Dye sensitized solar cells (DSSCs) are an attractive way to convert light into electricity. Its development requires a detailed understanding and kinetic optimization of various electronic processes, especially those occurring at the mesoporous TiO2/dye/electrolyte interface. This dissertation work is focused on the exploration of the various electronic processes at the sensitized-electrode/electrolyte interface by using various electrochemical and photochemical methods. Firstly, an alternative redox couple—TEMPO/TEMPO·+ with a relatively high positive redox potential—is explored, aiming to reduce the energy loss during the dye regeneration process. Despite the fast dye regeneration, the charge recombination between the electrons in the conduction band of mesoporous TiO2 and the oxidized redox species is found to be the limiting factor of the device. Further, a more efficient tandem-electrolyte system is developed, leading to DSSCs with the power conversion efficiency of 10.5 % and 11.7 % at 1 sun and 0.5-sun illumination, respectively. An electron-transfer cascade process during dye regeneration by the redox mediators is discovered to be beneficial. Further stability studies on the device suggest the crucial role of TiO2/dye/electrolyte interfaces in the long-term stability of cobalt bipyridyl electrolyte-based DSSCs. On the fundamental level, the local electric field and Stark effects at the TiO2/dye/electrolyte interface are investigated in various aspects—including the charge compensation mechanism, the factors affecting the electric field strength, as well as its impact on charge transfer kinetics. These results give further insights about the TiO2/dye/electrolyte interface, and contribute to the further development and understanding of DSSCs.

Dye-sensitized solar cells (DSSCs) or Grätzel cells are electrochemical devices in where physicochemical properties of different materials are combined to obtain electric energy. These photoconversion devices have evolved from a pioneering concept of molecular photovoltaics to industrial development with confirmed record efficiencies of 14.3%. Their efficiency combined with low-cost production methods and a high aesthetic interest enables the production of DSSC products for consumer electronics market. The strengths of this technology and the fact that its drawbacks are not limiting for this application makes consumer electronics and DSSC a perfect match for the development of self-powered devices. Some companies have already spot a potential market and are currently launching different consumer electronics and other devices with embedded DSSC. This thesis provides an overview of the operation principles of DSSC and the possible routes to improve the efficiency of these devices to emerge and thrive. Additionally, improvements in efficiency, stability and manufacturing needed to be addressed in the near future for this technology are discussed and its suitability to represent a breakthrough in the market of consumer electronics. An overview of the main companies developing DSSC and current prototypes and products is included.

This work studies the subject of perovskite solar cells. The structure of perovskites is described along with a portion of photovoltaics history that led to the employment of perovskites as absorpsion layers. Further, methods of measurement for solar cells such as impedance spectroscopy, photospectroscopy and load characteristics are summarized. Lastly there is a description of already done analytics of perovskite solar cells with the summary of the results. In the practical part, there are measurements made to find out any effect of cell topology or light source wavelength on the cell performance.

MSc (Physics) / Department of Physics / Currently the dye sensitized solar cells have attracted more attention due to their low cost, transparency and flexibility. These types of solar cells use the dye molecule adsorbed on TiO2 semiconductor in Nano architecture with the role of absorbing photons, in recent research attempts are being made to shifts the absorption spectral of TiO2 to visible and near infrared–region of solar spectrum to achieve maximum photo absorption which yields to an increase in the efficiency of the dye sensitized solar cells. In the current study, density functional theory (DFT) was used to model two croconate dyes (CR1 and CR2), one with an electron donating methyl group (CR1) and the other with an electron –withdrawing caboxyl group (CR2). The geometric, electronic and optical properties of these dyes were compared. The adsorption behaviour of the two dyes on (010 and 100) anatase TiO2 surfaces were investigated in this study by employing first principle calculation based on DFT using a plane-wave pseudo potential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke Ernzerhof to describe the exchange -correlation function as implemented in the CASTEP package in Material Studio of BIOVIA. The adsorption results shows a spontaneous electron injection followed by efficient regeneration of the oxidized dye molecules by the electrolyte and strong binding ability of CR2 to the TiO2 surface, but also shows a comparable binding strength of CR1. The results of this study will help in the design of high efficient dye for DSSCs. / NRF

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.