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Lee, Wi Hyoung,Lee, Seung Goo,Kwark, Young-Je,Lee, Dong Ryeol,Lee, Shichoon,Cho, Jeong Ho American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.24
<P>In organic field-effect transistors (OFETs), surface modification of the gate-dielectric is a critical technique for enhancing the electrical properties of the device. Here, we report a simple and versatile method for fabricating an ultrathin cross-linked interlayer (thickness ∼3 nm) on an oxide gate dielectric by using polymeric silsesquiazane (SSQZ). The fabricated siloxane film exhibited an ultrasmooth surface with minimal hydroxyl groups; the properties of the surface were chemically tuned by introducing phenyl and phenyl/fluorine pendent groups into the SSQZ. The growth characteristics of two semiconductorspentacene (p-type) and <I>N</I>,<I>N</I>′-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C13, n-type)on this ultrathin film were systematically investigated according to the type of pendent groups in the SSQZ-treated gate dielectric. Pentacene films on phenyl/fluorine groups exhibited large grains and excellent crystalline homogeneity. By contrast, PTCDI-C13 films exhibited greater crystalline order and perfectness when deposited on phenyl groups rather than on phenyl/fluorine groups. These microstructural characteristics of the organic semiconductors, as well as the dipole moment of the pendent groups, determined the electrical properties of FETs based on pentacene or PTCDI-C13. Importantly, compared to FETs in which the gate dielectric was treated with a silane-coupling agent (a commonly used surface treatment), the FETs fabricated using the tunable SSQZ treatment showed much higher field-effect mobilities. Finally, surface treatment with an ultrathin SSQZ layer was also utilized to fabricate flexible OFETs on a plastic substrate. This was facilitated by the facile SSQZ deposition process and the compatibility of SSQZ with the plastic substrate.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-24/am507003n/production/images/medium/am-2014-07003n_0007.gif'></P>
Lee, Wi Hyoung,Kwak, Donghoon,Anthony, John E.,Lee, Hwa Sung,Choi, Hyun Ho,Kim, Do Hwan,Lee, Seung Goo,Cho, Kilwon WILEY‐VCH Verlag 2012 Advanced functional materials Vol.22 No.2
<P><B>Abstract</B></P><P>The phase‐separation characteristics of spin‐cast difluorinated‐triethylsilylethynyl anthradithiophene (F‐TESADT)/poly(methyl methacrylate) (PMMA) blends are investigated with the aim of fabricating transistors with a high field‐effect mobility and stability. It is found that the presence of PMMA in the F‐TESADT/PMMA blends prevents dewetting of F‐TESADT from the substrate and provides a platform for F‐TESADT molecules to segregate and crystallize at the air–film interface. By controlling the solvent evaporation rate of the spin‐cast blend solution, it is possible to regulate the phase separation of the two components, which in turn determines the structural development of the F‐TESADT crystals on PMMA. At a low solvent evaporation rate, a bilayer structure consisting of highly ordered F‐TESAT crystals on the top and low‐trap PMMA dielectric on the bottom can be fabricated by a one‐step spin‐casting process. The use of F‐TESADT/PMMA blend films in bottom gate transistors produces much higher field‐effect mobilities and greater stability than homo F‐TESADT films because the phase‐separated interface provides an efficient pathway for charge transport.</P>
Lee, Wi Hyoung,Min, Honggi,Park, Namwoo,Lee, Junghwi,Seo, Eunsuk,Kang, Boseok,Cho, Kilwon,Lee, Hwa Sung American Chemical Society 2013 ACS APPLIED MATERIALS & INTERFACES Vol.5 No.16
<P>Research into printing techniques has received special attention for the commercialization of cost-efficient organic electronics. Here, we have developed a capillary pen printing technique to realize a large-area pattern array of organic transistors and systematically investigated self-organization behavior of printed soluble organic semiconductor ink. The capillary pen-printed deposits of organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), was well-optimized in terms of morphological and microstructural properties by using ink with mixed solvents of chlorobenzene (CB) and 1,2-dichlorobenzene (DCB). Especially, a 1:1 solvent ratio results in the best transistor performances. This result is attributed to the unique evaporation characteristics of the TIPS_PEN deposits where fast evaporation of CB induces a morphological evolution at the initial printed position, and the remaining DCB with slow evaporation rate offers a favorable crystal evolution at the pinned position. Finally, a large-area transistor array was facilely fabricated by drawing organic electrodes and active layers with a versatile capillary pen. Our approach provides an efficient printing technique for fabricating large-area arrays of organic electronics and further suggests a methodology to enhance their performances by microstructural control of the printed organic semiconducting deposits.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2013/aamick.2013.5.issue-16/am401698c/production/images/medium/am-2013-01698c_0010.gif'></P>
Lee, Wi Hyoung,Suk, Ji Won,Lee, Jongho,Hao, Yufeng,Park, Jaesung,Yang, Jae Won,Ha, Hyung-Wook,Murali, Shanthi,Chou, Harry,Akinwande, Deji,Kim, Kwang S.,Ruoff, Rodney S. American Chemical Society 2012 ACS NANO Vol.6 No.2
<P>Chemical doping can decrease sheet resistance of graphene while maintaining its high transparency. We report a new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq. Finally, the method presented here was used to fabricate flexible and a transparent graphene electrode on a plastic substrate.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2012/ancac3.2012.6.issue-2/nn203998j/production/images/medium/nn-2011-03998j_0003.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn203998j'>ACS Electronic Supporting Info</A></P>
Low Spin-Casting Solution Temperatures Enhance the Molecular Ordering in Polythiophene Films
Lee, Wi Hyoung,Lee, Hwa Sung,Park, Yeong Don Korean Chemical Society 2014 Bulletin of the Korean Chemical Society Vol.35 No.5
High-crystallinity poly(3-hexylthiophene) (P3HT) thin films were prepared by aging the precursor solutions, prepared using a good solvent, chloroform, at low temperatures prior to spin-casting. Lower solution temperatures significantly improved the molecular ordering in the spin-cast P3HT films and, therefore, the electrical properties of field-effect transistors prepared using these films. Solution cooling enhanced the electrical properties by shifting the P3HT configuration equilibrium away from random coils and toward more ordered aggregates. At room temperature, the P3HT molecules were completely solvated in chloroform and adopted a random coil conformation. Upon cooling, however, the chloroform poorly solvated the P3HT molecules, favoring the formation of ordered P3HT aggregates, which then yielded more highly crystalline molecular ordering in the P3HT thin films produced from the solution.
Increased Environmental Stability of a Tungsten Bronze NIR-absorbing Window
Lee, Wi Hyoung,Hwang, Hyunmin,Moon, Kyunghwan,Shin, Kwonwoo,Han, Jong Hun,Um, Soong Ho,Park, Juhyun,Cho, Jeong Ho 한국섬유공학회 2013 FIBERS AND POLYMERS Vol.14 No.12
We developed a facile method for increasing the environmental stability of a tungsten bronze near-infrared (NIR)-absorbing window using tetraethyl orthosilicate (TEOS) and 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FDS). The environmental stability of the tungsten bronze NIR-absorbing window could be enhanced by applying a variety of protective layers (i.e., TEOS, fluoropolymer (CYTOP), FDS). The protective characteristics of each layer type are discussed. The protection of tungsten bronze surfaces by TEOS and FDS layers enormously enhanced the environmental stability of the NIR absorbing window, whereas an untreated tungsten bronze film rapidly lost its NIR absorption properties. The protection efficiency followed the order: TEOS/FDS>FDS>TEOS>CYTOP. The improved environmental stability arose from the closely packed structure of FDS, which can self-assemble on an oxide surface, such as the tungsten oxide or silicon oxide surfaces. The method developed here provides a simple, robust, and versatile way to improve the environmental stability of a NIR-absorbing window.
Semiconductor-Dielectric Blends: A Facile All Solution Route to Flexible All-Organic Transistors
Lee, Wi Hyoung,Lim, Jung Ah,Kwak, Donghoon,Cho, Jeong Ho,Lee, Hwa Sung,Choi, Hyun Ho,Cho, Kilwon WILEY-VCH Verlag 2009 Advanced Materials Vol.21 No.42
<B>Graphic Abstract</B> <P>A one-step process for the production of all-organic, all-solution-processed field-effect transistors (FETs) can be achieved using triethylsilylethynyl anthradithiophene (TES-ADT). TES-ADT has a lower surface energy than poly(methyl methacrylate) (PMMA), which results in a segregation and crystal formation of TES-ADT at the air–film interface after spin-casting and subsequent solvent annealing. The resulting FETs comprise vertically phase-separated semiconducting and dielectric layers and exhibit high performances. <img src='wiley_img/09359648-2009-21-42-ADMA200900277-content.gif' alt='wiley_img/09359648-2009-21-42-ADMA200900277-content'> </P>
Selective-Area Fluorination of Graphene with Fluoropolymer and Laser Irradiation
Lee, Wi Hyoung,Suk, Ji Won,Chou, Harry,Lee, Jongho,Hao, Yufeng,Wu, Yaping,Piner, Richard,Akinwande, Deji,Kim, Kwang S.,Ruoff, Rodney S. American Chemical Society 2012 Nano letters Vol.12 No.5
<P>We have devised a method to selectively fluorinate graphene by irradiating fluoropolymer-covered graphene with a laser. This fluoropolymer produces active fluorine radicals under laser irradiation that react with graphene but only in the laser-irradiated region. The kinetics of C–F bond formation is dependent on both the laser power and fluoropolymer thickness, proving that fluorination occurs by the decomposition of the fluoropolymer. Fluorination leads to a dramatic increase in the resistance of the graphene while the basic skeletal structure of the carbon bonding network is maintained. Considering the simplicity of the fluorination process and that it allows patterning with a nontoxic fluoropolymer as a solid source, this method could find application to generate fluorinated graphene in graphene-based electronic devices such as for the electrical isolation of graphene.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2012/nalefd.2012.12.issue-5/nl300346j/production/images/medium/nl-2012-00346j_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl300346j'>ACS Electronic Supporting Info</A></P>