Cham Kim

Modification of a TiO2 photoanode by using Cr-doped TiO2 with an influence on the photovoltaic efficiency of a dye-sensitized solar cell

This work reports on the preparation of Cr-doped TiO2 (Cr–TiO2) and its utilization in the photoanode of a dye-sensitized solar cell (DSSC). Pure TiO2 and 10% Cr–TiO2 sols were individually obtained viahydrolysis followed by a hydrothermal process at 250 °C for 12 h. The TiO2 sol was first coated on a fluorine-doped tin oxide (FTO) substrate using a polymer binder and sintered at 500 °C for 30 min. The Cr–TiO2 sol was then deposited on the TiO2 layer through the same procedure, thus forming two different layers on the substrate. We found that the photovoltaic efficiency was improved by ca. 18.3% when the double layer was adopted as the photoanode of a DSSC instead of only the TiO2 layer. We assume that this improvement was caused by the repression of electron recombination occurring in the double layer. Further characterizations were carried out to clarify the function of the double layer.

Significant enhancement in the thermoelectric performance of a bismuth telluride nanocompound through brief fabrication procedures.

A binary BiTe nanocompound for thermoelectric applications was prepared via a water-based chemical reaction under atmospheric conditions. We attempted to increase the carrier mobility of the nanocompound by adopting a post-thermal treatment consisting of calcination and reduction at different temperatures. We also tried to control the carrier density of the compound by adjusting the stoichiometry of the atomic constituents. We measured other transport properties (i.e., electrical resistivity, Seebeck coefficient, and thermal conductivity) and observed how these properties were affected by both the carrier mobility and the carrier density. We derived the thermoelectric performance, as captured by the figure of merit (ZT), from the transport properties and discussed the effect of such properties on the ZT value. The nanocompound exhibited a very competent ZT value (0.91 at 100 °C), which is one of the best thermoelectric performances of chemically synthesized BiTe materials.

Investigation of reaction mechanisms of bismuth tellurium selenide nanomaterials for simple reaction manipulation causing effective adjustment of thermoelectric properties.

We synthesized ternary n-type bismuth tellurium selenide nanomaterials for thermoelectric applications via a water-based chemical reaction under an atmospheric environment. In this work, bismuth nitrate was employed as a bismuth precursor and was hydrolyzed to form bismuth hydroxide in an aqueous solution. Ascorbic acid was used to dissolve the bismuth hydroxide and give a reactive bismuth source (Bi(3+) ions) that was able to react with anion sources (Te(2-)/Se(2-) ions). Ascorbic acid played a role in reducing bismuth hydroxide to an unreactive bismuth source (bismuth particles, Bi(0)). We confirmed that ascorbic acid dissolved or reduced bismuth hydroxide depending on the solution pH. Because either Bi(3+) ions or bismuth particles were generated depending on the pH, the nanomaterial stoichiometry was pH dependent. Nanomaterials prepared at various pH levels were individually sintered using a spark plasma sintering process to measure their thermoelectric transport properties (i.e., carrier concentration, electrical resistivity, Seebeck coefficient, and thermal conductivity). We observed how the transport properties were affected through adjustment of the pH of the reaction and found an appropriate pH for optimizing the transport properties, which resulted in enhancement of the thermoelectric performance.

Thermoelectric properties of c-axis aligned Bi-Te materials

Single crystalline Bi-Te thermoelectric materials have a rhombohedral structure with van der Waals bonding along c-axis direction. This structure gives intrinsic anisotropy in thermoelectric properties like electrical resistivity, thermal conductivity, Seebeck coefficient, as well as electron/hall mobility. Recent research works report that nano structures in polycrystalline thermoelectric materials improve thermoelectric performance by reducing thermal conductivity or by enhancing Seebeck coefficient. We investigated the effect of adjusting crystal orientation in the microstructures containing sub micro-sized grains on the thermoelectric properties for polycrystalline Bi-Te materials. Bi-Te powder, prepared through the conventional pulverization process, was sufficiently dispersed in an appropriate solvent, and then was formed into c-axis aligned green body under a designated high magnetic field. The green bodies were sintered with spark-plasma-sintering machine. The degree of crystal alignment of sinter...

Synthesis and Optical Properties of n-Type Polymers Containing Perylene Moieties

Three n-type polymers, Ppe01, Ppe02, and Ppe03, containing both perylene moieties and fluorine atoms were synthesized. The synthesiszed polymeric perylene derivatives with good compatibility for solution processes were chosen as new acceptor candidates for substitution of C60 derivatives in poly(3-hexylthiophene)-based organic photovoltaic application, by considering the results based on good thermal stability, high electron affinity ranging from 3.81 to 3.96 eV, absorption property in visible region, and effective photoluminescence quenching in the blend films with P3HT. The potential of the synthesized perylene polymers was investigated by measuring the current-voltage property of the polymeric P3HT: Ppe01 OPV device.

Effects of Insulation Coating with Metal Salt on the Performance of Organic-Inorganic Hybrid Solar Cells

This work reports on the preparation of modified TiO2 electrodes, which could be applied to the photoanodes in a dye-sensitized solar cells (DSSCs). TiO2 layer was formed on a FTO (fluorine-doped tin oxide) coated glass via doctor-blade method and post heat treatment. Then, the various metal hydroxides and metal oxides were selectively deposited onto the TiO2 layers via electrophoretic deposition and annealing process to give modified TiO2 layer in DSSCs. When metal hydroxide-deposited electrodes were employed in DSSCs, photovoltaic efficiencies were improved by ca. 3–16% compared to those of the cells with bare TiO2 electrodes. However, it was found that the efficiencies regressed when the metal hydroxides were converted to metal oxides via the annealing process.

Interfacial energy band and phonon scattering effect in Bi2Te3-polypyrrole hybrid thermoelectric material

We hybridized n-type Bi2Te3 with an inexpensive and abundantly available conducting polymer, polypyrrole, to obtain a bulk-structured hybrid material in which the interfacial energy band and the phonon scattering effects should occur at the interface of the two components. The obtained hybrid material inevitably exhibited a lower electrical conductivity than pristine Bi2Te3, which may be attributable to carrier scattering at the interfacial energy barrier. However, the hybrid material completely compensated for this loss in electrical conductivity with a significant increase in the Seebeck coefficient, and thus it retained the power factor with no loss. In addition, the hybrid material displayed a much lower thermal conductivity than pristine Bi2Te3 owing to the phonon scattering effect. The hybrid material exhibited significant decoupling of the electrical and thermal properties, thus affording state-of-the-art figures of merit (ZT ∼ 0.98 at 25 °C, ZTmax ∼ 1.21 at 100 °C, and ZTave ∼ 1.18 at 50–150 °C) that exceed those of most of the previously reported n-type Bi2Te3 or Bi2(Te,Se)3 materials.We hybridized n-type Bi2Te3 with an inexpensive and abundantly available conducting polymer, polypyrrole, to obtain a bulk-structured hybrid material in which the interfacial energy band and the phonon scattering effects should occur at the interface of the two components. The obtained hybrid material inevitably exhibited a lower electrical conductivity than pristine Bi2Te3, which may be attributable to carrier scattering at the interfacial energy barrier. However, the hybrid material completely compensated for this loss in electrical conductivity with a significant increase in the Seebeck coefficient, and thus it retained the power factor with no loss. In addition, the hybrid material displayed a much lower thermal conductivity than pristine Bi2Te3 owing to the phonon scattering effect. The hybrid material exhibited significant decoupling of the electrical and thermal properties, thus affording state-of-the-art figures of merit (ZT ∼ 0.98 at 25 °C, ZTmax ∼ 1.21 at 100 °C, and ZTave ∼ 1.18 at 50–150 °C) t...

Optical Properties of Cyclic Amine Type Dyes, and Their Electronic Band Structures with Molecular Microstructure

Three different cyclic amine dyes, 9-(2-chlorophenyl)-9H-carbazole (CPCz; C18H12ClN), 10-(2-chlorophenyl)-10H-phenoxazine (CPPo; C18H12ClNO), and 10-(2-chlorophenyl)-10H-phenothiazine (CPPt; C18H12ClNS), were synthesized to systematically investigate their optical properties and electronic band structures with molecular microstructure. By the insertion of oxygen or sulfur atom into carbazole moiety, the absorption and photoluminescence (PL) spectra showed bathochromic shift, and the Stokes shift was increased. Ionization potential of CPCz was measured as 5.75 eV, on the other hand, CPPo and CPPt showed their ionization potentials of 5.37 and 5.39 eV, respectively. The minute difference in chemical structure such as insertion of oxygen or sulfur atom caused huge change of optical property and electronic band structure.