Rüdiger Berger

Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell

Perovskite-sensitized solar cells have reached power conversion efficiencies comparable to commercially available solar cells used for example in solar farms. In contrast to silicon solar cells, perovskite-sensitized solar cells can be made by solution processes from inexpensive materials. The power conversion efficiency of these cells depends substantially on the charge transfer at interfaces. Here we use Kelvin probe force microscopy to study the real-space cross-sectional distribution of the internal potential within high efficiency mesoscopic methylammonium lead tri-iodide solar cells. We show that the electric field is homogeneous through these devices, similar to that of a p-i-n type junction. On illumination under short-circuit conditions, holes accumulate in front of the hole-transport layer as a consequence of unbalanced charge transport in the device. After light illumination, we find that trapped charges remain inside the active device layers. Removing these traps and the unbalanced charge injection could enable further improvements in performance of perovskite-sensitized solar cells.

Patchy Nanocapsules of Poly(vinylferrocene)-Based Block Copolymers for Redox-Responsive Release

Nanocapsules composed of a poly(vinylferrocene)-block-poly(methyl methacrylate) shell and a hydrophobic liquid core are prepared in water. The nanocapsule shells display a patchy structure with poly(vinylferrocene) patches with sizes of 25 ± 3 nm surrounded by poly(methyl methacrylate). The functional nanopatches can be selectively oxidized, thereby influencing the colloidal morphology and introducing polar domains in the nanocapsule shell. The hydrophobic to hydrophilic transition in the redox-responsive nanopatches can be advantageously used to release a hydrophobic payload encapsulated in the core by an oxidation reaction.

Photoswitching of glass transition temperatures of azobenzene-containing polymers induces reversible solid-to-liquid transitions

The development of polymers with switchable glass transition temperatures (Tg) can address scientific challenges such as the healing of cracks in high-Tg polymers and the processing of hard polymers at room temperature without using plasticizing solvents. Here, we demonstrate that light can switch the Tg of azobenzene-containing polymers (azopolymers) and induce reversible solid-to-liquid transitions of the polymers. The azobenzene groups in the polymers exhibit reversible cis-trans photoisomerization abilities. Trans azopolymers are solids with Tg above room temperature, whereas cis azopolymers are liquids with Tg below room temperature. Because of the photoinduced solid-to-liquid transitions of these polymers, light can reduce the surface roughness of azopolymer films by almost 600%, repeatedly heal cracks in azopolymers, and control the adhesion of azopolymers for transfer printing. The photoswitching of Tg provides a new strategy for designing healable polymers with high Tg and allows for control over the mechanical properties of polymers with high spatiotemporal resolution.

Antibacterial Strategies from the Sea: Polymer-Bound Cl-Catechols for Prevention of Biofilm Formation

Inspired by the amino acid 2-chloro-4,5-dihydroxyphenylalanine (Cl-DOPA), present in the composition of the proteinaceous glue of the sandcastle worm Phragmatopoma californica, a simple strategy is presented to confer antifouling properties to polymer surfaces using (but not releasing) a bioinspired biocide. Cl-Dopamine is used to functionalize polymer materials and hydrogel films easily, to prevent biofilm formation on them.

Electrical Modes in Scanning Probe Microscopy

Scanning probe microscopy methods allow the investigation of a variety of sample surface properties on a nanometer scale, even down to single molecules. As molecular electronics advance, the characterization of electrical properties becomes more and more important. In both research and industry, films made from composite materials and lithographically structured elements have already reached structure sizes down to a few nanometers. Here, we review the major scanning probe microscopy modes that are used for electrical characterization of thin films, that is, scanning conductive force microscopy, Kelvin probe force microscopy and scanning electric field microscopy. To demonstrate the possibilities and capabilities of these modes, reference samples were fabricated by means of focused ion beam deposition and analyzed using the described methods. Furthermore, two upcoming modes are presented that are based on: i) local current measurements while the SPM-cantilever is excited into torsional vibrations, and, ii) changes in a backscattered microwave that was coupled into a scanning probe microscopy-cantilever. The scanning-probe-based electrical modes are applicable for studies of functional layers used in soft matter electronic devices under realistic environmental conditions.

Synthesis, Characterization, and Hierarchical Organization of Tungsten Oxide Nanorods: Spreading Driven by Marangoni Flow

Tungsten oxide nanorods were synthesized by a soft chemistry approach using tungsten alkoxide and trioctyl amine and oleic acid as the surfactants. The optical properties of the nanorods were studied. The nanorods were found to be soluble in a wide range of solvents like chloroform, cyclohexane, and so on. Upon solvent evaporation, the nanorods formed hierarchically organized solid state structures. Depending on the solvent used, the nanorods organized in different mesostructures. Moreover, the organization of the nanorods from mixtures of polar and nonpolar solvents was studied. Here, the Marangoni effect resulting from differences in the surface tensions of the two solvents was found to play a role in the organization of the nanorods. Furthermore, dip coating of the nanorod solutions on a mica substrate resulted in the formation of a uniform thin film of the nanorods, which may be useful for a variety of applications such as in electrochromic devices and in organic light emitting devices (OLEDs) using tungsten oxide as a buffer layer.

Dendritic Star Polymers for Efficient DNA Binding and Stimulus-Dependent DNA Release

Water-soluble core-shell star polymers consisting of a dendritic polyphenylene core and an outer shell containing a defined number of amino groups have been synthesized via atom transfer radical polymerization (ATRP). All macromolecules efficiently interacted with a diverse set of DNA fragments, and stable complexes were formed and visualized by atomic force microscopy. The observed tight binding of DNA, which was found in the sub-nanomolar range, was mainly attributed to strong electrostatic interactions. Complex stoichiometries between the polyelectrolytes were controlled via the number of amino groups of the star polymers, and well-defined nanoscopic architectures were formed. DNA was released from the complexes after treatment with high concentrations of sodium chloride in aqueous solution. Such star polymers, which allow the binding and release of DNA, represent attractive candidates for the development of novel anion-exchange resins for DNA purification or as nonviral vector systems for gene delivery.

Local Time-Dependent Charging in a Perovskite Solar Cell

Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction, and interfacial trap states can prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on time scales of seconds cause hysteresis in the current-voltage characteristics. In this work, we localized and investigated these slow processes using frequency-modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers formed at the interfaces of the selective contacts with the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at the interfaces of MAPI with adjacent layers. Our results indicate that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states causes the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.

Read-out of micromechanical cantilever sensors by phase shifting interferometry

White light interferometry was applied to determine the bending of micromechanical cantilever sensors (MCS) with an error typically less than 1permille. Deflections smaller than 2nm could be resolved at a lateral resolution of 2μm. Absolute values for curvatures can be determined and suitable reference points can be chosen on the MCS support. This was demonstrated in experiments using plasma polymerized polyallylamine films, which cross link upon ultraviolet light irradiation. The results suggest that 100μm long segments are sufficient to estimate reliable curvature radii of 450μm long microcantilever sensors.

Effect of Nanoroughness on Highly Hydrophobic and Superhydrophobic Coatings

The effect of nanoroughness on contact angles and pinning is investigated experimentally and numerically for low-energy surfaces. Nanoroughness is introduced by chemical vapor deposition of tetraethoxysilane and was quantified by scanning force microscopy. Addition of a root-mean-square roughness of 2 nm on a flat surface can increase the contact angle after fluorination by a semifluorinated silane by up to 30°. On the other hand, nanoroughness can improve or impair the liquid repellency of superhydrophobic surfaces that were made from assembled raspberry particles. Molecular dynamics simulations are performed in order to gain a microscopic understanding on how the length and the surface coating density of semifluorinated silanes influence the hydrophobicity. Solid-liquid surface free energy computations reveal that the wetting behavior strongly depends on the density and alignment of the semifluorinated silane. At coating densities in the range of experimental values, some water molecules can penetrate between the semifluorinated chains, thus increasing the surface energy. Combining the experimental and numerical data exhibits that a roughness-induced increase of the contact angle competes with increased pinning caused by penetration of liquid into nanopores or between neighboring semifluorinated molecules.