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Band-like temperature dependence of mobility in a solution-processed organic semiconductor

Abstract

The mobility μ of solution-processed organic semiconductorshas improved markedly1,2 to room-temperature values of 1–5 cm2 V−1 s−1. In spite of their growing technological importance3, the fundamental open question remains whether charges are localized onto individual molecules or exhibit extended-state band conduction like those in inorganic semiconductors4. The high bulk mobility of 100 cm2 V−1 s−1 at 10 K of some molecular single crystals5 provides clear evidence that extended-state conduction is possible in van-der-Waals-bonded solids at low temperatures. However, the nature of conduction at room temperature with mobilities close to the Ioffe–Regel limit remains controversial6. Here we investigate the origin of an apparent ‘band-like’, negative temperature coefficient of the mobility (dμ/dT<0) in spin-coated films of 6,13-bis(triisopropylsilylethynyl)-pentacene. We use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states, but that a moderate lateral electric field is able to detrap them resulting in highly nonlinear, low-temperature transport. The negative temperature coefficient of the mobility at high fields is not due to extended-state conduction but to localized transport limited by thermal lattice fluctuations.

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Figure 1: Characteristics of a TIPS-pentacene top-gate transistor.
Figure 2: Temperature-dependent characteristics of TIPS-pentacene FETs with L=5 μm, W=100 μm and a 120-nm-thick dielectric.
Figure 3: Optical spectroscopy of charge carriers in TIPS-pentacene.
Figure 4: Drain-voltage dependence of the CMS spectra at 100 K of a TIPS-pentacene FET with L=5 μm and a 250-nm-thick dielectric.

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Acknowledgements

We thank M. Caironi and M. Bird for many useful discussions, and acknowledge financial support from the Technology Strategy Board (TSB) through the POSTED project.

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T.S. carried out the experiments. T.S. and H.S. developed the interpretation of the data and wrote the manuscript.

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Correspondence to Tomo Sakanoue or Henning Sirringhaus.

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The authors declare no competing financial interests.

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Sakanoue, T., Sirringhaus, H. Band-like temperature dependence of mobility in a solution-processed organic semiconductor. Nature Mater 9, 736–740 (2010). https://doi.org/10.1038/nmat2825

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