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A Study on the Thermal Oxidation and Wettability of Lead-free Solders of Sn-Ag-Cu and Sn-Ag-Cu-In
Hyunbok Lee,Sang Wan Cho 한국진공학회(ASCT) 2014 Applied Science and Convergence Technology Vol.23 No.6
The surface oxidation mechanism of lead-free solder alloys has been investigated with multiple reflow using X-ray photoelectron spectroscopy. It was found that the solder surface of Sn-Ag-Cu-In solder alloy is surrounded by a thin InOx layer after reflow process; this coating protects the metallic surface from thermal oxidation. Based on this result, we have performed a wetting balance test at various temperatures. The Sn-Ag-Cu-In solder alloy shows characteristics of both thermal oxidation and wetting balance better than those of Sn-Ag-Cu solder alloy. Therefore, Sn-Ag-Cu-In solder alloy is a good candidate to solve the two problems of easy oxidation and low wettability, which are the most critical problems of Pb-free solders.
Interfacial electronic structure for high performance organic devices
Lee, Hyunbok,Cho, Sang Wan,Yi, Yeonjin Elsevier 2016 CURRENT APPLIED PHYSICS Vol.16 No.12
<P>Organic semiconductors (OSCs) are at the center of attention in a wide range of research fields since their unique advantages meet the requirements for next-generation optoelectronics applications. Since OSCs are lacking intrinsic carriers, charges for device operation have to be injected through organic/electrode and organic/organic interfaces. Therefore, the charge injection efficiency, which is determined by the energy level alignments at those interfaces, governs the device performance. In other words, high performance organic devices cannot be achieved without facilitating proper energy level alignments. Thus, the interfacial electronic structure, which should be determined from accurate measurements of the charge transport level, must be understood to establish the design strategy for high performance organic devices. In this review, various spectroscopic methods to investigate the surface and interface electronic structures, including direct photoelectron spectroscopy, inverse photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray emission spectroscopy, are discussed along with their fundamental principles. Examples of device performance enhancements with modification of the interfacial electronic structure in organic photovoltaics and organic light-emitting diodes are presented. (C) 2016 Elsevier B.V. All rights reserved.</P>
Electron transport mechanism of bathocuproine exciton blocking layer in organic photovoltaics
Lee, Jeihyun,Park, Soohyung,Lee, Younjoo,Kim, Hyein,Shin, Dongguen,Jeong, Junkyeong,Jeong, Kwangho,Cho, Sang Wan,Lee, Hyunbok,Yi, Yeonjin The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.7
<P>Efficient exciton management is a key issue to improve the power conversion efficiency of organic photovoltaics (OPVs). It is well known that the insertion of an exciton blocking layer (ExBL) having a large band gap promotes the efficient dissociation of photogenerated excitons at the donor-acceptor interface. However, the large band gap induces an energy barrier which disrupts the charge transport. Therefore, building an adequate strategy based on the knowledge of the true charge transport mechanism is necessary. In this study, the true electron transport mechanism of a bathocuproine (BCP) ExBL in OPVs is comprehensively investigated by in situ ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, density functional theory calculation, and impedance spectroscopy. The chemical interaction between deposited Al and BCP induces new states within the band gap of BCP, so that electrons can transport through these new energy levels. Localized trap states are also formed upon the Al-BCP interaction. The activation energy of these traps is estimated with temperature-dependent conductance measurements to be 0.20 eV. The Al-BCP interaction induces both transport and trap levels in the energy gap of BCP and their interplay results in the electron transport observed.</P>
Hyunbok Lee 한국진공학회(ASCT) 2018 Applied Science and Convergence Technology Vol.27 No.6
Organic light-emitting diodes (OLEDs) have received much attention for application the in next-generation display due to their many advantages. To increase the device performance of OLEDs, a host-dopant system in the emission layer has been used. In blue OLEDs, anthracene-based materials have been used as a host material. To understand the device behavior of OLEDs, a fundamental study on the electronic properties of organic semiconductors is necessary. In this study, theoretical calculations using density functional theory were performed to investigate the electronic structure and charge transport ability of 9,10-diphenyl-2-((3-trifluoromethyl)phenyl)anthracene (ATFP-Ph), 9,10-di([1,1’-biphenyl]-4-(trifluoromethyl)phenyl)anthracene (ATFP-BiPh), and 9,10-di(naphthalene-2-yl)-2-(3-(trifluoromethyl)phenyl)anthracene (ATFP-Naph). All molecules have similar lowest unoccupied molecular orbital and highest occupied molecular orbital energy levels, and thus their charge injection abilities from the adjacent layer to the host material are similar. However, ATFP-Ph has significantly lower hole and electron reorganization energy (λ) of 250 and 268 meV, compared to that of ATFP-BiPh which has hole and electron λ of 334 and 320 meV, and ATFP-Naph which has hole and electron λ of 324 and 361 meV, respectively. The origin of lower hole and electron λ is analyzed via charge distribution on the molecule. The change in charge on the phenyl moiety in ATFP-Ph during hole and electron injection is much smaller than the biphenyl moiety in ATFP-BiPh and the naphthyl moiety in ATFP-Naph, indicating low electron-phonon coupling. This low hole and electron λ of ATFP-Ph yield high hole and electron hopping rates, which result in the higher device performance of OLEDs.
Interfacial electronic structure for high performance organic devices
Hyunbok Lee,Sang Wan Cho,이연진 한국물리학회 2016 Current Applied Physics Vol.16 No.12
Organic semiconductors (OSCs) are at the center of attention in a wide range of research fields since their unique advantages meet the requirements for next-generation optoelectronics applications. Since OSCs are lacking intrinsic carriers, charges for device operation have to be injected through organic/electrode and organic/organic interfaces. Therefore, the charge injection efficiency, which is determined by the energy level alignments at those interfaces, governs the device performance. In other words, high performance organic devices cannot be achieved without facilitating proper energy level alignments. Thus, the interfacial electronic structure, which should be determined from accurate measurements of the charge transport level, must be understood to establish the design strategy for high performance organic devices. In this review, various spectroscopic methods to investigate the surface and interface electronic structures, including direct photoelectron spectroscopy, inverse photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray emission spectroscopy, are discussed along with their fundamental principles. Examples of device performance enhancements with modification of the interfacial electronic structure in organic photovoltaics and organic light-emitting diodes are presented.
Effect of Ar ion Sputtering on the Surface Electronic Structure of Indium Tin Oxide
Lee, Hyunbok,Cho, Sang Wan The Korean Vacuum Society 2016 Applied Science and Convergence Technology Vol.25 No.6
We investigated the effect of Ar ion sputtering on the surface electronic structure of indium tin oxide (ITO) using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements with increasing Ar ion sputtering time. XPS measurements revealed that surface contamination on ITO was rapidly removed by Ar ion sputtering for 10 s. UPS measurements showed that the work function of ITO increased by 0.2 eV after Ar ion sputtering for 10 s. This increase in work function was attributed to the removal of surface contamination, which formed a positive interface dipole relative to the ITO substrate. However, further Ar ion sputtering did not change the work function of ITO although the surface stoichiometry of ITO did change. Therefore, removing the surface contamination is critical for increasing the work function of ITO, and Ar ion sputtering for a short time (about 10 s) can efficiently remove surface contamination.
Hyunbok Lee 한국진공학회(ASCT) 2020 Applied Science and Convergence Technology Vol.29 No.6
Atomic substitution in an organic semiconductor alters the intermolecular and intramolecular interactions in the solid state. Thus, an understanding of such electronic interactions is crucial in designing an efficient molecule. In this study, the effect of the substitution of two hydrogen atoms in the tetracene core to sulfur in benzopyrazine-fused tetracene was investigated via theoretical methods. The reorganization energy and electronic coupling of 5,6-substituted benzopyrazine-fused tetracene and its 5,6,11,12-substituted disulfide were calculated using density functional theory. By sulfur substitution, the hole reorganization energy increases but the electron reorganization energy decreases. Meanwhile, the highest values of both hole and electron electronic couplings decrease. As a result, the calculated hole mobility decreased significantly. However, the calculated electron mobility increased slightly, indicating that the charge transport type changes from p-type to ambipolar. These results indicate that atomic substitution significantly affects the charge transport ability of organic semiconductors.