Ji-Hoon Kim

Kyung Hee University, Sŏul, Seoul, South Korea

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Publications (6)17.25 Total impact

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    ABSTRACT: Using ultraviolet photoelectron spectroscopy (UPS), we have measured the energy level offset at the planar interface between poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM). Gradual deposition of PCBM onto spin-coated P3HT in high vacuum was made possible by using electrospray vacuum deposition (EVD). The UPS measurement of EVD-prepared planar interface resulted in the energy level offset of 0.91 eV between P3HT HOMO and PCBM LUMO, which is considered as the upper limit of Voc of the organic photovoltaic cells.
    03/2014; 104(16).
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    ABSTRACT: Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.
    ChemSusChem 02/2014; · 7.48 Impact Factor
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    ABSTRACT: Knowledge of molecular structures of emerging charge acceptors, such as hexaaza-triphenylene-hexacarbonitrile (HATCN), on metal surfaces is essential for their optoelectronic device applications. Here, we studied the two-dimensional molecular ordering of HATCN at submonolayer coverages on Au(111) using scanning tunneling microscopy (STM). Linear and hexagonal porous structures were observed at atomic steps and terraces, respectively, and our density functional theory calculations revealed that the structures were stabilized with CN···CN dipolar interactions. The hexagonal porous structures possess chirality, and they form only small (<1000 nm2) phase-separated chiral domains that easily change their structures in subsequent STM images at 80 K, which explains the no electron diffraction pattern reported previously.
    The Journal of Physical Chemistry C. 10/2013; 117(41):21371–21375.
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    ABSTRACT: The electronic structures of self-assembled hybrid chains comprising Ag atoms and organic molecules were studied using scanning tunneling microscopy (STM) and spectroscopy (STS) in parallel with density functional theory (DFT). Hybrid chains were prepared by catalytic breaking of Br-C bonds in 4,4″-dibromo-p-terphenyl molecules, followed by spontaneous formation of Ag-C bonds on Ag(111). An atomic model was proposed for the observed hybrid chain structures. Four electronic states were resolved using STS measurements, and strong energy dependence was observed in STM images. These results were explained using first-principles calculations based on DFT.
    Physical Chemistry Chemical Physics 04/2012; 14(20):7304-8. · 4.20 Impact Factor
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    ABSTRACT: Inverted top-emitting organic light-emitting diodes (ITOLEDs) with high power efficiency and near-ideal emission characteristics are demonstrated by using the combination of the following: 1) an electron-injection layer composed of Cs<sub>2</sub>CO<sub>3</sub>, which lowers the turn-on voltage; 2) an electron-transporting layer with optimal electron mobility, which enhances the electron current and thus improves the carrier balance; and 3) a dielectric/metal/dielectric multilayer electrode that works as a damage-free top transparent anode optimized to achieve high efficiency and ideal emission characteristics. By this approach, ITOLEDs with power efficiency values of 3.8 and 30 lm W<sup>-1</sup> are demonstrated in fluorescent and phosphorescent types, respectively, at a luminance value of 1000 cd m<sup>-2</sup> with little distortion in spectral/angular characteristics.
    IEEE Transactions on Electron Devices 02/2012; · 2.06 Impact Factor
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    ABSTRACT: A high efficiency blue fluorescent organic light-emitting diode without LiF electron injection layer was developed. Aluminum electrode was directly deposited on a phosphine oxide type electron transport layer and the observed quantum efficiency was as high as 6.13%. The ultraviolet photoemission spectroscopy data clearly indicated that the electron injection barrier (the offset between Al Fermi level and the lowest unoccupied molecular orbital of the organic layer) is less than 0.1 eV, which led us to believe that more efficient electron injection through the lower barrier is mainly responsible for the high efficiency.
    Applied Physics Letters 01/2011; 98. · 3.52 Impact Factor