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Nketia-Yawson, Benjamin,Kang, Hyojin,Shin, Eul-Yong,Xu, Yong,Yang, Changduk,Noh, Yong-Young Elsevier 2015 ORGANIC ELECTRONICS Vol.26 No.-
<P><B>Abstract</B></P> <P>We report the effect of an electron-donating unit on solid-state crystal orientation and charge transport in organic field-effect transistors (OFETs) with thienoisoindigo (TIIG)-based small molecules. End-capping of different electron-donor moieties [benzene (Bz), naphthalene (Np), and benzofuran (Bf)] onto TIIG (giving TIIG-Bz, TIIG-Np, and TIIG-Bf) is resulted in different electronic energy levels, solid-state morphologies and performance in OFETs. The 80°C post-annealed TIIG-Np OFETs show the best device performance with a best hole mobility of 0.019cm<SUP>2</SUP> V<SUP>−1</SUP> s<SUP>−1</SUP> and threshold voltage of −8.6±0.9V using top gate/bottom contact geometry and a CYTOP gate dielectric. We further investigated the morphological microstructure of the TIIG-based small molecules by using grazing incidence wide angle X-ray scattering, atomic force microscopy and a polarized optical microscope. The electronic transport levels of the TIIG-based small molecules in thin-film states were investigated using ultraviolet photoelectron spectroscopy to examine the charge injection properties of the gold electrode.</P> <P><B>Highlights</B></P> <P> <UL> <LI> OFETs with thienoisoindigo (TIIG) small molecules are reported. </LI> <LI> The morphology effect on device characteristics is studied. </LI> <LI> TOP gate devices are developed with CYTOP dielectric layer. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Nketia-Yawson, Benjamin,Jung, A-Ra,Nguyen, Hieu Dinh,Lee, Kyung-Koo,Kim, BongSoo,Noh, Yong-Young American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.38
<P>We report synthesis of a new poly(4-(4,4-bis(2-ethylhexyl)-4<I>H</I>-silolo[3,2-<I>b</I>:4,5-<I>b</I>′]dithiophene-2-yl)-7-(4,4-bis(2-ethylhexyl)-6-(selenophene-2-yl)-4<I>H</I>-silolo[3,2-<I>b</I>:4,5-<I>b</I>′]dithiophene-2-yl)-5,6-difluorobenzo[<I>c</I>][1,2,5]thiadiazole (PDFDSe) polymer based on planar 4,7-bis(4,4-bis(2-ethylhexyl)-4<I>H</I>-silolo[3,2-<I>b</I>:4,5-<I>b</I>′]dithiophen-2-yl)-5,6-difluorobenzo[<I>c</I>][1,2,5]thiadiazole (DFD) moieties and selenophene linkages. The planar backboned PDFDSe polymer exhibits highest occupied molecular orbital and lowest unoccupied molecular orbital levels of −5.13 and −3.56 eV, respectively, and generates well-packed highly crystalline states in films with exclusive edge-on orientations. PDFDSe thin film was incorporated as a channel material in top-gate bottom-contact organic thin-film transistor with a solid-state electrolyte gate insulator (SEGI) composed of poly(vinylidene difluoride-trifluoroethylene)/poly(vinylidene fluoride-<I>co</I>-hexafluroropropylene)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, which exhibited a remarkably high hole mobility up to μ = 20.3 cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP> corresponding to effective hole mobility exceeding 5 cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP> and a very low threshold voltage of −1 V. These device characteristics are associated with the high carrier density in the semiconducting channel region, induced by the high capacitance of the SEGI layer. The excellent carrier mobility from the PDFDSe/SEGI device demonstrates a great potential of semiconducting polymer thin-film transistors as electronic components in future electronic applications.</P> [FIG OMISSION]</BR>
Nketia-Yawson, Benjamin,Tabi, Grace Dansoa,Noh, Yong-Young American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.19
<P>We report on systematic mobility enhancements in electrolyte-gated organic field-effect transistors (OFETs) by thinning down the active layer and exploiting polymer solid-state electrolyte gate insulators (SEGIs). The SEGI is composed of homogeneous poly(vinylidene fluoride-<I>co</I>-hexafluoropropylene) [P(VDF-HFP)] polymer solution-ion gel blends of high areal capacitance of >10 μF cm<SUP>-2</SUP> at 1 Hz. By scaling up the poly(3-hexylthiophene) (P3HT) semiconducting layer by 1 order of magnitude (5-50 nm), an ultraviolet photoelectron spectroscopy examination reveals a downward vacuum-level shift generating a substantial hole injection barrier that originates from different interfacial dipole layer formations. The ultrathin (5.1 nm) P3HT FETs outperformed the other devices, exhibiting stable device characteristics with a highest field-effect mobility of >2 cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP> (effective mobility of 0.83 ± 0.05 cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP>), on/off ratio of ∼10<SUP>6</SUP>, low threshold voltage of <−0.6 V, and low gate-leakage current levels of ∼10<SUP>5</SUP> below the on-current levels in 10 μm channel length devices. We observed a positive threshold voltage shift in the P3HT/SEGI FETs with decreasing semiconductor thickness. The aforementioned mobility is at least 10 times greater than that of neat P(VDF-HFP) devices. The significant FET performance is attributed to a better insulator/semiconductor interface, efficient hole injection from the Au electrode resulting in a low contact resistance of <500 Ω cm, and boosted charge-carrier densities in the transistor channel. This work demonstrates an excellent approach for carrier mobility enhancement and reliability assessment in low-voltage-operated electrolyte-gated OFETs.</P> [FIG OMISSION]</BR>
Nketia-Yawson, Benjamin,Jung, A-Ra,Noh, Yohan,Ryu, Gi-Seong,Tabi, Grace Dansoa,Lee, Kyung-Koo,Kim, BongSoo,Noh, Yong-Young American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.8
<P>Understanding the sensing mechanism in organic chemical sensors is essential for improving the sensing performance such as detection limit, sensitivity, and other response/recovery time, selectivity, and reversibility for real applications. Here, we report a highly sensitive printed ammonia (NH3) gas sensor based on organic thin film transistors (OTFTs) with fluorinated difluorobenzothiadiazole-dithienosilole polymer (PDFDT). These sensors detected NH3 down to 1 ppm with high sensitivity (up to 56%) using bar-coated ultrathin (<4 nm) PDFDT layers without using any receptor additives. The sensing mechanism was confirmed by cyclic voltammetry, hydrogen/fluorine nuclear magnetic resonance, and UV/visible absorption spectroscopy. PDFDT-NH3 interactions comprise hydrogen bonds and electrostatic interactions between the PDFDT polymer backbone and NH3 gas molecules, thus lowering the highest occupied molecular orbital levels, leading to hole trapping in the OTFT sensors. Additionally, density functional theory calculations show that gaseous NH3 molecules are captured via cooperation of fluorine atoms and dithienosilole units in PDFDT. We verified that incorporation of functional groups that interact with a specific gas molecule in a conjugated polymer is a promising strategy for producing high-performance printed OTFT gas sensors.</P>
Nketia-Yawson, B.,Lee, H.S.,Son, H.J.,Kim, B.,Noh, Y.Y. Elsevier Science 2016 ORGANIC ELECTRONICS Vol.29 No.-
<P>We report high-performance organic thin-film transistors (OTFTs) with an ultrathin active layer of difluorobenzothiadiazole-dithienosilole copolymer (PDFDT) form by using the wire bar-coating process. The top-gate/bottom contact (TG/BC) OTFTs based on bar-coated PDFDT polymer as channel material and poly(methyl methacrylate) (PMMA) as gate dielectric show a hole mobility of up to 2.2 cm(2) V-1 s(-1) with a current ON/OFF ratio (I-on/I-off) of 10(4)-10(5), with the mobility being two times larger than that of the spincoated PDFDT based OTFTs. The higher mobility of the bar-coated PDFDT polymer films can be attributed to the well-organized fibril structures of the polymer chains. Importantly, two different molecular weight polymers (M-n = 23 and 34 kDa) were employed to conduct these experiments and both batches showed about the same performance, which mitigates the typical batch-to-batch variation in OTFT performance. Furthermore, we explored the operational stability of the bar-coated OTFTs in ambient air and nitrogen environments. The bias-stress and cycling tests between the ON/OFF states of the bar-coated devices showed high stability in both nitrogen and air. Conclusively, here we demonstrate that (i) a simple barcoating process is a better method to control and obtain good polymer morphology in comparison to spin-coating, and (ii) the PDFDT polymer has great potential to provide good reproducibility and stability in large-area OTFT devices. (C) 2015 Elsevier B.V. All rights reserved.</P>
Organic Thin Film Transistor with Conjugated Polymers for Highly Sensitive Gas Sensors
Benjamin Nketia-Yawson,노용영 한국고분자학회 2017 Macromolecular Research Vol.25 No.6
The ease in structural modification, redesign and development of multifunctional polymer semiconductors have led to significant advances in organic electronic device applications such as organic light emitting diodes (OLED), organic photovoltaics (OPV), organic thin film transistors (OTFT), and organic sensors. In this article, we reviewed recent techniques and strategies employed toward the optimization of conjugated polymer films for high sensing performance in OTFT-based gas sensors. Significant enhancements, particularly in gas selectivity, sensitivity, detection limit, response/recovery time of OTFT-based gas sensors by polymer film morphology and thickness control, functionalization of conjugated polymers with chemical groups and the use of doped/blended inorganic or dielectric materials and conjugated polymer composites as active layers are discussed here.