| Περίληψη: | Undoubtedly, dye-sensitized solar cells (DSSCs) are one of the most important 3rd generation photovoltaic technologies today, meeting the ever-growing demand of humanity for low-cost, efficient, and clean energy production. DSSCs are hybrid organic-inorganic photovoltaic devices, recently gaining considerable attention in the academic and industrial communities. From a scientific point of view, they are enthralling systems to investigate, develop, and optimize. These devices comprise various materials, while each of the components can be designed, fabricated, and explored individually. Nonetheless, it must be pointed out that the crucial parameter for their efficiency and stability is the interplay between their components. From an industrial point of view, the efficiency and long-term stability of DSSCs have been a subject of concern during the past years of development of this technology. To solve these problems, numerous research efforts have been devoted to the engineering and manufacturing of these devices, in order to meet the standards of the photovoltaics market, for various applications.
The present Ph.D. dissertation is merely a small contribution to the gigantic amount of data, models, and theories that have been made over the last years to improve DSSCs technology, but hopefully a valuable research effort to the scientific community. The main research objective of the dissertation is the application of hybrid nanotechnology in DSSCs, with the main goals of improving their energy conversion efficiency and stability, further reducing their manufacturing cost, and increasing their application range. Here, it must be noted that all manufacturing processes used during the present research effort were based on low-cost and simple techniques, capable of reproduction by conventional means. The obtained experimental results were interpreted thoroughly from the physicochemical point of view. The electrical characterization of solar cells, which were all fabricated in the laboratory, comes along with a large number of materials characterization experiments, where the morphology, the crystallinity, the chemical composition and structure, as well as the thermal, optical, electrical, and optoelectrical characteristics of the individual parts of the solar cells were examined. In addition, the one-diode model equivalent circuit analysis of DSSCs contributed to the interpretation of the results obtained from the electrical characterization of the materials as a system (solar cells). The solar cells were characterized both for their performance and their stability. Finally, the mechanical, dynamic mechanical, and viscoelastic behavior of composite materials, which simulate the structure of the solar cells, were investigated.
Starting from the first main goal, “enhancement of DSSCs efficiency”, the dissertation deals with the systematic investigation in the direction of replacing the materials and structure of the conventional anode of DSSCs with novel hybrid nanostructures, which exhibit unique and optimized characteristics for the aforementioned application. The modifications concerned interfacial engineering, regulation of the materials porosity, fabrication of composites aiming to improve the electrical and optical characteristics of the anode, enhancement of light scattering, and co-sensitization of the anode for enhanced light-harvesting. The results are very satisfactory since the fabrication of solar cells with an energy conversion efficiency of 13% was achieved, reaching DSSCs efficiency records.
Focusing on the second main goal, “enhancement of DSSCs stability”, two separate investigations were conducted. The first one deals with the development of high-efficiency quasi-solid state DSSCs employing novel advanced polymer electrolytes, which were prepared in the laboratory. The aim was the optimization of the characteristics of the polymer electrolytes for achieving a high performance to DSSCs. In all cases, the rest of the materials and structure of the solar cells were identical to the conventional DSSCs. More specifically, the investigation focused on the preparation of iodide-based electrolytes, using polymer blends as solidification agents, suitable for solar cells application. Further up, the extra use of chemical additives and iodide compound mixtures, which have proven to improve the performance of the corresponding liquid state electrolytes for the aforementioned application, were investigated. The results are very satisfactory since the quasi-solid state DSSCs presented a higher energy conversion efficiency and stability compared to their conventional counterparts, which employed a liquid state factory-available high-stability electrolyte. The second investigation on the DSSCs stability topic deals with the determination of the performance degradation of conventional DSSCs under extreme ageing conditions. The investigation also includes the accurate prediction of the degradation of the solar cells performance after all accelerating ageing conditions, by means of a semi-analytical model (residual property model, RPM) developed by Papanicolaou et al. This achievement is considered important in the direction of the fast and accurate determination of the lifetime and reliability of solar cells for various applications.
Concerning the third main goal of the dissertation, “further reduction of DSSCs costs”, a series of platinum-free DSSCs were fabricated, based on novel carbon-based counter electrodes. Even though the study was at the first stages, the platinum-free DSSCs performed equally well compared to the conventional DSSCs employing factory-available platinum-based counter electrodes. The results of this study are considered important in the direction of increasing the competitiveness of DSSCs technology in the photovoltaic market.
Finally, concerning the fourth main goal of the dissertation, “DSSCs wide commercialization”, three separate investigations were conducted. The first one deals with the development of high-efficiency back-side illuminated DSSCs, using simple and low-cost techniques, based on highly ordered and mesoporous materials, after their optimization for solar cells application. In this case, the achieved efficiency of the back-side illuminated DSSCs is quite satisfactory since it was higher than the corresponding of the conventional front-side illuminated DSSCs. At this point, it is worth mentioning that the active surface of the back-side illuminated DSSCs was four times larger than the one of the conventional front-side illuminated DSSCs. The results of this study are considered important in the direction of developing high-efficiency, high-stability, and low-cost flexible DSSCs. The second investigation concerns the evaluation of the limiting factors affecting large-sized flexible platinum-free DSSCs performance, which is considered as a topic with a literature gap. This study contributes to the increase of the competitiveness of DSSCs technology in the photovoltaic market for various novel applications. Finally, with the ever-increasing demand for high quality and reliable solar cells, DSSCs were investigated from a mechanical point of view. In this case, the mechanical, the dynamic mechanical, and the viscoelastic behavior of sandwich-like structured composite materials, which simulate the structure of a flexible quasi-solid state DSSC, were studied through three-point blending experiments at different strain rates, dynamic mechanical analysis experiments at different oscillation frequencies, and relaxation experiments at different strain levels. These structures were fabricated using materials already examined for their suitability for DSSCs application. The aforementioned research is a preliminary study in the direction of fabrication of high-mechanical strength solar cells, which is considered as a new hot topic in the field of photovoltaics.
All the above-presented concepts and their realizations made a valuable contribution to the DSSCs field. The studies presented hereby are comprehensive and elegant examples of a carefully and logically planned scrutiny, leading to a deeper understanding of the processes taking place in DSSCs. The extensive use of so many different investigation techniques and the successful combination of the obtained results allowed for drawing important conclusions, upon which the base for further optimization of DSSCs and/or other emerging photovoltaic technologies may be built.
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