Molecular dynamics simulations of organic photovoltaic materials: Investigating the formation of π-stacked thiophene clusters in oligothiophene/fullerene blends

The behavior of short oligothiophene molecules in the presence of a fullerene surface have been investigated via fully atomistic molecular dynamics studies. The simulations mimic the hybrid assemblies characteristic of bulk heterojunction structures that are ubiquitous in current state-of-the-art polymer-based solar cells. This manuscript focuses on the development of thiophene ring clusters, and demonstrates the manner in which the surface drives the orientation and stacking of rings into clusters in prototypical optoelectronic materials. The proximity of the substrate is shown to significantly hinder the development of charge transfer pathways in the polymer: simulations on bulk oligothiophenes show dramatically better cluster formation in comparison with oligothiophene/fullerene systems, indicated by higher populations, and larger cluster sizes. Furthermore, the properties of percolating clusters that are capable of transporting charges across the simulation box are degraded in the presence of the surface. The fullerene substrate also influences the temperature-dependent behavior of the polymer phase, with more subtle changes in π-stacked order in the presence of the C60, than in the bulk. Our results unveil molecular details of the structure of oligothiophene chains in the context of the nanoscale morphology in organic photovoltaic devices, and reveal how the presence of the acceptor filler can result in lower hole mobilities.