2000 Nobel Chemistry Prize was awarded to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa for their great achievement in discovery of conductive polymers and developed theories to understand the mechanism of polymer conductivity.
Mechanism
- Conductive polymers have conjugated carbon structure, e.g. alternative single -C-C-/double -C=C- bond structure
- The carbon is sp2 hybrided. Three sp2 orbitals form sigma bond in one plane; Pz orbital overlaps shoulder by shoulder, perpendicular to the sp2 plane
- Pz electrons delocalize in the polymer segments and form electrical bands, valence band (VB) and conduction band (CB).
- The energy different between bottom edge of CB (or LUMO) and top edge of VB (HOMO) is energy gap
- Therefore, polymers, or small molecules, with the above structure present semiconducting behavior. Heavily doped organic semiconductors can be conductive, e.g. PEDOT/PASS
Applications
The materials are semiconductors in nature and they can be used for electronics as conventional semiconductors do, such as:
- Organic light emitting diodes (OLED)
- Organic photovoltaics (OPV)
- Organic thin film transistors (OTFT)
- Memories, photodetectors, and sensors and others
Uniqueness compared with traditional semiconductors
Organic semiconductors can form thin films through thermal evaporation or simple solution coating.
Particularly, the organic semiconductor powders can be dissolved in solvents or even water, and then formulated into coatable fluids. Through coating techniques, including ink jet, slot die, gravure, etc, the fluids can be converted into solid films of the thickness from a couple of angstroms to hundreds of nanometers. With proper design of the device architecture, multilayer electronics can be made. The process is also called addition manufacturing; these devices are printed electronics.
Examples
The following are the organic solar cell modules
If transparent electrode is used, the solar cell can be transparent and colorful as well.
