Martin Pfannmöller

Visualizing a Homogeneous Blend in Bulk Heterojunction Polymer Solar Cells by Analytical Electron Microscopy

To increase efficiency of bulk heterojunctions for photovoltaic devices, the functional morphology of active layers has to be understood, requiring visualization and discrimination of materials with very similar characteristics. Here we combine high-resolution spectroscopic imaging using an analytical transmission electron microscope with nonlinear multivariate statistical analysis for classification of multispectral image data. We obtain a visual representation showing homogeneous phases of donor and acceptor, connected by a third composite phase, depending in its extent on the way the heterojunction is fabricated. For the first time we can correlate variations in nanoscale morphology determined by material contrast with measured solar cell efficiency. In particular we visualize a homogeneously blended phase, previously discussed to diminish charge separation in solar cell devices.

Unraveling the nanoscale morphologies of mesoporous perovskite solar cells and their correlation to device performance.

Hybrid solar cells based on organometal halide perovskite absorbers have recently emerged as promising class for cost- and energy-efficient photovoltaics. So far, unraveling the morphology of the different materials within the nanostructured absorber layer has not been accomplished. Here, we present the first visualization of the mesoporous absorber layer in a perovskite solar cell from analytical transmission electron microscopy studies. Material contrast is achieved by electron spectroscopic imaging. We found that infiltration of the hole transport material into the scaffold is low and inhomogeneous. Furthermore, our data suggest that the device performance is strongly affected by the morphology of the TiO2 scaffold with a fine grained structure being disadvantageous.

Visualizing physical, electronic, and optical properties of organic photovoltaic cells

The most interesting active layer system for organic photovoltaic cells is the bulk heterojunction (BHJ). The general structure of a BHJ is a network of domains, which contain blended donor and acceptor molecules, often in pure and mixed phases. Directly visualizing this nanoscale phase separation of BHJs and corresponding morphological parameters has attracted much interest and has led to novel insights into their physical properties. Imaging methods that have been proven to be of great importance are scanning probe, scanning X-ray, and electron microscopy. Early works successfully employed conventional contrast mechanisms at high resolution for correlation of structure and function. Many imaging techniques also offer analytical capabilities using specific interactions of the probes with different material domains. Resulting material contrasts can be converted into morphological maps by correlation with the intrinsically different physical, electronic, and optical sample properties. Spatially resolved visualization of these properties is crucial to provide answers to questions about fundamental processes that determine the solar cell performance. A major paradigm shift was initiated by the discovery that instead of a completely separated network of donor and acceptor domains, a multiphase system is beneficial for enhanced photo-physical properties. Thereby phases differ in ordering, orientation and composition. In this review, we provide an overview of techniques and recent approaches for imaging BHJ structures at the nanoscale. Advantages and disadvantages of the underlying contrast mechanisms are summarized. In addition, we introduce best practices for the application of transmission electron microscopy (TEM) by comparing enhanced conventional and analytical modes.

3D Geometry-Based Quantification of Colocalizations in Multichannel 3D Microscopy Images of Human Soft Tissue Tumors

We introduce a new model-based approach for automatic quantification of colocalizations in multichannel 3D microscopy images. The approach uses different 3D parametric intensity models in conjunction with a model fitting scheme to localize and quantify subcellular structures with high accuracy. The central idea is to determine colocalizations between different channels based on the estimated geometry of the subcellular structures as well as to differentiate between different types of colocalizations. A statistical analysis was performed to assess the significance of the determined colocalizations. This approach was used to successfully analyze about 500 three-channel 3D microscopy images of human soft tissue tumors and controls.

The influence of branched alkyl side chains in A–D–A oligothiophenes on the photovoltaic performance and morphology of solution-processed bulk-heterojunction solar cells

Besides providing sufficient solubility, branched alkyl chains also affect the film-forming and packing properties of organic semiconductors. In order to avoid steric hindrance as it is present in wide-spread alkyl chains comprising a branching point position at the C2-position, i.e., 2-ethylhexyl, the branching point can be moved away from the π-conjugated backbone. In this report, we study the influence of the modification of the branching point position from the C2-position in 2-hexyldecylamine (1) to the C4-position in 4-hexyldecylamine (2) connected to the central dithieno[3,2-b:2′,3′-d]pyrrole (DTP) moiety in a well-studied A–D–A oligothiophene on the optoelectronic properties and photovoltaic performance in solution-processed bulk heterojunction solar cells (BHJSCs) with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor material. Post-treatment of the photoactive layers is performed via solvent vapor annealing (SVA) in order to improve the film microstructure of the bulk heterojunction. The time evolution of nanoscale morphological changes is followed by combining scanning transmission electron microscopy with low-energy-loss spectroscopic imaging (STEM-SI), solid-state absorption spectroscopy, and two-dimensional grazing incidence X-ray diffraction (2D-GIXRD). Our results show an improvement of the photovoltaic performance that is dependent on the branching point position in the donor oligomer. Optical spacers are utilized to increase light absorption inside the co-oligomer 2-based BHJSCs leading to increased power conversion efficiencies (PCEs) of 8.2% when compared to the corresponding co-oligomer 1-based devices. A STEM-SI analysis of the respective device cross-sections of active layers containing 1 and 2 as donor materials indeed reveals significant differences in their respective active layer morphologies.

3D geometry-based quantification of colocalizations in three-channel 3D microscopy images of soft tissue tumors

We introduce a new model-based approach for automatic quantification of colocalizations in multi-channel 3D microscopy images. The approach is based on different 3D parametric intensity models in conjunction with a model fitting scheme to localize and quantify subcellular structures with high accuracy. The central idea is to determine colocalizations between different channels based on the estimated geometry of subcellular structures as well as to differentiate between different types of colocalizations. Furthermore, we perform a statistical analysis to assess the significance of the determined colocalizations. We have successfully applied our approach to about 400 three-channel 3D microscopy images of human soft-tissue tumors.

Detecting correlative light and electron microscopic labels by electron spectroscopic imaging

To bridge the gap in resolution and to localize molecules of interest in high-resolution light and electron microscopy a number of correlative approaches have been proposed. One possibility is the use of correlative markers, e.g. fluorescent Quantum Dots (Qdots), which in TEM are imaged and distinguished by their contrast and shape. In 2007 Mhawi et al. demonstrated a more direct possibility, the visualization of electron low energy loss, which represents the TEM equivalent of fluorescence excitation. Such spectral information in the low energy-loss range was used to identify doxorubicin introduced into cells to label nuclei.