Browsing by Author "Ahn, Kyunghan"

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    Engineering functionalized low LUMO [1]benzothieno[3,2-b][1]benzothiophenes (BTBTs): unusual molecular and charge transport properties
    (ROYAL SOC CHEMISTRY, THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND, 2020) Ozdemir, Resul; Ahn, Kyunghan; Deneme, Ibrahim; Zorlu, Yunus; Kim, Dojun; Kim, Myung-Gil; Usta, Hakan; 0000-0002-7957-110X; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü
    Diacene-fused thienothiophenes (DAcTTs) have provided an excellent pi-framework for the development of high mobility p-type molecular semiconductors in the past decade. However, n-type DAcTTs are rare and their electron transport characteristics remain largely unexplored. Herein, a series of functionalized low LUMO (lowest unoccupied molecular orbital) [1]benzothieno[3,2-b][1]benzothiophene (BTBT)-based small molecules, D(C7CO)-BTBT, C7CO-BTBT-CC(CN)(2)C-7, and D(C7CC(CN)(2))-BTBT, have been developed. Detailed structural, physicochemical, optoelectronic, and single-crystal characterization were performed. The new molecules exhibit large optical band gaps (similar to 2.8-3.1 eV) and highly stabilized (-Delta E-LUMO = 1.2-1.4 eV)/pi-delocalized LUMOs as compared to p-type DAcTTs. Symmetric functionalization is found to be important to enable strong intermolecular interactions in the solid-state. All molecules exhibit alternately stacked layers of "F-BTBT-F" and "S" (F: functional group/S: substituent) with strong herringbone-like interactions (2.8-3.6 angstrom distances) between pi-cores. While carbonyls, regardless of the substituent, adopt nearly coplanar pi-backbones with BTBT, dicyanovinylenes are found to be twisted (47.5 degrees). The conformational difference at the molecular level has unusual effects on the pi-electron deficiencies, frontier molecular orbital energetics, thermal/photophysical properties, and pi-electronic structures. Dicyanovinylenes at the 2,7 positions, despite twisted conformations, are shown for the first time to yield good electron transport in DAcTTs. The D(C7CC(CN)(2))-BTBT thin film exhibits large 2D plate-like crystalline grains (similar to 1-2 mu m sizes) of terraced islands and becomes a rare example of an n-type DAcTT in organic field-effect transistors (OFETs). Although a stabilized/pi-delocalized LUMO, largely governed by functional groups and intramolecular twists, is essential for electron transport, our findings suggest that it should be combined with proper substituents to yield a favorable three-dimensional BTBT/functional group pi-electronic structure and a low intramolecular reorganization energy. Combined with our first n-type DAcTT semiconductor D(PhFCO)-BTBT, a molecular library with systematically varied chemical structures has been studied herein for the first time for low LUMO DAcTTs. The molecular engineering perspectives presented in this study may give unique insights into the design of novel electron transporting thienoacenes for unconventional optoelectronics.
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    Optimized Activation of Solution-Processed Amorphous Oxide Semiconductors for Flexible Transparent Conductive Electrodes
    (WILEY, 111 RIVER ST, HOBOKEN 07030-5774, NJ USA, 2018) Choi, Byung Doo; Park, Joohyung; Baeg, Kang-Jun; Kang, Minji; Heo, Jae Sang; Kim, Seonhyoung; Won, Jongkook; Yu, Seungwoo; Ahn, Kyunghan; Lee, Tae Hoon; Hong, Jongin; Kim, Dong-Yu; Usta, Hakan; Kim, Choongik; Park, Sung Kyu; Kim, Myung-Gil; 0000-0002-0618-1979; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü
    Here, the preparation of transparent amorphous oxide semiconductor (AOS) films with unprecedented conductivity via an optimized activation process under hydrogen atmosphere for applications in solution-processed large-area optoelectronics is reported. Owing to their high cost and mechanical vulnerability, conventional vacuum-processed indium-tin oxide (ITO) electrodes are inappropriate for use in next-generation flexible and wearable electronic devices and systems. As an alternative to the ITO electrodes, solution-processed AOS films, such as alpha-IZO and alpha-ZITO, with an optimized composition and postreduction treatment under hydrogen show the highest electrical conductivity of approximate to 300 S cm(-1) and a high optical transmittance of over 90% at 550 nm. The microstructures and electrical properties of these AOS films are also studied in order to determine the optimized chemical composition and postreduction conditions. It is found that a controlled hydrogen reduction treatment of AOS films is critical for achieving high electrical conductivity by suppressing the surface morphology degradation and grain boundary disconnection. Furthermore, the alpha-IZO transparent conductive electrodes are successfully implemented for high efficiency organic photovoltaic cells based on the PTB7/PC71BM active layers. This technique promises the low-cost fabrication of high mobility and/or conductive AOSs for their applications in large-area transparent and flexible optoelectronics.