<p>Recently, materials such as hole conductors (CuI, CuSCN) and light absorbers (Ru-complexes, CuInS₂) have been actively investigated for application in nanocrystalline solar cells. In this thesis combinations of these materials have been studied.</p><p>In the first part of the thesis, various methods were applied to characterize the electronic structure and photoconversion mechanism of the dye molecule Ru(dcbpyH₂)₂(NCS)₂ when combined with materials for the use in photovoltaic devices. Specifically, the adsorption and electronic structure of the dye molecules adsorbed to semiconductors were investigated by means of photoelectron spectroscopy. The results indicate a chemical bond between the dye molecules and the hole conductors (CuI, CuSCN) via the NCS- groups. In addition, preparation of a TiO₂/Ru-dye/CuI solid state model system was studied <i>in situ</i>. These experiments showed a partial breaking of the TiO₂-dye bond caused by CuI evaporation. Photovoltage measurements were also performed. These investigations showed a shift in the light absorption threshold of the dye molecules adsorbed onto the hole conductors (CuI and CuSCN), indicating new defect states at the dye/CuSCN interface. Also, charge accumulation and transport in solar cells with CuSCN were compared to liquid electrolyte cells. Measurements showed that the lifetime and transport time of electrons in solar cells with CuSCN are much shorter than in electrolyte cells.</p><p>In the second part of the thesis, the deposition of CuInS₂ onto various metal oxides by spray pyrolysis has been studied with x-ray diffraction and photoelectron spectroscopy. The measurements showed that the morphologies of the substrates play a significant role in the formation of CuInS₂ layers. Also, the presence of CdS at TiO₂ has a positive influence on the formation of CuInS₂.</p>

Ink-jet printing offers an ideal answer to the emerging trends and demands of depositing at ambient temperatures picoliter droplets of oxide solutions into functional thin films and device components with a high degree of pixel precision. It is a direct single-step mask-free patterning technique that enables multi-layer and 3D patterning. This method is fast, simple, easily scalable, precise, inexpensive and cost effective compared to any of other methods available for the realization of the promise of flexible, and/or stretchable electronics of the future on virtually any type of substrate. Because low temperatures are used and no aggressive chemicals are required for ink preparation, ink-jet technique is compatible with a very broad range of functional materials like polymers, proteins and even live cells, which can be used to fabricate inorganic/organic/bio hybrids, bio-sensors and lab-on-chip architectures. After a discussion of the essentials of ink-jet technology, this thesis focuses particularly on the art of designing long term stable inks for fabricating thin films and devices especially oxide functional components for electronics, solar energy conversion, opto-electronics and spintronics. We have investigated three classes of inks: nanoparticle suspension based, surface modified nanoparticles based, and direct precursor solution based. Examples of the films produced using these inks and their functional properties are: 1) In order to obtain magnetite nanoparticles with high magnetic moment and narrow size distribution in suspensions for medical diagnostics, we have developed a rapid mixing technique and produced nanoparticles with moments close to theoretical values (APL 2011 and Nanotechnology 2012). The suspensions produced have been tailored to be stable over a long period of time. 2)In order to design photonic band gaps, suspensions of spherical SiO2 particles were produced by chemical hydrolysis (JAP 2010 and JNP 2011 - not discussed in the thesis). 3) Using suspension inks, (ZnO)1-x(TiO2)x composite films have been printed and used to fabricate dye sensitized solar cells (JMR 2012). The thickness and the composition of the films can be easily tailored in the inkjet printing process. Consequently, the solar cell performance is optimized. We find that adding Ag nanoparticles improves the ‘metal-bridge’ between the TiO2 grains while maintaining the desired porous structure in the films. The photoluminescence spectra show that adding Ag reduces the emission intensity by a factor of two. This indicates that Ag atoms act as traps to capture electrons and inhibit recombination of electron-hole pairs, which is desirable for photo-voltaic applications. 4) To obtain and study room temperature contamination free ferromagnetic spintronic materials, defect induced and Fe doped MgO and ZnO were synthesized ‘in-situ’ by precursor solution technique (preprints). It is found that the origin of magnetism in these materials (APL 2012 and MRS 2012) is intrinsic and probably due to charge transfer hole doping. 5) ITO thin films were fabricated via inkjet printing directly from liquid precursors. The films are highly transparent (transparency &gt;90% both in the visible and IR range, which is rather unique as compared to any other film growth technique) and conductive (resistivity can be ~0.03 Ω•cm). The films have nano-porous structure, which is an added bonus from ink jetting that makes such films applicable for a broad range of applications. One example is in implantable biomedical components and lab-on-chip architectures where high transparency of the well conductive ITO electrodes makes them easily compatible with the use of quantum dots and fluorescent dyes. In summary, the inkjet patterning technique is incredibly versatile and applicable for a multitude of metal and oxide deposition and patterning. Especially in the case of using acetate solutions as inks (a method demonstrated for the first time by our group), the oxide films can be prepared ‘in-situ’ by direct patterning on the substrate without any prior synthesis stages, and the fabricated films are stoichiometric, uniform and smooth. This technique will most certainly continue to be a versatile tool in industrial manufacturing processes for material deposition in the future, as well as a unique fabrication tool for tailorable functional components and devices. / <p>QC 20120907</p>

The interactions between the molecular constituents in dye-sensitized solar cells were studied with UV-VIS and IR spectroscopy, Raman scattering, conductivity and electron accumulation measurements. From stability studies of the dye, bis(tetrabutylammonium)cis-bis(thiocyanato) bis(2,2’-bipyridine-4-carboxylic acid, 4’-carboxylate) ruthenium(II), in the complete solar cell, the thiocyanate ion ligand was found to be lost from the dye. A method was developed to study mechanisms in a sealed dye-sensitized solar cell using resonance Raman scattering (RRS). RRS studies of a complete dye-sensitized solar cell including iodine and lithium iodide in the electrolyte indicate that triiodide exchange the SCN- ligand of the dye. It was proposed that an ion pair Li+…I3- formation occurred, which, by a reduced electrostatic repulsion between I3- and SCN- facilitated the exchange of these anions at Ru(II) of the dye. The additive 1-methylbenzimidazole suppressed the SCN-/I3- ligand exchange by forming a complex with Li+. In order to study charge transport in nanostructured TiO2 films permeated with electrolyte, a technique was developed for determining activation energies of conduction, electron accumulation and effective mobility. Two regions were distinguished from the relation between conductivity and electron concentration. In the first region (~1-20 electrons per TiO2 particle), which resembles best the region where the nanostructured dye-sensitized solar cell operates, the results can be fitted to some extent with a trapping/detrapping or a hopping model for charge transport, but not with a conduction band model. For the second region (&gt; 20 electrons per TiO2 particle), charge transport by electrons in the conduction band seems to be the most applicable model. Through this work many effects from the interplay between the solar cell components were observed. These observations emphasize the importance of well-balanced interactions in dye-sensitized solar cells.

Recently, materials such as hole conductors (CuI, CuSCN) and light absorbers (Ru-complexes, CuInS2) have been actively investigated for application in nanocrystalline solar cells. In this thesis combinations of these materials have been studied. In the first part of the thesis, various methods were applied to characterize the electronic structure and photoconversion mechanism of the dye molecule Ru(dcbpyH2)2(NCS)2 when combined with materials for the use in photovoltaic devices. Specifically, the adsorption and electronic structure of the dye molecules adsorbed to semiconductors were investigated by means of photoelectron spectroscopy. The results indicate a chemical bond between the dye molecules and the hole conductors (CuI, CuSCN) via the NCS- groups. In addition, preparation of a TiO2/Ru-dye/CuI solid state model system was studied in situ. These experiments showed a partial breaking of the TiO2-dye bond caused by CuI evaporation. Photovoltage measurements were also performed. These investigations showed a shift in the light absorption threshold of the dye molecules adsorbed onto the hole conductors (CuI and CuSCN), indicating new defect states at the dye/CuSCN interface. Also, charge accumulation and transport in solar cells with CuSCN were compared to liquid electrolyte cells. Measurements showed that the lifetime and transport time of electrons in solar cells with CuSCN are much shorter than in electrolyte cells. In the second part of the thesis, the deposition of CuInS2 onto various metal oxides by spray pyrolysis has been studied with x-ray diffraction and photoelectron spectroscopy. The measurements showed that the morphologies of the substrates play a significant role in the formation of CuInS2 layers. Also, the presence of CdS at TiO2 has a positive influence on the formation of CuInS2.

Artificial systems involving water oxidation and solar cells are promising ways for the conversion of solar energy into fuels and electricity. These systems usually consist of a photosensitizer, an electron donor and / or an electron acceptor. This thesis deals with the synthesis and photoinduced electron transfer of several donor-sensitizer-acceptor supramolecular systems. The first part of this thesis describes the synthesis and properties of two novel dinuclear ruthenium complexes as electron donors to mimic the donor side reaction of Photosystem II. These two Ru2 complexes were then covalently linked to ruthenium trisbipyridine and the properties of the resulting trinuclear complexes were studied by cyclic voltammetry and transient absorption spectroscopy. The second part presents the synthesis and photoinduced electron transfer of covalently linked donor-sensitizer supramolecular systems in the presence of TiO2 as electron acceptors. Electron donors are tyrosine, phenol and their derivatives, and dinuclear ruthenium complexes. Intramolecular electron transfer from the donor to the oxidized sensitizer was observed by transient absorption spectroscopy after light excitation of the Ru(bpy)32+ moiety. The potential applications of Ru2-based electron donors in artificial systems for water oxidation and solar cells are discussed. In the final part, the photoinduced interfacial electron transfer in the systems based on carotenoids and TiO2 is studied. Carotenoids are shown to act as both sensitizers and electron donors, which could be used in artificial systems to mimic the electron transfer chain in natural photosynthesis.

In dye-sensitized solar cells, the functions of light absorption and charge transport are separated. A photosensitive ruthenium-polypyridine dye in the cell absorbs light, injects an electron to a semiconductor and is then regenerated by a redox couple, typically iodide/triiodide. Quantum chemical calculations of the electronic structure of triiodide have been carried out with the restricted active space SCF method, including spin-orbit coupling, and with density functional theory. It was shown that the difference in charge density between the terminal and central atoms results in a splitting of the core levels. The calculations gave a value of the splitting of 0.8 - 1.0 eV for the 3d and 4d levels. Experimentally, the electronic structure has been investigated with photoelectronspectroscopy. The measured terminal/center splitting is 1.1 eV.The spin-orbit interaction of the 4d levels of triiodide has also been calculated. The splitting was determined to be 1.6 eV. The experimental value is 1.7 eV. An assignment of the peaks in the computed spectrum of triiodide was made and the features of the experimental spectrum have beenidentied.The theoretical valence spectrum of triiodide has been computed and assigned. The results can be used in the analysis of photoelectron spectra of the molecule. Information about the electronic structure of the redox couple can help in the understanding of the electron transfer processes and forfurther development of the solar cells.  Furthermore, the solvation structure of the prototype dye, the tris(bipyridine)ruthenium(II) complex, in water and its interaction with iodide and chloride has been studied by means of molecular dynamics simulations. The trajectory analysis showed that the water molecules in the first solvation shell form a chain in between the bipyridine ligands. It was found that the iodide ions are more likely than chloride to enter between the ligands, which can be important for the electron transfer processin the solar cell.

This thesis deals with development and synthesis of organic chromophores for dye sensitized solar cells. The chromophores are divided into three components; donor, linker and acceptor. The development of efficient organic chromophores for dye sensitized solar cells starts off with one new organic chromophore, D5. This chromophore consists of a triphenylamine moiety as an electron donor, a conjugated linker with a thiophene moiety and cyanoacrylic acid as an electron acceptor and anchoring group. Alternating the donor, linker or acceptor moieties independently, would give us the tool to tune the HOMO and LUMO energy levels of the chromophores. The following parts of this thesis regard this development strategy. The contributions to the HOMO and LUMO energy levels were investigated when alternating the linker moiety. Unexpected effects of the solar cell performances when increasing the linker length were revealed, however. In addition, the effect of an alternative acceptor group, rhodanine-3-acetic acid, in combination with different linker lengths was investigated. The HOMO and LUMO energy level tuning was once again successful. Electron recombination from the semiconductor to the electrolyte is probably the cause of the poor efficiencies obtained for this series of dyes. Finally, the development of functionalized triphenylamine based donors and the contributions from different substituents to the HOMO and LUMO energy levels and as insulating layers were investigated. This strategy has so far been the most successful in terms of reaching high efficiencies in the solar cell. A top overall efficiency of 7.79 % was achieved. / QC 20100716

In this study, we investigated the effect of electrolyte composition, photoanode thickness, and the additions of GuSCN (guanidinium thiocyanate), NMB (N-methylbenimidazole), and SiO2 on the photovoltaic performance of DSSCs (dye sensitized solar cells). A bifacial DSSC is realized and irradiated from front and rear sides. The devices give maximum photovoltaic efficiencies for 70% PMII (1-propyl 3-ethylimidazoliumiodide)/30%(EMIB(CN)4)(1-ethyl-3-methyl-imidazolium tetracyano borate) electrolyte composition and 10 &mu / m thick photoanode coating which is considered to be the ideal coating thickness for the diffusion length of electrolyte and dye absorption. A significant increase in the photocurrent for DSSCs with optimum molarity of 0.1 M GuSCN was observed due to decreased recombination which is believed to be surface passivation effect at photoanode electrolyte interface suppressing recombination rate. Moreover, optimum NMB molarity was found to be 0.4 for maximum efficiency. Addition of SiO2 to the electrolyte both as an overlayer and dispersed particles enhanced rear side illuminated cells where dispersed particles are found to be more efficient for the front side illuminated cells due to additional electron transport properties. Best rear side illuminated cell efficiency was 3.2% compared to front side illuminated cell efficiency of 4.2% which is a promising result for future rear side dye sensitized solar cell applications where front side illumination is not possible like tandem structures and for cells working from both front and rear side illuminations.

The principles of dye-sensitized solar cells were studied and are outlined in this thesis. An overview of the basic steps needed to create a DSC isfollowed by detailed experimental information on how to assemble the solar cells that were fabricated in this project. They were based on synthesizednanocrystalline ZnO thin films sensitized by chlorophyll a as well as isolated photopigments from spinach leaves. The nanocrystals werestudied using XRD, and it was confirmed that three different methods of synthesis resulted in ZnO crystals of a few nanometers. The solar cellswere assembled with Au electrodes in a sandwich configuration and their photovoltaic properties were measured. Overall light-to-electricity conversionwas low with the highest efficiency being 0.21 %. An astonishingly low efficiency of 0.0003 % was noted for a thin film which was not thermallytreated, and it is suggested that heat-treatment is of great importance. It was also found that photopigments from spinach can be extractedeasily and used as molecular sensitizer without any demanding purification steps.

Dye-sensitized solar cells (DSSCs), based on use of a black counter electrode (BCE) and thin TiO2 electrode (photoelectrode), have been developed to reduce related manufacturing costs. Despite their effectiveness in lowering manufacturing cost, the above DSSCs have a low photovoltaic performance, owing to their insufficient light harvesting efficiency. This work presents a novel metal-based light scattering layer (MLSL), which can be formed either on a black counter electrode or on a thin TiO2 electrode, to reflect the light passing through the latter. The proposed MLSL increases the light harvesting efficiency from the interior of the cell, thus enhancing the photovoltaic performance of DSSC. Experimental results indicate that the proposed MLSL also reduces the internal resistance, as well as increases the electron collection efficiency of DSSC, subsequently increasing the power conversion efficiency by 116%. This work also designs a low-cost external lightguide (EL), which is disposed on the exterior of photoelectrode of DSSC, to direct light towards the dye-covered nanoporous TiO2 film (D-NTF) of the photoelectrode. Incorporating EL can increase the light harvesting efficiency from the exterior of the cell, thus enhancing the photovoltaic performance of DSSC. Furthermore, in addition to increasing the light harvesting efficiency by 30.69%, the proposed EL increases the photocurrent density by 38.12% and power conversion efficiency by 25.09%.