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General observation of n-type field-effect behaviour in organic semiconductors

Abstract

Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes1: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity2,3,4 or low-bandgap5 organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric—such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6)—can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10-3 to 10-2 cm2 V-1 s-1 have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylenevinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor–dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO2 dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed.

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Figure 1: F8BT and OC1C10 - PPV n-channel FETs with BCB/SiO2 dielectric and Ca source-drain electrodes.
Figure 2: Evidence from vibrational spectroscopy for interfacial electron trapping on standard silicon oxide/silicon backgate device structures during attempted n-channel FET operation.
Figure 3: F8BT n-channel FETs with various siloxane self-assembled monolayers (SAMs) on SiO2 as dielectric, or with polyethylene as buffer dielectric.
Figure 4: Ambipolar F8T2 and regioregular-P3HT FETs.

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Acknowledgements

We thank A. Achen for providing the BCB material, which is commercially available as Dow Cyclotene, and Merck KGaA for P3HT. L-L.C. and J-F.C. thank the EPSRC Carbon-Based Electronics programme for support, and J.Z. thanks the Gates Cambridge Trust for support. P.K-H.H. thanks St John's College for a research fellowship, the National University of Singapore for research support, and S. J. Chua for technical discussions, support and access to laboratory facilities. We also thank C.-C. Chum for technical support.

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Correspondence to Peter K.-H. Ho or Richard H. Friend.

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Competing interests

H.S. and R.H.F. are directors of Plastic Logic Ltd, a University of Cambridge start-up company with activities within this broad field.

Supplementary information

Supplementary Figures S1-S3

Supplementary Figure S1 provides further examples of polymer gate dielectrics that have also yielded n-channel FETs. Supplementary Figure S2 provides evidence that long chain self-assembled monolayers slow down occupation but do not completely passivate the electron traps at the SiO2 interface. Supplementary Figure S3 provides further evidence for electron-trapping from the gate-threshold shifts in p-type FETs after positive gate biasing. (PDF 187 kb)

Supplementary Discussion

This section discusses the differing time scales for trapping of electrons, which is probed by decay of the electron field-effect current, and for advancement of the electrochemical reaction front across the channel length, which is probed by the ATR-FTIR experiment. (PDF 78 kb)

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Chua, LL., Zaumseil, J., Chang, JF. et al. General observation of n-type field-effect behaviour in organic semiconductors. Nature 434, 194–199 (2005). https://doi.org/10.1038/nature03376

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