Tuesday, May 18, 2004

[ODCAD] Junction and Vacuum Level in Organic Devices

[ODCAD] Junction and Vacuum Level in Organic Devices
The electrical performance of a device is effected by the junctions present. The energy barrier is a major factor to consider in modeling the junction. In device physics, the vacuum level is regarded as the common reference point to calculate the energy barrier. In inorganic semiconductors based on Si material, it is always regarded as truth that the vacuum level is the same for all of contacting layers (materials). This is called Vacuum level alignment.

In organic devices, it is common that function layer is organic semiconductor, and electrode layers are inorganic materials. Scientists found that this vacuum level alignment may not be applicable in the junction between organic layer and inorganic layer. For example, the junctions Ag/Alq3, Ag/Almq3 have been observed that the vacuum levels are not aligned for difference about 1.1 eV [1].

This non aligned vacuum level can happen for system with or without chemical bond, and doped and undoped. It means that it is not due to chemical reaction in the junction. Scientists explained that there is dipole behaving like internal field across the junction. The source of the dipole is due to significant difference between two materials. The molecules in the junction try to arrange their position to have minimum free energy of the whole system. This junction and the dipole are different from p-n or Schottky junction in terms of size and field. They will be discussed in the other topic.

The consequence of different vacuum levels is critical for the energy barrier. For example, the vacuum level of organic layer is usually reduced by a amount (say 0.5 eV). For hole injection from the junction into the organic layer, the energy barrier is increased by that amount, while the energy barrier is reduced by that amount for electron injection. That amount of energy barrier change can result in significant effect to electrical performance of the device.


1. I. G. Hill, Appl. Phys., Vol. 84, 3236 (1998)


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