Organic semiconductors (OSCs) represent the next generation of advanced optoelectronic materials. These materials are readily synthesized in solution under standard benchtop conditions, enabling an array of cost-effective routes towards high-yield syntheses and facile device fabrication. However, the performance of these materials, with charge carrier mobilities ranging from 0.1 – 40 cm2 V-1 s-1, is significantly lower than inorganic semiconductors with mobilities on the order of 103 – 104 cm2 V-1 s-1. Historically, it has been understood that thermally-induced large-amplitude intermolecular dynamics create inherent disorder in the electronic couplings throughout the lattice, which strongly hinders charge transport. Specifically, most OSCs exhibit weak intermolecular forces (i.e., van der Waals interactions) that ultimately result in low-frequency (terahertz) vibrational modes, which are often thermally excited and exhibit intermolecular motions. These dynamics produce fluctuations in the overlaps of the electronic wavefunctions, generating dynamic electronic disorder commonly referred to as electron-phonon coupling. This phenomenon has a demonstrable destructive effect upon charge carrier dynamics, and thus its comprehension has powerful implications for material design and optimization. In this work, terahertz time-domain spectroscopy is used to directly measure the low-frequency vibrational modes in a series of promising OSCs, including crystals specifically engineered to mitigate adverse mode-types. Coupling the experimental measurements to periodic density functional theory (DFT) simulations enables the assignment of the experimental spectra, and the obtained vibrational modes are used as the fundamental basis for determining a mode-resolved picture of electron-phonon coupling in the investigated materials. Additionally, through the use of crystalline orbital Hamiltonian population (COHP) calculations, we have resolved contributions from individual atomic orbitals that are most pivotal to detrimental electron-phonon coupling. Overall, this work provides an unprecedented level of insight into the origins of semiconducting behavior in this important class of materials.

Untangling the dynamic disorder of organic semiconductors with terahertz spectroscopy

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