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Posted: Apr 02, 2008
Using molecules as processable semiconductors
(Nanowerk Spotlight) The most common type of modern transistor, and the type of transistor used in integrated circuits, is called a field-effect transistor (FET). The FET is so named because it relies on an electric field to control the shape and hence the conductivity of a 'channel' (the charge carrier) in a semiconductor material. This field causes a second electrical current to flow across the semiconductor, identical to the first weak signal, but stronger. Since the invention of the first transistor in 1947, the vast majority of electronic devices have been based on inorganic semiconductors, which in most cases has been silicon. Due to the demand for lightweight, flexible opto-electronic devices such as displays, solar cells and lasers, organic materials have become an important new class of semiconductor as they combine the virtues of plastics, which can be easily shaped, with those of semiconductors which are the basis of all microelectronics.
Contrary to amorphous silicon, which is widely used in solar cells and flat screen displays, organic materials offer the benefits
that they can be deposited on plastic substrates at low temperature by employing solution-based printing techniques, which would result in a dramatic reduction of the manufacturing costs. Another benefit is the reduced environmental impact: Organic transistors and other printed electronics allow transistors to be formed by printing directly onto the substrate. This means that manufacturing processes can be dramatically simplified in comparison to conventional semiconductors; waste materials and carbon dioxide emissions generated through manufacturing processes can be reduced.
Organic field-effect transistors (OFETs) have been mainly based on two types of semiconductors: conjugated polymers and small conjugated molecules. A recent review, published in Chemical Society Reviews, provides a general introduction about the current standing in the area of OFETs focusing on the new processable small molecules that have been recently reported for their use as organic semiconductors ("Novel small molecules for organic field-effect transistors: towards processability and high performance" – free access article).
The review's two authors, Dr. Marta Mas-Torrent and Prof. Concepció Rovira from the Department of Molecular Nanoscience and Organic Materials at the Institut of Materials Science of Barcelona (ICMAB), Spain, say that – though the first OFETs did not transport charge as well as inorganic materials – the best ones nowadays are achieving charge carrier mobilities of the same order as amorphous silicon.
"Organic-based electronics will not replace high density and high speed silicon circuits, but might play an important role in applications such as identification tags, electronic bar codes or active matrix elements for displays."
Transistors made from organic materials have an advantage over normal silicon transistors. They can be constructed on flexible surfaces, such as plastic film, making organic circuits ideal for portable and mobile devices. Solution-based techniques such as drop-casting or spin-coating are used for depositing the organic semiconductor material on the device.
By combining these processes with stamping or printing techniques, it is possible to pattern organic semiconductors eliminating the use of lithography (and eliminating poisonous waste materials).
The authors point out that, since organic semiconductors are often not very soluble, an alternative deposition method is by sublimation of the organic material in a variety of vacuum depositions systems. Parameters such as pressure and substrate temperature determine the morphology and quality of the resulting films.
Charge Transport Mechanisms
The authors note that, despite all the intense work devoted to OFETs, charge transport mechanisms on organic semiconductors are still
uncertain and that further work needs to be performed to understand the transport mechanisms in OFETs.
This issue arises from the fact that the intermolecular forces in organic semiconductors are weak van der Waals interactions, which leads to completely different transport mechanisms than those that occur in inorganic semiconductors. In materials like silicon or germanium, atoms are held together by strong covalent bonds permitting charge transport to take place via delocalized states following a band transport regime. The transport is limited by the lattice vibrations and, thus, at lower temperatures the conductivity in these materials
In contrast, "it is generally agreed that, at least at room temperature, the charge mobility of semiconducting organic materials is determined by a hopping transport process. This transport mechanism is phonon assisted and thus is thermally activated. Hopping transport can be depicted as an electron or hole transfer reaction in which an electron or hole is transferred from one molecule to the neighboring one."
Gaining an improved understanding on how exactly charge transport mechanisms on organic semiconductors occur will be helpful for designing new organic semiconductors.
OFETs have been mainly based on two families of semiconductors – conjugated polymers and small conjugated molecules. Polymers are deposited from solution, allowing for low cost electronics.
Mas and Rovira explain that the higher molecular disorder in polymers limits their charge transport, typically resulting in lower mobilities compared to devices based on small molecules. "On the other hand, devices prepared with small conjugated molecules have to be prepared more expensively by evaporating organic materials, due to their low solubility in common organic solvents. Therefore, to promote the development and use of organic semiconductors, there is a clear need to find materials that can be processed in solution and that simultaneously achieve a high OFET mobility."
Most of the organic transistors that have been fabricated to date show only unipolar conduction (either holes or electrons), with the ones based on hole transport much more developed. There is a growing interest in developing electron or ambipolar devices (which conduct both electrons and holes) because this would allow to design robust circuits of low power consumption and a good noise margin following CMOS technology.
However, the development of ambipolar semiconductors, specially the ones that can be solution-processed, is still at its infancy. Carrier mobility is the main limiting factor in the operating frequency of complementary circuits, so there is an urgent need to find materials with much higher mobilities for both holes and electrons.
The paper describes active-matrix electronic paper as one of the first emerging devices realized with OFETs. "These systems consist of an organic transistor backplane in which each transistor functions as a switch that locally controls the display of microencapsulated electrophoretic 'inks' formed by charged pigments. Activating the transistors generate an electric field that causes movement of the pigment within the microcapsules, which changes the color of the pixel. The high potential of these devices is confirmed by their high stability and excellent paper-like contrast, making the commercialization of these products imminent."
There is a wide range of applications where OFETs, promising cheap, flexible and disposable plastic electronics, will be preferred over traditional silicon electronics. OFETs promise to be important in applications ranging from sophisticated medical diagnostics to 'smart' clothes that can display changing images. "New markets will undoubtedly appear in areas where electronics meets with information technology, biomedicine or optics" Mas and Rovira conclude their review.
Watch a video on the coming age of plastic electronics: