Roles of nonlocal electron-phonon coupling on the electrical conductivity and Seebeck coefficient: A time-dependent DMRG study

Organic molecular materials are potential high-performance thermoelectric materials. Theoretical understanding of thermoelectric conversion in organic materials is essential for rational molecular design for efficient energy conversion materials. In organic materials, nonlocal electron-phonon coupling plays a vital role in charge transport and leads to complex transport mechanisms, including hopping, phonon assisted, band, and transient localization. In this work, based on the time-dependent density matrix renormalization group method, we look at the role of nonlocal electron-phonon coupling on the thermoelectric conversion in organic systems described by the Holstein-Peierls model. We calculate the current-current correlation and the heat current-current correlation functions. We find that (i) nonlocal electron-phonon coupling has a very weak influence on the Seebeck coefficient because of the cancellation between the heat current-current correlation function and the current-current correlation function, but it has a strong influence on the conductivity through dynamic disorders; and (ii) doping concentration has a strong influence on both the conductivity and Seebeck coefficient, and the optimal doping ratio to reach the highest power factor is 3%–10% fillings when the Holstein-Peierls model is valid. These findings suggest that we can design organic materials with higher power factors by first enhancing mobility through rational design, and then searching for the optimal doping ratio.