Organic semiconductors are solids whose constituents are pi-bonded molecules or polymers composed of carbon and hydrogen atoms and, in some cases, heteroatoms such as nitrogen, sulfur, and oxygen. These are materials with semiconducting characteristics that are predominantly constituted of organic (carbon-based) molecules. They can be found as molecular crystals or as amorphous thin sheets. Organic semiconductors, as opposed to standard inorganic semiconductors such as silicon, which are extensively used in electronics, are composed of carbon, hydrogen, nitrogen, and occasionally additional elements such as oxygen or sulfur.
They are electrical insulators in general, but become semiconducting when charges are introduced from appropriate electrodes, doped, or photoexcited. Because of their unique features and prospective uses in various electronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVs), and organic field-effect transistors (OFETs), these materials have received a lot of interest in recent years.
The energy separation between the top of the valence band and the bottom conduction band in molecular crystals, i.e. the band gap, is typically 2.5-4 eV, but in inorganic semiconductors the band gaps are often 1-2 eV. This suggests that they are insulators rather than semiconductors in the traditional sense. Only when charge carriers are introduced from the electrodes or created by purposeful or incidental doping do they become semiconducting.
Charge carriers can also be produced during optical excitation. However, it is critical to understand that the fundamental optical excitations are neutral excitons with Coulomb-binding energies ranging from 0.5 to 1.0 eV. The reason for this is that the dielectric constants of organic semiconductors can be as low as 3-4. This impedes efficient charge carrier photogeneration in tidy systems in bulk. Due to charge transfer between donor and acceptor moieties, efficient photogeneration can only occur in binary systems.
Otherwise, neutral excitons decay either radiatively (producing photoluminescence) or non-radiatively to the ground state. Organic semiconductors typically have an optical absorption edge of 1.7-3 eV, which corresponds to a spectral range of 700 to 400 nm (the visible spectrum).
Applications of organic semiconductors include:
- Organic Light-Emitting Diodes (OLEDs): OLEDs are used in displays and lighting applications. Organic semiconductors emit light when an electric current is applied, making them suitable for high-quality, energy-efficient displays and lighting panels.
- Organic Photovoltaic Cells (OPVs): Organic solar cells convert sunlight into electricity. They are lightweight and flexible, making them suitable for applications where traditional rigid solar panels are impractical.
- Organic Field-Effect Transistors (OFETs): OFETs are used in flexible and low-power electronics, such as RFID tags, sensors, and flexible displays.
- Organic Thin-Film Transistors (OTFTs): OTFTs are utilized in electronic paper (e-paper) displays and other low-power electronic applications.
- Organic Sensors: Organic semiconductors can be used in various sensors, including gas sensors, chemical sensors, and biosensors.