Jae-Yeol Hwang

Improved Photovoltaic Response of Nanocrystalline CdS-Sensitized Solar Cells through Interface Control

Nanocrystalline CdS-sensitized solar cells (CdS-SSCs) based on mesoporous TiO(2) were fabricated by the spray pyrolysis deposition method. The energy conversion efficiency of these cells was drastically increased (156%) by modifying the junction structure through post-treatment that included soaking in a dilute TiCl(4) aqueous solution and subsequent thermal annealing. We propose that the post-treatment is responsible for an increased number of interconnections between TiO(2) and CdS, as well as surface passivation of the CdS sensitizer. The increase in the cell efficiency is attributed to the improved charge carrier transport, suppression of photoelectron recombination with holes both in the same sensitizer particle and in nearby ones, and suppression of photoelectron capture by the electrolyte.

Strong enhancement of the Faraday rotation in Ce and Bi comodified epitaxial iron garnet thin films

Ce and Bi comodified iron garnet (Ce2.2Bi0.8Fe5O12) thin films for magneto-optic applications were epitaxially grown on a (111)-oriented Gd3Ga5O12 substrate by pulsed laser deposition. We found that epitaxial film quality could be achieved under a low-pressure Ar atmosphere. Surprisingly, our 1 μm thick epitaxial films showed a record Faraday rotation as high as 0.55 deg/μm, a value strongly dependent on the concentration of Bi3+ ions.Ce and Bi comodified iron garnet (Ce2.2Bi0.8Fe5O12) thin films for magneto-optic applications were epitaxially grown on a (111)-oriented Gd3Ga5O12 substrate by pulsed laser deposition. We found that epitaxial film quality could be achieved under a low-pressure Ar atmosphere. Surprisingly, our 1 μm thick epitaxial films showed a record Faraday rotation as high as 0.55 deg/μm, a value strongly dependent on the concentration of Bi3+ ions.

Dielectric characterization of transparent epitaxial Ga2O3 thin film on n-GaN∕Al2O3 prepared by pulsed laser deposition

(2¯01)-oriented β-Ga2O3∕GaN thin films were epitaxially grown by pulsed laser deposition. These films have the specific in-plane orientation, which was confirmed by φ scans of Ga2O3 (111) and (3¯11) reflections. When oxygen flow rate was increased, the surface morphologies and roughness of β-Ga2O3 drastically changed. The β-Ga2O3∕GaN structure showed a stable and sharp interface and uniform elemental distribution in depth. The dielectric constant and memory window of β-Ga2O3∕GaN were about 13.9 and 0.50V for oxygen flow rate of 5SCCM (SCCM denotes cubic centimeter per minute at STP).

Hierarchically Structured Hole Transport Layers of Spiro-OMeTAD and Multiwalled Carbon Nanotubes for Perovskite Solar Cells

The low electrical conductivity of spiro-OMeTAD hole transport layers impedes further enhancements of the power conversion efficiency (PCE) of perovskite solar cells. We embedded multiwalled carbon nanotubes (MWNTs) in spiro-OMeTAD (spiro-OMeTAD/MWNTs) to increase carrier mobility and conductivity. However, direct electrical contact between CH3 NH3 PbI3 and the MWNTs created pathways for undesirable back-electron transfer, owing to the large work function of MWNTs, limiting enhancements of the PCE. A hierarchical structure of pure spiro-OMeTAD and spiro-OMeTAD/MWNTs was designed to block back-electron transfer and fully exploit the enhanced charge transport of spiro-OMeTAD/MWNTs. The enhanced fill factor, short-circuit current density, open-circuit voltage, and PCE (15.1 %) were achieved by using this hierarchical hole transport layer structure (MWNT concentration=2 wt %). The perovskite solar cells were fabricated by a low-temperature solution process, further decreasing their per-Watt cost.

Ferromagnetism of heteroepitaxial Zn1-xCuxO films grown on n-GaN substrates

Ferromagnetic Zn1-xCuxO (ZnO:Cu) films were epitaxially grown on n-GaN(0001)/Al2O3(0001) by radio-frequency magnetron sputtering at a substrate temperature as low as 700°C. X-ray diffraction θ-2θ scans, -scans, and surface morphology studies revealed the degree of crystallinity, epitaxiality, and grain size. The full-width at half-maximum of the peak for the ZnO:Cu films deposited on GaN was below 0.18° according to ω-scans. The chemical bonding states and depth profiles of the films were investigated by X-ray photoelectron spectroscopy (XPS) and with a glow discharge spectrometer (GDS). A magnetic property measurement revealed that Cu-doped ZnO films exhibit ferromagnetic behavior with a strong exchange interaction between sp-band carriers and localized d electrons at room temperature.

Phase transitions via selective elemental vacancy engineering in complex oxide thin films

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.

Growth and Characterization of (Ba0.5Sr0.5)TiO3 Films Epitaxially Grown on (002) GaN/(0006) Al2O3 Electrode

Epitaxial (Ba0.5Sr0.5)TiO3 (BST) films have been grown on an n-GaN/(0006) Al2O3 by pulsed-laser deposition. X-ray θ-2θ, ω-rocking curves, and -scans reveal the epitaxial growth of (111) BST/(002) GaN bilayers. The full-width at half-maximum (FWHM) of (111) BST deposited on GaN was 0.95°. The depth profile of the film was recorded by Auger electron spectroscopy (AES). The dielectric constant of the BST film was about 510 with a dielectric loss of 0.05 at 1 kHz. The memory window of the epitaxial BST film was about 2 V for an applied voltage range of -8 V to 3 V.

Graphene Substrate for van der Waals Epitaxy of Layer-Structured Bismuth Antimony Telluride Thermoelectric Film

Graphene as a substrate for the van der Waals epitaxy of 2D layered materials is utilized for the epitaxial growth of a layer-structured thermoelectric film. Van der Waals epitaxial Bi0.5 Sb1.5 Te3 film on graphene synthesized via a simple and scalable fabrication method exhibits good crystallinity and high thermoelectric transport properties comparable to single crystals.

Effects of doping on transport properties in Cu-Bi-Se-based thermoelectric materials.

The thermoelectric properties of Zn-, In-, and I-doped Cu1.7Bi4.7Se8 pavonite homologues were investigated in the temperature range from 300 to 560 K. On the basis of the comprehensive structural analysis using Rietveld refinement of synchrotron radiation diffraction for Cu(x+y)Bi(5-y)Se8 compounds with the inherently disordered crystallographic sites, we demonstrate a doping strategy that provides a simultaneous control for enhanced electronic transport properties by the optimization of carrier concentration and exceptionally low lattice thermal conductivity by the formation of point defects. Substituted Zn or In ions on Cu site was found to be an effective phonon scattering center as well as an electron donor, while doping on Bi site showed a moderate effect for phonon scattering. In addition, we achieved largely enhanced power factor in small amount of In doping on Cu site by increased electrical conductivity and moderately decreased Seebeck coefficient. Coupled with a low lattice thermal conductivity originated from intensified point defect phonon scattering by substituted In ions with host Cu ions, a thermoelectric figure of merit ZT of 0.24 at 560 K for Cu1.6915In0.0085Bi4.7Se8 was achieved, yielding 30% enhancement compared with that of a pristine Cu1.7Bi4.7Se8 at the same temperature.

Ferroelectric properties of mn-doped Bi3.6La0.4Ti3O12 thin films prepared under different annealing conditions

Ferroelectric Bi3.6La0.4Ti3O12 (BLT) and Mn-doped (0.1 wt%, 0.2 wt%, and 0.3 wt%) BLT thin films were prepared by sol-gel processing. In the X-ray diffraction (XRD) results, all films exhibited a randomly oriented perovskite phase. Concerning surface morphology, the grain boundaries of the Mn-doped (0.2 wt%) BLT thin film are more closely packed than those of the nondoped BLT film without the presence of many crevices and cracks around the grain boundaries due to the volatilization of the Bi components at high temperature. The remanent polarization of the stoichiometric, 0.1 wt%, 0.2 wt% and 0.3 wt% Mn-doped BLT films annealed at a heating rate of 50°C/h, were 14 µC/cm2, 18 µC/cm2, 31 µC/cm2 and 13 µC/cm2, respectively. The 0.2 wt% Mn-doped BLT thin film is a good candidate for ferroelectric memory cells and nonvolatile random access memory (NvRAM) applications.