Harim Jeon

Efficiency improvement of dye-sensitized solar cells using graft copolymer-templated mesoporous TiO2 films as an interfacial layer

Organized mesoporous TiO2 films with high porosity and good connectivity were synthesized via sol–gel by templating an amphiphilic graft copolymer consisting of poly(vinyl chloride) backbone and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM. The randomly microphase-separated graft copolymer was self-reorganized to exhibit a well-ordered micellar morphology upon controlling polymer–solvent interactions, as confirmed by atomic force microscope (AFM) and glazing incidence small-angle X-ray scattering (GISAXS). These organized mesoporous TiO2 films, 550 nm in thickness, were used an an interfacial layer between a nanocrystalline TiO2 thick layer and a conducting glass in dye-sensitized solar cells (DSSC). Introduction of the organized mesoporous TiO2 layer resulted in the increased transmittance of visible light, decreased interfacial resistance and enhanced electron lifetime. As a result, an energy conversion efficiency of DSSC employing polymer electrolyte was significantly improved from 3.5% to 5.0% at 100 mW cm−2.

Facile fabrication of vertically aligned TiO2 nanorods with high density and rutile/anatase phases on transparent conducting glasses: high efficiency dye-sensitized solar cells

We present a facile and effective method to prepare vertically aligned TiO2 nanorods (NRs) with a high density and rutile/anatase mixture phases on transparent conducting oxide (TCO) glasses. The anatase TiO2 nanoparticles grafted with hydrophilic poly(oxyethylene) methacrylate (POEM), which can coordinate with a TiO2 precursor such as Ti(BuO4), were introduced in the presence of glycine. Following application of a hydrothermal process and calcination at 450 °C, vertically well-aligned TiO2 NRs with diameters of 70 nm and lengths of 3 μm were generated, as confirmed by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The larger wavenumber shift observed with TiO2–POEM in FT-IR spectra suggests more favorable and stronger interactions that facilitate the nucleation and growth of NRs on the transparent conductive fluorine-doped tin oxide (FTO) substrates, resulting in increased NRs density. Dye-sensitized solar cells (DSSCs) fabricated using TiO2 NRs with a high density and rutile/anatase mixture phases exhibited improved energy conversion efficiency, irrespective of the type of electrolyte. When liquid electrolyte was used, the DSSCs exhibited an efficiency of 5.7% at 100 mW cm−2, which is the highest value for DSSCs fabricated with NRs directly grown on TCO substrates. High cell efficiencies of 4.5 and 3.7% were also obtained with quasi-solid-state and solid-state electrolytes, respectively, due to the reduced interfacial resistance of electrolyte/electrode and improved electron transport.

Nanocomposites with Graft Copolymer-Templated Mesoporous MgTiO3 Perovskite for CO2 Capture Applications

Mesoporous MgTiO3 perovskite with a high porosity and interfacial properties were synthesized via a solvothermal reaction at 150 °C for 10 h using a graft copolymer, i.e., poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) with a well-ordered micellar morphology as a structure-directing agent. A PVC-g-POEM graft copolymer with a wormlike morphology was utilized as a soft matrix to prepare a mixed matrix membrane (MMM) with mesoporous MgTiO3 perovskite through a solution-casting method. The structure and morphology of PVC-g-POEM graft copolymer was carefully tuned by controlling polymer-solvent interactions, as characterized by transmission electron microscopy (TEM). The average pore diameter of the MgTiO3 perovskite was 10.4 nm, which is effective in facilitating gas transport via Knudsen diffusion through mesopores as well as improving interfacial contact with the organic polymer matrix. Because of a high porosity (0.56), the density of mesoporous MgTiO3 (1.75 g/cm(3)) was much lower than that of dense nonporous MgTiO3 (4 g/cm(3)) and not significantly higher than that of PVC-g-POEM (1.25 g/cm(3)), leading to a uniform distribution of MgTiO3 in MMM. The permeability of MMM with MgTiO3 was greater than those of MMM with only MgO or TiO2, indicating the simultaneous improvement of solubility and diffusivity in the former, as supported by CO2 temperature-programmed desorption (TPD) measurements. The MMM with MgTiO3 25 wt % exhibited a CO2 permeability improvement of 140% up to 138.7 Barrer (1 Barrer = 1 × 10(-10) cm(3)(STP) cm cm(-2) s(-1) cmHg(-1)) without a large loss of CO2/N2 selectivity.

Well-organized meso-macroporous TiO2/SiO2 film derived from amphiphilic rubbery comb copolymer.

We report the facile synthesis of a well-organized meso-macroporous TiO2/SiO2 thin film with high porosity and good interconnectivity from a binary mixture (i.e., titania precursor and polymer template). Our process is based on self-assembly of the amphiphilic rubbery comb copolymer, poly(dimethylsiloxane)-g-poly(oxyethylene methacrylate) (PDMS-g-POEM) with titanium tetraisopropoxide (TTIP). SiO2 is self-provided by thermal oxidation of PDMS chains during calcination under air. The selective, preferential interaction between TTIP and the hydrophilic POEM chains was responsible for the formation of well-organized TiO2/SiO2 films, as supported by transmission electron microscopy, scanning electron microscopy, X-ray photospectroscopy, and X-ray diffraction analyses. We investigated in detail the effect of precursor content, solvent type, and polymer concentration on thin film morphology. Photodegradation of methyl orange by the well-organized meso-macroporous TiO2/SiO2 film was greater than that of a dense TiO2 film prepared without PDMS-g-POEM as well as a SiO2-etched TiO2 film. These results indicate that the well-organized structure and SiO2 doping of the TiO2 film play a pivotal role in enhancing its photocatalytic properties.

High-performance Polymer Membranes with Multi-functional Amphiphilic Micelles for CO2 Capture.

Herein, we report a high performance polymer membrane with simultaneously large improvements in the CO2 permeability and CO2/N2 selectivity. These improvements are obtained by incorporation of a multi-functional amphiphilic comb copolymer micelle, that is, poly(dimethylsiloxane)-g-poly(oxyethylene methacrylate) (PDMS-g-POEM), into a poly(amide-b-ethylene oxide) (Pebax) matrix. Both CO2 and N2 permeabilities continuously increased with PDMS-g-POEM content, whereas the CO2/N2 selectivity increased up to 40 wt % of PDMS-g-POEM, which enabled the maximum performance to approach the upper bound limit (2008). The membranes with PDMS-g-POEM exhibited greater CO2 permeability and CO2/N2 selectivity than those with a zeolitic imidazolate framework (ZIF-8), a well-known expensive inorganic filler, indicating the effectiveness of PDMS-g-POEM micelles for CO2 capture.

Ionic liquid crystals: Synthesis, structure and applications to I2-free solid-state dye-sensitized solar cells

AbstractA novel type of ionic liquid crystal (ILC) was synthesized and used as a solid electrolyte in I2-free solidstate dye-sensitized solar cells (ssDSSCs). In particular, the properties of two ILCs, 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium iodide (EBII) with a single aliphatic C=C bond and 1-[(4-ethenylphenyl)methyl]-3-vinyl-imidazolium iodide (EVII) with two aliphatic C=C bonds, were evaluated. The structures and morphologies of the ILCs were characterized using Fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). Ultraviolet (UV)-visible spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analyses revealed that EBII exhibited weaker π-π stacking interactions, longer d-spacing, and a lower melting temperature. The energy conversion efficiency of I2-free ssDSSC with EBII (4.7% at 100 mW/cm2) was higher than with EVII (3.8%) due to facile charge transport and lower electron recombination in the former, as supported by electrochemical impedance spectroscopy (EIS).

One-step Synthesis of Vertically Aligned Anatase Thornbush-like TiO2 Nanowire Arrays on Transparent Conducting Oxides for Solid-State Dye-Sensitized Solar Cells

Herein, we report a facile synthesis of high-density anatase-phase vertically aligned thornbush-like TiO2 nanowires (TBWs) on transparent conducting oxide glasses. Morphologically controllable TBW arrays of 9 μm in length are generated through a one-step hydrothermal reaction at 200 °C over 11 h using potassium titanium oxide oxalate dehydrate, diethylene glycol (DEG), and water. The TBWs consist of a large number of nanoplates or nanorods, as confirmed by SEM and TEM imaging. The morphologies of TBWs are controllable by adjusting DEG/water ratios. TBW diameters gradually decrease from 600 (TBW600) to 400 (TBW400) to 200 nm (TBW200) and morphologies change from nanoplates to nanorods with an increase in DEG content. TBWs are utilized as photoanodes for quasi-solid-state dye-sensitized solar cells (qssDSSCs) and solid-state DSSCs (ssDSSCs). The energy-conversion efficiency of qssDSSCs is in the order: TBW200 (5.2%)>TBW400 (4.5%)>TBW600 (3.4%). These results can be attributed to the different surface areas, light-scattering effects, and charge transport rates, as confirmed by dye-loading measurements, reflectance spectroscopy, and incident photon-to-electron conversion efficiency and intensity-modulated photovoltage spectroscopy/intensity-modulated photocurrent spectroscopy analyses. TBW200 is further treated with a graft-copolymer-directed organized mesoporous TiO2 to increase the surface area and interconnectivity of TBWs. As a result, the energy-conversion efficiency of the ssDSSC increases to 6.7% at 100 mW cm(-2) , which is among the highest values for N719-dye-based ssDSSCs.

Vertically aligned anatase TiO2 nanotubes on transparent conducting substrates using polycarbonate membranes

6 μm-thick, anatase TiO2 nanotubes (NTs) that were vertically aligned to transparent conducting oxide (TCO) substrates were prepared using a sol–gel process and track-etched polycarbonate (PC) membranes.

Amphiphilic Graft Copolymer Nanospheres: From Colloidal Self-Assembly to CO2 Capture Membranes

Colloidal nanosphere self-assembly effectively generates ordered nanostructures, prompting tremendous interest in many applications such as photonic crystals and templates for inverse opal fabrication. Here we report the self-assembly of low-cost, graft copolymer nanospheres for CO2 capture membranes. Specifically, poly(dimethylsiloxane)-graft-poly(4-vinylpyridine) (PDMS-g-P4VP) is synthesized via one-pot, free radical dispersion polymerization to give discrete monodisperse nanospheres. These nanospheres comprise a surface-anchored highly permeable PDMS layer and internal CO2-philic P4VP spherical core. Their diameter is controllable below the submicrometer range by varying grafting ratios. The colloidal dispersion forms a long-range, close-packed hexagonal array on a substrate by inclined deposition and convective assembly. The array shows dispersion medium-dependent packing characteristics. A thermodynamic correlation is determined using different solvents to obtain stable PDMS-g-P4VP dispersions and interpreted in terms of Flory-Huggins interaction parameter. As a proof-of-concept, the implementation of these nanospheres into membranes simultaneously enhances the CO2 permeability and CO2/N2 selectivity of PDMS-based transport matrixes. Upon physical aging of the solution, the CO2/N2 selectivity is improved up to 26, one of the highest values for highly permeable PDMS-based polymeric membranes.

Use of Amphiphilic Graft Copolymer as Dispersant for Carbon Nanotubes

Carbon nanotubes (CNTs) draw attention as promising materials due to their excellent electrical and mechanical properties. However, the intrinsic strong interaction between CNTs presents a challenge to their use in various applications. Here, we present a facile method to disperse single-walled carbon nanotubes (SWCNTs) in a polar solution using a graft copolymer, poly(vinyl chloride)- graft-poly(oxyethylene methacrylate), PVC-g-POEM. The graft copolymer was synthesized via atom transfer radical polymeri- zation (ATRP), as confirmed by gel permeation chromatography (GPC) and 1 H NMR spectroscopy. The SWCNTs were uniformly dispersed in a polar solvent such as dimethylsiloxane (DMSO) using PVC-g-POEM as a dispersant, due to interaction between CNT and the graft copolymer, as revealed by transmission electron microscopy (TEM) analysis. Upon removal of the solvent, free standing nanocomposite films with good homogeneity were obtained.