Organic Semiconductors and Devices


During the last fifteen years dramatic advances have been achieved in the performance of organic semiconductor devices. Organic light emitting diodes (OLEDs), for example, were first demonstrated in the late '80s and can already be found in cell phone displays. Moreover, OLED-based displays for laptops are scheduled to hit the market in 1-2 years. At the same time, organic thin film transistors (OTFTs) are being developed by numerous companies for "disposable" plastic electronics such as smart identification tags. This fast-paced progress can be attributed to unique properties of organic semiconductors such as ease of processing, their compatibility with mechanically flexible substrates and optoelectronic properties that can be tailored via chemical synthesis.

The electronic properties of organic semiconductors and their connection to device performance will be discussed by drawing examples from recent studies of OLEDs and TFTs. In OLEDs, optimization of charge injection from metal electrodes is of tantamount importance for achieving efficient and stable device operation. New materials that combine electronic and ionic conductivity have recently demonstrated extremely promising performance. The physics of these devices is determined by a complex interplay between ionic and electronic space charge. The role of the ionic space charge in determining the OLED performance will be discussed.

A different set of challenges is encountered in OTFTs, where the mobility of the organic semiconductor has to be as high as possible. Polycrystalline films from small molecules such as pentacene have been shown to exhibit mobilities comparable to those in amorphous silicon. The connection between the growth conditions of these films, their morphology and their electrical properties will be discussed. Finally, applications of OTFTs in gas and biosensors will be examined.