[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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[ODCAD] US Defence Dept.- Organic Photovotaics (PV)
Effective solar cell device definitly has great application in any electronic products that require power supply. Inorganic semiconductors like Si currently rule the market. Organic materials potentially have advantage of smaller size (thinner), light weight, effcient, flexible (bend without breaking), and low cost. Many experts, and labs bet the future of photovotaics (PV) on organic materials.

US Dept. of Defence may be the biggest investor in this technology. Lynne Samuelson, a researcher at the US Army's Natick Soldier Center, in Massachusetts, claimed that his lab is starting to make proto-type PV devices to try out in the field. That Lab teamed up with a chemistry lab at Univ. of Massachusetts Lowell. The structure and materials used in their cell may contain Titania (? it may mean Ti)-TiO2 particles/Polymer. Titania particle can be as small as 20nm. The polymer is polyethylene terephthalate. Its effciency has not been disclosed. It is estimated that it may be >4%. This lab is expecting 20% effciency in five years.

Some Information is from Spectrum of IEEE.
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[ODCAD] Nano conveyor of Metal
Source: Chemical Eng. News, May 3, 2004

Scientists from Univ. of Californian, Berkeley and Lawrence Berkeley National Lab have found a way to transport molten metal along nanotube.

Physics professor Alex Zettl, postdoc Christ Regan and their coworkers applied electrical current to a multiwalled carbon nanotube (MWNT), and the heat generated can melt contacting metal crystal. The metal then migrate along the tube from the anode to the cathode. The metal particles are shuttled along the surface of the nano tube in atomic form without evaporation. They observed that the metal can move over a greater distance than 2 um.

Potentially, this may be useful technology to make nano circuit or nano device combining metal with nano tube.