Mapping Structural Dynamics of Conjugated Polymers
- Date: Wednesday 9 December 2020, 14:00 – 15:00
- Location: Online
- Type: Soft matter physics, Seminars, Physics and Astronomy
- Cost: Free
Dr Anne Guilbert is an EPSRC Postdoctoral Fellow in the Department of Physics, Imperial College London. She is presenting some of her research to our Soft Matter Physics Research Group.
Abstract: Conjugated polymers have attracted a keen interest over the past decade for their potential applications as semiconductors in various types of devices: organic light emitting diodes, organic solar-cells, organic field-effect transistors, etc. Because polymers are soft materials, a range of dynamics occurs over an extended time scale, from femtosecond to millisecond, and are likely to affect the optoelectronic properties of the material. Vibrational dynamics have been evidenced to impact absorption, inner reorganization energy, charge transfer between similar molecules and between different molecules at a heterojunction, delocalization, and so more generally charge transport and charge separation processes. Slower dynamics on the picosecond to nanosecond time-scale includes side-chain reorientation and backbone torsion. These dynamics are impacting the conformation of the polymer chains and thus, affect both optical and electronic properties. Furthermore, these slow dynamics are temperature-dependent and therefore, can be activated during device operation. For instance, molecular diffusion becomes predominant above the glass transition of the material and is a known degradation mechanism.
We present a detailed microscopic study of the structure-dynamics relationship of both regio-regular (RR) and regio-random (RRa) poly(3-hexylthiophene) (P3HT) using elastic, quasi-elastic, spin-echo and inelastic neutron scattering techniques. We use deuteration to modulate the coherent and incoherent cross-sections of the materials, beyond a contrast variation purpose, allowing particularly to access both self-motions and collective dynamics of the materials. Measurements are underpinned by quantitative numerical simulations using classical molecular dynamics (MD) simulations, as well as molecular and periodic density functional theory-based quantum chemical (QC) calculations. MD simulations reproduced well the large structural features and slow motions but provided a limited description of molecular vibrations. MD shed light on differences in collective dynamics between Q-values linked with the π-π stacking and the lamellar stacking of the polymer, with the crystalline phase being the most impacted. Molecular QC described well the high-energy vibrational features, while periodic QC allowed to describe the mid-energy vibrational range. We show that this extensive combined approach of neutron-based measurements and multicomputational calculations allows us to fully map the structural dynamics of conjugated polymers such as P3HT. We further demonstrate[2,3]that the same methodology can be used to study blends of P3HT and fullerene derivatives for photovoltaic applications.
 A. A. Y. Guilbert, M. Zbiri et al., Chem. Mater. 2019, 31, 9635–9651.
 A. A. Y. Guilbert, M. Zbiri et al., J. Phys. Chem. Lett. 2016, 7, 2252–2257.
 A. A. Y. Guilbert, M. Zbiri et al., J. Phys. Chem. B 2017, 121, 9073–9080.
Host: Professor Helen Gleeson