Untangling the Fundamental Electronic Origins of Non-Local Electron-Phonon Coupling in Organic Semiconductors

Organic semiconductors with distinct molecular properties and large carrier mobilities are constantly developed in attempt to produce highly-efficient electronic materials. Recently, designer molecules with unique structural modifications have been expressly developed to suppress molecular motions in the solid state that arise from low-energy phonon modes, which uniquely limit carrier mobilities through electron-phonon coupling. However, such low-frequency vibrational dynamics often involve complex molecular dynamics, making comprehension of the underlying electronic origins of electron-phonon coupling difficult. In this work, we first generate a mode-resolved picture of electron-phonon coupling in a series of materials that were specifically designed to suppress detrimental vibrational effects. From this foundation, we develop a method based on the crystalline orbital Hamiltonian population analyses to resolve the origins -- down to the single atomic-orbital scale -- of surprisingly large electron-phonon coupling constants of particular vibrations, explicitly detailing the manner in which the intermolecular wavefunction overlap is perturbed. Overall, this approach provides a comprehensive explanation into the unexpected effects of less-commonly studied molecular vibrations, revealing new aspects of molecular design that should be considered for creating improved organic semiconducting materials.