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Polymer Solar Cells (PSCs) continue to be a promising third generation, low energy-budget, lead-free PV technology. Efficiencies have now improved to over 12% and a new class of materials (small molecule acceptors) have recently provided rapid improvements that promise further advances. Significant effort in the field is being spent on synthetic efforts to tune the electronic structure and in understanding charge generation. In contrast, we explore the correlation of fill factor to the purity of domains as measured with soft x-ray scattering, and how said purity is controlled by thermodynamically metastable morphologies. We will argue that ideal materials systems will have a Flory Huggins interaction parameter χ that naturally leads to mixed domains that have a composition close the fullerene/SMA percolation threshold. Systems that are too miscible will have excessive bimolecular recombination. Systems too immiscible need to be quenched for best performance, are thus unstable and eventually produce fullerene islands that trap charges. Although some of these concepts are known and are indirectly referred to as “miscibility”, quantitative relations remained largely elusive. Understanding molecular interactions is even more important for conventional ternaries devices. In some cases, the two donor polymer used can have unfavorable thermodynamic interactions (χ = - 0.56 at 296 °C) that prevent improved performance due to lack of phase separation and alloying of the two donors. Overall, we advocate a program to measure χ(T) in model systems in order to develop a frame-work that will eventually lead to computational approaches that allow predictions of χ as a function of molecular tuning before synthesis of a targeted compound is attempted.
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