Validating the Mott Formula with Self-Assembled Monolayer (SAM)-Based Large-Area Junctions: Effect of Length, Backbone, Spacer, Substituent, and Electrode on the Thermopower of SAMs

Abstract

Understanding how the Seebeck effect of organic thermoelectric devices is associated with the chemical structure of active molecules within the devices is a key goal in organic and molecular thermoelectrics. This paper describes a series of physical-organic studies that investigate structure-thermopower relationships in self-assembled monolayers (SAMs) through measurements of the Seebeck coefficient (S, mu V/K) using the eutectic gallium-indium (EGaIn)-based junction technique. Several hypotheses were derived from a transmission function-based simple toy model, the Lorentzian transmission function-based Mott formula. These hypotheses were tested by comparing values of S for simple alkyl and aryl molecules with different structures F- in terms of backbone, length, spacer, anchor, and substituent, and for different electrodes (Au vs Ag), and by monitoring responses of S to the structural modifications. Experimentally obtained S values were further reconciled with values simulated by the Mott formula and with interfacial electronic structure and molecule-electrode coupling strength, independently measured by ultraviolet photoelectron spectroscopy and transition voltage spectroscopy.