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

Abstract

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 study, first a mode‐resolved picture of electron–phonon coupling in a series of materials that are specifically designed to suppress detrimental vibrational effects, is generated. From this foundation, a method is developed based on the crystalline orbital Hamiltonian population (COHP) 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.

Lattice Dynamics: The Unexplored Multidimensional Dynamic Playground of Molecular Crystalline Materials

“…The studies on crystalline organic semiconductors have been burgeoning thanks to their easy processability, low cost, lightweight, high tunability, and inherent mechanical flexibility ̵making them ideal candidates for a broad range of electronic and optoelectronic applications. − However, the success of this class of materials is jeopardized by low charge carrier mobilities that rarely exceed 10 cm 2 V –1 s –1 , making it difficult for them to compete with their inorganic counterparts. , To overcome these limitations, controlling and reducing dynamic structural disorder has been identified as a key parameter to suppress detrimental electron–phonon coupling and, hence, to increase the charge transport properties of organic semiconductor crystals. , To do so, preliminary studies have highlighted how molecular, crystal, and lattice dynamics engineering strategies can be devised to suppress specific low-frequency vibrations (below 100 cm –1 ) that are highly excited at room temperature and seem to be responsible for most of the detrimental dynamic disorders of organic semiconductors. , However, the field is still in its infancy, and major interdisciplinary efforts combining chemists, solid state physicists, and materials scientists will be needed tackle this challenge.…”

Engineered solid-state aggregates in brickwork stacks of n-type organic semiconductors: a way to achieve high electron mobility

“… 28 The significance of such planar interactions and the resultant rigid frameworks in reducing the lattice vibrations and electron scattering would be essential for better carrier transport and electron mobility. 14 This report addresses the importance of such 3-D structural motifs in self-assembled structures of molecular semiconductors.…”

“…There are some recent reports in the abovementioned directions. [14][15][16][17][18] Some molecular semiconductors and OFETs derived from them have exhibited benchmark performance in laboratory scale devices. Most of them fall into p-type materials.…”