await to be solved. First of all, Ⅲ-Ⅴ solar cell devices are limited because of their complex fabrication. Secondly, the

Ⅲ-Ⅴ materials are still inherently very expensive that results in a enormous challenge for large-area fabrication. Moreover some of the Ⅲ-Ⅴ materials are highly toxic such as arsenic and its compounds. 69 The environment, health and safety aspects of gallium arsenide sources (such as trimethylgallium and arsine) and industrial hygiene monitoring studies of metallo-organic precursors have been reported. 96 The stability and disposal methods of solid arsenic bearing wastes are important for environment pollution due to the toxicity of arsenic compounds. In short, from a long-term environmental perspective, Ⅲ-Ⅴ based solar cells do not remain as the first option regardless of their decent photoelectric conversion efficiency.

Ⅰ.2.3.3. CdTe/CdS Thin Film Solar Cells

Cadmium telluride (CdTe) photovoltaics is known as the most commercial successful thin film photovoltaic technology, benefiting from some excellent physico-chemical peculiarities as follows [97][98][99][100][START_REF] Kay | Artificial photosynthesis. 1. Photosensitization of titania solar cells with chlorophyll derivatives and related natural porphyrins[END_REF][102][103][104][105] : (1) its band gap (1.45 eV) is almost a perfect match to the distribution of photons in the solar spectrum in terms of conversion to electricity, (2) 1 μm thick layer of CdTe can capture over 90% of the sunlight because of its high absorption coefficient in the visible part of the solar spectrum in the range of (10 4 -10 5 ) cm -1 , (3) it exhibits a great thermodynamic stability, ( 4) its high stability and congruent evaporation make it superior for the growth by means of various different preparation methods, ( 5) it is moderately p-type because it naturally grows with intrinsic stoichiometry defects, ( 6) its post-growth treatment can reduce defects and increase the crystallinity which make it boundaries electrically inactive.

The CdTe was deeply studied as a light absorber material in solar cells firstly by J. J.

Loferski in 1956. 106 The first all thin film solar cell exhibiting a 6% efficiency based on CdTe/CdS was reported by D. efficiency of 15.8% was reported by J. Britt and C. Ferekides in 1993. 111 From this research, a promising experience can be learned that the stability over-time issue could be settled by a very fine optimization of the back-contact formation. Recent years, the other layers were applied in CdTe/CdS solar cells such as cadmium stannate (CTO) to replace in indium tin oxide (ITO), aluminum doped zinc oxide (ZAO) and fluorine doped tin oxide (FTO) to improve the front-contact. And also, a second low-conductive and high-transparent layer such as zinc stannate (ZTO) layer, ZnO layer, and SnO2 layer was added to improve the stability of the front-contact. 112 In the last two decades, the efficiency of CdTe/CdS solar cells went through from 16% to 22.1% and lots of work have been done to push the conversion efficiency closer to the theoretical Shockley-Queisser limit of 33% 133,136 . In the meantime, the industrial development based on CdTe/CdS thin film solar cells has become mature enough since 2000s and nowadays, the CdTe production capacity covers about 6% of the total photovoltaic production. [103][104][105]126,137 interface defects and so on. 70,103,[129][130][131][132][133][134][135]

Ⅰ.2.3.4. CIGS Thin Film Solar Cells

Copper indium gallium (di)selenide (CIGS) with the chalcopyrite crystal structure is a I-Ⅲ-VI2 tetrahedrally bonded semiconductor material composed of copper, indium, gallium, and selenium. Since the technology was first deployed, CIGS has always been one of the three mainstream thin-film photovoltaic technologies, alongside the cadmium telluride (CdTe) and amorphous silicon (a-Si). CIGS layers are thin enough to be flexible, adapting them to be deposited on flexible substrates. [138][139][140] Upon heating from 805 °C to 1045 °C, CuInxGa1-xSe2 transforms from the zinc blende form (pure copper indium selenide, where x=1) to the chalcopyrite form (pure copper gallium selenide, where x=0) with bandgap from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide). 138,141 Although CIGS can be applied on flexible supports, the best performance comes from the band gaps of the absorbers can be widened and thus higher open circuit voltage can be achieved. By doping with Ga and S, the alloys Cu(In, Ga)Se2 and CuIn(S, Se)2 can be obtained which can also provide an optimal match to the solar spectrum resulting to an enhanced efficiency. [159][160][161] On the basis of this, a CdZnS/CuIn0.73Ga0.27Se2 heterojunction cell, manufactured by Boeing Aerospace Company, was reported to achieve an efficiency of 12.9%.

The significantly large open circuit voltage of the cell was 555 mV which was unattainable for non alloyed CIS solar cells. 162 Since 1990s, the CIS thin film solar cells has been rapidly developed. In 1993, L. established the foundation for further improvement of CIGS solar cell efficiencies. 164 Among the CIGS thin film solar cell devices, the best-performing one had a CIGS absorber layer including 20~30% Ga with respect to In + Ga. 159,165 The National Renewable Energy Laboratory (NREL) was the world leader in the field of small-area CIGS thin film solar cell devices and reported record efficiencies increasing steadily over the years. [166][167][168][169][170] The latest record efficiency of Cu(In, Ga)Se2 solar cells by NREL was 23.3% at 14.7 suns optical concentration, whereas the devices yielded an efficiency of 20.8% under low flux optical concentration. 171 The current world record for CIGS solar cell efficiency stands at 22.6% at 1 sun fabricated through a three-stage process reported by P. suns. 173 There is no doubt that the transfer of the lab-scale processes into an industrial production encounters huge challenges which is obvious from the reported efficiency values.

Even so, a number of companies such as Solar Frontier, AVANCIS, Solibro, Manz, Siemens Solar and others are pursing commercial development of CuInSe2 photovoltaic modules through employing a variety of process technologies and product design according to their perception of production costs and market demands. 159,[174][175][176][177][178] Moreover, flexible module manufacturing systems were fabricated through using polyimide or stainless steel as substrates reported by some companies with an efficiency above 20% achieved. The CIGS solar modules deposited on flexible substrates not only are advantageous for manufacturing

Ⅰ.2.3.5. Dye-Sensitized Solar Cells

The dye-sensitized solar cells (DSCs) are photoelectrochemical systems based on photo-sensitized semiconducting oxide anodes in contact with electrolytes and are low-cost solar cells belonging to the group of thin film solar cells. 7,10 One major difference between conventional thin film cells and DSCs lies in the parts where the light is absorbed. In the thin film cells, the light is absorbed in the semiconductor layer while in DSCs light absorption is achieved by dye molecules grafted onto highly porous nanostructured metal oxide electrodes. Compared to traditional thin film solar cells, DSCs have different operating principles regarding charge separation and photoelectric conversion process. With regard to silicon based solar cells, for example, the silicon plays the role of the photoelectron source, as well as the donor of the electric field to separate the charges and produce the photocurrent, while in DSCs, the bulk of the semiconductor is used only for charge transport and the separate photosensitive dye provides electrons to the semiconductor after light excitation.

Therefore, charge separation takes place at the interfaces defined by the semiconductor, the dye and the electrolyte in DSCs. As a result of the unparalleled differential operational principles, DSCs can work even under low-light conditions by which it means DSCs are able to operate under cloudy skies and non-direct sunlight condition. By contrast, the traditional solar cells such as silicon based solar cells degrade at some lower limit of illumination and when charge carrier mobility is low, the recombination becomes a major issue for these classical thin film cells. 69,[184][185][186][187][188] It counter electrode and a redox electrolyte, which are sandwiched in between two conductive substrate materials. In detail, the typical system comprises the following parts:

1. The anode, which supports the dye-sensitized semiconducting oxide layer and helps in the electron transfer, is typically made by transparent conducting oxides (TCO). 7,10,[185][186][187] The TCO-coated glass is used as the substrate for the nanostructured metal oxide electrodes and hence is an imperative component. Theoretically, it should have high transparency and low sheet resistance which is less than 20Ω/. Moreover, because of sintering of the electrode material (typically TiO2), which is usually carried out between 450 and 500 ℃, the sheet resistance should also be independent of temperature up to 500 ℃. The two main types of conducting glass used as substrates are fluorine-doped tin oxide (FTO) and indium-doped tin oxide (ITO).

Sometimes, it is difficult to choose a proper type due to the variety in cell configurations and materials. 189 High temperature processing during the device fabrication, in which a paste of TiO2 is sintered at 400-600 ℃, can degrade ITO properties because the exposure of ITO to temperature above 300℃ in air usually increases its sheet resistance, whereas FTO is much more stable at 400-600 ℃. 69,189 2. The nanostructured mesoporous oxide layer is deposited on the anode as electrodes to activate electronic conduction. The semiconductor electrode most commonly comprises nanocrystalline anatase TiO2 layer. The TiO2 thin film is deposited on the conducting glass with a thickness of 5-30 μm which plays an important part in both the exciton dissociation and the electron transfer process. The porosity and morphology of the TiO2 layer have crucial influence on the amount of dye sensitizers adsorbed on its surface which can harvest incident light and further determine the conversion efficiencies of the solar cell devices, and hence are dominant factors. In order to adsorb a reasonable amount of the sunlight, the semiconductor layer needs to be made much thicker than the dye molecules themselves. To address this problem, a nanostructured material is used as a scaffold to hold large numbers of the dye molecules, increasing the number of molecules adsorbed on the surface of the TiO2 layer. 7,10 Some alternatives to TiO2 have been reported such as ZnO, Nb2O5, WO3, Ta2O5, CdS, Fe2O3, SnO2 and so on. The more efficient semiconducting metal oxides reported so far are TiO2, ZnO, SnO2 and Nb2O5. [47][48][49]69,[190][191][192][193][194][195][196][197][198] 3. The monolayer of dyes which is covalently anchored to the surface of the nanostructured mesoporous oxide layer plays a very important role in the dye-sensitized solar cell devices. The dyes (photosensitizers or dye sensitizers) contribute to the primary steps of photon absorption and the subsequent electron injection. 7,10,69,[184][185][186] The historical context, molecular structures, classification, operational principles, recent progress and future outlook for the dyes are discussed in details in section Ⅱ.2., Ⅲ.2. and Ⅳ.2..

4.

The electrolyte (or a hole-transporter for all-solid-state DSCs) includes redox mediator which is a ubiquitous and indispensable component of all DSCs. It plays an important role in the inner charge carrier transport between electrodes through regenerating the dye during DSCs operation. Furthermore, the electrolyte has significant influence on the photovoltaic conversion efficiency, operational stability as well as long-term durability of dye-sensitized solar cell devices. 15,188,199 Numbers of important parameters in DSCs can be relevantly affected by the electrolytes such as photocurrent density (Jsc), photovoltage (Voc) and fill factor (FF). 200 As a material that provides pure ionic conductivity between the electrodes in solar cell devices, the electrolyte must fulfill several essential requirements. 15,188,[201][202][203] Firstly, the electrolyte must ensure the charge carriers transport between the photoanode and the counter-electrode. Secondly, the electrolyte must have high conductivity and possess good interfacial contact with the dye-sensitized mesoporous layer and the counter-electrode. Thirdly, long-term stabilities such as chemical, thermal, optical, electrochemical, and interfacial stability are necessary. Last but not least, the absorption spectrum of the electrolyte should not cover the visible region. The electrolytes can be divided into liquid electrolytes, quasi-solid-state electrolytes and solid-state transport electrolytes. The most popular electrolyte is still the iodide/triiodide (I -/I3 -) redox couple in terms of solar cell device efficiency and device stability. 7.10,15,188 5. The cathode, also called counter-electrode, is made of a glass sheet treated with a catalyst to facilitate electron collection and electron transfer. The cathode employed in DSCs can directly affect the fill factor of the solar cell devices. 7,10,69,186,200 Sputtered or calcined platinum (Pt) coated on a TCO substrate is popularly used as a cathode. [204][205][206][207][208] Also, carbon materials [209][210][211][212] , polymer materials [213][214][215] as well as other novel materials such as cobalt sulfide (CoS) 216 can be used as the cathodes instead of Pt.

As aforementioned in I.

Ⅰ.2.3.6. Perovskite Solar Cells

A promising new class of solar cells based on mixed organic-inorganic hybrid perovskite materials for converting solar energy into electrical energy have experienced unprecedentedly rapid emergence within less than a decade. 52,[217][218][219][220][221][222][223][224][225][226][227][228][229] The power conversion efficiencies of these solar cells are boosted from original 3.

      X B B A R R R R    2 / t
(2)

With respect to the high symmetry cubic structure under ideal conditions, the tolerance factor t should be close to 1. Otherwise, the perovskite material structures are pseudo-cubic because of distortion and the symmetry is degraded. For halide-structured perovskite materials, the tolerance factor t should be in the range of 0.81 through 1.11 and the defined octahedral factor μ (RB/RX) in the range of 0.44 through 0.90. At finite temperature, the cubic structure is likely when t lies in the range of 0.89-1.0 while lower symmetry tetragonal or orthorhombic structures arise when t < 0.89. Despite these constraints, transitions between these different structures occur when heated and the cubic structure usually exists at high temperature. In particular, for the organic-inorganic halide perovskites, the position A is occupied by a larger organic cation 234 which generally results in materials with good performance [235][236][237][238] . mesoporous scaffold (most commonly TiO2); 4) a perovskite material as a light absorber in the bulk of which most electron or hole transport occurs; 5) a hole transfer material; and 6) a conducting back contact. In the working process of PSCs, the perovskite material contributes to the primary steps of photoexcitation and charge separation. Under light illumination, the electron-hole pairs are created in the perovskite material. Then, the photogenerated electrons are injected into the mesoporous layer while the holes are driven into the hole transfer material. The injected electrons are further passed on to the back contact through the external circuit and at the same time the holes are transferred to the same back contact. Subsequently, the holes and the electrons recombine at the interface of the back contact and hole transfer layer which finally regenerates the system. For an effective charge extraction process, the band alignment of the perovskite material should satisfy the following requirements: 1) its conduction band must be higher than the conduction band edge of the electron transport layer and 2) its valence band must be lower than the valence band edge of the hole transport layer.

However, some detrimental charge recombination can occur during the charge-transfer process. Some photogenerated electrons can recombine with holes in the perovskite material layer and/or in the hole transfer material layer. What's more, the injected electrons may further recombine with the holes at the interface of electron transport layer/perovskite material layer and/or at the interface of electron transport layer/hole transfer material layer.

Therefore, charge generation and extraction must operate on much faster timescales than charge recombination in order to get high performance PCSs. [239][240][241][242] Research into halide perovskite materials can date back over a century 232 , while present emergence of organic-inorganic halide PSCs benefit from the investigations of related thin film transistors and light-emitting diodes based on the electricity-to-light conversion ability of halide perovskites reported by the Mitzi's group. 254,255 What's more, these perovskite materials are promising for flexible devices. 217 PSCs as a rising star in the third generation photovoltaics, have attracted much attention from both academic and industrial fields, indicating a great possibility of mass production and commercialization in the near future. However, the stability of PSCs in moist environments, the interplay between the organic and inorganic components, the fundamental reasons for alloy stabilization of the structures and the replacement of toxic Pb atoms by environment-friendly components are still crucial challenges that require further developments.

Ⅰ.3. TiO 2 -Based n-Type Dye-Sensitized Solar Cells

The third generation solar cells conquer the limits of single junction devices and display high efficiency compared to the same production costs of the first and second generation solar cells which effectively drive down the US$/W. 256 Dye-sensitized solar cells can be identified as a technology between the second and third generation solar cells, [START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF] and thus have been extensively studied since the pioneering work being reported by O'Regan and Grätzel in 1991. 19 Owing to their low fabrication cost, variety of colors, large-scale manufacturing processes, environmental friendliness, earth-abundant materials and relatively high power conversion efficiency, the DSCs can fulfill all of the desired properties of conventional silicon based solar cells. [7][START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF][9][10][11][12][13][14][15][185][186][187][188]199,200 Over the last few decades, TiO2-based n-type DSCs have attracted considerable attention and a substantial amount of experiments have been carried out to improve the device efficiency from 7.1% in 1991 19 to 14.3% in 2015. 257 In this section, n-type DSCs based on nanostructured mesoporous TiO2 layer and the iodide/triiodide (I -/I3 -) redox couple are introduced. was connected with a platinum black electrode through the external circuit. 260 From then on, TiO2 was widely recognized as a crucial semiconductor material in the fields of photovoltaic solar cells. However, in preliminary research, the dye-sensitized photoelectrochemical cell devices were fabricated based on smooth semiconductor surfaces resulting in a rather low light-harvesting ability. The main reason is that a monolayer of the photosensitizer can absorb even less than 1% of the AM 1.5G spectrum. 7,12,259 As to this background, before 1991 some researches were focused on the possibilities to increase the roughness of the semiconductor surface in order to increase the number of dye molecules adsorbed on the surface and in contact with a redox electrolyte. In 1977, M. with a monolayer of a charge-transfer dye for light harvesting resulting in exceptionally more than 80% efficiencies for the conversion of incident photons to electrical current and an overall energy conversion yield of 7.1-7.9% under simulated sunlight. Since then, DSCs have entered public view and garnered ever-increasing research attention over the following decades, with efficiencies over 14% at the lab scale. 257 Also, TiO2 was commonly deployed as the semiconductor of choice because of its various advantages for sensitized photo-chemistry and photo-electrochemistry (low-cost, broadly available, non-toxic, biocompatible, etc…) and great progress was incremental in a synergy of structure, substrate roughness and morphology, dye photophysics as well as electrolyte redox chemistry. [46][47][48][49]106,[185][186][187]264,265

Ⅰ.3.1. Historical Background

Ⅰ.3.2. Operational Principles and Electron Transfer Processes of n-Type

Dye-Sensitized Solar Cells

A scheme of typical liquid-state DSCs showing their basic configuration has been mentioned above (I.2. 3.5., Figure I-14). In 1993, cis -X2bis(2,2' -bipyridyl -4,4'dicarboxylate) ruthenium(Ⅱ) complexes were prepared and characterized in regard to their absorption, luminescence, and redox behavior by M. Grätzel et al., where X = Cl -, Br -, I -, CN - and SCN -. Among them, cis -di (thiocyanato) bis (2,2' -bipyridyl -4,4' -dicarboxylate) ruthenium(Ⅱ), coded as N3, exhibited the more promising properties including broad visible absorption spectrum, commendable photon-to-current conversion efficiency (IPCE), remarkable long excited state lifetime, strong adsorption on the semiconductor surface and superior solar-to-electric energy conversion efficiency when employed in DSCs. Because of its excellent properties, the classical N3 dye is commonly used as a reference and yielded record efficiencies. 7,200,266,267 On the other hand, as referred above (I.2.3.5.), the iodide/triiodide (I -/I3 -) redox couple is the most popular electrolyte because of different advantageous features such as easy preparation process, high conductivity, low viscosity and excellent interfacial wetting between the sensitized electrode and itself. 15 The basic electron When exposed to the sunlight, the dyes on the surface of TiO2 semiconductor film get excited. The electrons from photoexcited dyes are injected into the conduction band of TiO2, leaving the dyes in their oxidized state, and then flow through the mesoporous TiO2 thin film to the transparent conducting oxide (TCO) substrate. Finally, these generated electrons diffuse via electron migration through an external load to the counter-electrode in contact with the electrolyte containing the redox mediator I -/I3 -, while the oxidized dyes are regenerated to their ground state by the electrons diffused from the electrolyte to complete the cycle. 7,10,13,185,186,268,269 In the process, the regeneration of the dye by iodide (I -) intercepts the adverse recapture of the conduction band electrons directly by the oxidized dye and the electron flow lies on the incident intensity and trapping-detrapping effect. 7,185,270,271 The basic electron transfer processes can be represented with the following equations: 7,9,185,272,273 :

i. Under illumination, the dye is photoexcited, resulting in the transition of the sensitizer from the ground state (S) to the excited state (S * ). ( S * → S + hν (10) As mentioned above, the loss pathways of the electron transfer processes (3, 4 and 8) are indicated and must be minimized compared to the desired pathways of processes (1, 2, 5, 6 and 7). Process 8 represents the adverse recombination of the excited dye which can be characterized by the excited state lifetime while process 3 and process 4 represent the recombination of the conduction band electrons with either the oxidized dye or I3 -in the electrolyte, respectively. 7,9,185,272,273 Actually, there is an alternative possibility not mentioned above that electron transfer to I3 -can occur at the photoanode contact (transparent conducting oxides (TCO)) which borders on the electrolyte. However, this adverse electron transfer process can be prevented by using a compact TiO2 blocking layer deposited on the photoanode. Also, some other semiconductors and insulating materials such as Nb2O5, ZnO, WO3, CaCO3, BaCO3 and SnO2 can be used as blocking layers which can effectively suppress the back reaction of photoinjected electrons with I3 -ions in the electrolyte. 7,10,[190][191][192][193][194][195][196][197][198][274][275][276][277]

Ⅰ.3.3. Energetics of Operational Principles

In general, charge separation in n-type DSCs can be considered as a two-step redox reaction including the electron injection into the TiO2 semiconductor and the subsequent oxidation of the redox electrolyte respectively, while the latter reaction results in the dye sensitizers from the oxidized state to the ground state which is referred in I.3.2.. Also, the thermodynamically downhill for both electron injection and dye regeneration is schematized in Figure I-17. After charge separation processes, the transport processes of the photogenerated charges to both photoanode and counter-electrode are of great importance for charge collection processes which are primarily driven by diffusion processes because of concentration gradients. For an efficient n-type DSC device under optimum conditions (good TiO2 nanoparticle interconnections and low-viscosity electrolyte in the presence of the redox mediator I -/I3 -), the modest concentration gradients can efficiently drive the charge transport processes of both photogenerated electrons toward the photoanode and redox ions toward the counter electrode, and therefore result in small free energy losses. 7,9,272,273 Electron transfer to reduce I3 -to I -can occur by using an efficient catalyst at the counter electrode to enable the process to proceed with minimal overpotential. Analogously, the photogenerated electron diffusion toward the photoanode can occur through an excellent contact at the interface between TiO2 and TCO. Hence, the positions of the energy levels, in other words the energetics, at the interfaces of both TiO2/dye and dye/electrolyte fundamentally enable to determine the overall device output. The standard measurements of the positions of the energy levels can be found in the refs 278 and 279. individual components and the ordinate may represent the internal energy, that is to say, the conduction band energy is one-electron energy rather than a free energy. The meaningful configurational entropy of charge carriers caused by the number of accessible energy states can be totally different in the different phases. 7 The energy levels of the sensitizer are usually introduced in the form of HOMO/LUMO levels in some publications. 204,[280][281][282][283] For the solid-state physics, the energy scale with vacuum can be used as reference, while for solid-state electrochemistry, the potential scale with the standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE) can be used as reference. 7,9,272,273,278,279 In TiO2 semiconductor, the electrochemical potential of electrons corresponds to the energy at the Fermi level (EF), while in an electrolyte solution the electrochemical potential of electrons corresponds to the redox potential (Uredox). On the one hand, the efficient charge separation and collection by the photoanode and the counter electrode can determine the power output of DSCs and result in a photocurrent. On the other hand, the generation of the photovoltage can also determine the power output of DSCs which corresponds to the free energy difference between the photoanode and the counter electrode. At equilibrium, the electrochemical potential of the TiO2 semiconductor (or the Fermi energy of the TiO2 semiconductor) is equivalent to the midpoint potential of the redox couple, which finally results in zero output voltage. In the circumstances, the Fermi level of the TiO2 semiconductor mainly lies in the band gap of the TiO2 semiconductor, while the TiO2 film is effectually insulating which means almost a negligible electron density in the TiO2 conduction band. As known, the photoexcitation leads to the electron injection into the conduction band of TiO2 and subsequent the oxidation of the redox electrolyte. On this occasion, the oxidized and reduced species are highly concentrated in the electrolyte, and thus the electrolyte photooxidation process will not give rise to a significant change in the potential of the electrolyte. By comparison, the process of electron injection into the TiO2 conduction band can give rise to a dramatic increase of the electron density, which in turn can raise the TiO2 Fermi level to the edge of the conduction band. Furthermore, such shift of the TiO2 Fermi level signifies a significant increase in the stored free energy of the injected electrons which also plays a role in the photovoltage generation process in the external electrical circuit. 7,273,[278][279][280][281][282][283] The midpoint potential of the redox couple can be described by Nernst equation 273 , which hinges on the relative concentrations of iodide and iodine. An efficient n-type DSC device requires high concentrations of iodide and iodine. However, the kinetics of the dye regeneration process at the photoanode and the iodide regeneration process at the counter electrode inversely constrains the concentrations of iodide and iodine. [284][285][286][287][288][289] Typically, the concentrations of iodide and iodine are in the range of 0.1-0.7 M and 10-200 mM, respectively, which as mentioned above, constrains the midpoint potential of the electrolyte to approximately 0.3V versus NHE. 273,290,291 Additionally it is worthwhile to mention that the iodine is primarily present in the form of I3 -due to the presence of an excess of iodide, which results in the electrolyte usually being referred to as the iodide/triiodide (I -/I3 -) redox couple. 15,272,273,[292][293][294][295] Although the difference of an electron versus the NHE potential in vacuum is unmeasurable with thermodynamic rigor 7,296 , an estimated equation reported by R. Memming gives the relationship in any redox couple between the Fermi level (EF,redox) and the redox potential (Uredox) as follows 297 :

EF,redox [eV] = -(4.6 ± 0.1) -eUredox [V] (11)
The e in the equation refers to the elementary charge, with which the standard potentials of the other redox couples are able to be represented on the absolute scale. publications and references therein. 7,9,10,185,204,267,273,298,299 These charge transfer processes lead to an enhanced spatial separation of the electrons and the holes and thus increase the lifetime of the charge-separated state along with the reduction of the free energy stored in the state.

Ⅰ.3.4. Kinetics of Operational Principles

This mechanism is quite similar to the function of the photosynthetic reaction centers. For the natural photosynthesis, the quantum efficiencies of charge separation and collection result from the competitions between the different forward processes and the loss pathways, which are therefore considered as key factors determining energy conversion efficiency. 7,55,273,[300][301][302][303][304][305] It is worthy to note that the electron injection efficiency in DSCs hinges on the magnitude of the injection kinetics relative to the excited-state decay to the ground-state, rather than the absolute kinetics of the electron injection. The excited state lifetimes of the sensitizers substantially differ from picoseconds to nanoseconds, which thus require on the different kinetics of the electron injection resulting in efficient device function. 273,[306][307][308][309] Another point to consider is that the potential for electron injection comes not only from dye singlet but also triplet excited states. The triplet states are long-live and can be ascribed to intersystem crossing from the singlet excited state. However, the triplet states are less energetic than the relative singlet state. For example, the intersystem crossing from the singlet to triplet state based on ruthenium bipyridine coordination dyes can be as fast as 1 × 10 13 s -1 . [309][310][311][312] Generally, depending on the solvent/electrolyte environments, the singlet state lifetimes are in the range of 1 × 10 -13 to 1 × 10 -9 s, while the triplet state lifetimes are in the range of nanoseconds to milliseconds. 7,273,312,313 As discussed above, the efficiency of electron injection is dependent not only on the rate of electron injection but also on the kinetics of the excited-state decay.

The rate of electron injection from the dye excited state into the mesoporous TiO2

semiconductor depends both on the electronic coupling between the dye LUMO orbital and accepting states of TiO2, and on the large density of states (DOS) in the TiO2 energetically accessible from the dye excited state. 7,314,315 After the electron injection process, the injected electrons play a role in fast thermal relaxation down to the electron Fermi level of the electrode. The thermalization process contributes to an adverse loss in free energy, which results in a limitation on the efficiencies of DSCs. 7,203,273 As referred, the electron injection kinetics depends on the energy of TiO2 conduction band compared to the dye excited-state oxidation potential. The electron injection kinetics also depends on the concentrations of the oxidized and reduced species in the electrolyte. Particularly, the presence of acidic ions can lead to a high metal oxide energy conduction band and thus a lower density of accessible acceptor states. The consequent low density of accessible acceptor states eventually results in a slow rate of electron injection, a low quantum yield of charge injection and a low device photocurrent. [316][317][318][319] In order to obtain efficient dye regeneration, the rate of reduction of the oxidized dye cations by the redox couple must exceed that of charge recombination of the injected electrons with the oxidized dye cations. The dye regeneration process strongly depends on the iodide concentration in the electrolyte, the viscosity of the electrolyte and the dye structure.

Employing the best-performing Ru-complexes and a low viscosity electrolyte such as I -/I3 - dissolved in acetonitrile, the regeneration process is sufficiently fast to compete with the recombination process which thereby guarantee the regeneration process can be attained with unity quantum efficiency. 273,299,320 The recombination process is strongly dependent on the Fermi level of the TiO2 film and thus on the light intensity and solar cell voltage, accelerating by an order of magnitude between short circuit and circuit conditions. 315 Furthermore, the recombination process depends on the spatial separation of the dye cation from the electrode surface, with the rate constant decaying exponentially with the approximate distance of the excited state HOMO from the TiO2 surface according to the electron-tunneling theory. 280 In order to obtain efficient charge collection by the external circuit, the rate of the electron transport to the counter electrode must exceed the rate of charge recombination of injected electrons with the redox couple in the electrolyte. The electron trapping in localized sub-bandgap states plays a key role in the diffusive electron transport process. Therefore, the kinetics strongly hinges on the position of the TiO2 electron Fermi level, that is, the higher Fermi level toward the conduction band edge the more trap filling. 273,321,322 There are coupled relationships between the charge separation and collection kinetics and the energetic losses involved in the basic electron transfer processes in DSCs. The energetic losses can cause the device output voltage being remarkably less than its optical bandgap, which thereby results in the maximized device output voltage being in the range of 0.6 to 0.75 V, less than half absorption onset of the typical dye sensitizers. 7,273 The balance between the kinetics and the energetics have been reported in a number of publications 7,55,273,301,319,[323][324][325][326][327] which will not be considered further in this manuscript.

Ⅰ.3.5. Characterization Techniques

Characterization techniques are essential to investigate internal processes of complete solar cell devices under normal solar light conditions. These techniques are also useful to measure and assist in the development of theoretical models accounting for the various electron transfer and transport processes, explore the stability and reproducibility of the solar cell devices and optimize their manufacturing based on different material components. Some efficiency measurements and electrochemical methods are reviewed in the following sections.

Ⅰ.3.5.1. Efficiency Measurements

The current density-voltage (J-V) characteristics of DSCs are the most widespread method used to determine the power conversion efficiency (Ƞ) of DSCs. 7,[9][10][11][185][186][187] Because of high interfacial capacity, DSCs usually have a relatively slow electrical response. Therefore, the voltage scan should be at a slow speed to avoid errors happening in the current measurement. On the contrary, the currents from a rapid forward and reverse voltage scan can be at average speed. 328 Figure I-20 illustrates a typical J-V curve, where Jsc, Voc and Pmax are indicated.

P max V OC J SC Voltage,V (v) Current Density,J (mA•cm -2 ) Figure I-20: A typical J-V curve.
From the J-V curve, the overall power conversion efficiency is given by 10

in oc sc in P FF V J P JV   max  ( 12
)
where Pin represents the power density of the incident light and FF represents the fill factor.

Graphically, FF represents the shape of the J-V curve, also known as the "square-ness" of the solar cells, and its value is comprised between 0 and 1. A high value of FF reveals a more favorable rectangular shape. The short circuit current density Jsc, which corresponds to the highest photocurrent generated by the solar cell divided by the area illuminated, is determined at the V=0 intercept. The open-circuit potential Voc is determined at the J=0 intercept. 7,9,11,328 The FF is given by the equation 7,9,11,200 ) ( max oc scV J P FF  (13) where Pmax represents the maximum power of the solar cell per unit area. Besides the two other parameters (Voc and Jsc) the FF is another significant parameter which can reflect the quality of DSCs. A higher value of FF can be achieved by increasing the shunt resistance, decreasing the series resistance and reducing the overvoltage for diffusion and electron transfer processes. 269 Another fundamental method used to measure the performance of a solar cell is the "external quantum efficiency (EQE)" which is usually called the incident photon-to-current conversion efficiency (IPCE). The value of IPCE is relative to the Jsc produced in the external circuit divided by the photon flux that strikes the solar cell. The IPCE as a function of wavelength can be calculated by 7,10,11,15,200

      2 2 ) ( ) ( 1240 ) ( ) (      Wcm P nm Acm J e J IPCE in sc sc      ( 14
)
where e represents the elementary charge. The values of IPCE provide information about the monochromatic quantum efficiencies of a solar cell device. 7,200 With a view to the bandgap energy, the maximum voltage generated under illumination is dependent upon the difference between the quasi Fermi level of the TiO2 film and the electrolyte redox potential. The Voc value is dependent on the iodide concentration due to the recombination reaction occurring between the electrons on the conduction band of TiO2 and I3 -and thus can be given by 10,331,332   ) ln(

3 0 0 oc    I k n q kT V et  ( 15
)
where Ƞ represents the quantum yield of photogenerated electron for the given incident photon flux (Φ0) and n0 represents the electron density on the conduction band of TiO2 in the dark. The ket in equation ( 15) corresponds to the recombination reaction rate for a given I3 - concentration. Higher Voc can be achieved through using Co(Ⅱ/Ⅲ) redox couple which has more positive redox potential and thus increases the potential difference. 10,269,331,332 Similarly, the precise value of Jsc can be determined by integrating IPCE and the Φ0 over the spectral distribution which is given by 10,331,332

    d r IPCE e J )) ( 1 )( ( ) ( 0 sc     (16)
where r(λ) represents the incident light loss and e represents the elementary charge. According to equation ( 16), a good approach to improve Jsc is to increase the value of IPCE, which can be realized through using panchromatic dyes that can absorb broad sunlight covering visible to the near-infrared range in solar spectrum. 10,269,331,332 The IPCE can also be calculated by 7,9 )

( ) ( ) ( ) (        cc reg inj LHE IPCE  (17)
where LHE(λ) represents the light-harvesting efficiency at wavelength λ, φinj represents the quantum yield for electron injection from the excited sensitizer in the conduction band of the TiO2, φreg represents the quantum yield for dye regeneration and Ƞcc represents the efficiency for the charge collection where LHE is equal to 1-10 -A with A being the absorbance of the film. 7,9,304

Ⅰ.3.5.2. Electrochemical Methods

The individual components of DSCs can be characterized by a range of electrochemical methods. Such methods afford the significant information on the energy levels of the component, the reversibility and kinetics of the electrochemical processes. The standard equipment for electrochemical measurements comprise a potentiostat connected to a three electrode cell including one working electrode, one reference electrode and one counter electrode. 7,328,[333][334][335][336] The most widespread electrochemical method is cyclic voltammetry. In such method, the potential is swept at a constant rate and reversed at a certain point, and at the same time the current is persistently monitored. The monitored current can arise from both the Faradaic processes such as the electron transfer at the electrode and the non-Faradaic processes such as capacitive charging at the electrode/electrolyte interface. In normal cyclic voltammograms, one can test the potentials of redox processes and obtain the reversibility of the electron transfer processes. Also, because the current peaks depend on the scan rate, one can get information on whether the redox active species are free in the solution or adsorbed to the electrode surface. There are some other electrochemical methods such as differential pulse voltammetry and square wave voltammetry. However, unlike the cyclic voltammetry, these referred techniques cannot give information on the reversibility. 7,328,[333][334][335][336] The electrochemical impedance spectroscopy (EIS) is another powerful electrochemical method to fully understand the electronic and ionic processes occurring at the different interfaces in DSCs. Such method afford information on the series resistance, the charge transfer resistance of the counter electrode, the diffusion resistance of the electrolyte, the resistance of the electron transport process and recombination process in TiO2 as well as the chemical capacitance of the mesoporous TiO2 electrode. The impedance measured in a wide range of frequencies in a real system can be determined according to an equivalent circuit including the series and parallel connected elements (resistors, capacitors, inductors) and the Warburg element that indicates the diffusion processes. 205,334,337,[338][339][340] The electrochemical method can be better applied if combined with different kinds of spectroscopy such as UV-Vis. From some reported publications, the electrochemistry combined with the UV-Vis spectroscopy has been employed to study the accumulation of electrons in mesoporous TiO2 semiconductor electrodes. 341 The onset of the accumulation of electrons in TiO2 electrodes can be related to the conduction band edge of mesoporous TiO2.

Such onset based on a number of organic electrolytes have been reported by G. Redmond and D. Fitzmaurice. 342 They found that some cations in the electrolytes such as Li + and Mg 2+ can determine the position of the conduction band of TiO2 on account of their specific adsorption on the mesoporous TiO2 electrode. Particularly, the electrochemistry combined with spectroscopy can be employed to determine the spectra of the oxidized and reduced dye sensitizers, which is significant in laser spectroscopy studies. 7,[343][344][345]

Ⅰ.3.6. Recent Improvements

In this section, the various improvements achieved in the recent years in the field of In recent years, a lot of research work have been done on modification of photoanodes through using nanoparticles, nanorods, nanotubes 7,10,13,268,269,348,349 in order to enhance the light absorption, 350,351 scattering, 352-354 charge transport 355 and by improving the interfaces energy by restraining the charge recombination processes 356 . Ag nanowires and graphene can be used in order to modify the photoanodes on the FTO glass which was reported by H. Yan et al. in 2015. In this publication, a novel multistage structure of DSCs which was comprised of Ag nanowires, TiO2 nanoparticles and graphene wrapped TiO2 mesoporous microspheres was designed and fabricated by using simple spin coating methods. The resulting PCE based on the novel solar cell structure was up to 7.42% which was about twice as much as that for the solar cell devices only with a TiO2 nanoparticle layer. 357 Generally, both the dye adsorption and the optical absorption via surface plasmon resonance can be enhanced through employing metal nanoparticles such as Ag nanoparticles. [358][359][360] Because of the flexibility and low cost, the polymers such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) can act as an alternative to glass substrates. The solar cell devices by using PET coated with ITO showed an efficiency of 3.8% and the solar cell devices by using PEN coated with ITO showed an efficiency of 7.8%. 361,362 Apart from polymers, metals such as tungsten, stainless steel and titanium can also be used as substrates. [363][364][365] In order to obtain a better performance of DSCs, a novel Sr, Cr co-doped TiO2 xerogel film was employed as a constituent part of photoanode. The performances of solar cell devices varied with the different ratios of strontium and chromium along with the FTO glasses. 366 Similarly, the performance of DSCs can be optimized through employing a novel

Sr, V co-doped TiO2 film. 367 Also, the nanoparticles such as Au-TiO2 and Pt-TiO2 along with the FTO glasses can be employed as photoanodes to show a better performance compared to the solar cell devices using pure TiO2 along with the FTO glasses as phonoanodes. 268 The semiconductor layers of ZnO nanoparticles, ZnO powders and nanopowders along

Ⅰ.3.6.2. Improvements in Counter Electrode

Overall, the counter electrode is a key component of DSCs. 7,10,13,268,348,349 FTO or ITO substrate coated with standard Pt is used as the most common and effective counter electrode because of its remarkable catalytic activity and high conductivity. However, improvements are required on developing alternative Pt-free electrodes due to the high cost and scarcity of Pt. 186,268 For now, studies have been reported on modification and development of the counter electrode by using different types of nanoparticles, novel metals and surface modification such as single or double layer.

More specifically, the counter electrode can be modified by using different materials such as graphite, activated carbon, platinum, carbon black, single wall carbon nanotubes, polypyrrole, polyaniline and poly (3,4-ethylene dioxythiophene) which are used as catalysts for reduction of triiodide. 186,210,268 Numerous metals materials were tried as substrates in DSCs such as stainless steel, W, Ti, Co, Ni, Al, Pt, Cu and Zn because of their promising electrical conductivity, good flexibility and/or ductility, thermal stability withstanding high temperature treatment, or also lower sheet resistance compared to ITO or FTO. However, when used as substrate materials of counter electrodes, metal substances are demonstrably prone to corrosion in the aggressive redox electrolyte medium. 186,268,284,[378][379][380][381] Intrinsic conducting polymers may have metallic conductivity or semiconducting properties. The electrical properties of such compounds can be finely-tuned through organic synthesis and advanced dispersion techniques. 284 Most of the conductive polymers are derivatives of polyacetylene, polyaniline, polypyrrole or polythiophenes, the structures of which are usually conjugated double bonds for conduction. [381][382][383][384] The reported conductive polymers such as poly (3,4-ethylenedioxythiophene) Graphene-based composite materials along with FTO glass substrate were also reported to act as counter electrodes. [389][390][391][392][393][394][395] For example graphene modified vanadium pentoxide nanobelts can be employed as an efficient counter electrode for DSCs and showing efficiency as well as pristine graphene, the graphene decorated by Ni0.75Cu0.25 nanoparticles showed the best efficiency of 5.1%. 397

The single wall carbon nanotubes (CNTs) were used as a carbon counter electrode because of the high longitudinal conductivity and showed a conversion efficiency of 4.5%, as reported by S. Kazuharu et al.. 212 Carbon-black-loaded stainless steel electrode used as a novel counter electrode can also display a good efficiency if a suitable thickness of carbon black is chosen. Such counter electrode is low cost and can be used for large scale production, the efficiency of DSCs based on which showed a high efficiency of 9.15%, as reported by T.

N. Murakami et al. 398 Compared to stainless steel materials, the carbon catalysts on metal surface are suitable for counter electrode because of their low cost and high stability towards corrosion. 209 A low cost nanoscale carbon/TiO2 composite was reported to be used as counter electrode for DSCs with an efficiency of 5.5%. In such composite, carbon acted as a catalyst and the TiO2 as a binder. 399 A nickel incorporated carbon nanotube/nanofiber composite can also be used as a low cost counter electrode for DSCs, as reported by P. 4) a non-light-absorbing characteristic in the visible region. 7,15,188,199,205,285 Generally, the electrolytes used in DSCs can be classified into three types: ( 1 DSCs is liquid, the major drawbacks of liquid electrolytes are poor long-term stability due to volatility, leakage and flammability of the liquids. 7,15,186,295 In this context, electrolytes including liquid ionic as low-volatile solvents were successfully designed.

Ⅰ.3.6.3.2. Organic-Solvent-Based Electrolytes

An organic electrolyte is usually composed of a solvent, a redox couple and some additives. There are a number of redox couples for the organic electrolytes which have already been reported such as Br -/Br3 -, 405 SCN -/(SCN)2, SeCN -/(SeCN)2 406,407 and substituted bipyridyl cobalt(Ⅱ/Ⅲ) 408 . Anyway, the most ideal redox couple is still I -/I3 -because of its good solubility, rapid ion diffusion, low light absorption in the visible region, fast dye regeneration, appropriate redox potential and slow dynamics of recombination for electron injection into the semiconductor and the triiodide. 15,188,409 The solvents for the organic electrolytes play an important role in the electron diffusion and the dissolution of the iodide/triiodide ions. For now, many organic solvents for the organic electrolytes such as acrylonitrile (AcN), propylene carbonate (PC), ethylene carbonate (EC), 3-methoxypropionitrile (MePN) and N-methylpyrrolidone (NMP) have already been reported. 13,410 The solvents with large donor number can enhance the Voc but reduce the Jsc. By controlling the donor number one can control the equilibrium of iodine and triiodide ions in the solvents. 411 Therefore, the electric additives can optimize the photovoltaic properties of DSCs because the additives can adsorb on the photoanode/electrolyte interface and restrain the recombination of injected electrons with the triiodide ions. For now, 4-tert-butypyridine (TBP), guanidinium thiocyanate (GuNCS) and N-methylbenzimidazole (NMBI) are reported to be the most efficient additives. [412][413][414] Ⅰ.3.6.

Ionic Liquid Electrolytes

Due to their volatility, liquid electrolytes usually have high evaporation rate. The evaporation issue and the leakage of the liquid electrolyte further lead to poor long term stability. The use of ionic liquid-based electrolytes can minimize the above drawbacks.

Room-temperature ionic liquids can act as alternative electrolytes for DSCs and other electrochemical devices because of their excellent chemical and thermal stability, negligible vapor pressure, non-flammability, high ionic conductivity, and a wide electrochemical window. 415 The ionic liquid electrolytes are conventionally composed of various organic salts containing cations such as imidazolium, pyridinium and anions from the halides or pseudo halides. 416 The most used ionic electrolyte for DSCs contains N,N' bis-alkyl-substituted imidazolium iodides. The viscosity of the organic salts was proved to decrease with the reduction of the alkyl chain length due to the decrease of Van der Waals forces. On the other hand, if the viscosity increases, the conductivity of the ionic liquid electrolytes decreases. [415][416][417] Therefore, an unprecedented efficiency of 8.2% under AM1.5G illumination was obtained by M. Grätzel's group through using a solvent-free ionic liquid electrolyte including 3-alkylimidazolium based salts which opened up possibilities for large-scale outdoor applications of mesoscopic DSCs. 404

Ⅰ.3.6.3.4. Solid State Electrolytes

Solid state electrolytes have also been studied to overcome the leakage issue and improve the stability, compared to the liquid electrolytes. 7,338,418 In solid state electrolytes, a p-type semiconductor or a hole transfer material takes the place of the liquid electrolyte. The band gap of the p-type semiconductor should be compatible with the HOMO level of the dye and the conduction band of the n-type semiconductor. 7,10,268 Copper based compounds such as CuI, CuBr and CuSCN can be used as inorganic hole transfer materials because of their excellent conductivity. 188 However, organic hole transport materials are superior to their inorganic counterpart because of their easy processing characteristics. The amorphous organic hole transfer material 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene, commonly named as spiro-OMeTAD, was for the first time employed as organic hole transfer material in 1998 by U. The existing difficulty in penetrating into the mesopores of the semiconductor for solid state electrolytes limits their applications. Therefore, the quasi solid electrolytes rose in response to the difficulty. A quasi-solid electrolyte can be considered as a composite of a polymer and a liquid electrolyte. 7,186,188,268 The quasi-solid electrolytes display a better TiO2 based photoanode with a dye-sensitized photocathode were proposed to improve the conversion efficiency of DSCs. 7,18,421,[425][426][427] Although the pn-type tandem DSCs are calculated to reach the energy conversion efficiencies of approximate 43% in theory, the practical maximum energy conversion efficiency achieved is only 4.1%, 421,427-429 which is far from ideal. Similarly, the best up-to-date NiO based p-type DSCs device shows an efficiency of 2.51%. 430 The low efficiencies of tandem DSCs can be prevailingly attributed to the practical performances of the p-type semiconductors. Particularly, the low energy conversion efficiency of p-type DSCs can be attributed to the low Jsc and Voc produced by the p-type semiconductors. 7,18,430,[423][424][425][426][427][428][429] Up to now, the most frequently used semiconductor for p-type DSCs is nickel oxide (NiO). 421,423,424,431,432 However, some other inorganic oxide materials such as CuO 433-435 can be employed as p-type semiconductors, as well as delafossites (CuMO2, M = Al, Ga, Cr, Fe, In, Sc, etc.) 429,431,432 and non-delafossites materials (SrCu2O2, LaCuOCh where Ch = chalcogen, etc.). 431,432 In addition, the production of fuels from solar energy and other renewable raw materials is an attractive solution to the constant depletion of fossil fuels and satisfy the world's increasing energy demand. As an example, water splitting can produce molecular hydrogen with O2 as a side product, while light driven reduction of CO2 can produce carbon-based fuels, with a net zero-carbon footprint. Theoretically, these processes can be implemented in dye-sensitized photoelectrochemical cells (DSPEC). 16,437,438 To date, a number of architectures have been designed for DSPEC, all of which contains photoelectrodes. 439 In this thesis, in analogy to p-type DSCs, DSPEC based on NiO photocathode have been focused which display photoelectrochemical activity for hydrogen production.

In this section, the historical background, recent improvements and operational principles for NiO-based p-DSCs and NiO-based DSPEC are introduced. Characterization techniques of n-type DSCs described in section I.3. can be adapted to p-DSCs and thus will not be described further in this section. Relevant contents for the characterization techniques of NiO-based DSPEC will be introduced in Chapter Ⅳ. The charge transport studies of dye-sensitized NiO photocathodes are also important. For p-NiO semiconductor, the hole transport was reported to occur by a hopping mechanism between localized surface states which enhance the interfacial charge recombination losses with the electrolyte. [444][445][446][447] The enhancement of charge collection efficiency of p-DSCs can be realized by increasing the hole diffusion coefficient or decreasing the collection time. [448][449][450] In contrast to TiO2 based n-DSCs, it was investigated that the transport time in NiO based p-DSCs would not to vary significantly with the incident photo intensity. 444,446,451 Theoretically, the conductivity of the semiconductors should be enhanced by increasing the concentration of the photogenerated charges. This is often the case for the typical n-DSCs.

Ⅰ.4.1. Historical Background and Recent Improvements

However, for NiO based p-DSCs, the hole transport rate is almost invariant and the hole lifetime drops severely with the increase in the photogenerated charge concentration. And therefore, when the hole lifetime is close to the transport time, the effective diffusion length of the hole limits the thickness of the NiO photocathode to 2-3 μm. 444,446,451,452 The peculiar charge transport mechanism of dye-sensitized NiO photocathodes involves hole hopping between Ni(Ⅲ) cations located at the surface and also assists the recombination with the electrolyte. 446,453 Another drawback of NiO stems from its peculiar ability to catalyze oxidation reaction when employed in heterogeneous catalysts. Actually, it has been already investigated that the overpotential of iodide oxidation is reduced on dye-sensitized NiO photocathode compared to that on FTO or platinum electrodes. The photogenerated hole charges are excited on NiO and subsequently react quickly with the reduced form of the electrolyte, which finally results in the primary photocurrent loss especially at voltages close to the Voc. On the basis of the above theories, either the typical I -/I3 -electrolyte or the NiO photocathode should be replaced or modified in order to improve the efficiencies of p-type DSCs.

In 2009, E. A. In addition to the replacement of the typical I -/I3 -electrolyte, an alternative approach to bring further improvements to the efficiency of p-type DSCs is to improve the quality and conductivity of NiO based photocathodes. An excellent review was reported to explore the effect of the preparation of NiO surfaces on their efficiencies as photocathodes. 424 As referred in I.3., there are several optional semiconductors which can be employed as photoanodes in n-type DSCs. However, only few metal oxides can exhibit p-type semiconductivity. In this thesis, nickel oxide is employed as photocathode in p-type DSCs and DSPEC and some other p-type semiconductors or NiO-based semiconductors in doped forms will not be introduced.

Nickel oxide is an intrinsic nonstoichiometric p-type semiconductor which has a wide bandgap (Eg = 3.6-4.0 eV) and possesses excellent thermal and chemical stability. 18,420,421,422,424,443,444,448,450 The valence band of nickel oxide semiconductor is positioned at 0.54 V vs. NHE in neutral environment, 425,442 which endows itself a promising electron donating property for a number of dye sensitizers and also makes the reductive quenching process of the dye sensitizers easier. However, the low-lying valence band potential of nickel oxide results in a low Voc for NiO-based dye-sensitized solar cell devices.

Moreover, NiO exhibits lower electrical conductivity than n-type metal oxide semiconductors and the TCO such as fluorine-doped tin oxide and indium-doped tin oxide are mostly n-type.

The hole diffusion coefficient of NiO semiconductor films is in the range of 10 -8 to 10 -7 cm 2 •s -1 while that of TiO2 semiconductor films is in the range of 10 -6 to 10 -5 cm 2 •s -1 , which were determined by photoinduced absorption spectroscopy. 445,460 The slow hole diffusion rate results in a long time delay between hole injection process into NiO and hole collection process, which finally increases their potential scavenging by the redox couples. 18,420,421,424,445 In general, the nanostructured NiO photocathodes for p-type DSCs are prepared in two steps: ⅰ) synthesis of Ni(OH)2 precursor, and ⅱ) heat Ni(OH)2 sol-gel films at high temperature. 443 Simultaneously, some other alternative routes including chemical vapor deposition, electrodeposition, hydrothermal synthesis, sputtering and sol-gel were also reported. J. eV. [START_REF]TfOas a light brown powder in 93% yield[END_REF][467][468][469][470][471][472] On the other hand, the band gap of a suitable semiconductor must be smaller than 3.0 eV for the sake of visible light (λ > 400 nm) absorption. The difficulty of finding suitable semiconductor materials lies in the dilemma that few semiconductors can possess both a sufficiently narrow band gap and suitable energy levels for catalytic reactions. Most of the semiconductor materials have either poor absorption characteristic or additional undesirable external bias. 473 Also, the energy levels of inorganic semiconductor materials are intrinsically hard to be finely-tuned. The employment of molecular dye sensitizers can be a solution for extending absorption into the visible light region for semiconductors. 7,474 Anyway, as mentioned above, NiO can be employed as an effective semiconductor for DSPEC. Similarly general requirements to obtain functional dye-sensitized photocathodes. On one hand, in addition to the suitable absorption and stability characteristics, the HOMO level of the dye must be more positive than the valence band position of the p-type semiconductors for the sake of efficient hole transfer processes after photoexcitation. On the other hand, the LUMO level of the dye sensitizers must be more negative than the catalytic onset reduction potential of the hydrogen evolving catalysts. [START_REF]TfOas a light brown powder in 93% yield[END_REF]437 Some promising dye sensitizers for NiO-based DSPEC are introduced in the following. Up to now, a large number of hydrogen evolving catalysts for proton reductions in homogeneous systems have been designed and investigated. 437,[475][476][477][478][479] Here, the details of the hydrogen evolving catalysts will not be further introduced.

The first example of a p-type DSPEC for visible light-driven proton reduction based on the earth-abundant and inexpensive p-type NiO semiconductor was reported by L. Sun et al.

in 2012. 17 photocurrent density of 1.7 μA•cm -2 which subsequently increased to 3.9 μA•cm -2 after 4 h.

After 2 h illumination, the Faradaic efficiency of H2 generation was investigated to be 97 ± 7 %. Through using the dye-sensitized NiO photocathode and BiVO4 photoanode (which was selected due to its visible response up to λ = 500 nm and its good photocatalytic properties for O2 evolution reaction) connected in a tandem configuration, the tandem DSPEC showed a photocurrent density of 1.8 μA•cm -2 without any external bias in a 0.1 M Na2SO4 buffer solution at pH = 7. The photocurrent density increased up to 2.7 μA•cm -2 after 4h. Also, the Faradaic efficiency of H2 generation was investigated to be 79 ± 3 % after 4h. However, O2 evolution could not be detected in their work due to the low sensitivity of their gas chromatography detector to oxygen.

In 2013, Y. the reduced form of the redox mediator (M -) finally yields an electron to an external load to complete the whole circuit, and at the same time the redox mediator is oxidized back to its original state by the hole (h + /CE) (equation 22). 18,[START_REF]TfOas a light brown powder in 93% yield[END_REF]420,421,424,489 To sum up, the reactions

The free energies of the dye regeneration (ΔGreg) can be determined by the following equation:

ΔGreg = e[E(M/M -) -E(S/S -)] (24)
The free energies of the charge recombination (ΔGCR) can be determined by

ΔGCR = e[E(S/S -) -EVB(p-SC)] (25)
In equations 23-25, the EVB, E00(S * ), E(S/S -), E(M/M -) and e represent the valence band potential of the NiO semiconductor, the energy of the excited state dye sensitizer, the reduction potential of the dye sensitizer, the reduction potential of the redox mediator and the elementary charge, respectively. 420 According to these equations, the maximum open circuit voltage (Voc max ) can be determined by the difference of the Fermi levels (Ef) of both the photocathode and the counter electrode as follows 420 :

Voc max = E(M/M -) -EVB(p-SC) (26)
The concept of dye sensitization extends to the field of photosynthetic solar cells, making the emergence of the so-called DS-PEC. A NiO based p-DSPEC device operates in an analogous fashion to a NiO-based p-DSC except two catalytic reactions which take place in a DS-PEC. Instead of using redox mediators like in DSCs, hydrogen evolving catalysts coupled to the dye sensitizers are employed to act as electron acceptors in DS-PEC to restore the reduced forms of dye molecules to their ground states. Protons are subsequently reduced to generate hydrogen by the H2-evolving catalysts. The photogenerated holes migrate through an external circuit to the counter electrode, where water molecules are oxidized to generate oxygen and protons. 18,[START_REF]TfOas a light brown powder in 93% yield[END_REF]437,439,456,473,481 According to the Nernst equation, the free energy change for the conversion of one molecule of H2O to H2 and 1/2 O2 under standard conditions is ΔG = 237.2 kJ/mol, which corresponds to the fact that the reduction potential for both (H + /H2) and (O2/H2O) redox couples (E 0 ) is 1.23 V for one electron transfer. The photoelectrochemical process should generate two electron-hole pairs per molecule of H2 or four electron-hole pairs per molecule of O2. As discussed above, NiO semiconductor can drive the hydrogen evolution reaction and oxygen evolution reaction because its band gap energy is large enough to split water as well as its conduction band-edge energy and valence band-edge energy straddle the electrochemical potentials E 0 (H + /H2) and E 0 (O2/H2O). 437,442,[443][444][445][446]450,452,491

Ⅰ.5. Research Objectives

DSCs which is an innovative, modern and growing technology, has a number of attractive features. It is simple to manufacture using conventional coating techniques and most of the raw materials employed are low-cost. The semi-transparency and colour of the device can be controlled for aesthetic integration into a variety of surfaces including flexible substrates, which is not applicable to Si-based systems. Moreover, DSCs can work efficiently under low-light conditions, even during overcast periods, and exhibit no angular dependence.

Although its solar-to-electric power conversion efficiency is still lower than the best Si-based cells, in theory its price/performance ratio should be good enough to allow DSC to compete with fossil fuel electrical generation by achieving grid parity. 7,9,10,185,187 To achieve efficient light-harvesting, the DSC device therefore relies on pigment molecules that play the role of

Ⅱ.1. General Introduction

As discussed in the first chapter, n-type DSCs as sub-species of DSCs have attracted worldwide attention in the field of photovoltaics since the seminal paper reported by B.

O'Regan and M. Grätzel 1 due to cost-effective and easy manufacturing process. The extensive fascination n-type DSCs arouse primarily stems from the tunability of the main components of the solar cell device that stimulates huge emulation in various areas of research. In particular, advanced molecular engineering afforded finely tailored organometallic and fully-organic dyes that raised the current record power conversion efficiency above 13%. 2,3 To achieve efficient light-harvesting, the solar cell device therefore relies on pigment molecules that play the role of visible-light absorbing antennae. In this important respect, ruthenium can operate as a connector allowing electron flow to occur between different elements in trans-ditopic architectures. [4][5][6] Up to now, ruthenium coordination complexes such as N3 or N719 remain to be the most efficient chromophores which therefore represent one of the main families of dye sensitizers. However, ruthenium complexes usually have modest molar extinction coefficient as well as exclusively polypyridyl derivatives which finally limit the development of their applications for DSCs. [7][START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF][9] In order to overcome the drawback of moderate light absorption, organic dye sensitizers or so-called push-pull materials bearing electron-donating (D) and electron-withdrawing (A) fragments linked by π-conjugated bridges have been designed and reported for DSCs. [10][11][12][13] The organic dye sensitizers featuring a D-π-A architecture notoriously afford efficient charge transfer from their ground state to the excited state which results in the enhancement of their absorption properties. Furthermore, the prototypical push-pull design can allow reduction of the electronic recombination processes in the solar cell devices. The reason may be that the photons with short wavelengths will create electrons in the TiO2 which are closer to the back contact and thus less prone to recombination during transport. 14 Since the cutting-edge dye sensitizers play an important role in the future of DSCs, this thesis focuses on the design and synthesis of innovative ruthenium-diacetylide organometallic dye sensitizers to obtain efficient DSCs and DSPEC.

In this chapter, various dominant n-type dye sensitizers are reviewed, including ruthenium complexes, porphyrins, phthalocyanines and metal-free organic dyes. Then, our work dealing with the design and synthesis of innovative ruthenium-diacetylide organometallic dye sensitizers is described in section Ⅱ.3.. Optical and electrochemical properties, theoretical calculations as well as photovoltaic properties based on these novel dyes are then discussed.

Ⅱ.2. n-Type Dye Sensitizers

From the working principles of DSCs referred in the previous section, the dye-sensitizer is crucial for DSCs by playing an important role in the photovoltaic properties of the solar cell device. The reason for the importance of dye-sensitizers is because their photophysical and electrochemical properties can firsthand determine and influence the related factors of short-circuit current density such as LHE(λ), ηED, Φinj, ηreg and so on. 15 Consequently, rational design and optimization of the dyes have been the key factors of this field and opportunity-filled challenges. Theoretically, efficient photosensitizers should fulfill some essential characteristics 7,[16][17][18] :

i. The dye-sensitizers should have high molar extinction coefficients and their absorption spectra should have wide absorption spectrum which cover visible region and even the part of the near-infrared.

ii. The photosensitizers should have at least one anchoring group (-COOH, -H2PO3, -SO3H, -Si(OEt)3, etc.) to strongly bind the dye onto the semiconductor surface which make sure the photoinduced electron injection from the excited dye into the conduction band (CB) of the semiconductor.

iii. For n-type DSCs, the conduction band (CB) edge of semiconductor should be lower in energy than the excited level of photosensitizer to make sure that the efficient electron transfer process can take place between the conduction band (CB) of the semiconductor and the excited dye.

iv. The oxidized state level of the photosensitizer should be lower (more positive) than the redox potential of the electrolyte to ensure the process of dye regeneration.

v. Dye aggregation is unfavorable process which should be prevented through the molecular structure optimization or addition of cosensitizer. Controlling of dye aggregation is to the benefit of good performance of the photosensitizer.

vi. The ground state, excited state and oxidized state should have photostable, electrochemical and thermal stability which can enable the dye repetitive work for 10 8

times (equivalent to be exposed under natural daylight for 20 years).

Based on the above-mentioned essential requirements, many different photosensitizers have been designed and applied to n-type DSCs in the past decades. These published photosensitizers have various types and numerous examples exist that can be broadly separated into metal complexes (mainly ruthenium complexes), porphyrins, phthalocyanines and metal-free organic dyes. The above-mentioned requirements are also our clues when we designed, synthesized and optimized our target products.

Ⅱ.2.1. Ruthenium Complexes

Metal complexes, especially ruthenium complexes, have been extensively investigated in recent decades. Metal complexes have excellent performances such as their broad absorption spectra and outstanding photovoltaic properties when used for dye sensitized solar cell devices. The heart of metal complexes comprises a central metal ion and auxiliary ligands having at least one anchoring group. Charge separation and electron transport from the central metal ion to its ligand determine the absorption spectrum of the photosensitizer and kinetic competition which means that the central metal ion is a crucial part of the overall properties. 7,16,[19][20][21] On the other hand, the auxiliary ligands (typically bipyridines, terpyridines, etc.) of metal complexes play an important role in photophysical and electrochemical properties through changing different substituents (alkyl, aryl, heterocycle, etc.). [22][START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF][24][25] Meanwhile, the function of anchoring groups of metal complexes is to link the photosensitizer and the semiconductor and make sure that excited electron injects into the conduction band (CB) of the semiconductor. Therefore, modification of any part of metal complexes can tune the excited level of photosensitizers, optimize the electron injection, charge separation and regeneration kinetics. [26][START_REF]TfOas a light brown powder in 93% yield[END_REF][28][29][30] So far, lots of photosensitizers based on metal complexes have been designed and synthesized including most used ruthenium complexes and other complexes where the central metal ions are osmium (Os), [31][32][33] platinum (Pt), [34][35][36] rhenium (Re), 37 copper (Cu), [START_REF]TfOas a red powder in 88% yield[END_REF]39 and iron (Fe). 40,41 Here, ruthenium complexes are mainly introduced, while other metal complexes will not be detailed. equipped with an ultraviolet filter. 50,51 The solar cell device based on nanocrystalline TiO2 films covered by a mixed self-assembled monolayer of the Z907 dye and 1-decylphosphonic acid (DPA) as a co-absorbent showed a sustainable power conversion efficiency of over 7%

for 1000 h aging at 80 ℃ and displayed an excellent photostability when submitted to accelerated testing under a solar simulator at 100 mW•cm -2 illumination. 52 Despite the long-term thermal device stability, the molar extinction coefficient of the amphiphilic polypyridyl ruthenium (Ⅱ) complexes such as the Z907 dye are somewhat lower than the analogous N719 dye. 53,54 One approach to enhance the molar extinction coefficient is to increase the conjugation length of the ligand, and thus to increase the light-harvesting the IJ-1 dye showed a high power conversion efficiency of 10.3% and an incident photon-to-electron conversion efficiency of 87%. 65 The thiophene-derived species were investigated to be good candidates for increasing the conjugation length of the ancillary ligand to improve the light-harvesting ability and red-shift the MLCT band of a ruthenium complex.

Furthermore, the thiophene-derived units can be easily functionalized with a alkyl-substituted hole-transport moiety, such as bis(heptyl)carbazole. 62,64 Two new ruthenium based dyes, which was coded as CYC-B6S (see CYC-B6L showed photovoltaic power conversion efficiencies of 9.72% and 8.98%, respectively. [START_REF]UV-visible absorbance spectra of the sensitized films were recorded on an Agilent Cary 60 UV-Vis spectrometer equipped with a solid sample holder. Film sensitization. NiO electrodes were soaked in a 0.5 mM solution of SL6 or SL7 in MeCN for 24 h on an orbital stirring table. The electrodes were rinsed with MeCN and dried in air. Film sensitization. NiO electrodes were soaked in a 0.5 mM solution of SL6 or SL7 in MeCN for 24 h on an orbital stirring table. The electrodes were rinsed with MeCN and dried in air. Photoelectrochemical measurements. Chronoamperometric and linear sweep voltammograms were recorded with a Biologic SP 300 potentiostat under nitrogen at room temperature using a previously described specific cell in a three-electrode configuration. 22 The NiO electrode is[END_REF] The conjugated electron-rich species such as alkyl thiophene, alkyl furan, alkyl selenophene and alkyl thieno [3,2-b]thiophene can be applied in the ancillary ligands of heteroleptic ruthenium dye sensitizers to further enhance the molar extinction coefficient. 9,[67][68][69] In order to increase the optical absorptivity of mesoporous TiO2 film and charge collection yield in DSCs, a new heteroleptic polypyridyl ruthenium complex, coded as C101 (see Organometallic complexes have been considered as promising materials for light absorption due to their higher electroconductibility and weaker π-conjugated bridge length dependence than their purely organic counterparts of comparable length. Furthermore, the metal complexes can act as redox switchable units because of their strong ligand-mediated electronic effects. 10,[72][73][74][75] In this context, the incorporation of the ruthenium metal fragment into the π-conjugated scaffold of push-pull chromophores have been investigated to be a promising method to improve the intramolecular charge transfer and enhance the performance of DSCs. 6,10,76,77 In

Ⅱ.2.2. Porphyrins and Phthalocyanines

The limited absorption in the near-infrared region of the solar spectrum is one of the short boards of the ruthenium complexes as photosensitizers, while porphyrins and phthalocyanines have intense spectral response in the near-infrared region and exhibit excellent thermal stability, optoelectronic and electrochemical properties, which provide great advantage for photovoltaic applications. 7,[86][87][88]

Ⅱ.2.2.1. Porphyrins

In the last decade, many extensive research efforts have been devoted to the design, synthesis and characterization of porphyrin sensitizers for DSCs. As a consequence, porphyrin sensitizers exhibit remarkable solar cell performances which can be comparable to conventional ruthenium sensitizers such as N3 and N719. 89,90 At the very beginning, the research of porphyrins as potential sensitizers for DSCs focused on their intense absorption bands in the visible region, versatile modifications of their core, and facile tuning of the electronic structures. However, the overall power conversion efficiencies of DSCs based on typical porphyrin sensitizers were much lower than those based on conventional ruthenium sensitizers because of the poor light-harvesting ability at around 500 nm and beyond 600 nm of porphyrin sensitizers. To tackle this drawback, the push-pull structure and/or the elongation of porphyrin π-conjugated system were introduced in follow-up studies to improve the light-harvesting property which finally resulted in highly efficient porphyrin DSCs. 89-95 Some representative porphyrin photosensitizers can be found in showed the highest power conversion efficiency of 4.8% than those based on the other dyes and a higher IPCE value of 75% at max. This study paved the way for improving the efficiency of nanocrystalline injection solar cells through employing suitable porphyrins with a small band gap and which absorb in the visible and near IR regions of the solar spectrum. 100

In most cases, carboxylic acids such as benzoic acid and cyanoacrylic acid are used as anchoring groups for attaching dye sensitizers onto the semiconductors. However, under severe conditions such as exposure to aqueous and alkaline electrolytes, the carboxylic acids maybe dissociate from the semiconductor surface which results in the detachment of adsorbed dyes from TiO2 and finally results in adverse effect on the durability of DSCs for photovoltaic applications. [START_REF] Kay | Artificial photosynthesis. 1. Photosensitization of titania solar cells with chlorophyll derivatives and related natural porphyrins[END_REF]

Ⅱ.2.2.2. Phthalocyanines

Phthalocyanines are planar 18 π-electron macroheterocycles that consist of four isoindole subunits linked together through nitrogen atoms. 106 Their specially conjugated system endow them with intense absorption spectra, presenting two major bands including the Q-band (620-700 nm) and the Soret B band (near 350 nm). Therefore, phthalocyanines, with high molar extinction coefficients in a wide region of electromagnetic spectrum, can be promising targets as photosensitizers for DSCs. Through the addition of a metal into the ring and axial and/or peripheral substitution with a variety of ligands, the physical and optical properties of phthalocyanines can be easily tailored. Moreover, phthalocyanines are thermally and chemically stable and exhibit appropriate redox properties for sensitization of TiO2 films, thus representing perfect light-harvesting systems for solar-energy-to-electricity conversion devices. 7,92,[105][106][107][108][109][110][111] 72% at λ=600-720 nm, which corresponded to the Q band of zinc phthalocyanines. When used as a light harvesting dye on a TiO2 electrode under standard solar conditions, the sterically isolated zinc phthalocyanine dye 41 showed a power conversion efficiency of 5.3%. 115 The conjugated spacers with an anchoring group can result in significant changes in the overall power conversion efficiency for phthalocyanine sensitizers. Particularly, ethynyl bridges have been investigated to be very effective for the connection of the phthalocyanine π-system with the carboxylic anchoring group. 92,105,107,108 In this context, M.-E. Ragoussi et al.

developed a new zinc phthalocyanine substituted with peripheral diphenylphenoxy groups and an ethynyl bridge between the anchoring carboxy group and the phthalocyanine macrocycle

(see Figure Ⅱ-4, dye 42
). The solar cell device based on the dye 42 showed a power conversion efficiency of 5.5% (Jsc = 12.3 mA•cm -2 , Voc = 638 mV, FF = 0.7) under one sun irradiation, while the increased power conversion efficiency of 6.1% was attained based on the dye 42 sensitized solar cell under 9.5 mW•cm -2 irradiation. 116 They further optimized the devices to improve the power conversion efficiency up to 6.01% under one sun irradiation.

The optimization was associated with the use of an optimized photoanode obtained, for instance, from a thicker TiO2 film and using overnight dye sensitization. 117 The change of the bridge between the anchoring carboxylic acid group and zinc phthalocyanine core plays a key role in the electron injection efficiency from the lowest unoccupied molecular orbital (LUMO)

of the dye into the conduction band of TiO2. [118][119][120] In this context, a series of zinc phthalocyanine sensitizers with different adsorption sites were designed and synthesized in order to investigate the dependence of adsorption site structures on the solar cell performances in zinc phthalocyanine based DSCs. Among these phthalocyanine sensitizers, the one It is worth mentioning that, despite the refined characteristics and advantages compared to other dyes, the solubility of phthalocyanine sensitizers is usually poor and needs to be further improved by structural optimization in order to enhance the sensitization process. On the other hand, phthalocyanine sensitizers suffer from strong aggregation on the semiconductor surface and lack of directionality in the excited state, which unfavourably limit the power conversion efficiency of DSCs. Therefore, the conversion efficiencies attained by phthalocyanine-sensitized solar cells have not yet been able to match the values obtained by porphyrin-based devices. 7,92,105,[109][110][111]

Ⅱ.2.3. Metal-Free Organic Dye Sensitizers

Despite the broad absorption spectra and favorable photovoltaic properties, the scarcity and high cost of metal complexes have motivated the synthesis of novel dyes, for example dyes using inexpensive metal complexes, such as zinc porphyrins and phthalocyanines or fully organic dyes, such as indoline-, coumarin-, perylene-, phenoxazine-, or triarylamine-based dyes. 7,[86][87][88]110,[123][124][125] Among these alternative dyes, fully organic dyes are good candidates for photosensitizers due to their distinctive features such as high extinction coefficients, low synthetic and purification cost, tunable absorption spectra, fluorescence emission and energy levels via molecular structure tailoring and essentially no limitation of resources. [124][125][126][127] Generally, metal-free organic dyes are designed on the D-π-A system, featuring an electron donor part and an electron acceptor part bridged by a π-conjugated spacer to ensure efficient charge transfer from the ground state to the excited state. For n-type organic dyes, the electrons are injected from the excited dye into the conduction band of the semiconductor via the electron acceptor part. Therefore, the frontier molecular orbitals energy levels of the dye should match the corresponding energy levels of conduction band edge of semiconductor and electrolyte redox potential to ensure efficient electron injection and dye regeneration. The HOMO level is relative to the donor group and π-spacer bridge and the LUMO level is relative to the acceptor group and can be easily modified to tune the properties of organic dyes for photovoltaic applications. 7,[124][125][126][127][128] Besides the D-π-A architecture, the typical D-A-π-A and D-D-π-A have also been employed to design efficient organic dye molecules. [129][130][131] Up to date, hundreds of n-type fully organic dyes have been designed, prepared and obtained impressive efficiencies. Herein, a few metal-free organic dye molecular scaffolds such as triarylamine, coumarin, squaraine, carbazole, etc. are briefly introduced.

Some representative metal-free organic photosensitizers can be found in ). These triarylamine-based photosensitizers bearing one to three naphthyl units in place of phenyl rings were designed on the D-π-A system in order to explore the effect of the insertion of naphthyl units instead of the phenyl groups. An additional cyanovinyl unit was introduced into the conjugated spacer in order to improve the absorption wavelength of the three dyes for better light harvesting. It was found that the introduction of naphthyl units into the triarylamine core induced a slight blue-shift of the absorption maximum and a significant enhancement of the molar extinction coefficients. When employed in DSCs, the three dyes showed excellent spectral response with IPCE greater than 85% from 470 to 580 nm and overall power conversion efficiencies of 6.6% for the dye 48 (Jsc = 14.0 mA•cm -2 , Voc = 627 mV, FF = 0.745), 6.2% for the dye 49 (Jsc = 13.9 mA•cm -2 , Voc = 618 mV, FF = 0.717) and 6.4% for the dye 50 (Jsc = 13.9 mA•cm -2 , Voc = 625 mV, FF = 0.732). 136 Squaraine dyes are also good candidates for photovoltaic applications due to their excellent molar absorption coefficients in green to near-IR region. Up to date, quite a number of squaraine dyes have been designed and adopted for DSCs. Some of the squaraine dyes even showed high IPCE and efficiencies comparable to ruthenium and other organic dyes. 7,123,143 For example, J. In addition to the metal-free organic groups referred above, many other metal-free organic photosensitizers such as indole/indoline dyes, 129,160-163 perylene dyes, [164][165][166][167][168] N,N-dialkylaniline dyes, [169][170][171] anthracene/heteroanthracene dyes, 172-174 boradiazaindacene dyes, 175-177 tetrahydroquinoline dyes, 178,179 polymeric dyes, 180,181 triphenodioxazine dyes, 182 etc.

have been designed, synthesized and employed in DSCs. Despite the impressive progress that has been made in the development of metal-free organic dyes, new molecular design strategies for the construction of highly efficient devices are still essential and a profound understanding of the morphology of fully organic photosensitizers is indeed necessary in order to control their photovoltaic properties. 7,14,[123][124][125]128,129 The flexible molecular engineering of these organometallic complexes make them good candidates to extend absorption spectra of the dyes towards low energies while keeping high molar extinction coefficient, with the view to increasing the DSSC efficiency in the near-IR region. In particular, a ruthenium-diacetylide organometallic complex endowed with a benzothiadiazole (BTD) connector was prepared accordingly and used in DSCs. 25 However, although the BTD unit extends the absorption spectra of organic dyes, it also hampers electron collection by causing rapid back electron transfer phenomena, i.e. charges injected into the conduction band of TiO2 return to the oxidized dye molecules, thereby lowering the overall charge collection rates, photocurrent densities and power conversion efficiencies. To overcome this issue, it was shown that inserting a π-conjugated spacer between the BTD unit and the anchoring group hinders the back electron transfer phenomena. 183,184 Several studies were reported where the D-A-π-A architecture out-performs the D-π-A design. 185,186 Therefore this prompted us to target new low bandgap sensitizers featuring an appropriate D-[M]-A-π-A'

Ⅱ.3. Molecular Engineering of Ruthenium-Diacetylide Organometallic

Complexes for n-Type DSCs

design where [M] is the [Ru(dppe)2] metal fragment, A the BTD unit, π an aromatic conjugated spacer and A' represents both secondary acceptor and anchoring function.

Therefore, we designed and synthesized three novel organometallic dyes for DSCs (see

Ⅱ.3.3. Theoretical Calculations

The optimized geometry of complexes Concomitantly, the spatial distribution of the HOMOs allows a good accessibility for the redox shuttle in order to regenerate the dye's ground-state.

Additionally, time-dependent density functional theory (TD-DFT) calculations provided information about the main transitions energy and related oscillator strength (see Figure Ⅱ-12).

The data are given in Table Ⅱ-

Ⅱ.3.4. Photovoltaic Properties

The

. General Introduction

As discussed in Chapter I, the p-DSC technology is similar by many aspects to its elder sister, the n-DSCs, but as the semiconductor is a hole transporting material the operation principle is the reverse of that occurring in classical DSCs, the photoactive electrode being the cathode. Photoexcitation of the dye promotes an electron from the HOMO level (ground-state) to the LUMO level (excited-state), thus prompting hole-carrier generation into the semiconductor valence band. The reduced sensitizer is afterwards regenerated by a redox mediator contained in the electrolyte (usually the iodine/iodide couple) and the reduced mediator is oxidized back at the counter-electrode. [1][2][3][4] Therefore, the choice of sensitizer is key to maximizing the power output of DSC devices. Ideally, with the view to develop tandem solar cells, the p-type sensitizers employed on the photocathode should absorb light in the red region of the visible spectrum, which will minimize spectral overlap with the higher energy absorbing dyes on the TiO2 photoanode. Particularly, high molar extinction coefficients are prerequisite in p-DSCs because it is difficult to prepare thick NiO films as compared with those of TiO2. Determined by the necessary stability of the device over a long period of time, the sensitizer should be also photochemically and electrochemically stable. More important, the reduced form of the sensitizer should remain inert toward electrophilic substances. Furthermore, high hole injection quantum yield is important for p-type dyes imposing that the reduction potential of the excited state of the sensitizer lies above the valence band edge of the semiconductor while the dye regeneration with the mediator should be thermodynamically allowed imposing that the reduction potential of the sensitizer is more negative than that of the mediator. Also, the sensitizers should be functionalized with anchoring groups in order to both graft the dyes on the semiconductor surface and provide a strong electronic coupling with the semiconductor. 3,5,6 Up to now the performance of the p-DSC technology is still far behind that of the TiO2 champion cells, with maximum efficiency limited to 2.51%. 7 For now, several classes of dyes were designed for p-DSCs, including push-pull organic sensitizers, [START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF][9][10] perylene imides, 7,[11][12][13] diketopyrrolopyrroles, 14 squaraines, 15,16 4,17,18 porphyrins, 19-21 ruthenium [22][START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF][24][25] , iridium 26 and rhenium [START_REF]TfOas a light brown powder in 93% yield[END_REF] coordination complexes. In this chapter, some representative p-type sensitizers are introduced in section Ⅲ.2.. Our work for the design and synthesis of novel donor-π-acceptor dyes based on organometallic ruthenium diacetylide complexes are introduced in sections Ⅲ. 3., Ⅲ.4. and Ⅲ.5. The optical and electrochemical properties, theoretical calculations as well as photovoltaic properties based on these novel dyes are also investigated and discussed.

Ⅲ.2. p-Type Dye Sensitizers

As referred in Chapter Ⅱ, porphyrins and their analogues are good candidates for photovoltaic applications due to their excellent photo-, electrochemical and thermal stability. [28][29][30][31][32] Porphyrinoids have unique absorption and photoredox properties which endow them an important role in photosynthesis. [32][33][34] which can be attributed to supramolecular interaction between ZnTCPP and C60PPy. This was the first example in which a supramolecular assembly between an electron donor ZnTCPP (dye 73) and an electron acceptor fullerene C60 (C60PPy) was employed in p-DSCs.

With Co 2+ / 3+ (dtbp)3 as the electrolyte, the ZnTCPP/C60PPy-based DSC devices showed a highest efficiency of 0.13% and a highest Voc of 260 mV. Several push-pull zinc porphyrin derivatives were prepared and investigated as sensitizers for NiO-based p-type DSCs by A. Maufroy et al. in 2015. 21 The first series of these zinc porphyrin derivatives consisted of trans disubstituted push-pull porphyrins containing a nitrophenyl unit as an electron withdrawing group and a benzoic acid unit as an anchoring group, incorporating or not an ethynyl spacer between the trans meso aryl groups and the porphyrin core (see Perylene imides are ideal class of high-performing dyes which exhibit excellent thermaland photo-stability. 39 The perylene imide sensitizers contain imide moieties as the electron withdrawing group which makes them easily reduced either chemically or electrochemically in order to form stable radicals. 40,41 Furthermore, the perylene imide sensitizers possess some other characteristics such as easy modification, broad absorption in the visible region and relatively high fluorescence quantum yields. Therefore, perylene imides are good candidates for p-type sensitizers and have been studied widely in last few years. 1,[40][41][START_REF]Ru-diacetylide/Cobaloxime Photocatalytic System for H 2 Evolution from Dye-Sensitized Photoelectrochemical Cells In the sections Ⅲ.3. and Ⅳ.3. we have demonstrated the relevance of using [Ru(dppe)2] organometallic complexes as photosensitizers in NiO-based photocathodes for p-type dye-sensitized solar cells and photo-electrochemical cells. 41,57 On the other hand, in a recent study, our collaborators in the group of Dr. Vincent Artero (CEA Grenoble), reported the preparation of an innovative mono-molecular photocatalyst through covalent linkage of an organic dye and a cobalt diimine-dioxime catalyst. 30 Therefore we designed and prepared a new mono-molecular photocatalytic system via axial coordination of a cobalt-based H2-evolving catalyst, i.e. a cobaloxime complex, to a pyridyl-functionalized Ru-diacetylide photosensitizer coded as SL8. Figure Ⅳ-11 shows the representation of the Ru-diacetylide/Cobaloxime photocatalytic system anchored onto NiO nanoparticulate thin-film. The choice of the cobalt-based catalyst was motivated by the fact that, among cobaloximes, [Co(dmgBF2)2(OH2)is known as[END_REF] Through the introduction of two electron acceptor groups and the anchoring group on the donor moiety, light excitation and electron flow from the donor part to the acceptor part provided an efficient electron transfer pathway from the dye to the electrolyte. The p-type DSC devices based on the dye 87 showed an APCE of 30%, an IPCE of 18% at λmax and a PCE of 0.05% (Jsc = 1.52 mA•cm -2 , Voc = 110 mV, FF = 0.31) at 100 mW•cm -2 under AM 1.5 irradiation. Few years later, after optimization of the NiO films (i.e. double layered photocathodes prepared in two steps) p-DSCs based on the same dye 87 afforded higher photocurrent density (Jsc = 5.48 mA•cm -2 ), higher APCE and IPCE maxima (69% and 64%, respectively) and therefore a better PCE of 0.15%. 43 [START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF] The four dyes employed a triphenylamine donating unit and a dihexylbithiophen π-bridge. The p-DSC devices sensitized by the dye 92 with an iodide-based electrolyte exhibited the highest PCE of 0.08% (Jsc = 1.66 mA•cm -2 , Voc = 163 mV, FF = 0.28), which can be attributed to significantly slower charge recombination process in the p-DSCs sensitized by the dye 92, due to the torsion angle between the acceptor and the donor.

Although employing the strong electron-accepting tricyanofurane group, the dye 91 based p-DSCs showed a lower efficiency of only 0.01% which can be attributed to a low-lying LUMO energy level, thus resulting in an insufficient driving force for efficient dye regeneration.

Scheme Ⅲ-1: Schematic representation of Ru or Ir polypyridine complexes for p-DSCs. 1 Transition metal complexes such as ruthenium and iridium polypyridine, exhibiting metal-to-ligand charge-transfer transition (MLCT), have been widely investigated as photosensitizers in n-DSCs and several good examples such as N3, Z910, CYC-B6S, CYC-B6L and others have been introduced in Chapter Ⅱ. Actually, some typical ruthenium complexes such as N3 could also be employed as p-type sensitizers because their MLCT excited states are both reductants and oxidants. 1,45 For most of the polypyridine transition 25 Both dyes exhibited high extinction coefficients because the special ligand system dominated the optoelectronic properties of the two dyes. The p-DSCs using dye 113 sensitized nanocrystalline NiO as working electrode and an iodine-based electrolyte showed a better efficiency of 0.09% (Jsc = 2.91 mA•cm -2 , Voc = 96 mV, FF = 0.32) and a higher IPCE of 14% at λmax compared to those using the dye 114.

However, with the cobalt electrolyte, both dyes exhibited lower efficiencies which can be ascribed to the fast rate of charge recombination reaction between the reduced dye and the holes in the NiO.

Ⅲ.3.

Push-Pull Ruthenium Diacetylide Complexes Including

Triphenylamine Donor Moieties as New Dyes for p-Type Dye-Sensitized

Solar Cells

As mentioned above, ruthenium polypyridine complexes are the archetypal dyes for solar energy conversion as they have been abundantly used for water splitting, 46,47 and as sensitizers in conventional n-DSCs 48,49 and more recently in p-DSCs. [22][START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF][24] However, ruthenium organometallic dyes are less investigated for the latter purposes. We showed before that Ru-diacetylide complexes are valuable visible-light sensitizers and that they can exhibit high photoconversion efficiencies (PCEs) in conventional n-DSCs. 50,51,52 Ruthenium diacetylide complexes are neutral molecules, they display easily tunable electronic properties and their rod-like structure enables to prepare push-pull systems. All these properties make them attractive candidates for p-DSCs, because neutral dyes can reach much higher packing density on the semiconductor than charged dyes and a strong push-pull character enhances the charge transfer band which is also favorable for charge injection. 53 All the organometallic complexes were characterized by means of 31 P, 1 H and 13 C NMR, HR-MS and FTIR. The full data set is in accordance with the expected structure of the organometallic dyes. The trans-ditopic geometry of the ruthenium center in SL1 and SL2 was evidenced by 31 P NMR as the spectra show a singlet for the four equivalent phosphorus atoms, with δ ≈ 53 ppm characteristic of the ruthenium-diacetylide structure. 54 In addition, an intense band was observed on the FT-IR spectra of SL1 and SL2 at ca. 2038 cm -1 corresponding to the νC≡C stretch of the σ-diacetylide metal fragment. shows λmax = 603 nm, hence affording blue-coloured dye-bath solution. Both dyes present a rather high molecular extinction coefficient (ɛ) of about 20 000 M -1 •cm -1 at λmax. Overall, the good absorption properties of this couple of organometallic complexes make them good candidates for use as photosensitizers in DSCs. Although such organometallic complexes are only weakly emissive, fluorescence spectra of SL1 and SL2 could be recorded in dichloromethane solution (see Figure Ⅲ-8). The maximum emission wavelength was detected at λem = 618 nm and λem = 700 nm for SL1 and SL2, respectively. The energy of the excited state (E00) was estimated from the wavelength at the intersection of the normalized absorption and emission spectra. As expected, the blue dye SL2 presents a lower lying excited state (E00 = 1.89 eV) than that of the red dye SL1 (E00 = 2.18 eV). Ⅲ-1. In the anodic region, the voltammograms show two reversible monoelectronic processes, both stemming from oxidation of the central electron-rich π-conjugated system with strong contribution of the [Ru(dppe)2] metal fragment. In the cathodic region the voltammograms feature a reversible monoelectronic reduction process mostly centered on the electron withdrawing group (acrylic ester or rodhanine). SL1 exhibits a more cathodic reduction potential compared to that of SL2 in agreement with the stronger withdrawing acceptor strength of rodhanine compared to acrylic ester. The calculated reduction potential of the dye at the excited state at +0.79 V and +0.86 V vs. NHE for SL1 and SL2, respectively, is more positive than the valence band edge of NiO (EVB ~+0.54 V vs. NHE). 57 This indicates that there is a small but sufficient driving force (about 0.25-0.3 eV) for hole injection from the dye excited state into the valence band of the semiconductor (see Table 1). On the other hand, the reduction potential of the dyes is strongly negative, suggesting that regeneration of the dyes by I3 -/I2 -(E°= -0.08 V vs. NHE) is a very exergonic process (|ΔG| > 0.9 eV) and should operate efficiently within the device. the second oxidation process is only partly reversible.

Ⅲ.3.2. Optical and Electrochemical Properties

Ⅲ.3.3. Theoretical Calculations

To gain further insights into the electronic properties of SL1 and SL2, the Theoretical calculations reveal that, for the two dyes, the HOMO is spread over the extended π-conjugated system from the triphenylamine to the bithiophene unit, with a strong contribution of the central C≡C-Ru-C≡C fragment. It also appears that, due to the presence of the two carboxylic-acid functions, the electron density at the ground-sate is only partially located on the triphenylamine donor motif. Nonetheless, upon grafting the dyes on NiO one could expect that the withdrawing effect of the COOH groups will decrease and the electron density will subsequently slide towards the surface. On the other hand, the LUMO of the dyes is well localized on the electron-withdrawing part of the complexes, i.e. on the methylester groups in SL1 and on the rhodanine motif in SL2. As a consequence, upon photoexcitation of the dye, an electron will be promoted from the metal center towards the remote part of the molecule, thus favoring electron capture by the electrolyte. Simultaneously, electron injection to the excited dye's HOMO will be possible by hopping from NiO to the triphenylamine in contact with the metal-oxide surface. Ⅲ-2. For both dyes the simulated absorption spectrum is in good agreement with the experimental one, showing two main absorption bands, one in the visible region and one around 360 nm. In accordance with the experience, the maximum wavelength calculated for the first allowed transition, λge, is located at 544 nm for SL1 and strongly red-shifted for SL2, up to 612 nm, due to increased electron withdrawing effect of rhodanine. Overall, the small deviation between the calculated and experimental data is attributable to the large size of such organometallic complexes. From the calculated data (Table Ⅲ-2) we could further assign the origin of the main absorption bands observed on the experimental spectra. As expected, the band in the visible region owns a major HOMO → LUMO character, but also some HOMO-1 → LUMO character which explains the broadness of the absorption in the visible region. As shown in Figure Ⅲ-10 the electronic distribution of the HOMO-1 is fairly similar to the HOMO and both orbitals are close in energy. Consequently, the observed visible absorption band owns a full MLCT character. This is confirmed by the calculated quantity of transferred charge, qCT, which is almost equal to 1. The band at higher energy also stems from multiple electronic transitions mainly from the HOMO and HOMO-1 to the LUMO+1 which is entirely located on the external phenyl rings of the triarylamine motif.

Ⅲ.3.4. Photovoltaic Measurements in p-Type DSCs

The photovoltaic performances of the dyes SL1 and SL2 were subsequently investigated in sandwich solar cells consisting of a 3 μm thick nanocrystalline layer of NiO, a platinized The best performing dye is the red SL1, which produces a higher short-circuit current density (Jsc) and a larger open-circuit voltage (Voc) than SL2 (see Table Ⅲ-3). The IPCE spectra show that SL2 produces photocurrent above 700 nm, with a moderate efficiency, while SL1 is more active but in a narrower bandwidth and at shorter wavelength. The IPCE is the factor of the light harvesting efficiency (LHE) by the injection quantum yield (ϕinj), the regeneration quantum yield (ϕreg) and the charge collection efficiency (ϕcollect) according to the following equation 27:

collect reg LHE IPCE        inj (27)
Looking at the HOMO in SL1 and SL2, we can infer that the electronic coupling of the dye excited state with the NiO valence band must be quite similar as these orbitals are roughly similarly distributed over the molecule (see Figure Ⅲ-10). Moreover, the injection driving force with SL2 is a bit larger (albeit quite close) than that of SL1, indicating that the injection quantum yield is certainly not the main factor controlling the IPCE. The regeneration reaction is most certainly very high as the driving force is huge in both cases (see Table Ⅲ -10).

Desorption experiments were made and they revealed that the dye loading is 31.9 nmol•cm -2 and 10.1 nmol•cm -2 for SL1 and SL2, respectively. These values are in the same range as other NiO dyes anchored with the same group N,N-di(4-benzoic acid)phenylamine (around 10-40 nmol•cm -2 ) [START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF]58,59 but much lower than the highly performing P1 dye (123 nmol•cm -2 ). 60 Changing the anchoring group can certainly improve the dye loading of these complexes and consequently increase their photoconversion efficiency. In addition, the dye loading of SL1 which is three fold higher than that of SL2, can explain the higher Jsc of SL1, which also most certainly comes from the higher absorbance of this dye around 500 nm, where the incoming solar flux in the largest. Finally, a lower dye loading means a higher degree of naked NiO surface exposed to the electrolyte promoting more interfacial charge recombination, thus lowering the charge collection efficiency. This is evidenced by the significantly lower Voc monitored for SL2 based devices.

Ⅲ.3.5. Conclusions

In this section we have described the synthesis and characterization of the first ruthenium diacetylide complexes for p-DSCs. While these dyes were not perfectly optimized yet, their photovoltaic performances were relatively high if compared with the reported ruthenium polypyridine complexes for the same application. [22][START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF][24]25 Interestingly, these two dyes displayed moderate injection driving force while it is accepted that the density of states of NiO valence band is lower than that of TiO2 conduction band, that is why the sensitizers for NiO must exhibit high driving force (>0.6 eV) to efficiently operate. 61,62 We therefore anticipated that much better performing ruthenium diacetylide dyes could be developed by lowering the energy position of the HOMO levels to enhance injection Gibbs free enthalpy.

Ⅲ.4. Push-Pull Ruthenium Diacetylide Complexes With Modified

Electron-Donating Group

As discussed above, we reported the synthesis and characterization of the first ruthenium diacetylide complexes for p-DSCs. 63 The optical and electrochemical properties of two new photosensitizers SL3 and SL4

were assessed and theoretical calculations were performed to rationalize the experimental data.

The dye SL1 was used as the reference for comparison in photovoltaic measurements in p-DSCs.

Ⅲ.4.1. Synthesis of the Dyes SL3 and SL4

As referred above, ruthenium diacetylide complexes SL3 and SL4 were prepared through already established procedures involving successive activation of terminal alkynes by the conventional electron-deficient metal center [RuCl(dppe)2] [TfO]. 54 First, the two alkyne ligands 33 and 37 corresponding to the electron-donating units were synthesized following synthetic routes shown on Scheme Ⅲ-4. To avoid side reactions of the carboxylic acid anchoring groups with the metal center during the following synthesis steps, protection of the COOH function was taken into consideration. As previously, the silyl-ester protecting group TMSE was employed for this purpose since it allows deprotection in mild conditions. 55 More importantly, for SL3, the less intense band centred around λ = 382 nm can be tentatively ascribed to electronic transitions involving the phenyl-containing ligand, while for SL4, the intense band centred around λ = 409 nm can be tentatively ascribed to electronic transitions involving the thiophene-containing electron-donor ligand. A similar feature of these dyes is the broad absorption band observed in the visible part of the spectrum. Similarly to SL1 and SL2, this intense absorption band corresponds to multiple MLCT processes, with main contribution of the photoinduced transition from the metal-centred HOMO to the LUMO located on the electron-withdrawing dimethylester moiety. The maximum absorption wavelength of SL3 (λmax = 514 nm) is similar to that of SL4 (λmax = 507 nm) due to the same electron-withdrawing group for both, which makes SL3 and SL4 red pigments. Both dyes present a rather high molecular extinction coefficient (ɛ) of about 20 000 M -1 cm -1 at λmax. Once again, the good absorption properties of this couple of organometallic complexes make them good candidates for use as photosensitizers in p-DSCs. Eox1 (V vs.

NHE) b

Eox2 (V vs. Due to the very weak emission properties of these organometallic complexes, the fluorescence spectra of SL3 and SL4 have not been measured. Still, the energy of the excited state (E00) was estimated from the onset of their absorption spectra, leading to E00 = 2.10 eV for the two new dyes.

NHE)

The electrochemical properties of SL3 and SL4 were investigated by cyclic voltammetry in CH2Cl2 solution (see Figure Ⅲ-15), the corresponding data are gathered in Table Ⅲ-4. In the anodic region, the voltammograms show two reversible monoelectronic processes. The first one is characteristic of the Ru Ⅱ →Ru Ⅲ oxidation process occurring in trans-diacetylide complexes. 63 The second wave can be attributed to the central π-conjugated system. In the cathodic region no clear reduction process was observable within the window allowed by the solvent.

As expected, the modified electron-donor groups of SL3 and SL4 led to a first oxidation process at higher potential compared to SL1: Eox1 = +0.68 V (vs. NHE), Eox1 = +0.71 V (vs. NHE) and Eox1 = +0.64 V (vs. NHE) for SL3, SL4 and SL1, respectively. This therefore corresponds to a lower position of the HOMO energy level, as expected.

Ⅲ.4.3. Theoretical Calculations

To obtain more insights into the electronic properties of SL3 and SL4, the Theoretical calculations revealed that, for the dye SL3, the HOMO includes the [Ru(dppe)2] metal fragment, however no electron density is again observed on the anchoring group. For the dye SL4, the HOMO is spread over the whole extended π-conjugated system, with a strong contribution of the central C≡C-Ru-C≡C fragment. Moreover, the calculated HOMO energy level of the two dyes (-5.27 eV for SL3 and -5.35 eV for SL4) is lower than that of SL1 (-5.19 eV), which also suggests more efficient hole injection from the dye excited state into the valence band of NiO for both dyes compared to SL1. Furthermore, considering these calculations, one can also expect a slightly larger injection driving force with SL4 than with SL3. Interestingly, the HOMO of SL4 presents more electronic density on the anchoring group which is favorable to charge transfer process from NiO. On the other hand, the LUMO of the dyes is well localized on the methylester groups of the complexes. As a consequence, upon photoexcitation of the dye, an electron will be promoted from the metal center towards the remote part of the molecule, thus favoring electron capture by the electrolyte.

Simultaneously, for the two dyes, electron injection to the excited dye's HOMO will be possible by transfer from NiO to the acetylide ligand in contact with the metal-oxide surface. Ⅲ.4.

Conclusions

In this section we have described the synthesis and characterization of two ruthenium diacetylide complexes based on the [Ru(dppe)2] moiety for p-DSCs which contain simple electron-donating groups. The introduction of either a benzene or thiophene as electron-donating units afforded lower HOMO energy levels compared to our former dye SL1, which was expected to enhance the charge injection rates. However, the efficiencies of the p-DSCs based on the two new dyes SL3 and SL4 are lower than those based on the reference dye SL1 which can be attributed to the lower dye loading amount, and thus lower photocurrent densities. We therefore anticipated that better performing ruthenium diacetylide dyes could be developed by changing the anchoring function, for instance by using a phosphonic acid group.

Ⅲ.5. Phosphonic Acid as a New Anchoring Function of Ru-Based Dyes for p-Type DSCs

As discussed above, we described the synthesis and characterization of two ruthenium diacetylide complexes based on the [Ru(dppe)2] moiety for p-DSCs which contain simple electron-donating groups. However, the main drawback for these dyes is the insufficient amount of dye grafted on NiO that limits their photovoltaic performances. Therefore, another The optical and electrochemical properties of SL5 were assessed and theoretical calculations were performed in order to rationalize the experimental data.

Ⅲ.5.1. Synthesis of the Dye SL5

The ruthenium diacetylide complex SL5 was prepared following the same procedures described above for SL1-SL4. The synthetic steps to the target organometallic complex SL5 are represented in Scheme Ⅲ-6. to n→π* and π→π* transitions from the dppe ligands. 55 The less intense band with maximum at λ = 424 nm for the dye SL5 can be ascribed to electronic transitions involving the polythiophene-based π-conjugated system. A broad absorption band is again observed in the visible part of the spectrum corresponding to MLCT processes, mainly stemming from the metal-centred HOMO to the LUMO located on the electron-withdrawing methylester group.

This band presents a maximum absorption wavelength at λmax = 490 nm with a rather high molecular extinction coefficient (ɛ) of over 34 000 M -1 cm -1 . The good absorption properties of the dye SL5 again make it a good candidate for use as photosensitizer in p-DSCs.

Ⅲ.5.3. Theoretical Calculations

Theoretical calculations were also performed to gain more insights into the electronic properties of SL5. The spatial representation of the frontier molecular orbitals is shown in

Ⅲ.5.4. Photovoltaic Measurements in p-Type DSCs

The photovoltaic performances of the dye SL5 were again investigated in sandwich solar cells, in collaboration with Fabrice Odobel, under the conditions described above but also using a new disulfide/thiolate redox couple (T2/T -) 64 (see Experimental part for details).

The photovoltaic parameters of the solar cells are gathered in Table Ⅲ-7 along with those of SL1, SL3 and SL4 for comparison. When the iodide/triiodide redox mediator is used, the dye SL5 shows enhanced photovoltaic performance compared to SL3 and SL4. The higher efficiency observed (PCE = 0.046%) is mainly due to a higher short-circuit photocurrent density (Jsc = 1.59 mA•cm -2 ) that can be related to the expected higher dye-loading amount. Here, it is worth mentioning that, although is was not possible to quantify the amount of dye grafted on NiO with SL5, the coloration of the NiO-based electrode observed after dye-bath treatment with the phosphonic-acid dye was significantly more intense than with SL3 and SL4.

Yet, the data obtained with SL5 are lower than those previously obtained with SL1 with the same electrolyte indicating that other phenomena might hamper the charge injection processes.

On the other hand, it is known that the triiodide/iodide redox couple suffers from two major disadvantages: 1) the electrolytes that contain triiodide/iodide corrode electrical contacts made of silver; 2) the triiodide absorbs visible light resulting in low light-harvesting efficiency. 64 Thus, the use of the disulfide/thiolate redox couple, that has negligible absorption in the visible spectral range and no corrosion properties, was examined. Moreover, this organic redox couple presents a higher standard potential compared of I3 -/I -(E°= 0.485 V 64 vs. E°= 0.35 V 65 ). This property should help to increase the photovoltage of p-type DSC devices.

Therefore, a novel electrolyte based on the disulfide/thiolate redox couple was employed with SL5 in order try to improve the photovoltaic performance of the dye.

Eventually, the p-DSC devices sensitized by SL5, combined with disulfide/thiolate-based redox mediator, showed a significantly higher open circuit voltage (Voc) of 149 mV and the best efficiency of the series, PCE = 0.125%.

Ⅲ.5.5. Conclusions

In this section we have described the synthesis and characterization of a new ruthenium diacetylide complex for p-DSCs. The new dye, featuring the [Ru(dppe)2] moiety, was endowed with a phosphonic acid anchoring group in the view of improving the density of dye on the photoelectrode. When using the standard iodide/triiodide redox mediator, the efficiency of p-DSC devices sensitized by SL5 showed better performance in comparison with those sensitized by SL3 and SL4 which can be attributed to the enhanced dye loading amount on NiO. Moreover, the p-DSC devices sensitized by SL5, combined with a disulfide/thiolate-based electrolyte, eventually showed higher open-circuit photovoltage and the best performance of the series, leading to PCE = 0.125%.

Ⅲ.6. General Conclusion

In this section we have described the synthesis and characterization of the first ruthenium diacetylide complexes for p-DSCs.

The first two complexes, SL1 and SL2, are endowed with a bidentate anchoring group based on a triphenylamine motif as electron-donating unit (D) and functionalized with an electron-accepting group (A) connected to the [Ru(dppe)2] metal fragment by a bithiophene linker. Thus simple modification of the alkynyl ligand on the acceptor side allowed easy fine-tuning of the optical properties of the complexes, leading to red and blue dyes featuring strong visible-light absorption properties. Spectroscopic and electrochemical studies of the two dyes further confirmed suitable optical and electronic properties (i.e. appropriate energy levels) for use in p-type DSC devices. Additionally, theoretical calculations were performed to rationalize the experimental data. The photovoltaic performance of these dyes were subsequently investigated in NiO-based p-type DSCs, in collaboration with Fabrice Odobel (University of Nantes). Interestingly, although these dyes were not perfectly optimized yet (i.e. moderate electron injection driving force) their photovoltaic performances were relatively high compared with other organometallic dyes reported for the same application (i.e. ruthenium polypyridine complexes). [22][START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF][24][25] In our study the best performing dye (the red dye SL1) afforded photocurrent density of 2.25 mA•cm -2 and maximum IPCE of 18 %.

These promising results paved the way towards other finely tuned organometallic efficient dyes for such application. We indeed anticipated that much better performing ruthenium diacetylide dyes could be developed by lowering the energy position of their HOMO level in order to enhance electron injection rate from the semiconductor.

Therefore, two novel Ru-based dyes were designed and prepared, in which the electron-donating unit is replaced by a simple phenyl (SL3) or thiophene (SL4) unit. The two dyes are functionalized with the same electron-withdrawing ligand as SL1, the dimethylmalonate group. As expected, the use of less electron-rich units as donors afforded lower HOMO energy levels compared to the dye SL1, which was expected to enhance the charge injection rates. However, the efficiencies of the p-DSCs based on the two new dyes SL3 and SL4 were lower than those based on SL1, which was attributed to lower dye loading amount with the two new dyes.

We therefore looked for better a performing Ru-diacetylide dye by replacing the anchoring function. The phosphonic acid group was chosen for this purpose, in order to improve the density of dye on the photoelectrode. The complex SL5, endowed with a phosphonic acid anchoring group on one side, was also functionalized by the dimethylmalonate electron-withdrawing group on the acceptor side. As expected, enhanced dye loading amount on NiO was observed with SL5, compared to SL3 and SL4. In p-DSC devices, using the standard iodide/triiodide redox mediator, the dye SL5 showed slightly enhanced photovoltaic performance compared to SL3 and SL4 but lower performance than those obtained with SL1. However, the dye SL5, combined with a disulfide/thiolate-based electrolyte, eventually showed higher open-circuit photovoltage and the best performance of the series, leading to PCE = 0.125%.

The good results obtained recently with SL5 and a disulfide/thiolate-based electrolyte gave us further impetus to design a new Ru-based complex with optimized structure in view of increasing both the dye-loading amount on the electrode and the charge injection rate in p-type DSC devices. The synthesis of this new dye is currently in progress in our group, in collaboration with the group of Fabrice Odobel.

Chapter Ⅳ: Organometallic Ruthenium Complexes for NiO Sensitization and Photoelectrochemical Applications

Ⅳ.1. General Introduction

The development of renewable energies is a major issue to be addressed in response to the problem of profuse and environmentally friendly energy for large-scale sustainable development. A realistic energy conversion method is the use of efficient catalytic systems that can convert the energy of the sun into storable chemical fuels by splitting water into oxygen and hydrogen. 1 In particular, hydrogen presents many advantages of being produced and used as a 'solar fuel', 2-4 e.g. it shows a high heat of combustion and can be used in fuel cells that produce a harmless combustion product, that is water. 5 Moreover, the use of hydrogen limits the use of fossil fuels and subsequent emissions of greenhouse gases. For all these reasons, a strong incentive remains for the development of new molecular synthetic systems able to convert the solar energy into chemical fuels. For this purpose efficient catalytic systems were reported for the photogeneration of hydrogen by molecular triads using dye/catalyst molecular assemblies in association with a sacrificial electron donor (SED) such as triethylamine (TEA) or triethanolamine (TEOA) in aqueous solutions. [6][7][START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF][9] Here it is worth mentioning that efficient molecular catalysts for light-driven H2 production are mostly based on cobalt complexes or other earth-abundant metals like nickel or iron. [10][11][12][13] In particular, among cobalt-based H2-evolving catalysts, cobaloximes are candidates of choice for both efficiency and stability reasons. [6][7][START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF][9]14,15 A more recent approach to convert sunlight into chemical fuel consists in building photoelectrocatalytic systems where the SED is replaced by a wide band gap semiconductor. 16 For instance, the use of NiO-based photocathodes to construct dye-sensitized photoelectrochemical cells (DSPECs) sparks growing interest in the research field of solar fuel production. Thus, several examples of molecular photocatalytic H2-evolving systems were reported for use in combination with NiO. Different strategies were developed such as a bimolecular dye/catalyst system where the dye is grafted on NiO and combined with a non-coordinated catalyst in solution, 17 co-grafted bimolecular dye/catalyst systems [18][19][20] or layer-by-layer assembly of a dye/catalyst system on NiO electrode. 21 Besides, a mono-molecular photocatalytic system was described involving axial coordination of the cobalt catalyst by the dye. 22 In this chapter, several representative dyes utilized in NiO-based DSPECs are reviewed.

Then, our work dealing with the design and synthesis of innovative Ru-based alkynyl-allenylidene organometallic dyes and a pyridyl-functionalized Ru-diacetylide photosensitizer are described in section Ⅳ. 3. and Ⅳ.4., respectively. Optical and electrochemical properties, theoretical calculations as well as photoelectrochemical properties of the organic-inorganic hybrid systems based on these novel dyes are then discussed.

Ⅳ.2. Sensitizers Utilized in NiO-Based DSPECs for Solar Hydrogen Production

Photocathodes are the key components of DSPECs, the fabrication of which is feasible by dye-sensitization of wide-band-gap p-type semiconductor oxides to drive proton reduction reactions in water splitting cells. [START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF]24 Apart from the mentioned properties in section Ⅲ.1. that a dye in p-DSCs should possess, the dye should also fulfil some general requirements to make functional dye-sensitized photocathodes in DSPEC devices. On one hand, the HOMO level of the dye should be more positive than the valence band position of the p-type semiconductor to facilitate efficient hole transfer after photoexcitation. On the other hand, the LUMO level of the dye should be more negative than the operation potential of the catalyst. In addition, in DSPEC devices the dyes should be strongly attached on the electrode, making it more difficult for the aqueous or the acidic media to wash the dyes off the semiconductor surface. 1,10,16,[START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF]24 There are three possible configuration types which are generally employed to combine a dye and a catalyst in DSPECs, as shown in Figure Ⅳ-2. [START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF] In the first configuration type, the catalyst is either dissolved in the reaction medium, or deposited onto the already chemisorbed dye on the surface of NiO. 17,18,[25][26][START_REF]TfOas a light brown powder in 93% yield[END_REF] The second approach is to co-adsorb both the dye and catalyst onto the surface of NiO. 28,29 The catalyst in this simple configuration type should be functionalized with an anchoring group. In the last strategy, the catalyst is covalently linked to the dye, [30][31][32][33] which is our approach to design the dye SL7 for DSPECs (see the section Ⅳ.4.).

Despite its more challenging synthetic routes, the last configuration type usually provides higher overall efficiency due to the slower charge recombination reaction, the greater quantum yield for electron transfer (from the reduced dye to the catalyst) and the higher molecular organization on the surface of the NiO. 23 -3, dyes 118, 122 and 123). 26 They found that increasing the number of anchoring groups from two to four was beneficial for the grafting efficiency. In addition, the transient absorption study revealed that the presence of two charge recombination pathways for each of the sensitizers and evidenced a stabilized charge separated state in the dipyridophenazine derivative, supporting its superior photoelectrochemical activity. Overall, the design of photocathodes for DSPECs has derived from the dyes used in p-type DSCs and is still in its early stage. The conditions for the efficient conversion of solar energy for hydrogen production from water splitting still need to be optimized. The enhancement of stability of the photosensitizers adsorbed onto NiO semiconductor surface for light-driven H2 production when immersed in aqueous and acidic medium still remains full of challenges.

Ⅳ.3. Organometallic Dyes for NiO Sensitization and Photoelectrochemical

Applications

In analogy to p-type DSCs, [START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF]24,34,35 NiO-based photocathodes have been obtained by sensitization of nanostructured thin films with organic or metallo-organic dyes, thus displaying photoelectrochemical activity in aqueous media. [17][18][19]22,[36][37][START_REF]TfOas a red powder in 88% yield[END_REF] In such photocathodes the dye plays a crucial in the sunlight harvesting and the inception of electron-transfer processes. In this section, we focus on the design and preparation of innovative dye structures with suitable redox properties for this purpose. The dyes obtained were further implemented in NiO-based photocathodes and tested as photocurrent generators under pertinent aqueous conditions in association with an irreversible electron acceptor (IEA). which showed a singlet for the four equivalent phosphorus atoms, with δ ≈ 43 ppm being characteristic of the mixed Ru-alkynyl-allenylidene structure. [START_REF]Ru-diacetylide/Cobaloxime Photocatalytic System for H 2 Evolution from Dye-Sensitized Photoelectrochemical Cells In the sections Ⅲ.3. and Ⅳ.3. we have demonstrated the relevance of using [Ru(dppe)2] organometallic complexes as photosensitizers in NiO-based photocathodes for p-type dye-sensitized solar cells and photo-electrochemical cells. 41,57 On the other hand, in a recent study, our collaborators in the group of Dr. Vincent Artero (CEA Grenoble), reported the preparation of an innovative mono-molecular photocatalyst through covalent linkage of an organic dye and a cobalt diimine-dioxime catalyst. 30 Therefore we designed and prepared a new mono-molecular photocatalytic system via axial coordination of a cobalt-based H2-evolving catalyst, i.e. a cobaloxime complex, to a pyridyl-functionalized Ru-diacetylide photosensitizer coded as SL8. Figure Ⅳ-11 shows the representation of the Ru-diacetylide/Cobaloxime photocatalytic system anchored onto NiO nanoparticulate thin-film. The choice of the cobalt-based catalyst was motivated by the fact that, among cobaloximes, [Co(dmgBF2)2(OH2)is known as[END_REF]43 The presence of an allenylidene carbon-rich chain was also evidenced by characteristic 13

Ⅳ.3.2. Crystallographic Study

Good quality crystals were obtained by slow diffusion of pentane into a concentrated solution of complex 48 in CH2Cl2 solution. The crystal structure of 48 was thus resolved by X-ray diffraction analyses. Cyclic voltammetry analyses of the organometallic complexes were performed in dichloromethane solution; the corresponding data are reported in Table Ⅳ-1. The cationic allenylidene-acetylide complexes show a well-defined reversible mono-electronic wave located at -0.99 V and -0.89 V vs. Fc + /Fc for SL6 and SL7, respectively. This electronic process is ascribed to the reduction of the cumulenic ligand. 43 Assuming that these potentials are not significantly affected when shifting from dichloromethane to acetonitrile 48 and considering that E°(Fc+/Fc) = +0.53 V vs. NHE in CH3CN, 37,46 we could estimate the LUMO energy levels of the dyes to be -4.11 eV and -4.21 eV for SL6 and SL7, respectively.

The HOMO energy level was calculated accordingly by subtracting the optical bandgap energy from the LUMO energy, EHOMO = ELUMO + E0-0. The HOMO energy was therefore estimated to be ca. -5.7 eV for SL6 and SL7. As a consequence, the HOMO energy level of the dyes is lower than the edge of the valence band of NiO (EVB(NiO) = -5.0 eV), 21,49,50 indicating that sufficient driving force exists for hole injection from the photoexcited dyes to the semiconducting metal oxide. On the other hand, the electron promoted to the LUMO upon photoexcitation is at a sufficiently high energy to be transferred to an irreversible electron acceptor (IEA) such as [Co(NH3)5Cl]Cl2 (E°Co(Ⅲ)/Co(Ⅱ) = -4.5 eV). 51 In that configuration, photoinduced electron transfers from NiO to the IEA, mediated by the mixed allenylidene-acetylide ruthenium complexes SL6 and SL7, should be highly favourable.

Ⅳ.3.4. Theoretical Calculations

Quantum chemical calculations were performed in order to obtain information about the molecular orbital distribution and the electronic transitions occurring upon photoexcitation of the dyes SL6 and SL7. These calculations were performed by Michele Pavone and Ana B. the expected small deviation between calculated and experimental data is attributable to the large size of such organometallic complexes and to the well-known limits of TD-DFT for charge-transfer excitations. 53 The maximum wavelength calculated for the lowest energy transition is 676 nm and 687 nm for SL6 and SL7, respectively, corresponding to pure charge transfer. We have also analysed the electron-density rearrangement upon excitation (see -8) and the computed average charge-transfer distance (DCT) was similar for both dyes, with a slightly longer extent for SL7 than that for SL6. Furthermore the calculated energy of the HOMO is in good agreement with that obtained experimentally, i.e. EHOMO calc. = Transition assignment reveals that this low-energy transition has a major HOMO → LUMO character whereas the transition at higher energy has a HOMO-1 → LUMO character.

These transitions therefore present a net MLCT (Metal-to-Ligand Charge Transfer) character since both HOMO and HOMO-1 strongly involve the central ruthenium-based fragment. This is further confirmed by the calculated quantity of transferred charge (qCT), which is very close to 1.

Ⅳ.3.5. Electrode Preparation and Characterization

This work was realized in collaboration with the group of Dr. Vincent Artero from CEA Grenoble. The NiO substrates were purchased from Dyenamo 38 and sensitized by soaking in a 0.5 mM CH3CN solution of SL6 or SL7 for 24 hours. The samples were rinsed with CH3CN and dried in air before characterization. optical pathway 1 cm) solution spectra of SL7 (dashed red line).

Assuming that the dyes retain their electronic structure features upon adsorption on p-NiO. 54 and that the dye-NiO interactions do not significantly modify the molar absorbance coefficient Ɛmax of the dye, the surface concentrations of SL6 and SL7 were estimated using the following equation:

Ⅳ.4.1 Synthesis and Characterization

The synthetic route to the new photocatalytic system and the relevant precursors is The binding energy relative to the formation of SL8-Co according to equation ( 29) was investigated through theoretical calculations.

O H C SL O H SL 2 2 o 8 53 8      (29)
The calculated enthalpy of the reaction is ΔH = -1.14 eV and the calculated Gibbs free-energy of the formation of the system is ΔG = -0.14 eV. Both values indicate that the formation of SL8-Co through pyridyl-coordination of the cobaloxime 53 is strongly favoured.

TD-DFT calculation parameters relative to the main photoinduced electronic transitions are given in Table Ⅳ-5. The two main transition energies calculated for SL8 in THF are in good accordance with the experiment, the small deviation being attributable to the large size of the complex and to some limitations of TD-DFT for charge-transfer excitation. 53 The transition assignment reveals that the lowest-energy transition, corresponding to λcalc = 445 nm, owns a major HOMO→LUMO character, while the higher-energy transition, corresponding to λcalc = 332 nm, presents a HOMO-1→LUMO+1 character. The calculations give very similar results when the solvent considered is H2O. The main transition energies calculated for SL8-Co in H2O also match the experiment, the lowest-energy transition being predicted at λcalc = 455 nm. TD-DFT calculation were also pursued on SL8-CoH, the reaction intermediate for hydrogen evolution reaction (HER). 11,15 Interestingly the formation of the HER intermediate SL8-CoH does not affect the optical properties of the photocatalytic system. The two main transition energies are conserved with λcalc = 445 nm and 330 nm, as well as the nature of the frontier orbitals involved in the lowest-energy transition which presents a major HOMO→LUMO character.

Ⅳ.4.4. Electrode Preparation and Characterization -XPS Analyses

The photocatalytic system SL8-Co was then used to prepare a photocathode based on NiO nanoparticulate thin film designed for H2 evolution. The F108-templated NiO electrode was prepared as previously reported.

Ⅳ.4.5. Photoelectrochemical Properties

The photocathodes SL8-Co@NiO were subsequently used for photocurrent generation and photoelectrochemical H2 generation in aqueous solution (acetate buffer, pH 4.5). Linear

Ⅳ.4.6. Conclusions

In this section, we designed and prepared a new mono-molecular photocatalytic system via axial coordination of a cobalt-based H2-evolving catalyst on a pyridyl-functionalized

Ru-diacetylide photosensitizer. The photocatalytic system SL8-Co showed suitable optical and electrochemical properties for use in NiO-based DSPECs. Thus this photocatalytic system was subsequently used to prepare a photocathode based on NiO nanoparticulate thin-film designed for H2 evolution. Long-term electrolysis in acetate buffer (0.1 M, pH 4.5) and under 1 sun visible-light irradiation led to photo-electrochemical generation of H2 with a faradaic efficiency of ~27%. This efficiency is comparable to other reports in the literature 21,22,30 and thus fully validates our approach consisting in using organometallic ruthenium complexes as photoactive species to build efficient NiO-based DSPECs.

SL8-Co

SL8

Ⅳ.5. Outlook

As discussed above, the photocatalytic system SL8-Co, comprising a Ru-diacetylide sensitizer in combination with a cobaloxime complex as H2-evolving catalyst, showed a very interesting faradaic efficiency of about 27% when employed in DSPEC Thus, we further designed a new mono-molecular photocatalytic system via coordination of a platinum-based H2-evolving catalyst to a bipyridyl-functionalized Ru-diacetylide complex as photosensitizer. Odobel (University of Nantes). Interestingly, although these two dyes were not perfectly optimized (i.e. moderate electron injection driving force) their photovoltaic performances were relatively high compared with other organometallic dyes reported for the same application (i.e. ruthenium polypyridine complexes). In our study the best performing dye (the red dye SL1) afforded photocurrent density of 2.25 mA•cm -2 and maximum IPCE of 18 %.

These promising results paved the way towards other finely tuned organometallic efficient dyes for such application. We indeed anticipated that better performing ruthenium diacetylide dyes could be developed by lowering the energy position of their HOMO level in order to enhance electron injection rate from the semiconductor.

Therefore, two novel Ru-based dyes were designed and prepared, in which the electron-donating unit was replaced by a simple phenyl (SL3) or thiophene (SL4) unit. The two dyes were functionalized with the same electron-withdrawing ligand as SL1, the dimethylmalonate group. As expected, the use of less electron-rich units as donors afforded lower HOMO energy levels compared to the dye SL1, which was expected to enhance the charge injection rates. However, the efficiencies of p-DSCs based on SL3 and SL4 were lower than those based on SL1, which was attributed to lower dye loading amount with the two new dyes.

We therefore looked for a better performing Ru-diacetylide dye by replacing the anchoring function. The phosphonic acid group was chosen for this purpose, in order to improve the density of dye on the photoelectrode. The complex SL5, endowed with a phosphonic acid anchoring group on one side, was also functionalized by the dimethylmalonate electron-withdrawing group on the acceptor side. As expected, enhanced dye loading amount on NiO was observed with SL5, compared to SL3 and SL4. In p-DSC devices, using the standard iodide/triiodide redox mediator, the dye SL5 showed slightly enhanced photovoltaic performance compared to SL3 and SL4 but lower performance than those obtained with SL1. However, the dye SL5, combined with a disulfide/thiolate-based electrolyte, eventually showed higher open-circuit photovoltage and the best performance of the series, leading to PCE = 0.125%.

On the basis of the good results obtained with SL5 and a disulfide/thiolate-based electrolyte, we eventually designed new Ru-based complexes with optimized structure in view of increasing both the dye-loading amount on the electrode and the charge injection rate in p-type DSC devices. The synthesis of new dyes is thus currently in progress in our group, in collaboration with the group of Dr. Fabrice Odobel.

Our third objective was to extend the scope of application of ruthenium-based organometallic sensitizers to NiO-based DSPEC for H2 evolution.

Thus, the Chapter IV describes the design, preparation and study of innovative organometallic chromophores featuring an original π-conjugated structure for binding onto a semi-conducting metal oxide in view of photochemical applications. In particular, we explored the use of mixed allenylidene-acetylide ruthenium complexes for the photosensitization of NiO and photocurrent generation in dye-sensitized photoelectrochemical devices.

The new dyes present a highly conjugated structure including the [Ru(dppe)2] metal fragment with a delocalized allenylidene ligand on one side and a σ-alkynyl ligand bearing an electron-rich group, i. e. thiophene or triphenylamine unit, and one or two anchoring functions, on the other side. The optoelectronic, electrochemical and photoelectrochemical properties of the two dyes were systematically investigated. A broad photoresponse was observed with absorption maximum at 600 nm. Furthermore, the X-ray crystal structure of one precursor was obtained and theoretical calculations were performed in order to address the photophysical properties of the new dyes. These photosensitizers were further implemented into NiO-based photocathodes and tested as photocurrent generators under pertinent aqueous conditions in association with [Co(NH3)5Cl]Cl2 as an irreversible electron acceptor. The dye-sensitized photocathodes provided good photocurrent densities (40-60 µA•cm -2 ) at neutral pH in phosphate buffer and a high stability was observed for both dyes.

Overall, this study evidenced the relevance of push-pull organometallic dyes for the construction of photocathodes operating in water.

Thus, following this work, we designed and prepared a new mono-molecular photocatalytic system via axial coordination of a cobalt-based H2-evolving catalyst (i.e. cobaloxime) on a pyridyl-functionalized Ru-diacetylide photosensitizer. The photocatalytic system SL8-Co showed suitable optical and electrochemical properties for use in NiO-based DSPECs. Thus photocathodes based on NiO nanoparticulate thin-film were prepared using our this [Ru-Co] photocatalytic system. Long-term electrolysis in acetate buffer (0.1 M, pH 4.5) and under 1 sun visible-light irradiation led to photoelectrochemical generation of H2 with a faradaic efficiency of ~27%. This efficiency, comparable to other reports in the literature, [1][2][3] fully validated our approach consisting in using organometallic ruthenium complexes as photoactive species to build efficient NiO-based DSPECs.

Finally, we recently designed a new mono-molecular photocatalytic system via coordination of a platinum-based H2-evolving catalyst to a bipyridyl-functionalized Ru-diacetylide complex as photosensitizer. The optical and electrochemical properties of this new photocatalytic system are currently under investigations, then the new photocatalytic system will be employed for the construction of NiO-based photocathodes and subsequent H2 generation in DSPECs.

Ⅵ.1. Materials and Methods

The reactions were carried out under inert atmosphere using Schlenk techniques.

Solvents were dried from appropriate drying agents (sodium for pentane, diethyl ether and THF; calcium hydride for dichloromethane, chloroform and methanol) and freshly distilled under nitrogen before use. All reagents were obtained from commercially available sources and used without further purification. One to two microliters solution of the compound were deposited on a 13µm emitter wire.

FT-IR spectra were recorded on a Perkin Elmer Spectrum 100 spectrometer using KBr pellets.

UV-visible absorption spectra were recorded on a UV-1650PC SHIMADZU spectrophotometer. Cyclic voltammetry analyses were performed using a potentiostat/ galvanostat Autolab PGSTAT100 and a three-electrode system (working electrode: Pt disc;

reference electrode: Ag/AgCl, calibrated with decamethylferrocene as internal reference;

counter electrode: Pt) with 0.1M Bu4NPF6 as salt support at a scan rate of 100 to 400 mV•s -1 .

Potentials were afterwards referred to NHE by addition of 130 mV. 5 DFT and TD-DFT calculations for [Ru]1-[Ru]3 and SL1-SL5 were performed by Dr.

Laurent Ducasse (ISM, Bordeaux) with Gaussian09. 6 B3LYP/LANL2DZ optimized geometries in vacuo conditions were used to perform TD-DFT calculations using the LANL2DZ basis set together with the MPW1K XC functional containing 42.8% of HF exchange, 7 while taking into account the solvent within the integral equation formalism of the polarizable continuum model (IEF-PCM). [START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF]9 DFT and TD-DFT calculations for SL6, SL7, SL8 and SL8-Co were performed by Dr.

Michele Pavone and Dr. Ana-Belen Muñoz-García (University of Naples) with the Gaussian09 suite of programs for quantum chemistry. 6 The PBE0 hybrid density functional 10 for ground state calculations was employed, including the semi-empirical dispersion term proposed by Grimme (D3BJ). 11 The standard Pople's 6-31G(d,p) basis set, 12 for H, C, N, O, P, and S atoms, and the SDD effective core potential and basis set for Ru 13 provided the best compromise between accuracy and computational feasibility. The polarizable continuum model (PCM) of solvation 14 has been applied to model the dichloromethane solvent. The Ru-based dyes under investigation have been purposely designed to undergo electronic excitation with long-range intra-molecular charge transfer from the ground to the excited states. This represents the worst-case scenario for the state-of-the-art time-dependent DFT (TD-DFT) methods. 15 Thus in Chapter Ⅳ several density functional models were tested for the TD-DFT calculations (see Table Ⅳ-3) and the long-range corrected LC-ωPBE density functional was chosen. 16 Analysis of electron density rearrangement upon vertical excitation has been performed according to the charge-transfer indexes developed by Ciofini and coworkers. 17 JH-H = 8.5 Hz), 0.10 (9H, s). 13 C NMR (300 MHz, CDCl3): 166. 5, 155.5, 152.9, 141.0, 134.4, 133.9, 130.9, 129.9, 129.3, 128.0, 115.6, 84.3, 79.4, 63.6, 17.5, -1.2 7.7 Hz),7.73 (m,8H),7.58 (d,1H,3 JH-H = 7.6 Hz), 20H),14H),8H),6.35 (d,1H,4.45 (m,2H),2.98 (m,4H), 2.67 (m, 4H), 1.18 (m, 2H), 0.11 (s, 9H). 13 C NMR (100 MHz, CD2Cl2): δ = 163.2, 157. 5, 155.4, 154.3, 153.8, 143.5, 141.4, 137.5, 137.1, 134.7, 134.3, 133.7, 132.8, 131.7, 131.3, 130.6, 130.0, 129.7, 129.6, 129.4, 129.3, 128.8, 128.6, 127.6, 127.3, 126.5, 126.1, 125.7, 124.4, 123.5, 120.5, 120.0, 118.1, 116.3, 110.2, 102.6, 65.4, 31.9, 17.6, -1.4. HR-MS FD + (m/z): 1594.3813

[M] + (calcd. 1594.3790 for [C95H80N4O2P4RuSSi] + ). FT-IR (KBr): νC≡N = 2220 cm -1 , νC≡C = 2032 cm -1 , νC=O = 1722 cm -1 , νP-Ph = 1092 cm -1 .

Synthesis of [Ru]2. To a solution of 14 (150 mg, 0.094 mmol, 1 equiv.) in dry THF (18 mL) and under inert atmosphere was added TBAF (1M sol. in THF, 0.14 mL, 1.5 equiv.). The JH-H = 7.9 Hz), 8.17 (d,4H,7.83 (m,8H),7.77 (d,1H,14H),19H),6.80 (m,8H),6.45 (d,1H,3 JH-H = 7.3 Hz), 3.09 (m, 4H), 2.81 (m, 4H). 13 C NMR (200 MHz, THF-d8): δ = 164. 9, 158.8, 156.0, 155.0, 153.4, 143.5, 142.5, 138.9, 138.3, 136.0, 135.8, 135.5, 135.0, 134.1, 132.5, 132.2, 131.3, 131.2, 130.6, 130.3, 130.2, 129.8, 128.6, 128.3, 127.5, 127.3, 127.0, 125.4, 124.8, 121.6, 121.4, 121.0, 119.5, 119.0, 117.8, 111.2, 33. ) in dry CHCl3 (50 mL) under nitrogen, were added piperidine (0.34 mL, 3.44 mmol, 4 equiv.) and dimethyl malonate (0.2 mL, 1.72 mmol, 2 equiv.). The solution was heated at reflux for 24 h and then evaporated to dryness. The crude product was purified on silica gel column (pentane/ethyl acetate (9 : 1, v/v)) to obtain 22 as an orange powder in 74% yield (0.26 g, 0.64 mmol). 1 H NMR (300 MHz, CD2Cl2): δ = 7.80 (s, 1H), 7.29 (d, 1H, 3 JH-H = 4.0 Hz), 7.16 (d, 1H, 3 JH-H = 4.0 Hz), 7.15 (s, 1H), 7.14 (s, 1H), 3.92 (s, 3H), 3.80 (s, 3H), 0.25 (s, 9H). 13 C NMR (75 MHz, CDCl3): δ = 166. 5, 164.8, 143.2, 137.4, 136.6, 135.7, 134.9, 133.6, 124.8, 124.4, 123.7, 120.8, 101.4, 96.9, 53.0, 52.8, -0.2.

Synthesis of 23.

Compound 22 (0.26 g, 0.64 mmol, 1 equiv.) was dissolved in dry THF (10 mL) under nitrogen atmosphere. Potassium carbonate (7 mg, 0.054 mmol, 0.1 equiv.) was subsequently added and the suspension was stirred at RT for 24 h. The reaction mixture was then concentrated, poured into pure water and the solution was extracted with CH2Cl2. After drying over MgSO4, the organic layer was evaporated to dryness to afford 23 in 69% yield (0.15 g, 0.44 mmol). 1 H NMR (300 MHz, CDCl3): δ = 7.82 (s, 1H), 7.26 (d, 1H, 3 JH-H = 4.0 Hz), 7.17 (d, 1H, 3 JH-H = 3.9 Hz), 7.12 (d, 1H, 3 JH-H = 4.0 Hz), 7.09 (d, 1H, 3 JH-H = 3.9 Hz),

3.94 (s, 3H), 3.83 (s, 3H), 3.45 (s, 1H). 13 C NMR (75 MHz, THF-d8): δ = 166. 4, 164.8, 143.0, 138.4, 137.3, 136.3, 135.2, 134.8, 125.7, 125.4, 123.4, 122.7, 85.1, 76.8, 52.6, 52.4.

Synthesis of 24.

In a Schlenk flask under nitrogen were sequentially introduced 5'-trimethylsilylethynyl-[2,2']bithiophene-5-carbaldehyde (21) (0.25 g, 0.86 mmol, 1 equiv.), 3-methyl rhodanine (0.14 g, 0.95 mmol, 1.1 equiv.), ammonium acetate (0.02 g, 0.26 mmol, 0.3 equiv.) and acetic acid (5 mL). The reaction mixture was left under stirring at 120 ℃ for 3 h. After cooling down to RT the reaction mixture was filtered and the solid was dissolved in CH2Cl2. The resulting solution was washed with pure water and then evaporated to dryness.

The crude product was purified on silica gel column (CH2Cl2/pentane (1 : 1, v/v)) to obtain 24 as an orange powder in 63% yield (0.23 g, 0.54 mmol). 1 H NMR (300 MHz, CDCl3): δ = 7.82 (s, 1H), 7.30 (d, 1H, 3 JH-H = 4.0 Hz), 7.20 (d, 1H, 3 JH-H = 4.0 Hz), 7.15 (s, 2H), 3.51 (s, 3H), 0.26 (s, 9H). 13 C NMR (75 MHz, CDCl3): δ = 192. 1, 167.4, 144.4, 137.1, 136.9, 135.2, 133.7, 125.4, 125.0, 124.1, 123.2, 120.9, 101.8, 96.8, 31.4 Synthesis of 36. To a solution of 35 (1.28 g, 4.18 mmol, 1 equiv.), PdCl2(PPh3)2 (73.3 mg, 0.1 mmol, 2.5%) and copper(I) iodide (39.8 mg, 0.2 mmol, 5%) in dry THF (50 mL) under nitrogen, were added Et3N (50 mL) and (trimethylsilyl)acetylene (0.65 mL, 4.6 mmol,

1.1 equiv.). The solution was stirred at RT for 12 h. After evaporation of the solvent, the resulting crude product was purified on silica gel column (CH2Cl2/pentane (8 : 1, v/v)) to obtain 36 as a pale yellow powder in 76% yield (1.03 g, 3.17 mmol). 1 H NMR (300 MHz, CD2Cl2): δ = 7.60 (d, 1H, 3 JH-H = 6.0 Hz), 7.17 (d, 1H, 3 JH-H = 6.0 Hz), 4.37 (m, 2H), 1.10 (m, 2H), 0.25 (s, 9H), 0.08 (s, 9H).

Synthesis of 37.

Compound 36 (1.36 g, 4.18 mmol, 1equiv.) was dissolved in dry MeOH (30 mL) under nitrogen atmosphere. Potassium carbonate (57.6 mg, 0.418 mmol, 0.1 equiv.)

was subsequently added and the suspension was stirred at RT for 24 h. The reaction mixture was then concentrated, poured into pure water and the solution was extracted with CH2Cl2.

After drying over MgSO4, the organic layer was evaporated to dryness to afford 37 as a dark red powder in 78% yield (0.82 g, 3. 26 mmol). 1 H NMR (300 MHz, CDCl3): δ = 7. 62 (d, 1H, 3 JH-H = 6.0 Hz), 7.20 (d, 1H, 3 JH-H = 6.0 Hz), 4.38 (m, 2H), 3.44 (s, 1H), 1.10 (m, 2H), 0.07 (s, 9H). 13 C NMR (75 MHz, CDCl3): δ = 161. 63, 135.18, 133.38, 132.79, 128.29, 83.68, 76.38, Synthesis of 44. To a solution of 43 (0.71 g, 2.07 mmol, 1 equiv.), palladium acetate (4.64 mg, 0.02 mmol, 1.0%) and dppf (12.6 mg, 0.023 mmol, 1.1%) in dry CH3CN (15 mL) under nitrogen, were added diethyl phosphite (0.32 mL, 2.48 mmol, 1.2 equiv.) and Et3N (0.37 mL, 2.69 mmol, 1.3 equiv.). The reaction mixture was refluxed for 24 h and then evaporated to dryness. The organics were further washed with degassed water, extracted with ethyl acetate. Subsequently, the solution was washed with brine for two times. After evaporation of the solvent, the resulting crude product was purified on silica gel column (ethyl acetate/hexane (4 : 1, v/v)) to obtain 44 as a yellow powder in 52% yield (0.60 g, 1.5 mmol). 60, 137.45, 133.96, 129.17, 125.38, 125.27, 125.16, 123.84, 101.34, 97.04, 63.19, 63.12, 16.53, 16.44, 0.20.

Synthesis of 45.

Compound 44 (0.14 g, 0.34 mmol, 1equiv.) was dissolved in dry MeOH (10 mL) under nitrogen atmosphere. Potassium carbonate (4.72 mg, 0.034 mmol, 0.1 equiv.)

was subsequently added and the suspension was stirred at RT for 24 h. The reaction mixture was then concentrated, poured into pure water and the solution was extracted with ethyl acetate. After drying over MgSO4, the organic layer was evaporated to dryness to afford 45 as a yellow powder in 89% yield (0.10 g, 0.30 mmol). 31 P NMR (120 MHz, CD2Cl2): δ = 10.2. 35, 144.24, 137.88, 137.46, 134.55, 129.36, 125.22, 122.54, 83.24, 76.60, 63.22, 63.15, 16.54, 16.45. [TfO]. In a Schlenk tube under inert atmosphere, [RuCl(dppe)2] [TfO] ( [26][TfO]) (1.082 g, 1.0 mmol, 1 equiv.) and 23 (0.40 g, 1.2 mmol, 1.2 equiv.) were dissolved in dry CH2Cl2 (25 mL). The mixture was stirred for 24 h at RT. After removal of the solvent, the crude product was washed with freshly distilled pentane (2 × 30 mL). (0.14 g, 0.43 mmol, 1.2 equiv.) and NaPF6 (0.12 g, 0.72 mmol, 2 equiv.) in dry CH2Cl2 (25 δ = 52.9. 1 H NMR (300 MHz, THF-d8): δ = 10.82 (s, 2H), 8.56 (m, 2H), 7.97-6.85 (m, 60H), 2.47 (m, 8H). 13 C NMR (100 MHz, THF-d8): 166.21, 165.98, 151.02, 150.29, 150.02, 142.08, 141.11, 137.17, 136.76, 134.42, 133.92, 131.93, 132.35, 131.20, 131.11, 131.03, 130.85, 130.70, 129.74, 128.67, 128.47, 128.30, 127.86, 126.98, 126.34, 125.82, 125.56, 124.97, 124.64, 124.56, 124.42, 124.19, 123.73, 123.61, 123.50, 122.00, 121.57, 29.74

Synthesis of [46]

Ⅵ.3.2. DSC Preparation and Characterization for SL1-SL5

For the p-type photovoltaic devices, conductive glass substrates (F-doped SnO2) were purchased from Solaronix (TEC15, sheet resistance 15 ohm/). The latter were successively cleaned by sonication in soapy water, then acidified ethanol for 10 min before being fired at 450 ℃ for 30 min. Once cooled down to room temperature, a dense layer of NiO was deposited on the FTO plates by spin coating a solution of nickel acetate (0.5 M) and triethanolamine (0.5 M) in methoxyethanol (2000 RPM, 30s) and firing at 500 ℃ for 30 min.

Mesoporous NiO was then screen printed on the latter substrates using a home-made paste.

Briefly, the NiO screen-printing paste was produced by preparing a slurry of 3 g of NiO nanopowder (Inframat) suspended in 10 mL of distilled ethanol and ball-milled (500 rpm) for 24 h. The resulting slurry was mixed in a round-bottom flask with 10 mL of 10 wt% ethanolic ethyl cellulose (Sigma Aldrich) solution and 20 mL terpineol, followed by slow ethanol removal by rotary evaporation. The dried film was calcined in air at 400 ℃ for 30 min. Once back at room temperature, the substrates were treated in a nickel acetate ethanolic solution (20 mM) with 1 mM triethanolamine at 60 ℃ for 30 min and subsequently fired at 200 ℃ for 30 min. The prepared NiO electrodes for the dyes SL1 to SL5 were finally soaked while still hot

Ⅵ.4. Determination of Dye-loading Amounts

A solution of dye (0.3 mM) and cheno-deoxycholic acid (1 mM) in dichloromethane was prepared and utilized to sensitize a nanoparticulate TiO2 thin-film (thickness = 6.8 µm ;

surface area = 1 cm -2 ) as described above. UV-visible absorption spectrum of the dye solution was recorded prior to and after sensitization. The amount of dye loaded onto TiO2 was deduced from the difference between the two sets of data.

Figure I :

 I Figure I: Structure moléculaire des complexes Ru-diacétylures [Ru]1-[Ru]3

Figure II :

 II Figure II: Courbes J/V de [Ru]1-[Ru]3 et N3 en DSSC.

  Figure Ⅱ et les paramètres photovoltaïques sont rassemblés dans le Tableau I. Le colorant de référence N3 (cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthénium a été testé dans les mêmes conditions pour comparaison. L'introduction d'un espaceur π-conjugué aromatique entre le BTD et le semi-conducteur, en empêchant la recombinaison de charges et le retour d'électrons de la surface, a permis d'améliorer les performances de [Ru]2 par rapport au sensibilisateur Ru-BTD rapporté précédemment. 2 Les colorants [Ru]2 et [Ru]3, associant BTD et fonction acide cyanoacrylique, ont donné des performances supérieures à celles du colorant [Ru]1, doté d'une fonction acide carboxylique simple, montrant ainsi que le choix du groupement d'ancrage est d'une grande importance pour optimiser le fonctionnement du dispositif. Dans l'ensemble, cette étude montre la pertinence d'utiliser des complexes organométalliques de type « push-pull » pour la conception de colorants à faible bandgap pour des dispositifs DSSC performants et une collecte efficace des photons de basse énergie.

Figure III :

 III Figure III: Structure moléculaire des colorants organométalliques SL1 et SL2.

Figure IV :

 IV Figure IV: Structure moléculaire des colorants organométalliques SL3 et SL4.

Figure V :

 V Figure V: Structure moléculaire du colorant organométallique SL5.

  photo-electrochemical cells) à base de NiO pour la production d'H2. Ainsi, le Chapitre IV décrit la conception, la préparation et l'étude de chromophores organométalliques innovants comportant une structure π-conjuguée originale et pouvant s'adsorber sur un oxyde métallique semi-conducteur en vue d'applications photochimiques.

Figure Ⅵ :

 Ⅵ Figure Ⅵ: Structure moléculaire des colorants organométalliques SL6 et SL7.

Figure VII :

 VII Figure VII: Photocourants cathodiques mesurés sous illumination intermittente avec des électrodes de NiO sensibilisées par SL6 (bleu) et SL7 (rouge) en présence de [Co(NH3)5Cl]Cl2 (20 mM) en tampon phosphate (0.1 M ; pH 7) à + 0.2 V vs. NHE.

Figure VIII :

 VIII Figure VIII: Représentation du système photocatalytique Ru-diacétylure/Cobaloxime SL8-Co chimisorbé sur un film mince nanoparticulaire de NiO.

Figure IX :

 IX Figure IX: Mesures chrono-ampérométriques sous illumination visible intermittente (400-800 nm, 65 mV•cm -2 , 1 sun) obtenues avec des électrodes de NiO sensibilisées par SL8-Co (rouge) et SL8 seul (orange) en tampon acétate (0.1 M, pH 4.5) avec un potentiel appliqué de -0.2 V vs. NHE (-0.4 V vs.

Figure X :

 X Figure X: Représentation du système photocatalytique Ru-diacétylure/PtCl2 chimisorbé sur un film mince nanoparticulaire de NiO.

Figure I- 1

 1 Figure I-1 lists the human development index (HDI) of more than 60 countries comprising over 90% of the global population and their per capita amount of electric power demand. 1

Figure I- 1 :

 1 Figure I-1: Human Development Index (HDI), a measure of basic human well-being used by the United Nations, reaches a plateau at about 4000 kilowatt hours of annual electricity use per capita.

Figure I- 2 :

 2 Figure I-2: The effective temperature, or black body temperature, of the Sun (5777 K) is the temperature a black body of the same size must have to yield the same total emissive power. (cited from Wikipedia)

Figure

  Figure I-3.

Figure I- 3 :

 3 Figure I-3: The path length in units of air mass, changes with the zenith angle.

  Figure I-4 gives these standard solar spectra.39

Figure I- 4 :Figure I- 5 :

 45 Figure I-4: Standard spectral irradiance spectra of AM 0 (black line), AM 1.5G (blue line) and AM 1.5D (red line). 39

  Figure I-6 52 , solar cells can fall into three generations on the basis of their development stages.

Figure I- 6 :Figure I- 7 :Figure I- 8 :

 678 Figure I-6: Classification of photovoltaic technologies with their current market share in percentage until now. 52

  Figure I-9: (left) Single-junction pin solar cell with a stainless-steel (ss) substrate. In a pin structure in the dark, excess electrons are donated from the n-type doped layer to the p-type layer, establishing the space charges and electric fields as shown. Under illumination, each photon absorbed in the undoped,

  Soonafterwards, a 6% GaAs p-n junction solar cell was first reported by RCA laboratories in New Jersey, USA.86 In 1970, Z. I.Alferov (2000 Nobel Prize in Physics) and co-workers at the Ioffe Institute developed a heteroface GaAlAs/GaAs solar cell which pointed the way to new device structures. 87 At the initial stage of study, liquid phase epitaxy (LPE) is used to manufacture the GaAs solar cells. However, the main issue for GaAs solar cells made by LPE is the high surface recombination velocity of GaAs material, which can greatly decrease the short circuit currents of GaAs solar cells. Until 1973, H. J. Hovel and J. M. Woodall put forward a way to add a thin, epitaxial layer of p-type Ga1-xAlxAs on the GaAs p-n junction surface which can solve the above problem (the high surface recombination velocity of GaAs material). 88 Recently, with the rapid increase of laminated cell efficiencies, the development of concentrating solar cell technology and the continuous improvement of installations, the terrestrial applications of GaAs solar cells have become into reality. 89 R. R. King and co-workers of multijunction solar cell team at Spectrolab in USA reported a metamorphic three-junction GaInP/GaInAs/Ge cell which had an efficiency of 40.7% under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm 2 , AM 1.5D, low aerosol optical depth, 25 °C) in late 2007. 90 In January 2009, Fraunhofer ISE in Germany announced to obtain the efficiency of 41.1% for triple-junctions cells based on Ⅲ-Ⅴ semiconductor materials. 91 Subsequently, in August 2009, Spectrolab in USA launched a press release announcing an efficiency of 41.6 % for a triple-junction concentrator solar cell which broke the record of 41.1% conversion efficiency. 92 In June 2010, the excellent efficiency was demonstrated with InGaP/GaAs/InGaAs 3-junction solar cell with 35.8% and 42.1% at 1-sun and 230-suns, respectively by Sharp. 93 Figure I-10 shows theoretical conversion efficiencies of single-junction and multi-junction solar cells in comparison with experimentally realized efficiencies.94 Finally, the record efficiency of about 46% at 234 suns was reached with InGaP/GaAs/InGaAs/InGaAs four-junction solar cell device structure.95

Figure I- 10 :

 10 Figure I-10: Theoretical conversion efficiencies of single-junction and multi-junction solar cells in comparison with experimentally realized efficiencies. (Bule: single-junction solar cells; Red: multi-junction solar cells) 94

  Bonnet and H. Rabenhorst more than 45 years ago.107 At the very beginning, a lot of early work on the CdTe/CdS solar cells -in which CdS as n-type partner for the junction formation with the p-type CdTe absorber -focused on the growth of the layers and the formation of the junction. However, with the post-deposition technology adapted to the as-deposited CdTe/CdS layer stacks, it has been obvious that the growth of the absorber and the formation of the junction were not of central importance. During the 1970s, the single crystals growth of Ⅱ-Ⅵ elements in halogen was reported by C. Paorici et al. which was crucial and promising research result for the CdTe/CdS stack treatment in halogen atmosphere. 108,109 In 1982, an efficient CdTe/CdS solar cell was developed in which CdTe/CdS films were prepared by a close-spaced-sublimation technique. The efficiency of this kind of CdTe/CdS solar cell was 10% which was considered as a threshold value for the large-scale development of thin film technology. 110 And thereafter, by using CdS films prepared by chemical bath deposition and p-CdTe films deposited by close-spaced-sublimation, a CdTe/CdS solar cell of greater than 1cm 2 area with an AM 1.5

Figure I- 11 :

 11 Figure I-11: Not in scale view of the sequence layers in a CdTe thin film solar cell highlighting the order of deposition: (a) superstrate configuration, (b) substrate configuration. 103

Figure I- 12 :

 12 Figure I-12: Basic structure of a typical CIGS solar cell, with examples of the most commonly used materials. 142The development history of CIGS solar cells can date back to the 1960s and 1970s, during that period the electrical, optical and structural properties of the semiconducting ternary chalcopyrites based on (Cu, Ag)(Al, Ga, In)(S, Se, Te)2 were firstly reported in the 1960s and 1970s.[147][148][149] The first CuInSe2/CdS p-n heterojunction photovoltaic detectors by

  Stolt et al. employed a quadrupole mass-spectrometer feedback system to make a CuInSe2 thin film by co-evaporating elemental sources and then on the CuInSe2 thin film covered a thin (10~20 nm) CdS buffer layer by a wet chemical dip process, which reached an active area efficiency of 14.8% with the cell structure n-ZnO/n-CdS/p-CuInSe2 deposited on a soda-lime glass substrate. The achievement described the highest efficiency single junction thin-film cell reported to date at that time because the CdS layer deposited by chemical bath deposition improved the p-n junctions which finally resulted in reduced recombination rates, hence higher open circuit voltage (490 mV) and fill factor (71%).163 In the same year, homogeneous bandgap (0.95 to 1.43 eV) and graded bandgap Cu(In, Ga)(Se, S)2 (CIGSS) multinary absorber layers were fabricated by D. Tarrant and J. Ermer using a two-stage method. The thin film solar cell devices based on the structure of soda-lime glass/ Mo electrode/ graded bandgap CIGSS absorber/ CdS (50 nm) buffer layer/ ZnO window layer/ MgF2 anti-reflection layer achieved the efficiency of 15.1% with an open circuit voltage of 728 mV. In this work, sulfur incorporation at grain boundaries or within the grains decreased the charge recombination and Ga incorporation forming an effective back surface field. The participation of sulfur and gallium, which both increased the band gap and improved the deposition,

(

  photovoltaics and concepts for solar electricity generation.

Figure I- 13 :

 13 Figure I-13: Building-integrated photovoltaic product concepts based on flexible, lightweight monolithic CIGS solar modules on polyimide, from top left to bottom left: residential roof tile, architectural standing seam roof, and metal façade cladding elements. Bottom right: fully rollable module for application on rooftop membrane. 142

7 Figure I- 14 :

 714 Figure I-14: Schematic diagram of the typical dye-sensitized solar cells. 7

1 ,

 1 DSCs can be divided into n-type DSCs, p-type DSCs and tandem DSCs according to their operational principles. The historical contexts, the specific operational principles, recent progresses and future outlooks based on n-type DSCs and p-type DSCs are discussed in sections I.3. and I.4., respectively.

  8% 230 in 2009 up to 22.1% 231 (the highest power conversion efficiency reported in a scientific article) in 2017, making them the fastest-advancing solar cells to date. The breakthrough solar cells fabricated based on perovskite materials are called perovskite solar cells (PSCs), which have revolutionized the third generation photovoltaic devices.

Figure I- 15 :

 15 Figure I-15: (a) Crystal structure of cubic metal halide perovskite with generic chemical formula ABX3 (A site: the large organic cations; B site: the small metal cations; and X site: halogen ions). (b)Organolead halide perovskite materials in PSCs.229

Figure I- 16 :

 16 Figure I-16: The multilayer arrangement of a typical mesoporous perovskite solar cell. 220

  photoelectrochemical cells.259 In 1972, A. Fujishima and K. Honda constructed an electrochemical cell to fulfill the hydrogen generation in which an electrode based on TiO2

  transfer processes and various potentials in a DSC device based on the N3 dye attached to the surface of the nanocrystalline TiO2 semiconductor and the iodide/triiodide (I -/I3 -) redox couple in the electrolyte are depicted in Figure I-17.

Figure I- 17 :

 17 Figure I-17: Basic electron transfer processes and potentials in a DSC device. The basic electron transfer processes are denoted by numbers 1 to 8 and the potentials indicated correspond to the iodide/triiodide (I -/I3 -) redox couple, the HOMO/LUMO of N3 dye and the TiO2 conduction band edge. Loss pathways are represented by red arrows. (Adapted from reference 7)

Figure

  injection occurs from the excited dye (S * ) to the conduction band (CB) of TiO2, leaving the dye in its oxidized state (S + ). (Figure I-17, Process 2) S * → S + + e -(CB) (4) iii. The oxidized state (S + ) is unfavourably regenerated by the conduction band electrons (e -(CB)). (Figure I-17, Process 3) S + + e -(CB) → S (5) iv. The adverse electrons transfer to I3 -from the conduction band (CB) of TiO2. conduction band electrons (e -(CB)) diffuse through the mesoporous TiO2 thin film to the transparent conducting oxide (bc: back contact), and then migrate to the external load. (Figure I-17, Process 5) e -(CB) → e -(bc) (7) vi. I3 -ions diffuse to the counter electrode at which electrons transfer to reduce I3 -ions to I -ions. (Figure I-17, Process 6) I3 -+ 2e -→ 3I - (8) vii. The oxidized dye (S + ) is regenerated and the I3 -ions form by oxidation of I -ions. adverse excited state (S * ) decay occurs resulting in the dye to the ground state (S). (Figure I-17, Process 8)

Figure I- 18

 18 Figure I-18 can be used to interpret and analyze the typical values for the interfacial energetic situation in DSCs. These values can be derived from the measurements of the

A

  schematic state diagram representing the kinetics of DSCs operating principle is illustrated in Figure I-18 273 which gives the sequence of electron transfer processes, while the more detailed kinetic data for the diverse electron transfer processes corresponding to the processes 1 to 8 (see Figure I-17) taking place at the TiO2/dye/electrolyte interface for the state-of-the-art DSCs are shown in Figure I-19 7 . Figure I-18 illustrates not only the forward processes of light absorption, electron injection, dye regeneration and charge transport but also the unfavorable loss pathways (processes 3, 4 and 8; see Figure I-17). As referred in I.3.2., the unfavorable loss pathways comprise decay of the sensitizer excited state to ground, and charge recombination of injected in the TiO2 with either oxidized dye or the redox couples in the electrolyte. The kinetic data in Figure I-19 have also been summarized in a number of

Figure I- 18 :

 18 Figure I-18: State diagram representation of the kinetics of DSCs function. Blue arrows represent the forward processes of light absorption, electron injection, dye regeneration and charge transport. Gray arrows represent the unfavorable loss pathways. The ordinate corresponds to the free energy stored in the charge separated states. The free energy of injected electrons depends on the Fermi level of the TiO2, assuming that the TiO2 Fermi level is 0.6 V above the chemical potential of the redox electrolyte. 273

Figure I- 19 :

 19 Figure I-19: Overview of processes and typical time constants under 1 sun simulated irradiation in DSCs based on Ru complex dye with iodide/triiodide electrolyte. Loss pathways are represented by red arrows. 7

Ⅰ. 3 . 6 . 1 .

 361 DSCs are exposed. The rapid strides in the development of DSCs performance now demand attention to be paid to architectural improvements on individual component. With supporting data, the recent improvements on different components of DSCs including photoanodes, electrolytes, counter electrodes and some parameters of DSCs are compared to provide ideas for further modification. These comparative studies can help to compile and apply a few of the discussed works and give suggestions to enhance the overall power conversion efficiency of DSCs. Furthermore, the progress of DSCs has stimulated research efforts for the development of feasible commercial products including this technology. Also, the life-time of DSCs is a key factor which holds back their developments. Therefore, some important research works referred in this section have been done in terms of enhancing the life-time of DSCs. The improvements on various sensitizers are introduced in Ⅱ.2., Ⅲ.2. and Ⅳ.2..Improvements in PhotoanodeAs indicated in I.2.3.5., the photoanode which supports the sensitizers, is involved in the electron transfer processes, and thus plays as an important role on the overall power conversion efficiency and is an indispensable component of DSCs. Most commonly, TiO2, ZnO and SnO2 are used to coat on TCO substrates such as FTO and ITO.186,268 Fundamentally, compared with the SnO2, the lower isoelectric point of TiO2 can restrain the adsorption of sensitizers with carboxyl groups.346 However, the DSCs based on SnO2 semiconductor films are prone to severe recombination losses. That the reason why SnO2-based core-shell heterostuctures have been investigated. Thus, solar cell devices based on SnO2/MgO films where SnO2 crystallites are covered with an ultra-thin shell of MgO, showed a maximum overall power conversion efficiency (PCE) of 7%.347

  photoanode was formed through screen-printing the ZnO powders and nanopowders onto the under-layer. The best results of DSCs based on such photoanodes were achieved 50 sparking cycles for ZnO powders and ZnO nanopowders over-layers with the efficiencies of 1.11% and 1.14%, respectively.368 The hybrid films such as carbon nanotubes/ZnO by blending multi walled carbon nanotubes or single wall carbon nanotubes with ZnO can be employed to act as photoanodes and enhance the efficiencies of DSCs.[369][370][371][372] Notably, few high efficiency ZnO-based DSCs have been reported because of Zn 2+ -dye aggregations at the interface between ZnO and the dye that result in blocking of the electron transfer processes.373 To solve this problem, L. Loh et al. reported a heterogeneous architecture of ZnO and bismuth ferrite prepared through using a technique which allows the synthesis using purely chemical solution techniques. The performance of DSCs can be improved by using bismuth ferrite with the ZnO

  photoanode, a Pt counter electrode deposited by electrochemical deposition method on ITO/polyethylene naphthalate of 5 mM H2PtCl6 aqueous solution showed an efficiency of 7.2%. 362 J. Burschka et al. proposed a PEDOT film deposited onto ITO-polyethylene terephthalate(PET) substrates as the counter electrode by electro polymerization proving that polymer PET films are suitable for counter electrode.387 In another publication, the deposition of platinum was reported to result in the lower charge transfer resistance and lower ohmic serial resistance. The charge transfer resistance and ohmic serial resistance were 0.175 Ωcm 2 and 3.16 Ωcm 2 for solar cell devices using electrospun carbon nanofibers (ECNs)/Pt as counter electrodes, 1.15 Ωcm 2 and 3.31 Ωcm 2 for Pt, 0.44 Ωcm 2 and 4.12 Ωcm 2 for ECNs.388

of 4 .

 4 94%. 396 NiCu bimetallic nanoparticle decorated graphene prepared by hydrothermal treatment can be used to act as an efficient and alternative Pt-free counter electrode for DSCs, as reported by M. Motlak et al.. In their study, the performance of the prepared modified graphene as counter electrode was strongly dependent on the composition of the metallic nanoparticles. Compared to the other introduced formulations(Ni-, Ni0.25Cu0.75 and Ni0.6Cu0.4)

Ⅰ. 3 . 6 . 3 .

 363 Joshi et al.. In their study, polyacrylonitrile (PAN) was employed to fabricate Ni(AcAc)2 nanofibers by electrospinning and a mixture of Ar and C2H4 was employed to form carbon nanotubes (CNTs) and carbon nanofibers (CNFs) which were expected to significantly increase the surface area for triiodide reduction. The solar device based on the Ni-CNTs-CNFs composite counter electrode yielded an efficiency of 7.96%. 400 X. Mei et al. reported a high performance dye-sensitized solar cell device based on a binder-free film of CNTs including both single walled and multi walled CNTs as the counter electrode. Compared to the solar cell device based on multi walled CNTs, the solar cell device based on single walled CNTs showed a slightly higher efficiency of 7.81%. 401 Improvements in Electrolyte As referred in I.2.3.5., the electrolyte is a ubiquitous and indispensable component in DSCs. The main function of the electrolyte is to inject electrons into the conduction band of the semiconductor to accomplish the dye regeneration and to transfer positive charges to the counter electrode. The properties of the electrolyte are responsible for the photovoltaic conversion efficiency, operational stability and long-term durability of DSCs. Therefore, the electrolyte should have (1) a high electrical conductivity and a low viscosity for electron diffusion processes, (2) an excellent interfacial contact with the semiconductor and the counter electrode, (3) a non-desorption characteristic in oxidization surface and the degradation of the dye sensitizer, and (

  et al. reviewed the liquid electrolytes in 2011. 295 According to their work the liquid electrolytes can be further classified into two types: (1) the organic solvent based electrolytes and (2) the ionic liquid based electrolytes. Although the conventionally used electrolyte in

  photoelectrochemical devices paves the way for the conversion of solar energy into both electricity and chemical energy.420 In contrast to conventional n-type DSCs, p-type DSCs operate in an inverse mode where the rapid electron transfer process from a p-type semiconductor to the dye sensitizer occurs after dye excitation.269 The photoelectrochemical devices share some structural similarities with the conventional n-type DSCs due to their mirror-image relationship. However, they use different materials and have different charge transfer kinetics. In p-type DSCs, the photoexcited dye sensitizer is reductively quenched by hole injection into the valence band of a p-type semiconductor. Therefore, the research on the p-type DSCs plays an important role in understanding both the key factors which control the rate of hole injection and the rational design of efficient p-type DSCs.[420][421][422] Moreover, improving the performance of p-type DSCs is a crucial part of the development of high performance tandem DSCs. The tandem DSCs which can be fabricated by pairing a typical

  Through a hydrothermal process, D.-B. Kuang et al. designed and prepared the hierarchical β-Ni(OH)2 microspheres with 6-8 μm diameter assembled from nanoplate/nanosheet building blocks in the presence of an anion surfactant poly(ethylene glycol) 4-nonylphenyl 3-sulfopropylether potassium salt. They also prepared NiO microspheres assembled from nanoplate/nanosheet through thermal decomposition of the as-synthesized β-Ni(OH)2 at 320 ℃ for 3 h in air. These novel hierarchical single crystalline Ni(OH)2 and NiO microsphere structures were all investigated to be suitable for p-DSCs. 465 Y. Y. Xi et al. in 2008 also reported their synthesis of NiO nanostructured thin films through both hydrothermal and electrodeposition methods at different deposition temperatures. For both fabrication methods, annealing was indispensable treatment for conversion of the films to NiO. 466In the photocatalytic or DSPEC systems for light driven water splitting, the semiconductor materials should meet certain requirements. Theoretically, the conduction band position of the suitable semiconductor should be more negative than the redox potential of H + /H2 which is ~-0.42 V vs. NHE at pH = 7. Simultaneously, the valence band of the suitable semiconductor should be more positive than the redox potential of O2/H2O which is ~0.81 V vs. NHE at pH = 7. As a result, the band gap of a suitable semiconductor must be minimum 1.23 eV. Moreover, if the overpotentials which drive the oxygen and hydrogen evolution reactions is taken into account, then the value of band gap should be larger than1.23

  to DSCs, DSPEC based on dye-sensitized n-type semiconductors or dye-sensitized p-type semiconductors are both possible. Compared to the p-DSCs, hydrogen evolving catalysts instead of redox mediators are employed as electron acceptors in p-DSPEC based on p-type semiconductors, such as NiO. Our work concerning the design, fabrication and study of p-type DSPEC based on NiO semiconductor and metallo-organic dye sensitizers will be described in Chapter IV. The dye sensitizers used in NiO-based DSPEC should fulfill some

  photocathode by photogenerated dye anions. In connection with a Pt wire anode, such dye-sensitized photocathode in Na2SO4 buffer solution (pH 7) with zero bias exhibited a

  photoelectrode.484 Although some of efficient catalytic systems reported for solar hydrogen production employed the dye sensitizer molecular assemblies containing a sacrificial electron donor such as triethylamine or triethanolamine in aqueous solutions,[485][486][487] the use of NiO-based photocathodes to fabricate DSPEC is sparking growing interest in the research field of solar fuel production. An excellent review reported by J.Willkomm et al. summarized strategies and challenges for the assembly of functional dye-sensitized photocatalysis systems and experimental techniques for their evaluation based on NiO-based photocathodes.488

  photogenerated charge carriers in NiO semiconductor, which is affected by charge recombination at the interfaces of both dye/NiO and electrolyte/NiO.445,490 On the basis of the above operational principles, the free energies of the hole injection (ΔGinj) can be determined by the equation as follows: ΔGinj = e[EVB(p-SC) -(E00(S * ) + E(S/S -))](23)

  photoelectrochemical properties of the new target dyes. The second objective of this part was to design and synthesize a mono-molecular photocatalytic system comprising a Ru-diacetylide photosensitizer and a cobalt-based H2-evolving catalyst, and to subsequently implement the new photocatalytic system in NiO-based photocathodes for H2 generation tests in DSPEC.

Figure Ⅱ- 1 :

 1 Figure Ⅱ-1: Some representative ruthenium complex photosensitizers.

  photovoltaic power conversion efficiency of 8.4%. Furthermore, the DSC devices displayed prominent stability when submitted to long-term high-temperature stress and light-soaking tests.57 On the other hand, S.-R.Jang et al. designed, synthesized and characterized two novel ruthenium sensitizers, Ru (4,4' -dicarboxyvinyl) -2,2' -bipyridine) ( 4,4' -dinonyl -2,2'bipyridine ) (NCS)2 (coded as K9, see Figure Ⅱ-1, dye 8) and Ru (4,4' -dicarboxy (phenylethenyl) -2,2' -bipyridine) (4,4' -dinonyl -2,2' -bipyridine) (NCS)2 (coded as K23, see Figure Ⅱ-1, dye 9), which contained hydrophobic alkyl chains and an extended π-conjugation bridge between carboxylic acid groups and the ruthenium chromophore center.Both K9 and K23 displayed enhanced red response and higher molar extinction coefficients compared to the Z907 dye. The nanocrystalline TiO2-based thin film solar cell devices based on the K9 and K23 dye sensitizers showed short-circuit photocurrent densities (Jsc) of 7.70 and 8.31 mA/cm 2 , and power conversion efficiencies of 4.14 and 4.41%, respectively. The investigated higher Jsc of the devices based on the K9 and K23 dye sensitizers compared to

Figure Ⅱ- 1 ,

 1 dye 11) and CYC-B6L (see Figure Ⅱ-1, dye 12), were reported by C.-Y. Chen et al. in 2008. In the structures of the two ruthenium based dyes, the alkyl-substituted carbazole moieties were incorporated in the thiophene-substituted bipyridine ligands. The absorption data of CYC-B6S and CYC-B6L were in parallel which indicated that the difference in the alkyl chain on carbazole was of no importance for the light-absorption ability of dye molecules. The solar cell devices based on CYC-B6S and

Figure Ⅱ- 1 ,

 1 dye 13), was reported by M. Grätzel et al.9 The solar cell devices based on the C101 dye and an acetonitrile-based electrolyte showed notably high efficiencies of 11.0-11.3% under AM 1.5 global sunlight. Significantly, when incorporating a low volatility 3-methoxypropionitrile electrolyte and a solvent-free ionic liquid electrolyte, the devices based on the C101 dye showed corresponding efficiencies of more than 9.0%, which indicated that over 95% of the initial performance after 1000 h full sunlight soaking at 60℃ was retained. Through introducing 2-thiophene-2-yl-vinyl into the ancillary ligand of the ruthenium complexes, S. Yanagida et al. designed and synthesized a novel hydrophobic ruthenium complex, which was coded as HRS-1 (see Figure Ⅱ-1, dye 14).70 The molar extinction coefficient of the lowest energy absorption band of the HRS-1 dye was increased by 33% and its absorption maximum wavelength was red-shifted of about 10 nm compared to that of the N719 dye. Under standard AM 1.5 sunlight, 100 mW•cm -2 irradiation, the solar cell device based on the HRS-1 dye showed an overall conversion efficiency of 9.5% (Jsc = 20 mA•cm -2 , Voc = 680 mV, FF = 0.69). To facilitate dye/redox couple interaction and dye regeneration in nanocrystalline TiO2-based DSCs, J. Faiz et al. introduced a cyclodextrin unit into the ancillary ligand of the ruthenium tri-bipyridyl core to obtain a supramolecular complex which was coded as 1-ɑ-CD (see Figure Ⅱ-1, dye 15). Incorporating a composite polymer electrolyte, the solar cell devices based on the 1-ɑ-CD dye showed a higher short-circuit photocurrent (Jsc = 4.21 mA•cm -2 ), open-circuit photovoltage (Voc = 590 mV), and overall conversion efficiency (1.64%) compared to those of solar cell devices based on the dye without the cyclodextrin unit (Jsc = 3.84 mA•cm -2 , Voc = 521 mV, Ƞ = 1.17%). The enhanced performances of the solar devices based on the 1-ɑ-CD dye can be attributed to guest-host interactions between the dye and the iodide/triiodide redox couple. This study indicated that the regeneration of the dye can be facilitated through binding of the iodide/triiodide redox couple into the CD cavity. 71

Figure Ⅱ- 2 :

 2 Figure Ⅱ-2: Several representative asymmetrical σ-dialkynyl ruthenium complexes containing original D-π-Ru-π-A architectures

  view of the advantage of the linear geometry of the trans-ditopic ruthenium-based unit such as[Ru(dppe)2] (dppe: bisdiphenylphosphinoethane) or[Ru(dppm)2](dppm: bisdiphenylphosphinomethane) and their π-conjugated characters, such organometallic complexes may present outstanding electronic and structural properties that lead to a wide range of applications.7,77 Several representative asymmetrical σ-dialkynyl ruthenium complexes containing original D-π-Ru-π-A architectures are shown in Figure Ⅱ-2.In 2011, A. Colombo et al. reported a novel dinuclear ruthenium(Ⅱ) complex where two Ru atoms were separated by a bridge consisting of a 2,1,3-benzothiadiazole acceptor moiety flanked on either side by 2,5-thienyl donor units. This rather simple rodlike diruthenium-acetylide complex was the first example which employed Ru-acetylide as a tool for the design of donor materials. The bulk heterojunction solar cell device based on this dye showed a power conversion efficiency of 0.1% upon blending with a methanofullerene derivative.78 This study paved the way for the employment of rodlike Ru-acetylide complexes as the dye sensitizers for DSCs. Despite the reported unsymmetrical diacetylide-transition-metal complexes, asymmetric diacetylide push-pull complexes containing strong donor and/or acceptor groups have barely been reported before 2014.[79][80][81][82][83][84] In 2014, the first example of chromophores for DSCs presenting a ruthenium-diacetylide moiety embodied within a push-pull architecture (see FigureⅡ-2, complex 16) was reported by our group. Such an innovative photosensitizer based on the D-π-[M]-π-A design concept in which[M] was the [Ru(dppe)2] metal fragment, appropriately end-capped with an anchoring carboxylic acid function, was further adsorbed onto a semiconducting TiO2 porous thin film to serve as a photosensitizer in hybrid solar cells. The solar cell devices based on the complex 16 along with chenodeoxycholic acid as a co-adsorbent showed a good spectral response with a broad IPCE and overall power conversion efficiency of 7.32% (Jsc = 15.56 mA•cm -2 , Voc = 680 mV, FF = 0.692).6 Due to the decent optoelectronic and photovoltaic properties, this asymmetric Ruthenium-diacetylide organometallic complex paved the way to a new generation of organometallic photosensitizers for photovoltaic applications. A series of dipolar π-delocalized Ru(Ⅱ) dialkynyl complexes were designed, synthesized and characterized by F.Nisic et al. in 2015 in order to get a better understanding of the 2,1,3-benzothiadiazole as acceptor moiety and to study new "Donor-phenylalkynyl-Ru-alkynylphenyl-Acceptor" architectures.85 In this study, the second-order nonlinear optical properties and the dipole moments of these complexes were investigated by the electric field induced second harmonic generation technique and density functional theory calculations. Among these complexes, two most promising push-pull ruthenium diacetylide complexes (see Figure Ⅱ-2, complex 17 and 18) bearing a carboxylic acid anchoring group were investigated as photosensitizers in DSCs.The solar cell devices based on the complex 17 and the appropriate electrolyte showed a best overall power conversion efficiency of 1.5% (Jsc = 4.6 mA•cm -2 , Voc = 578 mV, FF = 0.561) while the devices based on the complex 18 showed that of 0.3% (Jsc = 1.5 mA•cm -2 , Voc = 432 mV, FF = 0.3). This study indicated that D-π-[M]-π-A structured metal acetylide complexes were positively suitable for dye sensitizers in DSCs with the aim to obtain the right compromise between the best efficiency together with a satisfactory stability and the introduction of selected functional groups along the push-pull chain may play a key role in acquisition of good efficiencies in DSCs. Recently, R. J. Durand et al. reported a series of dipolar π-delocalized Ru(Ⅱ) dialkynyl chromophores (see FigureⅡ-2, complexes 19 to 26) based on the innovative asymmetrical D-π-[Ru]-π-A push-pull design concept which incorporated pyranylidene ligands as pro-aromatic electron-donating groups and formaldehyde, indane-1,3-dione, pyrimidine or pyrimidinium as electron-withdrawing groups separated by ruthenium bis-acetylide fragments and π-conjugated linkers.10 Through the investigation of their electrochemical, photophysical and second-order nonlinear optical properties compared to those of purely organic compounds, it can be concluded that the [Ru(dppe)2] metal fragment merged into the π-conjugated core of pyranylidene-N-methylated pyrimidine ring push-pull complexes resulted in the enhancement of the second-order nonlinear optical responses and the nonlinear optical properties of the easily modulated push-pull diacetylide ruthenium-based complexes in return resulted in versatility of such complexes. Complex 25 and 26 containing pyrimidinium species displayed significantly high μβ values which indicated that the excited states of the two complexes were more polarized than their ground states. Section Ⅱ.3. of this manuscript focuses on our work relative to new Ru-diacetylide organometallic complexes designed for n-type DSCs and their relevant optical and electrochemical properties, theoretical calculations as well as photovoltaic characterization.

  Figure Ⅱ-3.

Figure Ⅱ- 3 :

 3 Figure Ⅱ-3: Some representative porphyrin photosensitizers.

  In 2012, H. He et al. for the first time reported their work to employ 8-Hydroxylquinoline (HOQ) as a strong alternative anchoring group porphyrin dyes to improve the long-term stability of solar cells. Through the comparison of an 8-hydroxylquinoline modified porphyrin, TPPZn-HOQ, (see Figure Ⅱ-3, dye 31) and a benzoic acid modified porphyrin, TPPZn-COOH (see Figure Ⅱ-3, dye 32), it was found that HOQ was a more reliable anchoring group compared to the benzoic acid group for porphyrin dyes. Moreover, when employing a complementary dye boron dipyrromethene (BET), the overall power conversion efficiency can be significantly improved. 102 Two years later, they reported another three porphyrin dyes including DPZn-HOQ and DPZn-COOH (see Figure Ⅱ-3, dyes 33 and 34) for DSCs. Both DPZn-HOQ and DPZn-COOH exhibited a D-π-A configuration through employing N,N-dimethylaniline as a donor group and HOQ and para-benzoic acid (BZA) as acceptor groups. As a better photosensitizer, DPZn-HOQ showed a broader and stronger light absorption in the red region than DPZn-COOH. Under the same conditions, the solar cell device based on DPZn-HOQ exhibited a higher power conversion efficiency compared to that based on DPZn-COOH, which was 3.09% and 1.76%, respectively. When employing a complementary dye boron dipyrromethene (BET), the efficiency of DPZn-HOQ-sensitized solar cells was further improved up to 3.41%. 103 A possible strategy to achieve a better light-harvesting ability in the visible and near infrared (NIR) regions for porphyrin sensitizers is to develop π-extended porphyrins. For example, C.-Y. Lin et al. designed and prepared a series of acene-modified zinc porphyrins (benzene to pentacene, denoted as LAC-1 to LAC-5, see Figure Ⅱ-3, dye 35) and studied their absorption spectra, electrochemical properties, and photovoltaic properties. With respect to the performances of DSCs using LAC serial porphyrins, the solar cell devices based onLAC-1 to LAC-5 showed the overall power conversion efficiencies of 2.95%, 3.31%, 5.44%, 2.82% and 0.10%, respectively. The IPCE data indicated that the gap between B and Q bands of LAC-3 was minimized due to its broadened absorption bands, which resulted in the best performance of DSCs. The very poor performance of LAC-5 was deduced to be result from rapid non-radiative relaxation of the molecule in the singlet excited state.104 As referred above, porphyrin sensitizers have been research hotspot from their birth until now. Up to date, the record efficiency for solar-energy-to-electricity conversion based on porphyrin sensitizers associated with a cobalt-based redox mediator is 13% and was reported by S.Mathew et al. in 2014. In their study, a molecularly engineered porphyrin dye, coded as SM315 (see FigureⅡ-3, dye 36), which features the prototypical structure of a D-π-A was employed in order to maximize electrolyte compatibility and improve light-harvesting properties. 2 However, there are still remaining challenges for practical applications of porphyrin-based DSCs such as further improvement of the light-harvesting ability in the visible and NIR regions, suppression of porphyrin aggregation on TiO2 to reduce undesirable quenching of the excited singlet state, optimization of electron injection and inhibition of charge recombination processes at the interface by controlling the adsorption structures on TiO2 and so on.7,[86][87][88][89][90]

  Some phthalocyanine sensitizers used in DSCs are shown in Figure Ⅱ-4.

Figure Ⅱ- 4 :

 4 Figure Ⅱ-4: Some phthalocyanine sensitizers used in DSCs.

  possessing a single carboxylic acid directly attached to the zinc phthalocyanine ring and bearing six 2,6-diisopropylphenoxy units (see Figure Ⅱ-4, dye 43) showed a best power conversion efficiency of 5.9% when used as a light-harvesting dye on a TiO2 electrode under one sun condition. 121 T. Ikeuchi et al. reported an asymmetric zinc phthalocyanine sensitizer bearing propoxy groups in the 2 and 6 positions of peripheral phenoxy units with alkyl chain substituents which was coded as PcS20 (see Figure Ⅱ-4, dye 44). The 2,6-diisopropylphenoxy units were also used in order to prevent aggregation of the dye on TiO2 surface. Under simulated AM 1.5 global sunlight, the solar cell device based on PcS20 in contact with an electrolyte containing 0.6 M DMPImI, 0.1M LiI, 0.05 M I2 and 0.5 M tBP in acetonitrile led to a record power conversion efficiency of 6.4% (Jsc = 15.1 mA•cm -2 , Voc = 600 mV, FF = 0.71). The high efficiency can be attributed to high absorption properties and the high density adsorption of PcS20 on TiO2. 122

Figure

Figure Ⅱ- 5 :

 5 Figure Ⅱ-5: Some representative metal-free organic photosensitizers.

  photocurrent of 13.06 mA•cm -2 , and FF of 0.77 were obtained, resulting in PCE of 10%.138 The coumarin-based dye, coded as coumarin 343 (see Figure Ⅱ-5, dye 52), was employed to explore the charge injection dynamics from the surface-bound dye to the conduction band of TiO2 for the first time in 1996. The ultrafast fluorescence dynamics indicated that the charge injection from the surface-bound dye to the conduction band of TiO2 proceeded on a time scale of about 200 fs, and such an excellent charge injection efficiency can be attributed to strong electronic coupling between the dye and TiO2 energy levels.139 From then on, coumarin-based dyes, as one of the most promising organic photosensitizers, have come into sight because their excellent photoresponse in the visible region, good long-term stability and appropriate LUMO levels matching with the conduction band of TiO2.7,[123][124][125] K. Hara et al. reported a novel coumarin-based dye whose carboxyl group was directly connected to the conjugated methine chain (see Figure Ⅱ-5, dye 53). The structure of the dye 53 was advantageous for improving the injection of electrons into semiconductors and its cyano group connected to the methine chain and the carboxyl group might be effective for electron injection due to its strong electron acceptability. When embedded in TiO2-based DSCs, the coumarin-based dye 53 showed a high power conversion efficiency of 5.6% (Jsc = 13.8 mA•cm -2 , Voc = 630 mV, FF = 0.63) under standard AM 1.5 irradiation which was comparable to the Ru(dcbpy)2(NCS)2 system. 140 The thiophene moieties can be introduced into coumarin-based dyes to improve the photovoltaic performance of dye-sensitized nanocrystalline TiO2 solar cells. In this context, a coumarin-based dye (see Figure Ⅱ-5, dye 54) endowed with a bithiophene moiety into the methine chain was reported to attain a solar-energy-to-electricity conversion efficiency of 7.7% (Jsc = 14.3 mA•cm -2 , Voc = 730 mV, FF = 0.74) when employed in DSCs under standard AM 1.5 sunlight, 100 mW•cm -2 irradiation. 141 The transient absorption spectroscopy measurements of the dye 54 revealed an ultrafast electron injection (<100 fs) from the dye into the conduction band of TiO2, which was much faster than the emission lifetime of the dye (1.0 ns), thus showing a highly efficient electron injection yield of near unity. 142

  Figure Ⅱ-5, dye 55). 144 When embedded in a sandwich cell, the dye 55 showed a very high IPCE of 85% and an overall power conversion efficiency of 4.5% (Jsc = 10.5 ± 0.20 mA•cm -2 , Voc = 603 ± 30 mV, FF = 0.71 ± 0.03) under standard global AM 1.5 solar condition. It was found in their study that creating directionality and inhibiting self-quenching of the excited state of the sensitizer played a key role in the unprecedented efficiency of squaraines. The synthesis and optical, electronic, and photovoltaic properties of an unsymmetrical bis(indoline) squaraine sensitizer incorporating a promising extended conjugation and a strong π-accepting carboxycyanovinyl group as a surface-anchoring group instead of a directly attached carboxylic acid (see Figure Ⅱ-5, dye 56), coded as YR6, was reported by Y. Shi et al. The well-designed structure of the dye 56 resulted in a red-shift of the absorption maximum andthe presence of additional higher-energy bands that contributed to panchromatic absorption of the dye 56. As a result, the dye 56 sensitized cell with a standard I3/I -liquid electrolyte showed a power conversion efficiency of 6.74% (Jsc = 14.8 mA•cm -2 , Voc = 642 mV, FF = 0.71) under standard global AM 1.5 simulated solar conditions. 145 However, most of the squaraine dyes showed low efficiencies because of their dye aggregation, lack of directionality in the excited state and low stability. Therefore, molecular engineering of near-IR squaraine-based photosensitizers with high photovoltaic properties and an improved stability remains strongly required.123,125,146 Carbazoles and their derivatives have been investigated as electroluminescent, non-linear optical and photorefractive materials.147,148 The carbazole-based dyes are good candidates for reaching high Voc values and thus constructing photovoltaic devices because of the wide band gap of carbazole.[149][150][151][152] Many efforts have been done by our group to design and synthesize various carbazole-based dyes. In particular, in 2013, a series of organic oligocarbazole-based chromophores (see FigureⅡ-5, dyes 57 to 60) designed on the D-π-A model were reported by our group for efficient thin-film DSCs. In order to enrich the spectral response, multiple visible-light-absorbing motifs and a naphthyl group in place of a phenyl ring in the π-conjugated bridge were introduced in these chromophores. When embedded in TiO2-based DSCs, dyes 58 and 59 achieved power conversion efficiency values of up to 5.8% in the presence of the electrolyte Z960 under standard AM 1.5 sunlight, 100 mW•cm -2 irradiation.153 A rational molecular engineering strategy was also proposed by our group in 2015 in order to prepare an original series of efficient carbazole-based photosensitizers for DSCs. Every of these novel carbazole-based dyes with high light-harvesting capacity contained a fluorene

Figure Ⅱ- 6 :

 6 Figure Ⅱ-6: Several representative metal-free organic materials used for DSCs with the cell efficiencies of > 10%.

Figure Ⅱ- 7 :

 7 Figure Ⅱ-7: Organometallic complexes featuring an original D-[M]-A architecture.6,25

Figure Ⅱ- 8 )

 8 Figure Ⅱ-8). The dye [Ru]1 bears a phenyl spacer between the BTD unit and the carboxylic

Figure Ⅱ- 8 :

 8 Figure Ⅱ-8: Molecular structure of the Ru-diacetylide complexes [Ru]1-[Ru]3

Figure Ⅱ- 9 : 3 .

 93 Figure Ⅱ-9: (a) Electronic absorption spectra of [Ru]1-[Ru]3 in CH2Cl2 (C ≈ 3 × 10 -5 M); (b)

a 3 .

 3 Absorption maxima in CH2Cl2 solution (C = 3 × 10 -5 M). b Visible range absorption maximum on 3-µm TiO2 transparent film. c Oxidation potentials in CH2Cl2 solution with FeCp * 2 as internal reference. Potentials referred to NHE by addition of 130 mV. 188 d ΔEopt estimated from the onset of the absorption spectra. e Considering that ΔEopt (V) = [ΔEopt (eV) / q ] = [ΔEopt (eV) / 1] = ΔEopt (eV) The redox properties of the dyes were investigated through cyclic voltammetry analyses performed in dichloromethane solutions using Bu4NPF6 as salt support. The corresponding voltammograms are shown in Figure Ⅱ-10 and data reported in Table Ⅱ-1. The three organometallic complexes undergo one reversible monoelectronic oxidation process at equivalent potential (Eox ≈ +0.70 V vs. NHE) corresponding to the Ru Ⅱ →Ru Ⅲ oxidation process. As a consequence, the first oxidation potential of the three dyes is more positive than the Nernst potential of the conventional redox mediator used in DSCs (i. e. I -/I3 -, E °= +0.35V vs. NHE).7 And thus, sufficient driving force is observed (≥ 0.35 V) to ensure effective regeneration of the dye's ground-state following the photoinduced electron transfer to TiO2.The optical band-gap (ΔEopt) of the dyes was estimated from the onset of the absorption spectra in solution, leading to ΔEopt[Ru]1 > ΔEopt [Ru]2 > ΔEopt [Ru]It is worth noting that [Ru]3, by showing an optical band-gap of 1.62 eV, appears as a low-band gap sensitizer, as expected. Finally, the difference Eox -ΔEopt gave us an estimation of the oxidation potential in the excited state.7 The values reported in Table Ⅱ-1 are more negative than the conduction band edge of TiO2 (ca. -0.50 V vs. NHE) 189 thus creating sufficient driving force for electron injection from the photo-excited dyes to the semiconducting oxide. In addition, reduction processes are observed in the cathodic region that can be attributed to the benzothiadiazole ligand.

Figure Ⅱ- 10 :

 10 Figure Ⅱ-10: Cyclic voltammograms of [Ru]1-[Ru]3 in CH2Cl2 (working electrode: Pt disc; reference electrode: Ag/AgCl, counter electrode: Pt wire); salt support = 0.1 M Bu4NPF6; scan rate = 100 mV•s -1 .

[Ru] 1 -

 1 [Ru]3 was obtained using density functional theory (DFT) (see experimental part for details). The calculations attest the linearity of the molecular backbone. The two acetylide ligands, in trans position with regard to the [Ru(dppe)2] fragment, form an angle close to 180°. This configuration provides a good directionality for the intramolecular charge transfer processes.

Figure Ⅱ- 11 :

 11 Figure Ⅱ-11: Electron-density distribution of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of [Ru]1-[Ru]3.

  2. Calculations confirm that the broad absorption band observed in the visible range (see Figure Ⅱ-9a) represents the envelope of several transitions, however with major contribution of the HOMO→LUMO transition. Considering the localization of the two frontier orbitals, the main absorption band of the Ru-based dyes therefore owns a strong MLCT character. The computed transferred charge qCT is close to 1 and the distance of the charge transfer is 3.2 Å for [Ru]1 and 3.8 Å for [Ru]2 and [Ru]3.

a

  ΔEge = main transition energy. b λge = calculated λmax. c fge = oscillator strength. d Only the transitions with coefficients higher than 0.15 are given for. e Ʌ = spatial overlap. f qCT = quantity of transferred charge. g DCT = distance between the barycentres of the density depletion and density increment zones related to the CT excitation.

  dyes [Ru]1-[Ru]3 were further adsorbed onto TiO2 nanoparticulate transparent thin-films, in the presence of cheno-deoxycholic acid as co-adsorbent, and the photoanodes were incorporated in solar cell devices following reported procedure. 190 The photocurrent density-voltage (J/V) curves are shown in Figure Ⅱ-12 and the photovoltaic parameters are gathered in Table Ⅱ-3. The benchmark dye N3 (cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'dicarboxylato) ruthenium as reference dye was tested under the same conditions for comparison. First observation was that [Ru]1 is the least effective photosensitizer of the series with Jsc = 7.13 mA•cm -2 , Voc = 577 mV and PCE = 3.11 %. The limited performance afforded by [Ru]1 mainly arises from the low photocurrent density value which can be imputed to the carboxylic acid anchoring group. Due to the poor electron-withdrawing character of the -COOH function, the electron-density of the dye's LUMO remains principally on the BTDunit and only few electron-density is observed on the anchoring group. As a consequence, the molecular structure of this complex apparently impedes fast charge injection from the excited dye to TiO2. Moreover, the amount of dye adsorbed on TiO2 surface, determined by absorption spectroscopy, indicates that the carboxylic acid anchor leads to lower dye-loading amount for [Ru]1 (DLA = 1.40 × 10 -7 mol•cm -2 ) than for the two other dyes. This can be ascribed to the weak acidity of [Ru]1. The pKa of benzoic acid is known to be around 4 whereas pKa of α-cyanoacrylic acid is around 1-2. Since the carboxylate group is known to form chelating and bridging linkage with TiO2, 7 the more acidic carboxylic acids are, the more stable complexes they form with the surface.191

Figure Ⅱ- 12 :

 12 Figure Ⅱ-12: J/V characteristics of [Ru]1-[Ru]3 and N3 in DSCs.

  Photovoltaic data measured at full sunlight (AM 1.5G, 100 mW•cm -2 ): Jsc = short-circuit current density, Voc = open-circuit voltage, ff = fill factor, PCE = power conversion efficiency. TiO2 film thickness = 10 µm (transparent layer) + 5 µm (scattering layer). Active area = 0.159 cm 2 (defined by a mask). Data from the best of 5 devices (standard deviation on the efficiency +0.06%). DLA = dye-loading amount measured on 1 cm 2 TiO2 electrode.

Figure Ⅱ- 13 :

 13 Figure Ⅱ-13: IPCE spectra of [Ru]1-[Ru]3 in DSCs

  However, many opportunities for improvement remain because so far only little attention has been given to this technology. In this context, Ru-based diacetylide complexes again represent serious candidates for the sensitization of NiO due to their excellent optical properties and a good matching of the energy levels. Molecular engineering should undoubtedly provide new examples of organometallic chromophores suitable for the p-DSC technology.

  Some representative porphyrins used as sensitizers in NiO based p-DSCs are shown in Figure Ⅲ-1.

Figure Ⅲ- 1 :

 1 Figure Ⅲ-1: Some representative porphyrins used as sensitizers in NiO based p-DSCs.

  photovoltaic performances were significantly enhanced through the introduction of a secondary electron acceptor next to the porphyrin which can be attributed to the shifted electron from the ZnP to the secondary acceptor slowing down the charge recombination. The dye 78 (ZnP-CO2H-eNDI) exhibited a PCE of 0.056% (Jsc = 1.38 mA•cm -2 , Voc = 127 mV, FF = 0.32) with the I -/I3 -electrolyte under stimulated AM1.5G solar light, 100 mW•cm -2 irradiation.

Figure Ⅲ- 2 :

 2 Figure Ⅲ-2: Two representative structures of BODIPY dyes used as sensitizers in NiO based p-DSCs.

Figure Ⅲ- 3 :

 3 Figure Ⅲ-3: Several representative perylene imide sensitizers used in p-DSCs.

  Several representative perylene imide sensitizers used in p-DSCs are shown in Figure Ⅲ-3. A. Morandeira et al. for the first time reported a perylene-imide sensitizer (PMI) and a covalently linked perylene imide-naphthalene diimide dyad (PMI-NDI) (see Figure Ⅲ-3, dye 82 and dye 83). 12 For both dyes upon light excitation of the perylene imide unit, an electron shift from the valence band of NiO to the dye with an average time constant of approximately 0.5 ps was investigated. In the case of the dye 83, the excess electron was shifted further onto the naphtalene-diimide unit, producing a new charge separated state. The dye 83 displayed a substantial retardation of the charge recombination between the hole and the reduced molecule in comparison with the dye 82. The sandwich NiO-based DSC devices sensitized by the dye 83 showed an absorbed-photon to current conversion efficiency (APCE) of 45%, which was three times higher than those sensitized by the dye 82. Based on this work, L. L. Pleux et al. designed and prepared two novel perylene imide sensitizers comprising a PMI moiety connected to a secondary electron accepting unit (naphthalene diimide (NDI) or fullerene (C60), see Figure Ⅲ-3, dyes 84 and 85) for application in NiO-based p-DSCs. 13 Either dye 84 or dye 85 exhibited a longer lifetime of the charge separated state by about five orders of magnitude compared to the dye 82 which can be attributed to the presence of the secondary electron acceptor group. The p-DSC devices sensitized by the dye 85 showed the highest IPCE of 57% at λmax and the Jsc of 1.88 mA•cm -2 with the iodide electrolyte, which were ascribed to the slightly stronger reducing power of C60 relative to NDI, resulting in more efficient reduction of the mediator in the electrolyte. In 2015, I. R. Perera et al. developed an electrolyte based on the tris(acetylacetonato) -iron(Ⅲ)/(Ⅱ) redox couple ([Fe(acac)3] 0/1-) for p-DSCs. 7 When applied [Fe(acac)3] 0/1-as redox mediator in p-type DSSCs in conjunction with the former reported PMI-6T-TPA sensitizer 9 (see Figure Ⅲ-3, dye 86) and incorporating a NiO blocking layer and chenodeoxycholic acid as an electrolyte additive, a world record PCE of 2.51 ± 0.08% (Jsc = 7.65 ± 0.39 mA•cm -2 , Voc = 645 ± 12 mV, FF = 0.51 ± 0.01) was achieved. Some sensitizers discussed above are based on the prototypical push-pull design. As referred in Chapter Ⅱ, the prototypical push-pull design notoriously affords efficient charge transfer from the ground-state to the excited-state of the dye, thus enhancing its absorption properties. This design also allows reduction of the electronic recombination processes in the device. Such structure has been applied to numerous examples of organic dyes featuring various donor and acceptor parts (see Chapter Ⅱ, ref. 12 and 14). Therefore, molecular engineering of push-pull organic dyes for p-DSCs can be a meaningful area of research. Several representative push-pull organic sensitizers used in p-DSCs are shown in Figure Ⅲ-4. In 2008, P. Qin et al. designed and prepared a novel push-pull organic dye which employed the triphenylamine moiety as the electron donor, malononitrile moiety as the electron acceptor, and a thiophene unit as the conjugated chain (see Figure Ⅲ-4, dye 87). 10

Figure Ⅲ- 4 :

 4 Figure Ⅲ-4: Several representative push-pull organic sensitizers used in p-DSCs.

  photophysical and photovoltaic properties in NiO-based p-DSCs (see FigureⅢ-4, dye 90 to 93).[START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes[END_REF] The four dyes employed a triphenylamine donating unit and a dihexylbithiophen

  metal complexes, the HOMO is positioned on the anchoring group while the LUMO is positioned on the ancillary ligands, decreasing the charge recombination by alienating the electron away from the NiO surface. Scheme Ⅲ-1 represents the best performing structure of Ru or Ir polypyridine complexes for p-DSCs.1

Figure Ⅲ- 5 :

 5 Figure Ⅲ-5: Several representative ruthenium coordination complexes developed for p-DSCs.

Figure Ⅲ- 6 :Ⅲ. 3 . 1 .

 631 Figure Ⅲ-6: Molecular structures of the push-pull organometallic dyes SL1 and SL2.

  UV-visible absorption spectra of SL1 and SL2, recorded in dichloromethane solution, are shown in Figure Ⅲ-7 and the corresponding values are listed in TableⅢ-1. The intense short wavelength absorption band observed in the UV region is characteristic of n→π* and π→π* transitions from the dppe ligands.55 The less intense band centred around λ = 360 nm can be tentatively ascribed to electronic transitions involving the triphenylamine electron donor and the origin of this band is discussed below. However the most interesting feature of these dyes is the broad absorption band observed in the visible part of the spectrum. This intense absorption band corresponds to multiple MLCT processes, mainly stemming from the metal-centred HOMO to the LUMO located on the electron-withdrawing alkynyl ligand (see DFT calculations below). The maximum absorption wavelength, λmax, of SL1 in the visible is centred at 521 nm, making SL1 a red pigment. Elongation of the π-conjugated system through the rhodanine acceptor group in SL2 led to red shifted absorption band. The spectrum of SL2

Figure Ⅲ- 7 :

 7 Figure Ⅲ-7: Absorption spectra of SL1 and SL2 in CH2Cl2 (C ≈ 3 × 10 -5 M).

a

  Absorption maxima in CH2Cl2 solution (C = 3 × 10 -5 M). b Emission maximum in CH2Cl2 solution (C = 3 × 10 -5 M). c ΔEopt estimated from the intercept of the normalized absorption and emission spectra. d Potentials measured in THF solution with FeCp2 as internal reference and referred to NHE by addition of 630 mV. 56 e Reduction potential at the excited state estimated from Ered + E00.

Figure Ⅲ- 8 :

 8 Figure Ⅲ-8: Normalized electronic absorption (plain) and fluorescence emission (dashed) spectra of SL1 and SL2 in CH2Cl2.

Figure Ⅲ- 9 :

 9 Figure Ⅲ-9: Cyclic voltammograms of SL1 and SL2 recorded in THF/nBu4NPF6 (0.1M) vs. Fc/Fc+ at 200 mV s -1 . The blue dotted line represents the reversibility of the first oxidation process isolated;

  electron-density distribution of the frontier molecular orbitals (MOs) were determined by density functional theory (DFT) using the B3LYP/LANL2DZ hybrid functional. Spatial representation of the calculated transition-involved MOs is shown in Figure Ⅲ-10.

Figure Ⅲ- 10 :

 10 Figure Ⅲ-10: Electron-density distribution of the transition-involved frontier molecular orbitals of SL1 and SL2.

Figure Ⅲ- 11 :

 11 Figure Ⅲ-11: TD-DFT simulated absorption spectra of SL1 and SL2. Absorption bands enlarged using Gaussian functions with full-width at half-height (FWHH) of 5 nm to reproduce the experimental spectra.

a

  ΔEge = main transition energy. b λge = calculated λmax. c ƒge = oscillator strength. d Only the transitions with coefficients higher than 0.15 are given. e Λ = spatial overlap. ƒ qCT = quantity of transferred charge. g DCT = distance between the barycentres of the density depletion and density increment zones related to the CT excitation.

Table Ⅲ- 3 :Figure Ⅲ- 12 :

 312 Figure Ⅲ-12: Photoaction spectra of the dyes SL1 and SL2 in NiO based p-DSC recorded in short-circuit conditions.

  However, the indicated driving force of SL1 and SL2 for hole injection from the dye excited state into the valence band of NiO is small. Therefore, two other Ru-based dyes based on the [Ru(dppe)2] moiety were further designed by changing the electron-donating unit to simple phenyl and thienyl units in order to lower the HOMO energy level and to enhance the charge injection rate. The molecular structure of the two new Ru-based dyes is depicted in Figure Ⅲ-13. The bis(σ-arylacetylide) photosensitizers SL3 and SL4 are endowed with a carboxylic acid anchoring group separated by a benzene or thiophene linker. The two dyes are functionalized with the same electron-withdrawing ligand as SL1 bearing a dimethylmalonate group.

Figure Ⅲ- 13 :

 13 Figure Ⅲ-13: Molecular structures of the push-pull organometallic dyes SL3 and SL4.

Scheme Ⅲ- 4 :

 4 Scheme Ⅲ-4: Synthesis of the alkyne ligands 33 and 37. Reaction conditions: (a) trimethylsilylacetylene, PdCl2(PPh3)2, CuI, Et3N, THF; (c) K2CO3, MeOH; (b) trimethylsilylethanol, HBTU, DIPEA, DMF.

Figure Ⅲ- 14 :

 14 Figure Ⅲ-14: Absorption spectra of SL3 and SL4 in CH2Cl2 (C ≈ 3 × 10 -5 M).

a

  Absorption maxima in CH2Cl2 solution (C = 3 × 10 -5 M). b Potentials measured in CH2Cl2 solution with FeCp*2 as internal reference and referred to NHE by addition of 130 mV. 56

Figure Ⅲ- 15 :

 15 Figure Ⅲ-15: Cyclic voltammograms of SL3 and SL4 recorded in CH2Cl2/nBu4NPF6 (0.1M) vs. Ag/AgCl at 200 mV s -1 .

  electron-density distribution of the frontier molecular orbitals (MOs) were determined by density functional theory (DFT) using the B3LYP/LANL2DZ hybrid functional. Spatial representation of the calculated transition-involved MOs is shown in Figure Ⅲ-16.

Figure Ⅲ- 16 :

 16 Figure Ⅲ-16: Electron-density distribution of the frontier molecular orbitals of SL3 and SL4.

  Ru-based dye was designed which included a phosphonic acid anchoring group for improving the density of dye on the photoelectrode. The molecular structure of the new dye is depicted in Figure Ⅲ-17. The ruthenium diacetylide complex SL5 is endowed with a phosphonic acid anchoring group on one side, and functionalized with a dimethylmalonate electron-withdrawing group on the other side, both separated by two bithiophene linkers and the [Ru(dppe)2] electron-rich metal center.

Figure Ⅲ- 17 :

 17 Figure Ⅲ-17: Molecular structure of the push-pull organometallic dye SL5.

Scheme Ⅲ- 6 :

 6 Scheme Ⅲ-6: Synthesis of the organometallic complex SL5. Reaction conditions: (a) trimethylsilylacetylene, PdCl2(PPh3)2, CuI, DIPEA, THF; (b) diethyl phosphite, Et3N, Pd(OAc)2, dppf, CH3CN; (c) K2CO3, MeOH; (d) CH2Cl2; (e) NaPF6, Et3N, CH2Cl2; (f) N,O-bis(trimethylsilyl) acetamide, iodotrimethylsilane, CH2Cl2.

a

  Absorption maxima in CH2Cl2 solution (C = 3 × 10 -5 M).b Potentials measured in CH2Cl2 solution with FeCp*2 as internal reference and referred to NHE by addition of 130 mV.56 The electrochemical properties of SL5 were investigated by cyclic voltammetry in CH2Cl2 solution (see FigureⅢ-19), the corresponding data are given inTable Ⅲ-6. Similarly to the dyes SL1-SL4, the voltammogram of SL5 shows two reversible monoelectronic processes in the anodic region. The first one can be attributed to the Ru Ⅱ →Ru Ⅲ oxidation process while the second one can be ascribed to the central π-conjugated system.Interestingly, the new modification made in the electron-donor group of SL5 yielded a first oxidation process comparable to that of SL1: Eox1 = +0.65 V vs. NHE and Eox1 = +0.64 V vs. NHE for SL5 and SL1, respectively. However the second oxidation process of SL5 (Eox2 = +1.32 V vs. NHE) occurs at higher potential compared to SL1 (Eox2 = +1.05 V vs. NHE) and is finally similar to that of SL3 and SL4 (Eox2 = +1.32 V vs. NHE).

Figure Ⅲ- 19 :

 19 Figure Ⅲ-19: Cyclic voltammograms of SL5 recorded in CH2Cl2/nBu4NPF6 (0.1M) vs. Ag/AgCl at 200 mV s -1 .

Figure Ⅲ- 20 .

 20 Figure Ⅲ-20. Theoretical calculations reveal that the HOMO of SL5 is well spread over the extended π-conjugated system from the phosphonic acid anchoring group to the remote end of the molecular backbone, again with a strong contribution of the central C≡C-Ru-C≡C fragment. On the other hand, the LUMO of SL5 is well localized on the alkyne ligand bearing the electron-withdrawing group. It is worth underlining that the HOMO of SL5 presents an electronic density coming very close to the surface of NiO which seems favorable to charge transfer process from the semiconducting oxide.

Figure Ⅲ- 20 :

 20 Figure Ⅲ-20: Electron-density distribution of the frontier molecular orbitals of SL5.

  Figure Ⅳ-1a 23 and Ⅳ-1b 24 show the working principle of DSPEC based on NiO photocathodes and the energy diagram of a dye-sensitized photocathode in a DSPEC, respectively.

Figure Ⅳ- 1 :

 1 Figure Ⅳ-1: (a) Operation principle of DSPEC based on NiO photocathodes. Cat = catalyst; 23 (b)

Figure Ⅳ- 2 :

 2 Figure Ⅳ-2: Schematic representation of the three possible configuration types that are generallyemployed to combine a sensitizer with a catalyst in DSPEC.[START_REF] O'reilly | Oxidation-reduction potential of the ferro-ferricyanide system in buffer solutions[END_REF]

Figure Ⅳ- 3 :

 3 Figure Ⅳ-3: Some representative dyes utilized in NiO-based DSPECs for light-driven H2 production.

Figure Ⅳ- 4 :

 4 Figure Ⅳ-4: Molecular structures of the dyes SL6 and SL7.

Scheme Ⅳ- 1 :

 1 Scheme Ⅳ-1: Synthetic routes to SL6 and SL7. Conditions: (a) CH2Cl2; (b) diphenyl-2-propyn-1-ol, NaPF6, Et3N, CH2Cl2; (c) tetrabutylammonium fluoride, THF.

  Figure Ⅳ-5 shows the cationic organometallic unit from different views along with an appending triflate anion. The crystallographic data are detailed in the Experimental Part. The crystal structure of 48 confirms the trans position of the two carbon-rich ligands with regard to the metal centre, providing a linear arrangement of the carbon chains with a Cα-Ru-C'α angle of 179.1(7)°. The linearity of the chains extends well beyond, over 9.27 Å from Cγ to C'γ, with a Cγ-Ru-C′γ angle of 178.7(2)°. The different bond lengths are consistent with the presence of a cumulenic chain on one side and an alkynyl chain on the other side. 42 The corresponding distances are 1.933, 1.261 and 1.364 for Ru-Cα, Cα-Cβ and Cβ-Cγ of the allenylidene ligand, and 2.081, 1.197 and 1.436 for Ru-C'α, C'α-C'β and C'β-C'γ of the acetylide ligand. Note that the Ru-Cα distance is longer in the alkynyl chain than that in the cumulenic one, and conversely Cα-Cβ is much shorter in the alkynyl ligand, thus presenting a strong C≡C character. The crystal structure also shows how the metal atom and linked carbon chains are surrounded by the diphosphine ligands, which shelter the central π-conjugated system and, through their bulkiness, cause a de-aggregating effect.

Figure Ⅳ- 5 :Ⅳ. 3 . 3 .

 533 Figure Ⅳ-5: Crystal structure of 48 (top and side views). Proton and solvent molecules were removed for clarity.

Figure Ⅳ- 6 :

 6 Figure Ⅳ-6: Absorption spectra of SL6 (blue line) and SL7 (red line) in CH2Cl2 (C ~3×10 -5 M; optical pathway 1 cm).

  Muñoz-García from the University of Naples Federico Ⅱ.A spatial representation of the frontier molecular orbitals HOMO and LUMO calculated at the PBE0 level of theory is shown in FigureⅣ-7. The calculations show that the HOMO of the dyes is delocalized over the acetylide ligand bearing the thiophene (SL6) or triphenylamine unit (SL7) and on the anchoring group(s), which should greatly favour hole injection into the valence band of NiO.52 The HOMO also shows a strong contribution from the metal centre, and in the case of SL6 a substantial extension onto the cumulenic carbon chain. On the other hand, the LUMO of such mixed allenylidene-acetylide metal complexes is mainly localized on the allenylidene ligand, thus involving the external phenyl rings, the cumulenic chain and some contribution from the metal fragment. As a consequence, the HOMO and LUMO of the dyes are well positioned on the metal complexes for hole injection into NiO on one side and electron donation to the cobalt-based electron acceptor on the other side.

Figure Ⅳ- 7 :

 7 Figure Ⅳ-7: Electron-density distribution of the transition-involved frontier molecular orbitals of SL6 (left) and SL7 (right).

Figure Ⅳ- 8 :

 8 Figure Ⅳ-8: Charge density difference iso-surface plots between excited-(S1) and ground-state (S0) electron densities of SL6 (left) and SL7 (right). The yellow surface represents positive charge density values (i.e., excited electron localization in S1), the cyan surface represents negative charge density values (i.e., hole in the ground-state).

  Figure Ⅳ-9a displays typical absorbance spectra of a dye-sensitized electrode, showing new features as compared with the spectrum of the blank NiO substrate (measured on the same sample before sensitization) that correspond to the absorbance of SL6 or SL7 dyes grafted onto the NiO surface (Figure Ⅳ-9b andc). For both dyes, a blue shift (33 nm for SL6 and 53 nm for SL7) of the absorbance band is observed, in agreement with previous reports.47,50 These modifications result either from small structural rearrangements of the dye molecules upon grafting on the NiO substrate or from intermolecular interactions between adjacent grafted dyes.

Figure Ⅳ- 9 :

 9 Figure Ⅳ-9: (a) Absorbance spectra of a blank NiO electrode (black line) and the same electrode after sensitization with SL6 (dashed blue line). The difference between these two spectra is shown as a plain blue line. (b) Comparison of corrected spectra recorded on NiO films (plain blue line) and CH2Cl2 (C ~3 × 10 -5 M; optical pathway 1 cm) solution spectra of SL6 (dashed blue line). (c) Comparison of corrected spectra recorded on NiO films (plain red line) and CH2Cl2 (C ~3 × 10 -5 M;

Figure Ⅳ- 11 :

 11 Figure Ⅳ-11: Representation of the Ru-diacetylide/Cobaloxime photocatalytic system SL8 anchored onto NiO nanoparticulate thin-film.

  Scheme Ⅳ-2: The synthetic route to the new photocatalytic system and the relevant precursors. Conditions: (a) 4-ethynylpyridine hydrochloride, PdCl2(PPh3)2, CuI, diisopropylamine, THF; (b) K2CO3, MeOH ; (c) NaPF6, Et3N, CH2Cl2; (d) TBAF, THF; (e) THF.

Figure Ⅳ- 12 :

 12 Figure Ⅳ-12: Absorption spectra of SL8, SL8-Co and 53 in THF (C ≈ 1 × 10 -5 M).

  spectra in THF. a 0-0 transition energy estimated from the onset of the absorption spectra. b Redox potentials in DMF solution. c Redox potentials referred to NHE by addition of 730 mV. 58 d Oxidation potential of the excited state obtained from Eox-E0-0.

Figure Ⅳ- 13 :

 13 Figure Ⅳ-13: Cyclic voltammograms of SL8, SL8-Co and 53 in DMF solution. (Working electrode: Pt disc; reference electrode: Ag/AgCl, calibrated with decamethylferrocene as internal reference; counter electrode: Pt; salt support: 0.1M Bu4NPF6; scan rate: 100 mV•s -1 ).

Figure Ⅳ- 14 :

 14 Figure Ⅳ-14: Energy diagram of a NiO-based DS-PEC including SL8-Co (S stands for the sensitizing part of the photocatalytic system, i.e. the Ru-based dye).

Figure Ⅳ- 15 :

 15 Figure Ⅳ-15: Ground-state minimum-energy molecular structures of SL8 (a) and SL8-Co (b).

Figure Ⅳ- 16 :

 16 Figure Ⅳ-16: Isodensity surface plots of the HOMO and LUMO of SL8 and SL8-Co (contour value set to 0.005 a.u.). Color legend: C atom green, O atom red, N atom light blue, Ru atom magenta, H atom white, S atom yellow, P light purple; isodensity positive and negative values are depicted in yellow and cyan, respectively.

  Figure Ⅳ-17: Electronic absorption spectra of blank NiO film (gray dashed) and of NiO electrode sensitized by SL8-Co (red).

Figure Ⅳ- 18 :

 18 Figure Ⅳ-18: XPS survey spectra of pristine powders of SL8, 53 and SL8-Co.

Figure Ⅳ- 19 :

 19 Figure Ⅳ-19: XPS survey spectra of neat NiO electrode and NiO electrodes sensitized by SL8 and SL8-Co.

Figure Ⅳ- 20 :

 20 Figure Ⅳ-20: XPS core level spectra of (top) SL8-Co@NiO in the Ru3d-C1s region and (bottom) comparison of SL8-Co powder, SL8-Co@NiO, SL8@NiO and neat NiO in the Co2p region.

  sweep voltammograms were recorded in the dark and under visible-light irradiation (Figure Ⅳ-21). As expected a cathodic photocurrent is observed under light with a density of ca. 5.6 µA•cm -2 at -0.2 V vs. NHE (-0.4 V vs. Ag/AgCl). This behavior is due to photoinduced electron transfers from the p-type semi-conductor NiO, to the cobaloxime catalyst, through excitation of the sensitizer part in SL8-Co.

Figure Ⅳ- 21 :

 21 Figure Ⅳ-21: Linear sweep voltammograms (scan rate = 10 mV•s -1 ) of NiO electrode sensitized by SL8-Co recorded in acetate buffer (0.1 M, pH 4.5) in the dark (black line) or under visible-light irradiation (red line, 400-800 nm, 65 mV•cm -1 , 1 sun).

Figure Ⅳ- 23 :

 23 Figure Ⅳ-23: Chronoamperometric measurements under chopped-light using NiO electrodes sensitized with SL8-Co (red line) or SL8 only (orange line) in acetate buffer (0.1 M, pH 4.5) at an applied potential of -0.2 V vs. NHE (-0.4 V vs. Ag/AgCl) under visible-light irradiation (400-800 nm, 65 mV•cm -2 , 1 sun).

Figure Ⅳ- 24

 24 Figure Ⅳ-24 shows the representation of the new Ru-diacetylide/Platinum photocatalytic system anchored onto NiO nanoparticulate thin-film. This photocatalytic system, coded SL9-Pt, was prepared recently and the relevant measurements of optical and electrochemical properties as well as theoretical calculations are still in progress. The new photocatalytic system will be further employed for the construction of NiO-based photocathodes and subsequent H2 generation in DSPECs under pertinent aqueous conditions.

Figure Ⅳ- 24 :

 24 Figure Ⅳ-24: Representation of a Ru-diacetylide/Platinum photocatalytic system anchored onto NiO nanoparticulate thin-film.

[ 11 ] 1 H

 111 [TfO] 1 , the alkynyl ligands 7 and 10, 2 [RuCl(dppe)2][TfO] ([26][TfO]) 3 and 5'-trimethylsilylethynyl-[2,2']-bithiophene-5-carbaldehyde (21)4 were synthesized according to literature procedures. NMR,13 C NMR and 31 P NMR analyses were performed on Bruker Avance I 300 MHz, Avance Ⅱ 400 MHz and Avance Ⅲ 600 MHz spectrometers. Chemical shift values are given in ppm with reference to solvent residual signals. HR-MS analyses were performed by the CESAMO (Bordeaux, France). Field desorption (FD) measurements were carried out on a TOF mass spectrometer AccuTOF GCv using an FD emitter with an emitter voltage of 10 kV.

XSynthesis of 4 .

 4 -ray photoelectron spectroscopy surface analyses were performed at PLACAMAT (UMS 3626) with a K-Alpha spectrometer (ThermoFisher Scientific) for powders pressed onto indium foil. The monochromatized AlKα source (hν=1486.6eV) was activated with a spot size 200 μm in diameter. The full spectra (0-1100 eV) were obtained with a constant pass energy of 200 eV and high resolution spectra with a constant pass energy of 40 eV (dwell time 200 ms). Fitting and quantitative analyses of XPS spectra were performed using the AVANTAGE software (ThermoFisher Scientific). To the residue of 3 was added HBTU (437 mg, 1.15 mmol), DMF (15 mL), DIPEA (836 µL, 4.80 mmol) and 2-(trimethylsilyl)ethanol(227 µL, 1.58 mmol). The reaction mixture was stirred for 18h at RT. The reaction mixture was evaporated to dryness.The residue was dissolved in CH2Cl2 and the solution was washed with saturated NH4Cl aqueous solution. The organic phase was evaporated to dryness. After adsorbed on silica gel in advance, the crude solid was purified by silica gel chromatography with CH2Cl2 as an eluent, to afford 4 as a red-brown solid in 35 % yield (117 mg, 0.30 mmol).1

  reaction mixture was stirred overnight at RT. After removal solvent the resulting solid was dissolved in CH2Cl2 and thoroughly washed with degassed citric acid aqueous solution(10 % m) and pure water. The organics were evaporated to dryness and the solid was further washed with pentane. Slow crystallization from a CH2Cl2/pentane mixture afforded pure [Ru]2 as a deep violet powder in 75 % yield (105 mg, 0.07 mmol). 31 P NMR (240 MHz, THF-d8): δ = 53.5. 1 H NMR (600 MHz, THF-d8): δ = 8.39 (d, 2H,3 JH-H = 7.9 Hz),8.36 (s, 1H), 8.23 (d, 2H, 3

Synthesis of 15 .

 15 0. HR-MS FD + (m/z): 1450.3182 [M-CO2] + (calcd. 1450.3158 for [C89H68N4P4RuS] + ). FT-IR (KBr): νC≡N = 2212 cm -1 , νC≡C = 2031 cm -1 , νC=O = 1705 cm -1 , νP-Ph = 1093 cm -1 . To a solution of [11][TfO] (337 mg, 0.25 mmol, 1 equiv.), compound 10 (74 mg, 0.275 mmol, 1.1 equiv.) and NaPF6 (84 mg, 05 mmol, 2 equiv.) in dry CH2Cl2 (30 mL) and under inert atmosphere, was added distilled Et3N (105 µL, 0.75 mmol, 3 equiv.). The reaction mixture was stirred for 24h at RT. The organics were further washed with degassed Synthesis of 22. To a solution of 5'-trimethylsilylethynyl-[2,2']bithiophene-5-carbaldehyde (21) (0.25 g, 0.86 mmol, 1 equiv.

  1 H NMR (300 MHz, CD2Cl2): δ = 7.52 (d, 1H,3 JH-H = 6.0 Hz), 7.10 (d, 1H,3 JH-H = 3.0 Hz), 4.37 (m, 2H), 1.10 (m, 2H), 0.08 (s, 9H).

1 H

 1 NMR (300 MHz, CD2Cl2): δ = 7.50(m, 1H), 7.23 (m, 2H), 7.15 (d, 1H, 3 JH-H = 3.0 Hz), 4.13 (m, 4H), 3.51 (s, 1H), 1.33 (m, 6H).13 C NMR (75 MHz, CD2Cl2): δ = 144.

3 FTO

 3 photo-generated current out from the cell. Action spectra of incident photon-to-current conversion efficiency (IPCE) were realized using a Xe lamp associated with a monochromator (Triax 180, Jobin Yvon) to select and increment wavelength irradiation to the cell. No bias light was employed to illuminate the cell. The current produced was measured by steps of 5 nm after 2 s of radiation exposure with a Keithley 6487 picoammeter in order to be in steady state conditions. The incident photon flux was measured with a 6-in. diameter calibrated integrated sphere and a silicon detector.

(

  80 ℃) in a 0.3 mM solution of each dye during 16 h. A mixture of distilled acetonitrile and methanol was used (9/1, v/v) for each bath. Platinum counter electrodes were prepared by drop casting a few drops of an isopropanol solution of hexachloroplatinic acid in distilled isopropanol (10 mg per mL) on FTO plates (TEC7, Solaronix). Substrates were then fired at 375 ℃ for 30 min. The photocathode and the counter electrode were placed on top of each other and sealed using a thin transparent film of Surlyn polymer (DuPont, 25 mm) as spacer.A drop of electrolyte was introduced through a predrilled hole in the counter electrode by vacuum backfilling. The electrolyte for the dyes SL1 to SL5 is composed of: 0.6 M 1,2-dimethyl-3-butylimidazolium iodide, 0.03 M I2, 0.5 M 4-tert-butylpyridine and 0.1 M guanidinium thiocyanate in acetonitrile. The hole was then sealed by a glass stopper with Surlyn gasket. The cell had an active area of 0.25 cm 2 . Specially, another novel electrolyte using disulfide/thiolate (T2/T -) molecules as redox mediators was employed for the dye SL5 to improve the photovoltaic performances.18

Ⅵ. 5 .PCE

 5 Crystallographic Study for Compound 48Data collection was performed at the IECB X-ray facility (UMS3033) on a high flux microfocus Rigaku (MM007, 1.2 kW) rotating anode at the copper kα wavelength. The experimental setup is composed of osmic Varimax mirrors and a semi-cylindrical R-Axis spider IP detector. The crystal was mounted on a cryo-loop after quick soaking on Paratone-N

Organometallic Ruthenium Complexes for NiO Sensitization and Photoelectrochemical Applications p.209

	Table of Content Table of Content Table of Content
	Table of Content Ⅰ.3.6.3.5. Quasi Solid State Electrolytes Ⅲ.3.3. Theoretical Calculations Dye-Sensitized Photoelectrochemical Cells	p.52 p.182 p.227
	Ⅰ.4. NiO-Based p-Type Dye-Sensitized Solar Cells and NiO-Based Dye-Sensitized Ⅲ.3.4. Photovoltaic Measurements in p-Type DSCs Ⅳ.4.1 Synthesis and Characterization	p.185 p.228
	Chapter Ⅰ: Introduction Photoelectrochemical Cells Ⅲ.3.5. Conclusions Ⅳ.4.2. Optical and Electrochemical Properties	p.1 p.53 p.187 p.230
	Ⅰ.1. General Introduction Ⅰ.4.1. Historical Background and Recent Improvements Ⅲ.4. Push-Pull Ruthenium Diacetylide Complexes With Modified Electron-Donating Ⅳ.4.3. Theoretical Calculations	p.2 p.54 p.233
	Ⅰ.2. Overview of Solar Energy and Solar Cells Ⅰ.4.2. Operational Principles Group Ⅳ.4.4. Electrode Preparation and Characterization -XPS Analyses	p.4 p.60 p.187 p.236
	Ⅰ.2.1. Solar Energy Ⅰ.5. Research Objectives Ⅲ.4.1. Synthesis of the Dyes SL3 and SL4 Ⅳ.4.5. Photoelectrochemical Properties	p.5 p.63 p.188 p.240
	Ⅰ.2.2. Solar Radiation and Solar Spectrum Ⅰ.6. References Ⅲ.4.2. Optical and Electrochemical Properties Ⅳ.4.6. Conclusions	p.6 p.65 p.190 p.242
	Ⅰ.2.3. Solar Cells Chapter Ⅱ: Ruthenium-Diacetylide Organometallic Complexes for n-Type p.9 Ⅲ.4.3. Theoretical Calculations p.192 Ⅳ.5. Outlook p.243
	Ⅰ.2.3.1. Silicon-Based Solar Cells Ⅲ.4.4. Photovoltaic Measurements in p-Type DSCs Ⅳ.6. References	p.12 p.193 p.244
	Ⅰ.2.3.1.1. Monocrystalline Silicon (mono-Si) Solar Cells Dye Sensitizers Ⅲ.4.5. Conclusions Chapter Ⅴ: General Conclusion	p.12 p.105 p.194 p.251
	Ⅰ.2.3.1.2. Polycrystalline Silicon (poly-Si) Solar Cells Ⅱ.1. General Introduction Ⅲ.5. Phosphonic Acid as a New Anchoring Function of Ru-Based Dyes for p-Type	p.13 p.106
	Ⅰ.2.3.1.3. Amorphous Silicon (a-Si) Solar Cells Ⅱ.2. n-Type Dye Sensitizers DSCs Chapter Ⅵ: Experimental Part	p.14 p.107 p.195 p.257
	Ⅰ.2.3.2. Ⅲ-Ⅴ Solar Cells Ⅱ.2.1. Ruthenium Complexes Ⅲ.5.1. Synthesis of the Dye SL5 Ⅵ.1. Materials and Methods	p.15 p.108 p.195 p.258
	Ⅰ.2.3.3. CdTe/CdS Thin Film Solar Cells Ⅱ.2.2. Porphyrins and Phthalocyanines Ⅲ.5.2. Optical and Electrochemical Properties Ⅵ.2. Synthesis	p.18 p.117 p.197 p.259
	Ⅰ.2.3.4. CIGS Thin Film Solar Cells Ⅱ.2.2.1. Porphyrins Ⅲ.5.3. Theoretical Calculations Ⅵ.2.1. Synthetic Routes to the Dyes [Ru]1-[Ru]3	p.20 p.117 p.199 p.259
	Ⅰ.2.3.5. Dye-Sensitized Solar Cells Ⅱ.2.2.2. Phthalocyanines Ⅲ.5.4. Photovoltaic Measurements in p-Type DSCs Ⅵ.2.2. Synthetic Routes to the Dyes SL1 and SL2	p.24 p.121 p.200 p.264
	Ⅰ.2.3.6. Perovskite Solar Cells Ⅱ.2.3. Metal-Free Organic Dye Sensitizers Ⅲ.5.5. Conclusions Ⅵ.2.3. Synthetic Routes to the Dyes SL3 and SL4	p.27 p.124 p.201 p.269
	Ⅰ.3. TiO2-Based n-Type Dye-Sensitized Solar Cells Ⅱ.3. Molecular Engineering of Ruthenium-Diacetylide Organometallic Complexes Ⅲ.6. General Conclusion Ⅵ.2.4. Synthetic Routes to the Dye SL5	p.31 p.201 p.272
	Ⅰ.3.1. Historical Background for n-Type DSCs Ⅲ.7. References Ⅵ.2.5. Synthetic Routes to the Dye SL6 and SL7	p.32 p.133 p.203 p.274
	Ⅰ.3.2. Operational Principles and Electron Transfer Processes of n-Type Dye-Sensitized Solar Cells Ⅰ.3.3. Energetics of Operational Principles Ⅰ.3.4. Kinetics of Operational Principles Ⅱ.3.1. Synthesis of the Dyes [Ru]1-[Ru]3 Ⅱ.3.2. Optical and Electrochemical Properties Ⅱ.3.3. Theoretical Calculations Ⅱ.3.4. Photovoltaic Properties Ⅵ.2.6. Synthetic Routes to the Dye SL8 and SL8-Co Ⅵ.3. Photovoltaic Characterization Ⅵ.3.1. DSC Preparation and Characterization for [Ru]1-[Ru]3 Chapter Ⅳ: Ⅳ.1. General Introduction Ⅵ.3.2. DSC Preparation and Characterization for SL1-SL5	p.33 p.36 p.38 p.134 p.137 p.139 p.141 p.276 p.278 p.278 p.210 p.279
	Ⅰ.3.5. Characterization Techniques Ⅱ.3.5. Conclusions Ⅳ.2. Sensitizers Utilized in NiO-Based DSPECs for Solar Hydrogen Production Ⅵ.4. Determination of Dye-loading Amounts	p.41 p.144 p.211 p.280
	Ⅰ.3.5.1. Efficiency Measurements Ⅱ.4. References Ⅳ.3. Organometallic Dyes for NiO Sensitization and Photoelectrochemical Ⅵ.5. Crystallographic Study for Compound 48	p.41 p.146 p.280
	Ⅰ.3.5.2. Electrochemical Methods Chapter Ⅲ: Ruthenium-Diacetylide Organometallic Complexes for p-Type p.43 Applications p.215 Ⅵ.6. Electrode preparation method for SL6 and SL7 p.282
	Ⅰ.3.6. Recent Improvements Ⅳ.3.1. Synthesis of the Dyes SL6 and SL7 Ⅵ.7. Electrochemical and Photoelectrochemical Measurements for SL8-Co and	p.45 p.216
	Ⅰ.3.6.1. Improvements in Photoanode Dye Sensitizers Ⅳ.3.2. Crystallographic Study SL8-Co@NiO	p.45 p.165 p.218 p.282
	Ⅰ.3.6.2. Improvements in Counter Electrode Ⅲ.1. General Introduction Ⅳ.3.3. Optical and Electrochemical Properties Ⅵ.8. References	p.47 p.166 p.219 p.284
	Ⅰ.3.6.3. Improvements in Electrolyte Ⅲ.2. p-Type Dye Sensitizers Ⅳ.3.4. Theoretical Calculations	p.49 p.167 p.221
	Ⅰ.3.6.3.1. Liquid Electrolytes Ⅲ.3. Push-Pull Ruthenium Diacetylide Complexes Including Triphenylamine Donor Ⅳ.3.5. Electrode Preparation and Characterization	p.50 p.224
	xiv xvi	xv

Ⅰ.

3.6.3.2. Organic-Solvent-Based Electrolytes p.50 Ⅰ.3.6.3.3. Ionic Liquid Electrolytes p.51 Ⅰ.3.6.3.4. Solid

State Electrolytes p.51 Moieties as New Dyes for p-Type Dye-Sensitized Solar Cells p.176 Ⅲ.3.1. Synthesis of the Dyes SL1 and SL2 p.177 Ⅲ.3.2. Optical and Electrochemical Properties p.179 Ⅳ.3.6. Photoelectrochemical Properties of Dye-Sensitized NiO Films p.225 Ⅳ.3.7. Conclusions p.227 Ⅳ.4. Ru-diacetylide/Cobaloxime Photocatalytic System for H2 Evolution from

  Si material (over 99.999%) and quite demanding hot processing technology. Therefore, the theoretical boundary of efficiency based on mono-Si solar cells is 29%.63 Moreover, it is a hardly practical option in terms of current technology to improve the efficiency of mono-Si solar

cells. Although having high efficiencies and high market share, mono-Si solar cells have high production cost because they require highly purified mono-Si material and quite demanding hot processing technology. To reduce the production costs and simultaneously obtain high conversion efficiency, poly-Si solar cells and a-Si solar cells are the backup options for subsequent development in this field.

  the polycrystalline CIS thin-film solar cells with exceptionally high photocurrents (>35 mA•cm -2 ), low open circuit voltages (about 400mV) and high quantum yields (>0.8) in the spectral range from 600 nm to 900 nm which reached the efficiency up to 10.6% in 1982 156 , 10.98% in1984 157 and 11.9% in 1986 158 . After years of constant development, the emphasis of the CIS thin film turned into exploring the ways to increase the open circuit voltage while maintaining short circuit current. Through the use of wider energy

	band gap materials CuGaSe2 (1.67 eV) and CuInS2 (1.5 eV) compared to CuInSe2 (1.04 eV),
	Au-metallized glass substrate and then the CdS film (approximately 6 μm) was evaporated as
	window layer to form the heterojunction. 152 However, there was limited interest in CIGS thin
	film solar cells because of underestimated significance of this p-n junction devices to
	photovoltaic field, 153,154 until the first high-efficiency (9.4%) polycrystalline thin-film solar

[147][148][149]

The first CuInSe2/CdS p-n heterojunction photovoltaic detectors by evaporating n-type CdS onto a single crystal of p-type CuInSe2 which displayed uniform quantum efficiencies between 0.55 and 1.25 μm were reported by S.

Wagner et al. in 1974 and

at the same time the first solar device based on a single CuInSe2 crystal was reported by them with a power conversion efficiency of 5%. 150 Subsequently, J. L. Shay and S. Wagner carried on this research through optimizing the device for application as a photovoltaic solar cell with an active area of 0.79 mm 2 and the solar intensity of 92 mW•cm -2 and finally obtained a better power conversion efficiency of 12%.

151

In 1976, L. L. Kazmerski from university of Maine reported thin-film CuInSe2/CdS heterojunction solar cells with the efficiency of 4%~5% for the first time which opened the door to Copper indium diselenide (CIS) thin film solar cells.

The CIS thin film was prepared by a two-source (CuInSe2 + Se) co-evaporation technique in order to control carrier type. The p-CuInSe2 film (5~6 μm) was subsequently grown on a cell was reported in 1981 by R.A. Mickelsen of Boeing Aerospace Company.

155

One year later, R. A. Mickelsen, W. S. Chen and co-workers evaporated Cd1-xZnxS instead of CdS as a buffer layer to attain

  Jackson et al.. The alkali elements rubidium and cesium are used in this kind of CIGS solar cells in the place of sodium and potassium in the alkali post deposition treatment.172 The development of CIGS photovoltaic modules can date from the year 2003, when the efficiency of small-area CIGS thin film solar cells has already overwhelm 23.3% under 15

  Up to now, spiro-OMeTAD is still used in state-of-the-art dye-sensitized solar cell devices. The solid state p-type DSCs were reported to obtain a best efficiency of 7.51% employing spiro-OMeTAD as a hole transport photocathode and a novel benzothiadiazole-based D-A-π-A organic material as a dye sensitizer.455 However, there are some drawbacks of the molecular hole conductor spiro-OMeTAD which are low hole-conductivity, difficult infiltration into mesoporous photoanodes and high material cost.

	Therefore, alternative hole conductors have garnered growing interest. J. Zhang et al. reported
	a conducting polymer poly(3,4-ethylenediothiophene) (PEDOT) hole transport material in
	2016. The solid state dye-sensitized solar cell device based on PEDOT and a
	diketopyrrolopyrrole-based dye showed an efficiency of 5.5% while the device based on the
	materials such as [Co(bpyPY4)](OTf)2 and Co(bpyPY4)](OTf)3 where bpyPY4 represents the
	hexadentate ligand 6,6'-bis(1,1-di(pyridin-2-yl)ethyl)-2,2'-bipyridine and OTf represents the
	trifluoromethanesulfonate anion were also reported. 460

Bach et al.

showing a low efficiency of 0.74% when a

Ru-complex

was used as sensitizer.

419

Further development of tailored-made sensitizers combined with the introduction of new dopants allowed for a continuous increase in the PCE of the corresponding cells over 7%. small molecular spiro-OMeTAD hole conductor showed a lower efficiency of 2.9%. The higher power conversion efficiency for PEDOT-based p-type DSCs can be attributed to the significantly enhanced charge collection efficiency due to the three-order-of-magnitude higher hole conductivity compared with that of the hole conductor spiro-OMeTAD.

456

A low-cost spiro[fluorene-9,90-xanthene] based organic hole transport material, which is the first example of an easily synthesized spiro-structured hole conductor, was designed and employed for highly efficient solid state p-type DSCs by B. Xu et al.. Solar devices based on such hole transport material showed an best efficiency of 7.30% under 100 mW•cm -2 AM1.5G solar illumination. Furthermore, the simple synthesis of spiro[fluorene-9,90-xanthene] from cheap commercially available starting material paved the way for the large-scale industrial production of solid state p-type DSCs in future. 457 The copper complexes as redox mediators in DSCs were first reported by Fukuzumi et al. which showed an efficiency of 2.2%. 458 Bis(2,9-dimethyl-1,10-phenanthroline)-copper(I/Ⅱ) (Cu(dmp)2) in the solid phase were reported to replace the typical I -/I3 -electrolyte and employed as efficient molecular hole transport material. This Copper phenanthroline complex-based solar cell device showed the best efficiency of 8.2% in all Cu-mediated DSCs. 459 Some other Cu-mediated hole transport Ⅰ.3.6.3.5. Quasi Solid State Electrolytes

  CuSCN film.441 A seminal publication was then reported by J.He et al., in which p-type nanostructured NiO film was prepared by depositing nickel hydroxide slurry on conducting glass as a photocathode for DSCs. The highest IPCE of solar cell devices based on tetrakis(4-carboxyphenyl)porphyrin (TPPC) and erythrosin B-coated NiO films were 0.24% and 3.44%. The valence band of NiO was approximately 0.54 V vs. NHE at pH = 7.0. 442Another publication was subsequently reported by the same group in 2001 in which nanostructured NiO thin film was further investigated and the conclusions were given that nanostructured NiO film was optically transparent with a band gap of ~3.55 eV. In this study, the nanostructured NiO photocathodes were obtained by heating Ni(OH)2 sol-gel films at a high temperature of 300-320 ℃.443

	to adsorb thiocyanated cationic dyes and show high photoresponse in aqueous KCNS. 440 In
	1995, a method for the preparation of thin polycrystalline films of cuprous thiocyanate onto
	ITO glass substrates by electrochemical deposition was reported by K. Tennakone et al.. In
	this study, CuSCN was employed as a p-type semiconductor which was sensitized with SCN -

Compared to classical n-type DSCs, the field of p-type DSCs is much younger, with far fewer studies being reported: less than one hundred publications dealing with this type of cells since 2012, and before 2005 only 2 publications referring to DSCs based on the sensitization of p-type metal oxide.

423

However, scientific interest has been rapidly growing over the past decade.

The earliest study on dye-sensitization of p-type semiconductors can date back to 1984. K. Tennakone et al. reported cuprous thiocyanate as a p-type semiconductor, which was found salts of cationic dyes and the typical I -/I3 -redox couple was used. Illuminated by a 60 W tungsten filament lamp, the constructed photoelectrochemical cell device displayed low photocurrents of approximate 100 μA•cm -2 which was attributed to the low porosity of the

  Gibson et al. presented a three-fold increase in Voc (0.35 V) while maintaining a Jsc of 1.7 mA•cm -2 for a p-type NiO dye-sensitized solar cell device. Contrary to

	earlier publications, a novel electrolyte composed of Co(Ⅱ/Ⅲ) tris(4,4'-di-tert-butyl-2,2'-
	dipyridyl) perchlorate (0.1 M/ 0.1 M) and LiClO4 (0.1 M) in propylene carbonate was
	employed to replace the typical I -/I3 -electrolyte. The efficiency of NiO-based p-type DSCs
	based on the novel electrolyte and a carefully designed "dyad" dye sensitizer was up to
	0.20%. 454

  onto FTO glass substrates. These NiOx semiconductor films exhibited anodic electrochromism, changing color from transparent white-yellowish to dark brown.461 Through spin-coating, dipping and electrochemically, F. Vera et al. fabricated nickel oxide (NiO) thin films onto ITO glass substrates. The NiO grown onto ITO glass substrates by dipping method exhibited excellent morphological properties. The semiconductor properties of these nanostructured NiO thin films showed p-type behaviors characterized by photoelectrochemical and optical experiments.462 A nanostructured NiO photoelectrode was also synthesized through initial deposition of layered basic nickel acetate having a yarn-ball-shaped morphology and subsequent heat-treatment at 500 ℃ in oxygen, which was reported by Y.Mizoguchi and S. Fujihara in 2008. 463 Porous and nanostructured NiO thin films can be fabricated onto FTO glass substrates by annealing nickel acetate films obtained by recrystallization under hydrothermal conditions in the presence of hexamethylenetetramine.

Velevska et al.

reported their methods to prepare nickel oxide (NiOx) thin films through a low vacuum evaporation

The excellent reproducibility of this method offered a reliable strategy to explore various dye sensitizers and other parameters to improve the power conversion efficiency of p-DSCs.

464

  Wu et al. designed and prepared a stable sensitizer-catalyst dyad assembly, in which the Ru polypyridyl sensitizer acted as a molecular bridge between the alumina coated NiO film and the cobaloxime H2-evolving catalyst CodmgBF2.481 On one side of the dye molecule, a phenylpyridyl ligand with a carboxylic acid anchoring group was designed to be covalently attached onto the surface of the nanostructured NiO film to facilitate the hole injection to NiO. On the other side of the dye molecule, a vinylpyridyl substituent at the 4,4'-bipyridine ligand was designed to axially coordinate to the cobalt metal center of CodmgBF2. The oxidation potential of the bifunctional cyclometalated Ru sensitizer was from the dye to NiO as well as electron transfer from the dye to CodmgBF2 are thermodynamically favorable. Irradiation with a 300 W Xe lamp on the sensitizer-catalyst dyad photocathode based on the bifunctional cyclometalated ruthenium sensitizer and CodmgBF2 catalyst in a neutral aqueous solution, led to a stable photocurrent density of 9 μA•cm -2 under an applied bias of 0.1 V for 1h. Hydrogen generation was also measured through gas chromatography to exhibit a Faradaic efficiency of 68% under an applied bias of 0.1 V for 2.5 h. In 2014, K.Fan et al. in 2014 reported a Pt-free tandem molecular DSPEC device, for the first time, employing molecular ruthenium-and cobalt-catalysts with strong dipicolinic acid anchoring groups on the respective photoanode (TiO2 was used) and photocathode (NiO was used) for total water splitting.482 Before this work, all the dye-sensitized photoanodes reported employed passive Pt cathodes to fabricate DSPEC.

investigated to be 0.6 V vs. NHE, which was more positive than the valence band of NiO (0.54 V). The first reduction potential of the dye sensitizer was investigated to be -1.36 V vs. NHE, which was more negative than the redox potential of CodmgBF2 (-0.46 V). Therefore, hole injection Therefore, their study paved the way for the Pt-free tandem molecular DSPEC for light-driven total-water splitting. A novel dye-sensitized p-type NiO photocathode with a hexaphosphonated Ru(2,2'-bipyridine)3 based dye and a tetraphosphonated molecular [Ni(P2N2)2] 2+ type proton reduction catalyst (NiP) for light-driven H + reduction to H2 was reported by E.

Reisner et al. in 2016. 483

They employed a layer-by-layer deposition method

Table Ⅱ - 1 :

 Ⅱ1 Optical and electrochemical properties.

	Dye λabs@TiO2 [Ru]1 λabs a / nm ε / M -1 .cm -1 239, 343, 94 200, 39 800, 550	+ 0.70	1.96	-1.26
	545		20 000
	240,	340,	84 000, 41 800,
	[Ru]2		554	+ 0.73	1.82	-1.09
	564		18 200
	238,	340,	94 000, 42 000,
	[Ru]3
	448,	650	15 200, 33 000

b / nm Eox c / V ΔEopt d / eV Eox -ΔEopt e / V

Table Ⅱ - 2 :

 Ⅱ2 Theoretical data. Dye Ege /eV a ge /nm b fge c Transition assignment (coefficient) d  e qCT / e f DCT / Å g

	[Ru]1 2.04	605	0.73	HL (0.63); H-2L (0.22)	0.58 0.91	3.2
	3.47	357	0.90	HL+1 (0.59); H-1L+1 (0.21); H-2L+1 (0.21)
	4.08	304	0.82	HL+4 ( 0.51); H-1L+1 ( 0.24)
	[Ru]2 1.93	642	1.14	HL (0.62); H-2L (0.20)	0.58 0.90	3.8
	2.76	448	0.33	HL+1 ( 0.53); HL (-0.23); H-2L+1 (0.22)
	2.81	441	0.31	H-3L ( 0.48); HL+1 (0.28); H-2L (-0.25)
	4.07	304	1.39	HL+4 ( 0.39); HL+6 ( 0.29)
	[Ru]3 1.73	713	1.51	HL (0.64); H-1L (0.22)	0.60 0.84	3.8
	2.58	481	0.13	H-2L (0.44); HL+1 (-0.37)
	2.64	469	0.46	H-1L+1 (0.27); HL+1 (0.58)
	4.07	304	0.88	HL+3 (-0.28); HL+5 (0.29)

Table Ⅱ -

 Ⅱ

	Dye	J SC / mA.cm	-2	V OC /mV	ff /%	PCE/%	DLA (mol.cm -2 )
	[Ru]1	7.13		577	75.6	3.11	1.40 x 10 -7
	[Ru]2	13.48		650	73.7	6.45	1.94 x 10 -7
	[Ru]3	11.77		598	74.4	5.23	2.12 x 10 -7
	N3	17.42		680	71.4	8.46	-

3: Photovoltaic performance of DSCs using [Ru]1-[Ru]3 and N3 dyes. Dye-loading amount on TiO2.

Table Ⅲ -

 Ⅲ

1: Optical and electrochemical properties Dye λmax a

Table Ⅲ -2: Theoretical data

 Ⅲ

	Dye	ΔEge a (eV)	λge b (nm)	ƒge c	Transition assignment (coefficient) d	Λ e qCT/e ƒ DCT/Å g
	SL1 2.276	544	1.842	H→L (0.62); H-1→L (-0.26)	0.59	0.88	4.7
		3.359	369	0.580	H→L+1 (-0.22); H-1→L+1 (0.56);
					H-2→L+1 (-0.21)
	SL2 2.024	612	1.968	H→L (0.62); H-1→L (0-0.20)	0.58	0.90	5.1
		3.371	367	0.601	H→L+1 (0.46); H-1→L+1 (-0.39);
					H-2→L+1 (0.23); H-3→L+1 (-0.26)

Table Ⅲ - 5 :

 Ⅲ5 Photovoltaic characteristics of the dyes SL3 and SL4 in p-DSC recorded under simulated solar spectrum (AM 1.5). Voc) (see TableⅢ-5). However, the efficiencies of the p-DSCs based on SL3 and SL4 are lower than those based on the reference dye SL1, mainly due to much lower photocurrent density values (Jsc). This seems to indicate that two COOH anchoring groups (SL1) are probably better than only one (SL3 and SL4) to get a higher dye-loading amount and charge injection yield. It is worth to mention that only small intensity of coloration of the NiO-based electrode was observed after dye-bath treatment with SL3 and SL4, compared to

	Dye	Jsc (mA•cm -2 )	Voc (mV)	FF (%)	PCE (%)
	SL1	2.50	112.1	34.4	0.096
	SL3	1.28	97.6	31.1	0.039
	SL4	1.22	89.7	30.2	0.033
	The photovoltaic performances of SL1 are similar to those in section Ⅲ.3.4. meaning
	that the data can be directly compared. The dye SL3 showed slightly better photovoltaic
	performances compared to the dye SL4, which produces notably larger open-circuit
	photovoltage (SL1.

Table Ⅲ - 6 :

 Ⅲ6 Optical and electrochemical properties of SL5.

	Dye	λmax a (nm)	ɛ (M -1 cm -1 )	Eox1 (V vs. FeCp*) b	Eox2 (V vs. FeCp*) b	Eox1 (V vs. NHE) b	Eox2 (V vs. NHE) b
	SL1	364 37 500 521 20 300	-	-	+0.64	+1.05
	SL5	424 35 100 490 34 400	+0.52	+1.18	+0.65	+1.32

Table Ⅲ - 7 :

 Ⅲ7 Photovoltaic characteristics of the dyes SL1, SL3, SL4 and SL5 in p-DSCs recorded under simulated solar spectrum (AM 1.5).

	Dye	Electrolyte	Jsc (mA•cm -2 )	Voc (mV)	FF (%)	PCE (%)
	SL1	I3 -/I -	2.50	112	34	0.096
	SL3	I3 -/I -	1.28	98	31	0.039
	SL4	I3 -/I -	1.22	90	30	0.033
	SL5	I3 -/I -	1.59	92	32	0.046
	SL5	T2/T -	2.40	149	35	0.125

Table Ⅳ - 1 :

 Ⅳ1 Optical and electrochemical properties. onset of the absorption spectra.b Reduction potential in V vs. Fc + /Fc. c Estimated LUMO level in V vs. NHE, obtained from the reduction potential considering E°(Fc+/Fc) = +0.53 V vs. NHE.37,46 d Estimated LUMO energy in eV, considering E°(Fc+/Fc) = -5.1 eV. 47 e Estimated HOMO level in V vs. NHE, obtained from ELUMO+E0-0 ƒ Estimated HOMO energy in eV, obtained from ELUMO -

	Dye	λmax (nm)	Ɛ (M -1 cm -1 )	E0-0 a eV	ERed b V	ELUMO c / V vs. NHE	ELUMO d eV	EHOMO e /V vs. NHE	EHOMO ƒ eV
	SL6	340 602	20 100 12 500	1.58 -0.99	-0.46	-4.11	+1.12	-5.69
	SL7	350 598	41 000 11 500	1.53 -0.89	-0.36	-4.21	+1.17	-5.74
	Absorption spectra and cyclic voltammograms were recorded in CH2Cl2. a 0-0 transition energy
	estimated from the E0-0.

Table Ⅳ - 2 :

 Ⅳ2 DFT and TD-DFT calculated electronic properties in CH2Cl2. ΔEcalc is the main transition energy. b λcalc is the calculated λmax. c f is the oscillator strength. d Only those transitions with coefficients higher than 0.15 are given. e EHOMO calc. is the calculated energy of the HOMO.

	Dye	ΔEcalc a / eV	λcalc b / nm	ƒ c	Transition assignment d	qCT / e	DCT / Å	EHOMO calc. e / eV
	[Ru]1	1.83 2.93	676 423	0.005 0.841	HOMO → LUMO ; HOMO-2 → LUMO HOMO-1 → LUMO	0.961 1.98	-5.66
	[Ru]2	1.80 2.93	687 423	0.005 0.856	HOMO → LUMO HOMO-1 → LUMO ; HOMO-2 → LUMO	0.965 2.06	-5.39

a

DFT and TD-DFT calculation parameters relative to the main photoinduced transitions are summarized in Table Ⅳ-2. In accordance with the experiment, two main transitions are predicted for SL6 and SL7 in the 300-800 nm region. Table Ⅳ-3 shows TD-DFT validation test of different exchange-correlation density functional models for predicting the vertical excitation energies of SL6 dye in dichloromethane. The maximum wavelengths and oscillator factors calculated for the two main transitions are consistent with the experimental spectra;

Table Ⅳ - 3 :

 Ⅳ3 TD-DFT validation test of different exchange-correlation density functional models for predicting the vertical excitation energies of SL6 dye in dichloromethane.

	SL6 (a)	1 st excitation (nm)	2 nd excitation (nm)
	B3LYP	861	550
	PBE0	820	525
	mPW1PBE	818	524
	mPW1PW91	818	524
	M06-2X	753	455
	CAM-B3LYP	707	448
	ωB97X	696	441
	LC-ωPBE	676	423
	Exp.	602	340

Table Ⅳ - 4 :

 Ⅳ4 Optical and electrochemical properties.

		λmax (nm)	(M	Ɛ -1 •cm	-1 )	E0-0 (eV)	Fc + /Fc) b	Eox (V vs. Fc + /Fc) b	Ered (V vs. NHE) c	Eox (V vs. NHE) c	E +/* (V vs. NHE) d
	SL8	463 356	11 870 26 055	2.25	-	+ 0.02	-	+ 0.75	-1.50
	SL8-Co	463 355	12 230 24 836	2.25	-1.05	-0.02	-0.32	+ 0.71	-1.54

a Ered (V vs.

Table Ⅳ - 5 :

 Ⅳ5 TD-DFT calculated electronic properties (CAM-B3LYP/SDD/6-31G(d)/PCM=THF/H2O)

		Ecalc /eV a	/ nm b λcalc	f c	Electronic transition assignment d
	SL8	2.79	445	2.023	HOMO  LUMO (72%)
	(THF)	3.74	332	0.589	HOMO-1  LUMO+1 (37%)
	SL8	2.80	443	2.013	HOMO  LUMO (71%)
	(H2O)	3.74	331	0.576	HOMO-1  LUMO+1 (36%)
	SL8-Co	2.73	455	2.180	HOMO  LUMO (68%)
	(H2O)	3.76	330	0.584	HOMO-1  LUMO+3 (35%)
	SL8-CoH	2.67	465	2.235	HOMO  LUMO (66%)
	(H2O)	3.76	330	0.585	HOMO  LUMO+4 (37%)

a ΔEcalc = main transition energy. b λcalc = calculated λmax. c ƒ = oscillator strength. d Main Kohn-Sham orbital contribution to the electronic transition.

  . CI-HRMS (m/z): 381.1096[M-H] + (calcd. 381.1093 for C20H21N2O2SSi). mixture was refluxed for 48h. The organics were further washed with degassed water and evaporated to dryness. Precipitation from a CH2Cl2/pentane mixture afforded pure 14 as a purple solid in 72 % yield (150 mg, 0.094 mmol). 31 P NMR (120 MHz, CDCl3): δ = 53.7.1 H NMR (300 MHz, CDCl3): δ = 8.30 (s, 1H), 8.25 (d, 2H, 8.16 (d, 4H, 3 JH-H =

  , -0.2. reaction mixture was washed with saturated aqueous NH4Cl solution and water, and then dried over MgSO4 and evaporated to dryness. The crude product was purified on silica gel column (CH2Cl2/pentane (1:1, v/v) to CH2Cl2) to obtain 33 as a pale yellow oil in 72 % yield (2.42 g, 9.22 mmol).1 H NMR (CDCl3, 300 MHz): 8.00 (d, 2H3 JH-H = 12 Hz), 7.55 (d, 2H, -diisopropylethylamine (4.21 mL, 24.2 mmol, 5 equiv.) were added subsequently. The solution was stirred at RT for 24 h. After dilution with CH2Cl2 (20 mL), the reaction mixture was washed with saturated aqueous NH4Cl solution and water, and then dried over MgSO4 and evaporated to dryness. The crude product was purified on silica gel column (CH2Cl2/pentane (1 : 1, v/v) to CH2Cl2) to obtain 35 as a pale yellow powder in 77% yield(1.14 g, 3.72 mmol).

	3 JH-H=12), 4.42 (m, 2H), 3.22 (s, 1H), 1.13 (m, 2H), 0.07 (s, 9H).
	Synthesis of 35. 5-Bromo-2-thiophenecarboxylic acid (1.0 g, 4.8 mmol, 1 equiv.) was
	dissolved in dry DMF (15 mL) under nitrogen atmosphere, and HBTU (2.20 g, 5.8 mmol, 1.2
	equiv.),	2-(trimethylsilyl)ethanol	(0.97	mL,	6.7	mmol,	1.4	equiv.)	and
	N,N

  Precipitation from a CH2Cl2/pentane mixture afforded pure [46][TfO] as a red powder in 92% yield (1.30 g, 0.92 mmol). 31 P NMR (120 MHz, CD2Cl2): δ = 39.1 (s, PPh2).1 H NMR (300 MHz, CD2Cl2): δ = 7.79 (s, 1H),7.42-6.77 (m, 45H), 3.92 (s, 3H), 3.81 (s, 3H), 2.87 (m, 8H). HR-MS APCI + (m/z): 1264.1550 [M-H] + (calcd. 1264.531 for [C68H59ClO4P4RuS2] + ).

Synthesis of 47. To a solution of [46]

[TfO] (0.51 g, 0.36 mmol, 1 equiv.), compound 45

MeOH; (g) 3-methylrodhanine, ammonium acetate, acetic acid.

Acknowledgements

This work has been performed in Molecular Chemistry and Materials group, Institute of Molecular Sciences, University of Bordeaux. Here, I hope to express my sincere gratitude to some organizations and a number of people, without whom it would have not been possible for me to undertake such a novel and huge academic programme during my four years in France. First of all, I sincerely thank "China Scholarship Council" for providing me financial support. I could have the opportunity to study abroad and finally get my doctor degree because of the strong support of my country. I will keep in mind the ardent expectations of my country and take them as an encouragement to keep working hard so as to serve the Motherland. I would like to thank my thesis supervisors Prof. Dr. Thierry Toupance and Dr. Céline Olivier for their continuous support, encouragement and insightful guidance during my doctoral work in the last four years. I am very lucky and happy to be able to work with them. I sincere thank Dr. Thierry Toupance and Dr. Céline Olivier for their reasoning and constructive criticisms that motivated me to delve deeper into my research. When I made mistakes in my work, they express their precious understanding and patience. I will never forget their unconditional support in and out of the lab to facilitate my stay in France. This work is the result of a teamwork and collaboration of many people without whom this work could not have any real chance to execute well. I would like to thank our colleagues who helped me to characterize my samples: Samuel De Sousa, Laurent Ducasse, Yoann Farré, Yann Pellegrin, Fabrice Odobel, Tomofumi Hamamura, Julien Massin, Michele Pavone, Ana B. Muñoz-García, Brice Kauffmann, Murielle Chavarot-Kerlidou, Vincent Artero. I am very happy in my last four years because I met many good colleagues in our lab. I

Chapter Ⅱ: Ruthenium-Diacetylide Organometallic Complexes for n-Type Dye Sensitizers

This methodology afforded surface concentration estimations of 21.5 ± 0.8 and 28.7 ± 0.8 nmol•cm -2 for SL6 and SL7, respectively (average values determined for 3 samples). The variation in the grafting density between the two dyes likely reflects the fact that SL7

possesses two carboxylate anchoring groups whereas SL6 has only one.

Ⅳ.3.6. Photoelectrochemical Properties of Dye-Sensitized NiO Films

The photoelectrochemical properties of dye-sensitized NiO films were subsequently investigated in the presence of [Co(NH3)5Cl]Cl2 (20 mM) acting as an irreversible electron acceptor (IEA) in solution. 37,55 The substrates were used as the working electrode in a three-electrode configuration. Potassium phosphate buffer (KPi, pH 7; 0.1 M) was used as an electrolyte and linear sweep voltammograms (LSV) were recorded under chopped-irradiation conditions (400-800 nm filtered Xe lamp light; 100 mW•cm -2 ) corresponding to a visible fraction of 1.5 sun (Figure Ⅳ-10a). A photocurrent was established with the onset at +0.78 V vs. NHE. Such a behaviour was directly related to the presence of SL6 or SL7 at the surface of the films since non-sensitized NiO films show very little photocurrent under the same conditions. This was attributed to the establishment of photoinduced electron transfers from NiO to IEA, mediated by the excited dyes. Maximum photocurrent densities (~40 μA•cm -2 for SL6 and ~60 μA•cm -2 for SL7) were obtained from +0.20 V vs. NHE (0 V vs.

Ag/AgCl). 56 Therefore this potential was applied for the whole series of experiments described below.

Under such conditions, NiO electrodes sensitized with SL7 display a significantly higher photocurrent density (58 μA•cm -2 ) than the same electrodes sensitized with SL6 (43 μA•cm -2 ) Chronoamperometric measurements were performed with the photocathodes SL8-Co@NiO under chopped-light at different applied potentials (+0.2 V, 0.0 V, -0.2 V vs.

NHE) over a period of 10 minutes showing steady-state cathodic photocurrent generation (Figure Ⅳ-22). The maximum magnitude was obtained at the most cathodic potential, i.e.

-0.2 V vs. NHE (-0.4 V vs. Ag/AgCl), with a photocurrent density of ca. Control experiment using a NiO-electrode sensitized with SL8 only confirmed steady-state photocurrent generation however with a slightly lower magnitude, ca. 2.4 µA•cm -2 at -0.2 V vs. NHE, due to electron transfers from NiO to the excited dye. Long-term electrolysis over a period of 5h30 led to a cathodic charge of Q = 144 mC and an amount of H2 close to zero was detected in the headspace with a faradaic yield of 0.3%. And thus, this confirms that the photocatalytic system SL8-Co is the active specie for H2 evolution in our NiO-based DSPEC.

Chapter Ⅵ: Experimental Part Synthesis of 7. Compound 6 (200 mg, 0.594 mmol) and K2CO3 (82 mg, 0.59 mmol)

were dissolved in MeOH (150 mL). The mixture was stirred for 2h at RT. The reaction mixture was diluted with H2O and the product was extracted with CH2Cl2. The organic phase was dried with MgSO4 and evaporated to dryness to afford 7 as a red solid in 99 % yield (155 mg, 0.58 mmol). 1 H NMR (300 MHz, CDCl3): δ 10.11 (1H, s), 8.12 (2H, m), 8.05 (2H, m), 7.91 (1H, m), 7.76 (1H, d, 3 JH-H = 7.4 Hz), 3.67 (1H, s). 13 C NMR (300 MHz,CDCl3): 191.9,155.6,153.6,142.7,134.4,133.9,132.6,130.1,130.1,128.3,116.0,84.6,79.3 JH-H = 7.5 Hz), 7.87 (1H, d, 3 JH-H = 6.9 Hz), 7.86 (1H, d, 3 JH-H = 4.1 Hz), 3.71 (1H, s). 13 C NMR (300 MHz, CDCl3): δ 183. 4, 155.3, 152.2, 147.9, 144.3, 137.5, 133.8, 130.0, 126.5, 126.0, 117.6, 84.2, 80 13 C NMR (100 MHz, CD2Cl2): δ = 166. 8, 157.5, 153.9, 153.0, 142.9, 141.5, 137.6, 137.2, 134.9, 134.8, 134.5, 134.4, 133.8, 132.8, 131.5, 131.3, 130.1, 129.9, 129.7, 129.6, 129.3, 129.2, 128.8, 127.5, 127.3, 126.6, 126.4, 126.3, 126.2, 123.9, 123.5, 120.5, 120.0, 118.0, 117.4, 110.3, 63.5, 31.9, 17.7, -1.3 (m, 4H). 13 C NMR (100 MHz, CD2Cl2): δ = 183. 3, 159.9, 157.3, 152.8, 150.8, 142.4, 141.5, 137.7, 137.6, 137.1, 134.9, 134.3, 133.2, 133.0, 131.4, 130.1, 129.5, 129.0, 128.7, 127.7, 127.4, 126.7, 126.3, 125.1, 123.6, 120.6, 120.1, 119.7, 119.4, 118.7, 110.4, 32.0 8, 162.3, 157.3, 152.9, 151.6, 146.7, 141.6, 139.7, 137.6, 137.1, 135.2, 135.0, 134.3, 133.1, 131.5, 130.2, 129.8, 129.6, 129.1, 127.8, 127.5, 126.8, 126.3, 125.4, 123.7, 120.7, 120.1, 119.5, 119.2, 117.1, 110.5, 97.6, 65.2, 32.1, 18.2 7.90 (m, 8H), 7.49 (d, 2H, 3 JH-H = 8.2 Hz), 7.44-7.38 (m, 12H), 7.28-7.25 (m, 6H), 7.49 (d, 2H, 3 JH-H = 8.3 Hz), 7.13-7.05 (m, 12H), 6.79 (m, 8H), 6.36 (d, 1H, 3 JH-H = 7.7 Hz), 3.12 (m, 4H), 2.81 (m, 4H). 13 C NMR (200 MHz, THF-d8): δ = 164. 9, 162.4, 158.4, 153.8, 151.6, 146.9, 142.5, 140.0, 138.9, 138.1, 136.6, 136.1, 135.3, 134.3, 134.0, 132.5, 131.2, 131.1, 130.4, 129.9, 129.7, 128.7, 128.4, 127.8, 127.6, 127.1, 126.1, 124.9, 121.5, 121.0, 120.8, 120.2, 117.6, 111.2, 100.0, 33.

Ⅵ.2.2. Synthetic Routes to the Dyes SL1 and SL2

Synthesis of 17. Methyl 4-bromobenzote (4.30 g, 20 mmol, 4 equiv.), cesium carbonate (4.89 g, 15 mmol, 3 equiv.), palladium(Ⅱ) acetate (67 mg, 0.3 mmol, 6%) and aniline (0.45 mL, 5.0 mmol, 1 equiv.) were sequentially introduced in a Schlenk flask under nitrogen. Then dry toluene (50 mL) and tri-tert-butyl-phosphine (0.14 mL, 0.6 mmol, 12%) were added sequentially. The suspension was heated at 90 ℃ for 3 days. After that, the reaction mixture was diluted with CHCl3 and filtered. The resulting solution was evaporated to dryness. The crude product was purified on silica gel column (pentane/ethyl acetate (9 : 1, v/v)) to obtain 17 as a pale yellow powder in 63% yield (1.14 g, 3.15 mmol). 1 H NMR (300 MHz, CDCl3): δ = 7.90 (d, 4H, 3 JH-H = 9.0 Hz), 7.35 (t, 2H, 3 JH-H = 15.0 Hz), 7.22-7.14 (m, 3H), 7.08 (d, 4H, 3 JH-H = 9.0 Hz), 3.89 (s, 6H). 13 C NMR (75 MHz, CDCl3): δ = 166. 7, 151.1, 146.1, 131.1, 129.9, 126.6, 125.5, 124.2, 122.5, 52.1. and denoted below as Ag/AgCl. The counter electrode was a Pt wire. The supporting electrolyte was 0.1 M nBu4NPF6 in dry DMF. The supporting electrolyte was degassed with a flow of N2 at least for 5 min before the measurements. The N2 flow was removed from the solution but let in the headspace of the cell for the duration of the experiment. The concentration of the compound of interest was 1 mM. Cyclic voltammograms were typically recorded at a scan rate of 100 mV•s -1 . The potential of the reference electrode was calibrated after each experiment by adding in the supporting electrolyte an internal reference (ferrocene for organic solvent, K4FeCN6 for aqueous solution), the potential of which was measured against the Ag/AgCl reference (0.43V for ferrocene and 0.24V for K4FeCN6).

Synthesis

Irradiation was carried out with a 300W ozone-free Xe lamp (Newport) operated at 280

W and mounted with a water-filled Spectra-Physics 6123NS liquid filter for elimination of IR (λ > 800 nm) irradiation and a Spectra-Physics 59472 UV cut-off filter (λ < 400 nm). The power density was calibrated using a Newport PM1918-R power-meter. The photocurrent measurements were carried out in a specifically designed three-electrode cell, using the NiO-sensitized film as the working electrode (1.2 to 2 cm 2 ), Ag/AgCl as the reference