Organic Field Effect Transistors
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Overview : Organic Field Effect Transistors
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.
Stretchable electronics have recently emerged as an important technology for the realization of next-generation electronic applications. Device elasticity under tensile stress, i.e., stretchability, beyond flexibility and bendability, is required in a wide range of future electronic applications. Despite the considerable efforts applied toward developing stretchable electronics, few intrinsically stretchable semiconductors have been reported that retain the original electrical characteristics under stretching. For these reasons, stretchable electronic devices, which consist solely of intrinsically stretchable materials, need to be developed as the ultimate concept device. .
Our studies introduces an intrinsically stretchable and transparent organic semiconducting layer by blending self-assembled organic semiconductor with an elastomeric and transparent polymer substrate. The stretchable and transparent active material was designed to have the structure of 1D organic semiconducting nanowire (NW) networks embedded in a transparent elastomeric matrix, which was demonstrated by blending self-assembled poly(3-hexylthiophene) (P3HT) NWs with poly(dimethylsiloxane) (PDMS). Compared to the homo-P3HT NWs, the blends were more suitable as active materials in OTFTs by systematically investigating the morphologies, crystalline structures, and electrical properties of blends with various P3HT contents .
Hard segments of self-organized nanocrystals and soft rubber segments was introduced to develop polymer dielectric layer. The segmented elastomeric network inside an end-functionalized reactive liquid rubber with a high dielectric constant was developed as dielectric layer by using an organosilane cross-linking agent .

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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The use in low-power soft electronics of the appropriate insulating polymer materials with a high dielectric constant (k) is considered a practical alternative to that of inorganic dielectric materials, which are brittle and have high processing temperatures. However, the polar surfaces of typical high-k polymer insulators are problematic. Further, it is a huge challenge to control their surface properties without damage because of their soft and chemically fragile nature.

In our study, a heat-assisted photoacidic oxidation method was used to oxidize the outermost surfaces of high-k rubbery polymer films without degradation. The oxidized surfaces prepared with the developed method contain large numbers of hydroxyl groups that enable the subsequent growth of dense and ordered self-assembled monolayers (SAMs) consisting of organosilanes. The whole process modifies the surface characteristics of polymer dielectrics effectively. The resulting surface-tailored rubbery dielectrics exhibit superior electrical characteristics when used in organic transistors.

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The bias stability of OFET devices remains a critical obstacle to their commercial use. The microstructural origins of charge traps inside OFET devices are not yet clearly understood. We investigated the correlation between the molecular orientations of the conjugated polymer thin films and the bias stress stabilities in OFETs fabricated using structurally controlled films by exploiting the unique electrical properties of P(NDI2OD-T2). A relatively high bias stress stability was observed in the P(NDI2OD-T2) FETs prepared with a face-on structure, compared to the FETs prepared with an edge-on film structure. The trapped charges induced by the bias stress were predominantly present in the P(NDI2OD-T2) layer, particularly near the interface with the source/drain electrodes. The measured trap DOS profiles indicated that the intensities of the energy states in the mid-gap region (1.15 and 1.25 eV) differed significantly, depending on the structure (edge-on or face-on) of the P(NDI2OD-T2) thin films. These results indicated the formation of localized polaron pairs, and aliphatic alkyl chains present in the edge-on structured P(NDI2OD-T2) film appeared to present a huddle to vertical charge transport, thereby increasing the density of the bipolarons during the bias stress. We believe that these results pave the way for the fabrication of practical and useful OFETs with a high degree of bias stress stability suitable for the realization of commercially available OFET-based products.
Written by OTFT Team ( Seon Baek Lee , Sangsik Park, Jinsung Kim, Seong Hyun Kim, Sanghyo Kim, Min Kyu Kim )
Edited by Sanghyo Kim ( 2019.08.12 )

이용약관 | 개인정보취급방침 | 이메일주소무단수집거부 | 청소년보호정책 | 책임의한계와법적고지 | 검색결과수집거부
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