Research Projects
Our research is focussed on the charge transport physics of organic semiconductors, and their application in field-effect transistor devices fabricated by solution-processing and direct printing techniques. In contrast to the field of polymer electroluminescence, which has been a very active area of research for more than 15 years, the field of organic transistors or "printable electronics" has only recently received more serious attention.
"Printable electronics" promises to deliver a new manufacturing technology for electronic circuits based on solution processing and additive, direct-write printing (as opposed to vacuum deposition, and subtractive photolithographic patterning as in conventional manufacturing).
One of the fascinating aspects of this research is that it allows combining fundamental physics research on the still relatively poorly understood transport physics of organic semiconductors and electronic properties of polymer-polymer heterointerfaces with device-oriented research that can make a significant impact on the technological, industrial development of the field. In many cases advances in device technology have been enabled by better understanding of the underlying device and materials physics. This provides a stimulating environment for academic research.
The research is highly interdisciplinary. It involves close collaboration with synthetic chemistry groups in several universities and research institutes as well as industry. The understanding of the physics of polymer devices is greatly helped by the interaction with more traditional branches of polymer physics as well as with groups focusing on structural polymer characterization.
Our research encompasses the following main directions:
Ink-jet printing of high performance integrated circuits
Printable electronics based on solution-processed organic semiconductors is a new paradigm for manufacturing of electronic integrated circuits. In contrast to conventional manufacturing processes which are based on a combination of vacuum deposition and photolithography, direct-write printing deposits material only where it is wanted, and is attracting a lot of attention for applications in large-area, low-cost electronics on flexible substrates. This approach is successfully being commercialized by a spin-off company, Plastic Logic Ltd. We have developed a technology platform to fabricate high performance printed electronic circuits. We have developed a printing technique, called Self Aligned Printing, which allows to achieve submicrometer resolution as a pathway to short-channel organic field effect transistors. In order to reduce the gate overlap parasitic capacitance, we have developed a technique, called Self Aligned Gate, which permits to selectively pattern a second thick dielectric layer on top of the source/drain electrodes of field effect transistors, which maintaining a thin dielectric on top of the channel region. We are able to integrate these high performance field effect transistors with printed interconnections and resistors, to fabricate integrated circuits comprising up to 100 transistors. This research is performed in collaboration with the Cambridge Integrated Knowledge Centre and Plastic Logic Ltd. For more information on this project, please contact Dr Enrico Gili .
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 (a) Optical microscopy image of a complete and functional transistor, with gold Self Aligned Printed source and drain contacts and silver gate contact. (b) Schematic cross-sectional diagram of the top-gate device architecture. (c) Transfer and (d) output characteristic curves of the device shown in (a) with L 200 nm and a gate dielectric thickness of 50 nm.
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Selected References:
- Caironi, M., Gili, E., and Sirringhaus, H.; " Inkjet Printing of Downscaled Organic Electronic Devices ", Organic Electronics II: More Materials and Applications, Edited by Hagen Klauk, Wiley-VCH (2011)
- Caironi, M., Gili, E., Tomo Sakanoue, Xiaoyang Cheng and Sirringhaus, H.; " High Yield, Single Droplet Electrode Arrays for Nanoscale Printed Electronics ", ACS Nano 4(3), 1451 (2010)
- Gili, E., Caironi, M., and Sirringhaus, H.; " Picoliter printing ", Handbook of Nanofabrication, Edited by Gary Wiederrecht, Elsevier (2009)
- Noh, Y.-Y., Zhao, N., Caironi, M. and Sirringhaus, H.; " Downscaling of self-aligned, all-printed polymer thin-film transistors ", Nature Nanotechnology 2(12), 784 (2007)
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Scanning probe characterization of nanoscale charge transport
In order to understand charge transport in molecular semiconductors it is important to be able to probe electrical properties on a scale comparable to the molecular length or the length scale over which the molecules self-assemble. We have developed scanning probe microscopy techniques, such as Scanning Kelvin Probe Microscopy (SKPM) that can be performed on operational devices and provide local, nanoscale information about the device physics of polymer field-effect transistors and other devices that cannot be obtained with other techniques. SKPM uses a conducting AFM tip to measure the local electrical field between tip and sample. The technique is capable of directly measuring, for example, the electrostatic potential profile along the channel of a transistor while the device is in operation yielding quantitative information about contact resistance at the source-drain electrodes, field dependence of the mobility, and information about local inhomogeneities in the transport properties of the organic semiconductor.
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 Measuring the potential profile of a polymer transistor.
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Selected References:
- Lukas Bürgi, Tim J. Richards, Richard H. Friend, and Henning Sirringhaus; "Close look at charge carrier injection in polymer field-effect transistors", J. Appl. Phys. 94, 6129 (2003)
- Lukas Bürgi, Richard H. Friend, and Henning Sirringhaus; "Formation of the accumulation layer in polymer field-effect transistors", Appl. Phys. Lett. 82, 1482 (2003)
- Lukas Bürgi, Henning Sirringhaus, and Richard H. Friend; "Noncontact potentiometry of polymer field-effect transistors", Appl. Phys. Lett. 80, 2913 (2002)
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Charge transport and device physics of organic semiconductors
Conjugated polymers are inherently one-dimensional organic semiconductors characterized by strong electron-phonon, and electron-electron interactions
giving rise to pronounced polaronic and excitonic effects. They can be self- assembled from solution into ordered structures by making use of molecular self-organisation
mechanisms, such as phase separation, liquid-crystallinity, or hydrogen bonding.
By careful control of intra- and intermolecular interactions highly ordered molecular assemblies can be produced when a polymer film, or a multilayer
structure comprising several polymer layers is grown from solution. Using a broad range of structural, electrical, optical and electro-optical techniques we investigate the fundamental charge transport and device physics of self-assembled organic
semiconductors, as well as the electronic properties at polymer-polymer heterointerfaces.
Often a better understanding of the relationship between microstructure and electronic
properties of novel materials synthesized by our organic chemistry partners results in devices
with enhanced performance.
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Examples of projects:
- Correlation between molecular structure, microstructure in the solid state, and electrical and optical properties.
- Investigation of the role of dimensionality on transport properties.
- Electronic structure and polaronic relaxation at polymer-polymer heterointerfaces.
- Polymer physics of solution-fabricated semiconducting polymer multilayers.
- Device modeling
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 X-ray investigation of P3HT as a function of regioregularity
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Selected References:
- Anick M. van de Craats, Natalie Stutzmann, Oliver Bunk, Martin M. Nielsen, Mark Watson, Klaus Müllen, Henri D. Chanzy, Henning Sirringhaus, and Richard H. Friend; "Meso-epitaxial solution-growth of self-organizing discotic liquid-crystalline semiconductors" , Adv. Mater. 15, 495 (2003)
- Peter J. Brown, Henning Sirringhaus, M. Harrison, M. Shkunov, and Richard H. Friend; "Optical spectroscopy of field-induced charge in self-organized high mobility poly(3-hexylthiophene)", Phys. Rev. B 63, 125204 (2001)
- Henning Sirringhaus, Richard J. Wilson, and Richard H. Friend; "Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid crystalline phase", Appl. Phys. Lett. 77, 406 (2000)
- Antonio Facchetti, Yvonne Deng, Anchuan Wang, Yoshihiro Koide, Henning Sirringhaus, Tobin J. Marks, and Richard H. Friend; "Tuning the semiconducting properties of sexithiophene by a,w-substitution - a,w-diperfluorohexylsexithiophene: the first n-type sexithiophene for thin-film transistors" , Angew. Chem. Int. Ed. 39, 4547 (2000)
- Henning Sirringhaus, Peter J. Brown, Richard H. Friend, Martin M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig, and D. M. de Leeuw;
"Two-dimensional charge transport in self-organized, high-mobility conjugated polymers", Nature 401, 685 (1999)
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Study of n-type and ambipolar transport in organic semiconductors
In order to exploit the full potential of organic field-effect transitors n-type as well as p-type semiconductors are needed. Although electron transport is readily obtained in polymer LEDs it has been elusive for many materials in field-effect structures. We found that the nature of the dielectric plays a major role in the realisation of organic n-type transitors. Electrons can be irreversibly trapped by hydroxyl groups which are present on e.g. silicon dioxide which renders the transistor non-functional. Using dielectrics that do not have any electron trapping groups (e.g. BCB) as well as appropriate injecting electrodes allows observation of electron transport in FETs in a wide range of semiconductung polymers that were previously thought to be only p-type.
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 Schematic structure and performance characteristics of n-type polymer transistors with BCB as a trap-free dielectric.
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Selected References:
- Lay-Lay Chua, Jana Zaumseil, Jui-Fen Chang, Eric C.W. Ou, Peter K.H. Ho, Henning Sirringhaus and Richard H. Friend, "General Observation of n-Type Field-Effect Behaviour in Organic Semiconductors", Nature 434, 194 (2005)
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