Ohmic current in organic metal-insulator-metal diodes revisited

In the classical analysis of charge transport in solids by Lampert in 1956 an Ohmic current is attributed to the presence of a background carrier density due to (unintentional) doping. We demonstrate that Ohmic currents are also observed in undoped semiconductors as a result of diffusion of charge carriers from the contacts into the semiconductor. This Ohmic diffusion current shows an enhanced thickness scaling and is governed by the charge-carrier mobility. Specific for organic semiconductors the charge-carrier density dependence of the mobility at zero electric field can be accurately determined from the Ohmic diffusion current.

Figure 1

Temperature-dependent $J$-$V$ characteristics of a symmetric MEH-PPV (100 nm) device. The dashed and solid lines represent fits with Eqs. (2) and (1), respectively.

Figure 2

(a) $J$$\text{−}V$ characteristics of symmetric MEH-PPV devices of different layer thicknesses at 295 K. Open symbols represent data from symmetric Au/MEH-PPV/PEDOT:PSS devices (see Supplemental Material [6]). The current density is multiplied by $L$ (b) and $L^3$ (c), to show the layer-thickness dependence of the current. The inset shows a schematic band diagram of the device.

Figure 3

(a) Carrier-density distribution as a function of position in a 100 nm symmetric and asymmetric device. (b) Corresponding valence-band edge, indicating the band-bending parameter $b$′.

Figure 4

Mobility extracted from the linear regime [Eq. (9)] vs layer thickness (a) and effective carrier density (b) at 295 and 255 K. The solid lines represent the mobility-density relation of the EGDM at zero electric field with the mobility parameters determined by numerical fitting of the J-V characteristics.

Figure 5

Scaled J-V characteristics for a range of layer thicknesses, confirming the $1/L^3$ scaling of the current in combination with a density-dependent mobility.