Organic Semiconductors: Optical Processes and Devices

ORGANIC SEMICONDUCTORS: OPTICAL PROCESSES AND DEVICES

In the last 1980s the interest in organic semiconductors demonstrated by the invention of electroluminescent devices and FETs made from polythiophene.

Polymer-based devices are now used for back-lights of liquid crystal displays, displays of devices, such as cell phones or watches. It is used in that commercial technologies, such as televisions, solar cells, etc., will benefit from new advances.

Polymer LEDs were first demonstrated in 1990. They are very attractive because of potential large area applications and mechanical flexibility.

The organic diode is usually not fabricated by doping the polymer itself. Instead an undoped polymer film is placed between an anode (indium tin oxide) and a cathode (e.g., calcium).

In organic semiconductors, the nature of atomic bonding results in very narrow range energy levels.

Figure 4.26 shows a diagram of a metal-organic- semiconductor junction.

When the Fermi energy of the metal is close to the HOMO level, electrons from the this level can move into the metal (a hole is injected into semiconductor. Thus in this case the metal acts as a p-type contact.

When the metal work functions are close to the LUMO state, as shown in fig. 4.26 an n-type contact results. The current flow in a p - n diode using an organic semiconductor film is shown in fig.(c).

Thus, the n-contact injects an electron, while diffuses into the semiconductor. Then, eventually recombining with a hole in the HOMO level. The electron finally leaves the HOMO state (i.e., a hole is injected).

It is also possible to have heterojunctions between different organic semiconductors.

The process of photon emission by electron-hole

recombination in organic semiconductors significantly different from that of inorganic semiconductors. The difference is due to the formation of exciton state. It is a bound state of electron and hole is a e - h interacting through Coulombic interaction.