• International Journal of Naval Architecture and Ocean Engineering
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• v.8 no.4
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• pp.375-385
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• 2016

Given the rapid progress made in understanding the dynamics of an offshore floating body in an ocean environment, the present study aimed to simulate ocean waves in a small-sized wave flume and to observe the motion of a cylindrical floating body placed in an offshore environment. To generate regular ocean waves in a wave flume, we combined a wave generator and a wave absorber. In addition, to precisely visualise the oscillation of the body, a set of light-emitting diode illuminators and a high-speed charge-coupled device camera were installed in the flume. This study also focuses on the spectral analysis of the movement of the floating body. The wave generator and absorbers worked well to simulate stable regular waves. In addition, the simulated waves agreed well with the plane waves predicted by shallow-water theory. As the period of the oncoming waves changed, the movement of the floating body was substantially different when tethered to a tension-leg mooring cable. In particular, when connected to the tension-leg mooring cable, the natural frequency of the floating body appeared suddenly at 0.391 Hz as the wave period increased.

• Journal of Radiation Protection and Research
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• v.13 no.2
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• pp.1-8
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• 1988

The energy losses of ${\alpha}$-particle with 3 to 5.4 MeV energy were measured as a function of gas absorber thickness and ${\alpha}$-particle energy in three light gaseous media; He, Ar, and $N_2$. The stopping powers and stopping cross-sections were determined by analyzing these data. For Ar gas, the experimental values are very well consistent with the corresponding values of Srivastava's stopping-power theory with the condition of the completely and partially stripped ion. For $N_2$ and He gases, these experimental values express the inconsistencies of about $13{\sim}36%$ and $27{\sim}28%$, respectively.

• Journal of Powder Materials
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• v.25 no.6
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• pp.514-522
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• 2018

Layered-double hydroxide (LDH)-based nanostructures offer the two-fold advantage of being active catalysts with incredibly large specific surface areas. As such, they have been studied extensively over the last decade and applied in roles as diverse as light source, catalyst, energy storage mechanism, absorber, and anion exchanger. They exhibit a unique lamellar structure consisting of a wide variety of combinations of metal cations and various anions, which determine their physical and chemical performances, and make them a popular research topic. Many reviewed papers deal with these unique properties, synthetic methods, and applications. Most of them, however, are focused on the form-factor of nanopowder, as well as on the control of morphologies via one-step synthetic methods. LDH nanostructures need to be easy to control and fabricate on rigid substrates such as metals, semiconductors, oxides, and insulators, to facilitate more viable applications of these nanostructures to various solid-state devices. In this review, we explore ways to grow and control the various LDH nanostructures on rigid substrates.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.431-431
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• 2016

Typical Cu(In,Ga)Se2 (CIGS)-based solar cells have a buffer layer between CIGS absorber layer and transparent ZnO front electrode, which plays an important role in improving the cell performance. Among various buffer materials, chemical bath deposition (CBD)-ZnS is being steadily studied to alternative to conventional CdS and the efficiency of CBD-ZnS/CIGS solar cell shows the comparable values with that of CdS/CIGS solar cell. The intriguing thing is that reversible changes occur after exposure to illumination due to the metastable defect states in completed ZnS/CIGS solar cell, which induces an improvement of solar cell performance. Thus, it implies that the understanding of metastable defects in CBD-ZnS/CIGS solar cell is important issue. In this study, we fabricate the ITO/i-ZnO/CBD-ZnS/CIGS/Mo/SLG solar cells by controlling the NH4OH mole concentration (from 2 M to 3.5 M) of CBD-ZnS buffer layer and observe their conversion efficiency with and without light soaking for 1 hr. From the results, NH4OH mole concentration and light exposure can significantly affect the CBD-ZnS/CIGS solar cell performance. In order to investigate that which layer can contain metastable defect states to influence on solar cell performance, impedance spectroscopy and capacitance profiling technique with exposure to illumination have been applied to CBD-ZnS/CIGS solar cell. These techniques give a very useful information on the density of states within the bandgap of CIGS, free carriers density, and light-induced metastable effects. Here, we present the rearranged charge distribution after exposure to illumination and suggest the origin of the metastable defect states in CBD-ZnS/CIGS solar cell.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.427-427
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• 2016

Organic-inorganic hybrid perovskite have attracted significant attention as a new revolutionary light absorber for photovoltaic device due to its remarkable characteristics such as long charge diffusion lengths (100-1000nm), low recombination rate, and high extinction coefficient. Recently, power conversion efficiency of perovskite solar cell is above 20% that is approached to crystalline silicon solar cells. Planar heterojunction perovskite solar cells have simple device structure and can be fabricated low temperature process due to absence of mesoporous scaffold that should be annealed over 500 oC. However, in the planar structure, controlling perovskite film qualities such as crystallinity and coverage is important for high performances. Those controlling methods in one-step deposition have been reported such as adding additive, solvent-engineering, using anti-solvent, for pin-hole free perovskite layer to reduce shunting paths connecting between electron transport layer and hole transport layer. Here, we studied the effect of alkali metal halide to control the fabrication process of perovskite film. During the morphology determination step, alkali metal halides can affect film morphologies by intercalating with PbI2 layer and reducing $CH3NH3PbI3{\cdot}DMF$ intermediate phase resulting in needle shape morphology. As types of alkali metal ions, the diverse grain sizes of film were observed due to different crystallization rate depending on the size of alkali metal ions. The pin-hole free perovskite film was obtained with this method, and the resulting perovskite solar cells showed higher performance as > 10% of power conversion efficiency in large size perovskite solar cell as $5{\times}5cm$. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma optical emission spectrometry (ICP-OES) are analyzed to prove the mechanism of perovskite film formation with alkali metal halides.

• Journal of Korean Society on Water Environment
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• v.23 no.3
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• pp.319-323
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• 2007

The Advanced Oxidation Process (AOP) is being increasingly used to oxidize complex organic constituents in treated effluents from domestic wastewater treatment plants. Generally, ${NO_3}^--N$ concentrations ranges between 5 and 8 mg/L for biologically well-treated effluents. However, nitrate ions, ${NO_3}^-$, affects on oxidation as not only a well-known strong absorber of UV light below 250 nm of wavelength but also as an OH radical scavenger. The objective of this study was to evaluate the AOP systems for degradation of 2,4-DCP, and to delineate the effect of nitrate ions on UV oxidation of 2,4-DCP by conducting a bench-scale operation at various reaction times and initial concentrations of $H_2O_2$. The experimental results indicated that 2,4-DCP could be completely oxidized by $UV/H_2O_2$ process with an initial $H_2O_2$ concentration of 20 mg/L at a retention time of 1.0 min or longer. Nitrate ions did not show any adverse effect on 2,4-DCP oxidation at this high $H_2O_2$ concentration, and the practical initial $H_2O_2$ concentration and reaction time for the 80% oxidation turned out to be 5 mg/L and 1.0 min, respectively.

• Current Photovoltaic Research
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• v.5 no.1
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• pp.15-19
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• 2017

The fossil fuel power consumption generates $CO_2$, which causes the problems such as global warming. Also, the increase in energy consumption has accelerated the depletion of the fossil fuels, and renewable energy is attracting attention. Among the renewable energies, the solar energy gets a lot of attention as the infinite clean energy source. But, the supply level of solar cell is insignificant due to high cost of generation of electric power in comparison with fossil fuels. Thus several researchers are recently doing the research on ultra-low-cost solar cells. Also, $Cu_2O$ is one of the applied materials as an absorption layer in ultra-low-cost solar cells. Cuprous oxide ($Cu_2O$) is highly desirable semiconductor oxide for use in solar energy conversion due to its direct band gap ($E_g={\sim}2.1eV$) and a high absorption coefficient that absorbs visible light of wavelengths up to 650 nm. In addition, $Cu_2O$ has several advantages such as non-toxicity, low cost and can be prepared with simple and cheap methods on large scale. In this work, we fabricated the $Cu_2O$ thin films by reactive sputtering method. The films were deposited with a Cu target with variable parameters such as substrate temperature, rf-power, and annealing condition. Finally, we confirmed the structural properties of thin films by XRD and SEM.

• ETRI Journal
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• v.38 no.2
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• pp.265-271
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• 2016

We analyzed the interface characteristics of Zn-based thin-film buffer layers formed by a sulfur thermal cracker on a $Cu(In,Ga)Se_2$ (CIGS) light-absorber layer. The analyzed Zn-based thin-film buffer layers are processed by a proposed method comprising two processes - Zn-sputtering and cracker-sulfurization. The processed buffer layers are then suitable to be used in the fabrication of highly efficient CIGS solar cells. Among the various Zn-based film thicknesses, an 8 nm-thick Zn-based film shows the highest power conversion efficiency for a solar cell. The band alignment of the buffer/CIGS was investigated by measuring the band-gap energies and valence band levels across the depth direction. The conduction band difference between the near surface and interface in the buffer layer enables an efficient electron transport across the junction. We found the origin of the energy band structure by observing the chemical states. The fabricated buffer/CIGS layers have a structurally and chemically distinct interface with little elemental inter-diffusion.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.134.1-134.1
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• 2014

콜로이달 리소그래피는 나노미터 크기의 나노구를 자가조립에 의해 정렬시킴으로써, 파장이하 크기의 주기 구조를 저비용으로 쉽게 구현할 수 있는 패터닝 기법이다. 콜로이달 리소그래피나 소프트 리소그래피와 같이 대면적 패터닝이 가능한 공정을 태양전지를 위한 반사방지 및 광 포획 증대 구조에 적용함으로써, 기존 성능을 크게 향상시켰다. 본 연구에서는, 유한차분 시간영역 수치해석법을 이용하여 반사 방지 및 광 포획 증대 구조에 대한 이론적 검증 및 설계를 진행하였고, 콜로이달 리소그래피 및 반도체 공정을 통해 샘플을 제작하였으며, 제작된 샘플의 성능을 적분구를 겸비한 자외선 가시광 근적외선 영역 분광기를 통해 평가하였다. 반사방지 나노섬을 겸비한 나노 원뿔대 언덕형 굴절률 소자를 구현함으로써, 300나노미터 이하의 구조체를 사용하지 않고도 근자외선 영역을 포함하는 태양광 에너지의 손실을 최소화할 수 있는 광대역 방사방지 구조체를 제시하였다. 나노 원뿔대가 격자상수 이상의 파장에 대한 언덕형 굴절률을 제공하고, 4분의 1파장 나노섬 반사방지막이 격자 상수 이하의 근자외선 태양광을 추가적으로 흡수하여, 근자외선 영역에서의 평균 반사율을 3.8% 수준으로 달성 할 수 있었다. 또한, 낮은 양호계수를 갖는 속삭임 회랑 공진기 어레이를 이용하여, 박막 태양전지에 적합한 유전체 기반 광포획 증대 나노구조를 제시하였다. 나노반구, 나노고깔, 나노구, 함몰형 나노구 어레이 형태를 가지며, 500nm의 주기를 갖는 유전체 표면 텍스쳐드 구조를 초박형 비정질 실리콘 필름(100nm) 위에 제작하여 광대역 광 포획 증대 효과를 실험적으로 평가하였다. 구조들 중 함몰형 나노구 어레이가 결합된 비정질 실리콘 박막이 가장 높은 성능을 보였으며, 구조가 없는 경우 대비 약 67.6%의 가중 흡수율 증가를 나타내었다. 특히, 함몰형 나노구 어레이 구조 중 폴리메틸메타아크릴레이트로 제작된 평판형 함몰층은 나노구 비정질 박막 실리콘 사이의 접착력 및 기계적 강성을 향상시켰을 뿐 아니라, 함몰층 내부로 회절되고 산란된 빛들이 도파모드 효과에 의해 부가적인 광 포획 증대를 가져옴으로써, 가장 높은 광 포획 효과를 얻을 수 있었다. 유전체 기반 나노 구조들은 간단하고 저비용이며, 대면적으로 쉽게 제작할 수 있는 자가 조립 기반 콜로이달 리소그래피 및 소프트 리소그래피 기술을 이용하여 제작되었다.

• Current Photovoltaic Research
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• v.3 no.3
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• pp.75-79
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• 2015

Recently, $Cu_2ZnSn(S_xSe_{1-x})_4$ (CZTSSe) has been received a tremendous attraction as light absorber material in thin film solar cells (TFSCs), because of its earth abundance, inexpensive and non-toxic constituents and versatile material characteristics. Kesterite CZTSSe thin films were synthesized by sulfo-selenization of sputtered Cu/Sn/Zn stacked metallic precursors. The sulfo-selenization of Cu/Sn/Zn stacked metallic precursor thin films has been carried out in a graphite box using rapid thermal annealing (RTA) technique. Annealing process was done under sulfur and selenium vapor pressure using Ar gas at $520^{\circ}C$ for 10 min. The effect of tuning Se/(S+Se) precursor composition ratio on the properties of CZTSSe films has been investigated. The XRD, Raman, FE-SEM and XRF results indicate that the properties of sulfo-selenized CZTSSe thin films strongly depends on the Se/(S+Se) composition ratio. In particular, the CZTSSe TFSCs with Se/(S+Se) = 0.37 exhibits the best power conversion efficiency of 4.83% with $V_{oc}$ of 467 mV, $J_{sc}$ of $18.962mA/cm^2$ and FF of 54%. The systematic changes observed with increasing Se/(S+Se) ratio have been discussed in detail.