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.

The electronic and geometrical structure of single difluoro-bora-1,3,5,7-tetraphenyl-aza-dipyrromethene (aza-BODIPY) molecules adsorbed on the Au(111) surface is investigated by low temperature scanning tunneling microscopy and spectroscopy in conjunction with ab initio density functional theory simulations of the density of states and of the interaction with the substrate. Our DFT calculations indicate that the aza-BODIPY molecule forms a chemical bond with the Au(111) substrate, with distortion of the molecular geometry and significant charge transfer between the molecule and the substrate. Nevertheless, most likely due to the low corrugation of the Au(111) surface, diffusion of the molecule is observed for applied bias in excess of 1 V. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

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.

Dans le contexte de la mise au point de dispositifs photovoltaïques efficaces, bon marché et respectueux de l’environnement, la synthèse d’oxydes métalliques semi-conducteurs tels que SnO2, Zn2SnO4 et WO3 de morphologies et textures diverses a été développée afin d’élaborer des photoanodes poreuses pour cellules solaires à colorant. D’après les études réalisées par différentes méthodes (MEB, MET, DRX et BET), les matériaux obtenus présentent des caractéristiques texturales, morphologiques et structurales appropriées pour l’application visée. Des cellules solaires à colorant ont donc été réalisées à partir de ces oxydes, puis différents paramètres influençant leurs performances ont été optimisés afin d’améliorer l’efficacité de la conversion photovoltaïque. Notamment l’influence positive de différents traitements des photoanodes (i.e. solution aqueuse de TiCl4 ou traitement à l’eau) sur les rendements de conversion énergétique et la stabilité des dispositifs a été démontrée. Ainsi, des performances comparables ou supérieures à l’état de l’art ont été atteintes pour les systèmes à base de SnO2. Ces performances ont ensuite été interprétées en déterminant les processus électroniques et ioniques ayant lieu dans ces cellules par différentes méthodes physiques (mesures de tension de seuil et de décroissance de circuit-ouvert, spectroscopie d’impédance). Enfin, des électrodes réalisées à partir de WO3 déposé sur substrats flexibles ont démontré des propriétés électrochromes très prometteuses ce qui ouvre de nouvelles perspectives dans le domaine de l’affichage. / In the context of the development of efficient, low-cost and environmentally friendly photovoltaic devices, the synthesis of metal-oxide semiconductors such as SnO2, Zn2SnO4 and WO3 with various textures and morphologies have been developed in order to achieve nanoporous photoanodes for dye-sensitized solar cells. According to studies carried out by different characterization methods (SEM, TEM, XRD and BET), the resulting materials show interesting features for the expected application. Dye solar cells were then fabricated from photoanodes processed with these oxides and several parameters influencing their performance were optimized to improve the overall conversion efficiency. In particular, the beneficial effect of different treatments of the photoanodes (ie aqueous TiCl4 or water treatment) on the power conversion efficiency and the stability of the devices has been evidenced. Thus, state-of-the art or, even, record efficiencies were reached in the case of SnO2-based systems. These performances were then rationalized by determining the electronic and ionic processes occurring in these devices by various physical methods (threshold voltage and open-circuit photovoltage decay measurements, electrochemical impedance spectroscopy). Finally, electrodes based on WO3 and deposited on flexible substrates have shown very promising electrochromic properties, which opens up new prospects in the field of smart displays.

Les cellules solaires à colorant (DSSC) sont une alternative sérieuse aux cellules à base de silicium. Le principe de fonctionnement repose sur la photosensibilisation d'un semi-conducteur par un colorant qui est en général un complexe polypyridinique du ruthénium (II). La modulation des propriétés de ces complexes permet d'optimiser les performances des cellules solaires correspondantes. Dans cette thèse, nous avons synthétisé et étudié l'effet de nouveaux ligands bipyridiniques à substituants électro-donneurs [pi]-délocalisés à base de dithiénylpyrroles (DTP). Ces motifs induisent, dans les complexes homoleptiques, bis- et tris-hétéroleptiques du Ru(II), des effets bathochromes (lorsque les motifs DTP sont liés par leur cycle thiophénique à la bipyridine) et d'importantes augmentations des coefficients d'extinction molaires. Les nouveaux composés ont été caractérisés par spectroscopies, électrochimie, photophysique et calcul théorique. Deux complexes hétéroleptiques ont été testés en cellule DSSC. Si la collecte de photons est excellente, les performances restent en dessous de celles de colorants de référence. Comme en attestent les courbes J/V et les courbes IPCE. Ce résultat peut-être dû à une limitation lors de l'injection dans la bande de conduction ou encore à une gêne stéréo-électronique provoquée par le ligand lors de la réduction du colorant oxydé (Ru(III) par le médiateur / Dye-sensitized Solar Cells (DSSC) appear to be promising devices. Operation principle relies on the photosensitization of a wide-gap semiconductor with a dye, the latter typically being a polypyridinyl ruthenium(II) complex. Modulation of the properties of such complexes enables the optimization of the corresponding solar cells' performances. In the present work, we synthesized and investigated the effect of new bipyridine ligands bearing electron-donating dithienylpyrroles (DTP). These moieties induced red-shifts of the absorption spectra in homoleptic, bis- and tris-heteroleptic Ru(II) complexes especially when the DTP was bound by its thiophene unit to the bipyridine ligand. A notable increase of the molar extinction coefficients was also obtained. All new compounds have been characterized by using spectroscopic, electrochemical, photophysical and computational chemistry techniques. Two heteroleptic complexes have been tested in DSSCs. Despite excellent light harvesting properties, performances were found lower than those of standard dyes as revealed by J/V and IPCE curves. Stereoelectronic effects could be involved since the bulky DTP moiety could impede an efficient access of the mediator to Ru(III) centers

Dye-sensitized solar cells (DSSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Electron transport is one of the prominent processes in the cell and it is further a complex process because the transport medium is a mesoporous film. The gaps in the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, the electron transport in these so called state-of-the-art systems has a practical limit because of the low electron diffusion coefficient (Dn) in this mesoporous film photoanode. This work focuses on the influence of the advanced core-shell nanoarchitecture geometry on electron transport and also on the influence of electron-ion interactions. In order to achieve the proposed goals, DSSCs based on ordered, highly aligned, 3D radial core-shell Au-TiO2 hybrid nanowire arrays were fabricated, using three different approaches. J-V, IPCE, and EIS characteristics were studied. The efficiency, light scattering and charge transport properties of the core-shell nanowire based devices were compared to TiO2 nanotube as well as TiO2 mesoporous film based DSSCs. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the TCO substrate and improved transport of electrons between the TiO2 and the TCO. The optical effects were studied by IPCE measurement which demonstrated that Au-TiO2 nanowires showed an improved light harvesting efficiency, including at longer wavelengths where the sensitizer has weak absorption. The metal nanostructures could enhance the absorption in DSSCs by either scattering light enabling a longer optical path-length, localized surface plasmon resonance (LSPR) or by near-field coupling between the surface plasmon polariton (SPP) and the dye excited state. Rapid, radial electron collection is of practical significance because it should allow alternate redox shuttles that show relatively fast electron-interception dynamics to be utilized without significant sacrifice of photocurrent. A combination of improved electron transport and enhanced light harvesting capability make Au-TiO2 core-shell nanowire arrays a promising photoanode nanoarchitecture for improving photovoltaic efficiency while minimizing costs by allowing thinner devices that use less material in their construction.

Orientador: Prof. Dr. André Sarto Polo / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Ciência & Tecnologia - Química, 2014. / O trabalho desenvolvido teve como foco sintetizar compostos tris-heterolepticos de rutenio(II), o cis-[Ru(phen)(dcbH2)(NCS)2], o cis-[Ru(Me2-phen)(dcbH2)(NCS)2] e o cis-[Ru(Ph2-phen)(dcbH2)(NCS)2]. Estes compostos foram sintetizados pela rota one-pot que forneceu rendimentos superiores a 60%. A escolha dos substituintes nas posicoes 4 e 7 da fenantrolina foram feitas com o intuito de analisar o efeito causado por um grupo retirador de eletrons (Ph - fenila) e por um grupo doador de eletrons (Me - metila). Os compostos sintetizados foram caracterizados por analise elementar, espectroscopia eletronica, espectroscopia vibracional, emissao e ressonancia magnetica nuclear de 1H. Pelo espectro vibracional constatou-se que o aumento da densidade eletronica causado pelo grupo doador de eletrons desloca o ¿Þ(CN)NCS para uma regiao de maior energia e apresenta um efeito contrario para o grupo retirador de eletrons. Com o auxilio da ressonancia magnetica nuclear de 1H foi possivel comparar os sinais dos protons H2 e verificou-se que o composto com o substituinte com o grupo doador de eletrons foi deslocado para campo alto em relacao ao composto com o substituinte com o grupo retirador de eletrons. Os grupos substituintes influenciam muito pouco nas propriedades eletronicas dos compostos sintetizados. Os espectros de emissao dos compostos apresentam bandas largas e nao estruturadas, tipicas de emissao proveniente do estado thexi 3MLCT. Os tres compostos foram utilizados como corantes sensibilizadores de celulas solares. Com este trabalho observou-se que pela escolha adequada dos grupos substituintes e possivel modular as propriedades dos corantes sensibilizadores. O desempenho fotoeletroquimico foi avaliado por testes de curvas de corrente potencial e apresentaram os seguintes resultados: cis-[Ru(phen)(dcbH2)(NCS)2]: Voc = 0,733 V, Jsc = 8,67 mA cm-2, ff = 0,61 e £b = 3,78%; cis-[Ru(Me2-phen)(dcbH2)(NCS)2]: Voc = 0,683 V, Jsc = 12,72 mA cm-2, ff = 0,55 e £b = 4,60%; cis-[Ru(Ph2-phen)(dcbH2)(NCS)2]: Voc = 0,738 V, Jsc = 13,28 mA cm-2, ff = 0,63 e £b = 6,18%. / The developed work focused on synthesizing compounds tris-heteroleptic ruthenium(II), cis-[Ru(phen)(dcbH2)(NCS)2], cis-[Ru(Me2-phen)(dcbH2)(NCS)2] and were synthesized cis-[Ru(Ph2-phen)(dcbH2)(NCS)2]. These compounds were synthesized by one-pot route which provided higher yields at 60%. The choice of the substituents at positions 4 and 7 of phenanthroline were made in order to analyze the effect caused by an electron withdrawing group (Ph - phenyl) and an electron donating group (Me - methyl). The synthesized compounds were characterized by elemental analysis, electronic spectroscopy, vibrational spectroscopy, emission and 1H nuclear magnetic resonance. For the vibrational spectrum it was found that increasing the electron density caused by the electron donating group displaces the ¿Þ(CN)NCS for a higher energy region and has an opposite effect to the electron withdrawing group. With the aid of 1H nuclear magnetic resonance was possible to compare the signals of the protons H2, and it was found that the compound of the substituent with electron donating group was shifted upfield compared to the compound with the substituent withdrawing group electrons. The substituent groups very little influence on the electronic properties of the synthesized compounds. Emission spectra of the compounds show broad and unstructured bands, typical of emission from the 3MLCT thexi state. The three compounds were used as sensitizing dyes of solar cells. With this work it was observed that with the appropriate choice of substituents is possible to modulate the properties of the dyes. The photoelectrochemical performance was evaluated by testing current and potential curves showed the following results: cis-[Ru(phen)(dcbH2)(NCS)2]: Voc = 0.733 V, Jsc = 8.67 mA cm- 2, ff = 0.61 and £b = 3.78 %; cis-[Ru(Me2-phen)(dcbH2)(NCS)2]: Voc = 0.683 V, Jsc = 12.72 mA cm-2, ff = 0.55 and £b = 4.60 %; cis-[Ru(Ph2-phen phen)(dcbH2)( NCS)2]: Voc = 0.738 V, Jsc = 13.28 mA cm-2, ff = 0.63 and ç = 6.18 % .

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Complexos de bipiridinas de rutÃnio sÃo bastante estudados na literatura por apresentarem propriedades de interesse em diversas Ãreas como estudos fotoquÃmicos e fotofÃsicos, aplicaÃÃo em sistemas biolÃgicos e como fotosensibilizadores em cÃlulas solares. No presente trabalho, os complexos do tipo cis-[Ru(bpy)(dcbH2)(L)Cl], onde L = Azul do Nilo (NB), Azul de Toluidina (TBO), 9-Aminoacridina (9AA), Azure B (AB) e Violeta de Cresila (VC) foram sintetizados e caracterizados por tÃcnicas espectroscÃpicas e eletroquÃmica para aplicaÃÃo em cÃlulas solares sensibilizadas por corante (DSCs). AlÃm destes, o complexo cis-[Ru(dcbH2)(bpy)(TCNE)Cl] (Ru-TCNE) tambÃm foi testado como sensibilizador em DSC. Estes compostos apresentaram bandas de transferÃncia de carga do tipo MLCT na regiÃo do visÃvel e potenciais redox termodinamicamente favorÃveis para as reaÃÃes de transferÃncia de carga que ocorrem no dispositivo fotoeletroquÃmico. A adsorÃÃo quÃmica dos complexos sensibilizadores na superfÃcie do TiO2 foi evidenciada pelo deslocamento das bandas de MLCT para regiÃes de menor energia quando comparadas aos espectros em soluÃÃo. Os desempenhos fotovoltaicos dos complexos como sensibilizadores em DSC foram avaliados atravÃs das curvas corrente versus potencial, obtidas em condiÃÃes padrÃo AM 1,5. As DSCs contendo os sensibilizadores Ru-TBO e Ru-AB apresentaram os menores desempenhos fotovoltaicos com eficiÃncia global de 0,02 e 0,06%, respectivamente. JÃ as cÃlulas solares sensibilizadas pelos corantes Ru-NB e Ru-VC obtiveram um rendimento de 0,11% com baixos valores de eficiÃncia de incidÃncia de conversÃo de fÃtons a corrente, IPCE. Os melhores resultados foram para as cÃlulas contendo os corantes Ru-9AA e Ru-TCNE, as quais apresentaram rendimentos de 0,54 e 2,01%, respectivamente, com valores de IPCE iguais a 10% para Ru-9AA e 48% para Ru-TCNE. Todos os complexos apresentaram eficiÃncia global de conversÃo de energia solar em elÃtrica inferiores ao complexo padrÃo N3. / Bipyridines ruthenium complexes are widely studied in the literature for presenting interesting properties in various fields such as photochemical and photophysical studies, applications in biological systems and as photosensitizers in solar cells. In this work, the complexes of the type cis-[Ru(bpy)(dcbH2)(L)Cl], where L = Nile blue (NB), Toluidine blue (TBO), 9-aminoacridine (9AA), Azure B (AB) and Cresyl Violet (VC) were synthesized and characterized by spectroscopic and electrochemical techniques for application in dye-sensitized solar cells (DSC). In addition, the complex cis-[Ru(dcbH2)(bpy)(TCNE)Cl] (Ru-TCNE) was also tested as a sensitizer DSC. These compounds showed bands of charge transfer type MLCT in the visible region and thermodynamically favorable redox potentials for the charge transfer reactions which occur in the photoelectrochemical device. The adsorption of the chemical sensitizers complexes on the surface of TiO2 was evidenced by displacement of MLCT bands to lower-energy when compared to the spectra in solution. The photovoltaic performances of the complexes as sensitizers in DSC were evaluated through current versus potential curves obtained in standard AM 1.5 conditions. The DSC sensitizers containing Ru-TBO and Ru-AB had the lowest overall efficiency with photovoltaic performances of 0.02 and 0.06%, respectively. As for the dye-sensitized solar cells by Ru-NB and Ru-VC obtained a yield of 0.11% with low efficiency values of incident conversion of photon to current, IPCE. The best results were for cells containing the dyes Ru-9AA and Ru-TCNE, with energy conversion efficiency of 0.54 and 2.01%, respectively, with IPCE values equal to 10% for Ru-9AA and 48% for Ru-TCNE moieties. All complexes showed overall efficiency of converting solar energy into electricity below the N3 complex pattern.