Karl C. Gödel

Blue-Green Color Tunable Solution Processable Organolead Chloride–Bromide Mixed Halide Perovskites for Optoelectronic Applications

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425–570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1–x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride–bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.

Enhancing photoluminescence yields in lead halide perovskites by photon recycling and light out-coupling.

In lead halide perovskite solar cells, there is at least one recycling event of electron–hole pair to photon to electron–hole pair at open circuit under solar illumination. This can lead to a significant reduction in the external photoluminescence yield from the internal yield. Here we show that, for an internal yield of 70%, we measure external yields as low as 15% in planar films, where light out-coupling is inefficient, but observe values as high as 57% in films on textured substrates that enhance out-coupling. We analyse in detail how externally measured rate constants and photoluminescence efficiencies relate to internal recombination processes under photon recycling. For this, we study the photo-excited carrier dynamics and use a rate equation to relate radiative and non-radiative recombination events to measured photoluminescence efficiencies. We conclude that the use of textured active layers has the ability to improve power conversion efficiencies for both LEDs and solar cells.

High Open-Circuit Voltages in Tin-Rich Low-Bandgap Perovskite-Based Planar Heterojunction Photovoltaics

Low-bandgap CH3 NH3 (Pbx Sn1-x )I3 (0 ≤ x ≤ 1) hybrid perovskites (e.g., ≈1.5-1.1 eV) demonstrating high surface coverage and superior optoelectronic properties are fabricated. State-of-the-art photovoltaic (PV) performance is reported with power conversion efficiencies approaching 10% in planar heterojunction architecture with small (<450 meV) energy loss compared to the bandgap and high (>100 cm2 V-1 s-1 ) intrinsic carrier mobilities.

Simulation framework for coherent and incoherent X-ray imaging and its application in Talbot-Lau dark-field imaging.

A simulation framework for coherent X-ray imaging, based on scalar diffraction theory, is presented. It contains a core C++ library and an additional Python interface. A workflow is presented to include contributions of inelastic scattering obtained with Monte-Carlo methods. X-ray Talbot-Lau interferometry is the primary focus of the framework. Simulations are in agreement with measurements obtained with such an interferometer. Especially, the dark-field signal of densely packed PMMA microspheres is predicted. A realistic modeling of the microsphere distribution, which is necessary for correct results, is presented. The framework can be used for both setup design and optimization but also to test and improve reconstruction methods.

Size and Energy Level Tuning of Quantum Dot Solids via a Hybrid Ligand Complex

The performance of quantum dots (QDs) in optoelectronic devices suffers as a result of sub-bandgap states induced by the large fraction of atoms on the surface of QDs. Recent progress in passivating these surface states with thiol ligands and halide ions has led to competitive efficiencies. Here, we apply a hybrid ligand mixture to passivate PbSe QD sub-bandgap tail states via a low-temperature, solid-state ligand exchange. We show that this ligand mixture allows tuning of the energy levels and the physical QD size in the solid state during film formation. We hereby present a novel, postsynthetic path to tune the properties of QD films.

Luminescent and geometric concentrators for building integrated photovoltaics

In developed countries 60% of the electricity consumed is attributable to commercial and public buildings. Even in the UK, the solar energy incident on buildings is more than 7× the electrical energy they consume. This represents a problem (the management of solar heat gain and glare) but also an opportunity that may be taken advantage of using complementary concentrator technologies. We are investigating conventional geometric and luminescent concentrators that may be combined to optimally harvest the direct and diffuse components of sunlight within a double glazed window unit. Initial results suggest that the combined system can achieve power conversion efficiencies approaching 20% under standard AM1.5g illumination at normal incidence.

Energy-resolved interferometric x-ray imaging

Interferometric X-ray imaging becomes more and more attractive for applications such as medical imaging or non-destructive testing, where a compact setup is needed. Therefore a so-called Talbot-Lau interferometer in combination with a conventional X-ray tube is used. Thereby, three different kinds of images can be obtained. An attenuation image like in conventional X-ray imaging, an image of the differential phase-shifts caused by the object and the so-called dark-field image. The dark-field image shows information about the objects granularity even in sub-pixel dimensions what especially seems very promising for applications like mammography. With respect to optimizing the output of interferometric X-ray imaging in any application, it is inevitable to know the energy response of the interferometer as well as the energy dependence of the interactions of X- rays with matter. In this contribution, simulations and measurements using a Medipix 2 and a Timepix detector are presented.

Investigations on the origin of the darkfield signal in X-Ray Talbot interferometry

Recently, darkfield imaging has been established as third imaging technique in X-Ray Talbot interferometry. The darkfield image shows the reduction of visibilities of the interferometer relative to the visibilities gained in reference measurements. The information perceivable from darkfield images can be different from what is observable in absorption and phase images. While the image formation for the absorption and phase image is widely understood, this is not the case for the darkfield signal. There are several attempts for an explanation. To pursue these explanations and, moreover, to get a comprehensive understanding of the darkfield image formation, an investigation based on simulations is carried out. These simulations include wave field propagation as well as Monte Carlo codes. The results suggest that small angle scattering and the micro structure of an object can not be the exclusive origins of the dark field signal. At least diffraction effects at steep edges and possibly visibility changes due to beam hardening effects also have to be accounted for.

Grating-based dark-field breast imaging

Grating-based X-ray phase-contrast imaging (XPCI) is a promising modality to increase soft-tissue contrast in medical imaging and especially in the case of mammography. Several groups worldwide have performed investigations on grating-based Talbot-Lau X-ray imaging of breast tissue, but in most cases focussed on the soft tissue contrast enhancement of the differential phase image. In this contribution, we present promising measurements with a Talbot-Lau interferometer of several mastectomy breast tissue samples especially focussed on the sensitivity of the dark-field signal of microcalcifications and with a comparable dose value to conventional mammography. We can present a contrast improvement for calcifications in surrounding breast tissue for the dark-field image by a factor of 10 related to the attenuation image.

Spectrum optimization of a Talbot-Lau interferometer towards clinical application

X-ray phase-contrast imaging (XPCI) using a Talbot-Lau interferometer is a promising modality to increase soft-tissue contrast in medical imaging and has drawn the attention of many groups worldwide. Nevertheless, due to the many free parameters to be optimized in a XPCI setup, theres a long way to go to find the optimal geometry, design energy and spectrum for a future clinical application. In this contribution, we present a fast procedure to optimize the visibility response of a Talbot-Lau Interferometer with respect to an introduced filter material. This is done by performing a virtual filtering of the known reference tube spectrum, followed by calculating the visibility using the simulated detector and interferometer responses. Additionally, our procedure can also be used to optimize the general setup lengths, the grating properties or the design energy. We present recent results of the optimization process of our lab setup, where the simulations predict a visibility increase of approximately 72% compared to the non-optimized state. In the future, we are going to extend the functionality of our optimization algorithm to perform simulations that allow the prediction of best suitable spectrum for a given application at a certain noise level tolerated and at the lowest dose possible.