Modern civilization highly depends on energy and finding alternative sources to fossil fuels becomes more and more necessary. The sun is the most abundant energy source available and exploiting it efficiently would result in a great environmental and economic breakthrough. Among the photovoltaic devices, dye-sensitized solar cells (DSCs) emerged for their tremendous commercial potential deriving from a combination of low-cost production and attractive features, such as flexibility and transparency, for indoor and outdoor applications. In the DSCs, a dye anchored to a semiconductor layer (typically TiO2) is responsible for capturing the sunlight and converting it into electricity. Nevertheless, many commercially available dyes for DSCs are based on a very rare metal, ruthenium, and its replacement with a cheaper, more abundant metal is desirable. A good alternative to ruthenium could be copper, which possesses similar photophysical properties in coordination with diimine ligands, but it is considerably cheaper and relatively earth-abundant. In this work, a particular “on-surface self-assembly” strategy was employed to form, on the surface of TiO2, heteroleptic copper (I) complexes with a “push-pull” design which facilitates the electron transfer from the copper (I) complex into the conduction band of TiO2 and enhances the performance of the photovoltaic devices.This thesis focuses on the investigation of the properties of five new heteroleptic copper (I) complexes bearing the same anchoring ligand but different ancillary ligands. Because of the method employed for their synthesis, a solid-state characterization of the optical and electrochemical properties on TiO2 was performed employing tools such as UV-Vis spectroscopy, cyclic voltammetry (CV), and differential pulse voltammetry (DPV). As internal benchmark through the entire characterization, the homoleptic copper (I) complex with the anchoring ligand was also studied. Some patterns between the heteroleptic complexes on TiO2 and their respective homoleptic complexes in solution were found, opening the possibility to predict the behaviour of unknown heteroleptic complexes starting from their corresponding homoleptic. Furthermore, the characterization was necessary to ensure that the complexes were fulfilling the requirements to be employed as dyes. The performances of the heteroleptic, and of the anchoring-ligand homoleptic, copper (I) complexes were then investigated as photosensitizers in DSC devices mainly by measuring the current density-voltage (J-V) characteristics at different light intensities and in the dark, the incident photon-to-current efficiency (IPCE), and electron lifetimes. As benchmark for the degree of effectiveness of the device, the state-of-the-art ruthenium (II) complex N719 was also studied. All the copper (I) complexes showed an overall similar behaviour. The J-V characteristics showed a power conversion efficiency up to 2,05% for the best performing device, which is 25% of the efficiency of DSCs based on N719. On the other hand, the least performing heteroleptic copper (I) complex studied showed an efficiency of 1,23%. From a general analysis combining all the results obtained, it may be concluded that a reason for the limited photocurrent measured through these devices can be due to incomplete dye coverage of TiO2. Despite the lower performance compared to the standard dye N719, the simplicity of the system is promising, and its considerable economic advantage could pave the way to the use of DSCs in everyday life applications. / Modernt samhälle är mycket beroende på energi och det blir allt mer akut att hitta alternativa källor till fossila bränslen. Solen är den mest riklig energikällan som finns och att utnyttja den effektivt skulle resultera i stora miljö- och ekonomiska genombrott. Färgämne sensibiliserade solceller (”dye-sensitized solar cells”; DSC) utvecklade i 90-talet för sin breda kommersiella potential som härrör från en kombination av låg kostnadsproduktion och attraktiva egenskaper, såsom möjligheter för flexibilitet och transparens. I DSC är ett färgämne förankrat till den ytan av en halvledare (vanligtvis TiO2). Färgämnet är ansvarigt för att fånga solljuset och överföra elektronerna till halvledaren för att producera el. Många kommersiella färgämnen för DSC är baserade på rutenium, en mycket sällsynt metall. Ersättning av rutenium med en billigare, rikligmetall är önskvärt mot mer hållbara DSC. En bra alternativmetall till rutenium är koppar. Komplexen av koppar(I) har liknande fotofysiska egenskaper till rutenium (II) men koppar är mer vanlig och mindre dyr än rutenium. Heteroleptiska koppar(I) komplexen med en "push-pull" design syntetiserades på ytan av TiO2 genom "ytan assisterade självmontering". "Push-pull" designen underlättar elektronöverföring från koppar(I)-komplexet till ledningsbandet av TiO2. Denna avhandling fokuseras på undersökning av egenskaperna av fem nya heteroleptiska koppar(I) komplex med den samma förankrings ligand men olika distala ligander. På grund av metoden som används i syntesen av heteroleptiska koppar(I) komplex, var karakterisering av komplexen vid optiska och elektrokemiska metoder utfördes på TiO2. Metoderna för karakterisering var UV-Vis-spektroskopi, cyklisk voltametri (CV) och differentialpuls voltametri (DPV). Som en intern standard genom hela karaktäriseringen studerades även homoleptiska koppar(I) komplex med förankringsliganden. Egenskaperna på heteroleptiska koppar(I) komplexen på TiO2 ytan kunde förutsägas från mätning av egenskaperna på homoleptiska koppar(I) komplexen. Koppar(I) komplexen är undersöktes som fotosensibiliserare i färg-sensibiliserade solceller. Effektiviteten av solcellerna med koppar(I) komplexen eller rutenium (II) komplex (N719) utvärderades genom att mäta fotokurrentdensitetsspänningen (J-V) vid olika ljusintensiteter, incidentfoton-till-ström effektiviteten (”incident photon-to-current efficiency”; IPCE) och laddningsrekombinationen (elektronlivstiden). Koppar(I) komplexen hade övergripande liknande egenskaper i solceller. En kraft omvandlingseffektivitet av 2,05% nås för den bästa solcellen med ett koppar(I) komplex. Medan den bästa effektiviteten med N719 färgämnet var 7,57%. En svaghet i självmonteringen av koppar(I) komplexen på ytan av TiO2 är den ofullständiga bindningen till ytan men självmonteringen metoden var enkel och kunde skapa många, olika färgämnen i kort tid. Trots den lägre prestandan jämfört med standardfärgen N719 är systemets enkelhet lovande, och dess stora ekonomiska fördel kan bana vägen till användningen av DSC i vardagsläget. / La civiltà moderna è fondata sull’uso dell’energia e trovare fonti alternative ai combustibili fossili è diventato sempre più necessario. La radiazione proveniente dal sole è la risorsa energetica più abbondante e disponibile sul nostro pianeta e sfruttarla al massimo comporterebbe una svolta decisiva per l’ambiente e l’economia. Tra i dispositivi fotovoltaici, le celle di Grätzel, conosciute anche come DSC dall’acronimo inglese (dye-sensitized solar cell), sono emerse per il loro enorme potenziale commerciale, dovuto alla combinazione tra basso costo di produzione e interessanti caratteristiche, come la loro flessibilità e trasparenza, che gli conferiscono la possibilità di integrazione negli edifici e l’uso in applicazioni “indoor”. Nelle DSC, un foto-sensibilizzatore, detto anche dye, ancorato ad uno strato di un materiale semiconduttore (tipicamente TiO2), è responsabile della cattura della luce solare e della sua conversione in elettricità. Tra i dye commercialmente disponibili per le DSC vi sono per lo più complessi di coordinazione basati su un metallo molto raro, il rutenio; la sua sostituzione con un metallo più abbondante ed economico è auspicabile per la diffusione di questa tecnologia. Una buona alternativa al rutenio potrebbe essere fornita dal rame, che possiede proprietà foto-fisiche molto simili al primo quando in coordinazione con diimmine; in più è abbastanza economico e relativamente abbondante sulla Terra. Una particolare strategia di "autoassemblaggio sulla superficie" è stata impiegata per formare, sullo strato di TiO2, complessi eterolettici di rame (I) con un design “push-pull” che facilita il trasferimento di elettroni dal complesso di rame (I) alla banda di conduzione di TiO2, migliorando così le prestazioni dei dispositivi fotovoltaici. Questa tesi si concentra sullo studio delle proprietà di cinque nuovi complessi eterolettici di rame (I) che possiedono lo stesso legante di ancoraggio ma diverso legante secondario. A causa del metodo impiegato per la loro sintesi, è stato necessario eseguire la caratterizzazione delle loro proprietà ottiche ed elettrochimiche direttamente sulla superficie del TiO2, utilizzando strumenti come la spettroscopia UV-Visibile, la voltammetria ciclica (CV) e la voltammetria ad impulsi differenziali (DPV). Come riferimento interno durante l'intera caratterizzazione, è stato studiato anche il complesso di rame (I) omolettico con il legante di ancoraggio. Tra i complessi eterolettici su TiO2 e i loro rispettivi complessi omolettici in soluzione, è stato individuato un trend con la possibilità di prevedere il comportamento dei primi a partire dal loro corrispondente omolettico. Inoltre, la caratterizzazione ottica ed elettrochimica è necessaria per garantire l’adeguatezza dei complessi come dye. Le prestazioni dei complessi eterolettici e del complesso omolettico con il legante di ancoraggio, sono state quindi studiate come foto-sensibilizzatori nei dispositivi DSC; in particolare è stata misurata la curva di densità di corrente – voltaggio (J-V) a diverse intensità di luce e al buio, l’efficienza quantica esterna (EQE o dall’acronimo inglese incident photon-to-current efficiency, IPCE) e infine il tempo di vita dell’elettrone nella banda di conduzione del semiconduttore. Come standard interno, per verificare l’efficacia del dispositivo, è stato anche studiato il ben noto complesso di rutenio (II), N719. Generalmente, tutti i complessi di rame (I) hanno mostrato un comportamento simile. Le curve caratteristiche J-V hanno presentato un'efficienza pari fino al 2,05% per il complesso di rame che ha dato le prestazioni migliori (25% dell'efficienza di N719). Da un'analisi generale che combina tutti i risultati ottenuti, si può concludere che una ragione per cui la foto-corrente risulta limitata potrebbe essere data dall’incompleta copertura del TiO2 da parte del complesso di rame (I) e quindi dallo scarso adsorbimento del dye. Nonostante le prestazioni inferiori rispetto allo standard N719, la semplicità del sistema è promettente e il suo notevole vantaggio economico potrebbe aprire la strada all'utilizzo delle DSC nelle applicazioni della vita quotidiana.

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<p>Dye-sensitized Solar Cells (DSSCs) with liquid electrolyte lack long term stability because of volatility of the electrolyte and assembly problems. Replacement of the volatile liquid-state electrolyte with solid-state hole conductor thus becomes necessary. A small molecule based hole conductor, Copper Phthalocyanine (CuPc), is proposed here to replace the liquid electrolyte, for its intrinsic thermal and chemical stabilities. However, a lower short circuit current was found in the CuPc solid state device from I-V curve, which is closely related to the inefficient hole transport in the CuPc thin film. Therefore, Two-Dimensional Grazing Incidence X-ray Diffraction (2D GIXRD) is utilized to study the phase and texture of CuPc thin film. It is found that the CuPc thin film has a cystallinity of greater than 80%, which is good for hole conducting. However, the <em>β</em>-phase formation lowers the overall hole conductivity. The hole conductivity of <em>β</em>-phase CuPc is two orders of magnitude smaller than that of <em>α</em>-phase CuPc, due to a less overlap in the <em>π-π</em> stacking. As a result, the low hole conductivity of <em>β</em>-phase CuPc is the reason that leads to an inefficient hole transport and reduces the short-circuit current of the solid-state DSSC. Therefore, future work will be necessary to isolate <em>α</em>-phase CuPc, in order to be successfully applied into the solid-state DSSCs.</p> / Master of Science (MSc)

Dye sensitized solar cells (DSSCs) have typically been produced using organic liquids such as acetonitrile as the electrolyte solvent. In real world situations water can permeate into the cell through sealing materials and is also likely to be introduced during the fabrication process. This is a problem as the introduction of water into cells optimized to use an organic solvent tends to be detrimental to cell performance. In this work DSSCs which are optimized to use water as the main electrolyte solvent are produced and characterized. Optimization of aqueous DSSCs resulted in cells with efficiencies up to 3.5% being produced. In terms of characterization, it is generally seen in this work that aqueous DSSCs produce a lower photocurrent but similar photovoltage compared to DSSCs made using acetonitrile and reasons for this are examined in detail. The decreased ability of the aqueous electrolyte to wet the nanoporous TiO2 compared to an acetonitrile electrolyte is found to be a key difficulty and several possible solutions to this problem are examined. By measuring the photocurrent output of aqueous cells as a function of xy position it can be seen that there is some dye dissolution near to the electrolyte filling holes. This is thought to be linked to pH and the effect of 4-tert-butylpyridine and may also decrease the photocurrent. It is found that there is little difference between the two types of cells in terms of the conduction band position and the reaction of electrons in the semiconductor with triiodide in the electrolyte, explaining the similarity in photovoltage. By altering the pH of the electrolyte in an aqueous cell it is found to be possible to change the TiO2 conduction band position in the DSSC. This has a significant effect on the open circuit voltage and short circuit current of the cell, though the pH range available is limited by the fact that dye desorbs at high pH values.

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.