Jihoon Ahn

Sub-0.5 nm equivalent oxide thickness scaling for Si-doped Zr1-xHfxO2 thin film without using noble metal electrode.

The dielectric properties of the Si-doped Zr1-xHfxO2 thin films were investigated over a broad compositional range with the goal of improving their properties for use as DRAM capacitor materials. The Si-doped Zr1-xHfxO2 thin films were deposited on TiN bottom electrodes by atomic layer deposition using a TEMA-Zr/TEMA-Hf mixture precursor for deposition of Zr1-xHfxO2 film and Tris-EMASiH as a Si precursor. The Si stabilizer increased the tetragonality and the dielectric constant; however, at high fractions of Si, the crystal structure degraded to amorphous and the dielectric constant decreased. Doping with Si exhibited a larger influence on the dielectric constant at higher Hf content. A Si-doped Hf-rich Zr1-xHfxO2 thin film, with tetragonal structure, exhibited a dielectric constant of about 50. This is the highest value among all reported results for Zr and Hf oxide systems, and equivalent oxide thickness (EOT) value of under 0.5 nm could be obtained with a leakage current of under 10(-7) A·cm(-2), which is the lowest EOT value ever reported for a DRAM storage capacitor system without using a noble-metal-based electrode.

A new class of chiral semiconductors: chiral-organic-molecule-incorporating organic–inorganic hybrid perovskites

An organic–inorganic hybrid perovskite incorporating chiral organic molecules is demonstrated as a new class of chiral semiconductors. Chiral perovskites exhibit oppositely-signed circular dichroism (CD) according to the S- and R-configurations of chiral organics. The CD signals can be also varied by changing the crystalline orientation and thickness of the chiral perovskite films.

Thermally driven in situ exsolution of Ni nanoparticles from (Ni, Gd)CeO2 for high-performance solid oxide fuel cells

In situ exsolution of metal nanoparticles affords a high content and uniform distribution of metal nanocatalysts without complex synthetic processes. To implement this strategy in practical electrodes for solid oxide fuel cells, understanding the exsolution process in terms of synthesis temperature and atmosphere is a prerequisite. Herein, we demonstrate that Ni:Gd co-doped ceria (GNDC) can be effectively used as an in situ exsolution system, from which substitutionally doped Ni is thermally exsolved as NiO nanoparticles strongly attached to the surface of GNDC, the host oxide, and subsequently reduced to a Ni nanocatalyst under anodic operation conditions. The exsolution procedures were characterized by X-ray diffraction, Raman spectroscopy, and transmission electron spectroscopy, which revealed that the evolution of Ni nanoparticles could be solely controlled by thermal treatment. The thermally exsolved Ni nanocatalyst from the 5 mol% Ni-doped GNDC electrode exhibits a polarization resistance comparable to that of the mechanically mixed Ni-GDC composite electrode, with a significant increase in the calculated triple phase boundary density despite having a low Ni volume fraction. By employing the GNDC layer as a functional layer of an anode-supported SOFC, we demonstrated the utilization of the thermally exsolved Ni nanocatalyst combined with fluorite-structured doped-ceria as an electrode of SOFCs at low temperature.

Investigating Recombination and Charge Carrier Dynamics in a One-Dimensional Nanopillared Perovskite Absorber

Organometal halide perovskite materials have become an exciting research topic as manifested by intense development of thin film solar cells. Although high-performance solar-cell-based planar and mesoscopic configurations have been reported, one-dimensional (1-D) nanostructured perovskite solar cells are rarely investigated despite their expected promising optoelectrical properties, such as enhanced charge transport/extraction. Herein, we have analyzed the 1-D nanostructure effects of organometal halide perovskite (CH3NH3PbI3- xCl x) on recombination and charge carrier dynamics by utilizing a nanoporous anodized alumina oxide scaffold to fabricate a vertically aligned 1-D nanopillared array with controllable diameters. It was observed that the 1-D perovskite exhibits faster charge transport/extraction characteristics, lower defect density, and lower bulk resistance than the planar counterpart. As the aspect ratio increases in the 1-D structures, in addition, the charge transport/extraction rate is enhanced and the resistance further decreases. However, when the aspect ratio reaches 6.67 (diameter ∼30 nm), the recombination rate is aggravated due to high interface-to-volume ratio-induced defect generation. To obtain the full benefits of 1-D perovskite nanostructuring, our study provides a design rule to choose the appropriate aspect ratio of 1-D perovskite structures for improved photovoltaic and other optoelectrical applications.

Facile Sol–Gel-Derived Craterlike Dual-Functioning TiO2 Electron Transport Layer for High-Efficiency Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) are considered promising materials for low-cost solar energy harvesting technology. An electron transport layer (ETL), which facilitates the extraction of photogenerated electrons and their transport to the electrodes, is a key component in planar PSCs. In this study, a new strategy to concurrently manipulate the electrical and optical properties of ETLs to improve the performance of PSCs is demonstrated. A careful control over the Ti alkoxide-based sol-gel chemistry leads to a craterlike porous/blocking bilayer TiO2 ETL with relatively uniform surface pores of 220 nm diameter. Additionally, the phase separation promoter added to the precursor solution enables nitrogen doping in the TiO2 lattice, thus generating oxygen vacancies. The craterlike surface morphology allows for better light transmission because of reduced reflection, and the electrically conductive craterlike bilayer ETL enhances charge extraction and transport. Through these synergetic improvements in both optical and electrical properties, the power conversion efficiency of craterlike bilayer TiO2 ETL-based PSCs could be increased from 13.7 to 16.0% as compared to conventional dense TiO2-based PSCs.