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ORGANIC TRANSISTORS

A polarized responseThe material used for the dielectric layer in organic fi eld-effect transistors strongly affects the effi ciencies of the resulting devices. The reasons behind this connection, and opportunities to tune the device performance by changing the dielectric material, are now revealed.

VEACESLAV COROPCEANU ANDJEAN-LUC BRÉDAS are at the School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics of the Georgia Institute of Technology, Atlanta, Georgia 30332–0400, USA.

e-mail: veaceslav.coropceanu@chemistry.gatech.edu; jean-luc.bredas@chemistry.gatech.edu

Organic fi eld-eff ect transistors (OFETs) are attracting major interest as these devices are potentially low-cost components that could be

used in a wide range of electronics applications, but improvements in performances are needed before they become commercially viable. In an OFET, the transport of the charge carriers — electrons and their positive counterparts, holes — takes place within a conducting channel in the semiconductor, just a few nanometres thick, located next to the dielectric layer (Fig. 1a). It therefore does not come as a surprise that the carrier mobility depends not only on the intrinsic properties of the semiconductor material but also on the nature of the dielectric material and the chemical and physical properties of the interface between the two. It was found, for instance, that the interface chemical purity, the composition of the dielectric material and the interface roughness have a dramatic impact on the charge transport1.

An intriguing eff ect observed recently in OFETs is that the charge carrier mobility at room temperature decreases with increasing dielectric constant of the insulating layer2–4. On page 982 of this issue, Hulea et al.5 report the use of temperature-dependent measurements of charge mobility in a series of rubrene single-crystal OFETs to not only confi rm the earlier room-temperature observations4, but also demonstrate that the charge carriers in the organic semiconductor couple to and cause movements of the dipoles in the dielectric material. Th eir work underlines the importance of a physical mechanism that has a great impact on the performance of organic transistors, the understanding of which could be exploited to improve devices.

A fi eld-eff ect transistor (Fig. 1a) consists of a semiconducting layer, in which charges move between source and drain electrodes, with their movement modifi ed by the potential applied to

a third, or gate, electrode, separated from the active material by a dielectric, or insulating, layer. Dielectric materials are commonly ionic materials such as SiO2, Si3N4 or Al2O3, and this layer can be considered as consisting of a set of dipoles, or a polarizable ionic lattice. Th e dependence of charge transport on dielectric polarizability was fi rst observed by Veres et al. in polymer OFETs2,3. Polymer systems are usually highly disordered so charge transport cannot progress smoothly and is thought to take place via hopping of charge carriers between sites. As the thermal activation energy required for this hopping was found to increase with the dielectric polarizability, it was suggested that the randomly oriented dipoles present in the dielectric increase the disorder in the semiconductor conducting channel (Fig. 1b); thus, the more polar the dielectric, the larger the static dipolar disorder eff ect it produces in the polymer.

Th e measurements by Hulea et al. were carried out on rubrene single-crystal devices. In such single-crystal materials, disorder is signifi cantly reduced compared with conducting polymers, enabling the observation of intrinsic charge-transport characteristics of the molecules themselves6. For instance, single-crystal OFETs show a high level of

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Figure 1 Transistor structure and the effect of a passing electron on the dielectric ionic lattice. a, A fi eld-effect transistor consists of a semiconducting thin fi lm contacted by two parallel planar electrodes, the source and drain. A third electrode, the gate, separated from the semiconductor fi lm by a dielectric material, induces a conducting channel within the semiconductor. b, The disorder pattern of the dipoles in an dielectric material. c, The presence of a charge carrier in the conducting channel of the organic semiconductor induces a polarization within the dielectric that, in turn, affects the carrier motion.

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NEWS & VIEWS

930 nature materials | VOL 5 | DECEMBER 2006 | www.nature.com/naturematerials

reproducibility, and carrier mobilities can be up to a few orders of magnitude larger than in thin-fi lm polymer devices, where disorder masks the intrinsic properties. Taking advantage of the quality of these rubrene single-crystal devices, it was possible from the experiments performed to conclude that the dependence of mobility on dielectric polarizability has a dynamic origin owing to the coupling of the charged carriers in the organic conducting channel to the ionic lattice of the dielectric, rather than the static dipolar disorder mechanism suggested previously.

Th e concept of interactions between charge carriers and the ionic lattice to which Hulea et al. refer was fi rst discussed in condensed matter physics by Pekar7 and then by Fröhlich et al.8 some 60 years ago. Th e idea is simple: the Coulomb interaction due to an electron (or hole) in the conducting channel displaces the positive and negative ions of the dielectric (which corresponds to inducing a vibration of the ionic lattice) and creates an ionic polarization (Fig. 1c). Th is polarization, in turn, acts on the charge carrier in the conducting channel and modulates its motion. As the electron moves through the conducting channel on the organic side of the interface, the induced polarization on the dielectric side moves with it. Th is coupled electron–polarization cloud can be pictured as a quasi-particle and is referred to as a ‘Fröhlich polaron’. Th e possibility of Fröhlich-polaron formation at such interfaces has been recently described theoretically by Kirova and Bussac9 and by Houili et al.10; the work by Hulea et al. confi rms their predictions experimentally.

In general, understanding the temperature dependence of the charge mobility is diffi cult because it is due to the interactions of the charge carriers with both the organic semiconductor and the dielectric material. An important achievement of the work by Hulea et al. is that the authors are able to separate

these two types of contributions and to show that the dielectric contributions are explained by a Fröhlich-type polaron model. Also, the authors elegantly demonstrate that when the dielectric polarizability increases, the charge mobility can switch from a band-like character, where mobility increases with decreasing temperature, to a hopping character, where it has an associated activation energy.

From a practical point of view, this work provides a clear confi rmation and explanation of previous demonstrations that the speed of organic transistors can be enhanced by using low-polarity dielectric materials. It gives even more credence to the deposition of ultra-thin organic buff er layers on the dielectric11; such layers could prove useful in the present context by separating the conducting channel from the ionic dielectric, thereby reducing the Fröhlich interactions and improving the mobility and effi ciency of the device. Finally, this work presents a new tool to study the charge-transport mechanisms in OFETs. By tuning the microscopic parameters, bipolarons — the fusion of two polarons — could be formed and other exotic eff ects could be observed. Th is work will no doubt stimulate further experimental and theoretical investigations along these lines.

REFERENCES1. Sirringhaus, H. Adv. Mater. 17, 2411–2425 (2005).2. Veres, J., Ogier, S. D., Leeming, S. W., Cupertino, D. C. & Khaff af, S. M.

Adv. Funct. Mater. 13, 199–204 (2003).3. Veres, J., Ogier, S. D, Lloyd, G. & de Leeuw, D. Chem Mater. 16, 4543–4555 (2004).4. Stassen, A. F., de Boer, R. W. I., Iosad, N. N. & Morpurgo, A. F. Appl. Phys. Lett.

85, 3899–3901 (2004).5. Hulea, I. N. et al. Nature Mater. 5, 982–986 (2006).6. Podzorov, V. et al. Phys. Rev. Lett. 93, 086602 (2004).7. Pekar, S. J. Phys. (USSR) 10, 341 (1946).8. Fröhlich, H., Peltzer, H. & Zienau, S. Phil. Mag. 41, 221 (1950).9. Kirova, N. & Bussac, M. N. Phys. Rev. B 68, 235312 (2003).10. Houili, H., Picon, J. D., Zuppiroli, L. & Bussac, M. N. J. Appl. Phys.

100, 023702 (2006).11. Chua, L. L. et al. Nature 434, 194–199 (2005).

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