Tuning Thermoelectricity in Molecular Junctions via Quantum Interference*

Studies of molecular junctions, created by trapping a single molecule or multiple molecules between metallic electrodes, not only provide fundamental knowledge of charge transport at the atomic scale, but also offer unique opportunities in developing molecule-based devices for energy conversion. It has been theoretically proposed that quantum interference effects can be employed to obtain impressive thermoelectric performance in molecular junctions. Toward this goal, we took advantage of the destructive interferences which arise in conjugated molecules to allow high-energy electrons to pass while blocking low-energy electrons. Specifically, we investigated this effect in oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring (para-OPE3) and meta-connected central ring (meta-OPE3). Experiments on both single molecules and monolayers reveal a two-fold increase in thermopower in meta-OPE3 (∼20 μV/K) junctions compared with para-OPE3 (∼10 μV/K) junctions, which agrees with our initio modeling. Our results illustrate how enhancements in thermopower can be achieved in molecular junctions via quantum interferences.