Jung-Ho Yun

Resistive Switching Behavior in Organic–Inorganic Hybrid CH3NH3PbI3−xClx Perovskite for Resistive Random Access Memory Devices

The CH3 NH3 PbI3- x Clx organic-inorganic hybrid perovskite material demonstrates remarkable resistive switching behavior, which can be applicable in resistive random access memory devices. The simply designed Au/CH3 NH3 PbI3- x Clx /FTO structure is fabricated by a low-temperature, solution-processable method, which exhibits remarkable bipolar resistive switching and nonvolatile properties.

Stable Hematite Nanosheet Photoanodes for Enhanced Photoelectrochemical Water Splitting

A vertically grown hematite nanosheet film modified with Ag nanoparticles (NPs) and Co-Pi cocatalyst exhibits a remarkably high photocurrent density of 4.68 mA cm(-2) at 1.23 V versus RHE. The Ag NPs leads to significantly improved light harvesting and better charge transfer, while the Co-Pi facilitates a highly stable oxygen evolution process. This photoelectrode design provides more efficient photoelectrochemical systems for solar-energy conversion.

Stable and Low‐Cost Mesoscopic CH3NH3PbI2Br Perovskite Solar Cells by using a Thin Poly(3‐hexylthiophene) Layer as a Hole Transporter

Mesoscopic perovskite solar cells using stable CH3 NH3 PbI2 Br as a light absorber and low-cost poly(3-hexylthiophene) (P3HT) as hole-transporting layer were fabricated, and a power conversion efficiency of 6.64 % was achieved. The partial substitution of iodine with bromine in the perovskite led to remarkably prolonged charge carrier lifetime. Meanwhile, the replacement of conventional thick spiro-MeOTAD layer with a thin P3HT layer has significantly reduced the fabrication cost. The solar cells retained their photovoltaic performance well when they were exposed to air without any encapsulation, presenting a favorable stability. The combination of CH3 NH3 PbI2 Br and P3HT may render a practical and cost-effective solid-state photovoltaic system. The superior stability of CH3 NH3 PbI2 Br is also promising for other photoconversion applications.

Wrapping the walls of n-TiO2 nanotubes with p-CuInS2 nanoparticles using pulsed-electrodeposition for improved heterojunction photoelectrodes

The CuInS(2) (CIS) nanoparticles were wrapped uniformly throughout the inner and outer walls of TNTs (TNT) by using square wave pulsed-electrodeposition. This structure enables the CuInS(2)-TiO(2) (CIS-TNT) to exhibit p-n junction diode behavior and enhanced photoelectrochemical properties.

Combined electrophoretic deposition-anodization method to fabricate reduced graphene oxide-TiO2 nanotube films

We directly constructed reduced graphene oxide–titanium oxide nanotube (RGO–TNT) film using a single-step, combined electrophoretic deposition–anodization (CEPDA) method. This method, based on the simultaneous anodic growth of tubular TiO2 and the electrophoretic-driven motion of RGO, allowed the formation of an effective interface between the two components, thus improving the electron transfer kinetics. Composites of these graphitic carbons with different levels of oxygen-containing groups, electron conductivity and interface reaction time were investigated; a fine balance of these parameters was achieved.

An Electrochemically Treated BiVO4 Photoanode for Efficient Photoelectrochemical Water Splitting

BiVO4 films with (040) facet grown vertically on fluorine doped SnO2 (FTO) glass substrates are prepared by a seed-assisted hydrothermal method. A simple electrochemical treatment process drastically enhances the photocatalytic activity of BiVO4 , exhibiting a remarkable photocurrent density of 2.5 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE) under AM 1.5 G illumination, which is approximately 10-fold higher than that of the pristine photoanode. Loading cobalt borate (CoBi) as cocatalyst, the photocurrent density of the BiVO4 photoanode can be further improved to 3.2 mA cm-2 , delivering an applied bias photon-to-current efficiency (ABPE) of 1.1 %. Systematic studies reveal that crystal facet orientation also synergistically boosts both charge separation and transfer efficiencies, resulting in remarkably enhanced photocurrent densities. These findings provide a facile and effective approach for the development of efficient photoelectrodes for photoelectrochemical water splitting.

Bifunctional resistive switching behavior in an organolead halide perovskite based Ag/CH3NH3PbI3−xClx/FTO structure

Organolead halide perovskite materials open up a new era for developing low-cost and high efficiency solar cells due to their simple and inexpensive fabrication process, superior light absorption coefficient, and excellent charge mobility. In addition to solar cells, hybrid perovskites have also seen dynamic advances with rapidly expanded applications to many other exciting fields including electronic and optical devices. Here, we demonstrate a new type of bifunctional resistive switching memory device based on a very simple bilayer structure of Ag and a CH3NH3PbI3−xClx perovskite material on an FTO substrate with both digital and analog resistive switching characteristics. The bi-stable resistive switching behavior with reliable endurance over 103 times and a retention time of 4 × 104 s demonstrates that the Ag/CH3NH3PbI3−xClx/FTO device can be a promising candidate for RRAM. In the low voltage sweeping region, surprisingly, analog resistive switching behavior with potentiation and depression characteristics was also observed, which can be useful in neuromorphic computing device applications. The possible Ag conducting filaments formed by redox reactions of the Ag electrode may play a key role in this newly observed resistive switching phenomenon.

Transition from the Tetragonal to Cubic Phase of Organohalide Perovskite: The Role of Chlorine in Crystal Formation of CH3NH3PbI3 on TiO2 Substrates

The role of chlorine in the superior electronic property and photovoltaic performance of CH3NH3PbI(3-x)Clx perovskite has attracted recent research attention. Here, we study the impact of chlorine in the perspective of the crystal structure of the perovskite layer, which can provide important understanding of its excellent charge mobility and extended lifetimes. In particular, we find that in the presence of chlorine (PbCl2 or CH3NH3Cl), when CH3NH3PbI3 films are deposited on a TiO2 mesoporous layer instead of a planar TiO2 substrate, a stable cubic phase rather than the commonly observed tetragonal phase is formed in CH3NH3PbI3 perovskite at room temperature. The relative peak intensity of two major facets of cubic CH3NH3PbI3 crystals, (100)C and (200)C facets, can also be easily tuned, depending on the film thickness. Furthermore, compared with pristine CH3NH3PbI3 perovskite films, in the presence of chlorine, CH3NH3PbI3 crystals grown on planar substrates exhibit strong preferred orientations on (110)T and (220)T facets.

Sodium Fluoride-Assisted Modulation of Anodized TiO2 Nanotube for Dye-Sensitized Solar Cells Application

This work reports the use of sodium fluoride (in ethylene glycol electrolyte) as the replacement of hydrofluoric acid and ammonium fluoride to fabricate long and perpendicularly well-aligned TiO₂ nanotube (TNT) (up to 21 μm) using anodization. Anodizing duration, applied voltage and electrolyte composition influenced the geometry and surface morphologies of TNT. The growth mechanism of TNT is interpreted by analyzing the current transient profile and the total charge density generated during anodization. The system with low water content (2 wt %) yielded a membrane-like mesoporous TiO₂ film, whereas high anodizing voltage (70 V) resulted in the unstable film of TNT arrays. An optimized condition using 5 wt % water content and 60 V of anodizing voltage gave a stable array of nanotube with controllable length and pore diameter. Upon photoexcitation, TNTs synthesized under this condition exhibited a slower charge recombination rate as nanotube length increased. When made into cis-diisothiocyanato-bis(2,2̀-bipyridyl-4,4̀-dicarboxylato) ruthenium(II) bis (tetrabutyl-ammonium)(N719) dye-sensitized solar cells, good device efficiency at 3.33 % based on the optimized TNT arrays was achieved with longer electron time compared with most mesoporous TiO₂ films.

A hybrid photoelectrode with plasmonic Au@TiO2 nanoparticles for enhanced photoelectrochemical water splitting

Herein we present a new p–n heterojunction photoelectrode design utilizing Cu2O and TiO2-P25 loaded with only 1 wt% Au@TiO2 plasmonic core–shell structure for the photoelectrochemical (PEC) process. It is found that the photoelectrode with a sandwich-like layer design, i.e. TiO2-1 wt% Au@TiO2/Al2O3/Cu2O, not only promotes light harvesting but also improves charge carrier separation, resulting in drastically improved photocurrent density up to −4.34 mA cm−2 at −0.2 V vs. Ag/AgCl under simulated AM 1.5 solar illumination, ∼20 fold enhancement compared to that obtained from a TiO2-P25/Cu2O photoelectrode. It is also revealed that the photoelectrochemical performance is closely related to the Au nanoparticle size, induced by surface-enhanced scattering of the plasmonic nanoparticles. It should also be noted that these multilayer photoelectrodes also exhibit high stability through rational design of the PEC system to avoid the photocorrosion on Cu2O layers.