Molecular Insights into the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions for Organic Photovoltaic Applications

We investigate the mechanical properties of π-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C60 using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C60 composites with varied degrees of polymerization and C60 mass fractions to obtain their stress–strain responses. Decomposition of a stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on nonbonded interactions and on bonded interactions associated with bond stretching, while the addition of C60 leads to an increase in the tensile modulus originating from enhanced nonbonded interactions associated with C60. Additionally, the tensile strength of the P3HT:C60 samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with an increasing C60 mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids. These results provide a fundamental understanding of the molecular determinants of the mechanical properties of π-conjugated polymer–fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.

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