Reliable semi-transparent pentacene thin-film transistors with polymer gate dielectric layers cured at an optimum temper
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0905-DD05-15.1
Reliable semi-transparent pentacene thin-film transistors with polymer gate dielectric layers cured at an optimum temperature D. K. Hwang1, Ji Hoon Park2, Jeong-M. Choi1, Jiyoul Lee1, S. H. Jeong1, Eugene Kim2, Jae Hoon Kim1, and Seongil Im1 1 Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea 2 Department of Information and Display, Hongik University, Seoul 121-791, Korea ABSTRACT We report on the insulator-charging-effects of poly-4-vinylphenol (PVP) gate dielectric on the reliabilities of pentacene thin-film transistors (TFTs). Our PVP films were prepared by spin coating and curing at various temperatures (155, 175, and 200 ℃). Evaluated using Au/PVP/p+Si structures, the dielectric strength of PVP films cured at 175 ℃ was superior to those of the other PVP films cured at different temperatures. Although saturation current and field mobility (~0.13 cm2/Vs) obtained from a TFT with PVP film cured at 200 ℃ appeared higher than those (~0.07 cm2/Vs) from the device with 175 ℃-cured polymer film, the TFT prepared at 200 ℃ revealed a low on/off current ratio of less than 104 due to its high off-state current and also unreliable saturation behavior under repetitive gate voltage sweep. The unreliable behavior is due to the dielectric-charging caused by gate-electron-injection. We thus conclude that there are some optimal PVP-curing conditions to improve the reliability of pentacene TFT. INTRODUCTION To make use of the full advantages of organic thin-film transistors (OTFTs), namely, large area coverage, mechanical flexibility, and low-cost processing, it is necessary to employ polymer gate dielectric materials using solution processes. Poly-4-vinylphenol (PVP) is one of the popular gate materials. OTFTs with PVP gate dielectric and opaque noble-metal source-drain (S/D) contacts displayed a good performance with a field effect mobility of ~0.1 cm2/Vs and an on/off ratio over 105 [1,2]. However, the charging capacitances of the polymer gate dielectric are still inferior to those of inorganic dielectric materials. Moreover, sensitivity to gate bias stress in OTFTs with the polymer dielectric, resulting in threshold voltage change depending on the direction of gate voltage sweeping has been often found and reported [1]. Few studies were reported on the reasons of the threshold voltage shift or of the unreliable behavior [3]. Hence no report has been found on optimum PVP curing conditions for cross-linking of polymer chains, which may substantially influence the electrical properties of polymer gate dielectrics films and
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further influence the reliability of the OTFTs with polymer gate dielectric. In the present study, we have fabricated pentacene TFTs with a PVP dielectric and NiOx S/D electrode, systematically varying PVP-curing temperatures to achieve more reliable gate behavior from the pentacene-based TFTs with polymer dielectric layers. EXPERIMENTAL PVP films were then prepared from solution of PVP and poly (melamine-co-formaldehyde), as a cross-linking agen
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