Predicting the Mechanical Properties of Organic Semiconductors Using Coarse-Grained Molecular Dynamics Simulations

Abstract

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. Here, we describe the use of coarse-grained molecular dynamics simulations to predict the density, tensile modulus, Poisson ratio, and glass transition temperature for poly­(3-hexyl­thiophene) (P3HT) and its blend with C60. In particular, we show that the resolution of the coarse-grained model has a strong effect on the predicted properties. We find that a one-site model, in which each 3-hexyl­thiophene unit is represented by one coarse-grained bead, predicts significantly inaccurate values of density and tensile modulus. In contrast, a three-site model, with one coarse-grained bead for the thiophene ring and two for the hexyl chain, predicts values that are very close to experimental measurements (density = 0.955 g cm–3, tensile modulus = 1.23 GPa, Poisson ratio = 0.35, and glass transition temperature = 290 K). The model also correctly predicts the strain-induced alignment of chains as well as the vitrification of P3HT by C60 and the corresponding increase in the tensile modulus (tensile modulus = 1.92 GPa, glass transition temperature = 310 K). We also observe a decrease in the radius of gyration and the density of entanglements of the P3HT chains with the addition C60 which may contribute to the experimentally noted brittleness of the composite material. Although extension of the model to poly­(3-alkyl­thiophenes) (P3ATs) containing side chains longer than hexyl groupsnonyl (N) and dodecyl (DD) groupscorrectly predicts the trend of decreasing modulus with increasing length of the side chain measured experimentally, obtaining absolute agreement for P3NT and P3DDT could not be accomplished by a straightforward extension of the three-site coarse-grained model, indicating limited transferability of such models. Nevertheless, the accurate values obtained for P3HT and P3HT:C60 blends suggest that coarse graining is a valuable approach for predicting the thermomechanical properties of organic semiconductors of similar or more complex architectures.

Bulk Heterojunction Morphologies with Atomistic Resolution from Coarse-Grain Solvent Evaporation Simulations

“…The morphologies can subsequently serve as a starting point for QM calculations on excitonic properties, or be used to compute the mechanical response 59,60 of BHJ materials.…”

Glass Transition Phenomenon for Conjugated Polymers

“…Recently, there have been increasing interest of utilizing simulations to study conjugated polymers, with the focus on the morphology, charge‐transport property, solubility, etc. However, the study of the T g for conjugated polymers is limited . Lipomi and co‐workers investigated the thermomechanical property of P3HT as well as three donor–acceptor polymers (PDTSTPD, PTB7, and TQ1) with molecular dynamic simulations, where the predicted properties including density, modulus, and T g exhibit good agreement with experimental results .…”

Modelling the morphology and thermomechanical behaviour of low-bandgap conjugated polymers and bulk heterojunction films

“…52 Interestingly, we found a direct correlation between T g and density in these materials; this finding is consistent with the free-volume model for glass transitions. 8 The shift can ultimately be attributed to the high dispersion forces arising from the fullerene cage and the spherical shape, which enables efficient packing and causes it to act as an anti-plasticizing agent. T g is an important parameter for the BHJ because it represents the upper bound for the operating temperature at which a device can function without undergoing detrimental morphological rearrangements.…”