Se Jin Ahn

Seoul National University, Seoul, Seoul, South Korea

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Publications (12)18.82 Total impact

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    ABSTRACT: We developed a simple and effective method for the large scale formation of self-assembled Cu(In,Ga)Se2 (CIGS) nanocrystals by ion beam irradiation. The compositional changes and morphological evolution were observed as a function of the irradiation time. As the ion irradiation time increased, the nano-dots were transformed into a nano-ridge structure due to the competition between sputtering and diffusion processes during irradiation. In terms of the stoichiometry of the CIGS nano-dots, an increase in the Cu content was observed while the Se content decreased. The PL peak of the nano-dots formed CIGS thin film exhibited a blue-shift. Uniformly formed crystalline CIGS nano-dots can be adopted to increase the p–n junction area and the size confinement effect between the CdS and CIGS film in solar cell systems. This simple method can be exploited for band-gap engineering and enhancing photovoltaic properties.
    Solar Energy Materials and Solar Cells 10/2012; 105:119. DOI:10.1016/j.solmat.2012.05.034 · 5.03 Impact Factor
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    ABSTRACT: To evaluate the diagnostic performance of computed tomography (CT) colonography (CTC) reconstructed with different levels of adaptive statistical iterative reconstruction (ASiR, GE Healthcare) and Veo (model-based iterative reconstruction, GE Healthcare) at various tube currents in detection of polyps in porcine colon phantoms. Five porcine colon phantoms with 46 simulated polyps were scanned at different radiation doses (10, 30, and 50 mA s) and were reconstructed using filtered back projection (FBP), ASiR (20%, 40%, and 60%) and Veo. Eleven data sets for each phantom (10-mA s FBP, 10-mA s 20% ASiR, 10-mA s 40% ASiR, 10-mA s 60% ASiR, 10-mA s Veo, 30-mA s FBP, 30-mA s 20% ASiR, 30-mA s 40% ASiR, 30-mA s 60% ASiR, 30-mA s Veo, and 50-mA s FBP) yielded a total of 55 data sets. Polyp detection sensitivity and confidence level of 2 independent observers were evaluated with the McNemar test, the Fisher exact test, and receiver operating characteristic curve analysis. Comparative analyses of overall image quality score, measured image noise, and interpretation time were also performed. Per-polyp detection sensitivities and specificities were highest in 10-mA s Veo, 30-mA s FBP, 30-mA s 60% ASiR, and 50-mA s FBP (sensitivity, 100%; specificity, 100%). The area-under-the-curve values for the overall performance of each data set was also highest (1.000) at 50-mA s FBP, 30-mA s FBP, 30-mA s 60% ASiR, and 10-mA s Veo. Images reconstructed with ASiR showed statistically significant improvement in per-polyp detection sensitivity as the percent level of per-polyp sensitivity increased (10-mA s FBP vs 10-mA s 20% ASiR, P = 0.011; 10-mA s FBP vs 10-mA s 40% ASiR, P = 0.000; 10-mA s FBP vs 10-mA s 60% ASiR, P = 0.000; 10-mA s 20% ASiR vs 40% ASiR, P = 0.034). Overall image quality score was highest at 30-mA s Veo and 50-mA s FBP. The quantitative measurement of the image noise was lowest at 30-mA s Veo and second lowest at 10-mA s Veo. There was a trend of decrease in time required for image interpretation as the percent level of ASiR increased, and ASiR or Veo was used instead of FBP. However, differences from comparative analyses of overall image quality score, measured image noise, and interpretation time did not reach statistical significance. ASiR and Veo showed improved diagnostic performance with excellent sensitivity and specificity with less image noise and good image quality compared with FBP reconstruction of same radiation dose. Our study confirmed feasibility of low-dose CTC with iterative reconstruction as a promising screening tool with excellent diagnostic performance similar to that of the standard-dose CTC with FBP.
    Journal of computer assisted tomography 09/2012; 36(5):596-601. DOI:10.1097/RCT.0b013e31826092be · 1.38 Impact Factor
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    Se Jin Ahn, In-One Kim
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    ABSTRACT: Radiation-induced spinal cord gliomas are extremely rare. Since the first case was reported in 1980, only six additional cases have been reported.; The radiation-induced gliomas were related to the treatment of Hodgkin's lymphoma, thyroid cancer, and medullomyoblastoma, and to multiple chest fluoroscopic examinations in pulmonary tuberculosis patient. We report a case of radiation-induced spinal cord glioblastoma developed in a 17-year-old girl after a 13-year latency period following radiotherapy for nasopharyngeal rhabdomyosarcoma. MRI findings of our case are described.
    Korean journal of radiology: official journal of the Korean Radiological Society 09/2012; 13(5):652-7. DOI:10.3348/kjr.2012.13.5.652 · 1.32 Impact Factor
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    ABSTRACT: A histologic grade in head and neck squamous cell carcinoma (HNSCC) is clinically important because of its association with prognosis. The purpose of this study was to investigate the efficacy of histographic analysis of apparent diffusion coefficient (ADC) maps on the basis of the entire tumor volume in differentiating histologic grades in HNSCC at standard (b = 1000 s/mm(2)) and high (b = 2000 s/mm(2)) b values. Fifty-four patients with HNSCC, including well-differentiated (WD; n = 35), moderately differentiated (MD; n = 13) and poorly differentiated (PD; n = 6) carcinomas, were retrospectively evaluated. ADC maps were obtained at two different b values (1000 and 2000 s/mm(2)) in each patient. Tumors were delineated on each slice of ADC maps, and data were collected to obtain a histogram for the entire tumor volume. Histographic parameters were calculated, including mean, standard deviation, kurtosis, skewness, and the ratio of the kurtosis measured at b values of 1000 and 2000 s/mm(2). These parameters were correlated with histologic grades. There was no significant correlation between tumor grades and histographic parameters obtained from ADC maps at b = 1000 s/mm(2). However, mean ADC at b = 2000 s/mm(2) was significantly higher in WD HNSCC (881 ± 131 × 10(-6) mm(2)/s) than in MD and PD HNSCC (770 ± 163 and 780 ± 158 × 10(-6) mm(2)/s, respectively) (P < .05). Kurtosis ratio was significantly higher in PD HNSCC (115 ± 10%) compared to WD and MD HNSCC (91 ± 21% and 86 ± 26%, respectively) (P < .05). Diagnostic accuracy was 100%, 76.9%, and 65.8% for PD, MD, and WD HNSCC, respectively. Histographic analysis of ADC maps on the basis of the entire tumor volume can be useful in differentiating histologic grades of HNSCC using mean ADC at b = 2000 s/mm(2) and kurtosis ratio.
    Academic radiology 07/2012; 19(10):1233-40. DOI:10.1016/j.acra.2012.04.019 · 2.09 Impact Factor
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    ABSTRACT: To assess the technical success and complication rates of the radiologic placement of central venous ports via the internal jugular vein. We retrospectively reviewed 1254 central venous ports implanted at our institution between August 2002 and October 2009. All procedures were guided by using ultrasound and fluoroscopy. Catheter maintenance days, technical success rates, peri-procedural, as well as early and late complication rates were evaluated based on the interventional radiologic reports and patient medical records. A total of 433386 catheter maintenance days (mean, 350 days; range 0-1165 days) were recorded. The technical success rate was 99.9% and a total of 61 complications occurred (5%), resulting in a post-procedural complication rate of 0.129 of 1000 catheter days. Among them, peri-procedural complications within 24 hours occurred in five patients (0.4%). There were 56 post-procedural complications including 24 (1.9%, 0.055 of 1000 catheter days) early and 32 (2.6%, 0.074 of 1000 catheter days) late complications including, infection (0.6%, 0.018 of 10000 catheter days), thrombotic malfunction (1.4%, 0.040 of 1000 catheter days), nonthrombotic malfunction (0.9%, 0.025 of 1000 catheter days), venous thrombosis (0.5%, 0.014 of 1000 catheter days), as well as wound problems (1.1%, 0.032 of 1000 catheter days). Thirty six CVPs (3%) were removed due to complications. Bloodstream infections and venous thrombosis were the two main adverse events prolonging hospitalization (mean 13 days and 5 days, respectively). Radiologic placement of a central venous port via the internal jugular vein is safe and efficient as evidenced by its high technical success rate and a very low complication rate.
    Korean journal of radiology: official journal of the Korean Radiological Society 05/2012; 13(3):314-23. DOI:10.3348/kjr.2012.13.3.314 · 1.32 Impact Factor
    This article is viewable in ResearchGate's enriched format
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    ABSTRACT: In this paper, we demonstrate, for the first time, the manufacture of a CuInSe2 thin film whose absorber layer is coated using an electrostatic spray deposition (ESD) technique; its complete transformation into a working device with measured conversion efficiency is presented. ESD is superior to pneumatic spraying because it produces nano-scaled, self-dispersive (non-agglomerating), highly wettable (electrowetting) and adhesive droplets to yield a uniform coating on a substrate. Furthermore, ESD’s extremely low material consumption rate holds promises for practical use in the solar cell industry. Copper and indium salts are added to various solvents, which are electrostatically sprayed onto a molybdenum-coated soda-lime glass substrate. The effect of substrate temperature on the thin film characteristics is examined. Our cell is completed by adding CdS and ZnO layers onto the CuInSe2 absorber layer. Light illuminated current-density voltage (J-V) characteristics demonstrate a power conversion efficiency of η = 1.75% ± 0.09 with an open-circuit voltage of VOC = 0.23 V, a short-circuit current density of JSC = 21.72 mA/cm2, and fill factor of FF = 0.34.
    Journal of The Electrochemical Society 02/2012; 159(4):H444. DOI:10.1149/2.jes113086 · 2.86 Impact Factor
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    ABSTRACT: CuInxGa1−xSeyS2−y (CIGS) thin films were synthesized on glass substrates by a paste coating of Cu, In, and Ga precursor solution with a three-step heat treatment process: oxidation, sulfurization, and selenization. In particular, morphological changes of CIGS films for each heat treatment step were investigated with respect to the kinds of glass substrates: bare, Mo-coated, and F-doped SnO2 (FTO) soda-lime glasses. Very high quality CIGS film with large grains and low degree of porosity was obtained on the bare glass substrate. Similar morphology of CIGS film was also acquired on the Mo-coated glass except the formation of an undesired Mo oxide interfacial layer due to the partial oxidation of Mo layer during the first heat treatment under ambient conditions. On the other hand, CIGS film with much smaller grains and higher degree of porosity was gained when FTO glass was used as a substrate, resulting in slight solar to electricity conversion behavior (0.20%). Higher power conversion efficiency (1.32%) was attained by the device with the CIGS film grown on Mo-coated glass in spite of the presence of a Mo oxide impurity layer.
    Applied Surface Science 10/2011; 258(1):120-125. DOI:10.1016/j.apsusc.2011.08.016 · 2.54 Impact Factor
  • Journal- Korean Physical Society 06/2007; 50. DOI:10.3938/jkps.50.1794 · 0.43 Impact Factor
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    ABSTRACT: The photovoltaic properties of CIGS cells on an alumina substrate were improved through the use of Na-doped Mo as the bottom layer of a Mo back contact. Na was supplied to the CIGS bulk region from an alumina/Na-doped Mo/Mo structure, similar to the Na diffusion from soda-lime glass. The diffusion of Na from the Na-doped Mo was controlled effectively compared to that from Soda-lime glass (SLG). The present results indicate that Na-doped Mo acts as a Na source material and that the Na amount can be controlled by adjustment of thickness of Na-doped Mo layer, without the use of an alkali barrier layer. The highest conversion efficiency of 13.34% (Jsc=34.62 mA/cm2, Voc=0.58 V and FF=66%) for an active area of 0.45 cm2 on an alumina substrate was obtained for 100 nm Na-doped Mo/1000 nm Mo.
    Thin Solid Films 05/2007; 515(15):5876-5879. DOI:10.1016/j.tsf.2006.12.156 · 1.87 Impact Factor
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    ABSTRACT: To make a dense CIGS absorber layer, spray deposited CIGS films were annealed in the two-zone RTP furnace in Se atmosphere. More Se supply by increasing Se evaporation temperature or by increasing the flow rate of carrier gas resulted in the larger CIGS grains. However, a thick MoSe 2 layer was formed between CIGS and Mo, as the Se supply increased, results in partial detachment of CIGS/MoSe 2 /Mo layers from the glass substrate. From the result, it was found that the short heat-treatment with high Se vapor pressure is better than the long heat-treatment with low Se vapor pressure. The large CIGS grains without peeling off, can be obtained from the following conditions; Se evaporation temperature of 450 o C, substrate temperature of 550 o C, annealing time of 5 min, and flow rate of carrier gas of 30 sccm. Introduction Advances in nanoparticle technology led to powerful low cost thin film techniques. Together with the established selenization method, these techniques open new possibilities for low cost solar cell production that does not require expensive vacuum deposition systems. Recent advances in nanoparticle synthesis and handling led to a resumption of prior work on non-vacuum deposition and sintering of a precursor material in the form particles 1) . The idea of this approach is to grow uniform CIGS layers of suitable quality on large area, using simple and fast techniques. Selenization of nanoparticle precursor layers has a promising potential that has already been indicated by Kapur et.al 2) . In this work, a novel approach to fabricate CIGS solar cells in which a non-vacuum process is used for depositing the CIGS absorber layer. CIGS nanoparticles with (~20 nm) size were synthesized by colloidal route and analyzed. The CIGS absorber layer for solar cells was obtained by spray deposition. The spray coated CIGS nanoparticles layer was heat-treated in the two-zone RTP (rapid thermal processing) furnace using Se pellets as a source of Se vapor. The effects of the selenium vapor on the sintering behavior and particles growth were discussed.
    Solid State Phenomena 01/2007; 124-126:983-986. DOI:10.4028/www.scientific.net/SSP.124-126.983
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    ABSTRACT: The wide band-gap CuIn1-xAlxSe2(CIAS) films were obtained by selenization process of metallic precursors. The metallic precursors were deposited sequentially by using sputtering system. As the ratio of Al/(Al+ln) in the precursors increased, the chalcopyrite (112) peak shifted to high value and the band-gap of CIAS layer increased to 1.38 eV. However the bilayer morphology with well crystallized large grain on the surface and small grain thin bottom layer was observed. Although the sequences of precursors were changed in order to get uniform layer, the distinguishable difference was not observed. Moreover the films were peeled off completely during the deposition of buffer CdS. The other process conditions such as selenizing at high temperatures, Se containing precursors and post annealing will be tried to remove the bilayer and to get homogeneous films

Publication Stats

59 Citations
18.82 Total Impact Points

Institutions

  • 2012
    • Seoul National University
      • College of Medicine
      Seoul, Seoul, South Korea
    • Seoul National University Hospital
      • Department of Radiology
      Sŏul, Seoul, South Korea
  • 2007–2012
    • Korea Institute of Energy Research
      • Solar Energy Department
      Sŏul, Seoul, South Korea