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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Processing Organic Semiconductors

Author: BAKLAR, Mohammed

Year: 2011

Supervisor(s): Natalie Stingelin, Ton Peijs

In recent years, there has been a considerable interest in organic semiconducting materials due to their potential to enable, amongst other things, low-cost flexible opto-electronic applications, such as large-area integrated circuitry boards, light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). Promisingly, improved electronic performance and device structures have been realized with e.g. OLEDs entering the market and organic field-effect transistors (OFETs) reaching the performance of amorphous silicon devices; however, it would be too early to state that the field of organic semiconductors has witnessed the sought-after technological revolution.

Initial progress in the field was mostly due to synthetic efforts in the form of enhanced regularity and purity of currently used materials, the creation of new molecular species, etc. In this thesis we show that the advancement of physico-chemical aspects – notably materials processing – and the realisation of increased order and control of the solid state structure is critical to realize the full intrinsic potential that organic semiconductors possess. We first investigated how the bulk charge-transport properties of the liquid-crystalline semiconductor poly(2,5-bis (3-dodecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) can be enhanced by annealing in the mesophase. To this end, temperature treatment of a period of hours was necessary to realize good bulk charge transport in the out-of-plane directions. This behaviour is in strong contrast to in-plane charge transport as measured in thin-film field-effect structures, for which it was shown that annealing times of 10 min and less are often sufficient to enhance device performance. Our observation may aid in future to optimize the use of pBTTT polymers in electronic devices, in which good bulk charge transport is required, such as OPVs.

In the second part of thesis, we explored ink-jet printing of pBTTT-C12, in order to realize precise deposition of this material into pre-defined structures. In organic electronic applications this can, amongst other things, enable deposition of different semiconductors or reduction of the unwanted conduction pathways that often result in undesirable parasitic ‘cross-talk’, for instance, between pixels in display products. We demonstrate the integration of ink-jet printed transistors into unipolar digital logic gates that display the highest signal gain reported for unipolar-based logic gates.

Finally, recognizing that a broad range of conjugated organic species fall in the category of “plastic crystals”, we explored the option to process this class of materials in the solid state. We find that solid-state compression moulding indeed can effectively be applied to a wide spectrum of organic small molecular and polymeric semiconductors without affecting adversely the intrinsic favourable electronic characteristics of these materials. To the contrary, we often observe significantly enhanced [bulk] charge transport and essentially identical field-effect transistor performance when compared with solution- or melt-processed equivalents. We thus illustrate that fabrication of functional organic structures does not necessitate the use of solution processing methods, which often require removal of 99 wt% or more of solvent, or precursor side-products, nor application of cumbersome vapour deposition technologies.

Summarizing, the expanding variety of processing schemes that also allow organic semiconductors to be deposited into functional architectures offers new exciting opportunities in the area of plastic electronics device technologies. In this thesis we explored the potential of ink-jet printing and annealing of liquid crystalline polymers to produce high mobility structures both in the in-plane and out-plane directions. In addition, this thesis illustrates the potential of solid-state processing methods. A multitude of scientific and technical applications readily can be envisioned for all the processing schemes described in this thesis. The final chapter of the present work focuses on three particular possible applications of them in the field of (organic) opto-electronic device technology

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