As the global warming and its side effects are rising, importance of technologies for alternative source of clean energies are becoming more on the focus of attention. Among all possible choices, solar energy is lauded as an inexhaustible source of pollution-free energy. From a technological point of view, sun light can be directly converted to thermal energy, electricity (Photovoltaics), or chemical energy such as in water splitting. However, the process, devices and involved materials still have to be optimized in a way to maximize the efficiency of the overall conversion process. This thesis work aims to introduce novel nanosturctures as photoelectrodes for this purpose. This included specifically hybrid organic-inorganic heterojuctions for hybrid solar cells and water splitting application. Originality of the work arises from current most important open issues for the development of efficient photoelectrodes, which concern the proper choice of material selection and the nano-scale morphology of the electrode junctions. As for the material selection, Transition Metal Oxides (TMOs) are found to be the best candidates due to their unique opto-electronic properties. TMOs offer wide range of energy levels that provide means for coupling with different types of conjugated polymer as photosensitive materials. In addition, they are charge selective materials which is an importance factor for boosting the device performance by minimizing the charge recombination at the organic-inorganic interface. Considering the nanoscale morphology, these materials can be nanostructured by facile methods which results in significant enhancement of the final device performance in comparison to the bulk form. Moreover opto-electronic properties can be tuned also by nanostructuring such as directionality of the structure for efficient charge transport and advanced light management by inducing scattering elements. Pulsed Laser Deposition (PLD), that is a well know and versatile technology for development of nanoassemblies directly from gas phase, is used to develop nanostructures in this work. The first part of this thesis is dedicated to quasi-1D hierarchical nanostructure of TiO2 as photoanode for hybrid polymer-oxide solar cells. Crystalline hierarchical assembly was fabricated and characterized. Hybrid solar cells were realized by coupling with P3HT. An average power conversion efficiency of 0.8%, exceeding 1% for the champion device, was obtained, which shows almost 3 fold increase in comparison with cells based on standard mesoporous titania structure with 0.34% efficiency. Such increase is attributed mainly to an increase in photogenerated current ascribed to the peculiar optical properties of the novel acceptor phase in maximizing the interfacial area and light absorption by enhanced scattering, close to optimal interpenetrating network for efficient photogenerated exciton separation, and higher crystallinity of P3HT chains inside the hierarchical nanostructure. The second part pf the theis is focused on development of oxides for photocathodic electrochemical water splitting application. This was performed in the framework of the European project "Photogenerated Hydrogen by Organic Catalytic Systems" (PHOCS) which aims to realize a new-concept photoelectrochemical system for hydrogen production, based on hybrid organic-inorganic and organic-liquid interfaces. As a first step, MoO3 was selected, having the highest value of work function among the non-soluble transition metal oxides In addition, MoO3, similar to other molybdenum calchogenides, is well known for its quasi-2D layered structure which if properly controlled at nanoscale can provided suitable means for charge transport while maintaining a large surface area. Nanolamellar α-MoO3 was developed, characterized, and by coupling with P3HT:PCBM a photoeletrochemical heterjunction for water splitting was realized. For the best devices, a 10 fold increase in the photogenerated current towards hydrogen evolution reaction with respect to the device based on blend as well as positive shift in open circuit voltage were observed, which confirm both the successful increase of interfacial area by nanostructuring and hole-selectivity characteristic of the oxide. Similar to majority of photocathodic oxides, MoO3 also tends to get reduced by negative voltage swap. Additionally the oxide is only stable in the range of pH from 0 to 1 that makes it far from an ideal photocathode for the PHOCS project which aims to produce hydrogen from sea water. In search for a more stable oxide, Palladium Oxide nanostructures were developed. Characterization of the oxide and its hybrid heterojuctions with APFO-3:PCBM reveal its high stability and photocathodic behavior. However the elechtrochemical characterization showed that this system fails to produce hydrogen, and all photogenerated currents are related to the reaction with dissolved oxygen in the electrolyte. Based on this, a photoelectrochemical oxygen sensor was developed for which precise sensing properties toward dissolved oxygen can be realized.

Con il riscaldamento globale ed i suoi effetti collaterali, la spinta verso la ricerca tecnologica sulle energie alternative a quelle fossili è sempre più al centro dell’attenzione. Tra tutte le possibili scelte, quella solare rappresenta una delle fonti principali, in quanto inesauribile e non inquinante. Da un punto di vista tecnologico, la luce solare può essere direttamente convertita in energia termica, energia elettrica (fotovoltaico), o energia chimica, come nella scissione dell'acqua. Tuttavia, i processi, i dispositivi ed i materiali coinvolti devono ancora essere ottimizzati in modo da massimizzare l'efficienza del processo di conversione complessiva. Questo lavoro di tesi si propone di introdurre nuovi elettrodi nanostrutturati per applicazioni nel campo fotovoltaico ed in quello del water splitting. In questo contesto si inquadrano anche le eterogiunzioni ibride organico-inorganico. La parte innovativa legata a questo lavoro nasce dalla necessità di sviluppare fotoelettrodi sempre più efficienti, con una appropriata scelta dei materiali e della morfologia degli stessi. Per le loro proprietà opto-eletroniche, gli ossidi di metalli di transizione appaiono i migliori candidati, dato che la loro struttura energetica si presta all’accoppiamento con differenti poilmeri coniugati da adottare come materiale fotosensibile. Essendo inoltre materiali charge-selective, essi migliorano le performances riducendo le perdite dovute alle ricombinazioni elettrone-lacuna all’interfaccia organico-inorganico. Per quanto riguarda la morfologia, essi possono essere facilmente nanostrutturati, incrementando ulteriormente l’efficienza del device finale. La tecnica di deposizione a laser pulsato (PLD) è ottimale per realizzare nanostrutture ed è stata utilizzata nel presente lavoro. Nella prima parte del lavoro, l’ossido di titanio (TiO2) è stato utilizzato come fotoanodo per celle solari ibride organico-inorganico. Nella seconda parte gli ossidi di molibdeno (MoO3) e di palladio (PdO) sono stati implementati come fotocatodi per il processo di water splitting.

Nanostructured oxide semiconductors for direct solar energy conversion: photovoltaics and water splitting

GHADIRZADEH, ALI

Abstract

As the global warming and its side effects are rising, importance of technologies for alternative source of clean energies are becoming more on the focus of attention. Among all possible choices, solar energy is lauded as an inexhaustible source of pollution-free energy. From a technological point of view, sun light can be directly converted to thermal energy, electricity (Photovoltaics), or chemical energy such as in water splitting. However, the process, devices and involved materials still have to be optimized in a way to maximize the efficiency of the overall conversion process. This thesis work aims to introduce novel nanosturctures as photoelectrodes for this purpose. This included specifically hybrid organic-inorganic heterojuctions for hybrid solar cells and water splitting application. Originality of the work arises from current most important open issues for the development of efficient photoelectrodes, which concern the proper choice of material selection and the nano-scale morphology of the electrode junctions. As for the material selection, Transition Metal Oxides (TMOs) are found to be the best candidates due to their unique opto-electronic properties. TMOs offer wide range of energy levels that provide means for coupling with different types of conjugated polymer as photosensitive materials. In addition, they are charge selective materials which is an importance factor for boosting the device performance by minimizing the charge recombination at the organic-inorganic interface. Considering the nanoscale morphology, these materials can be nanostructured by facile methods which results in significant enhancement of the final device performance in comparison to the bulk form. Moreover opto-electronic properties can be tuned also by nanostructuring such as directionality of the structure for efficient charge transport and advanced light management by inducing scattering elements. Pulsed Laser Deposition (PLD), that is a well know and versatile technology for development of nanoassemblies directly from gas phase, is used to develop nanostructures in this work. The first part of this thesis is dedicated to quasi-1D hierarchical nanostructure of TiO2 as photoanode for hybrid polymer-oxide solar cells. Crystalline hierarchical assembly was fabricated and characterized. Hybrid solar cells were realized by coupling with P3HT. An average power conversion efficiency of 0.8%, exceeding 1% for the champion device, was obtained, which shows almost 3 fold increase in comparison with cells based on standard mesoporous titania structure with 0.34% efficiency. Such increase is attributed mainly to an increase in photogenerated current ascribed to the peculiar optical properties of the novel acceptor phase in maximizing the interfacial area and light absorption by enhanced scattering, close to optimal interpenetrating network for efficient photogenerated exciton separation, and higher crystallinity of P3HT chains inside the hierarchical nanostructure. The second part pf the theis is focused on development of oxides for photocathodic electrochemical water splitting application. This was performed in the framework of the European project "Photogenerated Hydrogen by Organic Catalytic Systems" (PHOCS) which aims to realize a new-concept photoelectrochemical system for hydrogen production, based on hybrid organic-inorganic and organic-liquid interfaces. As a first step, MoO3 was selected, having the highest value of work function among the non-soluble transition metal oxides In addition, MoO3, similar to other molybdenum calchogenides, is well known for its quasi-2D layered structure which if properly controlled at nanoscale can provided suitable means for charge transport while maintaining a large surface area. Nanolamellar α-MoO3 was developed, characterized, and by coupling with P3HT:PCBM a photoeletrochemical heterjunction for water splitting was realized. For the best devices, a 10 fold increase in the photogenerated current towards hydrogen evolution reaction with respect to the device based on blend as well as positive shift in open circuit voltage were observed, which confirm both the successful increase of interfacial area by nanostructuring and hole-selectivity characteristic of the oxide. Similar to majority of photocathodic oxides, MoO3 also tends to get reduced by negative voltage swap. Additionally the oxide is only stable in the range of pH from 0 to 1 that makes it far from an ideal photocathode for the PHOCS project which aims to produce hydrogen from sea water. In search for a more stable oxide, Palladium Oxide nanostructures were developed. Characterization of the oxide and its hybrid heterojuctions with APFO-3:PCBM reveal its high stability and photocathodic behavior. However the elechtrochemical characterization showed that this system fails to produce hydrogen, and all photogenerated currents are related to the reaction with dissolved oxygen in the electrolyte. Based on this, a photoelectrochemical oxygen sensor was developed for which precise sensing properties toward dissolved oxygen can be realized.
BOTTANI, CARLO ENRICO
BOTTANI, CARLO ENRICO
LI BASSI, ANDREA
9-dic-2014
Con il riscaldamento globale ed i suoi effetti collaterali, la spinta verso la ricerca tecnologica sulle energie alternative a quelle fossili è sempre più al centro dell’attenzione. Tra tutte le possibili scelte, quella solare rappresenta una delle fonti principali, in quanto inesauribile e non inquinante. Da un punto di vista tecnologico, la luce solare può essere direttamente convertita in energia termica, energia elettrica (fotovoltaico), o energia chimica, come nella scissione dell'acqua. Tuttavia, i processi, i dispositivi ed i materiali coinvolti devono ancora essere ottimizzati in modo da massimizzare l'efficienza del processo di conversione complessiva. Questo lavoro di tesi si propone di introdurre nuovi elettrodi nanostrutturati per applicazioni nel campo fotovoltaico ed in quello del water splitting. In questo contesto si inquadrano anche le eterogiunzioni ibride organico-inorganico. La parte innovativa legata a questo lavoro nasce dalla necessità di sviluppare fotoelettrodi sempre più efficienti, con una appropriata scelta dei materiali e della morfologia degli stessi. Per le loro proprietà opto-eletroniche, gli ossidi di metalli di transizione appaiono i migliori candidati, dato che la loro struttura energetica si presta all’accoppiamento con differenti poilmeri coniugati da adottare come materiale fotosensibile. Essendo inoltre materiali charge-selective, essi migliorano le performances riducendo le perdite dovute alle ricombinazioni elettrone-lacuna all’interfaccia organico-inorganico. Per quanto riguarda la morfologia, essi possono essere facilmente nanostrutturati, incrementando ulteriormente l’efficienza del device finale. La tecnica di deposizione a laser pulsato (PLD) è ottimale per realizzare nanostrutture ed è stata utilizzata nel presente lavoro. Nella prima parte del lavoro, l’ossido di titanio (TiO2) è stato utilizzato come fotoanodo per celle solari ibride organico-inorganico. Nella seconda parte gli ossidi di molibdeno (MoO3) e di palladio (PdO) sono stati implementati come fotocatodi per il processo di water splitting.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10589/98428