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Instruments / Technical Area	Publication Details	Published
I07-Surface & interface diffraction	Interfacial width and phase equilibrium in polymer-fullerene thin-films E. L. Hynes , J. T. Cabral , A. J. Parnell , P. Gutfreund , R. J. L. Welbourn , A. D. F. Dunbar , D. Môn , A. M. Higgins Communications Physics 2 DOI: 10.1038/s42005-019-0211-z Diamond Proposal Number(s): [15486] Open Access Show Abstract ▼ Abstract: Domain composition and interfacial structure are critical factors in organic photovoltaic performance. Here, we report neutron reflectivity, grazing-incidence X-ray diffraction and atomic force microscopy measurements of polymer/fullerene thin-films to test a hypothesis that these partially miscible blends rapidly develop composition profiles consisting of co-existing phases in liquid-liquid equilibrium. We study a range of polymer molecular weights between 2 and 300 kg mol−1, annealing temperatures between 120 and 170 oC, and timescales up to 10 min, yielding over 50 distinct measurement conditions. Model bilayers of fullerene-derivatives and polystyrene enable a rigorous examination of theoretical predictions of the effect of polymer mass and interaction parameter on the compositions, ϕ, and interfacial width, w, of the coexistent phases. We independently measure ϕ and w and find that both Flory-Huggins mean-field-theory and key aspects of self-consistent-field-theory are remarkably consistent with experiment. Our findings pave the way for predictive composition and interface design in organic photovoltaics based on simple experimental measurements and equilibrium thermodynamic theory.	Sep 2019
I07-Surface & interface diffraction	Mixing in PCBM/P3HT bilayers, using in situ and ex situ neutron reflectivity Dyfrig Mon , Anthony M. Higgins , Philipp Gutfreund , David James Journal Of Materials Research 32, 1946 - 1956 DOI: 10.1557/jmr.2017.59 Diamond Proposal Number(s): [1519] Open Access Show Abstract ▼ Abstract: In situ and ex situ neutron reflectivity is used to characterize annealed regioregular-P3HT/PCBM bilayers. In situ annealing of a 20 nm PCBM/35 nm P3HT bilayer at 170 °C reveals rapid mixing of PCBM and P3HT to produce a polymer-rich layer that contains around 18–20% PCBM. Samples with three different thicknesses of P3HT layer are ex situ annealed at 140 °C. This again reveals migration of PCBM into the P3HT and vice versa, with the polymer-rich layer in the 20 nm PCBM/35 nm P3HT sample containing 19% PCBM. Complete migration of the entire PCBM layer into the P3HT layer is observed for a 20 nm PCBM/80 nm P3HT bilayer. The robustness of fitted model composition profiles, in comparison with real-space imaging of sample surface morphology and previous work on annealed P3HT/PCBM bilayer compositions, is discussed in detail.	May 2017
I07-Surface & interface diffraction	Bimodal crystallization at polymer-fullerene interfaces Dyfrig Môn , Anthony Higgins , David James , Mark Hampton , J. Emyr Macdonald , Michael B. Ward , Philipp Gutfreund , Samuele Lilliu , Jonathan Rawle Physical Chemistry Chemical Physics 17 (3), 2216 - 2227 DOI: 10.1039/C4CP04253K Open Access Show Abstract ▼ Abstract: The growth-kinetics of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) crystals, on two different length-scales, is shown to be controlled by the thickness of the polymer layer within a PCBM–polymer bilayer. Using a model amorphous polymer we present evidence, from in situ optical microscopy and grazing-incidence X-ray diffraction (GIXD), that an increased growth-rate of nanoscale crystals impedes the growth of micron-sized, needle-like PCBM crystals. A combination of neutron reflectivity and GIXD measurements, also allows us to observe the establishment of a liquid–liquid equilibrium composition-profile between the PCBM layer and a polymer-rich layer, before crystallization occurs. While the interfacial composition-profile is independent of polymer-film-thickness, the growth-rate of nanoscale PCBM crystals is significantly larger for thinner polymer films. A similar thickness-dependent behavior is observed for different molecular weights of entangled polymer. We suggest that the behavior may be related to enhanced local-polymer-chain-mobility in nanocomposite thin-films.	Nov 2014

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