Abstract
If high efficiency organic thermoelectric materials could be identified, then these would open the way to a range of energy harvesting technologies and Peltier coolers using flexible and transparent thin-film materials. We have compared the thermoelectric properties of three zinc porphyrin (ZnP) dimers and a ZnP monomer and found that the "edge-over-edge" dimer formed from stacked ZnP rings possesses a high electrical conductance, negligible phonon thermal conductance and a high Seebeck coefficient of the order of 300 μV K-1. These combine to yield a predicted room-temperature figure of merit of ZT ≈ 4, which is the highest room-temperature ZT ever reported for a single organic molecule. This high value of ZT is a consequence of the low phonon thermal conductance arising from the stacked nature of the porphyrin rings, which hinders phonon transport through the edge-over-edge molecule and enhances the Seebeck coefficient.
Highlights
Thermoelectric materials, which convert heat to electrical energy, could have an enormous impact on global energy consumption, but at present their efficiency is too low and the most efficient materials are toxic and have limited global supply
We present a comparative theoretical study of the thermoelectric properties of four different zinc porphyrin structures and elucidate a new strategy for simultaneously increasing their thermopower and reducing their thermal conductance leading to a high value of ZT
The geometry of each structure consisting of gold electrodes and a single zinc porphyrin molecule was relaxed to a force tolerance of 20 meV Å−1 using the SIESTA28 implementation of density functional theory (DFT), with a double-ζ polarized basis set (DZP) and generalized gradient functional approximation (GGA-PBE) for the exchange and correlation functionals,[30,31] which is applicable to arbitrary geometries
Summary
Thermoelectric materials, which convert heat to electrical energy, could have an enormous impact on global energy consumption, but at present their efficiency is too low and the most efficient materials are toxic and have limited global supply. In an effort to overcome these limitations, thermoelectric effects in low-dimensional structures and molecular-scale systems have begun to be investigated.[1,2,3,4,5,6,7,8,9,10,11,12,13,14] Nanostructures are promising, because transport takes place through discrete energy levels and in molecular-scale junctions, this leads to room-temperature quantum interference, which opens further avenues for enhancing the conversion of heat into electrical energy.[15]. If the Fermi energy lies in a region of high slope, close to a transmission resonance both G and S are enhanced.[20] On the other hand, to decrease the thermal conductance κ, which appears in the denominator of ZT, both electron and phonon transport must be engineered. Simultaneous consideration of both electron and phonon transport is needed to develop new materials for thermoelectricity
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