Systematic coarse-graining and dynamical simulations of anisotropic molecules with applications in organic semiconductors

Abstract

Organic semiconductors are used widely in different applications, including organic photovoltaics (OPVs), devices that convert solar energy to electricity. These devices, if applicable commercially, can help to supply the world’s energy needs without requiring complicated manufacturing and maintenance. Moreover, OPVs possess several useful physical properties such as being light weight, highly transparent, and flexible. This makes organic electronic devices advantageous over those made of inorganic hard materials, especially in applications in which these conditions are required. Although experimental studies show that organic semiconductors can potentially yield high performing devices, the electronic processes that govern the conversion of light to energy are not fully understood. Specifically, how free electrons are created and transferred within the device when a photon is absorbed is strongly debated in the literature. Many experimental and theoretical results have shown that microstructure at the interfaces between the component organic semiconductor materials that make up the device plays an important role in these processes. The microstructure can be induced by directional forces between generally anisotropic organic-semiconductor molecules, combined with translational symmetry breaking at interfaces. In Chapter 3, the interface of a high-performing electron donor–acceptor OPV system consisting of two small organic semiconductors benzodithiophene quaterthiophene rhodanine (BQR) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is studied using classical molecular dynamics (MD). Atomistic simulations at high temperatures indicate that the "face-on" configuration is more favorable at a liquid–solid interface between the materials. In addition, molecules close to the interface are less ordered with respect to one another than those far from the interface. These factors may benefit charge separation and transport, resulting in good device performance. In general, atomistic simulations are not feasible for studying donor–acceptor interface formation for the typical domain sizes found in devices. A solution to this is to use coarsegrained (CG) models, which increases the simulation efficiency by replacing a collection of atoms as a single interacting site. In Chapter 4, a new systematic methodology to generate CG models for MD simulations is introduced and validated, which constitutes the main result of this thesis. This algorithm is developed so that MD simulations can be simplified but still accurately represent the physical and thermodynamic properties of the simulated materials. More importantly, this method can produce models that capture the anisotropy of molecules, which is especially useful for theoretical studies of organic materials and has not previously been achieved via a systematic algorithm. To validate the method, a CG model of a simple anisotropic organic molecule (benzene) is produced in Chapter 5. Simulations using this model accurately describes the structural and thermodynamic properties of the FG model and is an improvement over previous CG benzene models. A future application of this method will be the study of the interface structure of materials in OPV systems on realistic time and spatial scales compared to experimental conditions. Ultimately, the studies presented in this thesis work towards the same goal, which is to discover optimal molecular design rules to increase the power conversion efficiency of OPVs.