Yong Chan Choi

Nanostructured TiO2/CH3NH3PbI3 heterojunction solar cells employing spiro-OMeTAD/Co-complex as hole-transporting material

For using 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as a hole conductor in solar cells, it is necessary to improve its charge-transport properties through electrochemical doping. With the aim of fabricating efficient mesoscopic TiO2/CH3NH3PbI3 heterojunction solar cells, we used tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tris(bis(trifluoromethylsulfonyl) imide)] (FK209) as a p-dopant for spiro-OMeTAD. The mixture of spiro-OMeTAD, FK209, lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and 4-tert-butylpyridine (TBP) exhibited significantly higher performance than mixtures of pristine spiro-OMeTAD, spiro-OMeTAD, and FK209, and spiro-OMeTAD, Li-TFSI, and TBP. Such a synergistic effect between the Co-complex and Li-TFSI in conjunction with spiro-OMeTAD effectively improved the power conversion efficiency (PCE) of the fabricated solar cells. As a result, we achieved PCE of 10.4%, measured under standard solar conditions (AM 1.5G, 100 mW cm−2).

Ferroelectricity in highly ordered arrays of ultra-thin-walled Pb(Zr,Ti)O3 nanotubes composed of nanometer-sized perovskite crystallites.

We report the first unambiguous ferroelectric properties of ultra-thin-walled Pb(Zr,Ti)O 3 (PZT) nanotube arrays, each with 5 nm thick walls and outer diameters of 50 nm. Ferroelectric switching behavior with well-saturated hysteresis loops is observed in these ferroelectric PZT nanotubes with P r and E c values of about 1.5 microC cm (-2) and 86 kV cm (-1), respectively, for a maximum applied electric field of 400 kV cm (-1). These PZT nanotube arrays (10 (12) nanotubes cm (-2)) might provide a competitive approach toward the development of three-dimensional capacitors for the terabyte ferroelectric random access memory.

Direct observation of alumina nanowire formation from porous anodic alumina membrane via the droplet etching method

We report the first direct observation of the formation process of an alumina nanowire (ANW) array from a porous anodic alumina (PAA) membrane employing a simple droplet etching method, not a general immersion etching method, using field emission scanning electron spectroscopy (FESEM). The PAA membrane is prepared by the two-step anodization method and subsequent lift-off process. The surface view SEM images of the PAA membrane show an array of highly ordered hexagonal pore distribution within the domains of 1–2 µm size, which are separated from neighbouring domains with different orientations of the pore lattice by grain boundaries. The FESEM analysis of the formation process of the alumina nanowire reveals that the formation process consists of three critical steps. The first step involves an image revealing that the outer parts of pore walls are dissolved. The image can be described by the different etching rates of oxide layers in the PAA membrane. Differences in the etching rate can be explained in terms of the different anion impurity concentration in the oxide layer of the PAA membrane. The second step produces an image of the PAA membrane with triangular triple points, surrounded by three adjacent pores. This image can be interpreted as the combined effects of the thickness difference of pore walls and the thermal annealing of the PAA membrane. The last step shows images where the triangular cross section ANWs transform into circular cross section ones as further etching occurs. This can result from the crystallization of the triple points during the annealing process.

Antisolvent-assisted powder engineering for controlled growth of hybrid CH3NH3PbI3 perovskite thin films

We develop antisolvent-assisted powder engineering for the controlled growth of hybrid inorganic-organic CH3NH3PbI3 (MAPbI3) perovskite thin films. The powders, which are used as the precursors for solution processing, are synthesized by pouring a MAPbI3 precursor solution into various antisolvents, such as dichloromethane, chloroform, diethyl ether, and toluene. Two types of powders having different colors are obtained, depending on the antisolvent used. The choice of the antisolvent used for synthesizing the powders strongly influences not only the phases of the powders but also the morphology and structure of the thin films subsequently fabricated by solution processing. This, in turn, affects the photovoltaic performance.

Effects of Wall Thickness on Morphology and Structure of Lead Titanate Nanotubes

We report the effects of wall-thickness on the morphology and structure of PbTiO3 nanotubes (PTO-NTs). The PTO-NTs were fabricated via template-directed growth in conjunction with sol-gel process and spin-coating technique. Wall-thickness was increased from 10 to 70 nm by repeating the process of ‘spin-coating and subsequent pyrolysis’. X-ray diffraction patterns showed that the PTO-NTs exhibited a tetragonal-structure. We found that their tetragonality increased from 1.0310 to 1.0487 as wall-thickness increased from 10 nm to 70 nm. Raman spectroscopy revealed that peak positions of E(1TO) phonon corresponding a-axis shifted toward higher frequency due to their residual stress in the PTO-NTs.

THE DEPENDENCE OF ALUMINA NANOWIRE FORMATION FROM POROUS ANODIC ALUMINA MEMBRANES ON THE ETCHING SOLUTION

We describe variations in the structure and formation mechanism of alumina nanowires (ANWs) formed from porous anodic alumina (PAA) membranes, depending on the composition of the etching solution. Two types of ANWs were synthesized by etching PAA membranes in etching solutions containing H3PO4 or NaOH. Not only did the ANW formation mechanism vary, but also the morphological properties and the surface chemical composition of the ANWs varied with the etching solution used. These results suggest that the ANW surfaces may be optimized by the appropriate selection of etching solution. The optimization of ANWs may potentially advance, for example, water purification technological developments because ANWs have, in recent studies, provided more effective filtration performances than nanoporous alumina membrane filters.

Synthesis and Characterization of Lead Zirconate Titanate Nanotubes

We report a template-directed growth of Pb(Zr x ,Ti1 − x )O3 (PZT) nanotubes in conjunction with sol-gel process and spin coating technique. Field emission scanning electron microscope (FESEM) images show that PZT nanotubes are well packed inside the pore of a porous alumina membrane (PAM). A Field emission transmission electron microscope (FETEM) image of a freestanding PZT nanotube shows that its outer diameter and length are estimated to be 50 nm and several microns, respectively. High resolution FETEM analysis reveals that PZT nanotubes have a wall thickness of 5 nm and consist of crystallites with a size in the order of 2–3 nm. The occurrence of electron diffraction ring patterns confirms that the typical PZT nanotube is polycrystalline.

Controlled growth of organic–inorganic hybrid CH3NH3PbI3 perovskite thin films from phase-controlled crystalline powders

High-quality CH3NH3PbI3 (MAPbI3) crystalline powders were synthesized from a dispersion of MAPbI3 solution (solvent: N,N-dimethylformamide (DMF)) in the antisolvent dichloromethane. They were used as starting chemicals for precursor solutions. The PbI2–DMF-, perovskite-, and PbI2–MAI (methylammonium iodide)–DMF-dominant phases were preferentially formed under conditions of excess PbI2, moderate, and high-excess MAI input ratios, respectively. The input ratio of powders in fabricating the thin MAPbI3 films strongly affected not only the morphology and structure of the films, but also the photovoltaic performance of the devices using them. The devices were constructed as follows: Au/hole-transporting material/MAPbI3/mesoporous TiO2 layer/TiO2 blocking layer/F-doped SnO2. The best device performance was obtained from the powder with a specific ratio of PbI2 : MAI = 1 : 1.6; the device exhibited a power conversion efficiency of ∼16% at 1.5G standard conditions. Our proposed method could provide a simple and versatile solution-based approach for a low-cost perovskite solar cell fabrication technology.

Lead Zirconate Titanate Nanowire Growth Via Spin Coating in Conjunction with Sol-Gel Process

Lead zirconate titanate (PZT) nanowires are successfully synthesized by a spin coating of sol-gel solution on a highly ordered porous alumina membrane (PAM). Field emission scanning electron microscope images show that PZT nanowires are well packed inside the nanochannels of the PAM. A selected area electron diffraction pattern reveals that they are polycrystalline. They exhibit a high aspect-ratio (up to 400) with diameters of ∼ 50 nm and lengths of ∼ 20 μ m. Our experiment suggests that a vortex motion induced by the spin coating plays an important role for the sol to enter the pores of membrane in order to achieve quality PZT nanowires. These results indicate that the spin coating of sol-gel solution on a PAM can be a simple, highly reproducible, and timesaving method to prepare functional complex oxide nanowires with high aspect-ratio.