The ability to pattern surfaces on a microscopic length scale is of importance for technological applications such as the fabrication of microelectronic circuits and digital storage media. Devices fabricated entirely from polymers are now available, opening up the possibility of adapting polymer processing technologies to fabricate cheap, large-area devices using non-lithographic techniques--for example, by exploiting dewetting and phase separation in thin films. But the final pattern adopted by the polymer film using such approaches requires a template printed onto the substrate by optical lithography, microcontact printing or vapour deposition. Here we describe a simple process for patterning surfaces that does not require a template. Our method involves the spinodal dewetting of a polymer surface by a thin polymer film, in which a liquid film breaks up owing to the amplification of thermal fluctuations in film thickness induced by dispersion forces. A preferred orientation is imposed on the dewetting process simply by rubbing the substrate, and this gives rise to patterns of remarkably well-aligned polymer lines. The width of these lines is well-defined, and is controlled by the magnitude of the dispersion forces at the interface, which in turn can be varied by varying the thickness of the polymer substrate. We expect that further work will make it possible to optimize the degree of order in the final morphology.

Blends of conjugated polymers are frequently used as the active semiconducting layer in light-emitting diodes and photovoltaic devices. Here we report the use of scanning near-field optical microscopy, scanning force microscopy and nuclear-reaction analysis to study the structure of a thin film of a phase-separated blend of two conjugated polymers prepared by spin-casting. We show that in addition to the well-known micrometre-scale phase-separated morphology of the blend, one of the polymers preferentially wets the surface and forms a 10-nm-thick, partially crystallized wetting layer. Using near-field microscopy we identify unexpected changes in the fluorescence emission from the blend that occurs in a 300-nm-wide band located at the interface between the different phase-separated domains. Our measurements provide an insight into the complex structure of phase-separated conjugated-polymer thin films. Characterizing and controlling the properties of the interfaces in such films will be critical in the further development of efficient optoelectronic devices.

We use optical and scanning force microscopy to explore the possibility of switching the stability of a bilayer of poly(methyl methacrylate) (PMMA) on polystyrene by simply changing the film thickness. We show that for thin PMMA layers on thicker polystyrene films the PMMA layer is unstable to thickness fluctuations. However, polystyrene layers are unstable when they are substantially thinner than the now stable PMMA film. Dewetting morphologies are cataloged as a function of the thickness of individual polymer layers by identifying which layer is unstable by which mechanism, be it spinodal dewetting or heterogeneous thermal nucleation. Our results are in good agreement with a linear stability analysis of the influence of long-range dispersion forces, but also indicate the influence of film preparation and small variations of material properties.

In-situ grazing incidence X-ray diffraction has been used to investigate the kinetics of crystallization at the surface of thin films of poly(ethylene terephthalate) (PET). By varying the angle of incidence of the X-ray beam around the critical angle of the film, the penetration depth can be tuned to allow a direct comparison of molecular ordering in the surface and bulk of the film. The results show that ordering occurs significantly faster at the surface in the temperature range 90−100 °C (close to the bulk glass transition temperature of 75 °C). The (0 1̄ 1) and (0 1 0) peaks narrow more rapidly and achieve a lower width, indicating that the crystallization progresses more rapidly in these crystallographic directions. This enhanced ordering is attributed to a combination of surface energy effects, which promote localized packing, and to an enhanced segmental mobility near the free surface.

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,9 -dioctylfluorene) (F8) and poly(9,9dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ∼4 nm at an F8 volume fraction φ = 0.5, dropping down to ∼1-2 nm for φ = 0.2 and 0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.

Poly(3,4 ethylene dioxythiophene)/poly(styrene sulfonate) (PEDT/PSS) blends are widely used in organic light‐emitting diodes (OLEDs) to enhance their performance. Using neutron reflectometry to study such blends, it is shown that substantial segregation of the blend components occurs at the surface (see Figure), a result that will have implications both for the performance and for the stability of optoelectronic devices using PEDT/PSS.

The stability and association of polymer-fullerene films upon thermal annealing depends strongly on exposure to light, even at ambient conditions. As a result, dewetting of nanocomposite films can be prevented and the characteristic lengthscales of phase separated morphologies finely tuned. Coupling photopatterning with either self-organization process provides a powerful route for the directed assembly of fullerene-based nanocomposites into functional "circuits".

The combined effects of illumination and thermal annealing on the morphological stability and photodimerization in polymer/fullerene thin films are examined. While illumination is known to cause fullerene dimerization and thermal stress their dedimerization, the operation of solar cells involves exposure to both. The competitive outcome of these factors with blends of phenyl-C61-butyric acid methyl ester (PCBM) and polystyrene (PS), supported on PEDOT:PSS is quantified. UV-vis spectroscopy is employed to quantify dimerization, time-resolved neutron reflectivity to resolve the vertical composition stratification, and atomic force microscopy for demixing and coarsening in thin films. At the conventional thermal stress test temperature of 85 °C (and even up to the PS glass transition), photodimerization dominates, resulting in relative morphological stability. Prior illumination is found to result in improved stability upon high temperature annealing, compatible with the need for dedimerization to occur prior to structural relaxation. Modeling of the PCBM surface segregation data suggests that only PCBM monomers are able to diffuse and that illumination provides an effective means to control dimer population, and thus immobile fullerene fraction, in the timescales probed. The results provide a framework for understanding of the stability of organic solar cells under operating conditions.