02/12/2020

Mapping the microscopic behaviour of conjugated polymers



Conjugated polymers have attracted broad academic and industrial interest over recent years for their potential use in a range of applications. Made up of a complex configuration of carbon and hydrogen, conjugated polymers are characterised by their unique backbone chain of carbon atoms in alternating single and double bonds.

This orientation enables electrons to flow along the chain axis, which leads to their intrinsic electrical and subsequent optical properties. As semiconductors with optoelectronic properties, they are promising for electronic devices and chemical engineering for energy applications that use or control light.

"Conjugated polymers can, for instance, be used in the fabrication of light emitting diodes, and has potential to be used as a major blend component to design light weight, low-cost and flexible organic solar cells. These materials have also been found to be promising candidates for the development of efficient metal-free photocatalytic water splitting technology", says Dr Anne Guilbert from Imperial College London.

As published in Chemistry of Materials, researchers from Institute Laue-Langevin (ILL), Imperial College London, and Queen Mary University of London, used the unique capabilities of instruments at ILL to map the structural dynamics of a specific conjugated polymer, P3HT. This work, highlighted as ACS Editor's Choice and selected as front cover of the December issue of Chemistry of Materials, helps to form a foundation for a better understanding of these polymers and their microscopic behaviour.

Using neutron scattering techniques, the researchers were able to both map where the atoms were placed and arranged, as well as what the atoms were doing over time, to gain a deeper understanding of the factors ultimately affecting the optoelectronic properties of the material. These dynamics can be complex and overlapping with different processes taking place from a femtosecond to a microsecond timescale.

"We made a synergistic use of the unique capabilities of the ILL instruments to perform this study. The suit of spectrometers IN1-Lagrange, IN6, IN11 and IN16B were used to probe different dynamics, i.e. relaxation and vibrations, occurring at different time scales ranging from the picosecond to nanosecond. The diffractometer D16 was used to look at the structure and its large scale characteristic. We should stress here the key role of the extensive multi-approach and multi-scale numerical simulations we carried out, which helped to underpin the various detailed neutron measurements" ", says Dr. Mohamed Zbiri from the ILL.

Polymers are soft materials and their optoelectronic properties are affected by the dynamical degrees-of-freedom of their microstructure, including vibrations, rotations and diffusive motion. For example, fast-motions like vibrations are shown to impact the transport and charge separation processes. Slower dynamics, such as side-chain reorientation and backbone torsion, can impact the efficiency of the optoelectronic processes.

P3HT is a reference in the field and it is one of the most studied conjugated polymers. The researchers explored the differences between two distinct types of P3HT. Regiorandom-P3HT (RRa-P3HT) and Regioregular-P3HT (RR-P3HT) were analysed in different phases and, as such, scientists were able to validate that Regiorandom-P3HT (RRa-P3HT) can be used as a model for the amorphous phase of Regioregular-P3HT (RR-P3HT). These behaviours have been largely assumed, but overlooked in previous research.

As RR-P3HT is a semi-crystalline polymer, for a film of RR-P3HT - such as those used in printed electronics - there will be a crystalline (ordered) phase and an amorphous (disordered) phase. As such, the clarification made through this research will be particularly useful for future study in materials for application. In the study, deuteration was used to reveal the structural components of P3HT, and associated dynamics.

"The structure of such organic material is complex and mainly made of carbons and hydrogens. Neutrons are quite sensitive to hydrogen and the signal from the hydrogenated parts of the material is too strong and "mask" the signal from the rest of the molecules. This is a unique and a fabulous feature of neutron scattering, since by substituting hydrogen with its isotope deuterium - whose neutron signal is much weaker than that of hydrogen - we can "switch on and off" different parts of the material, which helps to better "see" the different structural components", says Dr Zbiri.

"This progress is an exciting endeavour for conjugated polymers. Not only have we successfully mapped the structural dynamics of P3HT, but we should stress here the role of the extensive multi-approach in helping us achieve our goal. By complementing optical spectroscopy techniques with inelastic neutron scattering, we were able to underpin behaviour at a microscopic scale", says Dr Guilbert.

» Original publication

Source: Institut Laue-Langevin (ILL)




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