Development of High-Performance Organic Thin-Film Transistors for Large-Area Displays
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Development of HighPerformance Organic Thin-Film Transistors for Large-Area Displays Sangyun Lee, Bonwon Koo, Jae-Geun Park, Hyunsik Moon, Jungseok Hahn, and Jong Min Kim Abstract Organic thin-film transistors (OTFTs) are considered indispensable in applications requiring flexibility, large area, low processing temperature, and low cost. Key challenges to be addressed include developing solution-processable gate dielectric materials that form uniform films over large areas and exhibit excellent insulating properties, reducing contact resistance at interfaces between organic semiconductors and electrodes, and optimizing the patterning of organic semiconductors. High-performance pentacene-based OTFTs have been reported with polymeric gate dielectrics and indium tin oxide source/ drain electrodes. Using such OTFT backplates, a 15-in. 1024 ⫻ 768 pixel full-color active-matrix liquid-crystal display (AMLCD) and a 4.5-in. 192 ⫻ 64 pixel active-matrix organic light-emitting diode (AMOLED) have been fabricated. Keywords: dielectric, electronic material, organic, polymer, semiconducting.
Introduction Since the late 1940s, organic semiconductors have been continually studied, and organic thin-film transistors (OTFTs) have recently been of great interest for several electronics applications such as activematrix flat-panel displays, electronic paper, and chemical sensors, replacing the traditional inorganic semiconductor-based transistors.1 Among the many developed organic semiconductors, thermally evaporated pentacene has shown a carrier mobility reaching 3–5 cm2 V–1 s–1, which exceeds that of thin-film transistors (TFTs) based on amorphous Si.2 According to time-of-flight measurements, organic semiconductors can have intrinsic carrier mobilities ranging from 1 cm2 V–1 s–1 to 10 cm2 V–1 s–1.3 In addition, recently, rubrene showed a mobility in the range of 10 cm2 V–1 s–1 in single-crystal field-effect
MRS BULLETIN • VOLUME 31 • JUNE 2006
transistors.4,5 However, field-effect mobilities in thin-film transistors are lower than the theoretical maximum values, implying that the measured mobilities in thinfilm transistors are extrinsic ones that are mainly affected by the crystallinity of the semiconducting layer6 and interface characteristics in devices.7 There is still room for further improvement by optimizing the surrounding materials and processes used in the fabrication of OTFTs. Among many determining factors, the overall performance of OTFTs critically depends on the choice of the organic gate insulator8 and the contact resistance between organic semiconductors and source/drain materials.9 For large-area displays, processability must be taken into account. Solutionprocessable, cross-linked polymeric
dielectrics are promising materials that provide high-quality films over a large area, leading to a low fabrication cost. Subsequent thermal curing or photocuring affords a three-dimensional network of polymer dielectrics, leading to excellent dielectric properties. On the other hand, the choice of electrode materials is cr
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