Photovoltaic devices based on organic semiconductors (OPVs) are currently the subject of intense scientific interest as they offer the possibility of generating low-cost energy from sunlight. While the power conversion efficiency of the best devices presently stands at $7%, current thinking suggests that OPV devices having efficiencies exceeding 10% are practically achievable.[1] Such bulk heterojunction OPVs are usually composed of a blend of an electron accepting and an electron donating material. In an optimized device, such materials undergo self-organized phase separation into a network structure that provides separate charge conduction pathways for electrons and holes to the device electrodes. For efficient operation, it is necessary that such a phase separated network be structured on a length-scale commensurate with the exciton diffusion length (which is of the order of 10 nm), permitting the majority of excitons to reach an interface between the donor and acceptor materials and undergo disassociation into charge carriers. Previous studies have demonstrated that the morphology of such thin film blends can be tuned by adjusting a range of parameters, including relative donor-acceptor composition, the choice of casting solvent and the time over which the film dries. Thin-film structure can also be modified post-deposition via the use of thermal annealing or exposure to a solvent vapor. All of these processes are known to affect the performance and efficiency of devices created.[2-5] One issue of particular relevance is the possibility of self-stratification of the components perpendicular to the plane of the film; the frequent occurrence of self-stratification is a fundamental consequence of the importance of surface and interface effects in driving phase separation in thin polymer films. [6,7] In this work, we use neutron reflectivity (NR) [8][9][10] to study the depth dependent composition of a thin-film of the electrondonating polymer P3HT (poly(3-hexylthiophene)) and the electron accepting fullerene derivative PCBM ([6,6]-phenyl-C 61 -butyric acid methyl ester) that have been optimized for operation in an OPV. Although the maximum power conversion efficiency of devices based on P3HT/PCBM that we study is limited to between 4.5 to 5%, this system now serves as an excellent model for developing a full understanding of the strategies necessary to improve the efficiency of new and emerging OPV-applicable materials. [3,11,12] In particular, we seek to explore the composition of P3HT/PCBM blend thin films in a direction normal to the film surface. Previous work on this system using variable angle spectroscopic ellipsometry, [13] electron tomography, [14] and near edge X-ray absorption fine structure [15] has identified PCBM concentration gradients perpendicular to the film surface. In order to fully characterize self-stratification with quantitative depth profile information at nanometer resolutions, we have used the technique of neutron reflectivity to characterize selfstratification in P3HT/PCBM blend films...

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Graphene oxide membranes were recently suggested for applications in separation of ethanol from water using a vapor permeation method. Using isotope contrast, neutron reflectivity was applied to evaluate the amounts of solvents intercalated into a membrane from pure and binary vapors and to evaluate the selectivity of the membrane permeation. Particularly, the effect of D2O, ethanol and D2O-ethanol vapours on graphene oxide (GO) thin films (∼25 nm) was studied. The interlayer spacing of GO and the amount of intercalated solvents were evaluated simultaneously as a function of vapour exposure duration. The significant difference in neutron scattering length density between D2O and ethanol allows distinguishing insertion of each component of the binary mixture into the GO structure. The amount of intercalated solvent at saturation corresponds to 1.4 molecules per formula unit for pure D2O (∼1.4 monolayers) and 0.45 molecules per formula unit (one monolayer) for pure ethanol. This amount is in addition to H2O absorbed at ambient humidity. Exposure of the GO film to ethanol-D2O vapours results in intercalation of GO with both solvents even for high ethanol concentration. A mixed D2O-ethanol layer inserted into the GO structure is water enriched compared to the composition of vapours due to slower ethanol diffusion into GO interlayers.

Enabling control over macromolecular ordering and the spatial distribution of structures formed via the mechanisms of molecular self-assembly is a challenge that could yield a range of new functional materials. In particular, using the self-assembly of minimalist peptides, to drive the incorporation of large complex molecules will allow a functionalization strategy for the next generation of biomaterial engineering. Here, for the first time, we show that co-assembly with increasing concentrations of a highly charged polysaccharide, fucoidan, the microscale ordering of Fmoc-FRGDF peptide fibrils and subsequent mechanical properties of the resultant hydrogel can be easily and effectively manipulated without disruption to the nanofibrillar structure of the assembly.