arXiv subjects

Jonas Graetz

Publications and source records attributed to Jonas Graetz.

7 recordsLinked to original sources

arXiv2025

Understanding the effect of drying time in process-structure-performance relationships for PM6-Y6 organic solar cells

Making solution-cast organic solar cells industrially available generally comes at the cost of significant performance losses compared to device prototypes manufactured under laboratory conditions. Adjusting solvent evaporation kinetics is postulated to recover efficiency. Yet, a comprehensive characterization of their effect, independently of other property-defining parameters, is lacking. Thus, the present objective is to isolate the influence of the solvent drying rate on solution-deposited organic active layer nanomorphologies and performances. To this end, a specially designed gas quenching technique is employed to fabricate PM6:Y6 donor-acceptor films under systematic variations of evaporation conditions. Using an extensive investigation protocol that combines insights from numerical simulations and experimental measurements, process-structure-performance relationships are unraveled. It is found that higher drying rates imply finer and more dispersed nanomorphologies with increased fractions of amorphous material. This enhances electric charge generation, thereby improving short-circuit current density and overall cell performance. The open-circuit voltage is also boosted under accelerated evaporation due to changes in the aggregation mode of the Y6 small molecule that induce higher effective bandgaps. The results demonstrate that the developed gas-quenching technique is a valuable tool for optimizing the performance of upscaled organic photovoltaics, as it is readily compatible with high-throughput equipment, such as roll-to-roll coating machines.

cond-mat.mtrl-sci↗

arXiv2021

Considerations towards quantitative X-ray and neutron tensor tomography: on the validity of linear approximations of dark-field anisotropy

The validity of two approximative linear tensor models to be used for grating based X-ray or neutron dark-field tensor tomography is investigated in a simulation study. While the dark-field contrast originating from anisotropic microscopic mass distributions has, in a previous study, been confirmed to be in general a non-linear function of two orientations (optical axis and axis of interferometer sensitivity), linear approximations with a reduced parameter space (considering only one of the orientation dependencies) are highly preferable with respect to tomographic volume reconstruction from projections. By regarding isolated volume elements and systematically exploring the full range of possible anisotropies, direct correspondences are drawn between the respective tensors characterizing the complete model used for signal synthesization and the reduced linear models used for reconstruction. The tensors' dominant orientations are found to agree to a typical accuracy of 1{\deg}, with their eigenspectra exhibiting fuzzy, yet almost linear relations among each other. Although modeling only either of two orientation dependencies for the purpose of tensor reconstruction, the data acquisition scheme must nevertheless adequately address both dependencies.

physics.med-ph↗

arXiv2021

A Novel Nano Tomography Setup for Material Science and Engineering Applications

In a comprehensive study on several samples we demonstrate for our laboratory-based computed tomography system resolutions down to 150nm. The achieved resolution is validated by imaging com-mon test structures in 2D and Fourier Shell Correlation of 3D volumes. As representative application examples from nowadays material research, we show metallization processes in multilayer integrated circuits, ageing in lithium battery electrodes, and volumetric of metallic sub-micrometer fillers of com-posites. Thus, our laboratory system provides the unique possibility to image non-destructively struc-tures in the range of hundred nanometers, even for high density materials.

physics.ins-det↗

arXiv2020

Lenseless X-ray Nano-Tomography down to 150nm Resolution: On the Quantification of Modulation Transfer and Focal Spot of the Lab-based ntCT System

The ntCT nano tomography system is a geometrically magnifying X-ray microscopy system integrating the recent Excillum NanoTube nano-focus X-ray source and a CdTe photon counting detector from Dectris. The system's modulation transfer function (MTF) and corresponding point spread function (PSF) are characterized by analyzing the contrast visibility of periodic structures of a star pattern featuring line width from 150nm to 1.5$\mu$m. The results, which can be attributed to the characteristics of the source spot, are crosschecked by scanning the source's electron focus over an edge of the structured transmission target in order to obtain an independent measurement of its point spread function. For frequencies above 1000 linepairs/mm, the MTF is found to correspond to a Gaussian PSF of 250nm full width at half maximum (FWHM). The lower frequency range down to 340 linepairs/mm shows an additional Gaussian contribution of 1$\mu$m FWHM. The resulting resolution ranges at 3200 linepairs/mm, which is consistent with the visual detectability of the smallest 150nm structures within the imaged star pattern.

physics.ins-det↗

arXiv2020

Review and experimental verification of X-ray darkfield signal interpretations with respect to quantitative isotropic and anisotropic darkfield computed tomography

Talbot(-Lau) interferometric X-ray darkfield imaging has, over the past decade, gained substantial interest for its ability to provide insights into a sample's microstructure below the imaging resolution by means of ultra small angle scattering effects. Quantitative interpretations of such images depend on models of the signal origination process that relate the observable image contrast to underlying physical processes. A review of such models is given here and their relation to the wave optical derivations by Yashiro et al. and Lynch et al. as well as to small angle X-ray scattering is discussed. Fresnel scaling is introduced to explain the characteristic distance dependence observed in cone beam geometries. Moreover, a model describing the anisotropic signals of fibrous objects is derived. The Yashiro-Lynch model is experimentally verified both in radiographic and tomographic imaging in a monochromatic synchrotron setting, considering both the effects of material and positional dependence of the resulting darkfield contrast. The effect of varying sample-detector distance on the darkfield signal is shown to be non-negligible for tomographic imaging, yet can be largely compensated for by symmetric acquisition trajectories. The derived orientation dependence of the darkfield contrast of fibrous materials both with respect to variations in autocorrelation width and scattering cross section is experimentally validated using carbon fiber reinforced rods.

physics.app-ph↗

arXiv2018

Auto-Calibration of Cone Beam Geometries from Arbitrary Rotating Markers using a Vector Geometry Formulation of Projection Matrices

A method for the determination of the projection geometry of highly magnifying cone beam micro computed tomography systems based on few rotating fiducial markers of unknown position within the field of view is derived. By employing the projection matrix formalism commonly used in computer graphics, a very clear presentation of the resulting self consistent calibration problem can be given relating the sought-for matrix to observable parameters of the markers' projections. Both an easy to implement solution procedure for both the unknown projection matrix and the marker assembly as well as the mapping from projection matrices to real space positions and orientations of source and detector relative to the rotational axis are provided. The separate treatment of the calibration problem in terms of projection matrices on the one hand and the independent transformation to a more intuitive geometry representation on the other hand proves to be very helpful with respect to the discussion of the ambiguities occurring in reference-free calibration. In particular, a link between methods based on knowledge on the sample and those based on knowledge solely on the detector geometry can be drawn. This further provides another intuitive view on the often reported difficulty in the estimation of the detector tilt towards the rotational axis. A simulation study considering $10^{6}$ randomly generated cone beam imaging configurations and fiducial marker distributions within a range of typical scenarios is performed in order to assess the stability of the proposed technique.

eess.IV↗

arXiv2016

High performance volume ray casting: A branchless generalized Joseph projector

A concise and highly performant branchless formulation of a Joseph-type interpolating ray-casting algorithm for the computation of X-ray projections is presented. It efficiently utilizes the hardware resources of modern graphics processing units at the scale of their theoretic maximum performance reaching access rates of 600 GB/s within read-and-write memory, and is further shown to do so without compromising on image quality. The computation of X-ray projections from discrete voxel grids is an ubiquitous task in many problems related to volume image processing, including tomographic reconstruction and visualization. Although its central role has given rise to numerous publications discussing the optimal modeling of ray-volume intersections, a unique benchmark in this respect does not exist. Here, a 3D Shepp-Logan phantom is used, which allows the computation of analytic reference projections that can further serve as input to iterative reconstructions without committing the inverse crime. The proposed algorithm (GJP) is compared to the competing and widely adopted digital differential analyzer (DDA), which computes exact line-box intersections. It is thereby found to outperform the DDA on recent graphics processors in all respects: Despite accessing twice as much memory, the GJP is still able to calculate projections twice as fast. It further exhibits considerably less discretization artifacts, and neither oversampling of the DDA nor a smooth interpolation kernel within the GJP are able to improve on these results in any respect.

physics.med-ph↗