Erich Müller

Moving through the Phase Diagram: Morphology Formation in Solution Cast Polymer–Fullerene Blend Films for Organic Solar Cells

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the unfriendly solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.

Optimized Ar(+)-ion milling procedure for TEM cross-section sample preparation.

High-quality samples are indispensable for every reliable transmission electron microscopy (TEM) investigation. In order to predict optimized parameters for the final Ar(+)-ion milling preparation step, topographical changes of symmetrical cross-section samples by the sputtering process were modeled by two-dimensional Monte-Carlo simulations. Due to its well-known sputtering yield of Ar(+)-ions and its easiness in mechanical preparation Si was used as model system. The simulations are based on a modified parameterized description of the sputtering yield of Ar(+)-ions on Si summarized from literature. The formation of a wedge-shaped profile, as commonly observed during double-sector ion milling of cross-section samples, was reproduced by the simulations, independent of the sputtering angle. Moreover, the preparation of wide, plane parallel sample areas by alternating single-sector ion milling is predicted by the simulations. These findings were validated by a systematic ion-milling study (single-sector vs. double-sector milling at various sputtering angles) using Si cross-section samples as well as two other material-science examples. The presented systematic single-sector ion-milling procedure is applicable for most Ar(+)-ion mills, which allow simultaneous milling from both sides of a TEM sample (top and bottom) in an azimuthally restricted sector perpendicular to the central epoxy line of that cross-sectional TEM sample. The procedure is based on the alternating milling of the two halves of the TEM sample instead of double-sector milling of the whole sample. Furthermore, various other practical aspects are issued like the dependency of the topographical quality of the final sample on parameters like epoxy thickness and incident angle.

Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering

Electron holography in a transmission electron microscope was applied to measure the phase shift {delta}{phi} induced by Au clusters as a function of the cluster size. Large {delta}{phi} observed for small Au clusters cannot be described by the well-known equation {delta}{phi}=C{sub E}V{sub 0}t (C{sub E}, interaction constant; V{sub 0}, mean inner Coulomb potential (MIP) of bulk gold; and t, cluster thickness). The rapid increase of the Au MIP with decreasing cluster size derived from {delta}{phi} can be explained by the compressive strain of surface atoms in the cluster.

Low‐energy electron scattering in carbon‐based materials analyzed by scanning transmission electron microscopy and its application to sample thickness determination

High‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM) at low energies (≤30 keV) was used to study quantitatively electron scattering in amorphous carbon and carbon‐based materials. Experimental HAADF STEM intensities from samples with well‐known composition and thickness are compared with results of Monte Carlo simulations and semiempirical equations describing multiple electron scattering. A well‐defined relationship is found between the maximum HAADF STEM intensity and sample thickness which is exploited (a) to derive a quantitative description for the mean quadratic scattering angle and (b) to calculate the transmitted HAADF STEM intensity as a function of the relevant materials parameters and electron energy. The formalism can be also applied to determine TEM sample thicknesses by minimizing the contrast of the sample as a function of the electron energy.

Nanomorphology of P3HT:PCBM-Based Absorber Layers of Organic Solar Cells after Different Processing Conditions Analyzed by Low-Energy Scanning Transmission Electron Microscopy

In this study the nanomorphology of P3HT:PC61BM absorber layers of organic solar cells was studied as a function of the processing parameters and for P3HT with different molecular weight. For this purpose we apply scanning transmission electron microscopy (STEM) at low electron energies in a scanning electron microscope. This method exhibits sensitive material contrast in the high-angle annular dark-field (HAADF) mode, which is well suited to distinguish materials with similar densities and mean atomic numbers. The images taken with low-energy HAADF STEM are compared with conventional transmission electron microscopy and atomic force microscopy images to illustrate the capabilities of the different techniques. For the interpretation of the low-energy HAADF STEM images, a semiempirical equation is used to calculate the image intensities. The experiments show that the nanomorphology of the P3HT:PC61BM blends depends strongly on the molecular weight of the P3HT. Low-molecular-weight P3HT forms rod-like domains during annealing. In contrast, only small globular features are visible in samples containing high-molecular-weight P3HT, which do not change significantly after annealing at 150°C up to 30 min.

Advanced bimetallic In–Cu/Ag/Au nanostructures via microemulsion-based reaction

Bimetallic nanomaterials and nanostructures constituted of the coinage metals (Cu, Ag, Au) and indium with elaborate compositions and structures are realized via a microemulsion-based approach. In detail, this comprises Cu11In9@CuIn@In core@shell-A@shell-B nanoparticles, In-Ag Janushead-like nanoparticles, Ag0 hollow spheres, Ag3In@In core@shell nanoparticles, Au@AuIn2@In core@shell-A@shell-B nanoparticles and AuIn2 nanoparticles. To obtain these advanced architectures, two approaches are applied: (1) In0 nanoparticles—pre-synthesized in a microemulsion—were reacted in a follow-up reaction with CuCl2·2H2O, AgNO3 or KAuCl4; (2) simultaneous co-reduction of InCl3·4H2O together with CuCl2·2H2O, AgNO3 or KAuCl4 in a microemulsion. Characterization of the resulting advanced structures and compositions requires elaborate electron microscopy techniques, combined with energy dispersive X-ray spectroscopy for chemical analyses of single nanoparticles as well as X-ray powder diffraction and optical spectroscopy. The versatility of the experimental approach toward complex nanoparticle architectures is related to a precise control and fine-tuning of the experimental conditions. The resulting tool kit of In–Cu/In–Ag/In–Au-based bimetallic and intermetallic nanomaterials and, in general, of nanostructured metal architectures with such variability and complexity have not yet been described.

Backscattered electron SEM imaging of cells and determination of the information depth

Backscattered electron imaging of HT29 colon carcinoma cells in a scanning electron microscope was studied. Thin cell sections were placed on indium‐tin‐oxide‐coated glass slides, which is a promising substrate material for correlative light and electron microscopy. The ultrastructure of HT29 colon carcinoma cells was imaged without poststaining by exploiting the high chemical sensitivity of backscattered electrons. Optimum primary electron energies for backscattered electron imaging were determined which depend on the section thickness. Charging effects in the vicinity of the SiO2 nanoparticles contained in cell sections could be clarified by placing cell sections on different substrates. Moreover, a method is presented for information depth determination of backscattered electrons which is based on the imaging of subsurface nanoparticles embedded by the cells.

Composition quantification of electron-transparent samples by backscattered electron imaging in scanning electron microscopy

The contrast of backscattered electron (BSE) images in scanning electron microscopy (SEM) depends on material parameters which can be exploited for composition quantification if some information on the material system is available. As an example, the In-concentration in thin InxGa1-xAs layers embedded in a GaAs matrix is analyzed in this work. The spatial resolution of the technique is improved by using thin electron-transparent specimens instead of bulk samples. Although the BSEs are detected in a comparably small angular range by an annular semiconductor detector, the image intensity can be evaluated to determine the composition and local thickness of the specimen. The measured intensities are calibrated within one single image to eliminate the influence of the detection and amplification system. Quantification is performed by comparison of experimental and calculated data. Instead of using time-consuming Monte-Carlo simulations, an analytical model is applied for BSE-intensity calculations which considers single electron scattering and electron diffusion.

Contrast of Backscattered Electron SEM Images of Nanoparticles on Substrates with Complex Structure

This study is concerned with backscattered electron scanning electron microscopy (BSE SEM) contrast of complex nanoscaled samples which consist of SiO2 nanoparticles (NPs) deposited on indium-tin-oxide covered bulk SiO2 and glassy carbon substrates. BSE SEM contrast of NPs is studied as function of the primary electron energy and working distance. Contrast inversions are observed which prevent intuitive interpretation of NP contrast in terms of material contrast. Experimental data is quantitatively compared with Monte-Carlo- (MC-) simulations. Quantitative agreement between experimental data and MC-simulations is obtained if the transmission characteristics of the annular semiconductor detector are taken into account. MC-simulations facilitate the understanding of NP contrast inversions and are helpful to derive conditions for optimum material and topography contrast.

Investigation of the electrical activity of dislocations in ZnO epilayers by transmission electron holography

The electrical activity of threading dislocations in epitaxial n-ZnO layers was investigated by electron holography in a transmission electron microscope. By reconstructing the phase of the image wave in the vicinity of dislocations, the electrostatic potential associated with charged dislocations can be detected. Comparing the measured and theoretically expected potential, a line charge of 2 e/nm was found.