Current-voltage characteristics in donor-acceptor systems: Implications of a spatially varying electric field

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 nonequilibrium properties are calculated using Landauer’s formalism. The current-voltage characteristics are found to be asymmetric with a sharp negative differential resistance (NDR) 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.

Figure 1

The current-voltage characteristics for the 2-site system for ${ϵ}_2=-{ϵ}_1=0.5\mathrm{eV}$ and $t=0.1\mathrm{eV}$. Inset is the Tour molecule.

Figure 2

(a) The variation of the two levels of the 2-site system with the applied bias, for ${ϵ}_2=-{ϵ}_1=0.5\mathrm{eV}$ and $t=0.1\mathrm{eV}$. The dotted lines indicate the variation of the Fermi energies of the electrodes with bias. (b) The numerator of the Greens function matrix element for the corresponding energy levels shown in (a). 1 and 2 represent the site index and $k$ specifies the corresponding level. (c) The ${\mathrm{IPR}}_{\mathit{av}}$ for the levels shown in (a). See the text for definition.

Figure 3

The $I\text{−}V$ characteristics of a 20 sites system, for ${ϵ}_2=-{ϵ}_1=0.5\mathrm{eV}$ and $t=1.0\mathrm{eV}$. The inset shows the variation of the number density at the even sublattice for case i (triangles) and case ii (diamonds) discussed in the text. Similarly in the odd sublattice for case i (circles) and case ii (stars). Also shown are the pictorial representations of the corresponding eigenfunctions in site basis, very close to the critical bias. Filled circles indicate a large contribution to the eigenstate.

Figure 4

The $I\text{−}V$ characteristics for the 20 site system with ${ϵ}_2=-{ϵ}_1=0.5\mathrm{eV}$ and $t=1\mathrm{eV}$ with ramp potential (diamonds), $l=2$ drop (plus), and $l=8$ drop (circles) close to the interface. The inset shows the variation of the HOMO-LUMO gap with bias for the three cases.