Polymer Quantum Dot-Sensitized Solar Cell Incorporating Single-Walled Carbon Nanotubes

Lee, Junyoung,Lee, Jongtaek,Yang, Jonghee,Park, Taehun,Ahn, Sang Jung,Yi, Whikun American Scientific Publishers 2017 Journal of Nanoscience and Nanotechnology Vol.17 No.8

<P>Regioregular poly(3-hexylthiophene) (P3HT) of conjugated polymer is known to be p-type hole transports material. P3HT is used as a material to substitute for the liquid electrolyte in the quantum dot-sensitized solar cells (QDSSCs). In order to gain high performance of the cell, three different cells using P3HT, pristine P3HT, single-walled carbon nanotube (SWNT)-mixed P3HT (mixing), and SWNT-blended P3HT (blended) are made and their photovoltaic cells measure J-V curve, incident photon to current efficiency, and electrochemical impedance spectroscopy. Those values obtained from experiments are compared with each other.</P>

Some Features of Dye-sensitized Solar Cell Combining with Single-walled Carbon Nanotubes

Lee, Sanghun,Park, Hyunjune,Park, Taehee,Lee, Jongtaek,Yi, Whikun Korean Chemical Society 2014 Bulletin of the Korean Chemical Society Vol.35 No.3

A dye-sensitized solar cell (DSSC) was fabricated with a nanocrystalline $TiO_2$ film electrode on FTO glass, N719 dye, electrolytes (or $CsSnI_3$), and counter Pt electrode by incorporating it with single-walled carbon nanotubes (SWNTs). SWNTs were combined with $TiO_2$ film, $CsSnI_3$, Pt electrode, separately, and the SWNT-containing cell was compared with a pristine cell in cell performance. We also examined the performance change by pressing $TiO_2$ film, during cell fabrication, inside a high pressure chamber. Mostly, the change of conversion efficiency was compared for each cell, and an atomic force microscopy data were suggested to explain our results.

Current flow of single-walled carbon nanotubes upon the encapsulation of beta-carotene by using conducting probe atomic force microscopy.

Lee, Jongtaek,Park, Taehee,Lee, Jungwoo,Yi, Whikun American Scientific Publishers 2013 Journal of Nanoscience and Nanotechnology Vol.13 No.11

<P>Beta-carotene was inserted into single-walled carbon nanotubes (SWCNTs) by using the encapsulation method in a solution phase, and the energy transfer process was studied under irradiation of visible light. The encapsulation of beta-carotene inside SWCNTs was confirmed by ultraviolet (UV)/visible (Vis) and near-IR (N-IR) spectroscopy, and the stability of encapsulated beta-carotene was also confirmed by a UV irradiation experiment. The N-IR absorption spectrum revealed that the beta-carotene donated electrons to the SWCNTs upon encapsulation. We measured current flow through SWCNT bundles by using conducting probe atomic force microscopy (CP-AFM) while the samples were irradiated by green light (532 nm) and red light (650 nm). The current changed with the irradiation of 532 nm light, where the beta-carotene has its own absorption, but not with the irradiation of 650 nm light. From these results, we concluded that the encapsulated beta-carotene inside SWCNTs efficiently absorbed 532 nm light and excited electrons of beta-carotene might be transferred to the SWCNTs like an energy transfer process. Our conclusion was consistent with a previously suggested energy transfer theory between beta-carotene and SWCNTs.</P>

Sensitive and Stable Gas Ionization Sensor Based on 3-D Single-Walled Carbon Nanotube Networks Suspended on ZnO Nanorods

Jongtaek Lee,Taehee Park,Jungwoo Lee,Whikun Yi IEEE 2015 IEEE SENSORS JOURNAL Vol.15 No.1

<P>We fabricated a gas ionization sensor based on 3-D single-walled carbon nanotube (SWNT) networks suspended between ZnO nanorods using a chemical vapor deposition method. ZnO nanorods were grown on the pyramid-like protrusions of a textured Si substrate. The breakdown voltages and stability of the 3-D SWNT networks were measured and compared with those of a SWNT film synthesized on Si substrate. Our results show that 3-D SWNT networks on ZnO nanorods enable ionization gas sensors to be sensitive, selective, and highly stable devices.</P>

Field emission properties of a three-dimensional network of single-walled carbon nanotubes inside pores of porous silicon.

Lee, Jungwoo,Park, Taehee,Lee, Jongtaek,Yi, Whikun American Scientific Publishers 2014 Journal of Nanoscience and Nanotechnology Vol.14 No.8

<P>We report characteristic field emission (FE) properties of single-walled carbon nanotubes (SWCNTs) synthesized inside the pores as well as on the top surface of a porous silicon (PS) substrate. Turn-on fields and emission current densities were measured and compared with those of other types of SWCNTs in similar environments. Investigation of the FE properties of SWCNTs synthesized inside the pores of a PS substrate revealed a low turn-on field of approximately 2.25 V/μm at 10 μA/cm2 and a high field-enhancement factor (6182) compared with other samples. A life-time stability test was performed by monitoring the current density before and after repeated exposure to O2, suggesting that the pore channel can effectively prevent O2(+) ion etching from destroying SWCNTs within the pores of the PS layer.</P>

Fabrication of Three-Dimensional Single-Walled Carbon Nanotube Networks Suspended on Si Pillar Architecture

Lee, Jongtaek,Lee, Junyoung,Yang, Jonghee,Park, Taehun,Ahn, Sang Jung,Yi, Whikun American Scientific Publishers 2017 Journal of Nanoscience and Nanotechnology Vol.17 No.8

<P>We synthesized three-dimensional (3-D) SWNT networks suspended on Si pillar architecture. The pillar arrays on Si substrate were formed by a standard photolithography technique using a patterned chrome mask and plasma etching in a SF6:C4F8 gas mixture. Successively, 3-D SWNT networks were synthesized between pillar arrays by a thermal chemical vapor deposition. We investigated the optical properties and formation of 3-D SWNT networks suspended on Si pillar arrays through controlling catalyst deposition methods between pillar arrays by using scanning electron microscopy and Raman spectroscopy. We confirmed that the bridging and connecting of SWNTs between Si pillars, suggesting the effective growth of the 3-D SWNT networks on Si pillar architecture for application to large scale and well-organized devices.</P>

Terahertz Phonon Modes of Highly Efficient Electro-optic Phenyltriene OH1 Crystals

Kim, Jongtaek,Lee, Seung-Heon,Lee, Seung-Chul,Jazbinsek, Mojca,Miyamoto, Katsuhiko,Omatsu, Takashige,Lee, Yoon Sup,Kwon, O-Pil American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.42

<P>Understanding the origin of the phonon modes of highly efficient electro-optic crystals is very important for designing materials and for optimizing their photonic applications. Here we investigate the origin of phonon modes in the 0.1-15 THz range of the benchmark electrooptic OH1 (2-(3-(4-hydroxystyry1)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal, which is interesting due to its large electro-optic coefficient and high THz-wave generation efficiency. The phonon modes (and vibrational absorption properties) of OH1 crystals are evaluated theoretically by periodic density functional theory and also experimentally by THz absorption spectroscopy. The the oretical calculations are well-matched with experimental results. The THz absorption properties are highly anisotropic; the amplitude of the vibrational absorption is the largest along the polar c-axis compared to the other two crystallographic axes. For comparison, the vibrational absorption modes of the OH1 molecule in the gas phase are also calculated. The calculated vibrational absorption spectrum of OH1 crystalline powder appears similar to that of the OH1 molecule in the gas phase. However, the molecular vibrational motions in the crystalline state are coupled motions of vibrational motions in the gas phase. Interestingly, the vibrational mode of the torsion of the O-H bond with the largest absorption strength in the gas phase is in the crystal inhibited due to the crystal field effect. The origin of the intense phonon modes of OH1 crystals is mainly related to relatively strong distortions of the push pull pi-conjugated system including electron donor and acceptor groups.</P>

Some Features of Dye-sensitized Solar Cell Combining with Single-walled Carbon Nanotubes

Sanghun Lee,Hyunjune Park,Taehee Park,Jongtaek Lee,이휘건 대한화학회 2014 Bulletin of the Korean Chemical Society Vol.35 No.3

A dye-sensitized solar cell (DSSC) was fabricated with a nanocrystalline TiO2 film electrode on FTO glass, N719 dye, electrolytes (or CsSnI3), and counter Pt electrode by incorporating it with single-walled carbon nanotubes (SWNTs). SWNTs were combined with TiO2 film, CsSnI3, Pt electrode, separately, and the SWNTcontaining cell was compared with a pristine cell in cell performance. We also examined the performance change by pressing TiO2 film, during cell fabrication, inside a high pressure chamber. Mostly, the change of conversion efficiency was compared for each cell, and an atomic force microscopy data were suggested to explain our results.

Enhancement of Photocurrent Generation by C60-encapsulated Single-walled Carbon Nanotubes in Ru-sensitized Photoelectrochemical Cell

Jungwoo Lee,Taehee Park,Jongtaek Lee,Mira Jang,Seungjin Lee,Heesu Kim,한성환,이휘건 대한화학회 2012 Bulletin of the Korean Chemical Society Vol.33 No.8

Single-walled carbon nanotubes (SWNTs) and C60-encapsulated SWNTs (C60@SWNTs) are introduced to Rusensitized photoelectrochemical cells (PECs), and photocurrents are compared between two cells, i.e., an RuL2(NCS)2/DAPV/SWNTs/ITO cell and an RuL2(NCS)2/DAPV/C60@SWNTs/ITO cell. [L = 2,2'- bipyridine-4,4'-dicarboxylic acid, DAPV = di-(3-aminopropyl)-viologen, and ITO = indium-tin oxide] The photocurrents are increased by 70.6% in the presence of C60@SWNTs. To explain the photocurrent increase, the reverse-field emission method is used, i.e., RuL2(NCS)2/DAPV/SWNTs/ITO cell (or RuL2(NCS)2/DAPV/ C60@SWNTs/ITO cell) as an anode and a counter electrode Pt as a cathode in the external electric field. The improved field emission properties, i.e., β (field enhancement factor) and emission currents in the reverse-field emission with C60@SWNTs indicate the enhancement of the PEC electric field, which implies the improvement of the electron transfer rate along with the reduced charge recombination in the cell.

Evaluation of Target Deposit Insurance Fund Based on Credit Migration Approach for the Non-Life Insurance Sector in Korea

이창수(Changsoo Lee),홍지민(Jimin Hong),이종택(Jongtaek Lee) People&Global Business Association 2021 Global Business and Finance Review Vol.26 No.2

Purpose: In the case of non-life insurance business, the rate for the accumulation of fund in non-life insurance has fallen every year, making it difficult to reach the target fund level. Thus, we evaluate the appropriate target fund level for non-life insurers. Design/methodology/approach: The portfolio of the deposit insurer was constructed for 21 non-life insurers which were licensed non-life insurance businesses under the Insurance Business Act 4 (1) in Korea. We attempt to analyze the default probability using the Credit Migration method rather than the Merton. We derive the joint distribution for change in credit ratings and a loss distribution of the deposit insurer in non-life insurance sector by performing the Monte-Carlo simulation. The average value of the worst loss with a 0.5% probability in the loss distribution, TVaR99.5%, was adopted as an appropriate target fund level. Findings: The value of TVaR99.5% in this study was between 67 billion won and 2.434 trillion won. This amount of money is from 4.71% to 171.71% of the current deposit insurance fund for non-life insurers, and from 4.06% to 147.52% of the target fund level for non-life insurers. However, if a rather extremely conservative case that reflects the loss of deposit insurance funds due to the bankruptcy of up to 7 insurers among non-life insurers that consists of the portfolio is excluded, the distribution of the TVaR99.5% was between 67 billion won and 402 billion won. This money is from 4.71% to 28.28% of the current deposit insurance fund, and from 4.06% to 24.37% of the target fund level for non-life insurers. These results indicate that additional funding is unnecessary because the current level of deposit insurance funds will sufficiently cover future losses. Research limitations/implications: The limitation of this study is to evaluate the deposit fund under the RBC system, since it is controversial whether the RBC system adequately reflects the risk of non-life insurers. The risk of minimum guaranteed interest rate, catastrophe risk and liquidity risk are needed to be considered through further study. In particular, comparing the loss distribution for individual insurers under Solvency II and the RBC system is a future task of research. Originality/value: This study contributes to the evaluation of the target deposit insurance fund for non-life insurers by applying the credit migration method unlike the existing studies. This attempt is meaningful in that it provides an objective and practically easy-to-use alternative to the market participants such as the deposit insurer and non-life insurers.