Coarse-grained molecular dynamics simulations on mechanical properties of polymer composites for bulk heterojunction solar cells

We investigate mechanical properties of polymer composites of poly(3-hexylthiophene) (P3HT) and fullerene C₆₀ using coarse-grained molecular dynamics (CGMD) simulations, where the P3HT monomer unit and C₆₀ are represented by three CG beads and a single CG bead. Pure P3HTs with the degrees of polymerization (DPs) of 50, 100, and 150 exhibit almost identical tensile moduli, while the tensile strength increases with the DP. We quantify an increase in the number of molecular chain entanglements resulting from increasing DP, which in turn enhances the tensile strength. Meanwhile, the decomposition of molecular interactions contributing to stress indicates that the tensile modulus is primarily determined by non-bonded potentials and bond length potentials, almost independent of the chain entanglements. Furthermore, the addition of C₆₀ leads to higher tensile modulus and hence more brittle behavior of the composite in accordance with experiments. We find that an increase in C₆₀ mass fraction further inhibits molecular chain entanglements, leading to a significant reduction of the tensile strength. Meanwhile, an increase in the tensile modulus mainly originates from an increase in non-bonded interactions associated with C₆₀.