Miscibility-Function Relations in Organic Solar Cells: Significance of Optimal Miscibility in Relation to Percolation

Polymer solar cells (PSCs) continue to be a promising low-cost and lead-free photovoltaic technology. Of critical importance to PSCs is understanding and manipulating the composition of the amorphous mixed phase, which is governed by the thermodynamic molecular interactions of the polymer donor and acceptor molecules and the kinetics of the casting process. This progress report clarifies and defines nomenclature relating to miscibility and its relevance and implications to PSC devices in light of new developments. Utilizing a scanning transmission X-ray microscopy method, the temperature dependences of "molecular miscibility" in the presence of fullerene crystals, now referred to liquidus miscibility, are presented for a number of representative blends. An emphasis is placed on relating the amorphous miscibility of high-efficiency PSC blends at a given processing temperature with their actual device performance and stability. It is shown and argued that a system with an amorphous miscibility close to percolation exhibits the most stable morphology. Furthermore, an approach is outlined to convert liquidus miscibility to an effective Flory-Huggins interaction parameter . Crucially, determination of temperature-dependent amorphous miscibility paves a way to rationally optimize the stability and mixing behaviors of PSCs at actual processing and operating temperatures.