Empirical Parameter to Compare Molecule-Electrode Interfaces in Large-Area Molecular Junctions

This paper describesa simple model for comparing the degree ofelectronic coupling between molecules and electrodes across differentlarge-area molecular junctions. The resulting coupling parameter canbe obtained directly from current-voltage data or extractedfrom published data without fitting. We demonstrate the generalizabilityof this model by comparing over 40 different junctions comprisingdifferent molecules and measured by different laboratories. The resultsagree with existing models, reflect differences in mechanisms of chargetransport and rectification, and are predictive in cases where experimentallimitations preclude more sophisticated modeling. We also synthesizeda series of conjugated molecular wires, in which embedded dipolesare varied systematically and at both molecule-electrode interfaces.The resulting current-voltage characteristics vary in nonintuitiveways that are not captured by existing models, but which produce trendsusing our simple model, providing insights that are otherwise difficultor impossible to explain. The utility of our model is its demonstrativegeneralizability, which is why simple observables like tunneling decaycoefficients remain so widely used in molecular electronics despitethe existence of much more sophisticated models. Our model is complementary,giving insights into molecule-electrode coupling across seriesof molecules that can guide synthetic chemists in the design of newmolecular motifs, particularly in the context of devices comprisinglarge-area molecular junctions.