Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The initial charge transfer from dye molecules' excited states to the conduction band of a semiconductor, after absorption of visible light by the former, is critical to the performance of Dye sensitized Solar Cells (DSC). In a ZnO-based DSC sensitized by the organic indoline derivative D149, the dynamics associated with charge transfer are investigated with femtosecond transient absorption spectroscopy. The time-resolved measurement of the photo-initiated processes reveal electron transfer rates corresponding to excited state lifetimes of 100s of fs, consistent with previously measured high absorbed photon to current conversion efficiencies. The photo-electrode measured as an isolated system shows decay times of bound electrons in excited states of the dye to be ~150 fs and shows the subsequent emergence of absorption bands of the oxidized molecules. When the I-/I-3 redox couple is added to the system, these excited state lifetimes change and are found to be dependent on the cation in the electrolytic solution. Small cations like Li+ reduce the excited state lifetime to sub-100 fs, whilst larger cations like the organic tetrabutylammonium result in longer lifetimes of 240 fs. The action of the electrolyte can be observed by the reduced lifetime of the oxidized dye molecules' absorption bands. The effect of operating parameters and changes in the production protocol of the DSC on the primary charge injection are also investigated and reported on. / AFRIKAANSE OPSOMMING: Die aanvanklike ladingsoordrag vanuit kleurstofmolekules' opgewekte toestande tot in die leidingsband van 'n halfgeleier, na absorpsie van sigbare lig deur eersgenoemde, is van kritiese belang vir die uitset van halfgeleier-gebaseerde sonkragselle wat met kleurstowwe vir absorpsie verhoging, gebind is. In hierdie werk word hierdie proses en verwante fotodinamika in die geval van 'n ZnO sonkragsel gekleur met indolien D149 ondersoek d.m.v femtosekonde-tydopgelosde absorpsiespektroskopie. Hierdie metings onthul elektron-oordragstempos wat ooreenstem met lewenstye van opgewekte toestande in die orde van 100 fs. Hierdie is met voorheen-bepaalde hoë foton-tot-stroom omskakelingsdoeltreffendheid ooreenkomstig. Die foto-elektrode, as geïsoleerde sisteem beskou, toon afvalstye van gebonde elektrone in opgewekte toestande van ~150 fs, en die gevolglike opkoms van absorpsie deur geoksideerde molekules word waargeneem. As die I-/I-3 redoks oplossing tot die sisteem bygevoeg word, verander die opgewekte toestande se afvalstye en toon 'n katioon-afhanklikheid. Klein katioone soos Li+ verkort die afvalstye tot onder 100 fs, terwyl groter katioone soos die organiese tetra-butielammonium langer afvalstye (240 fs) tot gevolg het. Die werking van die elektrolitiese oplossing kan waargeneem word deur die verkorte lewenstyd van die absorpsiebande wat aan die geoksideerde molekules toegeken is. Die uitwerking van operasionele parameter asook veranderinge in die produksie protokol op die primêre ladingsoordrag word ondersoek en verslag daarop word gelewer.

<p>Currently, fossil fuels and nuclear power are our primary energy sources. However, both have critical disadvantages due to the limited supply and the hazard issues. Renewable energy research becomes one of most important research topics in the 21st century. Nanostructured materials show unique electrochemical properties in various energy conversion or storage devices. This dissertation starts with fundamental optical studies of nanomaterials (carbon nanotubes), followed by synthesizing novel nanomaterials for energy conversion (solar cells) and storage (lithium ion batteries) devices. </p><p> (1) There is an on-going debate concerning the ability of double walled carbon nanotubes (DWNTs) to exhibit photoluminescence (PL). We aim to clearly resolve this debate through the study of carefully separated DWNTs using density gradient ultra-centrifugation (DGU). Here, we clearly show that light is emitted from the inner wall of DWNTs. Interestingly, it was found that a very narrow range of diameters of the inner walls of DWNTs is required for photoluminescence (PL) to be observable. All other diameters led to complete PL quenching in DWNTs. (2) Inexpensive dye sensitized solar cells (DSSCs) on flexible plastic substrates have a bright future, but they require low temperature annealing (< 200°C). The method to fabricate low temperature DSSCs should resolve poor electron transfer between titanium dioxide (TiO2) nanoparticles (NPs) due to their incomplete contiguity and insulating layer of organic residues from binders in the photoactive film. Here, we have developed uniform CNTs/TiO2 composites for low temperature DSSCs by using modified sol gel method. DSSCs were fabricated to study incorporating functionalized few walled carbon nanotubes (f-FWNTs) effect on TiO2 NPs. Incorporating f-FWNTs can be beneficial for the low temperature annealing process of DSSCs to overcome extremely poor electron transport through TiO2 photoactive film. Incorporating f-FWNTs with TiO2 active layer improves electrons transport in some degree, but this advantage is limited. (3) Conductive fillers, such as amorphous carbon, carbon nanotube and graphene, have been mixed with nanostructured metal oxide materials to improve the performance of electrode materials in energy storage devices. However, ineffective junctions between conductive fillers are limiting the overall conductivity of the electrode. Therefore, we developed a convenient, inexpensive and scalable method for synthesizing hybrid carbon and titanium dioxide (C/TiO2) co-gels and co-aerogels to improve their electrochemical capacity in lithium ions batteries (LIBs). The monolith of the hybrid C/TiO2 co-aerogel can be directly used as active electrodes without the addition of binders. As a result, the capacitance of LIB anodes using the hybrid co-aerogel is significantly improved over current LIBs based on carbon/titanium oxide composite. Other metal oxides could also form co-gels with carbon to improve their potentials in numerous electrochemical, photocatalytic, and photoelectronic devices.</p> / Dissertation

This thesis improves our understanding of the charge-transfer dynamics in organic materials employed in dye-sensitized and nanotube-thiophene solar cells. For the purpose of this work, a femtosecond transient absorption spectroscopy setup was built. Additionally, microsecond transient absorption spectroscopy was utilised to explore dynamics on a longer time-scale. In the first study, the dependence of dye regeneration and charge collection on the pore- filling fraction (PFF) in solid-state dye-sensitized solar cells (DSSCs) is investigated. It is shown that while complete hole transfer with PFFs as low as ~30% can be achieved, improvements beyond this PFF are assigned to a stepwise increase in the charge-collection efficiency in agreement with percolation theory. It is further predicted that the chargecollection efficiency saturates at a PFF of ~82%. The study is followed by an investigation of three novel hole-transporting materials for DSSCs with slightly varying HOMO levels to systematically explore the possibility of reducing the loss-in-potential and thus improving the device efficiency. It is shown that despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. Furthermore, it is demonstrated that the design of the HTM has an additional impact on the electronic density of states present at the TiO₂ electrode surface, and hence influences not only hole- but also electron-transfer from the sensitizer. Finally, a study on a polymer-single-walled carbon nanotube (SWNT) molecular junction is presented. Results from femtosecond spectroscopic techniques show that the polymer poly(3-hexylthiophene) (P3HT) is able to transfer charges to the SWNT within 430 fs. Addition of excess P3HT polymer leads to long-lived free charges making these materials a viable option for solar cells.

Master of Science / Department of Chemistry / Jun Li / With the worldly consumption of energy continually increasing and the main source of this energy, fossil fuels, slowly being depleted, the need for alternate sources of energy is becoming more and more pertinent. One promising approach for an alternate method of producing energy is using solar cells to convert sunlight into electrical energy through photovoltaic processes. Currently, the most widely commercialized solar cell is based on a single p-n junction with silicon. Silicon solar cells are able to obtain high efficiencies but the downfall is, in order to achieve this performance, expensive fabrication techniques and high purity materials must be employed. An encouraging cheaper alternative to silicon solar cells is the dye-sensitized solar cell (DSSC) which is based on a wide band gap semiconductor sensitized with a visible light absorbing species. While DSSCs are less expensive, their efficiencies are still quite low compared to silicon. In this thesis, Grätzel cells (DSSCs based on TiO2 NPs) were fabricated and optimized to establish a reliable standard for further improvement. Optimized single layer GSCs and double layer GSCs showing efficiencies >4% and efficiencies of ~6%, respectively, were obtained. Recently, the incorporation of metallic nanoparticles into silicon solar cells has shown improved efficiency and lowered material cost. By utilizing their plasmonic properties, incident light can be scattered, concentrated, or trapped thereby increasing the effective path length of the cell and allowing the physical thickness of the cell to be reduced. This concept can also be applied to DSSCs, which are cheaper and easier to fabricate than Si based solar cells but are limited by lower efficiency. By incorporating 20 nm diameter Au nanoparticles (Au NPs) into DSSCs at the FTO/TiO2 interface as sub wavelength antennae, average photocurrent enhancements of 14% (maximum up to ~32%) and average efficiency enhancements of 13% (maximum up to ~23% ) were achieved with well dispersed, low surface coverages of nanoparticles. However the Au nanoparticle solar cell (AuNPSC) performance is very sensitive to the surface coverage, the extent of nanoparticle aggregation, and the electrolyte employed, all of which can lead to detrimental effects (decreased performances) on the devices.

<p>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.</p><p>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.</p><p>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.</p><p>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.</p>

<p>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 <em>3d</em> and <em>4d</em> 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 <em>4d </em>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.</p>

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

Because of its excellent and unique physical properties, ZnO nanowires have been widely used in numerous scientific fields such as sensors, solar cells, nanogenerators, etc. Although it is believed that single crystal ZnO has a much higher electron transfer rate than TiO₂, it was found that ZnO nanowire-based dye-sensitized solar cells (DSSCs) have lower efficiencies than TiO₂ nanoparticle-based DSSCs because the density and surface area of ZnO nanowires are usually lower than that of TiO₂ nanoparticles, limiting the cell's light absorption, and because the open-root structure of ZnO nanowires results in electron back transfer that causes charge shortage of the cell. Here, experimental studies were performed that utilize strategic manipulations of the design of the ZnO nanowire based DSSCs in efforts to address and solve its key challenges. It was shown that by incorporating various blocking layers into the design of the cell, the performance of the DSSC can be improved. Specifically, by placing a hybrid blocking layer of TiO₂-P4VP polymer between the substrate and the ZnO nanowires, the conversion efficiency of the cell was 43 times higher than that of a cell without this blocking layer due to the reduction of electron back transfer. Furthermore, in efforts to improve the surface area of the ZnO nanowire array, unique three dimensional structures of ZnO nanowires were fabricated. It was found that by significantly improving the overall density and surface area of the ZnO nanowire array through distinctive hierarchal nanowire structures, the light harvesting efficiency and electron transport were enhanced allowing the DSSC to reach 5.20%, the highest reported value for 3D ZnO NW based DSSCs. Additionally, the development of a theoretical model was explored in efforts to investigate how the geometry of ZnO nanowires affects the incident photon-to-current conversion efficiency of 1D ZnO nanowire-based N719-sensitized solar cells at the maximum absorption wavelength of 543 nm.