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Equilibration and thermal reversibility in mixtures of model OPV small-molecules and polymers
Journal of Materials Chemistry C, Volume: 11, Issue: 6, Pages: 2107 - 2119
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The thermal behaviour of small-molecule/polymer mixtures is of crucial significance in relation tothe operational stability of organic photovoltaics, and the equilibration (or otherwise) of domaincompositions and interfaces is of key importance for guiding design. Here, model phase-separatedmixtures...
|Published in:||Journal of Materials Chemistry C|
Royal Society of Chemistry (RSC)
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The thermal behaviour of small-molecule/polymer mixtures is of crucial significance in relation tothe operational stability of organic photovoltaics, and the equilibration (or otherwise) of domaincompositions and interfaces is of key importance for guiding design. Here, model phase-separatedmixtures of fullerene and polystyrene are studied in detail in a thin-film bilayer, to robustly examinewhether such systems satisfy two key requirements of thermodynamic equilibrium; (i) the attainment ofa state (at a given temperature) that minimises the free energy, independent of the starting state of thesystem, and (ii) the reversibility of transitions between such equilibrium states. In an extensive studyusing polystyrene molecular weights of 1.86, 4.73, and 278.2 kg mol1, depth profiles are measured as afunction of temperature using in situ neutron reflectivity, with initial sample composition profilescontaining layers that are either pure components or blends. Following thermal annealing at sufficientlyhigh temperatures we reproducibly observe changes in layer compositions, layer thicknesses andinterfacial roughnesses during temperature cycling that are reversible, irrespective of the startingcomposition profiles of the samples. This robust demonstration of equilibrium behaviour provides abenchmark for the understanding of mixing in small-molecule/polymer thin-films, with particularrelevance to the operation of organic photovoltaic devices
Data availability:Data can be downloaded using the DOIs for the two experiments: https://doi.org/http://doi.ill.fr/10.5291/ILL-DATA.9-11-1903 and https://doi.org/http://doi.ill.fr/10.5291/ILL-DATA.9-11-1983 after an embargo period of four years. Requests for data earlier than this should be emailed to the corresponding author.
Faculty of Science and Engineering
EH acknowledges Swansea University for funding her studentship.