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

TL;DR

This study uses molecular dynamics simulations to explore how polymer-fullerene composites' molecular interactions and microstructure influence their mechanical properties, revealing key factors like non-bonded interactions and microvoid coalescence.

Contribution

It provides new insights into the molecular determinants of mechanical behavior in polymer-fullerene composites, linking microstructure to tensile properties.

Findings

Tensile modulus increases with C60 content due to non-bonded interactions.

Tensile strength correlates with molecular chain entanglements.

Void coalescence relates to yield points during deformation.

Abstract

We investigate the mechanical properties of $\pi$-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C$_{60}$ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C$_{60}$ composites with varied degrees of polymerization and C$_{60}$ mass fractions to obtain their stress-strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions associated with bond-stretching, while the addition of C$_{60}$ leads to an increase in the tensile modulus originating from enhanced non-bonded interactions associated with C$_{60}$. Additionally, the tensile strength of the P3HT:C$_{60}$ 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 increasing C$_{60}$ 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 $\pi$-conjugated polymer-fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.