We demonstrate that rectification
ratios (RR) of ≳250 (≳1000) at biases of 0.5 V (1.2
V) are achievable at the two-molecule limit for donor–acceptor
bilayers of pentacene on C<sub>60</sub> on Cu using scanning tunneling
spectroscopy and microscopy. Using first-principles calculations,
we show that the system behaves as a molecular Schottky diode with
a tunneling transport mechanism from semiconducting pentacene to Cu-hybridized
metallic C<sub>60</sub>. Low-bias RRs vary by two orders-of-magnitude
at the edge of these molecular heterojunctions due to increased Stark
shifts and confinement effects