Identifying molecular processes involved in the electrical conductivity aspects of organic semiconductors

Organic semiconductor-based electronic devices offer societal and industrially relevant advantages over more commonly used inorganic ones. Crucial to improving their efficiency is understanding their fundamental behaviour at the microscopic level, according to Dr. Mohamed Zbiri from the Institut Laue-Langevin (ILL). Dr. Anne Guilbert from Imperial College London and Dr. Zbiri contributed the neutron scattering side of a collaborative project involving researchers from the University of Mons, the French National Centre for Scientific Research (CNRS), the Elettra and Consiglio Nazionale delle Ricerche (CNR) and led by a team coordinated by Prof. Emanuele Orgiu from the University of Strasbourg, and the Institut National de la Recherche Scientifique in Québec. This work examined the role of the inherent disorder in organic molecular crystals on their electrical conductivity and has recently been published in Advanced Materials.

 “Organic semiconductors are low cost, have limited toxicity and are easy to synthesise and process. As they are remarkably flexible and stretchable, they are useful under a wide range of operating conditions. Their potential applications include organic photovoltaics (OPV), organic field-effect transistors (OFET), biosensors and organic light emitting diodes (OLED). Some of these, such as OLED-based smart phone displays and more recently TVs, are now commercially available”, says Dr. Zbiri.

Other applications of organic semiconductors, however, have not yet reached the market. “Understanding the different molecular processes involved in the electrical conductivity of organic semiconductors could allow for the electronic behaviour of these devices to be better controlled from their chemical structure.”, says Dr. Guilbert.

To understand why two molecules with similar chemical structures might show a marked difference in how they conduct electricity, the researchers used various experimental techniques and simulations to study thoroughly the crystals of two small-molecule organic semiconductors with closely related chemical structures, but with different crystal structures.

“The aim was to picture the way electrons move in organic semiconductors from one molecular site to another, thereby promoting electronic conductivity”, says Dr. Zbiri.

As Dr. Guilbert explains, “electronic conductivity occurs via electrons ‘hopping’ between molecules. The frequency of these hopping events depends on the distance between molecules and their relative orientation”. In crystals, molecular distance and relative orientation are well defined, making them ideal to study the hopping process. At the length of the constituents of matter, molecules do not form solids through bonding, but have weak long-range interactions. Therefore, they can vibrate relatively freely around their equilibrium position. These vibrations impact the distance and relative orientation of the molecules over a very short timescale, sometimes promoting and often inhibiting the hopping of electrons. Depending on the arrangement of the crystals, vibrations will be easier in one direction than the other, according to Dr. Guilbert.

“In contrast to techniques like optical spectroscopy, where light is used as the probe and interact with the optical and electronic processes taking place in the material, neutrons do not interfere with the electronic behaviour of materials. Organic semiconductors absorb light and can emit light. Thus, using optical spectroscopy can be limited either by the absorption range of the material or by the measured signal containing a contribution from subsequent light emission of the material. Therefore, establishing neutron spectroscopy, like inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS), to study dynamics of organic semiconductors is crucial,” says Dr. Zbiri.

INS was used to characterise the vibrations of the crystals of two small-molecule semiconductors. Simulations were used to help assigning the measured vibrational modes and identifying the particular ones detrimental to the charge transport.

“As the materials being studied were molecular crystals, they consisted of molecules arranged following specific symmetry rules within a lattice. From a vibrational point of view, we can distinguish two aspects here, one related to the lattice and the other one related to the molecules themselves. Inelastic neutron scattering allows to ‘see’ all the molecular and lattice vibrational features. Neutrons probe how the atoms and the molecules interact by measuring the interatomic forces they are subject to, inducing their motions. The length and time scales associated with neutron spectroscopy match perfectly processes at the atomic and molecular levels,” explained Dr. Zbiri.

According to Prof. Orgiu, “our findings underline that there is a strong correlation between the molecular vibrations taking place at low energies, the molecular structure and the ability of the very same molecules to transport charge carriers when integrated as the active layer in a device.” Particularly, the team was able to understand how small differences in the chemistry of the two materials directly impacted their ability to conduct electricity, by ‘locking’ vibrations in one direction compared to another. As a result, this work suggests that combining crystal design with an understanding of the internal molecular chemistry of semiconductors can be a powerful way to improve the design of organic semiconductors.

The neutron spectroscopy work keeps progressing further by studying other organic semiconductors for new applications, such as biosensors and photocatalysis. In a recent separate study published in Physical Chemistry Chemical Physics, Dr. Guilbert, Dr. Zbiri and researchers from Queen Mary University of London and the University of Liverpool used INS and QENS to study bespoke semiconductors that combined biological (ionic) signalling with electronic conductivity.

According to Dr. Guilbert, “for such applications, we are looking at transducing a biological signal in an electronic signal. Biological systems do not rely on electrons but rather on ions to conduct signals. One part of the molecule is designed for electronic conductivity while the other is designed for ionic conductivity. Electronic conductivity is favoured by order and is governed partly, as we showed, by lattice vibrations. However, ionic conductivity is favoured in disordered materials. This new study helped to further understand how the physical behaviours of different parts of the molecules were coupled. We identified structural changes impacting the dynamics of the part of the molecules responsible for ionic conductivity without impacting the other molecular part responsible for electronic conductivity”.

The team plans to continue this line of work to help providing design rules for integrating efficiently different electronic and ionic functionalities in the same material.

Ref 1: Analysis of External and Internal Disorder to Understand Band‐Like Transport in n‐Type Organic Semiconductors. Marc-Antoine Stoeckel et al. (2021).

Ref 2: Effect of substituting non-polar chains with polar chains on the structural dynamics of small organic molecule and polymer semiconductors. Anne A. Y. Guilbert et al. (2021).

ILL Instrument: The neutron spectroscopy measurements were performed using the IN6 spectrometer.

Contacts: Dr. Mohamed Zbiri, ILL – Dr. Anne Guilbert, Imperial College London