Convergent equilibration at model polymer/small-molecule OPV interfaces?

Solution-processed organic photovoltaics (OPVs) have steadily increased in efficiency over the last 2 decades. However, they face barriers to commercialisation due to efficiency reduction during use. One aspect of this is the evolution of morphology and interfacial structure during operation (under thermal cycling). The equilibrium behaviour of mixed, amorphous polymer/small-molecule phases (‘domains’) within the active-layers of OPVs has also recently been linked directly to device efficiency and stability. Our interest is to see how far we can understand changes in phase compositions and interfacial width (both key to OPV efficiency) in terms of fundamental thermodynamics, with the aim of providing a solid basis for interpretation of equilibration measurements in polymer/small-molecule thin-film OPV systems. In this proposed experiment we seek to understand the non-equilibrium and equilibrium mixing behaviour that can occur in polymer/small-molecule bilayers on thermal annealing. The key parameter is the width of the buried interface, which can display hysteresis as a function of annealing temperature. The proposed experiment will establish whether samples with different starting points (composition profiles) that mix on heating, but exhibit different interfacial widths, converge to a single minimum as a function of annealing temperature.