Chang Kook Hong

Single-step synthesis of 3D nanostructured TiO2 as a scattering layer for vertically aligned 1D nanorod photoanodes and their dye-sensitized solar cell properties

In the present investigation we have successfully synthesized 1D vertically aligned rutile TiO2 nanorods (TNR) and 3D TiO2 nanostars (TNS) as a scattering layer by a single step hydrothermal route. The synthesized nanostructures were characterized by X-ray diffraction, scanning-and transmission electron microscopy and X-ray photoelectron spectroscopy. The 1D TiO2 nanorod and 3D TiO2 nanostar samples were further used for N-719 dye sensitized solar cells (DSSC) application. Compared to a nanorod based cell, the photovoltaic performance of the nanostars/nanorods TiO2 cell exhibits excellent DSSC performance, including superior light scattering, rapid electron transport and lower electron recombination rate. The 3D/1D TNS/TNR based DSSC cell exhibits 5.39% power conversion efficiency, which is remarkably higher than that of the bare 1D nanorod based (3.74%) photoelectrode. The detailed interface and transient properties of these nanorod and nanostar based photoanodes in DSSCs were analyzed by electrochemical impedance spectroscopy measurements and open circuit voltage decay measurements in order to understand the critical factors contributing to such high power conversion efficiency. The enhancement of the efficiency for the 3D/1D TNS/TNR photoanode based cell compared to the 1D TNR is mainly attributed to better light scattering capability, faster electron transport and lower electron recombination.

Room temperature deposition of nanostructured Bi2Se3 thin films for photoelectrochemical application: effect of chelating agents

Nanostructured bismuth selenide (Bi2Se3) thin films have been deposited by chemical bath deposition method at room temperature using three different chelating agents, trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid. The structural, morphological, optical and photoelectrochemical properties of Bi2Se3 thin films have been investigated as a function of different chelating agents. X-Ray diffraction studies revealed that the films were nanocrystalline in nature with a rhombohedral crystal structure. Trisodium citrate chelate resulted in Bi2Se3 thin films with poor crystallinity; further improvement in the crystallinity of the films was observed with triethanolamine and ethylenedimminetetraacetic acid chelates. From scanning electron microscopy, a uniform sphere-like morphology having an average sphere diameter of 90 nm was observed with trisodium citrate chelate. In the case of triethanolamine, a fibrous morphology with an average fiber thickness of 60 nm was observed, whereas for ethylenedimminetetraacetic acid chelate, a vertically arrayed petal-like morphology having petal thickness of 50–70 nm was observed. The UV-Vis absorption studies revealed that the band gap energy of the Bi2Se3 thin films with trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid chelates was 1.55, 1.48 and 1.40 eV, respectively. The maximum short circuit current densities (Jsc) of 0.158, 0.214 and 0.284 mA cm−2 and the corresponding open circuit voltages (Voc) of 196, 206 and 217 mV were obtained with trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid chelates, respectively. The Bi2Se3 thin films deposited using triethanolamine and ethylenedimminetetraacetic acid chelates show better photoelectrochemical performance as compared with trisodium citrate chelate.

Microwave assisted synthesis, characterization and thermoelectric properties of nanocrystalline copper antimony selenide thin films

In the present work, we have synthesized p-type copper antimony selenide (Cu3SbSe4) thin films in an aqueous alkaline medium using a microwave assisted synthesis technique. The deposited thin films were characterized by UV-Vis-NIR spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and thermoelectric techniques. On the basis of experimental results, a possible reaction mechanism has been discussed in detail. The band gap of the as deposited film is 1.94 eV and after annealing it reaches 1.87 eV for Cu3SbSe4. XRD results indicate that the as deposited thin films of CuSbSe2 have an orthorhombic crystal structure with secondary mixed phases and after annealing this is converted to Cu3SbSe4 having a pure tetragonal crystal structure. FESEM micrographs of Cu3SbSe4 showed a spherically diffused granular morphology having an average grain size of 25 nm. The HRTEM result of Cu3SbSe4 shows good crystallinity with a lattice spacing of 0.327 nm along the (112) plane. The EDS spectrum shows the presence of Cu, Sb and Se elements. The thermoelectric figure of merit (ZT) of the as deposited film is calculated to be 0.059 at 300 K and that of annealed Cu3SbSe4 is found to be 0.141 at 300 K.

Photoelectrochemically active surfactant free single step hydrothermal mediated titanium dioxide nanorods

Titanium dioxide (TiO2) nanorods have been successfully synthesized by a simple and cost-effective hydrothermal deposition method onto the conducting glass substrates. Effect of reaction temperature on the growth of TiO2 nanorods have been investigated by varying the reaction temperature from 140 to 200xa0°C. The optical, structural, compositional, morphological properties of the synthesized films are studied. X-ray diffraction patterns reveal the formation of polycrystalline TiO2 with the tetragonal crystal structure possessing rutile phase. The chemical composition and valence states of the constituent elements were analysed by X-ray photoelectron spectroscopy. Field emission scanning electron microscopy images shows the formation of nanorod-like structure with variation in diameter. The optical band gap energy was found to increase from 3.07 to 3.15xa0eV with the increase in reaction temperature exhibiting a blue shift. The films were photo electrochemically active with the maximum current density of 216xa0µA/cm2 for the sample prepared at 180xa0°C.

Hydrothermal synthesis of rutile TiO2 bottle brush for efficient dye-sensitized solar cells

Hydrothermal process has been employed to synthesize titanium oxide (TiO2) bottle brush. The nanostructured bottle brushes with tetragonal nanorods of ~75xa0nm diameter have been synthesized by changing the nature of the precursors and hydrothermal processing parameters. The morphological features and structural properties of TiO2 films were investigated by field emission scanning electron microscopy, X-ray diffraction, high-resolution transmission electron spectroscopy, Fourier transform Raman spectroscopy, and X-ray photoelectron spectroscopy. The influence of such nanostructures on the performance of dye-sensitized solar cells (DSSCs) is investigated in detail. The interface and transient properties of these nanorods and bottle brush-based photoanodes in DSSCs were analyzed by electrochemical impedance spectroscopic measurements in order to understand the critical factors contributing to such high power conversion efficiency. Surface area of sample was recorded using Brunauer–Emmett–Teller measurements. It is found that bottle brush provides effective large surface area 89.34xa0m2xa0g−1 which is much higher than TiO2 nanorods 63.7xa0m2xa0g−1. Such effective surface area can facilitate the effective light harvesting, and hence improves the dye adsorption and the photovoltaic performance of DSSCs, typically in short-circuit photocurrent and power conversion efficiency. A best power conversion efficiency of 6.63xa0% has been achieved. We believe that the present device performance would have wide interests in dye-sensitized solar cell research.