Abstract
Simultaneous engineering of electron and phonon transport through nanoscale molecular junctions is fundamental to the development of high-performance thermoelectric materials for the conversion of waste heat into electricity and cooling. Here, we demonstrate a systematic improvement of the room-temperature thermoelectric figure of merit (ZT) of molecular junctions. This is achieved by phonon interference (PI)-suppressed thermal conductance and quantum interference-enhanced electrical conductance and Seebeck coefficient. This strategy leads to a significant enhancement of ZT from low values ca. 10–6 in oligo(phenylene-ethynylene) (OPE2) to the record values of 2.4 in dinitro-functionalized OPE2 (DOPE2). The dinitro functionalization also considerably enhances ZT of biphenyl-dithiol (BDT) and bipyridyl molecular junctions. Remarkably, the energy levels of electron-withdrawing nitro groups are hardly changed from one molecule to the other. Because of this generic feature, a resonance transport in the vicinity of Fermi energy of electrodes is formed leading to a significant improvement of Seebeck coefficient and ZT of all derivatives. For example, the Seebeck coefficient enhances from 10.8 μV/K in BDT to −470 μV/K in dinitro-BDT (DBDT). In addition, destructive PI due to the nitro groups suppresses phonon thermal conductance, for example, from 20 pW/K in BDT to 11 pW/K in DBDT at room temperature. We also demonstrate that quantum and PI-enhanced single-molecule thermoelectric efficiency is conserved when parallel molecules are placed between gold electrodes. These results promise to remove the key roadblocks and open new avenues to exploit functionalized organic molecules for thermoelectric energy harvesting and cooling.
Highlights
Nearly 10% of the world’s electricity is used by computers and the internet
At the level of fundamental science, it was demonstrated recently that molecular wires can mediate longrange phase-coherent tunneling with remarkably low attenuation over sub-nanometer distances even at room temperature.[7]. This creates the possibility of using quantum interference (QI) in single- or few-molecular junctions[8−11] to engineer enhancement of thermoelectricity in molecular materials[12,13] and design new high-performance organic materials.[14−16] The aim of this paper is to demonstrate that room-temperature QI (RTQI) of electrons can be employed to obtain high-G, high-S materials, and simultaneously roomtemperature phonon interference (RTPI) can be used to suppress phonon thermal transport
Transport through the biphenyl-4,4′-dithiol (BDT) molecule is mainly through the highest occupied molecular orbital (HOMO) in agreement with the previous reports.[20−22] The measured conductance of parent BDT22,26 is in the range of 3 to 7.5 × 10−3 G0 in agreement with our calculation shown in Figure 2a for a wide range of energy around density functional theory (DFT) Fermi energy (EF = 0)
Summary
Nearly 10% of the world’s electricity is used by computers and the internet. This is expected to double over the decade. Most of this energy is converted to heat. This waste heat could be used to generate electricity economically, provided materials with a high thermoelectric efficiency could be identified.[1] efficient Peltier cooling using such materials would have applications to on-chip cooling of CMOS-based devices.[1] The demand for new thermoelectric materials has led to a worldwide race to develop materials with a high thermoelectric efficiency.[2] The efficiency of a thermoelectric device for power generation is characterized by the dimensionless figure of merit[3] ZT = GS2T/κ, where G is the electrical conductance, S is the Seebeck coefficient (thermopower), T is temperature, and κ = κel + κph is the thermal conductance[4] due to electrons κel and phonons κph. The interdependency of transport coefficients constrains the options for materials design and makes optimization a difficult task
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
