Sang Il Seok

CdS or CdSe decorated TiO2 nanotube arrays from spray pyrolysis deposition: use in photoelectrochemical cells

In the present work we report on an investigation of TiO(2) nanotube arrays sensitized by low band-gap materials, such as CdS and CdSe, prepared by the one-step spray pyrolysis deposition method, which is very suitable for mass production, and their performance in photoelectrochemical cells.

All solid state multiply layered PbS colloidal quantum-dot-sensitized photovoltaic cells

The combination of solution processable colloidal quantum dots (CQDs) and organic conjugated polymers has attracted growing interest in the past few years, because devices fabricated using this combination can have potentially low cost and high efficiency. In this paper, we propose a new device architecture of multiply layered PbS CQD-sensitized photovoltaic cells. The multiple layers of PbS CQDs were formed by the repeated spin-coating of oleic-acid-capped PbS CQD solution and 1 wt% 1,2-ethanedithiol (EDT) solution onto mesoporous (mp)-TiO2 as photoanode. Through the structural advantage of a sensitized type device and post-treatment in 10 wt% EDT solution for 16 h, we could fabricate efficient photovoltaic cells having an overall energy conversion efficiency of 2.9% under 1 sun illumination, and an external quantum efficiency of over 15% at zero bias with a signal attenuation to frequency of 10 kHz levels in the near-infrared (NIR) range.

Performance improvement of Sb2S3-sensitized solar cell by introducing hole buffer layer in cobalt complex electrolyte

We found that the insertion of the poly-3-hexylthiophene (P3HT) layer into the pristine device results in great improvement of the efficiency, from 2.0% to 3.3% at 1 sun illumination. The improvement was mainly attributed to the improved fill factor, which for the Sb2S3 sensitized solar cell was seriously degraded from 56.0% (at 0.1 sun) to 31.5% (at 1 sun), whereas the fill factor of the Sb2S3/P3HT sensitized solar cell was only moderately degraded, from 59.3% (at 0.1 sun) to 40.4%. The role of P3HT as a hole buffer layer was analyzed by electrochemical impedance spectroscopy (EIS).

Hole-conducting mediator for stable Sb2S3-sensitized photoelectrochemical solar cells

Iodide n redox (3I−/I3−), polysulfide redox (S2−/Sx2−), organic redox [TMTU/TMFDS2+: tetramethylthiourea/tetramethylformaminium-bis(trifluoromethanesulfonyl)imide], ferrocene redox (Fc/Fc+), nickel redox [Ni(II)/Ni(III)], and cobalt redox [Co(II)/Co(III)] hole conducting electrolytes were systematically investigated to determine their suitability for use in Sb2S3-sensitized photoelectrochemical solar cells. A long-term stability test and UV-visible spectral analyses revealed that Sb2S3-sensitized photoelectrochemical solar cells worked stably with Co(II)(o-phen)3(TFSI)2/Co(III)(o-phen)3(TFSI)3 [TFSI: bis(trifluoromethanesulfonyl)imide] as the redox mediator.

Peptide-templating dye-sensitized solar cells

A hollow TiO(2) nanoribbon network electrode for dye-sensitized solar cells (DSSC) was fabricated by a biotemplating process combining peptide self-assembly and atomic layer deposition (ALD). An aromatic peptide of diphenylalanine was assembled into a three-dimensional network consisting of highly entangled nanoribbons. A thin TiO(2) layer was deposited at the surface of the peptide template via the ALD process. After the pyrolysis of the peptide template, a highly entangled nanotubular TiO(2) framework was successfully prepared. Evolution of the crystal phase and crystallite size of the TiO(2) nanostructure was exploited by controlling the calcination temperature. Finally, the hollow TiO(2) nanoribbon network electrode was integrated into DSSC devices and their photochemical performances were investigated. Hollow TiO(2) nanoribbon-based DSSCs exhibited a power conversion efficiency of 3.8%, which is comparable to the conventional TiO(2) nanoparticle-based DSSCs (3.5%). Our approach offers a novel pathway for DSSCs consisting of TiO(2) electrodes via biotemplating.

Near-infrared responsive PbS-sensitized photovoltaic photodetectors fabricated by the spin-assisted successive ionic layer adsorption and reaction method

A PbS-sensitized photovoltaic photodetector responsive to near-infrared (NIR) light was fabricated by depositing monolayered PbS nanoparticles on a mesoporous TiO(2) (mp-TiO(2)) film via the spin-assisted successive ionic layer adsorption and reaction (SILAR) method. By adjusting the size and morphology of the PbS nanoparticles through repeated spin-assisted SILAR cycles, the PbS-sensitized photovoltaic photodetector achieved an external quantum efficiency of 9.3% at 1140 nm wavelength and could process signals up to 1 kHz.

ZnSe colloidal nanoparticles synthesized by solvothermal method in the presence of ZrCl4.

The nanocrystalline ZnSe was synthesized from precursors of zinc acetate and Se powder in a high boiling trioctylphosphine (TOP), oleic acid, and ZrCl(4) by solvothermal method. The produced ZnSe nanoparticles showed gradual absorption edge shifting towards blue wavelength region as well as transformation of crystal phase from cubic to hexagonal with increasing ZrCl(4) concentrations in precursor solutions. The particle size calculated from XRD measurements for ZnSe nanoparticles was decreased with increasing ZrCl(4) concentrations for a reaction time of 240 min, from 6.5 nm to 5.1 nm whereas from TEM measurements it was 7.0 to 6.1 nm at 0.0 and 22.5 mol% of ZrCl(4), respectively. No trace of zirconium was found in solid ZnSe nanoparticles by SEM-EDS analysis but the atomic percentage of Se with respective to Zn was decreased with increasing ZrCl(4). The absorption edge blue shifting was explained on the basis of decreased particle size. The crystal phase transformation may be due to the combined effect of small internal energy difference between the two phases and the decrease of crystallite size.

Precursor-driven selective synthesis of hexagonal chalcocite (Cu2S) nanocrystals: structural, optical, electrical and photocatalytic properties

The reaction of CuCl2·2H2O with the methyl ester of 3,5-dimethyl pyrazole-1-dithioic acid (mdpa) yields a blackish brown complex of composition, [Cu(mdpa)2][CuCl2]. The complex formed a well-defined crystal and is characterized by single-crystal X-ray diffraction, thermogravimetry (TG), and spectroscopic studies. The molecule possesses a distorted tetrahedral configuration with a CuN2S2 chromophore with +1 oxidation state of copper. The TG study reveals that the molecule is a suitable precursor for copper sulfide nanoparticles. The low temperature thermal decomposition of the single-source precursor produces hexagonal chalcocite (Cu2S) nanostructures in ethylene diamine and ethylene glycol. A selective synthesis of copper-rich high chalcocite was obtained using the new precursor. The size and morphology of the synthesized Cu2S nanoparticles are guided by the precursor and likely to be less dependent on the solvent used in the experiment. The nanoparticles were characterized by X-ray diffraction, scanning electronic microscope, and UV-Vis spectroscopic studies. The optical band gap of the as-synthesized Cu2S nanoparticles is measured to be 1.80–2.40 eV. The Cu2S nanoparticles are found to be good catalysts in UV photo catalytic decomposition (90%) of Congo red (CR) dye following first-order reaction kinetics, and the reusability study of the Cu2S catalyst also shows excellent catalytic performance (80%).

Colloidal TiO2 nanocrystals prepared from peroxotitanium complex solutions: Phase evolution from different precursors

We report the preparation of nanocrystalline anatase and rutile TiO(2) from aqueous peroxotitanium complex (PTC) solutions and their characterization by powder X-ray diffraction (XRD), infrared spectroscopy, and Raman spectroscopy. The phase evolution of TiO(2) prepared using PTC derived from different precursors, i.e., TiCl(4) and titanium tetraisopropoxide (TTIP), is related to the nature of the intermediate steps. Phase-pure nanoanatase was formed in PTC solution derived from TiCl(4), while a mixture of minor anatase and dominant rutile were prepared from PTC when TTIP was used as precursor. On the other hand, in the case of calcining PTC powders in air, a pure anatase phase of TiO(2) was obtained, regardless of the precursor used. Thus, the formation and attachment of hydrated TiO(6) units or TiO(2)·xH(2)O under a different environment, especially pH, plays a critical role in determining the phase during the crystallization of TiO(2).

Synthesis of uniform PS-b-P2VP nanoparticles via reprecipitation and their use as sacrificial templates for inorganic hollow nanoparticles

Uniform PS-b-P2VP (polystyrene-block-poly-2-vinylpyridine) block copolymer nanoparticles were successfully synthesized using a simple reprecipitation method. The PS-b-P2VP nanoparticles could serve as sacrificial templates to generate silica and titania hollow nanoparticles of ∼100 nm size.