Bias-Polarity-Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox-Active Molecular Junction

This paper describes the transition from the normal to inverted Marcus region in solid-state tunnel junctions consisting of self-assembled monolayers of benzotetrathiafulvalene (BTTF), and how this transition determines the performance of a molecular diode. Temperature-dependent normalized differential conductance analyses indicate the participation of the HOMO (highest occupied molecular orbital) at large negative bias, which follows typical thermally activated hopping behavior associated with the normal Marcus regime. In contrast, hopping involving the HOMO dominates the mechanism of charge transport at positive bias, yet it is nearly activationless indicating the junction operates in the inverted Marcus region. Thus, within the same junction it is possible to switch between Marcus and inverted Marcus regimes by changing the bias polarity. Consequently, the current only decreases with decreasing temperature at negative bias when hopping is “frozen out,” but not at positive bias resulting in a 30-fold increase in the molecular rectification efficiency. These results indicate that the charge transport in the inverted Marcus region is readily accessible in junctions with redox molecules in the weak coupling regime and control over different hopping regimes can be used to improve junction performance.Y.H., C.N., M.S.M., and S.K.K. contributed equally to this work. The authors acknowledge the Ministry of Education (MOE) for supporting this research under award no. MOE2019-T2-1-137. Prime Minister's Office, Singapore under its Medium sized centre program is also acknowledged for supporting this research. The authors also gratefully acknowledge the Australian Synchrotron (ANSTO) soft X-ray spectroscopy beamline where the SAM characterization was conducted. C.N. and E.d.B. acknowledge support from the US National Science Foundation (grant no. ECCS#1916874). D.Q. acknowledges the support of the Australian Research Council (Grant No. FT160100207). Authors also acknowledge the Marie-Sklodowska-Curie project ITN iSwitch (GA-642196), Spanish Ministry of Economy and Competitiveness (project FANCY CTQ2016-80030-R, GENESIS PID2019-111682RB-I00 and Severo Ochoa programme for Centers of excellence in R&D (SEV-2015-0496)). J.C. is a research director of the Belgian National Fund for Scientific Research (FNRS).Peer reviewe