Molecular Conformation and Dynamics of Conjugated Polymers using Neutron and X-ray Scattering and Simulations

Abstract

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.