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Category International Presentation
Paper Title Effect of hydrophobicity on stability of organic thin-film transistor
Conference name IC ME&D
Author Wan-Woo Noh, Sin-Hyung Lee, Gyujeong Lee, In-Ho Lee, Chang-Min Keum
Paper code Place Seoul
page Year/date 2015/05/22
Abstract Organic thin-film-transistors (OTFTs) have been extensively studied owing to their potential for low-cost, large-area, and flexible electronics. Recently, the field-effect mobility of the OTFT, being one of the important device parameters, has nearly reached or even exceeded that of an amorphous silicon-based transistor [1]. Nevertheless, the operational instabilities of the OTFT device such as the gate bias stress [1] and the hysteresis [2] have been considered as serious obstacles in use for practical applications. From the viewpoint of the charge traps, the surface properties of a gate insulator, which is interfaced with an active layer, such as the surface roughness [3], the density of hydroxyl groups [4], and the hydrophobicity [5] may significantly influence the stability of the OTFT. For example, the hydrophobicity of the gate insulator has been known to play a role in charge traps when the moisture in the atmosphere is concerned [6]. However, the effect of the hydrophobicity on the stability of the OTFT has not been systematically investigated so far since the modification of the hydrophobicity itself of the gate insulator should be made as a prerequisite. Note that the material replacement of the gate insulator is inevitably accompanied by the change of other surface properties as well [4, 5].
In this work, we report on the effect of the hydrophobicity of a gate insulator on the stability of the OTFT device, in particular, the shift of the threshold voltage by the gate bias stress. Two OTFTs comprising of the same material of the gate insulator, treated to produce different strengths of the surface hydrophobicity, were fabricated in a bottom gate-top contact configuration. A solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene layer was used as an active layer. The hydrophobic fluoropolymer, the CYTOP, was used for preparing the gate insulator layer and the hydrophobicity of the CYTOP was varied without changing other surface properties by means of the modification of the end functionalities as described elsewhere [7]. As shown in Fig. 1, it was found that on applying the gate bias stress of -20 V for 1000 s, the magnitude of the threshold voltage shift was decreased from 17.15 V to 1.98 V according to the change in the hydrophobicity of the gate insulator represented by the contact angle of water. This is attributed to the suppression of the trap formation owing to the water-repellency as well as non-polar end functionalities of the hydrophobic interface. Our results provide a first step toward understanding the physical mechanisms for the instabilities of the OTFT in operation and would be useful for designing the OTFTs for high-performance applications.

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