Semiconductors are materials that occur to have electric conductivities falling right in between conductors like metals i.e. copper, gold, and insulators like glass and most plastics. Basically, a semiconductor material is poorly conductor at its neutral state because the valence band is full. However, since the energy between the valence and the conduction band, also called bandgap (Eg), is low, there is a possibility, with a small input of energy, to promote an electron to the conduction band and create charges carriers upon the material leading to conduction properties. It is also possible to dope the semiconductor by oxidation (p-type) or reduction (n-type) which creates mobile charge carriers and generates current leading again to similar properties to conductor materials.
Figure 1: Band structure theory
Over the past decades, organic semiconductors (OSC) have gained in popularity for many reasons. Indeed, unlike their inorganic counterparts, these organic materials possess mechanical properties similar to those of plastics i.e. flexibility, lightweight, low-cost production. Also, organic materials are mostly composed of carbon, oxygen, hydrogen, sulfur and nitrogen atoms, some of the most abundant elements on Earth. The chemistry to synthesize molecules with those atoms is quite simple which allows endless possibilities for designing different arrangements of those elements to create conjugated organic molecules or materials with optimized energies of the valence and conduction bands as well as the bandgap. By doing so, the material can be adapted specifically for a certain application.
Another major advantage of organic semiconductors over their inorganic counterparts is the solubility of materials in common organic solvents. This solubility allows ink formulation to perform roll-to-roll printing. The roll-to-roll printing technique is fairly cheap and accessible since it is mainly the same technique used in the production of newspapers. This manufacturing method enables to print organic semiconductors on plastic films like polyethylene terephthalate (PET), an inexpensive flexible polymer, to produce low-cost, lightweight and flexible electronic devices.
Talking about flexible electronic devices and applications, technologies using organic semiconductors are extensively growing and are emerging on the market since the past couple of years. The most common commercialized applications are OLED (organic light-emitting diode), OPV (organic photovoltaic or organic solar cells), OTFT (organic thin-film transistors) and organic electrochromic devices.