Polymer-based solar cells (PSCs) require significant improvements in efficiency and life time in order to be commercially viable. Interfacial structure and morphology dictate the performance of PSCs, and these properties in turn depend on processing conditions and surface chemistry. To optimize device performance, detailed knowledge of the factors most critical to the molecular-level structure, morphology and dynamics of donor/acceptor systems at interfaces will be necessary. For one promising donor, poly(3-hexylthiophene (P3HT), we have utilized all-atom and coarse-grained molecular dynamics simulations to investigate such properties at liquid/vacuum, solid/liquid and solid/solid interfaces. At liquid/vacuum interfaces, static and dynamic properties of P3HT films and their dependence on temperature and molecular weight were studied. P3HT chains showed ordering through preferential exposure of side-chain at the interface, and surface tension showed strong dependence on temperature and molecular weight. Properties such as self-diffusion coefficients, chain end-to-end distance and torsion autocorrelations were also utilized to quantify the dynamics of the P3HT chains in the film. Both static and dynamic properties of P3HT were found to be in agreement with well-known models for polymers.Subsequent simulations of P3HT/water systems offered insight into the wetting behavior of P3HT and the nature of the solid-liquid interface in crystalline and amorphous P3HT. From contact angle calculations, different P3HT surfaces were determined to be hydrophobic. In the time scale of our simulations, no observable change in the orientation of the P3HT at interfaces was observed.Furthermore, the molecular ordering of P3HT close to substrates is expected to be the key to device performance. Ordering of P3HT chains at the interface can be tuned by altering the substrate surface chemistry. We investigated the effect of surface chemistry on the ordering of P3HT on self-assembled monolayers (SAMs) of n-alkanethiols. The results showed that the ordering of P3HT strongly depends on the P3HT-SAM interactions. The effect of solvent on the P3HT-SAM interactions was also studied. In addition, we characterized the surface properties of pure SAMs on gold 111. The end-functionalized network structure was found to be correlated to the adsorption sites. For P3HT/acceptor systems, all-atom simulations are challenging because of the need to access large spatial and temporal regimes. To overcome this, we developed a coarse-grained model for P3HT based on the all-atom force field. The coarse-grained model showed good agreement with bulk and interfacial properties obtained from the all-atom model and has a great potential for analyzing morphology and dynamics of P3HT/acceptor blends.

Conjugated polymers are a highly relevant class of materials due to their low processing cost and applicability in flexible electronic devices. A significant challenge in the field remains to relate the chemical structure of these materials with their morphology and dynamics, which in turn affect their charge transport characteristics. There are several experimental and computational methods that are commonly used when characterizing conjugated polymers. In this work, we attempt to synergistically apply these techniques so as to obtain a clear understanding of the properties that affect the structure and dynamics of conjugated polymers. A large experimental data set is used to validate molecular dynamics (MD) simulations of poly(3-hexylthiophene), a model conjugated polymer system. A sensitivity analysis of system and simulation parameters is performed, and the key factors affecting the conjugated polymer dynamics and configuration are found to be molecular weight, crystallinity, equilibration method, and force field parameters. Specifically, the atomistic partial charges are found to have the greatest influence on both properties. The identification of these key parameters will inform further studies of more complex conjugated polymer systems, elucidating the relationship between conjugated polymer chemistry and performance.

Conjugated polymers (CPs) are advantageous materials for lower-cost and flexible organic electronic devices, such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), bioelectronics, chemical sensors, flexible displays, and wearable electronics. Their pi-conjugated backbones enable charge transport along the chain or through the pi-orbital overlap of neighboring chains. The molecular dynamics and morphology in the crystalline and amorphous phases of both pure and blended CPs have a direct impact on these mechanisms and therefore, the macroscopic performance of the material. A thorough understanding of this relationship is important for the future development of improved materials and devices. In this work, we utilize neutron and X-ray scattering together with molecular dynamics (MD) simulations and density functional theory (DFT) for a powerful combined experimental and theoretical approach to probing the structure and dynamics in poly(3-hexylthiophene) (P3HT) and other polythiophenes. We first utilize quasi-elastic neutron scattering (QENS) to perform a critical assessment of MD simulation force fields for P3HT. Although these models capture system-level dynamics well, they fail to accurately represent characteristic motions along the polymer backbone which play a critical role in charge transport processes. Next, we utilize density functional theory (DFT) to explore the non-bonded, intermolecular interactions of P3HT and use MD simulations to understand their influence on in-silico dynamics. With selective deuteration, characteristic relaxation times are extracted from QENS data for a set of P3HT polymers and oligomers to probe the effect of molecular weight and crystallinity on backbone and side-chain dynamics. Pure CPs are still susceptible to limited environmental stability and low mechanical durability (e.g. cracking), but blending CPs with a commodity polymer, e.g. polystyrene, can improve the lifetime and mechanical robustness of these materials while maintaining electronic performance at low amounts of the conjugated material. In our final study, small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS) are used to characterize phase separation and self-assembly in these polythiophene-polystyrene blends, and correlate phase morphology with macroscopic conductivity.

Talks about various computer simulation techniques used for macromolecular materials. This book describes how to use simulation to explain experimental data and gain insight into structure and dynamic properties of polymeric structures. Explanations are given on how to overcome challenges posed by large size and slow relaxation polymer coils.