• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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• pp.310-310
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• 2010

Electronic and photovoltaic characteristics of two sensitizers (TA-BTD-CA and TA-BTD-St-CA), composed of a different $\pi$-conjugation in the linker group, have been investigated by theoretical and experimental methods. The electronic structure, transition dipole moment and oscillator strengths of two sensitizers have been scrutinized by using density functional theory (DFT) and time-dependent DFT (TD-DFT) method. The LUMO level and the oscillator strength of TA-BTD-St-CA was higher than that of TA-BTD-CA, which may facilitate the electron injection process as well as increase the absorption coefficient. The relative efficiencies of the electron injection from the excited sensitizer to nanocrystalline TiO2 and SnO2 films have also been investigated by nanosecond transient absorption spectroscopy. The relative electron injection efficiency of TA-BTD-St-CA exhibited similar injection efficiency for two different semiconductors. However, in the case of TA-BTD-CA sensitizer, electron injection into SnO2 was approximately three times larger than that into TiO2. This enhancement of electron injection of TA-BTD-CA for the SnO2 is due to the increment of the driving force caused by positive shift of conduction band of semiconductor, which was also confirmed from the investigation for the photovoltaic characteristics according to the electrolyte additive, such as LiI additive.

• 한국정밀공학회지
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• 제28권12호
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• pp.1419-1424
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• 2011

Nanoimprint lithography is a next generation lithography technology, which enables to fabricate nano to micron-scale patterns through simple and low cost process. Nanoimprint lithography has been applied in various industry fields such as light emitting diodes, solar cells and display. Functional patterns, including anti-reflection moth-eye pattern, photonic crystal pattern, fabricated by nanoimprint lithography are used to improve overall efficiency of devices in that fields. For these reasons, in this study, sub-micron-scaled functional patterns were directly fabricated on Si and glass substrates by thermal imprinting process using ZnO nano-particles dispersion resin. Through the thermal imprinting process, arrays of sub-micron-scaled pillar and hole patterns were successfully fabricated on the Si and glass substrates. And then, the topography, components and optical property of the imprinted ZnO nano-particles/resin patterns are characterized by Scanning Electron Microscope, Energy-dispersive X-ray spectroscopy and UV-vis spectrometer, respectively.

• 대한금속재료학회지
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• 제48권5호
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• pp.449-455
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• 2010

To enhance the efficiency of dye sensitized solar cells, we proposed crystalline anatase-$TiO_{2}$ by using a low temperature process ($150^{\circ}C{\sim}250^{\circ}C$). We successfully fabricated 30 nm-$TiO_{2}$ at a fixed atomic layer deposition condition of 1.0 sec of TDMAT pulse, 20 sec of TDMAT purge, 0.5 sec of H$_{2}$O pulse, and 20 sec of H$_{2}$O purge. In order to examine the microstructure, phase, and band-gap of the TiO$_{2}$ respectively, we employed a Nano-Spec, transmission electron microscope, high resolution XRD, Auger electron spectroscopy, scanning probe microscope, and UV-VIS-NIR. We were able to fabricate a crystalline anatase-phase of 30 nm-TiO$_{2}$ successfully at temperatures above $180^{\circ}C$. Our results showed that our proposed low temperature ALD process (below $200^{\circ}C$) might be applicable to glass and flexible polymer substrates.

• 천문학회보
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• 제45권1호
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• pp.42.4-43
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• 2020

The hydrogen-rich envelope mass of a dying massive star is the key factor that determines the type and properties of the resulting supernova. Emulating wind-driven mass loss of single stars with the MESA(Modules for Experiments in Stellar Astrophysics) stellar evolution code, we made a grid of models for a large parameter space of initial mass (12 M⊙ to 30M⊙), metallicity (solar, LMC and SMC), hydrogen envelope mass (0.01M⊙ to 10M⊙) for progenitor stars in their final step of evolution. Our results suggest the final luminosity of the progenitor is largely determined by the initial mass, which means there is luminosity degeneracy for stars with the same initial mass but with different hydrogen-rich envelope masses. Since we can break this degeneracy by correcting luminosity with surface gravity (spectroscopic HR diagram), we can infer the exact mass property of an observed progenitor. The surface temperature drastically varies near the envelope mass of ~0.1M⊙ and surface temperature of ~10000 K, where the demarcation between the hydrogen-rich envelope and the helium core lies, which explains the rarity of 'white' supergiants. There also exists a discontinuity in the chemical composition of the progenitor envelope around this critical hydrogen-rich envelope mass of ~0.1 M⊙, which can be tested in future observations of "flash spectroscopy" of supernovae.

• 천문학회보
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• 제45권1호
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• pp.45.1-45.1
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• 2020

Light bridges (LBs) are relatively bright structures that divide sunspot umbrae into two or more parts. Chromospheric LBs are known to be associated with various activities including fan-shaped jet-like ejections and brightenings. Although magnetic reconnection is frequently suggested to be responsible for such activities, not many studies presented firm evidence to support the scenario. We carry out magnetic field measurements and imaging spectroscopy of a LB where fan-shaped jet-like ejections occur with co-spatial brightenings at their footpoints. We study their fine photospheric structures and magnetic field changes using TiO images, Near-InfraRed Imaging Spectropolarimeter data, and Hα data taken by the 1.6 m Goode Solar Telescope. As a result, we detect magnetic flux emergence in the LB that is of opposite polarity to that of the sunspot. The new flux cancels with the pre-existing flux at a rate of 5.6×1018 Mx hr-1. Both recurrent jet-like ejections and their footpoint brightenings are initiated at the vicinity of the magnetic cancellation, and show apparent horizontal extension along the LB at a projected speed of 4.3 km s-1 to form the fan-shaped appearance. Based on these observations, we suggest that the fan-shaped ejections may have resulted due to slipping reconnection between the new flux emerging in the LB and the ambient sunspot field.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2016년도 제50회 동계 정기학술대회 초록집
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• pp.417-417
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• 2016

Global environmental deterioration has become more serious year by year and thus scientific interests in the renewable energy as environmental technology and replacement of fossil fuels have grown exponentially. Photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that can harvest light and use this energy directly to split water, also known as photoelectrolysis or solar water splitting, is a promising renewable energy technology to produce hydrogen for uses in the future hydrogen economy. A major advantage of PEC systems is that they involve relatively simple processes steps as compared to many other H2 production systems. Until now, a number of materials including TiO2, WO3, Fe2O3, and BiVO4 were exploited as the photoelectrode. However, the PEC performance of these single absorber materials is limited due to their large charge recombinations in bulk, interface and surface, leading low charge separation/transport efficiencies. Recently, coupling of two materials, e.g., BiVO4/WO3, Fe2O3/WO3 and CuWO4/WO3, to form a type II heterojunction has been demonstrated to be a viable means to improve the PEC performance by enhancing the charge separation and transport efficiencies. In this study, we have prepared a triple-layer heterojunction BiVO4/WO3/SnO2 photoelectrode that shows a comparable PEC performance with previously reported best-performing nanostructured BiVO4/WO3 heterojunction photoelectrode via a facile solution method. Interestingly, we found that the incorporation of SnO2 nanoparticles layer in between WO3 and FTO largely promotes electron transport and thus minimizes interfacial recombination. The impact of the SnO2 interfacial layer was investigated in detail by TEM, hall measurement and electrochemical impedance spectroscopy (EIS) techniques. In addition, our planar-structured triple-layer photoelectrode shows a relatively high transmittance due to its low thickness (~300 nm), which benefits to couple with a solar cell to form a tandem PEC device. The overall PEC performance, especially the photocurrent onset potential (Vonset), were further improved by a reactive-ion etching (RIE) surface etching and electrocatalyst (CoOx) deposition.

• 한국재료학회지
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• 제20권6호
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• pp.289-293
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• 2010

Silicon quantum dots (Si QDs) in a superlattice for high efficiency tandem solar cells were fabricated by magnetron rf sputtering and their characteristics were investigated. SiC/$Si_{1-x}C_x$ superlattices were deposited by co-sputtering of Si and C targets and annealed at $1000^{\circ}C$ for 20 minutes in a nitrogen atmosphere. The Si QDs in Si-rich layers were verified by transmission electron microscopy (TEM) and X-ray diffraction. The size of the QDs was observed to be 3-6 nm through high resolution TEM. Some crystal Si and -SiC peaks were clearly observed in the grazing incident X-ray diffractogram. Raman spectroscopy in the annealed sample showed a sharp peak at $516\;cm^{-1}$ which is an indication of Si QDs. Based on the Raman shift the size of the QD was estimated to be 4-6 nm. The volume fraction of Si crystals was calculated to be about 33%. The change of the FT-IR absorption spectrum from a Gaussian shape to a Lorentzian shape also confirmed the phase transition from an amorphous phase before annealing to a crystalline phase after annealing. The optical absorption coefficient also decreased, but the optical band gap increased from 1.5 eV to 2.1 eV after annealing. Therefore, it is expected that the optical energy gap of the QDs can be controlled with growth and annealing conditions.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2011년도 제41회 하계 정기 학술대회 초록집
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• pp.306-307
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• 2011

Franz Keldysh Oscillation (FKO)은 p-n 접합 구조의 공핍층(depletion zone)에서 전기장(electric field)에 의해 발생되며, Photoreflectance (PR) spectroscopy를 통하여 관측된다. InAs/GaAs 양자점 태양전지(Quantum Dot Solar Cells, QDSCs)에서 PR 신호에 대한 Fast Fourier Transform (FFT)을 통하여 FKO 주파수들을 관측할 수 있고, 각각의 FKO 주파수들은 태양전지 구조에 대응하는 표면 및 내부전기장(internal electric field) 들로 분류할 수 있다. InAs/GaAs 양자점 태양전지에서 AlGaAs potential barrier의 두께에 따른 내부전기장의 변화를 조사하기 위해, GaAs-matrix에 8주기의 InAs 양자점 층이 삽입된 태양전지를 molecular beam epitaxy (MBE) 방법으로 성장하였다. 양자점의 크기는 2.0 monolayer (ML)이며, 각 양자점 층은 1.6 nm에서 6.0 nm의 AlGaAs potential barrier들로 분리되어 있다. 또한 양자점 층의 위치에 따라 내부전기장 변화를 조사하기 위해, p-i-n 구조에서 양자점 층이 공핍층 내에 위치한 경우와 p+-n-n+ 구조에서 양자점 층이 공핍 층으로부터 멀리 떨어진 n-base 영역에 삽입하여 실험결과를 비교분석하였다. PR 실험결과로부터, p-i-n 구조에서 InAs 양자점 태양전지의 내부전기장 변화는 potential barrier 두께에 따라 다소 복잡한 변화를 보였으며, 이는 양자점 층이 공핍층 내에 위치함으로써 격자 불일치(lattice mismatch)로 발생된 응력(strain)의 영향으로 설명할 수 있다. 이러한 결과들을 각각의 태양전지 구조에서 표면 및 내부전기장에 대해 계산된 값들에 근거하여, p+-n-n+ 구조에서 양자점 층이 공핍 층으로부터 멀리 떨어진 영역에 삽입된 경우의 결과와 비교해 보면 내부전기장의 변화는 더욱 분명해진다. 즉, 양자점 층의 potential barrier의 두께를 조절하거나, 양자점 층의 위치를 변화시킴으로써 양자점 태양전지의 내부전기장을 조작할 수 있으며, 이는 PR 실험을 통해 FKO를 관측함으로써 확인할 수 있다.

• 공업화학
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• 제22권2호
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• pp.190-195
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• 2011

다중층을 형성하여 광자를 가두는 효과와 산란층의 효과를 보고, $TiCl_4$ 처리를 통해 전극에서의 전자의 재결합이 줄어드는 정도와 그에 따른 효과를 알아보기 위하여 여러 가지 방법으로 $TiO_2$ 전극을 형성하고, 가장 최적의 전극 조건을 알아보았다. 각 전극의 특성을 알기 위해서 I-V 곡선, UV-VIS 분광기, EIS, IPCE를 측정하였다. 그 결과, I-V 곡선을 통해 한 층 보다는 다중층이 효율이 더 높은 것을 확인할 수 있었고, 기판 표면과 전극표면에 $TiCl_4$ 처리를 함으로써 EIS분석을 통해 반응저항이 감소하여 효율이 증가함을 확인할 수 있었다. 여러 전극 조건 중 산란층을 지닌 전극이 기본 한 층을 사용한 전극의 효율보다 약 19% 정도 높아짐을 확인하였다. 이러한 효율의 증가는 장파장을 투과하는 빛이 산란층을 통과할 때 전자 이동 경로가 길어지게 되어 단락전류의 값을 증가시키기 때문이다. 이에 따라, $J_{SC}$는 약 10% 정도 증가하였으며, IPCE는 최대 피크에서 약 12%가 향상되는 특성을 보였다.

• 천문학회지
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• 제54권2호
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• pp.49-60
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• 2021

Observations of line of sight (LOS) Doppler velocity and non-thermal line width in the off-limb solar corona are often used for investigating the Alfvén wave signatures in the corona. In this study, we compare LOS Doppler velocities and non-thermal line widths obtained simultaneously from two different instruments, Coronal Multichannel Polarimeter (CoMP) and Hinode/EUV Imaging Spectrometer (EIS), on various off-limb coronal regions: flaring and quiescent active regions, equatorial quiet region, and polar prominence and plume regions observed in 2012-2014. CoMP provides the polarization at the Fe xiii 10747 Å coronal forbidden lines which allows their spectral line intensity, LOS Doppler velocity, and line width to be measured with a low spectral resolution of 1.2 Å in 2-D off limb corona between 1.05 and 1.40 RSun, while Hinode/EIS gives us the EUV spectral information with a high spectral resolution (0.025 Å) in a limited field of view raster scan. In order to compare them, we make pseudo raster scan CoMP maps using information of each EIS scan slit time and position. We compare the CoMP and EIS spectroscopic maps by visual inspection, and examine their pixel to pixel correlations and percentages of pixel numbers satisfying the condition that the differences between CoMP and EIS spectroscopic quantities are within the EIS measurement accuracy: ±3 km s-1 for LOS Doppler velocity and ±9 km s-1 for non-thermal width. The main results are summarized as follows. By comparing CoMP and EIS Doppler velocity distributions, we find that they are consistent with each other overall in the active regions and equatorial quiet region (0.25 ≤ CC ≤ 0.7), while they are partially similar to each other in the overlying loops of prominences and near the bottom of the polar plume (0.02 ≤ CC ≤ 0.18). CoMP Doppler velocities are consistent with the EIS ones within the EIS measurement accuracy in most regions (≥ 87% of pixels) except for the polar region (45% of pixels). We find that CoMP and EIS non-thermal width distributions are similar overall in the active regions (0.06 ≤ CC ≤ 0.61), while they seem to be different in the others (-0.1 ≤ CC ≤ 0.00). CoMP non-thermal widths are similar to EIS ones within the EIS measurement accuracy in a quiescent active region (79% of pixels), while they do not match in the other regions (≤ 61% of pixels); the CoMP observations tend to underestimate the widths by about 20% to 40% compared to the EIS ones. Our results demonstrate that CoMP observations can provide reliable 2-D LOS Doppler velocity distributions on active regions and might provide their non-thermal width distributions.