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Kim, Tae Gun,Lee, Hyunbok,Yi, Yeonjin,Lee, Seung Mi,Kim, Jeong Won American Institute of Physics 2015 Journal of Applied Physics Vol.118 No.2
<P>A three layer cathode is a promising stack structure for long lifetime and high efficiency in organic light-emitting diodes. The interfacial chemical reactions and their effects on electronic structures for alkaline-earth metal (Ca, Ba)/Alq(3) [tris(8-hydroxyquinolinato) aluminum] and Ca/BaF2/Alq(3) are investigated using in-situ X-ray and ultraviolet photoelectron spectroscopy, as well as molecular model calculation. The BaF2 interlayer initially prevents direct contact between Alq(3) and the reactive Ca metal, but it is dissociated into Ba and CaF2 by the addition of Ca. As the Ca thickness increases, the Ca penetrates the interlayer to directly participate in the reaction with the underlying Alq(3). This series of chemical reactions takes place irrespective of the BaF2 buffer layer thickness as long as the Ca overlayer thickness is sufficient. The interface reaction between the alkaline-earth metal and Alq(3) generates two energetically separated gap states in a sequential manner. This phenomenon is explained by step-by-step charge transfer from the alkaline-earth metal to the lowest unoccupied molecular orbital states of Alq(3), forming new occupied states below the Fermi level. (C) 2015 AIP Publishing LLC.</P>
Direct p-doping of Li-TFSI for efficient hole injection: Role of polaronic level in molecular doping
Kim, Kiwoong,Jeong, Junkyeong,Kim, Minju,Kang, Donghee,Cho, Sang Wan,Lee, Hyunbok,Yi, Yeonjin Elsevier 2019 APPLIED SURFACE SCIENCE - Vol.480 No.-
<P><B>Abstract</B></P> <P>Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) has been popularly employed as an efficient p-dopant that increases the conductivity of a hole transport layer (HTL) in perovskite solar cells and dye-sensitized solar cells. However, the working mechanism of the Li-TFSI dopant is a long-standing question. The hygroscopicity of Li-TFSI makes it difficult to isolate the exact doping mechanism. In this study, we unveil the role of Li-TFSI in the p-doping to the <I>N</I>,<I>N</I>′-di(1-naphthyl)-<I>N</I>,<I>N</I>′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) HTL. A series of systematic in situ measurements using ultraviolet and inverse photoelectron spectroscopy reveal that electron transfer from NPB to Li-TFSI occurs due to the lower-lying negative polaronic level of Li-TFSI rather than the positive polaronic level of NPB. The hole injection barrier between NPB and indium tin oxide is significantly reduced with Li-TFSI doping, enhancing the device performance of hole-only devices and organic light-emitting diodes dramatically. With excessive dopants, however, the agglomerative property of Li-TFSI became dominant, decreasing the doping efficiency. These results provide robust guidelines for developing an efficient doping method for a molecular system with high conductivity.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Electronic structure of Li-TFSI and NPB was investigated using in situ UPS and IPES. </LI> <LI> Electron transfer occurred from NPB to Li-TFSI through their polaronic levels. </LI> <LI> Hole injection barrier was reduced by 0.70 eV with Li-TFSI doping. </LI> <LI> Device performance of OLEDs was significantly enhanced with Li-TFSI doping. </LI> <LI> With excessive dopants, agglomeration of Li-TFSI decreased doping efficiency. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Modification of the characteristics of silkworm powder by treatment with alkaline protease
( Sungkuk Kim ),( Youyoung Jo ),( Kwanggill Lee ),( Hyunbok Kim ),( Yongsoon Kim ),( Wantaek Ju ),( Daeun Jung ),( Haeyong Kweon ) 한국잠사학회 2015 International Journal of Industrial Entomology Vol.31 No.1
Enzymatic modification of proteins is often used to increase the biological activity of materials. Silkworm powder has been investigated as a functional food resource, but no study has been performed on its modification by commercial food enzyme. Therefore, this study aimed to determine the feasibility of such modification of silkworm powder by alkaline protease. The activity of the enzyme was confirmed using an azocasein assay. Subsequently the silkworm powder was hydrolyzed by enzymatic treatment. UV visible spectrometry showed that the supernatant of silkworm powder subjected to enzymatic treatment had a stronger absorption band than the untreated powder. SDS-PAGE electrophoresis showed that the molecular weight of silkworm powder decreased on enzymatic treatment. Thus the results indicate that commercial enzymes might be used to modify the characteristics of silkworm powder.
Fabrication of poly(3-hexylthiophene-2,5-diyl) films with electrospray deposition method
Kim, Hyounggi,Lee, Hyunchan,Lee, Hyunbok Institute of Pure and Applied Physics 2018 Japanese Journal of Applied Physics Vol. No.
<P>A solution process to obtain a thin film of organic semiconductors allows for the fabrication of low-cost electronic devices without using expensive vacuum equipment. Thus, the development of an efficient method to deposit large area and patterned thin films via a solution process is of great importance. Among various deposition methods, electrospray is a promising candidate that can be employed in the industrial field as an alternative to spin coating. In this study, we fabricated poly(3-hexylthiophene-2,5-diyl) (P3HT) films using an electrospray deposition method while varying the deposition parameters. The effects of annealing, solution concentration, and acetonitrile additive on the film morphology were investigated with optical microscopy and atomic force microscopy measurement. The optoelectronic properties and the chemical states were also explored with UV-vis absorption and X-ray photoelectron spectroscopy measurement. As a consequence, we indicated that high-quality P3HT films can be obtained through the optimization of the deposition parameters in electrospray. (C) 2018 The Japan Society of Applied Physics</P>
Hole injection improvement using ultrathin Li-TFSI layer in organic light-emitting diodes
Kim Kiwoong,Yi Yeonjin,Lee Hyunbok 한국물리학회 2021 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.79 No.10
A high device performance of organic light-emitting diodes (OLEDs) can be achieved by minimizing the hole injection barrier (HIB) at the anode interface. However, the work function of indium tin oxide (ITO) is not sufficiently high; thus, the use of an appropriate hole injection layer (HIL) is necessary. In this study, we demonstrated that bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI) is an efficient HIL to decrease the HIB in OLEDs. The device performances of hole-only devices (HODs) and OLEDs were significantly enhanced by the insertion of the ultrathin Li-TFSI HIL between the N,N′- di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) hole transport layer and ITO. In situ ultraviolet photoelectron spectroscopy measurements revealed that the work function of ITO was considerably increased by the deposition of the Li-TFSI HIL. Accordingly, the HIB from ITO to NPB was remarkably reduced. This is the origin of the hole injection improvements in the HODs and OLEDs with the Li-TFSI HIL. Thus, Li-TFSI would be an efficient interface modifier to increase the work function of the electrode and decrease the HIB in various electronic devices.
Wonsik Kim,Seungsun Choi,Woojin Shin,Jaewon Oh,Hyesung Oh,Sehyun Jung,Moonseock Ko,Mee-Yi Ryu,Hyunbok Lee 한국진공학회 2021 한국진공학회 학술발표회초록집 Vol.2021 No.2
Ultraviolet-ozone (UVO) treatment modifies the surface properties of an organic film. However, the electronic properties of UVO-treated organic films have not understood sufficiently. In this study, the changes in the electronic structure of tetra-tert-butyl copper phthalocyanine (ttb-CuPc) films by various UVO treatment times were explored by using UV-vis absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). Significant oxidation is observed in both UV-vis and XPS spectra. The charge redistribution on the ttb-CuPc molecule is discussed using detailed fittings of the XPS spectra.