Jong Kwan Koh

Direct Assembly of Preformed Nanoparticles and Graft Copolymer for the Fabrication of Micrometer‐thick, Organized TiO2 Films: High Efficiency Solid‐state Dye‐sensitized Solar Cells

Solid-state dye-sensitized solar cell with 7.1% efficiency at 100 mW/cm(2) is reported, one of the highest observed for N719 dye. Excellent performance was achieved via a graft copolymer-templated, organized mesoporous TiO(2) film with a large surface area using spindle-shaped, preformed TiO(2) nanoparticles and solid polymer electrolyte.

Facile synthesis of size-tunable mesoporous anatase TiO2 beads using a graft copolymer for quasi-solid and all-solid dye-sensitized solar cells

Multi-functional mesoporous TiO2 (M-TiO2) beads with high porosity and good interconnectivity in the anatase phase were synthesized via a solvothermal reaction at low temperature (100 °C) using a graft copolymer, i.e., poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM), as a structure-directing agent. Field-emission scanning electron microscopy (FE-SEM), energy-filtering transmission electron microscopy (EF-TEM) and X-ray diffraction (XRD) revealed that the TiO2 beads consisted of 13 nm interconnected nanocrystallites and were monodisperse with tunable sizes of approximately 120, 250, 500 and 750 nm. The photoelectrodes fabricated with M-TiO2 beads showed a high surface area (86.5 m2 g−1) and a stronger light scattering effect, as confirmed by Brunauer–Emmett–Teller (BET) and incident photon-to-electron conversion efficiency (IPCE) measurements. The structures of M-TiO2 beads effectively offered better pore infiltration of the polymer electrolyte. Furthermore, the improved interconnectivity of M-TiO2 beads improved the electron diffusion coefficient and electron lifetime, resulting in an improvement in the light harvesting efficiency. Thus, quasi-solid-state polymer electrolyte dye-sensitized solar cells (DSSCs) with M-TiO2 beads showed a higher efficiency (4.8% at 100 mW cm−2) than those with conventional P25 (3.8%). A structure–property relation among M-TiO2 beads was investigated in terms of surface area and light scattering. Upon utilizing double layer structures and a solid polymerized ionic liquid (PIL), the efficiency was increased up to 6.7% at 100 mW cm−2, one of the highest values for all-solid-state DSSCs.

Graft copolymer directed synthesis of micron-thick organized mesoporous TiO2 films for solid-state dye-sensitized solar cells

Micron thick, well-organized mesoporous TiO(2) films with high porosity and good connectivity were synthesized by templating an amphiphilic graft copolymer for solid-state dye-sensitized solar cells.

A facile preparation method of surface patterned polymer electrolyte membranes for fuel cell applications

We report a facile patterning method that facilitates production of large-area platforms with well-arrayed micro/nanopatterns of a polymer electrolyte membrane (PEM) at low cost using an elastomeric mold at room temperature without hot-pressing. Membrane–electrode interfacial properties on the cathode side are controlled by the patterned structure of the membrane, which in turn directly affects the electrochemically active surface area (ECSA) and Pt utilization of the catalyst. This confirmed that electrochemical properties improve the performance of the membrane electrode assembly (MEA). A MEA fabricated with a 3 × 5 μm (width × gap) micropatterned Nafion membrane exhibits a current density of 1.79 A cm−2 at 0.6 V and a power density of 1.26 W cm−2 at 75 °C; these values are 53% and 59% greater than those of the corresponding MEA without a patterned membrane, respectively, and are among the highest performances reported for polymer electrolyte membrane fuel cells (PEMFCs). However, use of a nanopatterned membrane decreases the performance due to insufficient infiltration of the ionomer into the grooved surface, leading to a poor mechanical/electrical contact between the membrane and the electrode. Membrane morphology and the structure of the membrane–electrode interface are characterized by field emission scanning electron microscopy (FE-SEM), cyclic voltammetry (CV), and impedance spectroscopy.

Room Temperature Solid‐State Synthesis of a Conductive Polymer for Applications in Stable I2‐Free Dye‐Sensitized Solar Cells

A solid-state polymerizable monomer, 2,5-dibromo-3,4-propylenedioxythiophene (DBProDOT), was synthesized at 25 °C to produce a conducting polymer, poly(3,4-propylenedioxythiophene) (PProDOT). Crystallographic studies revealed a short interplane distance between DBProDOT molecules, which was responsible for polymerization at low temperature with a lower activation energy and higher exothermic reaction than 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) or its derivatives. Upon solid-state polymerization (SSP) of DBProDOT at 25 °C, PProDOT was obtained in a self-doped state with tribromide ions and an electrical conductivity of 0.05 S cm⁻¹, which is considerably higher than that of chemically-polymerized PProDOT (2×10⁻⁶ S cm⁻¹). Solid-state ¹³C NMR spectroscopy and DFT calculations revealed polarons in PProDOT and a strong perturbation of carbon nuclei in thiophenes as a result of paramagnetic broadening. DBProDOT molecules deeply penetrated and polymerized to fill nanocrystalline TiO₂ pores with PProDOT, which functioned as a hole-transporting material (HTM) for I₂-free solid-state dye-sensitized solar cells (ssDSSCs). With the introduction of an organized mesoporous TiO₂ (OM-TiO₂) layer, the energy conversion efficiency reached 3.5 % at 100 mW cm⁻², which was quite stable up to at least 1500 h. The cell performance and stability was attributed to the high stability of PProDOT, with the high conductivity and improved interfacial contact of the electrode/HTM resulting in reduced interfacial resistance and enhanced electron lifetime.

Fabrication of 3D interconnected porous TiO2 nanotubes templated by poly(vinyl chloride-g-4-vinyl pyridine) for dye-sensitized solar cells

Porous TiO(2) nanotube arrays with three-dimensional (3D) interconnectivity were prepared using a sol-gel process assisted by poly(vinyl chloride-graft-4-vinyl pyridine), PVC-g-P4VP graft copolymer and a ZnO nanorod template. A 7 µm long ZnO nanorod array was grown from the fluorine-doped tin oxide (FTO) glass via a liquid phase deposition method. The TiO(2) sol-gel solution templated by the PVC-g-P4VP graft copolymer produced a random 3D interconnection between the adjacent ZnO nanorods during spin coating. Upon etching of ZnO, TiO(2) nanotubes consisting of 10-15 nm nanoparticles were generated, as confirmed by wide-angle x-ray scattering (WAXS), energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (FE-SEM). The ordered and interconnected nanotube architecture showed an enhanced light scattering effect and increased penetration of polymer electrolytes in dye-sensitized solar cells (DSSC). The energy conversion efficiency reached 1.82% for liquid electrolyte, and 1.46% for low molecular weight (M(w)) and 0.74% for high M(w) polymer electrolytes.

Fabrication of Electrochromic Devices Using Double Layer Conducting Polymers for Infrared Transmittance Control

ABSTRACT: We report the performance improvement ofelectrochromic devices for modulating the transmittance contrast of long wavelength infrared light between 1.5 and 5.0 μm based on a double layer of conducting polymers. The device, fabricated with poly(3-hexylthiophene) (P3HT) and poly(3,4-ethylenedioxythiophene) (PEDOT) as the first and second layers, respectively, showed an transmittance contrast of 60% with a response rate under 5 s, which is greater than the transmittance contrast of cells based on only P3HT or PEDOT (approximately 40%). Electrochromism is defined as a reversible and visible change in the transmittance of a material by applying different voltages or electric currents, resulting in electrochemical oxidation or reduction [1]. Over the last several decades, a number of electrochromic devices based on various electrochromic materials such as tungsten oxide or nickel oxide have been discovered [2-6]. Electrochromic materials with the ability to modulate radiation in the visible and near infrared wavelengths have been used for optical displays, smart windows, and rear-view mirrors [7,8]. However, there have been few studies on infrared (IR) electrochromism, probably due to the relative difficulty in preparing IR transparent electrodes and the lack of interest in IR electrochromic devices [9]. Conducting polymer, known as p-conjugated polymers, have been extensively investigated due to their potential for application in various areas such as actuators [11], sensors [12], and light-emitting diodes [13]. In particular, conducting polymers have recently gained much attention for use in electrochromic devices because of their excellent coloration efficiency, rapid color-switching ability, and broad color availability [14,15]. A range of colors can be controlled and depend on the oxidized (doped) and reduced (undoped) states of the conducting polymers; thus, electroactive conducting polymers have the potential to be electrochromic materials. Here, we fabricated electrochromic devices using two kinds of conducting polymers, i.e., poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-hexylthiophene) (P3HT). These materials were made into thin films to modulate the transmittance of IR light between 1.5 and 5.0 μm. It has been known that PEDOT changes in color from faint blue (in its oxidized state) to deep blue (in the reduced state), while P3HT changes from transparent (in its oxidized state) to red-pink (in the neutral state) in the visible range. However, IR electrochromism of these materials has been poorly reported. In this work, double layers of conducting polymers were applied to the device and compared with single layer device performance. The transmittance contrast of electrochromic devices was measured under alternating potential steps (±2.7 V) in the infrared region between 1.5 and 5.0 μm. The electrochromic device structure is made up of a double polished Si wafer/patterned Au grid/conducting polymer/liquid electrolyte with spacer/patterned Au grid/double polished Si wafer, as shown in Fig. 1a. The use of a double polished Si wafer as a substrate is essential due to its transparency to IR light. In order for electrons to travel in the device, an electrically conducting material should be coated on the wafer substrate. However, most electrically conducting materials, such as metal, carbon, indium tin oxide (ITO), and F-doped tin oxide (FTO), are not transparent to the IR spectral region. Thus, the metal patterned grid was fabricated using Au via a photolithographic method to minimize the IR light absorption of the substrate (Fig. 1b). The Au patterned Si wafer, with a line width of 20 μm and a line repeat period of 500 μm, showed an IR transmittance of 90%. Conducting polymer thin films (Fig. 1c) on the Au grid/Si wafer were prepared as an electrochromic material via solution polymerization. For example, PEDOT thin films were formed by spin coating the solution, which consisted of 3,4-ethylenedioxythiophene monomer, pyridine retardant, Fe(III)-tosylate oxidant, and 1-butanol as the solvent. After spin coating, polymerization was carried out at 80°C for 5 min. The electrolyte consisted of 1.0 M LiClO

Graft Copolymer Directed TiO2 Films with Large Pores and Interconnectivity

This Journal is Korean Society of Photoscience 2012 C 44 I will introduce the synthesis of well organized mesoporous TiO ‐ 2 films templated by the organized graft copolymer, i.e. poly(vinyl chloride) graft poly(oxyethylene methacrylate) (PVCgPOEM) as a structure directing agent. ‐ ‐ ‐ ‐ 1 Well‐ organized mesoporous TiO2 films with high porosity and excellent channel connectivity were developed via the sol gel process using PVCgPOEM graft copolymer synthesized by atom transfer radical polymerization (ATRP). The careful ‐ ‐ adjustment of copolymer composition and solvent affinity using a THF/H2O/HCl mixture was used to systematically vary the material structure. Despite organized morphology, the thickness of sol gel derived TiO ‐ 2 film via a spin coating ‐ process is often limited to the submicron scale due to crack formation during calcination. Our group recently reported micron thick, mesoporous TiO ‐ 2 films templated by PVCgPOEM, via the addition of the P25 nanoparticles. ‐ ‐ 3 TiO2 nanospheres with hierarchical pores were also prepared using the combined process of ATRP and sol gel process. ‐ 4 In addition, 3 dimensional (3D) nanostructured TiO ‐ 2 photoelectrodes with interconnectivity, high surface area and bimodal pores were synthesized using a graft/crosslink polymerization and sol gel process. ‐ 5 Track etched polycarbonate (PC) ‐ membranes were also used as a soft template to synthesize mesoporous TiO2 nanowires or nanotubes and the relation between structure and efficiency was systematically investigated. Finally, we introduced solid state polymerizable ‐ conductive monomer with good conductivity and penetration to photoelectrode to I2 free solid state DSSCs, which ‐ ‐ involves easily accessible and widely applicable fabrication method. ‐ A graft copolymer templated, organized ‐ mesoporous TiO2 film with a large surface area was also prepared using spindle shaped, preformed TiO ‐ 2 nanoparticles.