Device Physics of Solution-Processed Organic Field-Effect Transistors

Following the initial demonstration of field-effect conduction in small organic molecules and conjugated polymers, the community of industrial and academic research groups that are interested in using organic semiconductors as the active layer in organic field-effect transistor (OFET) devices has been growing steadily, particularly over the last four to five years. The Institute for Scientific Information (ISI) Web of Science counts 393 scientific publications in the field of organic transistors in 2004, up from 304 in 2003, and 80 in 1999. The reasons for this surge of interest are manifold. The performance of OFETs, which is generally benchmarked against that of amorphous silicon (a-Si) thin-film transistors (TFTs) with field-effect mobilities of 0.5–1 cm V s and ON/OFF current ratios of 10–10, has improved significantly. Currently, the record mobility (l) values for thin-film OFETs are 5 cm V s in the case of vaccum-deposited small molecules and 0.6 cm V s for solution-processed polymers. As a result, there is now a serious level of industrial interest in using OFETs for applications that are currently incompatible with the use of a-Si or other inorganic transistor technologies. OFETs are most commonly manufactured using standard topgate (Fig. 1A) and bottom-gate TFT architectures. One of their main technological attractions is that all the layers of an OFET can be deposited and patterned at low/room temperature by a combination of low-cost solution-processing and direct-write printing, which makes them ideally suited for realization of low-cost, large-area electronic functions on flexible substrates (see the reviews by Sirringhaus et al. and Forrest). The first applications in which we can realistically expect OFETs to be used within the next three to five years are flexible, active-matrix electronic-paper displays, for which impressive demonstrations have been developed recently, and simple, low-cost, radiofrequency identification (RFID) tags and sensing devices. Other applications, such as activematrix liquid crystal or organic light-emitting diode (OLED) displays, or high-performance RFID tags compatible with existing communication standards, are also being envisioned, R EV EW