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Surface, Interface and Adhesion
Organic Field Effect Transistors
Organic Photovoltaics
 
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Overview : Organic Field Effect Transistor                                     
    In recent years, organic field-effect transistors (OFETs) based on conductive π-conjugated molecules have received significant attention because of their flexibility and low cost processing. The main reason is the fact that the performance of OFETs is now comparable to amorphous silicon thin-film transistors. In particular, the high field-effect mobility of organic π-conjugated molecules (i.e., poly(3-hexylthiophene), pentacene, triisopropylsilylethynyl pentacene) has stimulated a lot of interest in the utilization of this fascinating material as OFETs. Our research group studies various area of research field from fundamental aspects of charge transport mechanism in organic π-conjugated molecules to printing processes for realizing low-cost, large-area production of OFETs.
 
 
  
 One of the long-standing challenges in the development of OFETs is to produce organic single crystals of semiconducting materials. Organic single crystals are known not only to have superior device performance but also to provide a valuable resource for the investigation of the intrinsic parameters that determine charge transport efficiency. In particular, the highest mobilities have been found in single crystals because they are largely free of the grain boundaries and molecular disorder that reduce the mobility of the material, and thus the overlapping of the π-π orbitals is enhanced, which promotes efficient charge transport. It is well known that organic single crystal can be achieved by vacuum process. Alternatively, organic single crystals can be grown by using facile solution process. Our research group succeeded in fabricating organic single crystals (poly(3-hexylthiophene), triisopropylsilylethynyl pentacene) by solution process, which gives us possibility to study charge transport characteristics of FETs based on these materials. Now, we are investigating fundamental aspects of charge transport (i.e., anisotropic charge transport, charge transport mechanism) through temperature experiment.
  
 

 Controlled self-assembly of π-conjugated organic molecules (including conjugated polymers, oligomers, and small molecules) via π-π interaction has  attracted increasing attention in view of their potential utility for the fabrication of nano- or micro-structured building blocks in the application of OFETs. The thin-film morphologies and crystal structures of π-conjugated organic molecules are sensitive to processing conditions, such as the solvent drying rate, the solubility of molecules, and the surface properties of the substrate. In particular, the drying behavior during solution processing has a critical role in the self-organization of π-conjugated organic molecules. Our research group studies how to control the self-organization of π-conjugated organic molecules when these molecules are deposited onto the substrate. For example, the fabrication of one-dimensional crystal arrays of triisopropylsilylethynyl pentacene has been accomplished by controlling the evaporation behavior of the solvent during simple drop casting on a tilted substrate. Post-deposition treatment such as thermal or solvent-vapor annealing has been applied to enhance the molecular ordering of organic semiconductor film, which can effectively improve device performance of OFETs.

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In the operation OFETs, charge carrier transport is strongly dependent on the properties of two kinds of interface: interfaces between semiconductors and electrodes, where charge injection occurs from the electrodes into the semiconductors, and interfaces between semiconductors and dielectrics, where charge transport takes place in the semiconductor layer. In case of the interfaces between semiconductors and electrodes, hole injection barrier, energy barrier between HOMO level of semiconductor and work function of electrode, play a dominant role in the device performance. We have demonstrated that plasma or UV-ozone treatment of electrodes could be applied to lower the hole injection barrier, which results in enhancement of field-effect mobility. In the case of interfaces between semiconductors and dielectrics, surface modification of dielectric is efficient method for enhancing the performance of OFETs. To control this interface, we have used self-assembled monolayers (SAMs) with different chain length, functional group, and phase state (order/disorder). We have shown that the microstructures and morphologies of organic semiconductors could effectively be modulated, which results in the improvement of device performance.

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π-Conjugated organic molecules which have excellent conducting or semiconducting behavior open up new avenues in organic electronic technology, in which integrated circuits are produced by cheap solution processing and direct printing. Among other printing technologies such as gravure or screen printing, the inkjet printing is very promising due to the compatibility with various substrates because the ink material can be directly transferred to the substrate without contact with the surface. However, the inkjet printing of organic semiconductor films with uniform morphology and the desired crystalline microstructure become a major challenge for direct-write fabrication of high-performance organic devices because charge carrier transport in organic electronic devices is strongly related to the crystalline microstructures and morphologies of their organic semiconductor films. We have demonstrated the influence of evaporation-induced flow in a single droplet on the crystalline microstructure and film morphology of an inkjet-printed organic semiconductor by varying the composition of the solvent mixture.
 
    OFETs
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이용약관 | 개인정보취급방침 | 이메일주소무단수집거부 | 청소년보호정책 | 책임의한계와법적고지 | 검색결과수집거부
Department of Chemical Engineering, POSTECH, Hyoja-dong, San31, Pohang, 790-784, KOREA Tel : 054-279-2932 Fax : 054-279-8298
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