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Indium-tin-oxide-free organic photovoltaic devices and electrode structures
No full textPhotovoltaic cells based on organic semiconductors promise to provide low cost, flexible, lightweight and even semitransparent solar power for various applications. However, a cost effective fabrication of organic solar modules can only be achieved when large-scale roll-to roll (R2R) processing techniques as well as low-cost substrates and materials are used. As the transparent electrode is a major cost driver for R2R-fabricated organic photovoltaics (OPVs), this work presents the usage of a commercially available metal mesh film as cost effective transparent electrode for indium-tin-oxide (ITO)-free organic solar cells.
For this purpose, four different metal mesh based organic solar cell architectures were optimized and studied in detail, leading to the identification of two layer stacks matching the requirements for R2R production. Furthermore, a simple model for the optimization of the metal mesh geometry and the corresponding solar cell layer stack is demonstrated, compared to experimental data and used to achieve optimum device performance while ensuring R2R processing compatibility.
As an alternative approach, the Light-Bias Light-Beam-Induced-Current (LB-LBIC) technique, a new variant of the traditional LBIC measurement, which allows the visualization of lateral charge-extraction in metal mesh based solar cells is shown. An analytical model to explain the LB-LBIC measurement results is developed and used to verify the metal mesh optimization results.
In the final part of this work, the extension of the photoactive area ("‘photoactive area spreading"’) of solar cells, which is caused by unstructured layers, is introduced and a model to describe the effect is presented. The model is used to extract spreading-free photocurrents from measured solar cell current-voltage data and is shown to provide more accurate results compared to conventional shadow-masking
Unravelling the proteome and peptidome biochemistry in Theobroma cacao
No full textCocoa-derived products contribute largely to the human-luxury diet in many countries. According to the world cocoa foundation report 2015, worldwide there are 1.8 million tons of chocolate sold, 53 chocolate-producing companies with thousands of factories and more than 5 million cocoa farmers who depend upon cocoa as a cash crop. The high demand for the production of chocolate in the world has made food-oriented research very important. The development of a high-quality cocoa product requires a deep knowledge of the chemistry of the compounds found in the cocoa beans as well as high skills in the chocolate making process. Until recently, chocolate production was a completely empirical field, where the optimal process parameters were found after many trials. However, recent developments in chemical analysis offer the possibility to gain insight into and closely follow the chemical changes of cocoa during all production steps thereby allowing chocolate producers to direct their processes towards the desired features, i.e., taste, consistency, and color.
Chocolate and cocoa aroma as well as color derive from the chemical precursor content of the cocoa beans used as raw material and from the way this content is modified during fermentation and manufacturing. The composition of this metabolite content depends on the cocoa variety used, its geographical origin, cultivation method, and post-harvest processing. Final product characteristics are developed through reaction of these precursors during roasting leading to specific colors, flavors, and textures. From this perspective, the precursor content of the cocoa bean and its processing are the central points of importance
Work engagement as a key for unlocking performance: An investigation across different organizational levels
No full textSince people spend around one third of their day at work, the question of which factors enhance their well-being and their motivation at work is an important one. Moreover, organizations have to face several challenges, such as a quickly changing global economic market, digitalization, and continuous need for innovation (Cascio & Montealegre, 2016; Frese, 2008; Leibold & Voelpel, 2006). Work engagement is a construct that is an asset for both employees and organizations. This dissertation aims to answer the question which factors may foster work engagement within organizations and how this is related to individual and organizational performance across different organizational levels. In order to shed light on these questions, this dissertation investigates how constructive and destructive leadership is related to work engagement and which role leaders’ work engagement itself may play for followers’ tendency to engage in their work. Additionally, since a lot of organizations are interested in the topic of work engagement, a new engagement assessment – the Engagement Index (ENG-I) – is introduced that faces both, scientific and organizational requirements. Finally, the link between individual and organizational work engagement and performance is analyzed by considering causality issues. In sum, this dissertation can show that indeed, leadership is an important lever for work engagement, that the ENG-I shows to be a statistically validated and well accepted measurement of work engagement, and that individual and organizational work engagement leads to individual and organizational performance
The Role of Individual Differences in the Prediction of Cooperation, Deviance, and Performance
No full textThe present dissertation contributes to a deeper understanding about the effects of individual differences on three outcomes crucial for social and organizational functioning – cooperation, deviance, and performance. Chapter 1 describes the use of individual differences in predicting these behaviors in more detail, explains the most important individual difference (i.e., personality) and its conceptualizations, and provides an overview of the remaining chapters. In Chapter 2, it is shown that the relation between social value orientation (SVO) and cooperation is partially mediated by expectations of others’ behavior. In Chapter 3, the predictive validity of SVO for deviant behavior is demonstrated, whereas Chapter 4 meta-analytically examines the relations between broad personality domains and workplace deviance and shows that HEXACO Honesty-Humility is the strongest predictor out of all Big Five and HEXACO personality domains examined. In Chapter 5, it is demonstrated that the negative relation between age and workplace deviance is mediated by personality and negative affect. As such, findings of this chapter for the first time test the mechanisms underlying the negative relation between age and workplace deviance. The last empirical chapter of this dissertation examines how another important demographic characteristic – gender – relates to firm financial performance. Female representation on corporate boards and firm financial performance are neither positively nor negatively related. Together, these five empirical chapters provide valuable insights into the study of individual differences as predictors of behaviors that crucially determine social and organizational functioning. Chapter 7 summarizes the main findings, discusses theoretical and practical implications, and deduces directions for future research
Characterization of low molecular weight carbohydrates in dietary foods by chromatographic techniques coupled to mass spectrometry
No full textThe present thesis reports the development and use of chromatographic techniques coupled to mass spectrometry for the characterization of low molecular weight carbohydrates (LMWC) in dietary foods of economic relevance or identified as “functional food”.
The interest for the analysis of LMWC in different dietary foods is based on the involvement of these compounds in biological reactions in plants and the bioactive properties reported for some LMWC. A general overview of the carbohydrates, the analytical methodologies to perform their analysis and a brief description of the different dietary foods selected for this study (cocoa beans, commercial green tea and kale) are reported in the different chapters (chapter 1-3) of the introduction.
The study of LMWC in cocoa beans includes three chapters (chapter 4-6). The content of chapters 4 and 5 is a comprehensive characterization of the LMWC profile in cocoa beans using HILIC-ESI-TOF MS, HILIC-ESI-MSn and GC-MS, the quantification of the main and minor LMWC from different origins and a proposal of different indicators of fermentation. Chapter 6 covers a detailed chemometric and kinetic approach to monitor the LMWC changes during the spontaneous fermentation of cocoa beans.
The study of commercial green tea (chapter 7) evaluates the LMWC together with other physical and chemical established quality indicators (soluble solids, color and antioxidant capacity) to characterize CGT. This approach has resulted to be useful for the characterization of the samples according to the type of processing employed during the manufacturing.
Chapter 8 reports the identification for the first time of different LMWC in kale. This chapter also includes the analysis of the LMWC content in three types of kale during the development of the plant, as well as the monitoring of the changes produced as a consequence of cold temperature during farming
Understanding and engineering the cytosolic redox balance of Saccharomyces cerevisiae during glycerol catabolism
No full textThe valorization of carbon-rich industrial waste-streams, such as crude glycerol derived from the biodiesel industry, is a promising avenue to overcome the dependence on fossil resources for the production of certain chemicals. Glycerol has a higher degree of reduction compared to sugars, allowing higher theoretical yields of small reduced molecules, when used as a substrate in industrial biotechnological processes. Several microorganisms are able to catabolize glycerol, including the Saccharomyces cerevisiae strain CBS 6412-13A. In order to utilize glycerol’s degree of reduction for S. cerevisiae-based synthesis of fermentation products, metabolic engineering is required. A replacement of the native FAD-dependent L-glycerol 3-phosphate pathway by the artificial NADH-delivering dihydroxyacetone pathway is supposed to provide glycerol-derived electrons for the cytosolic formation of fermentation products. However, S. cerevisiae has potent respiratory mechanisms, that maintain cytosolic redox balance by transferring electrons from cytosolic NADH to oxygen. During growth with e.g. ethanol or low glucose concentrations, the external NADH dehydrogenases (Nde1/2) and the L-G3P shuttle (composed of Gpd1/2 and Gut2), are contributing to cytosolic NAD+ regeneration. Their contribution with glycerol as carbon source was not investigated so far. It is demonstrated here, that in particular Nde1 significantly contributes to cytosolic redox balance in S. cerevisiae catabolizing glycerol. S. cerevisiae’s major native fermentative mechanism to oxidize NADH is alcoholic fermentation, even when oxygen is available. However, glycerol is generally considered to be a ‘non-fermentable’ carbon source for S. cerevisiae. The current work demonstrates that at least S. cerevisiae strains catabolizing glycerol via the DHA pathway are able to perform alcoholic fermentation. The results of this thesis provide important insights into the redox metabolism of S. cerevisiae catabolizing glycerol
Biogeochemistry of Pacific deep-sea sediments and potential impacts of deep-sea polymetallic nodule mining
No full textThis cumulative PhD thesis explores trace metal distributions in deep-sea sediments and pore waters in two manganese nodule areas of the central equatorial Pacific, namely the Peru Basin and the Clarion Clipperton Zone (CCZ). In the light of new developments in the deep-sea mining industry and policy making, the environmental assessment of the possible impacts of deep-sea mining on the deep seafloor and the biogeochemical processes in the sediment and at the sediment-water interface is critical. For the analysis of mining impacts, a key area of focused research is on the Mn-oxide-rich surface layer, which is also rich in the associated metals Mo, Co, Ni, and Cu, and will be the layer most likely to be impacted by mining.
Overall, this thesis sheds light on the small-scale heterogeneity of deep-sea sediments and the contained pore waters in the Peru Basin, reflected in the depth distribution of POC, nitrate, Mn, and Fe. Based on the thorough geochemical analysis, oxygen and solid phase Mn were identified as key parameters for monitoring of potential future mining related impacts because they are representative of the redox zonation and the behavior of other metals. The work provides detailed baseline data for solid phase Al, Ca, Fe, Mn, Mo, Ni, Co, Cu, U, V, Cd, Pb, Zn, and REY and pore water Mn, Co, Cu, Mo, U, V, Cd, As, and REY that increases our knowledge of the biogeochemistry of trace metals in deep-sea sediments and provides a basis to assess future potential anthropogenic impacts, e.g., polymetallic nodule mining. It furthermore increases our understanding of the distribution of REY in deep-sea sediments, and the ongoing alteration of REYSN patterns during early diagenesis in the CCZ and the Peru Basin in a variety of redox conditions
Film growth and characterization of solution processed MoS2 semiconductor films for thin film transistors
No full textA novel wet-chemical synthesis of layered MoS2 thin films on silicon and sapphire substrate has been achieved. The gap in understanding solution processed MoS2 deposition needs to be closed to exploit all its excellent properties for low-cost applications. Both top-down and bottom-up approaches such as liquid phase exfoliation (LPE), dip coating, chemical bath deposition (CBD) and spin coating were explored for the synthesis of ultra-thin MoS2 films from solution phase. In this work, as deposited Mo-precursor thin films were prepared based on the solubility and coating properties of Molybdenum(V) chloride in 1-Methoxy-2-propanol. Subsequent annealing of the deposited amorphous Mo-precursor films in the presence of sulfur and H2 resulted in the formation of layered MoS2 films. Highly crystalline films are obtained on sapphire substrates, while inferior quality was obtained on Si/SiO2. Generally, improved crystallinity of the deposited films was achieved by increasing the process temperature and performing the post-annealing treatment. Post-annealing at temperatures above 900 °C increased the uniformity of multilayer films, together with the increase of MoS2 grain size. For charge transport analysis, top-gate top-contact thin film transistors (TFTs) based on these solution processed MoS2 films were fabricated. Ionic liquid gating of the TFT devices exhibited n-type semiconducting behaviour with field-effect mobility as high as 12.07 cm^2/Vs and Ion/Ioff ratio ~ 10^6. While with another precursor even mobility of 16.09 cm^2/Vs can be achieved. X-ray photoelectron spectroscopy measurements revealed that the films annealed between 900 °C and 980 °C had an average chemical composition of S/Mo ~ 1.84. This simple liquid phase synthesis method with centimeter-scale uniformity and closed films down to 2 ± 1 monolayer is suitable for low-cost preparation. This holds also for other transition metal dichalcogenides thin films in next-generation electronics
Wind stress forcing of the freshwater distribution in the Arctic and North Atlantic Oceans
No full textThe hypothesis of this study was that there is a potential for an oscillating covariability between the freshwater content of the Arctic Ocean and the Subarctic North Atlantic, and the redistribution between their basins is governed by wind stress forcing associated with large-scale patterns of atmospheric variability. In order to test this hypothesis, numerical model simulations were performed using the Max Planck Institute Earth System Model (MPI-ESM) with the objectives to 1) analyze the link between Arctic and Subarctic North Atlantic freshwater anomalies, to 2) identify key patterns of atmospheric variability that govern these anomalies through wind forcing, and to 3) explain the physical mechanisms of coupling between freshwater and near-surface winds associated with these key patterns.
The results showed that even though there is a stable sign of freshwater redistribution between the Arctic and the Subarctic North Atlantic on a multidecadal time scale, this sign is mostly obscured by large anomalies in the North Atlantic that are transported from the south.
A comprehensive statistical analysis revealed that the main statistical modes of large-scale atmospheric variability do not represent those modes that are best connected to freshwater anomalies. Such modes were identified in this work by performing a redundancy analysis of atmospheric variability and freshwater content, separately for its liquid and solid components.
The impact of wind stress forcing was demonstrated by further simulations. These used prescribed wind data based on observations, but unlike the otherwise identically set- up unconstrained fully coupled runs, they could reproduce the observed freshwater anomalies of the 1990s. This confirmed the key role of wind stress forcing. Additional experiments with prescribed idealized wind perturbations enabled the isolation of the effect of certain wind forcing patterns on freshwater variability
Reliability of photobioreactors in life support systems: an investigation of methods to restore Chlamydomonas reinhardtii cultures after heat stress
No full textMicroalgal cultures in photobioreactors are essential in biological life support systems for space flight. However, photobioreactor cultures are sensitive to environmental parameters outside of their tolerance range, and crew time for repair is limited. This work aimed to increase resilience and decrease vulnerability of photobioreactor cultures by exploring methods for restarting photobioreactor cultures after heat shock, with as little use of crew time as possible.
Using the microalga Chlamydomonas reinhardtii, two paths for restarting a photobioreactor culture were explored. In the first path we tested the heat resistance of C. reinhardtii biofilms, hypothesizing that microalgal biofilms would have a higher heat tolerance than planktonic cultures. That would make it possible to use a subset of microalgal culture, grown as a biofilm inside the planktonic photobioreactor culture, to restart the planktonic culture after a heat shock event, making it essentially self-restarting.
As biofilm substrate we used alumina (Al2O3). Several combinations of porosities and surface functionalization were tested for biofilm growth and potential harm to cells. Alumina with pore sizes around the size of a C. reinhardtii cell had the most cells attaching to them. To test heat tolerance of C. reinhardtii biofilm, and compare it to that of planktonic cultures, we designed a heat stress test setup. We could not, however, detect any difference in heat stress tolerance between C. reinhardtii biofilm and C. reinhardtii planktonic culture.
The second path explored the suitability of a silica sol-gel, manufactured with a novel method, for encapsulation of C. reinhardtii cells for medium term storage. The silica sol gel manufacturing method has been designed to be less stressful to biological cells and also consists of fewer steps than other similar methods described in literature. The method proved successful, the algal cells survived and stayed healthy for several weeks