Investigation of the minimum driving force for dye regeneration utilizing model squaraine dyes for dye-sensitized solar cells

Combined theoretical and experimental approaches were implemented to design model far-red sensitive unsymmetrical squaraine dyes in order to estimate the minimum energy barrier required for dye regeneration. Our logical molecular design indicated that it is possible to have a fine control on the energetics within 0.2 eV only by the judicious selection of substituents and alkyl chain length keeping the main π-molecular framework the same. The utilization of the LSDA functional under TD-DFT calculations offered an effective and economical computational method for the reliable prediction of the energetics as well as the absorption maximum of the sensitizers. Among the designed dyes under investigation, SQ-75 exhibited the best photovoltaic performance, having a short-circuit current density of 10.92 mA cm−2, open circuit voltage of 0.57 V and a fill factor of 0.67, leading to a photoconversion efficiency of 4.25% despite having photon harvesting mainly in the far-red region. The best photon harvesting by SQ-75, even with an energy difference of only 0.12 eV between the energy of its highest occupied molecular orbital and redox energy level I−/I3− electrolyte, corroborates the possibility for dye regeneration with such a small driving force

コバルト電解質を用いた色素増感型太陽電池用近赤外色素の分子構造の光電変換に及ぼす影響の研究

Dye sensitized solar cell (DSSC) falls under one of the splendid solar cells, which have amicably attracted mammoth research. The superlative of it amongst the other solar cells is its inspiration from natural photosynthesis, transparency and vivid colors imparting beauty and attraction. In addition, the cost of fabrication is expected to be relatively low compared to commercial solar cells owing to low cost raw materials and processes not demanding for use of high energy. Research in past two decades have demonstrated not only nearly quantitative photon harvesting in visible region but also the photoconversion efficiency (PCE) 12-14 %. This poses acute need for the design and synthesis of novel NIR dyes to have more photon flux harvested leading to further enhancement in the photoconversion efficiency (PCE). Beside the panchromatic photon harvesting for high current density (Jsc), there is also need for maximizing the open circuit voltage (Voc), which demands for use of electrolytes with deeper redox potential. The 1st chapter introduces the need of huge energy demand by the growing population and hence the necessity of the next generation solar cells. Keeping this mind, a brief discussion of the third generation solar cells like organic solar cells, DSSCs and perovskite solar cells has been done with emphasis on DSSCs. Importance of various dye sensitizers and redox electrolytes employed for DSSC justifying the need for squaraine dyes as NIR sensitizer and cobalt redox electrolyte. 2nd chapter deals with details about necessary chemicals and reagent for synthesis of sensitizers and their intermediates, basic instrumentation and various characterization techniques used for the entire research work carried out during doctoral course. 3rd chapter focusses on estimation of minimum energy barrier for dye regeneration using NIR sensitive squaraine dyes as sensitizers and most commonly used I-/I3- redox electrolytes redox electrolytes. It was found that the alkyl chain plays a crucial role in controlling the energetics. It has been demonstrated that it is possible to have dye regeneration with a minimum energy barrier of 0.12 eV. In the 4th chapter, optimization of DSSCs fabrication parameters was done using one of the model NIR sensitive squaraine dye (SQ-75) and Co(bpy)2+/3+ redox electrolyte was conducted. TiCl4 surface treatment was found to control the overall PCE and especially it was more effective when treatment was done on both of the FTO glass substrate and mesoporous TiO2 layer. At the same time, only 6 μm thick mesoporous TiO2 was found to be optimum considering the high molar extinction coefficient of the dye and bulky nature of the redox electrolyte. Dye aggregation owing to planarity of dye molecule demanded optimization of coadsorber for dye aggregation prevention and 20 times of its use with respect to the dye was found to be optimum for the best device performance. DSSCs using NIR dyes in combination of cobalt electrolyte needs strict surface passivation for both of conducting substrate as well as mesoporous TiO2 layer. Utilization of optimized surface passivation discussed in the previous chapter, dye structure level optimization was attempted in the 5th chapter. This chapter utilizes longer alkyl chain bearing NIR dyes to provide additional surface passivation needed for cobalt electrolyte based DSSCs. Newly designed squaraine dye with two long and branched alkyl chains (SQ-110) was found to perform well as NIR sensitizer providing best PCE amongst newly designed dyes. Considering the narrow wavelength and intense light absorption by newly designed squaraine dyes, efforts was directed for wide wavelength photon harvesting utilizing a dye cocktail of one of the best NIR dye (SQ-110) with cobalt electrolyte compatible visible sensitizer (D-35) in the 6th chapter. In the present dye-cocktail, DSSCs consisted of different ratios of the constituent dyes, mutual control of dye aggregation, complementary photon harvesting and FRET between two dyes are found to be responsible for synergistically enhanced photon harvesting in the wide wavelength region. It has been demonstrated that a compact TiO2 on FTO and a bilayer TiO2/MgO surface passivation was necessary for getting the optimum DSSC performance with optimum dye cocktail of D-35 and SQ-110 in (4:1). Finally, last and 7th chapter summarizes the general conclusion of the whole thesis along with the outlooks and future prospects of the present work.九州工業大学博士学位論文 学位記番号：生工博甲第332号 学位授与年月日：平成31年3月25日1 Introduction to Solar Cell|2 Experimental: (Instrumentation and Characterization)|3 Investigation on minimum driving force for the dye regeneration|4 Parametric Optimization of DSSCs Using Far red Sensitizing Dye with Cobalt Electrolyte|5 Development of Unsymmetrical Squaraine Dyes for DSSCs utilizing Cobalt Electrolyte|6 Wide Wavelength Photon Harvesting: Implication of dye cocktail and surface Passivation|7九州工業大学平成30年

コバルト電解質を用いた色素増感型太陽電池用近赤外色素の分子構造の光電変換に及ぼす影響の研究

九州工業大学博士学位論文（要旨）学位記番号：生工博甲第332号 学位授与年月日：平成31年3月25

Synthesis, Photophysical characterization and dye adsorption behavior in unsymmetrical squaraine dyes with varying anchoring groups

Four newly designed unsymmetrical squaraine dyes bearing different functional groups for their anchoring on the surface of mesoporous TiO2 were successfully synthesized aiming towards development of far-red sensitizers for dye-sensitized solar cells (DSSCs). The synthesized dyes were characterized by 1H NMR, fast ion bombardment mass, and subjected to photophysical investigations by electronic absorption/fluorescence emission spectroscopy and cyclic voltammetry. These dyes exhibited excellent solubility in a number of common organic solvents. Adsorption behavior of these dyes on the thin films of mesoporous TiO2 was investigated in detail and results indicated that rate of dye adsorption follows the order −COOH>-PO3H2>OH > SO3H. At the same time, dye desorption studies demonstrated that stability of adsorbed dyes on the mesoporous TiO2 follows the order -PO3H2>>OH>−COOH > –SO3H. Further, implication of the nature of anchoring groups of dye molecules upon the energy levels of their highest occupied molecular orbital and lowest unoccupied molecular orbital was evaluated by both of the combined theoretical and experimental approaches. In spite of enhanced dye loading, very high binding strength and favorable energetic cascade, SQ-143 bearing Phosphonic acid exhibited hampered photovoltaic performance as compared to that of SQ-138 with carboxylic acid anchoring group. Amongst various sensitizers used for present investigation, SQ-138 exhibited best photovoltaic performance having short-circuit current density, open circuit voltage and fill factor of 12.49 mA/cm2, 0.60 V, and 0.53, respectively, leading to power conversion efficiency of 4.07 % after simulated solar irradiation

Synthesis and Photophysical Characterization of Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells utilizing Cobalt Electrolytes

Development of novel near-infrared (near-IR) dyes compatible with cobalt complex based redox shuttles for their utilization as sensitizer is inevitable for the fabrication of high-efficiency dye-sensitized solar cells (DSSCs). A series of newly designed unsymmetrical squaraine dyes as a model of near-IR sensitizer were synthesized and characterized for their application as far-red sensitizers of DSSCs utilizing Co(bpy)2+/3+ redox electrolyte. It was shown that logical molecular design led to not only energetic tunability of the sensitizers but also the possibility of good far-red photon harvesting up to 750 nm. One of the newly designed sensitizers, SQ-110, bearing two long alkyl substituents in combination with an electron donating methoxy group directly linked to the aromatic ring was par excellent in terms of its photoconversion efficiency among the dyes utilized in this work. DSSC fabricated using SQ-110 as sensitizer and Co(bpy)2+/3+ redox electrolyte furnished a photoconversion efficiency of 1.98% along with good photon harvesting mainly in the far-red wavelength region. It was further demonstrated that dye molecular structure plays a rather more prominent role than their energetics in controlling the overall device performance of the DSSCs

Wide wavelength photon harvesting in dye-sensitized solar cells utilizing cobalt complex redox electrolyte: Implication of surface passivation

A combination of near infra-red (NIR) photon harvesting and cobalt electrolytes having deeper redox energy level are needed for the fabrication of high efficiency dye-sensitized solar cells (DSSCs). A logical molecular design of unsymmetrical squaraine dye (SQ-110) as a representative of NIR dyes has been demonstrated to function well in DSSC using cobalt complex redox electrolyte. Problem of mass transport limitations due to the bulky cobalt complex ions leading to relatively enhanced charge recombination was amicably solved by single as well as multiple compact metal oxide surface passivation on both of the transparent conducting oxide substrate as well as mesoporous TiO2. Complete absence of light absorption beyond 550 nm wavelength region by D-35 and utilization of a complementary light harvesting dye SQ-110 led to efficient wide wavelength photon harvesting. DSSC fabricated using a dye cocktail of D-35 and SQ-110 in 4:1 ratio resulted in to photoconversion efficiency (PCE) of 7.2%, which is much higher as compared to the constituent individual sensitizers D-35 (3.6%) and SQ-110 (1.9%). This synergistic enhancement in PCE by dye cocktail was associated with the mutual co-operation of respective dyes in terms controlling the dye aggregation and complementary photon harvesting. In this dye cocktail system, D-35 is involved in the prevention of dye aggregation, lower wavelength photon harvesting and energy transfer induced photocurrent enhancement

Global, regional, and national prevalence of adult overweight and obesity, 1990–2021, with forecasts to 2050: a forecasting study for the Global Burden of Disease Study 2021

Background: Overweight and obesity is a global epidemic. Forecasting future trajectories of the epidemic is crucial for providing an evidence base for policy change. In this study, we examine the historical trends of the global, regional, and national prevalence of adult overweight and obesity from 1990 to 2021 and forecast the future trajectories to 2050. Methods: Leveraging established methodology from the Global Burden of Diseases, Injuries, and Risk Factors Study, we estimated the prevalence of overweight and obesity among individuals aged 25 years and older by age and sex for 204 countries and territories from 1990 to 2050. Retrospective and current prevalence trends were derived based on both self-reported and measured anthropometric data extracted from 1350 unique sources, which include survey microdata and reports, as well as published literature. Specific adjustment was applied to correct for self-report bias. Spatiotemporal Gaussian process regression models were used to synthesise data, leveraging both spatial and temporal correlation in epidemiological trends, to optimise the comparability of results across time and geographies. To generate forecast estimates, we used forecasts of the Socio-demographic Index and temporal correlation patterns presented as annualised rate of change to inform future trajectories. We considered a reference scenario assuming the continuation of historical trends. Findings: Rates of overweight and obesity increased at the global and regional levels, and in all nations, between 1990 and 2021. In 2021, an estimated 1·00 billion (95% uncertainty interval [UI] 0·989–1·01) adult males and 1·11 billion (1·10–1·12) adult females had overweight and obesity. China had the largest population of adults with overweight and obesity (402 million [397–407] individuals), followed by India (180 million [167–194]) and the USA (172 million [169–174]). The highest age-standardised prevalence of overweight and obesity was observed in countries in Oceania and north Africa and the Middle East, with many of these countries reporting prevalence of more than 80% in adults. Compared with 1990, the global prevalence of obesity had increased by 155·1% (149·8–160·3) in males and 104·9% (95% UI 100·9–108·8) in females. The most rapid rise in obesity prevalence was observed in the north Africa and the Middle East super-region, where age-standardised prevalence rates in males more than tripled and in females more than doubled. Assuming the continuation of historical trends, by 2050, we forecast that the total number of adults living with overweight and obesity will reach 3·80 billion (95% UI 3·39–4·04), over half of the likely global adult population at that time. While China, India, and the USA will continue to constitute a large proportion of the global population with overweight and obesity, the number in the sub-Saharan Africa super-region is forecasted to increase by 254·8% (234·4–269·5). In Nigeria specifically, the number of adults with overweight and obesity is forecasted to rise to 141 million (121–162) by 2050, making it the country with the fourth-largest population with overweight and obesity. Interpretation: No country to date has successfully curbed the rising rates of adult overweight and obesity. Without immediate and effective intervention, overweight and obesity will continue to increase globally. Particularly in Asia and Africa, driven by growing populations, the number of individuals with overweight and obesity is forecast to rise substantially. These regions will face a considerable increase in obesity-related disease burden. Merely acknowledging obesity as a global health issue would be negligent on the part of global health and public health practitioners; more aggressive and targeted measures are required to address this crisis, as obesity is one of the foremost avertible risks to health now and in the future and poses an unparalleled threat of premature disease and death at local, national, and global levels. Funding: Bill & Melinda Gates Foundation

Investigating the Impact of Molecular Structure Near Infra-Red Sensitive Dyes on Photoconversion for Cobalt Electrolyte Based Dye-Sensitized Solar Cells

1 Introduction to Solar Cell||2 Experimental: （Instrumentation and Characterization）||3 Investigation on minimum driving force for the dye regeneration||4 Parametric Optimization of DSSCs Using Far red Sensitizing Dye with Cobalt Electrolyte||5 Development of Unsymmetrical Squaraine Dyes for DSSCs utilizing Cobalt Electrolyte||6 Wide Wavelength Photon Harvesting: Implication of dye cocktail and surface Passivation||7Dye sensitized solar cell （DSSC） falls under one of the splendid solar cells, which have amicably attracted mammoth research. The superlative of it amongst the other solar cells is its inspiration from natural photosynthesis, transparency and vivid colors imparting beauty and attraction. In addition, the cost of fabrication is expected to be relatively low compared to commercial solar cells owing to low cost raw materials and processes not demanding for use of high energy. Research in past two decades have demonstrated not only nearly quantitative photon harvesting in visible region but also the photoconversion efficiency （PCE） 12-14 %. This poses acute need for the design and synthesis of novel NIR dyes to have more photon flux harvested leading to further enhancement in the photoconversion efficiency （PCE）. Beside the panchromatic photon harvesting for high current density （Jsc）, there is also need for maximizing the open circuit voltage （Voc）, which demands for use of electrolytes with deeper redox potential. The 1st chapter introduces the need of huge energy demand by the growing population and hence the necessity of the next generation solar cells. Keeping this mind, a brief discussion of the third generation solar cells like organic solar cells, DSSCs and perovskite solar cells has been done with emphasis on DSSCs. Importance of various dye sensitizers and redox electrolytes employed for DSSC justifying the need for squaraine dyes as NIR sensitizer and cobalt redox electrolyte. 2nd chapter deals with details about necessary chemicals and reagent for synthesis of sensitizers and their intermediates, basic instrumentation and various characterization techniques used for the entire research work carried out during doctoral course. 3rd chapter focusses on estimation of minimum energy barrier for dye regeneration using NIR sensitive squaraine dyes as sensitizers and most commonly used I-/I3- redox electrolytes redox electrolytes. It was found that the alkyl chain plays a crucial role in controlling the energetics. It has been demonstrated that it is possible to have dye regeneration with a minimum energy barrier of 0.12 eV. In the 4th chapter, optimization of DSSCs fabrication parameters was done using one of the model NIR sensitive squaraine dye （SQ-75） and Co（bpy）2+/3+ redox electrolyte was conducted. TiCl4 surface treatment was found to control the overall PCE and especially it was more effective when treatment was done on both of the FTO glass substrate and mesoporous TiO2 layer. At the same time, only 6 μm thick mesoporous TiO2 was found to be optimum considering the high molar extinction coefficient of the dye and bulky nature of the redox electrolyte. Dye aggregation owing to planarity of dye molecule demanded optimization of coadsorber for dye aggregation prevention and 20 times of its use with respect to the dye was found to be optimum for the best device performance. DSSCs using NIR dyes in combination of cobalt electrolyte needs strict surface passivation for both of conducting substrate as well as mesoporous TiO2 layer. Utilization of optimized surface passivation discussed in the previous chapter, dye structure level optimization was attempted in the 5th chapter. This chapter utilizes longer alkyl chain bearing NIR dyes to provide additional surface passivation needed for cobalt electrolyte based DSSCs. Newly designed squaraine dye with two long and branched alkyl chains （SQ-110） was found to perform well as NIR sensitizer providing best PCE amongst newly designed dyes. Considering the narrow wavelength and intense light absorption by newly designed squaraine dyes, efforts was directed for wide wavelength photon harvesting utilizing a dye cocktail of one of the best NIR dye （SQ-110） with cobalt electrolyte compatible visible sensitizer （D-35） in the 6th chapter. In the present dye-cocktail, DSSCs consisted of different ratios of the constituent dyes, mutual control of dye aggregation, complementary photon harvesting and FRET between two dyes are found to be responsible for synergistically enhanced photon harvesting in the wide wavelength region. It has been demonstrated that a compact TiO2 on FTO and a bilayer TiO2/MgO surface passivation was necessary for getting the optimum DSSC performance with optimum dye cocktail of D-35 and SQ-110 in （4:1）. Finally, last and 7th chapter summarizes the general conclusion of the whole thesis along with the outlooks and future prospects of the present work.九州工業大学博士学位論文 学位記番号：生工博甲第332号 学位授与年月日：平成31年3月25日平成30年