Investigating charge mobility of alternating copolymers: The role of comonomers and electron-lattice interaction

Unraveling the intricate interplay between charge carriers and molecular vibrations is vital to enhancing charge transport in organic semiconductors. In this study, we employ a tight-binding model calibrated with density functional theory (DFT)-derived parameters to investigate the influence of intermolecular vibrations on the charge transport properties of two specific copolymers: Thienothiophene-Phenylene (Tt-Ph) and Thiophene-Pyrrole (Th-Py). Our findings reveal the emergence of self-trapped excitations with soliton-like profiles, whose mobility is primarily governed by both the electron-lattice interaction and the on-site energies of the comonomers. Stronger electron-lattice coupling reduces the velocity of such hybridized excitations, eventually rendering them dynamically localized (localized) for electron-lattice interactions above a critical strength χ_c. Extending the analysis to potential alternating copolymer chains, the power-law fitting $v_s\propto {\left({\chi }_c-\chi \right)}^{1∕2}$ for the velocity of soliton-like modes suggests a universal behavior of the underlying charge-lattice coupling. Furthermore, we unveil a nonlinear dependence between the critical carrier-lattice interaction χ_c and the effective difference in on-site energies of the comonomers, underscoring the delicate interplay between copolymer structure and charge-lattice interactions.