What are organic semiconductors?

All of us will be familiar with inorganic semiconductors like silicon, germanium and gallium arsenide. But most of us may not be familiar with organic semiconductors. Yet you may have come across television sets using organic light-emitting diode (OLED) display! There are also organic solar cells and organic field-effect transistors (OFET). Organic semiconductors are made up of carbon, hydrogen and sometimes nitrogen, sulphur and oxygen atoms. Some of them are even polymers. Structures could be either molecular crystals or thin amorphous layers. They are electrical insulators at basal state and become semiconductors when charges are injected with appropriate electrodes or with doping or photoexcitation. 2000 Nobel prize in Chemistry was for the discovery and development of conductive polymers. That was for Shirakawa et al who reported high conductivity in oxidized and iodine-doped polyacetylene in 1977.

The gap between the valence band and conduction band, known as band gap, is higher for molecular crystals. While it is 2.4 to 4 eV for molecular crystals, it is in the range of 1 to 2 eV for inorganic semiconductors. That is why they behave as insulators in the conventional sense, unless charge carriers are injected or generated by doping. Doping in semiconductor technology is adding a specific type of impurity in small amount to generate free charge carriers, to make them either n-type with free electrons or p-type with holes as charge carriers. Advantages mentioned for organic semiconductors are easy fabrication, mechanical flexibility, and low cost.