  We have studied the transport properties of a molecular device composed of
donor and acceptor moieties between two electrodes on either side. The device
is considered to be one-dimensional with different on-site energies and the
non-equilibrium properties are calculated using Landauer's formalism. The
current-voltage characteristics is found to be asymmetric with a sharp Negative
Differential Resistance at a critical bias on one side and very small current
on the other side. The NDR arises primarily due to the bias driven electronic
structure change from one kind of insulating phase to another through a highly
delocalized conducting phase. Our model can be considered to be the simplest to
explain the experimental current-voltage characteristics observed in many
molecular devices.
