Roles of non-local electron-phonon coupling on the electrical conductivity and Seebeck coefficient: a TD-DMRG study

Abstract: 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, non-local electron-phonon coupling plays a vital role in charge transport and leads to complex transport mechanisms, including hopping, phonon-assist current, band, and transient localization. In this work, based on time-dependent density matrix renormalization group (TD-DMRG) method, we look at the role of non-local electron-phonon coupling on the thermoelectric conversion in organic systems described by Holstein-Peierls model. We calculate the current-current correlation and the heat current-current correlation functions. We find that (i) non-local electron-phonon coupling has very weak influence on Seebeck coefficient because of the cancellation between heat current-current correlation function and the current-current correlation function, but it has strong influence on conductivity through dynamic disorders; (ii) doping concentration has a strong influence on both conductivity and Seebeck coefficient significantly, and the optimal doping ratio to reach the highest power factor is 3%-10% fillings.