Charge transport and injection in amorphous organic semiconductors

Organic electronic devices are receiving steady and increasing attention in optoelectronics in the past decade [1]. There are three broad classes of organic electronic devices that can be classified according to their functions. They include organic light-emitting diodes (OLEDs) [2], organic solar cells [3], and organic thin-film transistors (OTFTs) [4]. The technology has matured sufficiently, and commercial products are available, especially in the form of emissive flat panel displays. Irrespective of the functions of these devices, they generally have a sandwich structure of anode/organic material/cathode [5]. The entire device is, therefore, a stack of thin films. The total thickness is in the range of 50-200 nm. In the case of OLEDs, the device is commonly grown on a flat glass slide, which allows light viewing and provides mechanical support [2]. The active organic material in the middle is called an organic semiconductor. An organic semiconductor consists of aggregates of organic molecules bound by weak van der Waals forces. These molecules contain loosely bound φ-electrons that are ultimately responsible for electrical conduction [1]. In applications of OLEDs and solar cells, the organic semiconductors involved have conductivities that resemble an insulator more than a conventional semiconductor (e.g., Si). In OTFT applications, the conductivity of the organic semiconductor is higher, but it is still much lower than a crystalline semiconductor [4].