Equilibration and thermal reversibility in mixtures of model OPV small-molecules and polymers

Abstract

The thermal behaviour of small-molecule/polymer mixtures is of crucial significance in relation to the operational stability of organic photovoltaics, and the equilibration (or otherwise) of domain compositions and interfaces is of key importance for guiding design. Here, model phase-separated mixtures of fullerene and polystyrene are studied in detail in a thin-film bilayer, to robustly examine whether such systems satisfy two key requirements of thermodynamic equilibrium; (i) the attainment of a state (at a given temperature) that minimises the free energy, independent of the starting state of the system, and (ii) the reversibility of transitions between such equilibrium states. In an extensive study using polystyrene molecular weights of 1.86, 4.73, and 278.2 kg mol⁻¹, depth profiles are measured as a function of temperature using in situ neutron reflectivity, with initial sample composition profiles containing layers that are either pure components or blends. Following thermal annealing at sufficiently high temperatures we reproducibly observe changes in layer compositions, layer thicknesses and interfacial roughnesses during temperature cycling that are reversible, irrespective of the starting composition profiles of the samples. This robust demonstration of equilibrium behaviour provides a benchmark for the understanding of mixing in small-molecule/polymer thin-films, with particular relevance to the operation of organic photovoltaic devices.