Young Soo Kwon

Pohang University of Science and Technology, Geijitsu, North Gyeongsang, South Korea

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Publications (17)122.23 Total impact

  • Jongmin Choi, Young Soo Kwon, Taiho Park
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    ABSTRACT: Doubly open-ended TiO2 nanotube (NT) arrays decorated with a few nm-sized TiO2 nanoparticles (sNP@NT hybrid structure) were prepared and used as charge-collecting photoelectrodes in dye-sensitized solar cells (DSCs). The TiO2 nanoparticles (NPs) on the NT array surfaces increased the dye loading on the sNP@NT-based DSCs by 9% compared to the undecorated NT-based DSCs, thereby enhancing the light harvesting capabilities. The power conversion efficiency (PCE) of the sNP@NT-based DSC prepared with 11 μm thick NT arrays was 10.0%, which constituted a 47% improvement over the corresponding NT-based DSCs (which displayed a 6.8% PCE). Despite having a dye loading level that was 22% lower than the dye loading level in the conventional TiO2 NP-based DSCs, due to the limited internal surface area, the PCE of the sNP@NT-based DSC was 28% greater than that of the conventional TiO2 NP-based DSC (8.1% PCE) prepared with a light scattering layer. The high charge collection efficiency of the NT array and the good photovoltaic performance set a new record for efficiency among NT-based DSCs.
    J. Mater. Chem. A. 07/2014;
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    ABSTRACT: We have achieved 8% efficiency for the ruthenium(II) dye, SY-04, in quasi-solid state dye sensitized solar cells using a supramolecular oligomer-based electrolyte. The dyes in this study, SY-04 and SY-05, which were synthesized through highly efficient synthetic routes, showed better molar extinction coefficients compared to that of the Z907 dye. In the absorption spectra, SY-04 and SY-05 displayed better red shifted metal ligand charge transfer (MLCT) absorption bands at 533, 382 and 535, 373 nm, respectively, compared with 521, 371 nm of the Z907 dye. Also, SY-04 and SY-05 showed better molar extinction coefficients, 6691, 16189 M−1 cm−1 and 6694, 16195 M−1 cm−1 as compared with the Z907 dye, 4308, 4917 M−1 cm−1. When excited into the charge-transfer absorption bands of SY-04 and SY-05 in ethanol at 77 K, broad emission bands for SY-04 and SY-05 with a maximum at 788 nm and 786 nm, respectively, were observed compared to the emission band of Z907 at 797 nm. The current–voltage characteristics of the SY-04 sensitizer gave the best performance data, JSC = 18.0 mA cm−2, VOC = 0.662 V, ff = 0.663, and an η of 8.0%. The increased VOC value for SY-04 than Z907 is mainly attributed to high charge recombination resistance by effective dye coverage, which is confirmed by impedance spectroscopy.
    J. Mater. Chem. A. 07/2014;
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    ABSTRACT: A block copolymerization of non-functionalized conducting monomers was developed to enable the successful synthesis of a highly insoluble 3,4-(ethylenedioxy)thienyl-based all-conducting block copolymer (PEDOT-b-PEDOT-TB) that could encapsulate nanocrystalline dyed TiO2 particles, resulting in the formation of an all-conducting block copolymer bilayer hybrid nanostructure (TiO2/Dye/PEDOT-b-PEDOT-TB). Lithium ions were selectively positioned on the outer PEDOT-TB surface. The distances through which the positively charged dye and PEDOT-TB(Li+) interacted physically or through which the TiO2 electrode and the Li+ centers on PEDOT-TB(Li+) interacted ionically were precisely tuned and optimized within ca. 1 nm by controlling the thickness of the PEDOT blocking layer (the block length). The optimized structure provided efficient charge collection in an iodine-free dye-sensitized solar cell (DSC) due to negligible recombination of photoinduced electrons with cationic species and rapid charge transport, which improved the photovoltaic performance (η = 2.1 → 6.5 %).
    ACS Nano 06/2014; · 12.03 Impact Factor
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    ABSTRACT: A composite separator membrane (CSM) with an A/B/A type layered structure, composed of a microporous electrolyte-philic poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) gel layer (A) and a submicrometer porous polyethylene (PE) or a macroporous poly(ethylene terephthalate) (PET) non-woven matrix (B), is introduced in a dye-sensitized solar cell (DSSC). Commercially available PE and PET separator membranes (SMs) act as matrices that provide mechanical stability to the DSSC and permanent pore structures for facilitated ion transport. PVdF-HFP is used as a microporous gelator for improved interfacial contact between the solid SM and the electrodes. The PVdF-HFP gel impedes the charge recombination process between electron and I3− at the TiO2/electrolyte interface, resulting in improved electron lifetimes. The DSSC assembled with the CSM exhibits high initial solar energy conversion efficiency (η, 6.1%) and stable η values over 1400 h, demonstrating good long term stability. The behaviors of the DSSC are attributed to the synergistic factors of the CSM, such as improved ion conductivity, electrolyte affinity, electrolyte retention capability, effective interfacial contact, and plausible passivation of the dyes. This study demonstrates a practical combination of short- and long-term DSSC performance through the introduction of the CSM.
    Advanced Energy Materials 04/2014; · 14.39 Impact Factor
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    ABSTRACT: A series of liquid junction dye-sensitized solar cells (DSCs) was fabricated based on polymer membrane-encapsulated dye-sensitized TiO2 nanoparticles, prepared using a surface-induced cross-linking polymerization reaction, to investigate the dependence of the solar cell performance on the encapsulating membrane layer thickness. The ion conductivity decreased as the membrane thickness increased; however, the long term-stability of the devices improved with increasing membrane thickness. Nanoparticles encapsulated in a thick membrane (ca. 37 nm), obtained using a 90 min polymerization time, exhibited excellent pore filling among TiO2 particles. This nanoparticle layer was used to fabricate a thin-layered, quasi-solid state DSC. The thick membrane prevented short-circuit paths from forming between the counter and the TiO2 electrode, thereby reducing the minimum necessary electrode separation distance. The quasi-solid state DSC yielded a high power conversion efficiency (7.6 → 8.1%) and excellent stability during heating at 65 °C over 30 days. These performance characteristics were superior to those obtained from a conventional DSC (7.5 → 3.5%) prepared using a TiO2 active layer with the same thickness. The reduced electrode separation distance shortened the charge transport pathways, which compensated for the reduced ion conductivity in the polymer gel electrolyte. Excellent pore filling on the TiO2 particles minimized the exposure of the dye to the liquid and reduced dye detachment.
    Energy & Environmental Science 04/2013; 6(5):1559-1564. · 11.65 Impact Factor
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    ABSTRACT: Nanoporous network polymer nanocomposites with tunable pore size for size-dependent selective ion transport are successfully prepared via the surface-induced cross-linking polymerization of methyl methacrylate (MMA) and 1,6-hexanediol diacrylate (HDDA) on the surfaces of nanocrystalline TiO2 particles. The morphologies of the porous network polymer layer and nanopores were investigated by transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and Brunauer–Emmett–Teller (BET) experiments. The porous layer size-selectively screened the ions that contacted the nanocrystalline TiO2 particles, as demonstrated by ion conductivity measurements, electrochemical impedance spectroscopy (EIS), and transient absorption spectroscopy (TAS).
    Advanced Energy Materials 02/2013; 3(2). · 14.39 Impact Factor
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    ABSTRACT: A simple selective etching process easily removed a 2nd anodized TiO(2) nanotubes (TNTs) layer from a physically stable 1st anodized TNTs layer to produce noncurling, freestanding, large-area aligned doubly open-ended TNTs. These TNTs were easily transferred to a conducting glass for use in fabricating front-illuminated dye sensitized solar cells.
    Chemical Communications 06/2012; 48(70):8748-50. · 6.38 Impact Factor
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    ABSTRACT: The interfacial properties were systematically investigated using an organic sensitizer (3-(5'-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2']bithiophenyl-5-yl)-2-cyano-acrylic acid (D)) and inorganic sensitizer (bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2'-bipyridine-4,4'-dicarboxylato) ruthenium(II) (N719)) in a liquid-state and a solid-state dye-sensitized solar cell (DSC). For liquid-DSCs, the faster charge recombination for the surface of D-sensitized TiO(2) resulted in shorter diffusion length (L(D)) of ∼3.9 μm than that of N719 (∼7.5 μm), limiting the solar cell performance at thicker films used in liquid-DSCs. On the other hand, for solid-DSCs using thin TiO(2) films (∼ 2 μm), D-sensitized device outperforms the N719-sensitized device in an identical fabrication condition, mainly due to less perfect wetting ability of solid hole conductor into the porous TiO(2) network, inducing the dye monolayer act as an insulation layer, while liquid electrolyte is able to fully wet the surface of TiO(2). Such insulation effect was attributed to the fact that the significant increase in recombination resistance (from 865 to 4,400 Ω/cm(2)) but shorter electron lifetime (from 10.8 to 0.8 ms) when compared to liquid-DSCs. Higher recombination resistance for solid-DSCs induced the electron transport-limited situation, showing poor performance of N719-sensitized device which has shorter electron transport time and similar L(D) (2.9 μm) with D-sensitized device (3.0 μm).
    ACS Applied Materials & Interfaces 06/2012; · 5.90 Impact Factor
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    Advanced Energy Materials 01/2012; 2:219-224. · 14.39 Impact Factor
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    ABSTRACT: 3,4,5-Tris(dodecyloxy)benzoic acid (DOBA) and the Z907 dye were coadsorbed to form a light-sensitizing monolayer in a solid-state dye-sensitized solar cell (sDSC). Coadsorption of DOBA which has three hydrocarbon chains permitted preparation of a denser monolayer of dyes and DOBA. This dense monolayer formed interlayer between TiO2 and Spiro-OMeTAD (hole conductor), effectively preventing charge recombination, while half of the photocurrent was dissipated via recombination reaction when Z907 solely anchored on the surface of TiO2. Moreover, the DOBA induced a lower population of density-of-state (DOS) in the surface of TiO2, shifting the position of the conduction band (CB) toward negative values. This resulted in higher open-circuit voltage (VOC) for the device made with Z907 and DOBA than that of the Z907-sensitized device. These surface properties were investigated using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent/photovoltage spectroscopy (IMPS and IMVS).
    RSC Advances 01/2012; 2:3467–3472. · 3.71 Impact Factor
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    ABSTRACT: A series of organic dyes having an unsymmetrical geometry, 3-(5′-{4-[(4-tert-butyl-phenyl)-(4-fluoro-phenyl)-amino]-phenyl}-[2,2′]bithio-phenyl-5-yl)-2-cyano-acrylic acid (D-F), 3-(5′-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2′] bithiophenyl-5-yl)-2-cyano-acrylic acid (D-CH33), and 3-(5′-{4-[(4-tert-butyl-phenyl)-(4-methoxy-phenyl)-amino]-phenyl}-[2,2′]bithiophenyl-5-yl)-2-cyano-acrylic acid (D-OCH3), were designed and synthesized for use in solid-state dye-sensitized solar cells (sDSCs). The dye regeneration energy levels and surface properties were characterized to determine the hole transfer yield from the oxidized dye to the hole conductor (spiro-OMeTAD) by measuring the degree of pore-filling by the spiro-OMeTAD and the transient absorption spectra (TAS). An electrode sensitized with D-OCH3 exhibited the highest spiro-OMeTAD filling fraction and hole transfer quantum yield (Φ) to spiro-OMeTAD, resulting in an enhanced photocurrent and a power conversion efficiency of 3.56% in the sDSC, despite a lower energy driving force for hole transfer compared to those of D-F, or D-CH33. This result illustrates the importance of the chemical compatibility between the hole conductor and the dye on the surface of TiO2.
    Journal of Materials Chemistry 01/2012; 22:8641–8648. · 5.97 Impact Factor
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    ABSTRACT: TiO(2) electrodes, sensitized with the N719 dye at high immersion temperatures during the sensitization process, were found to have large fractions of weakly bound N719 on the electrode surface, which resulted in dye aggregation and decreased device longevity. These disadvantages were ameliorated using a low-temperature stearic acid (SA)-assisted anchoring method described here. The activation energy (ΔE(NS)(++)) and relative fraction of strongly bound N719 were twice as large as the respective values obtained without the use of SA. Slowing of adsorption, both by thermal means and through SA-mediated processes, effectively controlled the binding mode of N719 on the surface of TiO(2). The resulting sensitized electrodes displayed enhanced device longevity and improved generation of photoinduced electrons.
    Langmuir 12/2011; 27(23):14647-53. · 4.38 Impact Factor
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    ABSTRACT: A novel bis-EDOT-based monomer with ethylene glycol oligomers was synthesized and shown to exhibit strong ion solvation, good transport properties, and effective charge screening. These properties greatly improved J(SC) (4.2 to 7.0 mA cm(-2)) and η (1.6 to 2.9%) in an iodine-free solid state dye-sensitized solar cell (DSSC) employing a Z-907 sensitizer, compared with the corresponding values of DSSCs fabricated using the standard bis-EDOT.
    Chemical Communications 08/2011; 47(37):10395-7. · 6.38 Impact Factor
  • Jongchul Lim, Young Soo Kwon, Taiho Park
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    ABSTRACT: Stearic acid as a coadsorbent, which has a low dipole moment and high solubility, retarded the rate of dye adsorption during the competitive anchoring process on the TiO(2) layer in dye-sensitized solar cells (DSCs), thereby increasing the content of strongly bound dye on the TiO(2) surface. This resulted in an approximately 25% improvement in both J(SC) and the power conversion efficiency of the DSCs, even for much lower dye coverage.
    Chemical Communications 03/2011; 47(14):4147-9. · 6.38 Impact Factor
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    ABSTRACT: A series of carbazole-based D-π-A copolymers were synthesized to investigate the influences of conjugation length and structural distortion on intramolecular charge transfer (CT) complexation between the donor (D) and acceptor (A) components. Carbazole presents two possible linkage sites, the 2,7- and 3,6-positions, which led to significant differences in the thermal, photophysical, electrochemical, and electrical properties of the copolymers due to the positioning of the electron-rich nitrogen atom with respect to the copolymer backbone. The copolymers were comprehensively characterized using TGA, DSC, UV−vis, and photoluminescence spectroscopy, cyclic voltammetry, and DFT calculations. P(3,6C-DTBT), which was linked by a thienyl-2′,1′,3′-benzothiadiazole (DTBT) group at the 3,6-positions of the carbazoles so as to directly involve the electron-rich nitrogen atoms in conjugation, exhibited conjugation breaks in the middle of the carbazole units. The breaks resulted in a robust coplanar structure with an extraordinarily low oxidation potential and the ability to stably generate excitons, in contrast with P(2,7C-DTBT), which was linked by DTBT at the 2,7-positions of the carbazole. Two additional hexyl substituents at the 4-position of the thiophene in the DTBT groups of P(2,7C-HDTBT) and P(3,6C-HDTBT), which were identical to P(2,7C-DTBT) and P(3,6C-DTBT), respectively, except for the presence of the substituents, introduced steric hindrance between the D and A units, thereby breaking the coplanarity. Finally, the hole mobilities of the 3,6-carbazole-based copolymers were 1 order of magnitude higher than those of 2,7-carbazole-based copolymers, measured in hole-only devices. This result indicated the presence of stable radical cations and dications at the nitrogen atoms of the copolymers. This work deepens our understanding of carbazole-based D-π-A copolymers and provides insight into the design of novel materials for optoelectronic devices.
    Macromolecules 01/2011; 44(7):1909–1919. · 5.93 Impact Factor
  • Langmuir 01/2011; 27:14647–14653. · 4.38 Impact Factor
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    ABSTRACT: Size-tunable mesoporous spherical TiO2 (MSTiO2) with a surface area of 110 m2 g−1 have been prepared through combination of “dilute mixing”-driven hydrolysis of titanium(iv) tetraethoxide and solvothermal treatment. The hierarchically structured MSTiO2 are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MSTiO2 (587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO2 film, bi-layered dye-sensitized solar cells (DSCs) have been fabricated. Since the MSTiO2 particles are comprised of 10 nm nanocrystallites that cluster together to form large secondary spheres, they can function as light scatterers without sacrificing the surface area for dye-uptake. As a result, the present MSTiO2-based cells perform a noticeable improvement in the overall efficiency: maximum 9.37% versus 6.80% for the reference cell made of a TiO2 nanocrystalline film. This extraordinary result is attributed to the dual effects of enhanced dye loading and light scattering.
    Journal of Materials Chemistry 01/2011; 21:9582-9586. · 5.97 Impact Factor