Thermoelectric Enhancement in Single Organic Radical Molecules

Abstract

Organic thermoelectric materials have potential for wearable heating, cooling, and energy generation devices at room temperature. For this to be technologically viable, high-conductance (G) and high-Seebeck-coefficient (S) materials are needed. For most semiconductors, the increase in S is accompanied by a decrease in G. Here, using a combined experimental and theoretical investigation, we demonstrate that a simultaneous enhancement of S and G can be achieved in single organic radical molecules, thanks to their intrinsic spin state. A counterintuitive quantum interference (QI) effect is also observed in stable Blatter radical molecules, where constructive QI occurs for a meta-connected radical, leading to further enhancement of thermoelectric properties. Compared to an analogous closed-shell molecule, the power factor is enhanced by more than 1 order of magnitude in radicals. These results open a new avenue for the development of organic thermoelectric materials operating at room temperature.

Quantum Interference Controlled Spin-Polarized Electron Transmission in Graphene Nanoribbons

“…Apart from the electron transport, a few of the recent reports have also witnessed the spintronic applications of QI effects. An anomalous effect of QI in organic-radical based spin filters with the reversal of para-meta conductance was realized in our previous work which is recently certified by an experimental observation . Pal et al demonstrated the spin-polarized currents in a silver–vanadocene–silver single molecule junction stemming from the spin-dependent QI features .…”

“…An anomalous effect of QI in organic-radical based spin filters with the reversal of para-meta conductance was realized in our previous work 46 which is recently certified by an experimental observation. 47 vanadocene−silver single molecule junction stemming from the spin-dependent QI features. 48 53 Gehring et al experimentally demonstrated the QI effects in electronic transmission in graphene nanoconstrictions.…”

Flexible solar and thermal energy conversion devices: Organic photovoltaics (OPVs), organic thermoelectric generators (OTEGs) and hybrid PV-TEG systems

“…The obtained results revealed p-type conduction, 1.2 eV energy gap between the Fermi level and HOMO of BDT, and a positive linear relation between the Seebeck coefficient and the length of single molecules [426]. Recently, a combination of experimental and theoretical work revealed that the Seebeck coefficient and electrical conductance of single organic radical molecules can be simultaneously enhanced due to their intrinsic spin state [427]. 1,2,4-benzotriazin-4-yl (Blatter) radical molecules with pseudo-meta (meta) and pseudo-para (para) connections to two gold electrodes and the closed-shell (non-radical) quinazoline analogue have been designed to test this idea.…”

Redox‐Addressable Single‐Molecule Junctions Incorporating a Persistent Organic Radical**

“…[11,12] Despite the rich promise of radical-based molecular electronics, attempts to date to incorporate persistent organic radicals into single-molecule junctions at room temperature have resulted in loss of the open-shell character, [13] or in a modest enhancement of electrical conductance. [8,14] The significant increase in charge transport efficiency predicted by theoretical studies [6] has only been observed at cryogenic temperatures. [15,16] These observations are not entirely surprising, as the open-shell molecular components used in previous studies had either a highly delocalised radical state (e.g.…”

“…a resonance) at the Fermi level results in high charge transport efficiency. The high slope of T(E) vs E near the Fermi energy offered by radical systems is also predicted to give rise to enhanced Seebeck coefficients, [8] leading to enticing prospects for the design of molecular thermoelectric materials, [6,9,10] whilst the presence of an unpaired electron provides the basis from which to extend electronics to spintronics. [11,12] Despite the rich promise of radical-based molecular electronics, attempts to date to incorporate persistent organic radicals into single-molecule junctions at room temperature have resulted in loss of the open-shell character, [13] or in a modest enhancement of electrical conductance.…”