Kwang-Soon Ahn

All-solid-state electrochromic device composed of WO3 and Ni(OH)2 with a Ta2O5 protective layer

An all-solid-state electrochromic device composed of WO3 and Ni(OH)2 with a Ta2O5 protective layer was prepared by rf magnetron sputtering and lamination with a proton-conducting solid polymer electrolyte. This device had good durability, high transmittance modulation (18%–74%) and coloration efficiency (about 84 cm2 C−1), and good response times (8.5 and 18 s, respectively, during the bleaching and coloring processes). This indicates that Ta2O5 layers are electrochemically stable and can be used as protective layer for Ni(OH)2 as well as WO3.

Pt–WOx electrode structure for thin-film fuel cells

An electrode structure consisting of two phases of Pt and WOx for use in thin-film fuel cells was designed and fabricated using a cosputtering system with a Pt metal and a tungsten oxide target. The coexistence of a polycrystalline Pt nanosized phase and an amorphous tungsten oxide phase in the electrode layer was confirmed by transmission electron microscopic images and x-ray diffraction data. In addition, compared with a Pt thin-film electrode, the two-phase electrode of Pt and WOx showed excellent performance for the devices because of the improved activity of the Pt metallic phase and the spill-over effect of porous tungsten oxides.

Reactively sputtered nickel nitride as electrocatalytic counter electrode for dye- and quantum dot-sensitized solar cells

Nickel nitride electrodes were prepared by reactive sputtering of nickel under a N2 atmosphere at room temperature for application in mesoscopic dye- or quantum dot- sensitized solar cells. This facile and reliable method led to the formation of a Ni2N film with a cauliflower-like nanostructure and tetrahedral crystal lattice. The prepared nickel nitride electrodes exhibited an excellent chemical stability toward both iodide and polysulfide redox electrolytes. Compared to conventional Pt electrodes, the nickel nitride electrodes showed an inferior electrocatalytic activity for the iodide redox electrolyte; however, it displayed a considerably superior electrocatalytic activity for the polysulfide redox electrolyte. As a result, compared to dye-sensitized solar cells (DSCs), with a conversion efficiency (η) = 7.62%, and CdSe-based quantum dot-sensitized solar cells (QDSCs, η = 2.01%) employing Pt counter electrodes (CEs), the nickel nitride CEs exhibited a lower conversion efficiency (η = 3.75%) when applied to DSCs, but an enhanced conversion efficiency (η = 2.80%) when applied to CdSe-based QDSCs.

Bleached state transmittance in charge-unbalanced all-solid-state electrochromic devices

The bleached state transmittance of a charge-unbalanced, complementary electrochromic (EC) device may show residual coloration due to the presence of residual charges. In this study, EC devices were fabricated with configurations G/ITO/Ni(OH)2/Ta2O5/H+–SPE/Ta2O5/WO3/ITO/G and G/ITO/NiOOH/Ta2O5/H+–SPE/Ta2O5/HWO3/ITO/G (G=glass, H+–SPE=proton-conducting solid polymer electrolytes, and ITO=indium tin oxide). These devices, referred to as EC1 and EC2, were initially fabricated from fully bleached EC layers and from fully colored EC layers, respectively. The change in electrochromic properties as a function of charge capacity ratio (R) for each device was then compared. In comparison to EC2 devices, EC1 devices provided better bleached-state transmittances and higher coloration efficiencies over a wider range of R, and were less sensitive to changes in R value. This may arise because the absorbance caused by the residual charges in the colored state is greater and more sensitive to the charge capacity ratio than that in the bleached state.The bleached state transmittance of a charge-unbalanced, complementary electrochromic (EC) device may show residual coloration due to the presence of residual charges. In this study, EC devices were fabricated with configurations G/ITO/Ni(OH)2/Ta2O5/H+–SPE/Ta2O5/WO3/ITO/G and G/ITO/NiOOH/Ta2O5/H+–SPE/Ta2O5/HWO3/ITO/G (G=glass, H+–SPE=proton-conducting solid polymer electrolytes, and ITO=indium tin oxide). These devices, referred to as EC1 and EC2, were initially fabricated from fully bleached EC layers and from fully colored EC layers, respectively. The change in electrochromic properties as a function of charge capacity ratio (R) for each device was then compared. In comparison to EC2 devices, EC1 devices provided better bleached-state transmittances and higher coloration efficiencies over a wider range of R, and were less sensitive to changes in R value. This may arise because the absorbance caused by the residual charges in the colored state is greater and more sensitive to the charge capacity ratio th...

Joint Effects of Photoactive TiO2 and Fluoride-Doping on SnO2 Inverse Opal Nanoarchitecture for Solar Water Splitting.

Inverse opal (IO) films of tin dioxide (SnO2) were fabricated on polystyrene (PS) beads (diameter=350 nm (±20 nm) with a spin coating method. To compensate for the large band gap (Eg=3.8 eV), a thin TiO2 shell was deposited on the SnO2-IO films with atomic layer deposition (ALD), which produced shells with thicknesses of 10-40 nm. The morphological changes and crystalline properties of the SnO2 and TiO2-coated SnO2 (herein after referred to as TiO2/SnO2) IO films were investigated with field-emission scanning electron microscopy and X-ray diffraction, respectively. The photoelectrochemical (PEC) behavior of the samples was tested in a 0.1 M KOH solution under 1 sun illumination (100 mW/cm2 with an AM 1.5 filter). The highest PEC performance was obtained with the TiO2(10 nm)/SnO2 IO films, which produced a photocurrent density (Jsc) of 4.67 mA/cm2 at 0.5 V (vs NHE) and was sequentially followed by the TiO2(20 nm)/SnO2-IO, TiO2(30 nm)/SnO2-IO, TiO2 (40 nm)/SnO2-IO and SnO2 IO films. Overall, the thin TiO2 shell covered on the SnO2-IO core enhanced Jsc by 3 orders of magnitude, which in turn the PEC activity. This is mainly ascribed to the extremely low charge-transfer resistance (Rct) in the photoelectrode/electrolyte and at the TiO2/SnO2 interface, as well as the contribution of the photoactive TiO2 layer, which has an Eg of 3.2 eV. Moreover, to improve the electrical conductivity of the core SnO2 IO film, the films were doped with 10 mol % of F. The F- doped films were labeled as the FTO IO film. The Rct of the FTO-IO films decreased because of the improved electronic conductivity, enhancing the PEC performance of the TiO2(10 nm)/FTO-IO films by approximately 20%. The core-shell nanowire mesh nanoarchitecture is therefore suggested to provide an insight for designing the peculiar structure based on the materials properties and the engineering of their band gap energy for highly efficient PEC performance.

PtRu–WO3 nanostructured alloy electrode for use in thin-film fuel cells

A PtRu–WO3 nanostructured alloy electrode consisting of alloy nanophases and amorphous tungsten oxide for use in thin-film fuel cells was designed and fabricated using a multigun sputtering system with Pt and Ru metal and a tungsten oxide target. Alloy formation and the presence of nanophases in an amorphous tungsten oxide phase was confirmed by x-ray diffraction and transmission electron microscopy, respectively. The nanostructured alloy electrode, PtRu–WO3 showed the best performance in methanol electro-oxidation because of the presence of the alloy nanophases when compared to a Pt and PtRu thin-film electrode.

Thickness-dependent microstructural and electrochromic properties of sputter-deposited Ni oxide films

Thickness-dependent microstructural and electrochromic properties of sputter-deposited Ni oxide films were investigated as a function of growth time using x-ray diffraction, in situ transmittance measurements, and x-ray photoelectron spectroscopy. By increasing the thickness of the Ni oxide, the transmittance or optical differences during coloring/bleaching process were increased. However, thick Ni oxides showed a lower maximum bleached transmittance and coloration efficiency and a larger response time. Crystallinity developed with growth time, mainly due to the plasma heating effect. The evolution of crystallinity with the growth time resulted in the electrochromic inactive Ni oxide components even after several tens of potential cyclings, leading to a decreased maximum bleached transmittance and lower coloration efficiency.

Microstructural and electrochromic properties of sputter-deposited Ni oxide films grown at different working pressures

The electrochromic properties of Ni oxide films grown at different working pressures (2.7, 5, 15, and 20 mTorr) by rf sputtering were investigated by means of in situ transmittance measurement and the resulting data were related to the crystallographic structure, surface morphology, and film density. At working pressures of over 5 mTorr, the sputter-deposited Ni oxide films crystallized gradually due to the plasma heating effect at a low growth rate. Although the Ni oxide film grown at 2.7 mTorr had the same amorphous crystallographic structure as the film grown at 5 mTorr, the former had a considerably more inhomogeneous surface and a much lower film density due to the rapid growth rate, and resulted in a defect-rich Ni oxide film. The electrochromic properties, such as the transient cycling period, coloration efficiency, and coloring/bleaching response times, were best for the sample grown at 5 mTorr and they are discussed in terms of defect-rich and crystalline Ni oxide films.

The Effect of RF Power on the Electrochromic Response Time of Sputter-Deposited Ni Oxide Films

The effect of RF power on the electrochromic properties and response time of Ni oxide films grown by RF magnetron sputtering were investigated by in situ transmittance measurements with continuous potential cycling and a pulse potential cycling test. Although the coloration efficiency and crystallographic structure were independent of RF power, transmittance difference, transient cycling period, charge density, and response time during coloring/bleaching process were significantly affected. This can be attributed to the presence of electrochromic inactive components, produced as the result of the fast growth rate, high RF power, and low film density.

Electrochromic properties of SnO2-incorporated Ni oxide films grown using a cosputtering system

SnO2-incorporated Ni oxide (NiO:SnO2) films were grown by means of a cosputtering system, consisting of two rf sputter guns, and their electrochromic properties were compared with those of a Ni oxide film. The Ni oxide films crystallized with an increased film thickness due to a plasma heating effect, leading to a decreased maximum bleached transmittance and coloration efficiency (CE). However, the NiO:SnO2 films grown by cosputtering showed acceptable maximum bleached transmittance and CE values, which were independent of film thickness. This indicates that SnO2 adatoms generated by the side sputter gun interfere with the movement of Ni oxide adatoms deposited by the main sputter gun, preventing the crystallization of the films. This was verified by x-ray diffraction and scanning electron microscopic data. We propose that the cosputtering technique described herein has the capability of providing good maximum bleached transmittance and CE properties in thick electrochromic films with no degradation due t...