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.

La première partie du projet consiste à synthétiser de nouveaux chromophores de structure D-π-[M]-π-A pour des applications en cellules solaires à colorant. La synthèse de complexes symétriques contenant deux fragments métalliques [Ru(dppe)2] sera ensuite envisagée pour obtenir des architectures de type D-π-A-π-D pouvant être testées comme matériau de type n ou p en cellules solaires organiques. Enfin, comme il a été précédemment démontré que la communication électronique peut avoir lieu à travers plusieurs centres ruthénium, la longueur du complexe pourra être augmentée par addition de plusieurs unités métallo-organiques pour conduire à des structures oligomériques aux propriétés d’absorption de l’énergie lumineuse exaltées. / The first part of the research project will consist in the synthesis and study of new chromophores based on the D-π-[M]-π-A model. At that stage, easy synthesis of symmetrical complexes including two [Ru(dppe)2] metal fragments will be envisaged to afford D-π-A-π-D architectures to be tested in n-type solar cells and p-type solar cells. Later on, as it has been demonstrated that electronic communication may occur through several Ru-based metal centres,the complexes length might be incremented by addition of multiple metal-organic units to create oligomeric structures with strong light-harvesting properties.

abstract: Alternative energy technologies must become more cost effective to achieve grid parity with fossil fuels. Dye sensitized solar cells (DSSCs) are an innovative third generation photovoltaic technology, which is demonstrating tremendous potential to become a revolutionary technology due to recent breakthroughs in cost of fabrication. The study here focused on quality improvement measures undertaken to improve fabrication of DSSCs and enhance process efficiency and effectiveness. Several quality improvement methods were implemented to optimize the seven step individual DSSC fabrication processes. Lean Manufacturing's 5S method successfully increased efficiency in all of the processes. Six Sigma's DMAIC methodology was used to identify and eliminate each of the root causes of defects in the critical titanium dioxide deposition process. These optimizations resulted with the following significant improvements in the production process: 1. fabrication time of the DSSCs was reduced by 54 %; 2. fabrication procedures were improved to the extent that all critical defects in the process were eliminated; 3. the quantity of functioning DSSCs fabricated was increased from 17 % to 90 %. / Dissertation/Thesis / M.S.Tech Technology 2012

The literature surrounding organic small-molecule donor-acceptor systems is summarised for a range of optoelectronic applications (OLEDs, OPVs, OFETs etc.). There is a focus on the key building blocks: 1,3,4-oxadiazole (OXD), diphenylamine (DPA), carbazole (Cbz) and fluorene (F). The incorporation of such moieties into various donor-acceptor systems is discussed with further reference to selected alternative organic donor and acceptor systems. The syntheses of novel bipolar molecules based on a donor-spacer-acceptor (DPA/Cbz-F-OXD) structure and the incorporation of these molecules into single-layer OLEDs is presented. It is demonstrated how the emission colour can be tuned from green to deep blue by systematic manipulation of the structure. A significant result is that high efficiency accompanied with pure, deep blue emission in single-layer OLEDs can be achieved with this structural motif. The incorporation of these materials as part of a simple two-component blend to produce white OLEDs is presented and the modification of the materials to improve electron-transport properties is discussed. The synthesis of DPA-bridge-OXD wire systems is presented with the use of oligo-p-phenyleneethynylene units as a bridge of varying length to investigate the effect on charge transfer between the donor and acceptor. Photophysical studies demonstrate the change in absorption, emission and fluorescence lifetimes as the length scale of the molecules is altered. The synthesis of a series of planarised and twisted DPA-bridge-OXD systems based upon phenylene linkers is discussed. Finally, a series of DPA-F-OXD-anchor molecules is presented for incorporation into DSSC devices. The synthesis of these materials is described and the suitability of various anchoring groups for DSSCs is analysed through photophysical and device studies.

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This dissertation focuses on the design, synthesis, and characterization of a variety of organic dyes, semiconducting materials, and surface redox-active modifiers of potential interest to organic-based emerging photovoltaics. A discussion of the materials’ optoelectronic properties, their ability to modify and promote electron transfer through an organic/transparent conducting-oxide interface, and finally their effect on the photovoltaic properties of devices utilizing them as light-harvesters is provided where relevant. The first two research chapters discuss mono-chromophoric asymmetric squaraine-based sensitizers and covalently linked, dual-chromophoric, porphyrin-squaraine sensitizers as light absorbers in dye-sensitized solar cells (DSSCs), in an attempt to address two problems often encountered with DSSCs utilizing this class of near infra-red sensitizers; The lack of panchromatic absorption and aggregation on the surface. Also, this dissertation discusses the design and synthesis of asymmetric perylene diimide phosphonic acid (PDI-PA) redox-active surface modifiers, and reports on the electron-transfer rates and efficiencies across the interface of an ITO electrode (widely used in organic-electronic devices) modified with these perylene diimides. Finally two series of hole-transport materials based on oligothiophenes and benzodithiophenes are reported: optoelectronic properties and preliminary performance of organic photovoltaic (OPV) devices fabricated with them is discussed.

Στα πλαίσια της παρούσας διδακτορικής διατριβής μελετήθηκαν και αναπτύχθηκαν νανοδομημένα λεπτά υμένια διοξειδίου του τιτανίου (TiO2) και λευκόχρυσου (Pt) για χρήση σε ευαισθητοποιημένες φωτοβολταϊκές κυψελίδες. Αφού πραγματοποιήθηκε η μελέτη των υμενίων, παρασκευάστηκαν υμένια TiO2 και Pt και βελτιστοποιήθηκαν, ώστε να έχουν τις επιθυμητές ιδιότητες. Για το χαρακτηρισμό των υμενίων TiO2 χρησιμοποιήθηκε ηλεκτρονικό μικροσκόπιο σάρωσης (SEM) και περίθλαση ακτίνων X (XRD). Μάλιστα προτάθηκε η χρήση νιτρικού οξέος, ανάμεσα σε 4 διαφορετικά οξέα, για την αποτελεσματική διασπορά των σωματιδίων του TiO2 και την παρασκευή ομοιόμορφων υμενίων, χωρίς την παρουσία ρωγμών και με πάχος πάνω από 10 μm. Τα υμένια που παρασκευάστηκαν χρησιμοποιήθηκαν σε ευαισθητοποιημένες φ/β κυψελίδες και μελετήθηκε η επίδραση της δομής τους στην απόδοση των κυψελίδων. Για τα υμένια Pt αναπτύχθηκαν δυο διαφορετικοί τρόποι παρασκευής (θερμική διάσπαση αλάτων Pt, ηλεκτροαπόθεση). Τα υμένια που προέκυψαν αξιολογήθηκαν με βάση τη μορφολογία και τις καταλυτικές τους ιδιότητες ως προς την αναγωγή των ιόντων τριωδίου και προτάθηκαν τρόποι για τη βελτιστοποίησή τους. Ιδιαίτερη βαρύτητα δόθηκε στη σταθερότητα των υμενίων Pt κατά την παραμονή τους σε διάλυμα ηλεκτρολύτη, ίδιο με αυτό που χρησιμοποιείται στις ευαισθητοποιημένες κυψελίδες. Τέλος τα υμένια TiO2 και Pt ενσωματώθηκαν σε πρότυπες φωτοηλεκτροχρωμικές διατάξεις, στις οποίες η ενέργεια που απαιτείται για τη μεταβολή της διαπερατότητάς τους παρέχεται από την ενσωματωμένη φ/β κυψελίδα. Μάλιστα προτάθηκε και μελετήθηκε ένας νέος τύπος διάταξης. / In this PhD thesis we have studied and prepared nanostructured titanium dioxide (TiO2) and platinum (Pt) thin films, in order to use them for dye sensitized solar cells. The morphology and the structure of the TiO2 films were examined with scanning electron microscopy (SEM) and x-ray diffraction (XRD). We have proposed the use of nitric acid, among four other acids, in order to achieve a more efficient dispersion of TiO2 nanoparticles and to prepare uniform and crack free films, with thickness above 10 μm. The TiO2 films were used in dye sensitized solar cells and was examined the relation between the structure of the films and the efficiency of the cells. For the deposition of Pt films we have used two different methods (thermal decomposition of Pt salts and electrodeposition). The films were characterized according to their morphology and their catalytic activity towards triiodide ions reduction. Moreover we have proposed methods for improving the properties of Pt films and we have studied their stability, especially when the films were stored in the same electrolyte solution as that in dye sensitized solar cells. Finally the TiO2 and Pt films were used in photoelectrochromic devices, where the energy produced by the photovoltaic cell is used for the modulation of device’s transmittance. Also a new type of a photoelectrochromic device was introduced and studied.

Αντικείμενο της διπλωματικής εργασίας είναι μελέτη της ευαισθητοποίησης από κοινού (co-sensitization) με την χρήση απλών οργανικών χρωστικών με παρόμοιο φάσμα απορρόφησης. Για το λόγο αυτό αναπτύχθηκαν μείγματα διαφόρων χρωστικών ουσιών τα οποία χρησιμοποιήθηκαν για την ευαισθητοποίηση ηλιακών κυψελίδων (DSSCs) με ημιαγωγό νανοδομημένου ZnO. Οι χρωστικές που χρησιμοποιήθηκαν για την ευαισθητοποίηση ήταν απλές οργανικές, όπως η Rose-Bengal, η Rhodamine-B, η Eosin-B, η Coumarin 343 και η Malachite Green. Παρασκευάστηκαν μείγματα δυο και τριών χρωστικών ουσιών με σκοπό την επίτευξη υψηλότερης απόδοσης της ευαισθητοποιημένης ηλιακής κυψελίδας. / In this work we investigate co-sensitization effects by using simple organic dyes with complimentary absorption spectra. A combination of different organic dyes was used in this work to sensitize nanostructured ZnO films for Dye Sensitized Solar Cell (DSSC) devices. The dyes used to sensitize the films were the simple organic molecules Bengal Rose, Rhodamine B, Eosin B, Coumarin 343 and Malachite Green. Binary and ternary blends of these dyes were used in order to enhance the performance of ZnO DSSCs.

Dye-sensitized solar cells (DSSCs) and perovskite solar cells are emerging as promising potential low-cost alternatives to established crystalline silicon photovoltaics. Of the employed functional materials, however, many fundamental optoelectronic properties governing photovoltaic device operation are not sufficiently well understood. This thesis reports on a series of studies using ultrafast THz and photoluminescence spectroscopy on two classes of such materials, providing insight into the dynamics of charge-transport and recombination processes following photoexcitation. For TiO₂-nanotubes, which have been proposed as easy-to-fabricate electron transporters for DSSCs, fast, shallow electron trapping is identified as a limiting factor for efficient charge collection. Trapping lifetimes are found to be about an order of magnitude shorter than in the prevalently employed sintered nanoparticles under similar excitation conditions and trap saturation effects are not observed, even at very high excitation densities. In organo-lead halide perovskites - specifically CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x, which have only recently emerged as highly efficient absorbers and charge transporters for thin-film solar cells, carrier mobilities and fundamental recombination dynamics are revealed. Extremely low bi-molecular recombination rates at least four orders of magnitude below the prediction of Langevin's model are found as well as relatively high charge-carrier mobilities in comparison to other solution-processable materials. Furthermore a very low influence of trap-mediated recombination channels was observed. Due to a combination of these factors, diffusion lengths reach hundreds of nanometres for CH₃NH₃PbI₃ and several microns for CH₃NH₃PbI_3-xCl_x. These results are shown to hold for both, solution processed and vapour-deposited CH₃NH₃PbI_3-xCl_x and underline the superb suitability of the materials as absorbers in solar cells, even in planar heterojunction architectures. The THz-frequency spectrum of the conductivity of the investigated perovskites is consistent with Drude-like charge transport additionally exhibiting weak signatures of phonon coupling. These coupling effects are also reflected in the luminescence of CH₃NH₃PbI_3-xCl_x, where they are believed to be the cause of the observed homogeneous spectral broadening. Further photoluminescence measurements were performed at temperatures between 4 K and room temperature to study the nature of recombination pathways in the material.