Jongnam Park

Inverted Colloidal Quantum Dot Solar Cells

An inverted architecture of quantum dot solar cells is demonstrated by introducing a novel ZnO method on top of the PbS CQD film. Improvements in device characteristics stem from constructive optical interference from the ZnO layer that enhances absorption in the PbS CQD layer. Outstanding diode characteristics arising from a superior PbS/ZnO junction provide a further electronic advantage.

Ordered Mesoporous Carbon Supported Colloidal Pd Nanoparticle Based Model Catalysts for Suzuki Coupling Reactions: Impact of Organic Capping Agents

Recent advances in the field of nanoscience have enabled the preparation of high‐surface‐area supported catalysts with precise control over the individual structural components. As such, a range of factors that affect the catalytic reactivity, such as the size, shape, and composition of the nanoparticles (NPs), have been identified. Herein, high‐surface‐area model catalysts that were based on colloidal Pd NPs and a hexagonally ordered mesoporous carbon support were prepared and the impact of various organic capping agents for the Pd NPs on their catalytic activity towards Suzuki coupling reactions was investigated. Colloidal Pd NPs (diameter: 3 nm) were synthesized with different organic capping agents, oleylamine (OA) and trioctylphosphine (TOP), and they were subsequently incorporated into the mesopores of CMK‐3 mesoporous carbon to yield OA‐Pd/CMK‐3 and TOP‐Pd/CMK‐3 nanocatalysts, respectively. The OA‐Pd/CMK‐3 catalyst was treated with acetic acid to generate a supported catalyst with surfactant‐free Pd NPs (OA‐Pd/CMK‐3‐A). Structural characterization revealed that the Pd NPs were uniformly dispersed throughout the mesopores of the CMK‐3 support and the particle size and crystallinity of the Pd NPs were preserved following the incorporation. All of the Pd/CMK‐3 nanocatalysts exhibited higher activity than commercial activated carbon supported Pd catalysts in Suzuki coupling reactions. The catalytic activities of the three Pd/CMK‐3 nanocatalysts were in the following order: OA‐Pd/CMK‐3‐A>OA‐Pd/CMK‐3>TOP‐Pd/CMK‐3. This result suggested that the presence and type of surfactants had a significant effect on the catalytic activity. The OA‐Pd/CMK‐3‐A catalyst also showed high activity for various substrates and good recycling ability in Suzuki coupling reactions.

All-solid-state lithium-ion batteries with TiS2 nanosheets and sulphide solid electrolytes

Bulk-type all-solid-state lithium-ion batteries (ASLBs) using sulphide solid electrolytes (SEs) are considered as one of the promising alternative batteries because of their ultimate safety and scalable fabrication. However, they suffer from poor ionic contacts between active materials and SEs. Herein, we report, for the first time, the excellent electrochemical performances of sulphide-SE-based bulk-type ASLBs employing TiS2 nanosheets (TiS2-NSs) prepared by scalable mechanochemical lithiation, followed by exfoliation in water under ultrasonication. The TiS2-NS in all-solid-state cells exhibits an enhancement of reversible capacity which is attributed to the SE region in intimate contact with TiS2-NSs. Importantly, an exceptionally superior rate capability of the TiS2-NS compared to that of bulk TiS2 and even ball-milled TiS2, which is attributed to the ultrathin 2D structure (with short Li-ion diffusion length and intimate contacts between the TiS2-NS and SE) and high electronic conductivity, is highlighted.

Effects of Ionic Liquid Molecules in Hybrid PbS Quantum Dot–Organic Solar Cells

We investigated the effect of ionic liquid molecules (ILMs) in hybrid quantum dot-organic solar cells (HyQD-OSCs). The insertion of an ILM layer between PbS and phenyl-C61-butyric acid methyl ester (PCBM) can shift the band edge of PCBM closer to the vacuum level of PbS due to spontaneous dipole polarization. Because of this new architecture, improvements in device performance were achieved, including increases in open-circuit voltage (VOC, from 0.41 V to 0.49 V), fill factor (FF, from 0.48 to 0.59), and power conversion efficiency (PCE, from 1.62% to 2.21%), compared to reference devices under AM 1.5G illumination at 100 mW cm(-2). We observed that treatment of the PbS layer with ILMs causes a significant increase in work function from 3.58 eV to 3.93 eV. Furthermore, the ILMs layer minimizes the contact resistance between PbS and PCBM due to the improved compatibility between the two layers, confirmed as a decrease in charge transfer resistance, as measured by electrical impedance spectroscopy.

A new polymeric binder for silicon-carbon nanotube composites in lithium ion battery

AbstractWe introduced polyethyleneimine (PEI) as a new binder for silicon (Si)-carbon nanotube (CNT) anode materials in lithium ion batteries (LIBs). The PEI binder was chosen to enhance the binding of electrode material containing Si-CNT nanocomposites through the formation of a PEI thin layer on the surfaces of CNTs. It was expected that the spontaneous electrostatic interactions between weakly charged PEI molecules with CNT surfaces could promote the binding performance. In other words, the formation of solid-electrolyte interface (SEI) could be suppressed owing to the effect of dominant electrostatic interactions between PEIs and CNTs. Zeta potential analyses demonstrated the real presence of electrostatic interactions between PEIs and CNTs. Accordingly, lithium battery half-cell tests showed that improved capacity retention behavior was observed in the sample with PEI than that with polyvinyldifluoride (PVDF) binder. Remarkably, for the case of Si-CNT anode materials prepared without or with relatively less amount of CNT, a higher reduction in capacity was observed with PEI binder than with PVDF. An additional advantage of the incorporation of PEI binder is an increase of initial coulombic efficiency approximately 5%∼10%. Consequently, all these findings support that PEI is highly desirable as an alternative binder for electrode materials containing CNT.

High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Cobalt Ferrite Nanoparticles

Nano-floating gate memory (NFGM) devices are transistor-type memory devices that use nanostructured materials as charge trap sites. They have recently attracted a great deal of attention due to their excellent performance, capability for multilevel programming, and suitability as platforms for integrated circuits. Herein, novel NFGM devices have been fabricated using semiconducting cobalt ferrite (CoFe2O4) nanoparticles (NPs) as charge trap sites and pentacene as a p-type semiconductor. Monodisperse CoFe2O4 NPs with different diameters have been synthesized by thermal decomposition and embedded in NFGM devices. The particle size effects on the memory performance have been investigated in terms of energy levels and particle-particle interactions. CoFe2O4 NP-based memory devices exhibit a large memory window (≈73.84 V), a high read current on/off ratio (read I(on)/I(off)) of ≈2.98 × 10(3), and excellent data retention. Fast switching behaviors are observed due to the exceptional charge trapping/release capability of CoFe2O4 NPs surrounded by the oleate layer, which acts as an alternative tunneling dielectric layer and simplifies the device fabrication process. Furthermore, the NFGM devices show excellent thermal stability, and flexible memory devices fabricated on plastic substrates exhibit remarkable mechanical and electrical stability. This study demonstrates a viable means of fabricating highly flexible, high-performance organic memory devices.

Anomalous X-ray reflectivity study of metal oxide thin films

Anomalous X-ray reflectivity measurements have been performed to extract electron density profile as a function of depth. Using a model independent analysis scheme based on distorted wave Born approximation, we have demonstrated that element-specific density profiles in a film can be obtained from reflectivity measurements done at two different X-ray energies, one at an absorption edge of the corresponding metal and another one away from it. The merit of this technique has been demonstrated with the results on high dielectric constant metal oxide Ta 2 O 5 films on Si(001). Our results show different Ta profiles near interfaces for Ta 2 O 5 /Si interface and Ta 2 O 5 /SiO 2 interface, implying different kinetics at these interfaces during annealing process.

Solution-processed CdS transistors with high electron mobility

Solution-processed CdS field effect transistors (FETs) and solar cells are demonstrated via spin-coating and thermal annealing of soluble cadmium thiolate compounds. The synthesis is carried out in one simple step using cadmium oxide and tertiary alkane thiols. The cadmium thiolates are soluble in organic solvents such as chloroform and may be spin-coated, like organic semiconductors, to form thin films. The cadmium thiolate films decompose rapidly at 300 °C to yield semiconducting cadmium sulfide films. FETs are easily fabricated using these films and exhibit electron mobilities of up to 61 cm2 V−1 s−1, which compare favourably to FETs prepared from other solution-processed materials such as organic semiconductors, inorganic nanoparticles or chalcogenide films. Initial attempts to prepare hybrid bilayer solar cells were successfully realized by spin-coating a p-type semiconducting polymer layer on top of the n-type CdS film. These devices show significant photocurrent response from both the CdS and polymer layers, indicating that the CdS films are able to participate in photo-induced electron transfer from the polymer to the CdS layer as well as photo-induced hole transfer from CdS to the polymer layer.

Photon energy transfer by quantum dots in organic–inorganic hybrid solar cells through FRET

Organic–inorganic hybrid solar cells were fabricated with InP QDs (5 wt%) in a BHJ active layer (PTB7 + PC71BM). InP QDs were distributed on the top of the hybrid active layer (BHJ + QDs), determined through XPS depth profiling and AFM analyses. InP QDs showed strong emission characteristics at λmax = 650 nm in the PL spectra. They played an important role in increasing Jsc by Forster resonance energy transfer (FRET) to PTB7. The carrier recombination of the hybrid active layer (BHJ + QDs) was reduced by morphology control through introducing a PFN interlayer. The carrier mobility of the device with the hybrid active layer (BHJ + QDs) increased 1.5–1.58 times over that of the device with the BHJ active layer. By means of the synergy effect of InP QDs and the PFN interlayer, the PCE of the fabricated hybrid solar cells was enhanced from 4.9% (Jsc = 13.2 mA cm−2, FF = 60.0%) to 8.4% (Jsc = 14.5 mA cm−2, FF = 72.5%).

Surface engineered gold nanoparticles through highly stable metal–surfactant complexes

Monodispersed Au nanoparticles were synthesized by the reduction of Au-decyltrimethylammonium bromide (Au-DTAB), which was easily prepared via the reaction of HAuCl4 and DTAB. This Au-DTAB complex is highly stable in air and moisture, and suitable for large-scale synthesis of uniform-sized Au nanoparticles. The nanoparticles were characterized by transmission electron microscopy, optical absorption spectrometry, X-ray diffraction, and Fourier Transform infrared spectroscopy. The size of Au nanoparticles was controlled in the range of 5-10nm by changing the concentrations of reducing agent and Au precursor. The resulting Au nanoparticles were transferred to the aqueous phase after surface engineering using multidentate polymeric ligands with multiple imidazole functional groups. Polymeric imidazole ligands (PILs) demonstrated enhanced binding stability with the Au surface, and overcame the disadvantage of multidentate thiol ligand systems which have oxidative cross-linking and the formation of disulfide bonding. The colloidal stability of surface engineered Au nanoparticles with PILs was investigated by dynamic light scattering (DLS) characterization.