David Sparrowe

Polymerisable liquid crystalline organic semiconductors and their fabrication in organic field effect transistors

The performance of the semiconducting component in organic field effect transistors (OFETs) is a key parameter in the advancement of organic electronic devices. New semiconductors are required, which can be solution processed, possess high mobility and current modulation, and are stable in ambient conditions. This work provides the first demonstration of working field effect transistor devices fabricated from novel solution processible, polymerisable, small molecule liquid crystalline semiconductors, referred to as reactive mesogens. The design, synthesis, and performance of these materials in transistor devices are reported. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology was employed to control the liquid crystalline morphology, in an attempt to optimise organisation and packing. Alignment of the molecules in large homeotropic domains was achieved through surface treatment techniques, and the highly ordered mesophase was preserved by polymerisation of the reactive end groups, creating a crosslinked network.

Ink-jet printed p-type polymer electronics based on liquid-crystalline polymer semiconductors

Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTTs)—currently one of the most promising class of polymer semiconductors—are known to have a low-room-temperature solubility in a broad range of common organic solvents. By judicious selection of a suitable solvent for these rigid-rod-like, liquid-crystalline materials, we ink-jet printed poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C12) into discrete thin-film field-effect transistors (FETs) that display on/off ratios of up to 107 and charge-carrier mobilities in the range of 0.05–0.1 cm2 V−1 s−1. Compared to spin-coated devices, the ink-jet printed devices typically had lower current leakage; indeed, gate-leakage currents were 1 to 2 orders of magnitude lower in ink-jet printed transistors. These overall device performance of ink-jet printed pBTTT-C12 would be sufficient for, for instance, driving simple electrophoretic displays [T. N. Ng, S. Sambandan, R. Lujan, A. C. Arias, C. R. Newman, H. Yan and A. Facchetti, Appl. Phys. Lett., 2009, 94, 233307; G. H. Gelinck, T. C. T. Geuns and D. M. de Leeuw, Appl. Phys. Lett., 2000, 77, 1487–1489; H. E. A. Huitema, G. H. Gelinck, J. van der Putten, K. E. Kuijk, C. M. Hart, E. Cantatore, P. T. Herwig, A. van Breemen and D. M. de Leeuw, Nature, 2001, 414, 599–599; H. E. A. Huitema, G. H. Gelinck, J. van der Putten, K. E. Kuijk, K. M. Hart, E. Cantatore and D. M. de Leeuw, Adv. Mater., 2002, 14, 1201–1204.]. We demonstrate here integration of such ink-jet printed pBTTT-C12 transistors into unipolar digital logic gates, which produced a signal gain of more than 16. This is one of the highest gains reported for a unipolar-based logic gate and demonstrates the potential of pBTTT polymers for high performance application employing simple ink-jet printing procedures conducted in light and ambient air without any special precautions.

Hexyl‐Substituted Oligoselenophenes with Central Tetrafluorophenylene Units: Synthesis, Characterisation and Application in Organic Field Effect Transistors

A series of selenophene oligomers incorporating conjugated fluorinated phenylene units have been synthesised as potential semiconductor materials for organic field-effect transistors (OFETs). X-ray crystallography shows that the molecules are held in close proximity by several short intermolecular contacts, making them ideal candidates for OFET applications.

Stability of top- and bottom-gate amorphous polymer field-effect transistors

Performance and stability between the top- and bottom-gate field-effect transistor configurations are investigated in dual-gate transistor structures consisting of the same insulator and gate materials. The transistors behave similarly for both gate modes with on/off ratio in excess of 105, subthreshold swing of 0.5–1 V/decade, and mobility of 0.03–0.04 cm2/V s, retained over several months, with fabrication, storage, and characterization, performed in ambient conditions.

Designing solution-processable air-stable liquid crystalline crosslinkable semiconductors

Organic electronics technology, in which at least the semiconducting component of the integrated circuit is an organic material, offers the potential for fabrication of electronic products by low-cost printing technologies, such as ink jet, gravure offset lithography and flexography. The products will typically be of lower performance than those using the present state of the art single crystal or polysilicon transistors, but comparable to amorphous silicon. A range of prototypes are under development, including rollable electrophoretic displays, active matrix liquid crystal (LC) displays, flexible organic light emitting diode displays, low frequency radio frequency identification tag and other low performance electronics. Organic semiconductors that offer both electrical performance and stability with respect to storage and operation under ambient conditions are required. This work describes the development of reactive mesogen semiconductors, which form large crosslinked LC domains on polymerization within mesophases. These crosslinked domains offer mechanical stability and are inert to solvent exposure in further processing steps. Reactive mesogens containing conjugated aromatic cores, designed to facilitate charge transport and provide good oxidative stability, were prepared and their liquid crystalline properties evaluated. The organization and alignment of the mesogens, both before and after crosslinking, were probed by grazing incidence wide-angle X-ray scattering of thin films. Both time-of-flight and field effect transistor devices were prepared and their electrical characterization reported.

The influence of molecular weight on the microstructure and thin film transistor characteristics of pBTTT polymers.

A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.

Bulk charge transport in liquid-crystalline polymer semiconductors based on poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene)

The class of liquid-crystalline semiconducting polymers based on poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) recently has attracted significant interest in the field of organic electronics, predominantly due to their promising performance in field-effect transistor (FET) structures with device mobilities reaching—if not exceeding—those of amorphous silicon architectures. Less is known, however, about the bulk charge-transport properties of these interesting macromolecules. We therefore conducted time-of-flight (TOF) photoconductivity measurements on one particular material of this class of organic semiconductors—i.e. poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene), pBTTT-C12—and attempted to correlate the electronic bulk properties of this polymer with its microstructural development with temperature. The effect of annealing was also investigated.

Organic thin films with charge-carrier mobility exceeding that of single crystals

The performance of organic field-effect transistors (OFETs) depends heavily upon the intrinsic properties and microstructure of the semiconducting layer, the processes taking place at the semiconductor/dielectric interface, and the quality of contacts. In this article, we report on 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene single crystal and thin-film OFETs and compare their properties. We find that the single crystals exhibit a pronounced anisotropy in electrical characteristics, with a maximum field-effect mobility of 0.3 cm2 V−1 s−1. Through density functional theory (DFT) calculations we identified the main direction for hole transport, which was confirmed by X-ray diffraction (XRD) measurements as parallel to the plane of the single crystal facet where the transport was probed. By processing the material as a thin-film semiconductor, the content of high-mobility direction probed within the transistor channel was enhanced. The control of film morphology, coupled with a different design of the device structure allowed us to obtain an order of magnitude higher charge-carrier mobilities and a very small spread in device performance.

Stable semiconducting thiophene polymers and their field effect transistor characteristics

The development of p-type semiconducting polymers demonstrating good stability under ambient operation is of importance for the development of low cost, printed electronics. We present here the synthesis and full characterisation of two soluble terthiophene polymers, and examine the effect of introducing a fused aromatic heterocycle, thieno[2,3-b]thiophene, into a terthiophene polymer backbone. This heterocycle contains a cross-conjugated central double bond, and its inclusion was shown to have a marked influence on the optical, thermal and electrical properties of the terthiophene polymer. Transistors were fabricated from both polymers, and the operation and storage lifetime under ambient operation was compared.

Self-assembled liquid crystalline solution processable semiconductors

This work describes the development of solution processable liquid crystalline semiconductors and their applications in field-effect transistors. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology is employed to control the liquid crystalline morphology. The thermal, optical and electrical behaviour of these materials is characterised and X-ray diffraction scattering technique is used to reveal details of morphology and molecular orientation.