Conductive Polymers

Conductive polymers are organic polymers that conduct electricity. Their conductive properties can vary from fully conducting to semiconducting, depending on the band gap of the polymer. Due to the diversity of structures in organic chemistry, the conductive properties (i.e. the band gap) of these polymers can be tuned to the specific application. In addition to the tunable band gaps, another positive aspect of conductive polymers are that they can be processed in the liquid state, making processing cheaper than traditional solid-state semiconductor technologies. Because of the morphology of organic semiconductors, the active layers can often be thin enough so that they can be fabricated onto flexible substrates such as common plastics.

Organic molecules are conductive when they have delocalized electrons. As a simple rule, electrons are delocalized when adjacent atoms have hybridizations of SP2 or SP3 with lone pair(s); if there are multiple lone pairs on an SP3 atom, only one pair will be localized. The electrons are able to delocalize because the orbitals of these adjacent atoms overlap, allowing the electrons to flow freely between them; for SP3 with lone pairs, only one of the lone pairs will properly aligned for delocalization (the other pair will be perpendicular to the orbitals involved in delocalization).

Organic Semiconductor Function

At the heart of all semiconductor technologies is the P-N junction. P-type semiconductors are electron deficient/hole rich, while N-type semiconductors are hole deficient/electron rich. The most common semiconductor material is silicon. Silicon is "doped" by integrating other elements into the crystal structure that alter the band gap. To make P-type semiconductors, silicon is often doped with elements in column III on the periodic table because they have an empty orbital for holes, while doping with elements in the V column of the periodic table create an N-type semiconductor because they have extra valence electrons. A P-N junction is formed by placing P and N type semiconductors adjacent to each other between an anode and cathode (P-type next to anode, N-type next to cathode). In this arrangement, an electric field is created at the junction by the few holes in the N-type and the few electrons in the P-type to coming together. This electric field resists current flow between the anode and cathode until a large enough voltage is applied to overcome the electric field in the P-N junction. Virtually every semiconductor electronic device is created by combining these junctions in various arrangements. For example, in LEDs the applied voltage is always large enough to overcome the electric field in the P-N junction and the light that is emitted is from the electrons being transferred between P-N layers.

Organic semiconductors also function by creating P-N junctions, but instead of doping crystalline material to create P and N type semiconductors, two different molecules are used as the two layers. The N-type, electron-rich molecule is referred to as the donor, while the P-type, electron-deficient molecule is known as the acceptor. Oftentimes, instead of placing the donor (N-type) and acceptor (P-type) molecules in adjacent layers, organic semiconductors are often arranged as "bulk heterojunctions", with donor and acceptor being interdigitated in thin filaments. This bulk heterojunction arrangement minimizes the distance that excitons generated in the donor must travel to reach the acceptor, and is also a consequence of the liquid phase processing of organic semiconductors. Optimizing the filament thickness in bulk heterojunctions for the exciton mobility is a major aspect of improving design.

Applications

Organic semiconductors can theoretically be used anywhere that traditional semiconductor technologies are utilized, but difficulties in processing have inhibited large scale applications. As processing techniques improve, organic semiconducting technologies promise to be a more cost efficient technology than traditional semiconductors because of the low price of raw materials (often from petrochemicals) and low price of processing liquids when compared to solid-state technologies. Current research for organic semiconductors focuses on their applications in solar cells, printable electronics, OLEDs, chemical and biochemical sensors, flexible displays, and electromagnetic absorbant coatings with potential applications to stealth technology.

References

1. Inzelt, György, Conducting Polymers: A New Era in Electrochemistry, Springer, 2008.
2. B. Van Zeghbroeck. Principles of Semiconductor Devices. Department of Electrical and Computer Engineering. University of Colorado – Boulder. https://www.eletrica.ufpr.br/graduacao/e-books/Principles%20Of%20Semiconductor%20Devices.pdf
3. Sirringhaus, H., "Device Physics of Solution-Processed Organic Field-Effect Transistors", Advanced Materials 17 (20): 2411, 2005.