C. Gadermaier

Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation

While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct estimation of monolayer contents.

Electron-Phonon Coupling in High-Temperature Cuprate Superconductors Determined from Electron Relaxation Rates

We determined electronic relaxation times via pump-probe optical spectroscopy using sub-15 fs pulses for the normal state of two different cuprate superconductors. We show that the primary relaxation process is the electron-phonon interaction and extract a measure of its strength, the second moment of the Eliashberg function λ[ω2] = 800 ± 200 meV2 for La(1.85)Sr(0.15)CuO4 and λ[ω2] = 400 ± 100 meV2 for YBa(2)Cu(3)O(6.5). These values suggest a possible fundamental role of the electron-phonon interaction in the superconducting pairing mechanism.

Photophysics of poly(fluorenes) with dendronic side chains

Abstract Display technology requires full red, green, and blue electroluminescence colors in polymer light emitting diodes. For the realization of blue emission poly(fluorene)s (PFs) are possibly the most attractive class of materials. The color stability of poly(fluorene)s is, however, relatively poor. Introducing bulky side chains, such as oligo-phenyl dendrons is a promising method for improving the color stability of these materials. The present study deals with the photophysics of a dendronically substituted poly(fluorene) derivative. The excited state properties are investigated by means of photoinduced absorption (PA) techniques at either quasi steady-state conditions or with femtosecond time resolution.

Solution Processed Conjugated Polymer Multilayer Structures for Light Emitting Devices

We study the feasibility of semiconducting polymer nanospheres deposited from miniemulsions as an approach to form organic multilayer structures and devices from an all solution based process. A detailed study of the wetting and film forming properties of the dispersed semiconducting polymer nanospheres on different polar and non-polar organic surfaces is given. The transmission and fluorescence properties of the polymer multilayer structures are studied. Organic light emitting devices based on such multilayer structures are presented and their properties are discussed.

Functionalization of transparent conductive oxide electrode for TiO2-free perovskite solar cells

Many of the best performing solar cells based on perovskite-halide light absorbers use TiO2 as an electron selective contact layer. However, TiO2 usually requires high temperature sintering, is related to electrical instabilities in perovskite solar cells, and causes cell performance degradation under full solar spectrum illumination. Here we demonstrate an alternative approach based on the modification of transparent conductive oxide electrodes with self-assembled siloxane-functionalized fullerene molecules, eliminating TiO2 or any other additional electron transporting layer. We demonstrate that these molecules spontaneously form a homogenous monolayer acting as an electron selective layer on top of the fluorine doped tin oxide (FTO) electrode, minimizing material consumption. We find that the fullerene-modified FTO is a robust, chemically inert charge selective contact for perovskite based solar cells, which can reach 15% of stabilised power conversion efficiency in a flat junction device architecture using a scalable, low temperature, and reliable process. In contrast to TiO2, devices employing a molecularly thin functionalized fullerene layer show unaffected performance after 67 h of UV light exposure.

The influence of keto defects on photoexcitation dynamics in polyfluorene

Abstract The optical properties of two differently substituted types of polyfluorenes, 9-monoalkylated PF (mono-PF) and 9,9-dialkylated PF (bi-PF) where studied by means of photo-induced absorption (PIA) and ultrafast pump and probe measurements. The photo-induced absorption was complemented by measurements on a fluorene–fluorenone copolymer, which can be seen as a model substance for the polyfluorenes containing keto-defect sites. By differential transmission measurements we show that for the 9-monoalkylated PF measurements the singlet and triplet signal is strongly reduced compared to the 9,9-dialkylated PF. Instead, the polaron signal becomes the dominant feature.

Strain-Induced Enhancement of the Electron Energy Relaxation in Strongly Correlated Superconductors

We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate Γ1 of photoexcited carriers in cuprate and pnictide superconductors. We find that Γ1 increases monotonically with increased negative strain in the crystallographic a axis. Generally, the BardeenShockley deformation potential theorem and, specifically, pressure-induced Raman shifts reported in the literature suggest that increased negative strain enhances electron-phonon coupling, which implies that the observed direct correspondence between a and Γ1 is consistent with the canonical assignment of Γ1 to the electron-phonon interaction. The well-known nonmonotonic dependence of the superconducting critical temperature Tc on the a-axis strain is also reflected in a systematic dependence Tc on Γ1 ,w ith a distinct maximum at intermediate values (∼16 ps −1 at room temperature). The empirical nonmonotonic systematic variation of Tc with the strength of the electron-phonon interaction provides us with unique insight into the role of electron-phonon interaction in relation to the mechanism of high-Tc superconductivity as a crossover phenomenon.

Stimulated emission dynamics in a hexacatenar liquid crystal

A phasmidic liquid crystal incorporating an oligo(para)phenylene-vinylene core is a novel approach to achieve order and intermolecular separation in conjugated materials. Pump-probe experiments with 150-fs time resolution reveal efficient stimulated emission.

Optical characterisation of poly-2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene

We have investigated the electronic properties and the photoluminescence (PL) decay of the poly-2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene (PDHPT) copolymer made up by the regular alternation of high and low band-gap units. Experimental data show that the large twisting of the neighbouring constituents of the polymer chain leads to a considerable shift of the optical transitions towards high energies. This blue shift is much more pronounced than the expected band-gap lowering caused by the low-gap thiophene units. Transient PL experiments show that inter-chain migration of excitons towards centers where excitons decay non-radiatively reduces the PL quantum yield (QY).

Unlocking the Functional Properties in One-Dimensional MoSI Cluster Polymers by Doping and Photoinduced Charge Transfer

To improve functionalization of MoSI cluster polymers we have studied the effects of adsorption doping on the electrical transport, bundling, and optical absorption spectra. Doping results both in enhanced conductivity and aggregated bundles in dispersion. The different electronic properties of different bundle diameters can be ascribed to self-doping during the synthesis. Furthermore, doping shifts the characteristic absorption peaks and transfers oscillator strength to lower energies. Femtosecond optical spectroscopy shows that the spectral signature of adsorption and self-doping indeed originates from the population of electronic levels that are empty or absent in the undoped sample. The large spectral shifts and long lifetimes of photoinduced charges suggest efficient localization.