Molecular-scale thermoelectricity: a worst-case scenario.

Area of Science:

• Molecular electronics
• Thermoelectric materials
• Quantum transport

Background:

• Thermoelectric performance in single-molecule junctions is sensitive to fluctuations in frontier orbital energies relative to the electrode Fermi energy (E_F).
• A 'worst-case scenario theorem' indicates that junction asymmetry is crucial for non-zero average Seebeck coefficients (〈S〉) under significant energy fluctuations.

Purpose of the Study:

• To explore a novel strategy for designing molecules with high, fluctuation-resilient thermoelectric performance.
• To investigate the role of molecular and junction symmetry in optimizing thermoelectric transport coefficients.

Main Methods:

• Theoretical simulations using density functional theory (DFT) on 17 diverse molecules.
• Analysis of thermoelectric properties, focusing on the Seebeck coefficient and transmission coefficients at HOMO and LUMO resonances.
• Modeling the impact of energy fluctuations and symmetry breaking on charge transport.

Main Results:

• Junction asymmetry is confirmed as a necessary condition for high thermoelectric performance in the presence of energy fluctuations.
• Symmetric molecules with localized frontier orbitals can outperform asymmetric ones by minimizing contributions from 'silent' orbitals under slight symmetry breaking.
• Demonstrated high thermoelectric performance in symmetric 1,2,3-triazole, rotaxane-hexayne macrocycle, and phthalocyanine molecules.

Conclusions:

• Molecular design for robust thermoelectric performance can leverage junction asymmetry, even with symmetric molecules.
• Localized frontier orbitals in symmetric molecules offer a pathway to enhanced thermoelectric efficiency by directing transport.
• This strategy provides a new avenue for developing advanced thermoelectric materials for energy conversion applications.