Small molecules and polymers
There are two main types of organic semiconductors - small molecule versions, which are conductive in their pure state but typically insoluble and therefore are vacuum deposited or spin coated. One such material commonly used is Pentacene.
The second type - polymer - consists of long chains of carbon atoms. These are very poor conductors in their natural state, so a 'doping' agent is added, such as chlorine or iodine which vastly improves their conductivity. They are cheaper than small molecule types and can also be soluble, meaning that they can be printed using inkjet or other conventional printing methods, enabling low cost, relatively easy, high volume manufacturing of electronics. Most work is being done on these materials. A common material used is PEDOT (polyethylenedioxythiophene), which is soluble in water. However, this solution is corrosive and results in shortened circuit lifetime and can damage other components.
In April 2004 TDA Research, Colorado, announced their latest development with polymer semiconducting materials - Oligotron. This contains a PEDOT chain but with two non conducting molecules at the end of the chain which allow the molecule to be dissolved in non corrosive solvents. The material can be printed and then the patterned formed by shining UV light through a mask onto the surface, forming the conductive patterns where the light reaches.
The non conducting end molecules can be changed depending on the application, e.g. end pieces that convert solar energy into electricity can be used for photovoltaic applications.
For more information, see http://www.tda.com.