Jung Tae Park

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.

Preparation of TiO2 spheres with hierarchical pores via grafting polymerization and sol–gel process for dye-sensitized solar cells

Titania (TiO2) nanoparticles were surface-modified via atom transfer radical polymerization (ATRP) with hydrophilic poly(oxyethylene) methacrylate (POEM), which can coordinate to the TiO2 precursor, titanium(IV) isopropoxide (TTIP). Following application of a sol–gel process and calcination at 450 °C, TiO2 nanospheres with hierarchical pores were generated, as confirmed by the shifting of conduction bands in TiO2 using UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS). The particle size and morphology of TiO2 were characterized using wide angle X-ray scattering (WAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Brunauer–Emmett–Teller (BET) analysis revealed bimodal distribution of TiO2 pore sizes with peaks at 6 nm and 50 nm to afford better penetration of polymer electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS). Dye-sensitized solar cells (DSSC) made from TiO2 nanospheres with hierarchical pores exhibited improved photovoltaic efficiency (3.3% for low molecular weight (Mw) and 2.5% for high Mw polymer electrolytes), as compared to those from neat TiO2 nanoparticles (2.4% for low Mw and 1.3% for high Mw) at 100 mW/cm2, owing to the increased surface areas and light scattering.

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.

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.

Formation of mesoporous TiO2 with large surface areas, interconnectivity and hierarchical pores for dye-sensitized solar cells

TiO2 nanoparticles with anatase/rutile mixed phase and <100 nm in size were surface-modified using hydroxyethyl methacrylate (HEMA) and sulfosuccinic acid (SA), which can coordinate to the TiO2 precursor, titanium(IV) isopropoxide (TTIP). The HEMA in TiO2-HEMA nanoparticles underwent a graft/crosslink polymerization to poly(hydroxyethyl methacrylate) (PHEMA), i.e.TiO2-PHEMA. Following the application of a sol–gel process with TTIP, 3-dimensional (3D) nanostructured TiO2 photoelectrodes with interconnectivity, large surface area and bimodal pores were successfully obtained. The energy conversion efficiency of a polymer electrolyte dye-sensitized solar cell (DSSC) fabricated with TiO2-PHEMA/TTIP photoelectrode reached 3.5% at 100 mW cm−2, which was much higher than those of pristine TiO2 (1.4%), TiO2/TTIP (1.6%) and TiO2-HEMA/TTIP (2.0%) photoelectrodes. The higher cell performance of TiO2-PHEMA/TTIP is due to enhanced light harvesting, reduced charge recombination and excellent penetration of polymer electrolytes into the TiO2 pores.

Proton Conducting Crosslinked Membranes by Polymer Blending of Triblock Copolymer and Poly(vinyl alcohol)

Proton conducting crosslinked membranes were prepared using polymer blends of polystyrene-b-poly(hydroxyethyl acrylate)-b-poly(styrene sulfonic acid) (PS-b-PHEA-b-PSSA) and poly(vinyl alcohol) (PVA). PS-b-PHEA-b-PSSA triblock copolymer at 28:21:51 wt% was synthesized sequentially using atom transfer radical polymerization (ATRP). FT-IR spectroscopy showed that after thermal (120 oC, 2 h) and chemical (sulfosuccinic acid, SA) treatments of the membranes, the middle PHEA block of the triblock copolymer was crosslinked with PVA through an esterification reaction between the -OH group of the membrane and the -COOH group of SA. The ion exchange capacity (IEC) decreased from 1.56 to 0.61 meq/g with increasing amount of PVA. Therefore, the proton conductivity at room temperature decreased from 0.044 to 0.018 S/cm. However, the introduction of PVA resulted in a decrease in water uptake from 87.0 to 44.3%, providing good mechanical properties applicable to the membrane electrode assembly (MEA) of fuel cells. Transmission electron microscopy (TEM) showed that the membrane was microphase-separated with a nanometer range with good connectivity of the SO3H ionic aggregates. The power density of a single H2/O2 fuel cell system using the membrane with 50 wt% PVA was 230 mW/cm2 at 70 °C with a relative humidity of 100%. Thermogravimetric analysis (TGA) also showed a decrease in the thermal stability of the membranes with increasing PVA concentration.

Mesoporous TiO 2 Bragg Stack Templated by Graft Copolymer for Dye-sensitized Solar Cells

Organized mesoporous TiO2 Bragg stacks (om-TiO2 BS) consisting of alternating high and low refractive index organized mesoporous TiO2 (om-TiO2) films were prepared to enhance dye loading, light harvesting, electron transport, and electrolyte pore-infiltration in dye-sensitized solar cells (DSSCs). The om-TiO2 films were synthesized via a sol-gel reaction using amphiphilic graft copolymers consisting of poly(vinyl chloride) backbones and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM as templates. To generate high and low index films, the refractive index of om-TiO2 film was tuned by controlling the grafting ratio of PVC-g-POEM via atomic transfer radical polymerization (ATRP). A polymerized ionic liquid (PIL)-based DSSC fabricated with a 1.2-μm-thick om-TiO2 BS-based photoanode exhibited an efficiency of 4.3%, which is much higher than that of conventional DSSCs with a nanocrystalline TiO2 layer (nc-TiO2 layer) (1.7%). A PIL-based DSSC with a heterostructured photoanode consisting of 400-nm-thick organized mesoporous TiO2 interfacial (om-TiO2 IF) layer, 7-μm-thick nc-TiO2, and 1.2-μm-thick om-TiO2 BS as the bottom, middle and top layers, respectively, exhibited an excellent efficiency of 7.5%, which is much higher than that of nanocrystaline TiO2 photoanode (3.5%).

Synthesis and characterization of AgBr nanocomposites by templated amphiphilic comb polymer

A novel amphiphilic graft copolymer, poly(vinylidene fluoride-co-chlorotrifluoroethylene)-g-poly(4-vinyl pyridine) (P(VDF-co-CTFE)-g-P4VP) at 65:35 wt.%, respectively, was synthesized via atom transfer radical polymerization (ATRP), as confirmed by nuclear magnetic resonance (1H NMR) and transmission electron microscopy (TEM). Silver bromide (AgBr) nanoparticles were in situ generated within the self-assembled P(VDF-co-CTFE)-g-P4VP graft copolymer. TEM, UV-visible spectroscopy and X-ray diffraction (XRD) analyses support the successful formation of P(VDF-co-CTFE)-g-P4VP nanocomposites consisting of stabilized AgBr nanoparticles mostly 20-40 nm in size, which is presumably due to the capping action of the coordinating pyridine groups of the graft copolymer. The wavenumber of pyridine nitrogen in FT-IR spectra and the glass transition temperature (Tg) of the graft polymer measured by DSC shifted upon the formation of AgBr nanoparticles, indicating specific interactions between the nanoparticles and the graft copolymer matrix.

Bragg stack-functionalized counter electrode for solid-state dye-sensitized solar cells.

A highly reflective counter electrode is prepared through the deposition of alternating layers of organized mesoporous TiO(2) (om-TiO(2)) and colloidal SiO(2) (col-SiO(2)) nanoparticles. We present the effects of introducing this counter electrode into dye-sensitized solar cells (DSSCs) for maximizing light harvesting properties. The om-TiO(2) layers with a high refractive index are prepared by using an atomic transfer radical polymerization and a sol-gel process, in which a polyvinyl chloride-g-poly(oxyethylene) methacrylate graft copolymer is used as a structure-directing agent. The col-SiO(2) layers with a low refractive index are prepared by spin-coating commercially available silica nanoparticles. The properties of the Bragg stack (BS)-functionalized counter electrode in DSSCs are analyzed by using a variety of techniques, including spectroscopic ellipsometry, SEM, UV/Vis spectroscopy, incident photon-to-electron conversion efficiency, electrochemical impedance spectroscopy, and intensity modulated photocurrent/voltage spectroscopy measurements, to understand the critical factors contributing to the cell performance. When incorporated into DSSCs that are used in conjunction with a polymerized ionic liquid as the solid electrolyte, the energy conversion efficiency of this solid-state DSSC (ssDSSC) approaches 6.6 %, which is one of the highest of the reported N719 dye-based ssDSSCs. Detailed optical and electrochemical analyses of the device performance show that this assembly yields enhanced light harvesting without the negative effects of charge recombination or electrolyte penetration, which thus, presents new possibilities for effective light management.

Multifunctional Organized Mesoporous Tin Oxide Films Templated by Graft Copolymers for Dye‐Sensitized Solar Cells

The synthesis of organized mesoporous SnO2 films with high porosity, larger pores, and good interconnectivity, obtained by sol-gel templating with an amphiphilic graft copolymer, poly(vinyl chloride)-graft-poly(oxyethylene methacrylate), is reported. An improved performance of dye-sensitized solar cells (DSSCs) is demonstrated by the introduction of a 400 nm thick organized mesoporous SnO2 interfacial (om-SnO2 IF) layer between nanocrystalline TiO2 (nc-TiO2 ) and a fluorine-doped tin oxide substrate. To elucidate the improved efficiency, the structural, optical, and electrochemical properties of the devices were characterized by SEM, UV/Vis spectroscopy, noncontact 3D surface profilometry, intensity-modulated photocurrent/voltage spectroscopy, incident photon-to-electron conversion efficiency, and electrochemical impedance spectroscopy measurements. The energy-conversion efficiency of the solid polymerized ionic liquid based DSSC fabricated with the om-SnO2 IF/nc-TiO2 photoanode reached 5.9% at 100 mW cm(-2) ; this is higher than those of neat nc-TiO2 (3.5%) and organized mesoporous TiO2 interfacial/nc-TiO2 layer (5.4%) photoanodes. The improved efficiency is attributed to the antireflective property, cascadal energy band gap, good interconnectivity, and high electrical conductivity of the om-SnO2 IF layer, which results in enhanced light harvesting, increased electron transport, reduced charge recombination, and decreased interfacial/internal resistance.