Dissertations / Theses on the topic 'Dye-sensitized Solar Cells (DSSCs)'

ABSTRACT Background Solar energy plays a critical role in meeting the global energy challenge and represents one of the most promising energy sources for the future of the planet. Solar or photovoltaic cells are currently a hot topic on the market, these are devices that convert the energy of sunlight directly into electricity trough the photovoltaic effect. Strong and competitive research is currently devoted to lower the material costs of solar cells, and to increase their energy conversion efficiency. Up to now, commercially available photovoltaic technologies are based on inorganic materials, mainly crystalline silicon (first generation) and other semiconductors, such as gallium arsenide, indium phosphide and cadmium telluride (second generation In addition to high costs, also in terms of energy consuming, in fabrication processes, several of those materials are toxic and have low natural abundance. Therefore, in the last two decades the research focused on the development of a third generation of solar cells based on hybrid or organic materials, that offers a number of advantages, such as: high molar extinction coefficients, versatility of the chemical design for modulating the electronic properties, easy processability as well as low manufacturing costs. Although the efficiencies of organic-based photovoltaic cells ( 8%) are still at the moment a long way behind those obtained with purely inorganic based photovoltaic technologies ( 20%), the power conversion efficiency of organic solar cells have been significantly improved and there are expectations for more important results. Among the third generation of solar cells, the Dye-Sensitized Solar Cells (DSSC), also called Grätzel cells, have emerged as very promising candidates for low-cost alternative to conventional semiconductor photovoltaic devices. A DSSC cell scheme is shown in Figure 1. The cell components are: a mesoporous film of TiO2 (anode), a dye-sensitizer, an electrolyte, an electrochemical mediator and a cathode. The photovoltaic process in this cells can be resumed as follows: the dye-sensitizer (S), linked to semiconducting TiO2 surface (usually through a carboxylic group), absorbs a photon passing to the excited state S*, which transfers an electron to the conduction band of TiO2. The oxidized S+ thus obtained, is reduced by a redox mediator, generally I- from the couple I-/I3- dissolved in the electrolyte. The electron injected in TiO2 through the external circuit arrives to the cathode, where the reduction of I3- regenerates the iodide, closing the circuit. (Figure 1). Figure 1 The DSSC technology separates two requirements as: i) the charge generation, done at the semiconductor-dye interface and ii) the charge transport, done by the semiconductor and the electrolyte. Consequently, carrier transport properties can be improved by optimizing the semiconductor and electrolyte composition, while the spectral properties and thus charge generation can be improved by modifying the dye structure, that can be tailored in many ways by organic chemistry contribution. Many kinds of dyes have been studied for DSSCs application and in principle they could be divided in two classes (Figure 2): 1. metal complexes (N719, Zn-porphyrine e.g YD2-o-C8) , , 2. metal-free system Donor-Spacer-Acceptor (TA-St-CA) Figure 2 Up to now the best efficiencies (~11%) have been reached using ruthenium complexes, thanks to their large absorption range from visible to near infrared (NIR), and their capability to easily inject electrons in the conducting band of the semiconductor. The metal based chromophores still have several disadvantages such as not very high molar extinction coefficient and the presence of the expensive metal, such as ruthenium, which involves complicated synthesis and hard purification steps. On the contrary, metal-free dyes are simple and cheap to prepare and it is possible to easily modulate their photo- and electrochemical properties varying the functionalization, but very high efficiencies have not been achieved yet. The obtainment of new and more efficient dyes is therefore object of competitive international researches. Within this context, the present Ph.D. research project has focused on the synthesis of new metal-complexes and metal-free organic dyes characterized by a Donor-Spacer-Acceptor (D-π-A) structure, (Figure 3) in which the novelty is represented by the presence of benzo-condensed thiophene units as π bridge spacer. Figure 3 Aim of the work In such chromophores the π spacer plays a fundamental role, as it is responsible for the electronic communication between acceptor and donor moiety and for the extension of the conjugation that lead to wider and red-shifted absorption spectra. To date a number of new π-conjugated aromatic and heteroaromatic systems have been investigated and among these, thiophene or thienothiophene π-bridges have been reported to give remarkable efficiency. Benzodithiophenes systems BDT and BDT1 (Figure 4) attracted our attention because their rigid, π-conjugated, condensed-polycyclic structure , leads to unique electronic properties such as conductivity, high field effect mobility and tunable stacking in the solid state; rigid structures hamper the roto- vibrational modes responsible for the deactivation of the excited states in functional materials. Figure 4 In this Ph. D. work we investigated synthesis of suitably functionalized BDT and BDT1 derivatives as well as their use for the construction of two classes of dyes: 1) Zn-porphyrin based dyes (in collaboration with the research group of Prof. Pizzotti and Prof. Ugo) and 2) metal-free dyes and, Zn-porphyrin based dyes In addition the design of the new dyes have been oriented by preliminary theoretical calculations, done in collaboration with Dr. Filippo De Angelis of CNR-ISTM in Perugia, that allowed to gain insight into the molecular, ground and excited state electronic structure of the new chromophores. 1. Synthesis of new benzodithiophene containing Zn-porphyrins Metal porphyrins, characterised by very strong absorption bands around 450 nm (Soret band) and 600-700 nm (Q band) are potentially interesting as dyes for DSSC. For example, some push-pull type porphyrins bearing a carboxylic acid moiety as an anchoring group, have disclosed a remarkably high power conversion efficiency (6-7%), therefore in the recent years some research efforts have been devoted to the design, synthesis and application of new porphyrin-based chromophores for DSSC. , , The unique feature of these sensitizers is that the porphyrin chromophore itself constitutes the π-bridge of the D-π-A structure and with the aim of increasing the conjugation of the system, some new Zn porphyrins, containing the BDT1 unit (Figure 5), have been designed in our group. These porphyrin molecules are differently functionalized in 5,15 and 10,20 meso positions. In positions 5 and 15, aromatic rings bearing bulky groups are needed to avoid aggregation on the semiconductor surface, that drastically Figure 5 reduce the dye light-harvesting by a filtering effect. In 10,20 meso positions the structure presents two π-delocalized aromatic systems with opposite (electron-withdrawing or electron-donating) properties, in order to realize a push-pull system in which is possible to modulate the position and the intensity of the Q band and to favor the electron flow. The most promising structures were selected on the basis of preliminary theoretical calculations done by Dr. De Angelis and synthesized in collaboration with Prof. Ugo and Prof. Pizzotti’s research group. The novel Zn-porphyrin system 1 (Figure 6) was first synthesized, whose structure is characterized by the presence of BDT1 system in the acceptor part of the molecule. The suitable 2,6 di-functionalized BDT1 derivative was prepared and then linked to the porphyrin core. Figure 6 The resulting new Zn-porphyrin 1 was completely characterized from the analytical and photophysical point of view and used in preliminary tests as dye in Grätzel solar cells, giving an efficiency of 0.6%. Slightly optimization of the cell structure and in the composition of the electrolyte led to an increased efficiency of 2,54%. This result, although unsatisfactory, served as a starting point for the set-up of a number of synthetic protocols and for designing more targeted substitution and variation in the molecule structure. This part of the work is currently under progress. 2. Synthesis of benzodithiophene containing metal-free dye As already mentioned, the general structure of a metal-free dye, reported in Figure 3, presents a donor and an acceptor unit linked by a π-conjugate system. The most efficient structures reported in the literature contain triarylamines as donor unit, because of the prominent electron-donating ability and hole-transport of such molecules. Within this topic we designed novel metal-free triarylamine-containing organic dyes endowed with the innovative spacers BDT1 and its isomer BDT. Also in this case the design of the new compounds was oriented by preliminary TD-DFT calculations made by Dr. De Angelis, on two parent BDT1-containing structures 15 and 16, which differ from each other by the presence of a triple or a double bond. (Figure 7) With the aim to investigate the structure-performance relationship of the dyes in the cell, we designed a small library of structures, changing the BDT-bridge (17), the acceptor group (18) or the donor (19, 20) with respect of the model compound 16. (Figure 7). This allowed us to investigate the potentiality of BDT and BDT1 in the dyes in combination with double or triple bond in order to elongate the conjugation, and to obtain band gap reduction and enlarge the absorption spectra. In particular, the presence of the triple bond should ensures more planarity and therefore conjugation and avoids energy losses due to photoisomerization. The series of synthesized dyes are reported in figure 7. Figure 7 Almost all the dyes synthesized have also been characterized from a photophysical as well as electrochemical point of view, with the aim of identifying, among them, the most interesting and promising compounds for application in solar cells and try to clarify the relationship between the chemical structure and photovoltaic performances. Preliminary test in DSCs have been carried out for some of the dyes and among these dye 16 has emerged as the most promising one leading to an efficiency in liquid state cell of 5.11% and confirming the potential of BDT1 π-spacer for application in DSSCs. The cell efficiency found for 16, which is however still under optimization, allows us to say that this dye ranks among the promising dyes to date reported in literature. In addition, it must be pointed out that dye 16 seems to possess most of the essential chromophore characteristics required for obtaining high-performance DSSCs. The systematic study developed during the present Ph.D. thesis will be very useful for future improvement of the synthesized structures and their photovoltaic performances in DSSCs.

Dye-sensitized solar cell (DSSC) has attracted increasing interest as a promising hybrid organic-inorganic solar cell. At the heart of the device is a photosensitizer, which is anchored onto a wide-bandgap semiconducting metal oxide. It harvests solar light and transfers the energy via electron transfer to a suitable material (e.g. TiO2) to produce electricityas opposed to chemical energy in plant. The topic of this thesis focuses on the design and synthesis of metal-free organic dyes for applications in DSSCs. Specific attention has been paid to the correlation between the molecular structures and physical properties, as well as their performances in DSSCs. Chapter 1 presents the major components and working principle of DSSC, following by a brief overview of the development of organic dyes and their application in DSSCs. In chapter 2, we have designed two types of new phenothiazine-based dyes to investigate the positioning effect a donor group on the cell performance. The structural features of a donor aryl group at the C(7) position of phenothiazine core extend the π-conjugation of the chromophore and efficiently suppress the dye aggregation on TiO2 film. As a result, Type 1 dyes have better light harvesting properties in contact with TiO2 films, and give much better photovoltaic performance than Type 2 dyes. Chapter 3 presents the synthesis and characterization of a series of simple phenothiazine-based dyes, in which, a linear electron-rich (4-hexyloxy)phenyl group at C(7) of the phenothiazine periphery as the donor, and an alkyl chain with different length at N(10). The dye molecules show a linear shape which is favorable for the formation of a compact dye layer on the TiO2 surface, while their butterfly conformations can sufficiently inhibit molecular aggregation. Moreover, the alkyl substituents with different chain length at N(10) could further optimize the performance through complete shielding the surface of TiO2 from the Iˉ/I3ˉ electrolyte. Under simulated AM 1.5G irradiation, the PT-C6 based DSSC produces a short-circuit photocurrent of 15.32 mAcm−2, an open-circuit photovoltage of 0.78 V, a fill factor of 0.69, corresponding to a power conversion efficiency (PCE) of 8.18%. Moreover, we designed a stepwise approach for co-adsorption of the organic dye PT-C6 with a porphyrin dye (ZnP) for DSSCs. Upon optimization, the device made of the PT-C6 + ZnP system yielded Jsc = 19.36 mA cm-2, Voc =0.735 V, FF = 0.71 and η = 10.10%. In chapter 4, we further developed five organic dyes appended with T, TT, E, ET, or EE (T and E denote thiophene and 3,4-ethylenedioxythiophene (EDOT), respectively) on the C(7) atom of phenothiazine core as electron donors. We have also analyzed the structure-performance corelations of dye molecules in the aspect of dye aggregation, electron injection, dye regeneration and interfacial charge recombination of electrons with electrolytes and/or oxidized dye molecules, through DFT calculation, impedance analysis and transient photovoltage studies. In chapter 5, we extended our studies by using phenothiazine as a building block to construct 3D bulky organic dyes. We systematically investigated the influence of 3D bulky substituents on dye aggregation and charge recombination, as well as photovoltaic performance of DSSCs. The molecular design strategy demonstrates that high Voc can be realized by employing 3D-phenothiazine dyes featuring a bulky substituent, such as, hexylcarbazole and dihexylfluorene units. Impressively, the co-adsorbent-free DSSCs based on dye TP3 exhibits a photovoltaic performance with efficiency up to 8.00 %. In order to realize a panchromatic absorption and further enhance the energy conversion efficiency of DSSCs, we also designed a stepwise approach for co-adsorption of the organic dye TP3 with a NIR dye YR6 for co-sensitized DSSCs. Upon optimization, the device made of the TP3 + YR6 system yielded Jsc = 19.18 mA cm-2, Voc =0.721 V, FF = 0.712 and η = 9.84 %. The power-conversion efficiency is the highest reported efficiency for a squaraine dye-based co-sensitized panchromatic DSSCs. From chapters 6 and 7, a series of new simple panchromatic dyes based on thiadiazolo[3,4-c]pyridine (PyT) have been designed for panchromatic DSSCs. These new organic dyes exhibit broad absorption spectrum in the range of 300~850 nm and high molar extinction coefficients. The electrochemical analyses demonstrate that the incorporation of the auxiliary electron-deficient thiadiazole[3,4-c]pyridine unit can fine-tune the HOMO and LUMO energy levels and red-shift the absorption spectra to NIR region. The overall conversion efficiencies of liquid-electrolyte DSSCs based on these sensitizers range from 0.46 to 6.30 %. We draw some conclusions in chapter 8 together with the outlooks in DSSCs

Com foco nos últimos seis anos, o sistema bicíclico dicetopirrolopirrol tem sido cada vez mais utilizado como ”bloco de construção” de materiais (polímeros e moléculas pequenas) para utilização em células solares. Isso deve-se principalmente à sua alta estabilidade ambiental (principalmente fotoestabilidade) e capacidades de transferência de carga. Apesar dos estudos serem ainda recentes, os resultados já alcançados mostraram o tremendo potencial dos dicetopirrolopirróis em células solares. O trabalho descrito nesta tese de Mestrado envolveu a síntese de vários derivados de dicetopirrolopirrol com o objetivo de introduzir unidades fotossensibilizantes ligadas covalentemente ao sistema dicetopirrolopirrol. Os novos compostos podem vir a incorporar um grupo carboxílico para suportar o corante na superfície de um óxido semicondutor das células solares sensibilizadas por corantes (DSSCs). A primeira parte do trabalho consistiu na alquilação ou arilação dos grupos NH de dicetopirrolopirróis comerciais. Posteriormente, foram estudados métodos de funcionalização dos grupos arilo nas posições 3 e 6 dos DPP por reações catalisadas por paládio ou por clorossulfonação. Todos os dicetopirrolopirróis sintetizados foram caracterizados por ressonância magnética nuclear, espetrometria de massa e espectrofotometria de UV-visível. Alguns compostos foram também caracterizados por fluorescência; Abstract: With a focus on the last six years, the bicyclic diketopyrrolopyrrole (DPP) system has been increasingly used as an active building block in materials (polymers and small molecules) used in solar cells. That is mainly due to its high environmental stability (mainly photostability) and charge transfer capabilities. Despite its infancy, the results already achieved have shown the tremendous potential of diketopyrrolopyrroles in solar cells. The work reported in this Master thesis involved the synthesis of several diketopyrrolopyrrole derivatives aiming introducing photosensitizing units covalently linked to the diketopyrrolopyrrole system. The new compounds may be functionalized with carboxylic groups to support the dye firmly at the surface of a semiconductor oxide of dye-sensitized solar cells (DSSCs). The first part of the work consisted in the alkylation or arylation of the NH groups of commercially available DPP. Then, new methods for the functionalization of the aryl groups at 3 and 6 positions of DPP were studied, mainly by palladium catalysed reactions or by chlorosulfonation. All diketopyrrolopyrrole derivatives synthesized were characterized by nuclear magnetic resonance, mass spectrometry and UV-vis spectrophotometry. Some compounds were also characterized by fluorescence.

During the thesis the DSSCs optimization was analyzed mainly through two strategies: the study of new sensitizers and the study of alternatives materials for photo-cathode fabrication. Two class of sensitizers were be analyzed: squaraine dyes and cyclometalated-based dyes. Then a study on dye-loading process will be presented, with implication in an industrialization process. For the photo-cathode fabrication two di erent materials were studied, a carbon based material and a polymeric material. Then a part of the work concerned the study of devices analysis system. In particular electrochemical impedance spectroscopy was studied to propose a new set up to analyze electric processes in different cell components.3

Due to the strong increase in the world energy consumption, and need of exploiting carbon neutral energy sources, increasing efforts have been devoted to the exploitation of solar energy technology. For their unique properties, Dye Sensitized Solar Cells (DSSC) could complement the established silicon junctions. This Ph.D. thesis is mainly focused on the understanding of the (photo)/electrochemical properties of new components for DSSCs. The first chapter, realized in collaboration with the Prof. Stagni’s group, is about the characterization of new examples of Ru(II)-tetrazolato dyes as thiocyanate-free sensitizers for solar cell applications. Four complexes (D1-D4) have been analyzed together with the well know standard N719. The combination of the electrochemical and spectroscopic analyses revealed ground and excited states thermodynamic properties suitable for efficient interfacial charge separation. These features resulted in external quantum yield of photon to electron conversions higher than 80%. The best performances have been recorded in the case of D4 thanks to the combinations of the broader harvesting, efficient regeneration, and electron injection. Three chapters of my thesis report about the collaborative research carried out with the groups led by Dr. P.C. Gros and Dr. M.C. Pastore, involving the investigation of the electronic properties of Fe(II)NHC (NHC=N-Heterocyclic-Carbene) sensitizers. First, we tried to rationalize the charge transfer dynamics of C1 a homoleptic complex bearing σ-donating NHCs and π-accepting carboxylic groups, which initially reported rather low performances (0.13 % of PCE%). We achieved a substantial progress in cell efficiency (PCE = 1%). We estimated an injection quantum yield (Φinj) of ca. 50% that, is believed to be the main limitation for the rather low PCE. In consideration of the excited state energetics, nearly optimal for injection into TiO2, this relatively low Φinj could be due to a non-optimal electronic coupling arising from the symmetric design of the homoleptic C1. For this reason, we moved to Fe(II)NHC heteroleptic designs characterized by an asymmetric coordination sphere. The first complex was the asymmetric analogue of C1 named ARM13, while other design incorporated spacers between the anchoring moieties and the pyridine linked to the metal center, in particular, a thiophene in the case of ARM7 and a phenyl ring in the case of ARM11. The rationale behind such designs was to increase the electron-hole separation and the light harvesting capability. We were able to obtain the highest power conversion efficiency (ARM13 ca. 1.5%) ever reported for a Fe(II) sensitizer. In a third project, we designed, realized and characterized a new family of heteroleptic Fe(II)NHC complexes bearing electron withdrawing or donating substituents on the ancillary ligands. In particular, among the new series, ARM130 bearing a dimethoxyphenyl group, exhibited the best performance, thanks to its improved light harvesting capability introduced by the electron-donating -OMe moieties. We obtained a Power Conversion Efficiency of 1.83%. The last chapter of my thesis is about the investigations of alternative counter electrode (CE) materials for DSSCs based on the poly(3,4-ethylenedioxythiophene) (PEDOT) conductive polymer. The best and well-known electrocatalyst PEDOT/ClO4 (PER) involves the use of organic solvents, greener and sustainable alternative deposition routes are desirable. We explored the electrochemical properties of PEDOT/Nafion CE (NAF), produced through water- based electropolymerization. The electrocatalytic behavior of PER and NAF has been investigated in STLC by means of LSV and EIS, in the presence of either Co- or Cu- based electrolyte, NAF rivals the kinetic and mass transport properties of PER. This result was confirmed by the performance of D35 sensitized solar cells, where NAF counter electrodes generated comparable efficiency of those recorded for PER.
A causa dell’aumento della richiesta energetica e della necessità di esplorare risorse sostenibili, ingenti sforzi sono rivolti verso l’applicazione di tecnologia solare. Grazie alle loro peculiarità, le Celle Solari Sensibilizzate con Colorante (DSSCs) potrebbero essere uno strumento complementare alla tecnologia al silicio. Questa tesi di Dottorato è incentrata nella comprensione delle proprietà (foto)/elettrochimiche di nuovi componenti per DSSCs. Il primo capitolo sperimentale, realizzato in collaborazione con il gruppo del Prof. Stagni, ha avuto come scopo la caratterizzazione di nuovi sensibilizzatori di Ru(II)-tetrazolati come esempio di complessi privi di leganti tiocianati. Quattro complessi (D1-D4) sono stati studiati assieme al ben noto standard di rutenio N719. La combinazione dell’analisi elettrochimica e spettroscopica ha evidenziato come la termodinamica dello stato fondamentale ed eccitato sia in grado di favorire un’efficiente separazione di carica. Queste caratteristiche hanno portato ad una resa quantica di conversione di fotoni in elettroni superiore all’80%. D4 è risultato essere il complesso più efficiente grazie alla combinazione della più estesa estensione spettrale, efficiente rigenerazione ed efficiente iniezione di carica. Gran parte della mia attività, tuttavia, è stata rivolta allo studio di sensibilizzatori per DSSCs a base di ferro. Tre capitoli, in collaborazione con i gruppi del Dr. P. C. Gros e dalla Dr. M. C. Pastore, riportano l’investigazione delle proprietà elettroniche di sensibilizzatori di Fe(II)NHC. Nel primo di questi abbiamo studiato le proprietà di trasferimento dinamiche di un complesso omolettico denominato C1, caratterizzato da leganti NHC σ-donatori e gruppi carbossilici π-accettori, il quale aveva inizialmente restituito valori di efficienza dello 0.13%. Abbiamo ottenuto un sostanziale aumento di efficienza ottenendo valori vicini all’1%. Il rendimento quantico di iniezione di carica è risultato essere attorno al 50% e costituisce il principale fattore limitante per le DSSCs a base di ferro. L’energetica dello stato eccitato è risultata ottimale per un’efficiente iniezione di carica quindi, le limitate prestazioni esibite da C1 derivano dal suo design simmetrico che porta ad un accoppiamento elettronico non favorevole con la superficie. Abbiamo così analizzato complessi carbenici eterolettici, il primo di questi era l’analogo asimmetrico di C1, ARM13, altri due invece erano caratterizzati dall’introduzione di un anello tiofenico (ARM7) e uno fenilico (ARM11) aventi la funzione di spaziatori fra le funzionalità ancoranti e le piridine coordinate al metallo centrale. L’idea di questo nuovo design era quella di aumentare la separazione di carica ed incrementare la capacità di raccolta di fotoni. Abbiamo ottenuto la più alta efficienza di cella riportata in letteratura del 1.5% per ARM13. In un terzo progetto abbiamo analizzato una nuova famiglia di complessi eterolettici caratterizzati dall’introduzione di gruppi elettron-donatori o elettron-attrattori sui leganti ancillari. ARM130, caratterizzato da una funzionalità dimetossifenilica, ha restituito le migliori performances dell’1.83%. L’ultimo capitolo della mia tesi riguarda invece lo studio di un controelettrodo (CE) alternativo per DSSCs basato su polimeri conduttori a base di poli(3,4-etilendiossitiofene) (PEDOT), fra questi il ben noto PEDOT/ClO4 (PER), elettropolimerizzato da solventi organici, risulta essere il miglior materiale elettrocatalitico. Al fine di studiare soluzioni più sostenibile, abbiamo esplorato le proprietà elettrochimiche di CE a base di PEDOT/Nafion (NAF) prodotti in ambiente acquoso. Il comportamento elettrocatalitico di PER e NAF è stato investigato in celle simmetriche mediante LSV ed EIS e in celle solari in presenza di D35, quest’ultimo ha generato efficienze di cella comparabili a quelle registrate in presenza di PER.

En raison des problèmes d'instabilité à moyen termes des cellules solaires à colorant (DSSC), l'électrolyte liquide à base d'iodure a été remplacé par plusieurs types de matériaux solides transport de trous (HTM) pour obtenir des DSSCs à l'état solide (s-DSSCs). Parmi ces matériaux, l’utilisation des polymères conducteurs(PC) a attiré une attention considérable en raison de leur bonne stabilité, de leur haute conductivité et de la facilité de leur dépôt sur le semi-conducteur mésoporeux TiO2. Dans ce travail de thèse, plusieurs s-DSSCs basées sur des PC utilisés comme HTM ont été développés dans le but d'améliorer leurs performances photovoltaïques en tenant compte des deux objectifs suivants: (i) l'optimisation des processus de transfert inter facial de charge dans la cellule solaire, et (ii) l'optimisation du transport de charge dans le semi-conducteur d'oxyde de type n. Pour atteindre ces objectifs, chaque composant de la s-DSSC a été modifié afin d'étudier son effet sur les performances du dispositif final. En première tentative, une étude analytique est réalisée en faisant varier le sensibilisateur afin de déterminer les fragments de la structure du colorant, qui ont un effet important sur le processus de photopolymérization électrochimique in-situ (PEP) à la fois en milieu organique et en milieu aqueux mais aussi sur les performances des s-DSSCs. Sur la base de ces résultats, un nouveau concept a été développé et consiste en la suppression totale de l'interface entre le colorant et le HTM. Ceci est obtenu par la synthèse de nouveaux colorants liés de façon covalente à un monomère électroactif qui est co-polymérisé par la PEP in-situ. Le copolymère résultant, utilisé comme HTM, est lié de manière covalente au colorant. En outre, la nature de la liaison chimique, reliant le résidu triphénylamine TPA au monomère, est également étudiée comme un facteur clé dans les performances de s-DSSC. En outre, et pour optimiser les processus de transport de charges dans ce type de s-DSSC, de nouvelles s-DSSC basées sur ZnO ont été réalisées et étudiées
Due to instability problems of dye sensitized solar cells (DSSCs) in longtime uses, the iodine based liquidelectrolyte has been replaced by several types of solid hole transporting materials (HTM) to perform solidstate DSSCs (s-DSSCs). Among them, the substitution by conducting polymers (CP) has attractedconsiderable attention because of their good stability, high hole-conductivity and simple deposition withinthe mesoporous TiO2 semiconductor. In this thesis work, several s-DSSCs based on CPs used as HTM havebeen developed in order to improve their photovoltaic performances taking into account the following twoobjectives: (i) the optimization of the interfacial charge transfer processes within the solar cell, and (ii) theoptimization of the charge transport within the n-type oxide semiconductor. To reach these goals, eachcomponent that constitutes the device was varied in order to investigate its effect on the device’sperformances. As first attempt, an analytical study is carried out by varying the sensitizer in order todetermine the fragments of the dyes structures, that have an important effect on the in-situ photoelectrochemical polymerization process (PEP) both in organic and in aqueous media and hence on theperformances of the s-DSSCs. Based on these results, a new concept of removing completely the interfacebetween the dye and the HTM is developed. This is achieved by the synthesis of new dyes covalently linkedto an electroactive monomer which is co-polymerized by in-situ PEP. The resulting co-polymer, used asHTM, is covalently linked to the dye. In addition, the nature of the chemical bond linking the triphenylamineresidue TPA to the monomer is also investigated as a key factor in the s-DSSCs performances. Besides, andto optimize the charge transport processes within this type of s-DSSC, the elaboration of novel ZnO baseds-DSSCs has been achieved and investigated

The design of dyes for NiO-based dye-sensitized solar cells (DSSCs) has drawn attention owing to their potential applications in photocatalysis and because they are indispensable for the development of tandem dye-sensitized solar cells. The understanding of the electron transfer mechanisms and dynamics is beneficial to guide further dye design and further improve the performance of photocathode in solar cells and solar fuel devices. Time-resolved spectroscopy techniques, especially femtosecond and nanosecond transient absorption spectroscopy, supply sufficient resolution to get insights into the charge transfer processes in p-type dye sensitized solar cell and solar fuel devices. In paper I-V, several kinds of novel organic “push-pull” and inorganic charge transfer dyes for sensitization of p-type NiO, were systematically investigated by time-resolved spectroscopy, and photo-induced charge transfer dynamics of the organic/inorganic dyes were summarized. The excited state and reduced state intermediates were investigated in solution phase as references to confirm the charge injection and recombination on the NiO surface. The charge recombination kinetics is remarkably heterogeneous in some cases occurring on time scales spanning at least six orders of magnitude even for the same dye. In this thesis, we also proposed a novel concept of solid state p-type dye sensitized solar cells (p-ssDSSCs) for the first time (paper VI), using an organic dye P1 as sensitizer on mesoporous NiO and phenyl-C61-butyric acid methyl ester (PCBM) as electron conductor. Femtosecond and nanosecond transient absorption spectroscopy gave evidence for sub-ps hole injection from excited P1 to NiO, followed by electron transfer from P1●- to PCBM. The p-ssDSSCs device showed an impressive 620 mV open circuit photovoltage. Chapter 6 (paper VII) covers the study of electron transfer mechanisms in a covalently linked dye-catalyst (PB-2) sensitized NiO photocathode, towards hydrogen producing solar fuel devices. Hole injection from excited dye (PB-2*) into NiO VB takes place on dual time scales, and the reduced PB-2 (PB-2●-) formed then donates an electron to the catalyst unit.  The subsequent regeneration efficiency of PB-2 by the catalyst unit (the efficiency of catalyst reduction) is determined to ca. 70%.

This project employs the biological compounds hemin, melanin, and retinoic acid as photoactive dyes in dye-sensitized solar cells (DSSCs). These dyes are environmentally and economically superior to the standard ruthenium-based dyes currently used in DSSCs because they are nontoxic and widely available. Characterization by linear sweep voltammetry yielded averaged maximum overall conversion efficiency values of 0.059% for retinoic acid, 0.023% for melanin, and 0.015% for hemin. Absorption spectra of hemin and retinoic acid suggest that they would complement each other well when used in tandem in one cell because hemin has a secondary maximum absorption peak at 613nm and retinoic acid has maximum absorption at 352nm. Cells made with hemin or melanin performed better with the use of lower temperatures to seal the cells, and hemin cells performed exceptionally well with exclusion of the sealing procedure. These biologically-derived cells have the potential to advance the development of inexpensive and safer solar energy sources, which promise to serve as clean energy sources in the near future.

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.

Over the past decade, intensive academic and commercial interests have been paid on compounds possessing photochemical properties, namely for their preparation, chemical properties, high efficiency and potential low-cost. Compounds having intense photochemical properties gained great interest due to wide range of potential applications. The sensitizers are one of the key components for high power-conversion efficiency in the dye sensitized solar cells (DSSCs). They are the core components in the organic light-emitting devices (OLEDs) due to their ability to emit light with the wavelengths largely red- shifted from their absorption wavelength. Ruthenium based sensitizers have been tagged “molecular light switches” because, although the fluorescence of these complexes in aqueous solutions is negligible, it increases of greater than 10000 fold in the presence of DNA. Many polypyridyl and dipyrido phenazine ruthenium complexes have achieved high power conversion efficiencies and therefore are of practical interest. Several research groups stated that the dipyrido phenazine ligand may be thought of as comprising two components: a bipyridyl unit and a phenazine unit. These two subunits behave essentially separately, with many molecular orbitals being localised over only one subunit and a redox properties of central phenazine moiety in the dipyrido phenazine ligand are important for the photochemical applications. Therefore a phenazine ligand was selected as a model for the present investigation. The chemistry of phenazine ligand is mostly limited to the late transition metal and f - element complexes. Our laboratory has a rich backgroung in the aluminum and early transition metal chemistry. The aluminum chemistry and early transition metal chemistry are of great interest since aluminum and early transition metal complexes are environmentally friendlier and cheaper than the late transition metal compounds. Another drawback of the ruthenium-based sensitizers is the lack of absorption in the red region of the visible spectrum, and also low molar extinction coefficients. An essential requirement for efficient conversion of solar energy is the good spectral match of the sensitizer absorption to the emission spectrum of solar radiation. In this regard, the ruthenium sensitizers’ spectral response in the lower energy regions is not sufficient. The current project has three parts. In the first part we collected and reviewed known literature regarding the certain classes of non-innocent ligands containing the six-membered carbon- nitrogen heterocycles and regarding the ligands potentially important for the photochemical applications. We also reviewed all available to the data information about the complexes supported by the phenazine ligand. In the second part we have investigated interaction of alkylaluminum compounds and phenazine and observed reduction of phenazine accompanied by formation of dialuminum cage type compounds containing two formally mononegative phenazine ligand. The derivatization of phenazine has been also observed. It resulted in formation of compounds having a stable organic radical. In a third part of our project we have explored interaction of phenazine or thiophenazine with the alkylaluminum compounds and chromium dichloride. The reaction in the three component system resulted in reduction of phenazine ligand and lead to the heterometallic Cr(II) - aluminum complexes containing a formally dinegative phenazine or thiophenazine ligands. When a large excess of triethylaluminum was taken, reduction of phenazine and chromium has been observed leading to the heterometallic multinuclear Cr(I) - aluminum complex containing a formally dinegative phenazine ligands and two chromium atoms in one complex in the rare oxidation state one.

Esta tesis presenta un conjunto original de procedimientos para la síntesis de nanoestructuras de TiO2 y ZnO por biomimetización de membranas de cáscara de huevo obteniendo materiales valiosos para fotovoltaica como se muestra en su evaluación de rendimiento como ánodo en células solares sensibilizadas por colorante. "El manuscrito está dividido en 7 capítulos. En el primer capítulo, titulado Introducción, se presentan las bases teóricas para la comprensión de los procesos de biomimetización, membranas de cáscara de huevo, síntesis de ZnO y TiO2, y células solares sensibilizadas por colorantes (DSSC). Después del capítulo introductorio, el Capítulo 2 revela los objetivos generales y específicos de esta investigación. Posteriormente, el Capítulo 3 describe el procedimiento experimental utilizado para las síntesis y caracterizaciones de ZnO y TiO2, así como el procedimiento utilizado en el ensamblaje y la caracterización de las células fotovoltaicas. En el capítulo 4 se presentan y discuten los resultados obtenidos con las síntesis y la aplicación de los polvos como fotodoles en DSSC. En este capítulo, hemos decidido subdividirlo en secciones específicas para explicar cuestiones científicas específicas sobre el tema. En el capítulo 5 se presentan las conclusiones del estudio en vista de los diferentes aspectos: obtención de TiO2 biomimético y ZnO, diferencias entre los polvos sintetizados por biomimetización de las membranas de cáscara de huevo, y la caracterización de las células construidas con los polvos biomiméticos.
Esta tesi presenta un conjunt original de procediments per a la síntesi de nanoestructuras de TiO2 i ZnO per biomimetización de membranes de corfa d'ou obtenint materials valuosos per a fotovoltaica com es mostra en la seua avaluació de rendiment com a ànode en cèl·lules solars sensibilitzades per colorant. "El manuscrit està dividit en 7 capítols. En el primer capítol, titulat Introducció, es presenten les bases teòriques per a la comprensió dels processos de biomimetización, membranes de corfa d'ou, síntesi de ZnO i TiO2, i cèl·lules solars sensibilitzades per colorants (DSSC) . Després del capítol introductori, el Capítol 2 revela els objectius generals i específics d'esta investigació. Posteriorment, el Capítol 3 descriu el procediment experimental utilitzat per a les síntesis i caracteritzacions de ZnO i TiO2, així com el procediment utilitzat en l'acoblament i la caracterització de les cèl·lules fotovoltaiques. En el capítol 4 es presenten i discutixen els resultats obtinguts amb les síntesis i l'aplicació de les pols com fotodoles en DSSC. En este capítol, hem decidit subdividir-ho en seccions específiques per a explicar qüestions científiques específiques sobre el tema. En el capítol 5 es presenten les conclusions de l'estudi en vista dels diferents aspectes: obtenció de TiO2 biomimético i ZnO, diferències entre les pols sintetitzats per biomimetización de les membranes de corfa d'ou, i la caracterització de les cèl·lules construïdes amb les pols biomiméticos.
This thesis introduces an original set of procedures for the Synthesis of ZnO and TiO2 nanostructures by biomimetization of eggshell membranes obtaining valuable materiales for photovoltaic as shown on their performance evaluation as anode in Dye-Sensitized Solar Cells". The manuscript is divided into 7 chapters. In the first chapter, entitled Introduction, it is presented the theoretical bases for the understanding of the biomimetization processes, eggshell membranes, ZnO and TiO2 syntheses, and dye-sensitized solar cells (DSSC). After the introductory chapter, Chapter 2 reveals the general and specific objectives of this research. Subsequently, Chapter 3 describes the experimental procedure used for the syntheses and characterizations of ZnO and TiO2 as well as the procedure used in the assembly and characterization of the photovoltaic cells. In chapter 4 are presented and discussed the results obtained with the syntheses and application of the powders as photoanodes in DSSC. In this chapter, we have chosen to subdivide it into specific sections to explain specific scientific issues on the subject. In chapter 5 the conclusions of the study are presented in view of the different aspects: obtaining ZnO and biomimetic TiO2, differences between the powders synthesized by biomimetization of eggshell membranes, and the characterization of the cells constructed with the biomimetic powders.
Camaratta, R. (2018). Synthesis Of ZnO and TiO2 By Biomimetization Of Eggshell Membranes And Its Evaluation As Anode In Dye-Sensitized Solar Cells [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/113416
TESIS

Le Dye – Sensitized Solar Cells (DSSC) sono attualmente considerate tra le alternative più promettenti al fotovoltaico tradizionale. I ridotti costi di produzione e l’elevata versatilità di utilizzo rappresentano i punti di forza di questi dispositivi innovativi. Ad oggi la ricerca è concentrata prevalentemente sull’incremento delle prestazioni delle DSSC, ottenibile solamente attraverso un miglioramento delle funzioni dei singoli componenti e dell’interazione sinergica tra questi. Tra i componenti, ha recentemente assunto particolare interesse il blocking layer (BL), costituito generalmente da un film sottile di TiO2 depositato sulla superficie dell’anodo (FTO) e in grado di ottimizzare i fenomeni all’interfaccia FTO/TiO2/elettrolita. Nel corso di questo lavoro di tesi si è rivolta l’attenzione prevalentemente sulle caratteristiche del BLs (ad esempio proprietà morfologico – strutturali) cercando di mettere in correlazione il processo di deposizione con le caratteristiche finali del film ottenuto. A questo scopo è stato ottimizzato un processo di deposizione dei film via spin coating, a partire da soluzioni acquosa o alcolica di precursore (TiCl4). I film ottenuti sono stati confrontati con quelli depositati tramite un processo di dip coating riportato in letteratura. I BLs sono stati quindi caratterizzati tramite microscopia (SEM – AFM), spettrofotometria (UV.- Vis) e misure elettrochimiche (CV – EIS). I risultati ottenuti hanno messo in evidenza come i rivestimenti ottenuti da soluzione acquosa di precursore, indipendentemente dalla tecnica di deposizione utilizzata (spin coating o dip coating) diano origine a film disomogenei e scarsamente riproducibili, pertanto non idonei per l’applicazione nelle DSSC. Viceversa, i BLs ottenuti via spin coating dalla soluzione alcolica di TiCl4 sono risultati riproducibili, omogenei, e uniformemente distribuiti sulla superficie di FTO. Infine, l’analisi EIS ha in particolare evidenziato un effettivo aumento della resistenza al trasferimento di carica tra elettrodo FTO ed elettrolita in presenza di questi BLs, fenomeno generalmente associato ad un efficace blocking effect.

The present work is focused on the design, fabrication and characterization of a novel dye-sensitized solar cell architecture exploiting some microfluidic concepts. By using this innovative approach, it became possible to fabricate small laboratory test-cells with a high degree of assembly reproducibility, and develop some interesting applications, like the analysis of the cell components over time, without compromising the cell integrity. The proposed architecture has been used in the experimental test of some innovative materials. The study is of experimental nature and supported by different photoelectrochemical analytical techniques, in order to study the cell behavior.

This work investigates the correlation of structural and photovoltaic properties of dyes used in dye-sensitized solar cells. Experimental methods, including ultraviolet-visible spectroscopy, fluorescence spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy are employed to study optical and electrochemical properties of dye molecules. Computational methods, including density functional theory and time-dependent density functional theory, are used to validate and predict the optical and electronic properties of dye molecules, in their isolated state and once embedded into a working electrode device environment that comprises a dye...TiO2 interface. The results chapters begin with the presentation of a series of quinodimethene dyes that are experimentally validated for their photovoltaic application, and associated computational studies reveal that an inner structural factor - a phenyl ring rotation occurring during the optical excitation process - leads to the competitive photovoltaic device performance of these dyes. Carbazole-based dyes are then systematically studied by computation, especially considering charge transfer paths and binding modes of these dyes on a titania surface. The theoretical models for the basic building block of this chemical family of dyes, known as MK-44, successfully support and explain structural discoveries from X-ray diffraction and reflectometry that impact of their function. A benzothiadiazole-based dye, RK-1, is then systematically studied by both experimental and computational methods, and the results show that the π-bridge composed of thiophene, benzothiadiazole and benzene rings leads to excellent charge separation; and the rotation of these rings during the optical excitation process may well be consistent with the fluorescence spectrum. Finally, the well-known ruthenium-based dyes are theoretically studied to determine the properties of different ligands connected to the metal core of the complex. Conformations with different NCS ligands are calculated in terms of energy and explain well the corresponding results from X-ray diffraction. Acid-base properties of carboxyl groups connected to pyridine ligands in N3 and N749 are theoretically calculated based on thermodynamics and density functional theory. Implicit and explicit models are both adopted to predict these acid dissociative constant values, which are generally in a good agreement with the reported experimental data. The thesis concludes with conclusions and a future outlook.

In the last decades, the global perception of the energy crisis has encouraged the research to the development of novel technologies from renewable sources. In particular, great emphasis has been devoted to solar energy conversion devices. While the international photovoltaic market is still dominated by silicon-based solar cells, perovskite solar cells (PSCs), dye-sensitized solar cells (DSSCs) and luminescent solar concentrators (LSCs) are considered as promising alternative technologies in terms of improved manufacturability and for the possibility to reduce the costs and enhance the light trapping of high efficiency silicon-based solar cells. Additionally, in the last years PSCs have become potentially competitive to silicon-based solar cells for their high efficiency, while DSSCs and LSCs have the great advantage to work under diffused light and they can be easily incorporated into the building-integrated photovoltaics (BIPV). Despite the attractive features, these technologies still present some efficiency and instability issues related to their components. For this reason, the design of novel materials for highly performant and stable solar energy conversion devices has become an attractive and challenging issue involving worldwide researchers. In this context, this thesis addresses the design of novel organic compounds to be employed in PSCs, DSSCs and LSCs. In particular, the design of novel organic i) hole transport materials (HTMs) in PSCs, ii) sensitizers in DSSCs and iii) fluorophores in LSCs has been carried out with the aim of contributing to the development of potentially more efficient devices. To this end, state-of-the-art Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TDDFT) methods represent powerful tools for the discovery of promising materials in the solar energy field. Here, they have mainly been applied for the prediction of the most relevant physical-chemical properties of the designed compounds and for the investigation of the structure-property relationships. In such a way it is possible to make an assessment of their suitability to be employed in the aforementioned devices. The outcomes of these studies would provide the fertile ground for future projects that will hopefully contribute to accelerate the research in this field. Indeed, the increased knowledge on the behavior of these materials is the key for boosting the development of solar energy conversion devices in the near future.

In perfect harmony with the "Year of Light (IYL 2015)", the thesis has been entirely centered on the concept of Light involving the development of transition metal complexes for both the conversion of light into electric energy through dye-sensitized solar cells and, conversely, the production of light starting from electricity by fabrication of electroluminescent devices. The main part of the thesis has been devoted to the sunlight-to-electricity conversion, a target that is well contextualized within the global commitment for the progressive increase of the percentage of electric energy produced by renewable resources. In this context dye-sensitized solar cells, DSSCs, are promising devices alternative to the well established technology of silicon photovoltaics for energy production from the abundant solar light. DSSCs are devices able to harvest solar light and convert it into electricity employing a sensitizer (adsorbed on a semiconductor) and a redox couple properly chosen and combined. The project have concerned the design, synthesis and characterization of both sensitizers and redox mediators constituted by ruthenium and copper-based complexes respectively, together with their final assembly into laboratory-type DSSCs to evaluate their performance. In this way an all-round study has been carried out, from molecules on paper to test benches, passing through laboratory counters. The second, minor part, of the thesis has been focused on the diametrically opposed task, the generation of light. In this context some luminescent heteroleptic complexes based on the cheap and quite abundant copper element have been proposed. The final aim has been the synthesis of efficient luminophores for fabrication of devices able to generate light applying an electric potential across two electrodes such as in organic light-emitting diodes, OLEDs, or in analogue light-emitting electrochemical cells, LECs. The light production in LED-type devices is very efficient especially compared with other traditional artificial light sources like incandescent or fluorescent lamps, and so in line with the international policy of reducing energy consumption. In conclusion the thesis project can be schematically depicted as a circular pathway that joints together two opposite but strictly interconnected concepts (i.e. light and electricity) mutually corresponding to the task and the mean, the start and the end.

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.

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.

Electrocatalysis contributes to a huge extent in a large array of research fields and applications, including corrosion science, electroanalytical sensors, wastewater treatment, electro-organic synthesis and more importantly, energy conversion applications. Of the many electrocatalytic processes, the oxygen evolution reaction (OER) and triiodide reduction reaction (IRR) are of widespread importance in electrochemical cells and dye-sensitised solar cells (DSSCs). OER is a key half reaction in electrochemical water splitting, direct solar-to-electricity driven water splitting and metal-air batteries. The high cost of efficient benchmark electrocatalysts, such as RuO2 or IrO2, however, is a major drawback of OERs. While, IRR plays a significant role in DSSCs, which must be electrocatalysed at the counter electrode to complete the external circuit in real devices and thereby successfully convert solar energy to electricity. Traditionally, Pt is accepted as an ideal benchmark electrocatalyst for IRR, but its high cost and scarcity limits broad application of DSSCs. Thus, extensive effort has been made to find active alternative electrocatalysts with low-cost, high electrocatalytic activity and excellent stability for OER and IRR to the noble metals (Ru, Ir and Pt). Therefore, a rational design of earth-abundant and low-cost electrocatalysts for OER and IRR maintains a paramount significance for energy conversion applications to meet the constantly growing demand for energy supply.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

Chapter 1. Introduction a) Dye-sensitized solar cells: Dye-sensitized solar cells have emerged as potential device architecture to harvest the solar energy due to easy device fabrication and diversity in key components such as semiconductors, dyes, electrolytes and cathode materials. Ever since the discovery of homoleptic metallated ruthenium-based dyes with 7.12% device efficiency by O’Regan and Grätzel, last two and half decade research on the functional materials provided deeper understanding of device structure and various pathways associated with interfacial charge transfer and recombination processes. Dye design with panchromatic absorption, avoiding both charge recombination and dye aggregation achieved high device efficiencies by increasing the Jsc and Voc besides the choice of electrolyte having minimum loss-in-potentials. The structure-property relationship of organometallic and metal-free organic dyes have been carried out, so far the best conversion efficiency of about 13% was achieved for zinc-porphyrin dye and 12.5% for indenoperylene dye with cobalt (II/III) redox couple. One of the advantages of having D/A metal free organic dyes is that it utilizes simple precursors to tune the optical and electrochemical properties and which developed high molar absorptivity dyes with strong intermolecular charge transfer (ICT). Dyes with D--A, and D-A--A configurations provided good harvesting efficiency in the visible and far-red regions, respectively, whereas few chromophores such as polymethine, porphyrin, and phthalocyanine dyes have been explored for photo-current generation in near infrared (NIR) region. DSSC device contains dye coated TiO2 as photo anode, platinized FTO as cathode with electrolyte (Figure 1). Upon photo-excitation, a photo-excited electron is injected from the excited state (LUMO) of the dye into the conduction band of the TiO2. The oxidized dye molecules are then regenerated by the electrolyte leaving the oxidized electrolyte in the system. The injected electron in TiO2 percolates through the film and collected at a transparent conductive layer of fluorine-doped tin oxide (FTO) glass substrate, on which the TiO2 film is formed. After passing through an external circuit, the electron is reintroduced into the DSSC device at the Pt cathode, where the oxidized electrolyte is reduced to regenerate the electrolyte. b) Statement of Problem: Out of various interfaces in DSSC device, dye-TiO2 interface plays an important role for charge injection and dye regeneration besides aggregation of dye on the TiO2 surface. Charge injection from aggregated structure that contributes to the device performance is varied and which can be modulated by co-adsorbing the dyes with optically transparent CDCA. Hence, controlling the dye-dye interaction on the TiO2 surface has become one of the challenging tasks for synthetic chemists besides having desired optical band gap of dyes. Dimeric dyes that are connected through spiro--spacer showed better device efficiency than corresponding monomeric units. Generally, for panchromatic absorption, two or more dyes with complementary absorption properties were adsorbed to enhance the light harvesting properties over solar spectrum. In the present approach, this critical problem is solved by connecting two structurally different dyes through spiro-linkage. Spiro-based -spacer spiroBiProDOT has been explored to synthesize dimeric dyes with orthogonal dye orientation. Chapter 2. Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π-π interaction, self-aggregation of dyes leads to reduced cell performance. In this chapter, a series of homodimeric (D1-D1 and D2-D2) and heterodimeric (D1-D2 and D2-D4) donor/acceptor (D/A) dyes containing spiroBiProDOT-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and compared the device performance with model monomeric dyes, D1 and D2 (Figure 2). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye D1-D1 showed higher power conversion efficiency (PCE), of 7.6% with a Voc and Jsc of 0.672 V and16.16 mA/cm2, respectively. On the other hand, the monomeric D1 exhibited a PCE of 3.2 % (Voc of 0.64V and Jsc of 7.2 mA/cm2), which is lower by 2.2 fold compared to dimeric D1-D1 analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO2 surface due to four sp3-centers, which arrest the molecular motions after chemisorption. This study showed a new molecular approach to incorporate two different chromophores in the dimeric dye possessing complementary absorption characteristics towards panchromatic light absorption. The attenuated charge recombination at TiO2/Dye/redox couple interface in case of D1-D1, owing to better passivation of TiO2 surface due to the spiro spacer was elucidated through electrochemical impedance spectroscopy. The FT-IR spectrum of D1-D1 adsorbed on TiO2 film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO2. Chapter 3. Symmetrical and Unsymmetrical Bi-Chromophoric Squaraine Based Sensitizer for Dye-Sensitized Solar Cells Dimeric derivatives of symmetrical (Dsq-Dsq) and unsymmetrical (D1-Dsq) spiro-squaraine dyes were synthesized by using direct arylation method (Figure 3). Photo-physical, electrochemical properties were evaluated for the feasibility of charge injection and dye regeneration processes which is required for utilizing the synthesized dyes as an active material for photovoltaic device fabrication. A model squaraine dye Dsq was also synthesized and characterized. In Dsq-Dsq dye, two Dsq monomeric units were attached through spiroBiProDOT unit. Similarly, D1-Dsq contains two different dyes namely Dsq and D1 (donor-- acceptor) connected through spiro-spacer. Excitation energy transfer between photoexcited D1 and Dsq was explained by using Förster resonance energy transfer (FRET). Compared to Dsq-Dsq, unsymmetrical spiro-squaraine dye, D1-Dsq, is more efficient in terms of circuit voltage Voc = 0.607 V, short-circuit current density Jsc = 6.6 mA/cm2, fill factor (ff) = 69% and power conversion efficiency (η) = 2.8 % under 1 sun, AM1.5G, whereas D1-Dsq dye showed the Voc = 0.612 V, Jsc = 9 mA/cm2, ff = 71% and η = 3.9%. From the IPCE profile of D1-Dsq, absence of response in the visible region, where D1 absorb the photons, indicates that efficient energy transfer from exited D1 to Dsq. A new dye D1-Dsq has been developed to improve the performance of DSSCs by Förster resonance energy transfer (FRET), in addition to retarding the charge recombination for the improved DSSC device performance. Chapter 4. Effect of Position of Spiro-Bipropylenedioxythiophene-Spacer in D--A Dyes for Dye-Sensitized Solar Cell To understand the effect of position spiro-spacer in D--A dyes, spiro-spacer near (TT1) and away (T1T) from anchoring unit were designed, synthesized and characterized (Figure 4). The dimeric spiro-dye T1T showed higher power conversion efficiency (PCE), with 3.9 % with a Voc and Jsc of 0.593 V and 9.09 mA/cm2, respectively, under simulated AM 1.5G illumination (100 mW/cm2) which is 1.6 times higher than that of isomeric TT1. Further electrochemical impedance spectroscopy indicates that the charge transfer resistance for the T1T (8.62 ohm) is higher than that of TT1 (6.75 ohm). Accordingly, dye TT1T designed and synthesized which showed the PCE of 4.16 % (Voc and Jsc are 0.589 V and 9.79 mA/cm2 respectively). The positioning of branching spiroBiProDOT-spacer makes an impact on the device performance, which might be due to effective passivation, originates from strong interaction between the dye and TiO2 in the case of T1T. Chapter 5. Orthogonally Functionalized Molecular Diad Containing D--A Dye and Hole Transport Material for Dye-Sensitized Solar Cell An orthogonally functionalized molecular diad containing D--A dye and hole transport material (D1-HTM)for dye-sensitized solar cell were synthesized (Figure 5) and characterized by electrochemical, photo physical methods and fabricating photovoltaic devices. The main objective of this work is to embarrass the charge recombination by appending a hydrophobic triarylamine moiety, which is orthogonally attached to that D--A part through spiroBiProDOT-spacer. Dye regeneration may be faster due to faster hole transfer from dye (D--A) to I-/ I3- system through HTM part. Fluorescence quenching experiments indicate that electron transfer from hole transporting material (PTS1, HTM-1) to excited D1-D1 and D1 dye. Cyclic voltammetry was carried out to evaluate the HOMO energy level of D1-HTM and LUMO level was calculated from the band gap. LUMO energy level of this dye is higher than that of the conduction band of TiO2 and at the same time HOMO is lower than that of the energy level of the electrolyte which is required for an efficient charge injection and dye regeneration processes. Photo-voltaic studies shown that the best efficiency (2.24 % with a Voc and Jsc of 0.590 V and 5.27 mA/cm2) was obtained of D1-HTM by using 5 equivalent of CDCA.
Funding from NWP0054 (CSIR-TAPSUN), and SERB-EMR/2016/007114
AcSIR

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.

2012/2013
Dye-sensitized solar cells (DSSCs) represent a promising alternative to silicon-based technology. From the first publications about DSSCs in the 90s, they are considered an important breakthrough for achieving high efficiency by using relatively inexpensive and abundant materials. Stability and efficiency are two crucial points in the development of this new class of hybrid photovoltaic devices. Most of the DSSC studies carried out over the past twenty years are based on the optimization of these two aspects. In particular, no particular efficiency improvement was obtained in the last period, although many efforts have been made for the research of appropriate solutions able to allow to fabricate more efficient devices. In this scenario, the topic of interest for this thesis is to further enhance the photovoltaic performance of DSSCs by integrating a nano-engineered TiOx photoanode obtained by means of a new nanostructuring method. This novel method, called ASB-SANS (Auxiliary Solvent-Based Sublimation-Aided NanoStructuring) allows the fast nanostructuring of a material in conditions of room temperature and atmospheric pressure. The nanostructuring process occurs by means of an auxiliary sublimating substance that, after having influenced the spatial arrangement of the material to be nanostructured, sublimates away from the system spontaneously. So-obtained TiOx photoanodes are characterized by an inner surface area which is higher than that of commonly used photoanodes. This implies that higher dye loading values are possible, in turn meaning an increase of photogenerated charge carriers upon sunlight absorption, hence an overall increase of the DSSC efficiency. This thesis is structured as following: - Chapter 1 is a general introduction to the photovoltaics and dye-sensitized solar cells, such as the operating principles and the characteristics of the dye cell; - Chapter 2 presents the motivation and objectives of PhD work, with particular interest in the state of art on the semiconductor layer optimization; - Chapter 3 contains a description of the two instrumental systems assembled by the author and colleagues for the characterization of photovoltaic devices (current/voltage recording system and IPCE system). A particular focus is put on the development of a tool for the determination of the photovoltaic quantum efficiency obtained by the conversion of a common UV-Vis spectrometer; - Chapter 4 is focused on the description of two methods for the determination of the active sites (dye grafting points) of the TiOx surface: the first based on the acetic acid adsorption and the second on the dye molecules adsorption. These methods are used for the characterization of all fabricated photoanodes; - Chapter 5 starts with the proven effectiveness of the ASB-SANS method applied to nanostructuring, over relatively large areas, a semiconducting polymer widely used in organic solar cells. The chapter is then focused on the description of the ASB-SANS method applied to fabricate our nano-engineered photoanodes; - Chapter 6 presents the results obtained by the application of the nano-engineered photoanodes in photovoltaic devices; - Chapter 7 reports some final conclusions together with our outlooks in the future research and development of the nano-engineered photoanodes for dye-sensitized solar cells.
Le celle solari a colorante organico (DSSC) proposte da Grätzel rappresentano una promettente alternativa alle tecnologie basate sul silicio già in commercio. Dalle prime pubblicazioni negli anni 90 esse hanno reppresentato un importante passo avanti per raggiungere un’efficienza relativamente alta utilizzando materiali poco costosi e abbondanti in natura. Gli aspetti più importanti per lo sviluppo di questa tecnologia sono la stabilità e l’efficienza, su cui si fonda la maggior parte degli studi sulle DSSC condotti negli ultimi vent’anni. In particolare, nonostante gli sforzi enormi nella ricerca di soluzioni appropriate che consentissero di fabbricare dispositivi più efficienti, nessun particolare incremento di efficienze è stato registrato. In questo scenario, il presente lavoro di tesi ha come scopo il miglioramento della performance fotovoltaica di DSSC attraverso l’integrazione al loro interno di un fotoanodo di TiOx nanostrutturato utilizzando un nuovo metodo di fabbricazione. Questo metodo, denominato ASB-SANS (Auxiliary Solvent- Based Sublimation-Aided NanoStructuring) permette la nanostrutturazione di un materiale senza dispendio di tempo ed in condizioni di temperatura ambiente e pressione atmosferica. La nanostrutturazione di un materiale avviene per mezzo di un sublimante ausiliario che, dopo aver influenzato la disposizione spaziale del materiale, si allontana dal sistema spontaneamente per semplice sublimazione. I fotoanodi di TiOx così ottenuti presentano una superficie esposta all’attacco del colorante maggiore di quella esposta generalmente dai fotoanodi comunemente impiegati. Ciò comporta un aumento della quantità di colorante che il fotoanodo può adsorbire che si traduce in un aumento della quantità di portatori di carica che si possono generare per effetto dell’assorbimento della luce solare. Il miglioramento della corrente generata nel dispositivo influenzerà positivamente l’efficienza globale della cella DSSC. Il presente lavoro di tesi è strutturato nel seguente modo: - il Capitolo 1 costituisce l’introduzione alla tematica di interesse con un approfondimento descrittivo dei componenti di una DSSC e del suo funzionamento; - il Capitolo 2 espone la motivazione e gli obbiettivi del lavoro di dottorato con particolare interesse verso lo stato dell’arte inerente alla motivazione espressa; - il Capitolo 3 contiene la descrizione accurata dei sistemi di caratterizzazione di dispositivi fotovoltaici. Di particolare rilievo è la messa a punto di uno strumento per la determinazione dell’efficienza quantica. Quest’ultimo è stato ottenuto assemblando un comune spettrometro UV-Vis con un multimetro per la registrazione delle correnti generate dalla cella; - il Capitolo 4 improntato sulla descrizione di due metodi per la determinazione dei siti attivi (punti di attacco del colorante) presenti sulla superficie del TiOx: il primo basato sull’adsorbimento dell’acido acetico e il secondo sull’adsorbimento delle molecole di colorante. Tali metodi serviranno per la caratterizzazione dei fotoanodi nanostrutturati; - il Capitolo 5 si apre con la provata efficacia del metodo di nanostrutturazione ASB-SANS applicato su polimeri di interesse fotovoltaico. Il fulcro del capitolo è tutto rivolto alla descrizione del metodo applicato al sistema di nanoparticelle di TiOx, con tute le soluzioni tecniche adottate per renderlo altrettanto efficace su questo genere di sistemi; - il Capitolo 6 riporta i risultati ottenuti per l’applicazione dei fotoanodi del capitolo 5 all’interno dei dispositivi fotovoltaici; - il capitolo 7 conclude il lavoro e riporta le eventuali prospettive per il futuro.
XXVI Ciclo
1984

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

Today, due to the increasing global demands on energy, it is imperative that a renewable energy source be determined, that is cost effective and reliable. Solar cell technology has shown much promise over the years to replace the use of fossil fuels. However, with the current technology, the cost per watt is rather high due to the high cost of manufacturing silicon-based solar cells. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/34953

Energy and environment have become the two predominant scientific research areas in the 21st century, and in some ways they are closely interconnected. Fossil fuels can no longer represent the predominant energy supply for human being. Their use must be reduced and alternative sustainable energy resources have to be identified and rapidly exploited. In the coming decades, the exploited energy sources will not only affect economy and politics but, in fact, health itself. The most direct and obvious effect derived from the current intensive use of fossil fuels is linked to the global warming caused by greenhouse gas emissions. The World Health Organization has recently estimated an increase of five million patients and 150,000 deaths per year resulting from the recent global temperature increase. Indirect effects are also important, such as the increase of infectious diseases transmitted by insects (especially malaria) and the deterioration of overall health due to malnutrition, as a direct consequence of drought and famine. Finally, the continuous use of fossil fuels boosts global pollution, which in turn significantly increases the mortality for respiratory and cardiovascular diseases. Global energy supply system must be urgently reassessed exploiting the use of clean energy sources. To this purpose, investments for the development of renewable energy resources are increasing worldwide, with particular attention to the most mature technologies such as hydro, wind and solar power. In particular, photovoltaics stands out as the most effective technology to be intensively exploited, especially if one considers that the total solar energy absorbed by Earth in one hour is higher than the overall yearly energy use. Many different photovoltaic devices have been developed over the last sixty years, and the large-scale production of solar panels having good efficiencies has begun in the last decade and is rapidly growing. The major goal is to find a trade-off between efficiency, stability, cost and environmental impact of the solar cells. This has led to a lively scientific research in this direction, in a multidisciplinary environment that includes materials scientists, electronic engineers, technologists and experts of life cycle assessment. The dye-sensitized solar cell (DSSC) is a photoelectrochemical device proposed in 1991, composed of widely available and cheap materials. Due to its ease of manufacture, versatility in the choice of components, good efficiency even in the presence of low irradiation level and adaptability to flexible substrates, DSSC has received considerable attention from the scientific community. However, despite the record efficiency of 13% and the recent large-scale industrial production, DSSCs still suffer from poor long-term stability, mainly due to the presence of the volatile liquid electrolyte as well as photosensitive organic components. In such a scenario, the scope of this PhD Thesis is the development of innovative quasi-solid electrolytes and external coatings where specifically designed polymeric networks are able to impart both high stability and efficiency to the resulting DSSCs. In Chapter 1 the current global energy scenario is thoroughly presented, along with an overview of the technologies developed for the conversion of solar energy into electricity. The physical parameters useful for the evaluation of the photovoltaic device performance are detailed and the state of art efficiencies so far achieved by means of the current technologies are reviewed. Chapter 2 deals with the basic concepts for DSSCs; cell architectures, components and operating principle are detailed. The specific characterization methodologies developed for the study of DSSCs are also described. Chapter 3 is focused on DSSC stability, which represents a key issue of the current solar energy research. The two main strategies to achieve stable DSSCs (i.e., the replacement of liquid electrolytes with polymeric ones and the introduction of external multifunctional polymeric coatings) are reviewed. As regards the preparation of these materials, photopolymerization is presented as one of the most promising technique due to its unique features such as rapidity and environmental friendliness, which are highly desired in a low impact and cheap technology like DSSC. The experimental part of this Thesis deals with the research work carried out on the preparation, characterization and testing of photopolymerized electrolytes and coatings. Both of these components have been investigated by means of an approach that started with the identification of suitable UV-curable monomers, followed by the study of the relationship between materials and devices performance, and concluded with the optimization through the introduction of particular additives able to give the material a multifunctional feature. In Chapter 4 the preparation and characterization techniques used for the fabrication and analysis of cell components and devices are briefly described. The experimental work has been carried out in the Center for Space Human Robotics (Istituto Italiano di Tecnologia, Torino) and in the Department of Applied Science and Technology (Politecnico di Torino). In Chapter 5, UV-crosslinked polymer electrolyte membranes (PEMs) are proposed and demonstrated as efficient and stable DSSC electrolytes. Physico-chemical, thermal, viscoelastic and electrochemical techniques are used to investigate the correlation between chemical structure of PEMs and resulting DSSC performance, with a special focus on the transport phenomena within PEMs as well as at the interface with the cell electrodes. The experimental conditions for the preparation of the polymer electrolyte are optimized by a design of experiments approach, which is used in the DSSC research field for the first time. Light-to-electricity conversion efficiency values of the lab-scale DSSCs assembled with these polymer electrolytes are admirably almost equal to the corresponding liquid cells, moreover a remarkably better long-term stability is obtained. In Chapter 6, a step forward is proposed, where three unconventional approaches are exploited for the successful implementation of photocrosslinked PEMs, namely the fabrication of flexible devices, the preparation of PEMs having a gradient-tailored spatial composition and the in situ photopolymerization of electrolytes containing alternative redox couples. These three themes are definitely innovative in the DSSC field and represent important advances from a technological viewpoint. Since the use of functional fillers has been scarcely considered in the DSSCs literature so far, the idea of improving both cell efficiency and durability by their introduction in PEMs is proposed in Chapter 7. In this respect, metal-organic frameworks (MOF) and nanocellulose are introduced in UV-cured membranes, and their effect on photovoltaic and stability performance is investigated. In particular, a novel bio-sourced filler is demonstrated to cumulatively increase the photocurrent, the photovoltage and the long-term stability of a polymeric lab-scale DSSC. In Chapter 8, the protection of DSSCs from UV radiation and atmospheric agents by the application of photopolymerized coatings is proposed. Multifunctional coatings, able both to convert the harmful UV light into harvestable visible light by downshifting and to confer self-cleaning and water-repellent properties to the external side of the cells are investigated. For the first time, a general approach that simultaneously improves performance and weatherability of organic DSSC devices is presented, and it is noteworthy that these multipurpose coatings are obtained by means of a rapid and up-scalable photopolymerization process.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais
The electronic properties of push-pull substituted, zinc(II) (ZnPc), aluminum(III) (AlPc), and ruthenium(II) (RuPc), metal-phthalocyanine derivatives, presenting two electron donating groups (diethylamine) and two electron withdrawing groups (carboxylic) was studied using the Density Functional Theory (DFT) with B3LYP exchange-correlation functional in the vacuum and under the presence of solvent (DMSO), aiming their application in dye-sensitized solar cells (DSSC). For the excited states, the time-dependent approach of DFT (TD-DFT) was applied. In the transition for the excited state it was evidenced a charge transfer from donor to acceptor groups which results in large electronic rearrangement inducing the bathochromism when adding DMSO (a polar solvent). This electronic transfer is directed to the acceptor groups (benzoic groups), mainly in RuPc molecule, which enables bigger probability in the electronic injection into the semiconductor s conduction band. HOMO s energy to the ZnPc and AlPc are sufficiently below the redox potential of the electrolyte and LUMO s energy, in all compounds, is above the conduction band of the oxide. Those push-pull molecules present panchromism, important for the application of these compounds in DSSC since they can absorb photons in a large range of energies. NBO analysis suggests that the ruthenium presents strong coordination with the nitrogen atoms of the macrocycle, which allows a larger participation of this metal in the electronic transition. The ionization energy and electron affinity were calculated aiming to quantify the energetic barrier in the electron gain / loss.
As propriedades eletrônicas de derivados de ftalocianinas metaladas push-pull de zinco(II) (FtZn), alumínio(III) (FtAl) e rutênio(II) (FtRu) apresentando dois grupos doadores de elétrons (N,N-dietilanilina) e dois grupos retiradores de elétrons (ácido benzóico), foram estudadas usando a teoria do funcional de densidade (DFT) com o funcional híbrido B3LYP na presença do solvente (DMSO) e no vácuo, visando sua aplicação em células solares sensibilizadas por corante (CSSC). Para a descrição dos estados excitados usou-se a extensão dependente do tempo da DFT (TD-DFT). Na transição para o estado excitado destes compostos há uma transferência de carga dos grupos doadores para os grupos aceptores, que resulta em rearranjo eletrônico induzindo o efeito batocrômico observado com a adição de DMSO. Essa transferência eletrônica está direcionada para o grupo aceptor (grupo benzóico), principalmente na FtRu, que possibilita uma maior probabilidade na injeção eletrônica na banda de condução do semicondutor. A energia do HOMO para a FtZn e FtAl estão suficientemente abaixo do potencial redox do eletrólito e o LUMO, em todos os compostos, está acima da banda de condução do TiO2. Essas moléculas push-pull apresentam pancromismo, importante para a aplicação destes compostos em CSSC uma vez que podem absorver fótons de diferentes energias. Análises NBO sugerem que o rutênio apresenta forte coordenação com os nitrogênios do macrociclo que possibilita grande participação deste metal nestas transições eletrônicas. As energias de ionização e eletroafinidades foram calculadas com o objetivo de quantificar a barreira energética no ganho/perda de um elétron.
Mestre em Química

Aquesta tesi es basa en un tipus de dispositius fotovoltaics, les cel•les solars sensitivitzades amb colorant. Des de fa un parell de dècades, l’estudi d’aquests dispositius ha anat en augment i actualment ja s’han publicat resultats amb més d’un 13% d’eficiència. S’estudien els diferents components d’aquest dispositiuis, la seva funció i totes les reaccions i fenòmens físics que hi tenen lloc. S’explica com es prepara aquest tipus de dispositius i com es caracteritzen. Finalment, hi ha un recull de 6 articles publicats i entre ells es diferencien pel tipus de colorant utilitzat: porfirines, ftalocianines, colorants orgànics que tenen una estructura anomenada dadora-acceptora amb un pont tipus π entremig i complexes de ruteni.
This thesis is based on a type of photovoltaic devices; the dye sensitized solar cells (DSCs). In the last two decades, the study of these devices has been increased and currently results with over 13% efficiency have been published. The first chapter discusses the various components of this kind of device, its function and its components. It is also explained how these cells work and all the reactions and physical phenomena that take place. The second chapter explains how to prepare these devices and how are characterized. And the third, fourth, fifth and sixth chapters are based on diverse articles published and the difference between them is the kind of dye. In chapter 3, the dyes used are porphyrins, chapter 4 is based on phthalocyanines, chapter 5 is centred on organic dyes that have a structure called donor-acceptor with a π-bridge type in between and chapter 6 studies two ruthenium complexes.

The conversion of sunlight into electricity is one of the most promising energy sources because is renewable and available worldwide. Therefore, is desirable to develop low cost, stable and efficient photovoltaic devices that can compete with the expensive silicon solar panels. Dye-sensitized solar cells (DSC) are a promising photovoltaic technology based on low-cost materials and fabrication processes with power-conversion efficiencies over 12%. DSC harvest light thanks to a photoactive organic molecule, and extracts the charges through different materials. The analysis of the working principles of these complex devices is essential in order to achieve further improvements on the solar cell efficiency. The aim of this thesis is to analyze and identify the electrical interactions between the internal materials and interfaces of a DSC by means of impedance spectroscopy (IS). We provide useful interpretation of IS applied to DSC and insights on DSC development such as implementation of photonic crystals and up-scaling to modules, as a first step for a future commercialization.
Convertir la llum solar en electricitat és una de les maneres més prometedores d'obtindre energía, ja que és inesgotable i disponible arreu del món. Primer és necessari desenvolupar dispositius fotovoltaics què siguin eficients, estables i de baix cost, i així poder competir en el mercat amb les actuals cèl·lules solars de silici. Les cèl·lules solars sensitivitzades amb colorant (DSC) ‎ja han assolit eficiències per sobre del 12% amb materials i processos de fabricació senzills i barats. Les DSC recol·lecten la llum gracies a unes molècules fotoactives (colorant) i extrauen les càrregues a través de diferents materials semiconductors. Analitzar els principis de funcionament d'aquests dispositius és essencial per a assolir futures millores en eficiència i estabilitat. L'objectiu d'aquesta tesi és analitzar i identificar les interaccións elèctriques entre els materials i interfases internes d'una DSC mitjançant l'espectroscopía d'impedància (IS). El treball proporciona una interpretació pràctica de la IS aplicada a les DSC i també altres idees per al desenvolupament de les DSC com ara l'ús de cristalls fotònics i la fabricació de moduls de gran àrea, com a primer pas per a una futura comercialització.

Interfaces between semiconductors and adsorbed molecules form a central area of research in surface science, occurring in many different contexts. One such application is the so-called Dye-Sensitized Solar Cell (DSSC) where the nanostructured dye-semiconductor interface is of special interest, as this is where the most important ultrafast electron transfer process takes place. In this thesis, structural and electronic aspects of these interfaces have been studied theoretically using quantum chemical computations applied to realistic dye-semiconductor systems. Periodic boundary conditions and large cluster models have been employed together with hybrid HF-DFT functionals in the modeling of nanostructured titanium dioxide. A study of the adsorption of a pyridine molecule via phosphonic and carboxylic acid anchor groups to an anatase (101) surface showed that the choice of anchor group affects the strength of the bindings as well as the electronic interaction at the dye-TiO2 interface. The calculated interfacial electronic coupling was found to be stronger for carboxylic acid than for phosphonic acid, while phosphonic acid binds significantly stronger than carboxylic acid to the TiO2 surface. Atomistic and electronic structure of realistic dye-semiconductor interfaces were reported for RuII-bis-terpyridine dyes on a large anatase TiO2 cluster and perylene dyes on a periodic rutile (110) TiO2 surface. The results show strong influence of anchor and inserted spacer groups on adsorption and electronic properties. Also in these cases, the phosphonic acid anchor group was found to bind the dyes significantly stronger to the surface than the carboxylic acid anchor, while the interfacial electronic coupling was stronger for the carboxylic anchor. The estimated electron injection times were twice as fast for the carboxylic anchor compared to the phosphonic anchor. Moreover, the electronic coupling was affected by the choice of spacer group, where unsaturated spacer groups were found to mediate electron transfer more efficiently than saturated ones.

Dye sensitized solar cells (DSSCs) have received extensive attention among solar cells in recent years as the production cost is comparatively low and photovoltaic performance is good. Apart from TiO2, SnO2-based DSSCs are of great interest since SnO2 has a wide band gap and high mobility. Though the conversion efficiency of SnO2-based DSSCs is still not comparable to TiO2-based DSSCs, there is room for improvement to fabricate an efficient device. In this study, different commercial SnO2 nanoparticles have been compared. The number of SnO2 layers and paste formulation have been optimized. The effects of TiCl4 and TTIP treatments have been investigated. In order to further optimize the performance of SnO2-based DSSCs, different strategies have been adopted to increase dye loading, facilitate electron transport and enhance photon absorption. Different dopants (Zn, Mg and Ag) have been introduced to SnO2 pastes. It is found that cells with Zn dopants perform the best with increased dye uptake. SnO2 nanorods have been synthesized and mixed with SnO2 nanoparticles. More nanorods result in faster electron transport and hence increase the conversion efficiency. In addition, different gold nanostructures (nanostars, nanorods and nanocubic Au) have been synthesized and incorporated into SnO2 photoanodes to study the plasmonic effects. It can be observed that nanocubic Au demonstrates the largest improvement in conversion efficiency. The obtained results will be discussed in detail.
published_or_final_version
Physics
Master
Master of Philosophy

This thesis studies the application of copper(I) complexes as the sensitizing component of dye sensitized solar cells (DSCs). Ruthenium(II) polypyridyl complexes have been widely studied and shown great success for the past two decades; however the metal is rare and expensive. A copper(I) based DSC could offer a viable alternative to using ruthenium(II) dyes, taking into account the cost and sustainability advantages. Interest in copper(I) DSCs has reignited over the past five years and the work in this thesis begins by reproducing the synthesis of one of the first reported complexes, [Cu(6,6’-dimethyl-2,2’-bipyridine-4,4’-dicarboxylic acid)2][Cl]. A more detailed study of the dye and its properties will be described, including assessing the effect of TiO2 film dye time on DSC performance, electrochemical studies and coupling the dye with a Co2+/3+ mediator. In the following chapters, improvements to the basic 2,2’-bipyridine framework are investigated. An experimental and computational investigation with a [Cu(2,2'-biquinoline-4,4'-dicarboxylic acid)2][HNEt3] complex is presented, where the 2,2’-biquinoline ligand has been chosen as a bulkier, more conjugated alternative to the 2,2’-bipyridine ligand. Although DSC efficiencies with this complex are comparatively low, an investigation into possible reasons for this is described. This thesis then considers functionalisation of a 2,2’-bipyridine ligand with halide and thiophene substituents. Several new ligands and copper(I) complexes are described and characterised. A top DSC efficiency of 1.41% was obtained with a [Cu(6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(thiophene-2-carboxylic acid)2][PF6] dye. The synthetic route towards this complex and an analysis of its features, such as emissive behaviour, electrochemical properties and electron diffusion length, are described.

Due to climate change and its effects, alternative renewable energy sources are needed in the future human society. In the work of this thesis, the Dye-sensitized Solar Cell (DSC) has been investigated and characterized. DSCs are appealing as energy conversion devices, since they have high potential to provide low cost solar light to electricity conversion. The DSC is built up by a working electrode consisting of a conductive glass substrate with a dye-sensitized mesoporous TiO2 film, a counter electrode with a catalyst and, in between, the electrolyte which performs the charge transport by means of a redox mediator. The aim of this thesis was to develop and evaluate cheap and environmentally friendly materials for the DSC. An alternative polymer-based counter electrode catalyst was fabricated and evaluated, showing that the PEDOT catalyst counter electrode outperformed the platinum catalyst counter electrode. Different organic dyes were evaluated and it was found that the dye architecture affected the performance of the assembled DSCs. A partly hydrophilic organic triphenylamine dye was developed and applied in water-based electrolyte DSCs. The partly hydrophilic dye outperformed the reference hydrophobic dye. Small changes in dye architecture were evaluated for two similar dyes, both by spectroscopic and electrochemical techniques. A change in the length of the dialkoxyphenyl units on a triphenylamine dye, affected the recombination and the regeneration electron transfer kinetics in the DSC system. Finally, three water soluble cobalt redox couples were developed and applied in water-based electrolyte DSCs. An average efficiency of 5.5% (record efficiency of 5.7%) for a 100% water-based electrolyte DSC was achieved with the polymer-based catalyst counter electrode and an organic dye with short dimethoxyphenyl units, improving the wetting and the regeneration process.

博士
國立臺灣師範大學
化學系
98
We have explored three topics in this study. In the first topic, we synthesized eight organic dyes (4a、4b、4c、9、14a、14b、16、45 ) for dye-sensitized solar cells (DSSCs) applications. Their physical properties were investigated. Except for dyes 16 and 45, which serve as model compounds, the other dyes all possess 1,3-cyclohexdiene conjugated units in the structural skeleton. One obvious advantage of employing 1,3-cyclohexadiene unit in the framework of light-harvesting dyes is the essential planar conformation in the structural skeleton which yields more dense packing of the dyes adsorbed on TiO2 surface. All of these compounds were characterized by UV/vis, fluorescence spectroscopy, and cyclic voltammetry. The cell performances and incident photo-to-current efficiencies of these compounds were inspected using the AM 1.5 simulated solar light and irradiation of monochromic light system. The highest conversion efficiency of the DSSCs based on dye 14a can reach up to 4.4 %. (70 % of the standard N719 dye, η= 5.87 %) In the second topic, we designed dye 32 as a model for assessing the role of the twisted intramolecular charge transfer (TICT) state played in the conversion efficiency of the DSSCs. TICT state allows the efficient charge-separation and slows down the unwanted charge-recombination processes. Their physical properties were detailed investigated and correlated the TICT state with the photovoltaic process of DSSCs. Finally, we designed two types of D-D-π-A organic dyes for DSSCs. Different carbazolyl groups were inserted to the structure to form the D-D-π-A compounds. On one hand, we expected that the absorption region can be extended and the molar extinction coefficient can be enhanced comparing with the D-π-A structure. On the other hand, these dyes may benefit from lower tendency to aggregate and display better thermo-stability.

碩士
萬能科技大學
材料科學與工程研究所
102
In this study, ion-type network of polymer gel-state electrolyte (PGE) is synthesized. It has high-density conductive functional groups and ion-type network structure, which is favorable for electronic conductivity in solar cells. The synthesized PGE is vinyl/maleic anhydride-based copolymer. First, the vinyl/maleic anhydride were reacted with imidazole compound to form prepolymer with many overhang side-chains. Then this prepolymer was reacted with cross-linking reagents of different chain lengths of diiodo alkanes (eg. 1,10-diiodo decane, 1,5-diiodo pentane, and/or 1,3-diiodo propane). The synthesized PGE has a functional ion-type network, which is favorable for electronic conductivity. Thus an improved overall sunlight-to-energy conversion efficiency in DSSCs is expected. The conductivity of the synthesized PGE is decreases with an increase in chain length of crosslinkers; while shorter crosslinkers have denser networks, which are favorable for electronic conductivity, thus, a higher conductivity is observed. In this work, the conversion efficiency is dependent on the density of network of PEG. The higher density of network is obtained by using shorter crosslinker, which is favorable for exchanging reaction in conductivity, but is unfavorable for diffusion in conductivity. Experimental result indicates that the maximum conversion efficiency can be obtained by using crosslinker of di-iodo pentane to synthesize PEG with optimum density of network. The conversion efficiency of PEG synthesized from vinyl/maleic anhydride (PS-MA)-based copolymers is higher that vinyl/maleic anhydride (PE-MA)-based copolymer because the delocalizaion of electron conjugation in the structure of benzene ring of the former, which is favorable for electron conductivity. Conclusively, the maximum conversion efficiency in developed DSSCs can be reached to 6.85 %.

碩士
明志科技大學
化工與材料工程研究所
98
In this study, the Pt counter electrodes used as soft substrate (polyethylene terephthalate, PET) and glass substrates as the substrates were prepared and applied on dye-sensitized solar cells (DSSCs). The Pt counter electrodes were prepared by the pulsed plating and sputtering methods for dye-sensitized solar cells. The advantages of the Pt counter electrodes prepared by pulsed plating method have high activity, high specific surface area, fast preparation and easily mass production. The negative electrode was prepared by using the FTO/glass or the ITO/PET coated with P25 TiO2. Those TiO2 electrodes were assembled with Pt counter electrodes to form dye-sensitized solar cells. The surface morphology of as-prepared Pt electrodes was studied by using SEM/EDX; the chemical composition of the electrodes was measured by ICP-OES; the surface resistivity of the electrodes was measured by FPP; the surface morphology and roughness of the electrodes were measured by AFM; the charge transfer resistance of the electrodes was investigated by AC impedance; the electrochemical activity of the electrodes for I-/I3- electrolyte pair was studied by CV method. Finally, the electrical properties and optical conversion efficiency of the DSSCs were measured, respectively. It was found that the electrochemical activities of the pulsed plating Pt electrodes are much better than those of the direct current plated and the sputtered Pt counter electrodes. It was found that the optimal condition for preparation pulsed plating Pt electrodes on the DSSC is at D=0.2, ton = 0.1s, toff = 0.4s. The conversion efficiency of the DSSCs based on FTO/glass is 5.99%; However, the conversion efficiency of the flexible DSSCs based on ITO/PET is 1.96%.