Chasing the “Killer” Phonon Mode for the Rational Design of Low‐Disorder, High‐Mobility Molecular Semiconductors

Abstract

The last decade has witnessed drastic improvements of the electronic properties, environmental and operational stability, and processibility of organic semiconductors (OSCs). [1,2] Designing new materials with high carrier mobilities, μ, remains one of the main research objectives to enable faster operation and lower power consumption of circuits and addressing of advanced liquid crystal and organic lightemitting diode displays. [1,3] Yet despite exploring a wide range of material systems, charge carrier mobilities in excess of 10 cm 2 V −1 s −1 have only been achieved in very few molecular semiconductors and highly aligned polymers. [4][5][6] At present, despite significant general advances in the comprehension of transport physics, a Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron-phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high-mobility molecular semiconductors, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electronphonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a "killer" phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high-mobility molecular semiconductors is suggested.

Anharmonic Lattice Vibrations in Small‐Molecule Organic Semiconductors

“…These fluctuations stem from both intermolecular and intramolecular vibrations, which contribute to the dynamic disorder . Contemporary theoretical studies are in agreement that low‐frequency (<150 cm −1 ) lattice vibrations dominate charge carrier mobility in organic crystals via nonlocal electron–phonon interactions …”

“…Therefore, it is imperative to characterize the degree of lattice anharmonicity in small‐molecule organic crystals. The main experimental methods for measuring the lattice dynamics to infer about anharmonicity are THz spectroscopy, inelastic neutron scattering, and Raman spectroscopy …”

Dynamical localization corrections to band transport

“…The applicability of the transient localization approach, however, is by construction restricted to the regime of strong dynamical disorder. Materials with reduced localization effects, either featuring more isotropic two-dimensional band structures 17 or lower degrees of disorder 18 , are actively investigated. It can be expected that future organic compounds will progressively move away from the strong disorder regime, entering a crossover region for which there is yet no available theoretical description.…”

Charge transport in high-mobility conjugated polymers and molecular semiconductors

“…This can be achieved by acting principally on two factors. The first is the energetic disorder σ, which is unavoidable even in perfectly crystalline systems, and mostly originates from the thermal fluctuations of the intermolecular transfer integrals 31,38,39 . The mobility decreases with dynamical disorder as a power law, μ∝(σ/J) −ν , with J setting the scale of the electronic bandwidth and ν a material-dependent exponent that has been calculated to be in the range 1.5 < ν < 3.5 (ref.…”

“…In this respect, it has been shown recently that combining experimental spectral probes together with a theoretical analysis of the individual vibrational modes can provide key information to understand and control molecular disorder at the microscopic level 40 . There is evidence that the thermal disorder is dominated by few, in some systems just a single vibrational, 'killer' modes 39 . If these modes could be targeted by molecular design and their contribution to thermal disorder reduced, this could result in significant improvements in performance.…”