Marta Mas-Torrent and Concepció Rovira of the Institute of Science of Materials (CSIC) in Barcelona, Spain, look at how small molecules can be used as processable semiconductors

Our daily life involves the continuous use of field-effect transistors: they are the main logic units, functioning as either switches or amplifiers, controlling current flow in electronic circuits. A field-effect transistor is a three-terminal device in which current flows through a semiconductor from the 'source' terminal to the 'drain'. This flow is controlled at the third 'gate' terminal by a voltage that creates an electric field through the insulator (dielectric) on which the semiconductor is deposited. Since the invention of the first transistor in 1947 by John Bardeen, William Shockley and Walter Brattain, the vast majority of electronic devices have been based on inorganic semiconductors and, in particular, on silicon. 

Soluble organic molecules offer new perspectives as transistors in low-cost electronics

Over the past few years, however, organic field-effect transistors (OFETs) have attracted a great deal of interest due to their unique processing characteristics. Organic materials offer the benefit that they can be printed over large areas on plastic, flexible substrates at low temperature by solution-based techniques, which would result in a dramatic reduction of manufacturing costs. Though the first OFETs did not transport charge as well as inorganic materials, the best ones nowadays are achieving charge carrier mobilities of the same order as amorphous silicon. Organic-based electronics will not replace high density and high speed silicon circuits, but might play an important role in applications such as identification tags, electronic bar codes or active matrix elements for displays.

OFETs have been mainly based on two types of semiconductors that feature pi-pi interactions: conjugated polymers and small conjugated molecules. Polymers are deposited from solution, allowing for low cost electronics. However, the higher molecular disorder in polymers limits their charge transport, typically resulting in lower mobilities compared to devices based on small molecules. On the other hand, devices prepared with small conjugated molecules have to be prepared more expensively by evaporating organic materials, due to their low solubility in common organic solvents. Therefore, to promote the development and use of organic semiconductors, there is a clear need to find materials that can be processed in solution and that simultaneously achieve a high OFET mobility.

One way of imparting solubility to organic semiconductors is to prepare a precursor compound that can be converted into the parent semiconductor by heat or irradiation. An alternative strategy is to structurally modify organic semiconductors to impart solubility and, if possible, also achieve higher stability and increase pi-pi interactions. Semiconductors such as acenes and oligothiophenes (classed as p-type, since the charge carriers are mainly holes) have been processed by techniques such as spin coating and drop or zone casting, and very high performances have been achieved.

Currently, there is a growing interest in developing n-type semiconductors (where charge carriers are electrons) and ambipolar devices (which conduct both electrons and holes) in order to fabricate complementary circuits. The development in these devices is still far from the performance achieved with p-type materials, because transport in n-channel conductors is easily degraded by air and finding suitable metals for contacts is difficult.

Circuits based on organic transistors are being extensively investigated for a range of applications. One of the first emerging devices realised with OFETs is flexible electronic paper (see Chemistry World, April 2007, p15), where organic transistors switch pixels formed by charged pigments. OFETs also offer great promise for applications in chemical and biological sensing: organic semiconductors can interact with different analytes and it is possible to transduce the chemical information to electronic information, creating an 'electronic nose'. And the electrical switching of an OFET has been combined with an organic semiconductor's ability to generate light, to create organic light-emitting transistors (OLETs).

OFETs promise to be important in applications ranging from sophisticated medical diagnostics to 'smart' clothes that can display changing images. New markets will undoubtedly appear in areas where electronics meets with information technology, biomedicine or optics.

Read more in Mas-Torrent and Rovira's critical review in issue 4, 2008, of  Chemical Society Reviews.