Insight the molecular interactions governing the morphology of bulk heterojunctions in high-efficiency solar cells

Controlling the nanoscale morphology of bulk heterojunctions is one of the key parameters for printing high-efficiency organic solar cells. Especially in the case of ternary blends, where two donor polymers are mixed with an acceptor to form a bulk heterojunction with internal cascade charge carrier separation, the prediction of the resulting bulk heterojunctions is a major challenge. In general, the formation of bulk heterojunctions is determined by the miscibility of donor and acceptor in the solvent and the drying kinetics of the printed layer during the blending process. In this work, we use molecular dynamics atomistic simulations to model at the molecular level the miscibility of polymers in ternary blends as a function of density and relative concentration. The potential of such methods is largely unexplored and the dependence of miscibility on fullerene concentration or blend density is typically not considered in the literature. We take an important step forward beyond the state of the art by showing that miscibility depends significantly on density. Indeed, when polymer cells are processed from solution, typically by spin coating, rapid drying of the blend layer occurs and produces a nanoscale morphology under non-equilibrium conditions. The results clearly show that the non-equilibrium conditions during processing are crucial to set the miscibility and final morphology. This has important technological implications and may encourage experimenters to better investigate deposition parameters for more efficient and stable mixtures.