World’s First Discovery of Anomalous Pressure-Dependent Conductivity of Organic Semiconductors (Press Release)
- Release Date
- 28 Feb, 2013
- BL10XU (High Pressure Research)
Institute of Scientific and Industrial Research (ISIR), Osaka University
Japan Synchrotron Radiation Research Institute (JASRI)
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A joint research group led by Ken-ichi Sakai (research scientist) and Jun Takeya (professor) of ISIR, Osaka University, and Akihiko Fujiwara (chief scientist) of JASRI discovered a gigantic and anomalous pressure effect of conductivity in organic field-effect transistors (OFETs). They clarified the mechanism underlying this effect. The group developed an organic transistor structure that can operate at a high pressure and precisely measured its physical properties. The measurement results were combined with those of structural analysis using high-brilliance X-rays at SPring-8, enabling precise determination of a new physical property unique to organic semiconductors; the group was successful in determining the effect of molecular rotation induced by the application of pressure on the conductivity. These results indicate that organic semiconductor devices have potential for use in electronics in which the current is adjustable by pressurization and in pressure sensors with high sensitivity over a wide range of applied pressures. The achievement of this study was published in the online and printed versions of Physical Review Letters on 28 February and 1 March 2013, respectively. |
Background
Owing to their mechanical flexibility and the possibility of low-cost production by a printing technique, properties absent from electronics based on inorganic materials such as silicon (the most utilized material for current devices), organic semiconductors are being focused on as new next-generation electronic materials. The above properties have triggered intensive studies on various properties of OFETs such as electric conduction and field-effect characteristics. However, the effects of pressure on OFETs have not been investigated sufficiently despite their direct connection to mechanical flexibility and consequent applications such as flexible devices. With increasing external pressure, the conductivity of inorganic semiconductors such as silicon monotonically increases on the basis of a simple microscopic viewpoint, i.e., decreasing distance between the strongly covalently bonded atoms. In contrast, organic semiconductors are expected to exhibit a complicated pressure response, because the molecules with peculiar shapes can be easily rearranged by the external pressure owing to the weak intermolecular interaction.
Achievements of this study
(1) Development of a device structure that operates at high pressures
In general organic transistors, soft organic single crystals are formed on a hard inorganic substrate (silicon dioxide). Therefore, the organic single crystals are broken when the device is subjected to pressure, or shrinkage occurs owing to the large difference in the compressibility between the crystals and the substrate. To prevent this, all components of the device were fabricated using a polymer material as soft as the organic compounds to obtain uniform shrinkage on the entire device. As a result, it was shown that the newly designed devices exhibit a stable performance at high pressures exceeding 1 GPa without malfunctioning, and the conductivity of the devices was evaluated.
(2) Discovery of gigantic and anomalous pressure effect of DNTT*1 field-effect transistors
In this study, DNTT molecules, which contain sulfur atoms, were focused on to evaluate the properties of organic transistors at high pressures. Compared with typical organic semiconductors based on hydrocarbon systems, it was suggested that the sulfur atoms in DNTT molecules play key roles in the realization of the anomalous pressure effect because of their wider electron distribution than those of hydrogen and carbon atoms. As a result of this study, the following were found. (1) A gigantic pressure effect, in which the transport of celectrons between molecules is suddenly activated mainly because of the electron distribution of sulfur atoms. This is attributable to DNTT molecules approaching adjacent DNTT molecules while neatly arranging themselves to avoid repulsive interaction between the sulfur atoms. (2) A negative pressure effect, in which the transport of electrons between molecules becomes difficult owing to their rotation even though they are in close proximity (Figs. 1 and 2).
an organic semiconductor markedly increases with pressure at low pressures,
and negative pressure effect, in which the mobility decreases at high pressures
Future plans
In this study, a characteristic pressure effect of various molecules was demonstrated. The researchers are planning to carry out further fundamental research to find a larger pressure effect as well as a chemical pressure effect that changes the molecular shape and affects intermolecular bonding. They will also start to investigate the feasibility of employing organic semiconductors in pressure sensors and other devices.
<<Glossary>>
*1 DNTT
Dinaphtho-thieno-thiophene
*2 Mobility
Mobility is an indicator representing the ease of movement of charged carriers injected in a semiconductor.
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