The Seebeck effect of molecular junction in a hopping regime or tunneling-to-hopping transition remains uncertain. This paper describes the Seebeck effect in molecular epitaxy films (OPI_n where n = 1 – 9) based on imine condensation between aryl amine and aldehyde and investigates how Seebeck coefficient (S, μV/K) varies at the crossover region. The S value of OPI_n linearly increased with increasing the molecular length (d, nm), ranging from 7.2 to 38.0 μV/K. The increasing rate changed from 0.99 to 0.38 μV∙K⁻¹Å⁻¹ at d = 3.4 nm (OPI₄). Combined experimental and theoretical studies indicated that such a change stems from tunneling-to-hopping transition, and the small but detectable length-dependence of thermopower in the long molecules originates from the gradual reduction of tunneling contribution to the broadening of molecular orbital energy level, rather than its relative position to Fermi level. Our work helps to bridge the gap between bulk and nanoscale thermoelectric systems.

https://pubs.acs.org/doi/10.1021/acs.nanolett.2c03083