Organic thin film transistors: From theory to real devices
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The organic thin-film transistor (OTFT) is now a mature device that has developed tremendously during the last twenty years. The aim of this paper is to update previous reviews on that matter that have been published in the past. The operating mode of OTFTs is analyzed in view of recent model development. This mainly concerns the distribution of charges in the conducting channel and problems connected with contact resistance. We also delineate what differentiates n- and p-type semiconductors, and show how this concept differs from what it covers in conventional semiconductors. In the chapter devoted to fabrication techniques, emphasis is placed on solution-based techniques and particularly printing processes. Similarly, soluble materials are given a prominent place in the section dedicated to the performance of devices. Finally, special attention is given to devices at the nanoscale level, which demonstrate a new route toward molecular electronics.
I. INTRODUCTION
The concept of field-effect controlled current dates back to 1930.1 The idea was to replace vacuum lamps by solid-state devices in amplifiers. More than thirty years elapsed before the first device that really matched that early concept was realized, namely, the silicon-based metaloxide-semiconductor field-effect transistor (MOSFET).2 Nowadays MOSFETs are omnipresent in our environment. Millions of them are located in the processors that equip personal computers and other microelectronic devices. Besides its technological interest, the field-effect transistor can also be viewed as a tool for studying charge transport in solid materials. In particular, it gives direct access to charge mobility. For this reason, the electric field-induced effect has been used in low-mobility materials such as hydrogenated amorphous silicon (a-Si:H).3,4 In this case, an alternative geometry was used, the thinfilm transistor (TFT),5 which differs from the conventional MOSFET in that the conducting channel is constituted by an accumulation layer rather than an inversion layer. It was only later that the technological interest of a-Si:H TFTs emerged, when applications where large area is needed appeared. Today, a-Si:H TFTs are largely used in the active matrix of liquid crystal display (AM-LCD). Organic semiconductors have been known since the late 1940s.6 However, apart from a very small number of preliminary works on conjugated small molecules7,8 and
a) e-mail: [email protected] DOI: 10.1557/JMR.2004.0266
1946
J. Mater. Res., Vol. 19, No. 7, Jul 2004
polymers,9 the first transistor based on an organic semiconductor was only reported in 1986,10 with a device made on an electrochemically grown polythiophene film. Polythiophene belongs to the family of the conducting polymers that were discovered in the late 1970s.11 The inventors of polyacetylene, an archetypal model of conducting polymers, were awarded the Nobel Prize in chemistry in 2000. The possibility of fabricating organic TFT (OTFT) with small conjugated molecules was shown in 198912 with sexithiophene, an
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