Impact of Low‐Frequency Vibrations on Charge Transport in High‐Mobility Organic Semiconductors

Abstract

Despite decades of intensive studies of charge transport in organic semiconductors (OSs), understanding of mechanisms underpinning efficient charge transport in them remains elusive. Recently, it has been suggested that low‐frequency (LF) vibrations are a limiting factor of charge transport in high‐mobility OSs. Nevertheless, the relationship between the molecular structure, crystal packing, LF vibrations, and charge transport is still obscured. This hinders the focused search of high‐mobility OSs so that researchers rely mainly on trial‐and‐error method. This review presents theoretical and experimental approaches to studying the LF vibrations and their role in charge transport with a focus on recent results. It is anticipated that tight cooperation between experimentalists and theorists will yield an advanced understanding of LF vibrations in OSs and their impact on charge transport. This will guide the design of novel high‐mobility organic semiconductors for organic electronics.

Impact of N-substitution on structural, electronic, optical, and vibrational properties of a thiophene–phenylene co-oligomer

“…As mentioned above, LF Raman spectra are sensitive to the crystal structure and hence can monitor its changes. 46,56,57 Fig. 4 shows that the LF bands for CF 3 -PTzTzP-CF 3 have a considerably lower intensity than those for CF 3 -PTTP-CF 3 .…”

“…It was previously suggested that the LF Raman spectra are related to vibrational modulation of the charge transport integrals and the molecular polarizability: the more intense the LF Raman signal, the stronger this modulation. 46,58 Thus, the decrease in the LF Raman intensity can be attributed to a weaker impact of the corresponding vibrations on the charge transfer integrals between the molecules, i.e., the lower dynamic disorder. The latter is favorable for charge transport.…”

“…This decrease is expected to be favorable for charge transport. 46,47,59,60 Thus, the uorination and N-substitution not just increase the charge-transfer integrals but also inhibit their vibrational modulation.…”

Anharmonic Lattice Vibrations in Small‐Molecule Organic Semiconductors

“…5,23,[25][26][27][28] The anharmonic components of the lattice vibrations, i.e., phonon-phonon interactions 29 are therefore entirely neglected. However, organic crystals are known to be mechanically soft 30 , exhibit large molecular displacements 31 and have large thermal expansion coefficients 32 all of which are indicative of strong lattice anharmonicity. 33,34 Strongly anharmonic lattice dynamics are expected to have a disruptive effect on the electronic coupling between the molecules, and therefore they may profoundly impact charge transport in the organic crystal.…”

Computational Screening of Organic Semiconductors: Exploring Side-Group Functionalisation and Assembly to Optimise Charge Transport in Chiral Molecules

“…21,22 The relationship between molecular structure, crystal packing, low-frequency vibrations and charge transport still poses many open questions and there would be significant value in developing a framework to design these materials. 23,24 We are particularly interested in chiral helicene molecules as potential OSCs. Helicenes are axially fused benzene rings, as shown for [6]helicene in Figure 1a.…”