Mixing in non-fullerene acceptor (NFA)/polymer blends; Understanding equilibrium and non-equilibrium behaviour

Despite rapid increases in efficiency over the last five years or so, organic photovoltaics (OPVs) fabricated using non-fullerene electron acceptors (NFAs), face challenges to commercialisation due to efficiency reduction during use. A key aspect of this is the evolution of morphology and interfacial structure during operation; eg changes to the volumes, compositions and connectivity of mixed acceptor/donor phases (domains) at elevated temperatures. In recent years, we have developed a methodology, using in-situ neutron reflectometry, to explore phase composition, phase volume and interfacial width during thermal annealing in model fullerene/polymer thin-film samples, and examine their behaviour within the framework of equilibrium theories (Flory-Huggins, self-consistent field-theory). We now seek to apply this to recently developed NFA/polymer systems, and thoroughly evaluate key aspects of equilibrium behaviour; i) convergence to a global minimum from different starting points, ii) (thermal) reversibility and iii) MW dependence. The aim of our study is to complement device-optimisation and stability strategies in these commercially-relevant materials, by thoroughly investigating fundamental behaviour in model thin-film bilayers.