Inverse Rectification in Donor–Acceptor Molecular Heterojunctions

The transport properties of a junction consisting of small donor–acceptor molecules bound to Au electrodes are studied and understood in terms of its hybrid donor–acceptor–electrode interfaces. A newly synthesized donor–acceptor molecule consisting of a bithiophene donor and a naphthalenediimide acceptor separated by a conjugated phenylacetylene bridge and a nonconjugated end group shows rectification in the reverse polarization, behavior opposite to that observed in mesoscopic p–n junctions. Solution-based spectroscopic measurements demonstrate that the molecule retains many of its original constituent properties, suggesting a weak hybridization between the wave functions of the donor and acceptor moieties, even in the presence of a conjugated bridge. Differential conductance measurements for biases as high as 1.5 V are reported and indicate a large asymmetry in the orbital contributions to transport arising from disproportionate electronic coupling at anode–donor and acceptor–cathode interfaces. A semi-empirical single Lorentzian coherent transport model, developed from experimental data and density functional theory based calculations, is found to explain the inverse rectification.