P3HT-Fullerene Blends: a Classical Molecular Dynamics Simulation

Blends of polymer and C$_{60}$-derived molecules are in the spotlight in recent years for application in organic photovoltaics, forming what is known as bulk-heterojunction active layers. The character of the heterojunction is determinant, with clear relevance of morphology and phase separation. To better understand the morphology of the systems, we present a classical molecular dynamics (CMD) simulation of polymer/fullerene (P3HT/C$_{60}$) blends, coming from different starting points, using the specifically designed Nanomol Force Field based on the Universal Force Field. We use not-so-short regioregular polymers with 30 hexyl-thiophene units ($\sim$5kg$\cdot$mol$^{-1}$ molecular weight) and, adopting a $\sim$ 1:1 mass proportion for the polymer:molecule blend, we simulate cells of $\sim$ 40 thousand atoms, at room temperature and normal pressure conditions. We find that, independently of the starting-point spatial distribution of C$_{60}$ molecules relative to P3HT chains, segregated or isotropic, the fullerene molecules show a tendency to segregate, and phase separation is the dominating regime.