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 Fossil fuels can no longer represent the predominant energy supply for human being.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.

Photopolymers for dye-sensitized solar cells / Bella, Federico. - (2015).

Photopolymers for dye-sensitized solar cells

BELLA, FEDERICO
2015

Abstract

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.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11583/2594972
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