Markus Schindler

In Situ X-ray Study of the Structural Evolution of Gold Nano-Domains by Spray Deposition on Thin Conductive P3HT Films

Gold (Au) nanoparticles are deposited from aqueous solution onto one of the most used conductive polymers, namely poly(3-hexylthiophene) (P3HT), using airbrush deposition. We report on the structure formation and packing of the Au nanoparticles after a 5 s spray cycle. In situ grazing incidence small-angle X-ray scattering (GISAXS) measurements with 20 ms time resolution allow a real-time observation of the emergence and evolution of the microstructure during a spray cycle and subsequent solvent evaporation. The results reveal multistage nanoscale ordering of the Au nanoparticles during the spray cycle. Further ex situ atomic force microscopy measurements of the sprayed films showed the formation of Au monolayer islands on top of the polymer film. Our study suggests that the solvent-substrate interaction as well as solvent evaporation kinetics are important factors that need to be taken into consideration in order to grow a compact uniform monolayer film for the fabrication of ultrathin films using airbrush deposition.

Infiltration of Polymer Hole-Conductor into Mesoporous Titania Structures for Solid-State Dye-Sensitized Solar Cells

The degree of filling of titania nanostructures with a solid hole-conducting material is important for the performance of solid-state dye-sensitized solar cells (ssDSSCs). Different ways to infiltrate the hole-conducting polymer poly(3-hexylthiophene) (P3HT) into titania structures, both granular structures as they are already applied commercially and tailored sponge nanostructures, are investigated. The solar cell performance is compared to the morphology determined with scanning electron microscopy (SEM) and time-of-flight grazing incidence small-angle neutron scattering (TOF-GISANS). The granular titania structure, commonly used for ssDSSCs, shows a large distribution of particle and pore sizes, with porosities in the range from 41 to 67%, including even dense parts without pores. In contrast, the tailored sponge nanostructure has well-defined pore sizes of 25 nm with an all-over porosity of 54%. Filling of the titania structures with P3HT by solution casting results in a mesoscopic P3HT overlayer and consequently a bad solar cell performance, even though a filling ratio of 67% is observed. For the infiltration by repeated spin coating, only 57% pore filling is achieved, whereas filling by soaking in the solvent with subsequent spin coating yields filling as high as 84% in the case of the tailored titania sponge structures. The granular titania structure is filled less completely than the well-defined porous structures. The solar cell performance is increased with an increasing filling ratio for these two ways of infiltration. Therefore, filling by soaking in the solvent with subsequent spin coating is proposed.

Investigation of morphological degradation of P3HT:PCBM bulk heterojunction films exposed to long-term host solvent vapor

Solution-based processing procedures are widely used during the fabrication of polymer solar cells both on the lab scale and in industrial applications. The understanding of device stability in its host solvent vapor atmosphere is of great significance to the fabrication, encapsulation and storage. Solar cells with poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) active layers are prepared with different solvents of chlorobenzene, toluene, xylene and dichlorobenzene. The stability is investigated via exposure to their respective host solvent vapor for a long period. All solar cells strongly degrade after exposure to solvent vapor for long-term and only the dichlorobenzene-related device still shows reasonable function. The morphology of P3HT:PCBM BHJ films is probed using optical microscopy, atomic force microscopy, grazing incidence small and wide angle X-ray scattering and absorption measurements. The solvent induced PCBM crystallization is identified as the main reason for device failure.

Pressure-Sensitive Adhesives under the Influence of Relative Humidity: Inner Structure and Failure Mechanisms

Model pressure-sensitive adhesive (PSA) films of the statistical copolymer P(EHA-stat-20MMA), which comprises 80% ethylhexyl acrylate (EHA) and 20% methyl methacrylate (MMA), are studied. The PSA films are stored under different relative humidities from <2% to 96% for 24 h and subsequently investigated concerning the near-surface composition profile by measuring X-ray reflectivity (XRR) and tack performance. For both types of measurements, special custom-made sample environments are used, which ensure constant temperature and relative humidity during the XRR and tack measurements. Different failure mechanisms of the adhesive bond are found by adjusting the relative humidity. XRR measurements evidence enrichment layers in vicinity to and at the surface depending on the provided relative humidity during the postproduction treatment, which also influence the tack performance. This finding is supported by tack measurements using punches with different roughness.

Reorganization of the Near-Surface Composition in Pressure Sensitive Adhesive Films Stored in Nitrogen Atmosphere

The reorganization of the near-surface composition of a pressure sensitive adhesive (PSA) film upon exposure to nitrogen atmosphere is investigated. PSA films are prepared via solution casting of the statistical copolymer P(EHA-stat-20MMA) which consists of 80% ethylhexylacrylate as the sticky majority component and 20% methylmethacrylate as the glassy minority component. The resulting films are exposed to a nitrogen atmosphere representing an apolar and inert gas. This treatment causes a reorganization of the inner morphology in the PSA film. Using X-ray reflectivity measurements it is shown that the freshly prepared films show a solubility-driven enrichment of the PMMA component at the very surface. Due to the interaction with the apolar nitrogen atmosphere the less polar component of the copolymer, PEHA, enriches on top with time. The timescale of this internal rearrangement lies in the order of days; a complete inversion is found to take 12 days. An investigation of the local homogeneity of the sample surface on a nanoscale with atomic force microscopy proves a smooth and featureless surface right after casting.

Adhesive–adherent interfaces probed with grazing-incidence small-angle neutron scattering

The inner structure of a pressure-sensitive adhesive (PSA) is investigated at the surface as well as at the buried adhesive–adherent interface. Time-of-flight grazing-incidence small-angle neutron scattering (TOF-GISANS) is used to obtain depth-resolved structural information about the statistical copolymer poly(ethylhexylacrylate-stat-d-methylmethacrylate), which represents a well studied model PSA comprising 80% ethylhexylacrylate and 20% deuterated methylmethacrylate. Small and rare defects of around 40 nm in lateral size are found at the surface, while in the probed film volume a very low concentration of smaller structures of between 25 and 37 nm is found. Acidic and basic cleaning procedures are applied to the silicon adherent to alter the surface chemistry. At the buried interface of the adhesive and adherent no lateral structures are resolvable, irrespective of the surface treatment. The absence of dominant lateral structures shows that the statistical copolymer does not phase separate on a length scale that is of interest for applications. Furthermore, the findings prove the suitability of this kind of sample system for reflectivity measurements and demonstrate the suitability of TOF-GISANS for nondestructive investigations of buried interfaces in adhesion science.