Jin-Hyeok Kim

Efficient electrochromic performance of nanoparticulate WO3 thin films

This report highlights the suitability of electrodeposited nanoparticulate-WO3 (NP-WO3) electrodes for transmissive electrochromic devices (ECDs). The WO3 electrodes in the form of thin films are composed of 10–20 nm nanoparticles. An electrochromic (EC) device of dimensions 5 × 4 cm2 fabricated using NP-WO3 showed an Li insertion coefficient (x) of 0.43, which resulted in highest photopic transmittance modulation (88.51%), better Li-ion diffusion coefficient (∼3.16 × 10−9 cm2 s−1), fast electrochromic response time (5.2 s for coloration and 3.7 for bleaching) and excellent coloration efficiency (∼137 cm2 C−1). On reduction of WO3, the CIELAB 1931 2° color space coordinates show the transition from colorless to the deep blue state (Y = 97, a* = −1.93, b* = 0.46 and Y = 10, a* = 1.57, b* = −41.01) with steady decrease in relative luminance.

Electrochromic properties of dandelion flower like nickel oxide thin films

This report highlights a one pot, surfactantless, template free approach to grow novel dandelion flower like nickel oxide (NiO) thin films composed of nano-flakes with a highly porous structure. Good transmittance modulation, fast response time and excellent coloration efficiency make them a potential electrode material for electrochromic devices.

Single-step synthesis of 3D nanostructured TiO2 as a scattering layer for vertically aligned 1D nanorod photoanodes and their dye-sensitized solar cell properties

In the present investigation we have successfully synthesized 1D vertically aligned rutile TiO2 nanorods (TNR) and 3D TiO2 nanostars (TNS) as a scattering layer by a single step hydrothermal route. The synthesized nanostructures were characterized by X-ray diffraction, scanning-and transmission electron microscopy and X-ray photoelectron spectroscopy. The 1D TiO2 nanorod and 3D TiO2 nanostar samples were further used for N-719 dye sensitized solar cells (DSSC) application. Compared to a nanorod based cell, the photovoltaic performance of the nanostars/nanorods TiO2 cell exhibits excellent DSSC performance, including superior light scattering, rapid electron transport and lower electron recombination rate. The 3D/1D TNS/TNR based DSSC cell exhibits 5.39% power conversion efficiency, which is remarkably higher than that of the bare 1D nanorod based (3.74%) photoelectrode. The detailed interface and transient properties of these nanorod and nanostar based photoanodes in DSSCs were analyzed by electrochemical impedance spectroscopy measurements and open circuit voltage decay measurements in order to understand the critical factors contributing to such high power conversion efficiency. The enhancement of the efficiency for the 3D/1D TNS/TNR photoanode based cell compared to the 1D TNR is mainly attributed to better light scattering capability, faster electron transport and lower electron recombination.

Room temperature deposition of nanostructured Bi2Se3 thin films for photoelectrochemical application: effect of chelating agents

Nanostructured bismuth selenide (Bi2Se3) thin films have been deposited by chemical bath deposition method at room temperature using three different chelating agents, trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid. The structural, morphological, optical and photoelectrochemical properties of Bi2Se3 thin films have been investigated as a function of different chelating agents. X-Ray diffraction studies revealed that the films were nanocrystalline in nature with a rhombohedral crystal structure. Trisodium citrate chelate resulted in Bi2Se3 thin films with poor crystallinity; further improvement in the crystallinity of the films was observed with triethanolamine and ethylenedimminetetraacetic acid chelates. From scanning electron microscopy, a uniform sphere-like morphology having an average sphere diameter of 90 nm was observed with trisodium citrate chelate. In the case of triethanolamine, a fibrous morphology with an average fiber thickness of 60 nm was observed, whereas for ethylenedimminetetraacetic acid chelate, a vertically arrayed petal-like morphology having petal thickness of 50–70 nm was observed. The UV-Vis absorption studies revealed that the band gap energy of the Bi2Se3 thin films with trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid chelates was 1.55, 1.48 and 1.40 eV, respectively. The maximum short circuit current densities (Jsc) of 0.158, 0.214 and 0.284 mA cm−2 and the corresponding open circuit voltages (Voc) of 196, 206 and 217 mV were obtained with trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid chelates, respectively. The Bi2Se3 thin films deposited using triethanolamine and ethylenedimminetetraacetic acid chelates show better photoelectrochemical performance as compared with trisodium citrate chelate.

Effect of copper content on optostructural, morphological and photoelectrochemical properties of MoBi2−xCuxSe4 thin films

In the present investigation we have reported a facile chemical route for the deposition of MoBi2−xCuxSe4 (xxa0=xa00.0, 0.2, 0.4, 0.6, 0.8 and 1.0) thin films at room temperature by using a simple and self-organised arrested precipitation technique. The deposited samples were characterised for their structural, morphological, optical and photoelectrochemical properties. X-ray diffraction patterns revealed that, undoped MoBi2Se5 shows a rhombohedral crystal structure, while mixed rhombohedral and orthorhombic crystal structures were observed with shifting of diffraction peaks after copper doping. The scanning electron microscopy and transmission electron microscopy images revealed that the surface morphology was improved with copper content. Compositional analysis of all samples was carried out by using energy dispersive X-ray spectroscopy. The direct band gap energy of all the samples estimated from absorbance spectra varies from 1.26 to 1.60xa0eV. The photoelectrochemical properties of all samples were studied in I−/I3− redox electrolyte which demonstrated that the electrical conductivity was transformed from n-type to p-type after copper doping and photoelectrochemical response of p-type MoBi2−xCuxSe4 thin film electrode was improved with increasing copper content. The mechanism of change in the type of electrical conductivity and augmentation in photoelectrochemical response after copper doping are discussed.

Opto-structural and electrical properties of chemically grown Ga doped MoBi2Se5 thin films

An arrested precipitation route was developed to obtain gallium doped MoBi2Se5 thin films on substrate support. High purity organometallic complexes of Mo–triethanolamine (Mo–TEA), Bi–triethanolamine (Bi–TEA), Ga–triethanolamine (Ga–TEA) allow to react with sodium selenosulphite (Na2SeSO3) in the presence of sodium dithionite (Na2S2O4) as a reducing agent in an aqueous alkaline reaction bath. As deposited thin films were characterized by X-ray diffraction, which reveals that material is nanocrystalline with mixed phases of rhombohedral (Bi2Se3)-hexagonal (MoSe2)-hexagonal (GaSe) structures. Scanning electron micrographs show the grain of granular morphology decreases with increase in Ga concentration. Energy dispersive x-ray analysis shows presence of Mo, Bi, Ga and Se elements in stoichiometric ratio confirms the chemical formula MoBiGaSe5. Optical absorbance of the films show direct allowed transition in visible region having band gap energy in the range of 1.30–1.47xa0eV. Thermoelectric power and electrical conductivity measurements have been carried out for thin film samples in the temperature range 300–500xa0K and results revealed that n-type semiconducting behavior. It is interesting to note that Ga doping in MoBi2Se5 changes n to p type conductivity.

Epitaxial growth of (001)-oriented and (116)-oriented SrBi2Ta2O9 thin films by chemical solution deposition method

Abstract Epitaxial (0xa00xa01)-oriented and (1xa01xa06)-oriented SrBi 2 Ta 2 O 9 (SBT) thin films have been prepared by the chemical solution deposition method on (0xa00xa01)- and (0xa01xa01)-SrTiO 3 (STO) substrates, respectively. Crystal orientation and microstructure were characterized using four-circle X-ray diffractometer (XRD) and transmission electron microscopy (TEM). Only (0xa00xa0 l ) peaks in the XRD θ –2 θ scan and four sharp peaks for SBT {1xa01xa05} in the XRD pole-figure scan were observed in the SBT/(0xa00xa01) STO specimen annealed at 850°C/h. In the SBT/(0xa01xa01) STO specimen annealed at 850°C/h, two {0xa00xa08} peaks for SBT determined with the XRD pole-figure scan were consistent with the epitaxial orientation relationship of [ 1 1 0](1 1 6) SBT //[1 0 0](0 1 1) STO found by TEM selected area diffraction. The difference in the epitaxy nature was explained by using a coincident site lattice model. These results show the possibility to control the orientation, normal to a substrate surface, of a SBT thin film by choosing a proper substrate.

Laser Microfabrication of Multidirectional Side-fire Optical Fiber Tip

Currently, various optical fiber tips are used to deliver laser beam for endoscopic surgery. In this paper, we demonstrated multidirectional (forward and side) firing optical fiber tip using a femtosecond micromachining and laser polishing technology. We controlled the edge width of optical fiber tip, by modulating the condition of laser, to regulate the amount of side and forward emission. The distal end of the optical fiber with core/clad diameter of was microstructured with cone shape by using a femtosecond laser. And then the microstructured optical fiber tip was polished by laser beam result in smoothing and specular reflection at the surface of the cone structure. Finally, we operated the LightTools simulation and good agreement was generally found between the proposed model and experimental simulation.

Quantification of effective thermal conductivity in the annealing process of Cu2ZnSn(S,Se)4 solar cells with 9.7% efficiency fabricated by magnetron sputtering

Cu2ZnSn(S,Se)4 (CZTSSe) has attracted much attention as the absorption layer of photovoltaic devices because of its appropriate bandgap energy and low cost. However, CZTSSe solar cells have so far shown low power conversion efficiency compared to other solar cells and studies are being conducted to improve it. In this study, we achieved an improvement in the shunt resistance and power conversion efficiency (PCE) of CZTSSe solar cells with quantification of the effective thermal conductivity. A CZTSSe solar cell fabricated using a graphite box designed with high thermal conductivity exhibited a high shunt resistance of 413 Ω cm2 and a fill factor of 63.8%, and additionally, secondary phases such as Cu2S/Se and CuS/Se were not detected. On the other hand, a CZTSSe solar cell fabricated using a graphite box with low thermal conductivity showed a rougher surface and low shunt resistance mainly related to secondary phase formation. As a result, a PCE of 9.72% was achieved using a graphite box designed with high thermal conductivity.

The study of light waveguide effects on ZnO nanorod arrays

Two different types of ZnO nanorods (hexagonal pyramid-shaped nanorod and hexagonal prism-shaped nanorod) were integrated on light emitting diodes (LEDs) for the investigation of light waveguide effects, which were evaluated using electroluminescence (EL) and current-voltage (I–V) characteristics. EL intensity in LEDs with hexagonal prism-shaped ZnO nanorod arrays were improved by 18% while EL intensity in LEDs with hexagonal pyramid-shaped ZnO nanorods were reduced by 25% compared to bare LEDs. 3D-finite dimension time domain (3D-FDTD) programs were used to simulate the light waveguide effect on the two different shapes of ZnO nanorods. In addition, photonic crystals (PCs) effects in ZnO nanorod arrays were simulated to verify the light waveguide effect. It was found that light in a hexagonal prism-shaped ZnO nanorod propagated from the bottom to the top end. However, the light in a hexagonal pyramid-shaped ZnO nanorod was dissipated on the lateral face. The light extraction efficiency in ZnO nanorod arrays was dominated by the top end shape and planar density of ZnO nanorods.