• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.166.1-166.1
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• 2014

We have controlled the graphene surface in two ways to improve the device performance of optoelectronics based on graphene transparent conductive films. We controlled multilayer graphene (MLG) work function and localized surface plasmon resonance wavelength using a silver nanoparticles formed on graphene surface. Graphene substrates were prepared using a chemical vapor deposition and transfer process. Various size of silver nanoparticles were prepared using a thermal evaporator and post annealing process on graphene surface. Silver nanoparticles were confirmed by using scanning electron microscopy (SEM). Work functions of graphene surface with various sizes of Ag nanoparticles were measured using ultraviolet photoelectron spectroscopy (UPS). The result shows that the work functions of MLG could be controlled from 4.39 eV to 4.55 eV by coating different amounts of silver nanoparticles while minimal changes in the sheet resistance and transmittance. Also the Localized surface plasmon resonance (LSPR) wavelength was investigated according to various sizes of silver nanoparticles. LSPR wavelength was measured using the absorbance spectrum, and we confirmed that the resonance wavelength could be controlled from 396nm to 425nm according to the size of silver nanoparticles on graphene surface. To confirm improvement of the device performance, we fabricated the organic solar cell based on MLG electrode. The results show that the work function and plasmon resonance wavelength could be controlled to improve the performance of optoelectronics device.

• Korean Journal of Materials Research
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• v.25 no.12
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• pp.708-712
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• 2015

This study reports the effects of $H_2S$ gas concentration on the properties of $Cu_2ZnSnS_4(CZTS)$ thin films. Specifically, sulfurization process with low $H_2S$ concentrations of 0.05% and 0.1%, along with 5% $H_2S$ gas, was studied. CZTS films were directly synthesized on Mo/Si substrates by chemical bath deposition method using copper sulfate, zinc sulfate heptahydrate, tin chloride dihydrate, and sodium thiosulfate pentahydrate. Smooth CZTS films were grown on substrates at optimized chemical bath deposition condition. The CZTS films sulfurized at low $H_2S$ concentrations of 0.05 % and 0.1% showed very rough and porous film morphology, whereas the film sulfurized at 5% $H_2S$ yielded a very smooth and dense film morphology. The CZTS films were fully crystallized in kesterite crystal form when they were sulfurized at $500^{\circ}C$ for 1 h. The kesterite CZTS film showed a reasonably good room-temperature photoluminescence spectrum that peaked in a range of 1.4 eV to 1.5 eV, consistent with the optimal bandgap for CZTS solar cell applications.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.07a
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• pp.143-146
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• 2003

Al doped Zinc Oxide(ZnO:Al) films, which is widely used as a transparent conductor in optoelectronic devices such as solar cell, liquid crystal display, plasma display panel, thermal heater, and other sensors, were prepared by using the capacitively coupled DC magnetron sputtering method. The influence of the substrate temperature, working gas pressure and discharge power on the electrical, optical and morphological properties were investigated experimentally. The consideration on the effect of doping amounts of Al on the electrical and optical properties of ZnO thin film were also carried out. ZnO:Al films with the optimum growth conditions showed resistivity of $9.42{\times}10^{-4}\;{\Omeg}-cm$ and transmittance of 90.88% for 840nm in film thickness in the wavelength range of the visible spectrum.

• Journal of the Korean institute of surface engineering
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• v.29 no.6
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• pp.666-672
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• 1996

The optical property of CdTe thin film is important for applications such as the compound semiconductor type solar cells. CdTe films are prepared by RF sputtering at various substrate temperature between $25^{\circ}C$ and $300^{\circ}C$, then, annealed in argon gas environment at $400^{\circ}C$. The annealing process of the thin film caused variation in the film structure and the composition of films. The deformation of CdTe thin film was observed by X-ray diffractometry. After annealing, the grain size increased and the portion of the non-crystalline CdTe reduced. Futhermore, the structure of sputtered CdTe film grown at the substrate temperature more than $250^{\circ}C$ was enhanced in the (111) direction of zincblend structure. There was a discrepancy, in the spectroscopic ellipsometer spectrum, between the single crystal CdTe and the sputtered CdTe thin films, especially in the region over 3.2eV. An oxidation layer was found on the CdTe thin film by spectroscopic ellipsometry analysis.

• Current Photovoltaic Research
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• v.8 no.2
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• pp.50-53
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• 2020

GaN based photoelectrode has shown good potential owing to its better chemical stability and tunable bandgap with materials such as InN and AlN. Tunable bandgap allows GaN to make the maximum utilization of solar spectrum, which could improve photoelectrode performance. However, the problems about low photoelectrode performance and photo-corrosion still remain. In this study, we attempt to investigate the photoelectrochemical (PEC) properties of phosphor application to InGaN photoelectrode. Experimental result shows YAG:Ce³⁺ and beta-SiALON phosphor result in the highest photoelectrode performance of InGaN.

• Journal of Powder Materials
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• v.19 no.3
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• pp.177-181
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• 2012

Copper composite materials have attracted wide attention for energy applications. Especially $CuInS_2$ has a desirable direct band gap of 1.5 eV, which is well matched with the solar spectrum. $CuInS_2$ nanoparticles could make it possible to develop color-tunable $CuInS_2$ nanoparticle emitter in the near-infrared region (NIR) for energy application and bio imaging sensors. In this paper, $CuInS_2$ nanoparticles were successfully synthesized by thermo-decomposition methods. Surface modification of $CuInS_2$ nanoparticles were carried out with various semiconductor materials (CdS, ZnS) for enhanced optical properties. Surface modification and silica coating of hydrophobic nanoparticles could be dispersed in polar solvent for potential applications. Their optical properties were characterized by UV-vis spectroscopy and photoluminescence spectroscopy (PL). The structures of silica coated $CuInS_2$ were observed by transmission electron microscopy (TEM).

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2008.10a
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• pp.205-210
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• 2008

Titanium-doped indium oxide (ITiO) films were prepared on soda-lime glass substrate using a magnetic null discharge (MND) sputter source. The ITiO thin films containing 10wt.% Ti showed the minimum resistivity of $\rho=5.5{\times}10^{-3}{\Omega}-cm$. The optical transmittance increases from 70% at 450 nm to 80% at 700 nm in visible spectrum. Photoelectron peaks for In 3d, Ti 2p, O 1s and C1s were detected for the ITiO film in the binding energy range of 0 to 1100 eV. The surface roughness of the sample showed a change from 10 nm to 50 nm. The ITiO film used for TCO layer of DSCs exhibited an energy conversion efficiency of about 3.8% at light intensity of 100 mW/$cm^2$.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.425-425
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• 2008

The transparent conducting oxides(TCOs) are widely used as electrodes for most flat panel display devices(FPDs), electrodes in solar cells and organic light emitting diodes(OLED). Among them, indium oxide materials are mostly used due to its high electrical conductivity and a high transmittance in the visible spectrum. The present study reports on a study of the electrical and optical properties of IGZO thin films prepared on glass and PET substrates by the combinational magnetron sputtering. We use the targets of IZO and Ga2O3 for the deposition process. In some case the deposition process is coupled with the End-Hall ion-beam treatment onto the substrates before the sputtering. In addition we control the deposition rate to optimize the film quality and to minimize the surface roughness. Then we investigate the effects of the Ar gas pressure and RF power during the sputtering process upon the electrical, optical and morphological properties of thin films. The properties of prepared IGZO thin films have been analyzed by using the XRD, AFM, a-step, 4-point probe, and UV spectrophotometer.

• Journal of The Korean Astronomical Society
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• v.36 no.3
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• pp.177-187
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• 2003

Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electromagnetic spectrum, from radio waves to gamma-rays. There is much evidence that they are driven by gravitational accretion of stars, dust, and gas, onto central massive black holes (MBHs) imprisoning anywhere from $\~$1 to $\~$10,000 million solar masses; such objects may naturally form in the centers of galaxies during their normal dynamical evolution. A small fraction of AGNs, of the radio-loud type (RLAGNs), are somehow able to generate powerful synchrotron-emitting structures (cores, jets, lobes) with sizes ranging from pc to Mpc. A brief summary of AGN observations and theories is given, with an emphasis on RLAGNs. Preliminary results from the imaging of 10000 extragalactic radio sources observed in the MITVLA snapshot survey, and from a new analytic theory of the time-variable power output from Kerr black hole magnetospheres, are presented. To better understand the complex physical processes within the central engines of AGNs, it is important to confront the observations with theories, from the viewpoint of analyzing the time-variable behaviours of AGNs - which have been recorded over both 'short' human ($10^0-10^9\;s$) and 'long' cosmic ($10^{13} - 10^{17}\;s$) timescales. Some key ingredients of a basic mathematical formalism are outlined, which may help in building detailed Monte-Carlo models of evolving AGN populations; such numerical calculations should be potentially important tools for useful interpretation of the large amounts of statistical data now publicly available for both AGNs and RLAGNs.

• Proceedings of the KSRS Conference
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• 2009.03a
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• pp.254-259
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• 2009

In this study, Simplified Method for the Atmospheric Correction (SMAC) radiative transfer model (RTM) used to retrieve surface reflectance from MODIS Top Of Atmosphere (TOA) reflectance (MOD02). SMAC code provides coefficients which were previously yielded by Second Simulation of the Satellite Signal in the Solar Spectrum (6S) for each satellite sensor. We conducted error analysis of SMAC RTM using MOD02 over comparison with MODIS surface reflectance (MOD09) which was provided from 6S. It showed that low accuracy values such as, $R^2$ : 0.6196, Root Means Square Error (RMSE) : 0.00031, bias : - 0.0859. Thus sensitivity analysis of input parameters and coefficients was conducted to searching error sources. Coefficients about $\tau_p$ (average AOD) are more influence than any other coefficients of $\tau_{a550}$ (Aerosol Optical Depth at 550nm) from sensitivity test. Calibrated coefficients of $\tau_p$ from regression analysis were used to surface reflectance which showed that improve accuracy of surface reflectance ($R^2$ : 0.827, RMSE : 0.00672, bias : - 0.000762).