Organic thin ®lm transistors: from active materials to novel applications L. Torsi a, * , N. Cio a , C. Di Franco a , L. Sabbatini a , P.G. Zambonin a , T. Bleve-Zacheo b a Dipartimento di Chimica, UniversitaÕ degli Studi di Bari-4, via Orabona, I-70126 Bari, Italy b Istituto di Nematologia Agraria, Consiglio Nazionale delle Ricerche, 165/A, via Amendola, I-70126 Bari, Italy Received 22 March 2000 Abstract In this paper, a bird's eye view of most of the organic materials employed as n-channel and p-channel transistor active layers is given along with the relevant device performances; organic thin ®lm transistors OTFT) operation regimes are discussed and an interesting perspective application of OTFT as multi-parameter gas sensor is pro- posed. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Organic thin ®lm transistors; Gas sensor; Nanocomposites 1. Introduction Organic thin ®lm transistors OTFT) performan- ces have drastically improved in the last 10 years. In the late eighties ®eld eect mobilities l FET ) were typically 10 6 ±10 3 cm 2 /Vs and the current could be modulated by a factor of 10 3 at most [1±4]. Recently it has been demonstrated that mobilities as high as 10 2 ±10 1 cm 2 / Vs can be reached with current modulations larger than 10 6 . A key role to get high mobilities is the use of thin organic ®lms with high structural order as active layers; thin ®lms based on regioregular poly-3-hexyl-thiophene and pentacene exhibit this property along with mobili- ties of 0.1 and 0.3 cm 2 /Vs, respectively. In Fig. 1 the chemical structure of some of the or- ganic materials used in TFTs is reported, while in Table 1 device performances are listed. As it is evident from the entries of Table 1, organic materials already em- ployed in TFTs constitute a numerous class character- ised by quite dierent chemical and physical properties. As a matter of fact small molecules e.g. dierently substituted naphthalene and perylene as well as C 60 or metallophthalocyanines halogenated or not), oligomers e.g. thienylene oligomers end substituted or not and dierent oligoacenes), as well as polymers e.g. polythio- phenes substituted with alkyl chains of dierent length) can be used; as far as the latter class is concerned, par- ticularly interesting are the performances of regioregu- lar polythiophene thin ®lms because of their ordered structure [16±18]. All these materials are characterised by an extended system of delocalised p orbitals along which charged defects, called polarons and bipolarons, can move. The thin ®lms, obtained from their deposi- tion, are usually van der Waals type solids that have the mechanical ¯exibility typical of many plastic mate- rials. Depending on the chemical, and therefore elec- tronic, structure of the active material, this ®eld-eect transistors FET) can operate as n-type or p-type chan- nel devices. One of the appealing aspects of most of the materials employed in TFT is the fact that they can be deposited using very low cost procedures such as casting or spin coating. This is the case of soluble poly- mers such as regioregular polythiophenes. These depo- sition procedures are also thermally compatible with plastic substrates, because they are carried out at room Solid-State Electronics 45 2001) 1479±1485 * Corresponding author. Tel.: +39-0805-442019; fax: +39- 0805-442026. E-mail address: torsi@chimica.uniba.it L. Torsi). 0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0038-110100)00268-9