Hark Jin Kim

Size-dependent light-scattering effects of nanoporous TiO2 spheres in dye-sensitized solar cells

Submicron-sized monodispersed TiO2 spheres (SPs) with high porosity were synthesized by a controlled hydrolysis of titanium tetraisopropoxide (TTIP) and subsequent hydrothermal treatment at 230 °C. By adjusting the ratio of TTIP to water (the r-factor) in the hydrolysis reaction, the diameters of SPs were selectively controlled to 260, 350, 450, 560, 800, and 980 nm. The prepared SPs in the pure anatase phase were highly porous structures with crystallite sizes of ∼15 nm and surface areas of 101–121 m2g−1. The synthesized nanoporous SPs in different sizes were then applied as the light-scattering layer (LSL) of dye-sensitized solar cells (DSCs) for efficient utilization of solar spectrum, and the size-dependent light-scattering effects of those SPs were systematically investigated. The 450 nm sized SP (SP450) provided the highest light-scattering efficiency among those in the 260–800 nm range. Relatively higher efficiency is caused by the characteristic light-scattering effect based on its unique diameter and also by the photonic reflection effect originating from its size-uniformity and long-range ordering. As a result the photovoltaic conversion efficiency (η) of DSC was improved from 6.92 to 9.04% with introducing the nanoporous SP450 as LSL.

Formation of efficient dye-sensitized solar cells by introducing an interfacial layer of long-range ordered mesoporous TiO2 thin film.

Long-range ordered cubic mesoporous TiO 2 films with 300 nm thickness were fabricated on fluorine-doped tin oxide (FTO) substrate by evaporation-induced self-assembly (EISA) process using F127 as a structure-directing agent. The prepared mesoporous TiO 2 film (Meso-TiO 2) was applied as an interfacial layer between the nanocrystalline TiO 2 film (NC-TiO 2) and the FTO electrode in the dye-sensitized solar cell (DSSC). The introduction of Meso-TiO 2 increased J sc from 12.3 to 14.5 mA/cm (2), and V oc by 55 mV, whereas there was no appreciable change in the fill factor (FF). As a result, the photovoltaic conversion efficiency ( eta) was improved by 30.0% from 5.77% to 7.48%. Notably, introduction of Meso-TiO 2 increased the transmittance of visible light through the FTO glass by 23% as a result of its excellent antireflective role. Thus the increased transmittance was a key factor in enhancing the photovoltaic conversion efficiency. In addition, the presence of interfacial Meso-TiO 2 provided excellent adhesion between the FTO and main TiO 2 layer, and suppressed the back-transport reaction by blocking direct contact between the electrolyte and FTO electrode.

Double-heterojunction structure of SbxSn1-xO2/TiO2/CdSe for efficient decomposition of gaseous 2-propanol under visible-light irradiation

Highly crystallized antimony-doped tin oxide (ATO; SbxSn1-xO2, x = 0.1) of ∼50 nm size was prepared by co-precipitation of SnCl4·5H2O and SbCl3, followed by heat-treatment at 1000 °C. The prepared ATO nanoparticles of deep blue color revealed a profound light-absorption in the visible range. ATO/TiO2 composites were prepared by covering the surface of ATO nanoparticles with TiO2 using the sol–gel method. Under visible-light irradiation (λ ≥ 420 nm), the prepared ATO/TiO2 showed a notable photocatalytic efficiency in decomposing gaseous 2-propanol (IP), which seemed to be caused by the hole-transfer mechanism between the valence bands (VB) of ATO and TiO2, since the ATOs VB level is located lower than that of TiO2. Subsequently, a double-heterojunction ATO/TiO2/CdSe structure was prepared by loading CdSe quantum dots (QDs) onto the surface of the ATO/TiO2, which dramatically enhanced the visible-light photocatalytic efficiency. In fact, the catalytic activity of ATO/TiO2/CdSe in evolving CO2 from IP, was ∼3 times that of ATO/TiO2 and twice that of typical N-doped TiO2. The unexpectedly high efficiency of ATO/TiO2/CdSe seemingly is due to the unique band matching among these semiconductors. With sensitization of ATO and CdSe, not only the holes but also the electrons are generated in the VB and CB, respectively, of TiO2 under visible-light irradiation.

Effect of Layer-by-Layer Assembled SnO2 Interfacial Layers in Photovoltaic Properties of Dye-Sensitized Solar Cells

Ultrathin SnO(2) layers were deposited on FTO substrate by the layer-by-layer (LbL) self-assembly technique utilizing negatively charged 2.5 nm sized SnO(2) nanoparticles (NPs) and cationic poly(allylamine hydrochloride) (PAH). For the construction of dye-sensitized solar cells (DSC), the bulk TiO(2) layer was deposited over the (PAH/SnO(2))(n) (n = 1-10) and subsequently calcined at 500 °C to remove organic components. With introducing four layers of self-assembled SnO(2) interfacial layer (IL), the short circuit current density (J(sc)) of DSCs was increased from 8.96 to 10.97 mA/cm(2), whereas the open circuit voltage (V(oc)) and fill factor (FF) were not appreciably changed. Consequently, photovoltaic conversion efficiency (η) was enhanced from 5.43 to 6.57%. Transient photoelectron spectroscopic analyses revealed that the ultrathin SnO(2) layer considerably increased the electron diffusion coefficient (D(e)) in TiO(2) layer, but the electron lifetime (τ(e)) was decreased unexpectedly. The observed unusual photovoltaic properties would be caused by the unique conduction band (CB) location of the SnO(2), inducing the cascadal energy band matching among the CBs of TiO(2), SnO(2), and FTO.

Annealing-free preparation of anatase TiO2 nanopopcorns on Ti foil via a hydrothermal process and their photocatalytic and photovoltaic applications

Uniform and well-defined nanopopcorns of the tetragonal anatase TiO2 having an average diameter of 670 nm have been facilely grown on Ti foil via a hydrothermal method and characterized by analyzing electron microscopic images and electron diffraction patterns as well as X-ray photoelectron, photoluminescence, and Raman spectra. The morphology of TiO2 nanostructures on Ti foil has been controlled well by adjusting the volume ratio of H2O2 : HF : H2O, VR(H2O2 : HF : H2O). Truncated tetragonal pyramidal TiO2 nanocrystals exposing the {001} and {101} facets have grown on the surface of TiO2 nanostructures exposing the {001} facets to produce anatase TiO2 nanopopcorns. Without being treated via any annealing process, our well-defined TiO2 nanopopcorns on Ti foil have been directly employed for photocatalytic materials and dye-sensitized solar cells. Among our prepared samples, anatase TiO2 nanopopcorns grown on Ti foil at a VR(H2O2 : HF : H2O) of 1 : 1 : 1000 have shown the most reduced oxygen vacancy luminescence, the highest photocatalytic activity for the degradation of methylene blue, and the highest photovoltaic conversion efficiency of 3.98% as the working electrode of a dye-sensitized solar cell.

Interface control with layer-by-layer assembled ionic polymers for efficient low-temperature dye-sensitized solar cells

Herein we report a novel method for fabricating highly efficient dye-sensitized solar cells (DSCs) at low temperature (<150 °C). The ionic polymers of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were layer-by-layer deposited as an interfacial layer (IL) on the FTO substrate, before the nanoporous TiO2 layer was coated by a typical doctor-blade method. The presence of ultrathin (PAH/PSS)n IL created excellent adhesion of the TiO2 layer to the FTO substrate, leading to efficient transport of photo-injected electrons from TiO2 to FTO, as well as blocking the back-transport reaction from FTO to I3−. As a result, the fill factor (FF) was remarkably increased from 0.709 to 0.783, with a significant increase of the open circuit voltage (Voc) from 760 to 803 mV and the short circuit current (Jsc) from 8.078 to 8.768 mA cm−2, leading to the improvement of the photovoltaic conversion efficiency (η) from 4.41 to 5.52%. With optimization of the TiO2 electrode structure, η of the DSC fabricated at 140 °C was enhanced to 7.14%.

Photocatalytic Behavior of TiO2 Films : Thickness and Roughness Dependence

ABSTRACT Transparent TiO 2 films in various thicknesses were prepared by sol‐gel and MOCVD method, respectively, and their photocatalytic activities in decomposing gaseous 2‐propanol were evaluated. The surfaces and grain structures of the prepared films were characterized by FESEM, XRD, and AFM. It was found that the photocatalytic activities of TiO 2 films were greatly dependent on the film thickness and surface roughness: The photocatalytic activity increases with the increase of film thickness, while it decreases with the increase of surface roughness. We have proposed that these phenomena originate from the transfer of photogenerated electron and hole pairs from the bulk to the surface of TiO 2 film. Several experimental evidences supporting this mechanism have also been provided. INTRODUCTION TiO 2 photocatalysts in thin film form have promising industrial applications for the elimination of organic pollutants in aqueous solution or in gas phase, and they are also well‐known as self‐cleaning and super‐hydrophilic smart materials working under an irradiation of light.

Coupling of semiconductors for photocatalytic oxidation and CO2 reduction reactions under visible-light

This Journal is Korean Society of Photoscience 2012 C 40 Removal of environmental pollutants through photocatalytic reaction has drawn increasing attention over the last few decades. Photocatalysts have also been designed and investigated for the purpose of water splitting and CO2 reduction to generate clean energies. We fabricated several heterojunction structures between TiO2 and other visible light absorbing ‐ semiconductors, and found that relative energy band locations between TiO2 and sensitizer are a crucial factor in determining the efficiency of the photocatalytic reactions. First, we investigated several coupled structures of TiO2 and sensitizers, whose VB are lower than that of TiO2 (denoted to “typeB heterojunction”). With visible light irradiation, the ‐ electrons in the VB of the semiconductor are excited to its CB. Thereby, its VB is rendered partially vacant, and the electrons in the VB of TiO2 can be transferred to that of the semiconductor, since its VB is located at lower level. As a result, the holes generated in the VB of TiO2 have sufficient lifetime to initiate the photocatalytic oxidation reactions. Some of the coupled systems exhibited significantly higher photocatalytic efficiency than the typical Ndoped TiO ‐ 2 in decomposing gaseous 2propanol and several organic pollutants in aqueous solution. For further enhancement of visible light ‐ ‐ catalytic efficiency, we doubly combined the two different sensitizers with TiO2. That is, sensitizerB with lower VB ‐ position than that of TiO2 was designed to be located in the core of the TiO2 structure, whereas sensitizerA with higher ‐ CB was loaded onto the TiO2 surface. Under visible light irradiation, both the electrons and holes are generated in CB and ‐ VB of TiO2, and these active species induces remarkably high photocatalytic efficiency in evolving CO2. Second, we also found that some of the coupled photocatalytic systems can be used for the reduction of CO2 under visible light irradiation. ‐ We monitored the evolution of methanol and carbon monoxide, and it was also found that relative energy band positions between two semiconductors were critical for the photocatalytic CO2 reduction reactions. Coupling of semiconductors for photocatalytic oxidation and CO2 reduction reactions under visible light ‐