Nell’ambito della crescente richiesta di sorgenti di energia rinnovabile, lo sfruttamento dell’energia solare si è posto al centro della corsa globale verso un futuro sostenibile. Le celle solari sensibilizzate a colorante (Dye-Sensitized Solar Cells) hanno mostrato elevato potenziale per un approccio alla conversione elettrica dell’energia solare relativamente economico e facilmente realizzabile su ampia scala. La trasparenza, il design versatile e l’ampia gamma di colori disponibili per questi dispositivi offrono possibilità uniche nell’ambito dell’integrazione architettonica. Scopo di questa tesi è di contribuire allo sviluppo della tecnologia DSSC, puntando a migliorare le efficienze ottenibili e la comprensione dei meccanismi operativi di cella. L’approccio seguito è consistito nell’analizzare e modificare i diversi componenti delle DSSC: il fotoanodo sensibilizzato, il controelettrodo e il mediatore elettronico, i quali contribuiscono tutti insieme al globale funzionamento della cella. In seguito alla descrizione dei principi di funzionamento delle DSSC nel capitolo 1 e delle tecniche di caratterizzazione usate nel capitolo 2, nel capitolo 3 sono presentati gli sforzi tesi al miglioramento della componente fotoanodica. È stato considerato il trattamento della superficie fotoanodica con silani disponibili in commercio, come metodo di passivazione nei confronti del fenomeno di ricombinazione che coinvolge gli elettroni fotoiniettati nel biossido di titanio e la forma ossidata del mediatore. Analisi di tipo elettrochimico e fotoelettrochimico ne hanno rivelato un significativo abbattimento, in presenza del trattamento a base di silani. Successivamente, una nuova categoria di sensibilizzatori costituiti da porfirine β–monosostituite, è stata caratterizzata dal punto di vista fotoelettrochimico e fotofisico. Facendo un confronto con strutture porfiriniche analoghe, nella più nota forma meso-sostituita, quella β-sostituita ha prodotto prestazioni leggermente maggiori, con il vantaggio di un procedimento sintetico più semplice. Nel capitolo 4 si è focalizzata l’attenzione su un’altra componente fondamentale delle DSSC, quale il mediatore elettronico. Uno studio di spegnimento degli stati eccitati è stato condotto in soluzione, combinando sensibilizzatori di rutenio e mediatori di cobalto differentemente carichi, con l’obiettivo di razionalizzare l’importanza delle interazioni elettrostatiche tra le specie coinvolte nei processi di trasferimento elettronico responsabili del funzionamento delle DSSC. Risultati degni di nota si sono ottenuti impiegando come mediatore un complesso di cobalto carico negativamente, in associazione con un sensibilizzatore di rutenio esacationico, sistema che è stato ulteriormente studiato in cella tramite metodi fotoelettrochimici, ancorando il sensibilizzatore al TiO2. Gli sforzi impiegati nello sviluppo della parte catodica delle DSSC sono riportati nel capitolo 5. Inizialmente, è stato messo a punto un metodo per aumentare l’adesione del polimero conduttore PEDOT alla base elettrodica di vetro conduttore, che consiste nel modificare sinteticamente il monomero di partenza, EDOT, con gruppi terminali silossanici, permettendo così la produzione per via elettrochimica di un film di PEDOT stabilmente accoppiato alla superficie del catodo, che mostra migliore stabilità meccanica e ridotta resistenza al trasferimento di carica all’interfaccia polimero/vetro conduttore, fornendo dunque maggiori efficienze in cella. Infine, una categoria di nanomateriali a base di carbonio, chiamati single-walled nanohorns, è stata investigata come substrato catodico per DSSC impieganti mediatori elettronici a base di cobalto, mostrando buone caratteristiche di efficienza e stabilità che li rendono una valida alternativa a materiali catodici il cui impiego è già consolidato, quali platino e PEDOT.

With the increasing world demand of renewable energy sources, the exploitation of solar energy became a priority in the global race for a sustainable future. Dye-sensitized solar cells (DSSC) were recognized to hold great potential for a relatively cheap and easy-to-scale approach to direct solar-to-electrical power conversion. Their transparency, versatile design and wide color palette offers unique structural and architectural possibilities in the emerging field of building integration. Purpose of this thesis is to contribute to the DSSC technology improvement, aiming at higher efficiencies and making progress towards the comprehension of these devices operative mechanisms. The approach followed consisted in analyzing and modifying the different DSSC components: the sensitized photoanode, the counter electrode and the electronic mediator, contributing all together to the overall device performance. After describing the DSSC functioning principles in chapter 1 and the employed characterization techniques in chapter 2, in chapter 3 the efforts put in the photoanodic component improvement are presented. A passivation method was tested, consisting in a treatment of the titanium dioxide semiconductor surface with commercially available silanes. The investigation outcome, based on electrochemical and photoelectrochemical analyses, was a significant abatement of the recombination process involving the photoinjected electrons in TiO2 and the electronic mediator oxidized form, event that constitutes the main limit in DSSC containing common ruthenium sensitizers and cobalt redox couples. Subsequently a new category of β–monosobstituted porphyrin dyes was photoelectrochemically and photophysically characterized. Making a comparison with analogous porphyrinic structures in the well-known meso-substituted form, the β–substituted resulted in slightly higher performances, with the advantage of a simpler synthetic route. In chapter 4, attention was focused on another fundamental DSSC component, that is the electronic mediator. First, a quenching study was conducted in solution, by means of spectroscopic techniques, combining differently charged ruthenium sensitizers and cobalt mediators, aiming at a better understanding of the role electrostatic interactions could have in the charge transfer mechanisms involved in the oxidized sensitizer regeneration by cobalt mediators in DSSC. In particular, interesting results were obtained with a negatively charged cobalt complex, in association with a hexacationic ruthenium complex. This redox mediator was further studied in operative devices, anchoring the sensitizer to the TiO2 surface, through photoelectrochemical methods, leading to results that are consistent with the importance of electrostatic interactions between molecules involved in electron transfer processes responsible for DSSC functioning. The efforts put into the DSSC cathodic counterpart improvement are reported in chapter 5. A way to enhance the conductive polymer PEDOT adhesion to the fluorine doped tin oxide (FTO) covered electrode surface was explored. EDOT, the starting monomer for this well-known cathodic material, that can be electropolymerized directly on the conductive glass base, was synthetically modified with siloxane terminal groups. This, together with the silanization of the surface itself, led to decreased charge transfer resistance at the polymer/conductive glass interface, hence to improved cell efficiencies. Finally, an interesting category of carbon nanomaterials, called single-walled nanohorns, was investigated as cathodic substrate for DSSC employing cobalt based redox mediators. The good efficiency and stability characteristics observed make this material suitable as a viable alternative to the already consolidated counter electrodes materials like platinum and PEDOT.

Development of new molecular systems and innovative materials for regenerative photoelectrochemical cells

CASARIN, LAURA
2017

Abstract

With the increasing world demand of renewable energy sources, the exploitation of solar energy became a priority in the global race for a sustainable future. Dye-sensitized solar cells (DSSC) were recognized to hold great potential for a relatively cheap and easy-to-scale approach to direct solar-to-electrical power conversion. Their transparency, versatile design and wide color palette offers unique structural and architectural possibilities in the emerging field of building integration. Purpose of this thesis is to contribute to the DSSC technology improvement, aiming at higher efficiencies and making progress towards the comprehension of these devices operative mechanisms. The approach followed consisted in analyzing and modifying the different DSSC components: the sensitized photoanode, the counter electrode and the electronic mediator, contributing all together to the overall device performance. After describing the DSSC functioning principles in chapter 1 and the employed characterization techniques in chapter 2, in chapter 3 the efforts put in the photoanodic component improvement are presented. A passivation method was tested, consisting in a treatment of the titanium dioxide semiconductor surface with commercially available silanes. The investigation outcome, based on electrochemical and photoelectrochemical analyses, was a significant abatement of the recombination process involving the photoinjected electrons in TiO2 and the electronic mediator oxidized form, event that constitutes the main limit in DSSC containing common ruthenium sensitizers and cobalt redox couples. Subsequently a new category of β–monosobstituted porphyrin dyes was photoelectrochemically and photophysically characterized. Making a comparison with analogous porphyrinic structures in the well-known meso-substituted form, the β–substituted resulted in slightly higher performances, with the advantage of a simpler synthetic route. In chapter 4, attention was focused on another fundamental DSSC component, that is the electronic mediator. First, a quenching study was conducted in solution, by means of spectroscopic techniques, combining differently charged ruthenium sensitizers and cobalt mediators, aiming at a better understanding of the role electrostatic interactions could have in the charge transfer mechanisms involved in the oxidized sensitizer regeneration by cobalt mediators in DSSC. In particular, interesting results were obtained with a negatively charged cobalt complex, in association with a hexacationic ruthenium complex. This redox mediator was further studied in operative devices, anchoring the sensitizer to the TiO2 surface, through photoelectrochemical methods, leading to results that are consistent with the importance of electrostatic interactions between molecules involved in electron transfer processes responsible for DSSC functioning. The efforts put into the DSSC cathodic counterpart improvement are reported in chapter 5. A way to enhance the conductive polymer PEDOT adhesion to the fluorine doped tin oxide (FTO) covered electrode surface was explored. EDOT, the starting monomer for this well-known cathodic material, that can be electropolymerized directly on the conductive glass base, was synthetically modified with siloxane terminal groups. This, together with the silanization of the surface itself, led to decreased charge transfer resistance at the polymer/conductive glass interface, hence to improved cell efficiencies. Finally, an interesting category of carbon nanomaterials, called single-walled nanohorns, was investigated as cathodic substrate for DSSC employing cobalt based redox mediators. The good efficiency and stability characteristics observed make this material suitable as a viable alternative to the already consolidated counter electrodes materials like platinum and PEDOT.
BIGNOZZI, Carlo Alberto
CARAMORI, Stefano
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2488191
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