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A dramatic efficiency improvement of bulk heterojunction solar cells based on electron-donating conjugated polymers in combination with soluble fullerene derivatives has been achieved over the past years. Certified and reported power conversion efficiencies now reach over 9% for single junctions and exceed the 10% benchmark for tandem solar cells. This trend brightens the vision of organic photovoltaics becoming competitive with inorganic solar cells including the realization of low-cost and large-area organic photovoltaics. For the best performing organic materials systems, the yield of charge generation can be very efficient. However, a detailed understanding of the free charge carrier generation mechanisms at the donor acceptor interface and the energy loss associated with it needs to be established. Moreover, organic solar cells are limited by the competition between charge extraction and free charge recombination, accounting for further efficiency losses. A conclusive picture and the development of precise methodologies for investigating the fundamental processes in organic solar cells are crucial for future material design, efficiency optimization, and the implementation of organic solar cells into commercial products.
In order to advance the development of organic photovoltaics, my thesis focuses on the comprehensive understanding of charge generation, recombination and extraction in organic bulk heterojunction solar cells summarized in 6 chapters on the cumulative basis of 7 individual publications.
The general motivation guiding this work was the realization of an efficient hybrid inorganic/organic tandem solar cell with sub-cells made from amorphous hydrogenated silicon and organic bulk heterojunctions. To realize this project aim, the focus was directed to the low band-gap copolymer PCPDTBT and its derivatives, resulting in the examination of the charge carrier dynamics in PCPDTBT:PC70BM blends in relation to by the blend morphology. The phase separation in this blend can be controlled by the processing additive diiodooctane, enhancing domain purity and size. The quantitative investigation of the free charge formation was realized by utilizing and improving the time delayed collection field technique. Interestingly, a pronounced field dependence of the free carrier generation for all blends is found, with the field dependence being stronger without the additive. Also, the bimolecular recombination coefficient for both blends is rather high and increases with decreasing internal field which we suggest to be caused by a negative field dependence of mobility. The additive speeds up charge extraction which is rationalized by the threefold increase in mobility.
By fluorine attachment within the electron deficient subunit of PCPDTBT, a new polymer F-PCPDTBT is designed. This new material is characterized by a stronger tendency to aggregate as compared to non-fluorinated PCPDTBT. Our measurements show that for F-PCPDTBT:PCBM blends the charge carrier generation becomes more efficient and the field-dependence of free charge carrier generation is weakened. The stronger tendency to aggregate induced by the fluorination also leads to increased polymer rich domains, accompanied in a threefold reduction in the non-geminate recombination coefficient at conditions of open circuit. The size of the polymer domains is nicely correlated to the field-dependence of charge generation and the Langevin reduction factor, which highlights the importance of the domain size and domain purity for efficient charge carrier generation. In total, fluorination of PCPDTBT causes the PCE to increase from 3.6 to 6.1% due to enhanced fill factor, short circuit current and open circuit voltage. Further optimization of the blend ratio, active layer thickness, and polymer molecular weight resulted in 6.6% efficiency for F-PCPDTBT:PC70BM solar cells.
Interestingly, the double fluorinated version 2F-PCPDTBT exhibited poorer FF despite a further reduction of geminate and non-geminate recombination losses. To further analyze this finding, a new technique is developed that measures the effective extraction mobility under charge carrier densities and electrical fields comparable to solar cell operation conditions. This method involves the bias enhanced charge extraction technique. With the knowledge of the carrier density under different electrical field and illumination conditions, a conclusive picture of the changes in charge carrier dynamics leading to differences in the fill factor upon fluorination of PCPDTBT is attained. The more efficient charge generation and reduced recombination with fluorination is counterbalanced by a decreased extraction mobility. Thus, the highest fill factor of 60% and efficiency of 6.6% is reached for F-PCPDTBT blends, while 2F-PCPDTBT blends have only moderate fill factors of 54% caused by the lower effective extraction mobility, limiting the efficiency to 6.5%.
To understand the details of the charge generation mechanism and the related losses, we evaluated the yield and field-dependence of free charge generation using time delayed collection field in combination with sensitive measurements of the external quantum efficiency and absorption coefficients for a variety of blends. Importantly, both the yield and field-dependence of free charge generation is found to be unaffected by excitation energy, including direct charge transfer excitation below the optical band gap. To access the non-detectable absorption at energies of the relaxed charge transfer emission, the absorption was reconstructed from the CT emission, induced via the recombination of thermalized charges in electroluminescence. For a variety of blends, the quantum yield at energies of charge transfer emission was identical to excitations with energies well above the optical band-gap. Thus, the generation proceeds via the split-up of the thermalized charge transfer states in working solar cells. Further measurements were conducted on blends with fine-tuned energy levels and similar blend morphologies by using different fullerene derivatives. A direct correlation between the efficiency of free carrier generation and the energy difference of the relaxed charge transfer state relative to the energy of the charge separated state is found. These findings open up new guidelines for future material design as new high efficiency materials require a minimum energetic offset between charge transfer and the charge separated state while keeping the HOMO level (and LUMO level) difference between donor and acceptor as small as possible.
Donor-acceptor (D-A) copolymers have revolutionized the field of organic electronics over the last decade. Comprised of a electron rich and an electron deficient molecular unit, these copolymers facilitate the systematic modification of the material's optoelectronic properties. The ability to tune the optical band gap and to optimize the molecular frontier orbitals as well as the manifold of structural sites that enable chemical modifications has created a tremendous variety of copolymer structures. Today, these materials reach or even exceed the performance of amorphous inorganic semiconductors. Most impressively, the charge carrier mobility of D-A copolymers has been pushed to the technologically important value of 10 cm^{2}V^{-1}s^{-1}. Furthermore, owed to their enormous variability they are the material of choice for the donor component in organic solar cells, which have recently surpassed the efficiency threshold of 10%. Because of the great number of available D-A copolymers and due to their fast chemical evolution, there is a significant lack of understanding of the fundamental physical properties of these materials. Furthermore, the complex chemical and electronic structure of D-A copolymers in combination with their semi-crystalline morphology impede a straightforward identification of the microscopic origin of their superior performance. In this thesis, two aspects of prototype D-A copolymers were analysed. These are the investigation of electron transport in several copolymers and the application of low band gap copolymers as acceptor component in organic solar cells. In the first part, the investigation of a series of chemically modified fluorene-based copolymers is presented. The charge carrier mobility varies strongly between the different derivatives, although only moderate structural changes on the copolymers structure were made. Furthermore, rather unusual photocurrent transients were observed for one of the copolymers. Numerical simulations of the experimental results reveal that this behavior arises from a severe trapping of electrons in an exponential distribution of trap states. Based on the comparison of simulation and experiment, the general impact of charge carrier trapping on the shape of photo-CELIV and time-of-flight transients is discussed. In addition, the high performance naphthalenediimide (NDI)-based copolymer P(NDI2OD-T2) was characterized. It is shown that the copolymer posses one of the highest electron mobilities reported so far, which makes it attractive to be used as the electron accepting component in organic photovoltaic cells.\par Solar cells were prepared from two NDI-containing copolymers, blended with the hole transporting polymer P3HT. I demonstrate that the use of appropriate, high boiling point solvents can significantly increase the power conversion efficiency of these devices. Spectroscopic studies reveal that the pre-aggregation of the copolymers is suppressed in these solvents, which has a strong impact on the blend morphology. Finally, a systematic study of P3HT:P(NDI2OD-T2) blends is presented, which quantifies the processes that limit the efficiency of devices. The major loss channel for excited states was determined by transient and steady state spectroscopic investigations: the majority of initially generated electron-hole pairs is annihilated by an ultrafast geminate recombination process. Furthermore, exciton self-trapping in P(NDI2OD-T2) domains account for an additional reduction of the efficiency. The correlation of the photocurrent to microscopic morphology parameters was used to disclose the factors that limit the charge generation efficiency. Our results suggest that the orientation of the donor and acceptor crystallites relative to each other represents the main factor that determines the free charge carrier yield in this material system. This provides an explanation for the overall low efficiencies that are generally observed in all-polymer solar cells.
Permafrost, defined as ground that is frozen for at least two consecutive years, is a distinct feature of the terrestrial unglaciated Arctic. It covers approximately one quarter of the land area of the Northern Hemisphere (23,000,000 km²). Arctic landscapes, especially those underlain by permafrost, are threatened by climate warming and may degrade in different ways, including active layer deepening, thermal erosion, and development of rapid thaw features. In Siberian and Alaskan late Pleistocene ice-rich Yedoma permafrost, rapid and deep thaw processes (called thermokarst) can mobilize deep organic carbon (below 3 m depth) by surface subsidence due to loss of ground ice. Increased permafrost thaw could cause a feedback loop of global significance if its stored frozen organic carbon is reintroduced into the active carbon cycle as greenhouse gases, which accelerate warming and inducing more permafrost thaw and carbon release. To assess this concern, the major objective of the thesis was to enhance the understanding of the origin of Yedoma as well as to assess the associated organic carbon pool size and carbon quality (concerning degradability). The key research questions were:
- How did Yedoma deposits accumulate?
- How much organic carbon is stored in the Yedoma region?
- What is the susceptibility of the Yedoma region's carbon for future decomposition?
To address these three research questions, an interdisciplinary approach, including detailed field studies and sampling in Siberia and Alaska as well as methods of sedimentology, organic biogeochemistry, remote sensing, statistical analyses, and computational modeling were applied. To provide a panarctic context, this thesis additionally includes results both from a newly compiled northern circumpolar carbon database and from a model assessment of carbon fluxes in a warming Arctic.
The Yedoma samples show a homogeneous grain-size composition. All samples were poorly sorted with a multi-modal grain-size distribution, indicating various (re-) transport processes. This contradicts the popular pure loess deposition hypothesis for the origin of Yedoma permafrost. The absence of large-scale grinding processes via glaciers and ice sheets in northeast Siberian lowlands, processes which are necessary to create loess as material source, suggests the polygenetic origin of Yedoma deposits.
Based on the largest available data set of the key parameters, including organic carbon content, bulk density, ground ice content, and deposit volume (thickness and coverage) from Siberian and Alaskan study sites, this thesis further shows that deep frozen organic carbon in the Yedoma region consists of two distinct major reservoirs, Yedoma deposits and thermokarst deposits (formed in thaw-lake basins). Yedoma deposits contain ~80 Gt and thermokarst deposits ~130 Gt organic carbon, or a total of ~210 Gt. Depending on the approach used for calculating uncertainty, the range for the total Yedoma region carbon store is ±75 % and ±20 % for conservative single and multiple bootstrapping calculations, respectively. Despite the fact that these findings reduce the Yedoma region carbon pool by nearly a factor of two compared to previous estimates, this frozen organic carbon is still capable of inducing a permafrost carbon feedback to climate warming. The complete northern circumpolar permafrost region contains between 1100 and 1500 Gt organic carbon, of which ~60 % is perennially frozen and decoupled from the short-term carbon cycle.
When thawed and reintroduced into the active carbon cycle, the organic matter qualities become relevant. Furthermore, results from investigations into Yedoma and thermokarst organic matter quality studies showed that Yedoma and thermokarst organic matter exhibit no depth-dependent quality trend. This is evidence that after freezing, the ancient organic matter is preserved in a state of constant quality. The applied alkane and fatty-acid-based biomarker proxies including the carbon-preference and the higher-land-plant-fatty-acid indices show a broad range of organic matter quality and thus no significantly different qualities of the organic matter stored in thermokarst deposits compared to Yedoma deposits. This lack of quality differences shows that the organic matter biodegradability depends on different decomposition trajectories and the previous decomposition/incorporation history. Finally, the fate of the organic matter has been assessed by implementing deep carbon pools and thermokarst processes in a permafrost carbon model. Under various warming scenarios for the northern circumpolar permafrost region, model results show a carbon release from permafrost regions of up to ~140 Gt and ~310 Gt by the years 2100 and 2300, respectively. The additional warming caused by the carbon release from newly-thawed permafrost contributes 0.03 to 0.14°C by the year 2100. The model simulations predict that a further increase by the 23rd century will add 0.4°C to global mean surface air temperatures.
In conclusion, Yedoma deposit formation during the late Pleistocene was dominated by water-related (alluvial/fluvial/lacustrine) as well as aeolian processes under periglacial conditions. The circumarctic permafrost region, including the Yedoma region, contains a substantial amount of currently frozen organic carbon. The carbon of the Yedoma region is well-preserved and therefore available for decomposition after thaw. A missing quality-depth trend shows that permafrost preserves the quality of ancient organic matter. When the organic matter is mobilized by deep degradation processes, the northern permafrost region may add up to 0.4°C to the global warming by the year 2300.
Mars is one of the best candidates among planetary bodies for supporting life. The presence of water in the form of ice and atmospheric vapour together with the availability of biogenic elements and energy are indicators of the possibility of hosting life as we know it. The occurrence of permanently frozen ground – permafrost, is a common phenomenon on Mars and it shows multiple morphological analogies with terrestrial permafrost. Despite the extreme inhospitable conditions, highly diverse microbial communities inhabit terrestrial permafrost in large numbers. Among these are methanogenic archaea, which are anaerobic chemotrophic microorganisms that meet many of the metabolic and physiological requirements for survival on the martian subsurface. Moreover, methanogens from Siberian permafrost are extremely resistant against different types of physiological stresses as well as simulated martian thermo-physical and subsurface conditions, making them promising model organisms for potential life on Mars. The main aims of this investigation are to assess the survival of methanogenic archaea under Mars conditions, focusing on methanogens from Siberian permafrost, and to characterize their biosignatures by means of Raman spectroscopy, a powerful technology for microbial identification that will be used in the ExoMars mission. For this purpose, methanogens from Siberian permafrost and non-permafrost habitats were subjected to simulated martian desiccation by exposure to an ultra-low subfreezing temperature (-80ºC) and to Mars regolith (S-MRS and P-MRS) and atmospheric analogues. They were also exposed to different concentrations of perchlorate, a strong oxidant found in martian soils. Moreover, the biosignatures of methanogens were characterized at the single-cell level using confocal Raman microspectroscopy (CRM). The results showed survival and methane production in all methanogenic strains under simulated martian desiccation. After exposure to subfreezing temperatures, Siberian permafrost strains had a faster metabolic recovery, whereas the membranes of non-permafrost methanogens remained intact to a greater extent. The strain Methanosarcina soligelidi SMA-21 from Siberian permafrost showed significantly higher methane production rates than all other strains after the exposure to martian soil and atmospheric analogues, and all strains survived the presence of perchlorate at the concentration on Mars. Furthermore, CRM analyses revealed remarkable differences in the overall chemical composition of permafrost and non-permafrost strains of methanogens, regardless of their phylogenetic relationship. The convergence of the chemical composition in non-sister permafrost strains may be the consequence of adaptations to the environment, and could explain their greater resistance compared to the non-permafrost strains. As part of this study, Raman spectroscopy was evaluated as an analytical technique for remote detection of methanogens embedded in a mineral matrix. This thesis contributes to the understanding of the survival limits of methanogenic archaea under simulated martian conditions to further assess the hypothetical existence of life similar to methanogens on the martian subsurface. In addition, the overall chemical composition of methanogens was characterized for the first time by means of confocal Raman microspectroscopy, with potential implications for astrobiological research.