Yongsoo Jeong

Charge-transfer-based Gas Sensing Using Atomic-layer MoS2

Two-dimensional (2D) molybdenum disulphide (MoS2) atomic layers have a strong potential to be used as 2D electronic sensor components. However, intrinsic synthesis challenges have made this task difficult. In addition, the detection mechanisms for gas molecules are not fully understood. Here, we report a high-performance gas sensor constructed using atomic-layered MoS2 synthesised by chemical vapour deposition (CVD). A highly sensitive and selective gas sensor based on the CVD-synthesised MoS2 was developed. In situ photoluminescence characterisation revealed the charge transfer mechanism between the gas molecules and MoS2, which was validated by theoretical calculations. First-principles density functional theory calculations indicated that NO2 and NH3 molecules have negative adsorption energies (i.e., the adsorption processes are exothermic). Thus, NO2 and NH3 molecules are likely to adsorb onto the surface of the MoS2. The in situ PL characterisation of the changes in the peaks corresponding to charged trions and neutral excitons via gas adsorption processes was used to elucidate the mechanisms of charge transfer between the MoS2 and the gas molecules.

Template-based carbon nanotubes and their application to a field emitter

Anodic aluminum oxide (AAO) templates were fabricated by anodizing Al films. After the Co catalyst had been electrochemically deposited into the bottom of the AAO template, carbon nanotubes (CNTs) were grown by the catalytic pyrolysis of C2H2 at 650 °C. Overgrowth of CNTs on the AAO templates was observed. The diameter of the CNTs strongly depends on the size of the pores in the AAO template. The electron field emission measurements on the samples showed a turn-on field of 1.9–2.1 V/μm and a field enhancement factor of 3360–5200. Our observation concerning the low turn-on field and high field enhancement factors is explained in terms of a low field screening effect.

Ultrasmooth, extremely deformable and shape recoverable Ag nanowire embedded transparent electrode.

Transparent electrodes have been widely used in electronic devices such as solar cells, displays, and touch screens. Highly flexible transparent electrodes are especially desired for the development of next generation flexible electronic devices. Although indium tin oxide (ITO) is the most commonly used material for the fabrication of transparent electrodes, its brittleness and growing cost limit its utility for flexible electronic devices. Therefore, the need for new transparent conductive materials with superior mechanical properties is clear and urgent. Ag nanowire (AgNW) has been attracting increasing attention because of its effective combination of electrical and optical properties. However, it still suffers from several drawbacks, including large surface roughness, instability against oxidation and moisture, and poor adhesion to substrates. These issues need to be addressed before wide spread use of metallic NW as transparent electrodes can be realized. In this study, we demonstrated the fabrication of a flexible transparent electrode with superior mechanical, electrical and optical properties by embedding a AgNW film into a transparent polymer matrix. This technique can produce electrodes with an ultrasmooth and extremely deformable transparent electrode that have sheet resistance and transmittance comparable to those of an ITO electrode.

Graphene-based gas sensor: metal decoration effect and application to a flexible device

Roles of metal nanoparticles (NPs) on graphene-based devices were investigated in terms of gas-sensing characteristics of NO2 and NH3, and flexible gas sensing was also realized for future applications. The synergistic combination of metal NPs and graphene modulates the electronic properties of graphene, leading to enhancement of selectivity and sensitivity in gas-sensing characteristics. Introduction of palladium (Pd) NPs on the graphene accumulates hole carriers of graphene, resulting in the gas sensor being sensitized by NH3 gas molecular adsorption. In contrast, aluminum (Al) NPs deplete hole carriers, which dramatically improves NO2 sensitivity. Furthermore, the sensitivity of flexible graphene-based gas sensors was also enhanced via the same approach, even after 104 bending cycles and was maintained after 3 months.

Towards fabrication of high-performing organic photovoltaics: new donor-polymer, atomic layer deposited thin buffer layer and plasmonic effects

Using a novel polymer (polythienothiophene-co-benzodithiophenes 7 F-20) as a donor and phenyl-C71-butyric acid methyl ester as an acceptor of bulk heterojunction, inverted organic photovoltaics (OPVs) were fabricated. Wet-chemically prepared ZnO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were used as buffer layers. Particularly, for PEDOT:PSS deposition, no annealing step was employed. This inverted OPV showed a power conversion efficiency (PCE) of ∼7.0%, which is comparable to the hitherto reported highest efficiency of the inverted OPV with vacuum-deposited MoO3 as hole-collecting buffer layers without plasmonic enhancements. Incorporation of Au nanoparticles into PEDOT:PSS was performed for plasmonic enhancement of the electromagnetic field, whereas ZnO thin layers were deposited on ZnO using atomic layer deposition for quenching electron–hole recombination at surface defects of ZnO ripples. These additional treatments could be used for improving the performance of OPV, which ultimately resulted in a PCE of 7.9%.

High Efficiency Inorganic/Organic Hybrid Tandem Solar Cells

Hybrid tandem solar cells comprising an inorganic bottom cell and an organic top cell have been designed and fabricated. The interlayer combination and thickness matching were optimized in order to increase the overall photovoltaic conversion efficiency. A maximum power conversion efficiency of 5.72% was achieved along with a V(oc) of 1.42 V, reaching as high as 92% of the sum of the subcell V(oc) values.

Influence of surface roughness of aluminum-doped zinc oxide buffer layers on the performance of inverted organic solar cells

Aluminum-doped zinc oxide (AZO) films (70 nm thick) with dissimilar surface roughness were created on indium tin oxide coated glass and were used as electrodes for inverted organic solar cells. The photovoltaic performance of the devices depended strongly on the surface roughness of the AZO films. Increases in the surface root-mean-square roughness of AZO films from 2.5 to 10.9 nm enhanced power conversion efficiency from 0.5% to 1.4% due to increased contact area between electrode and active layer.

Surface Modification of a ZnO Electron-Collecting Layer Using Atomic Layer Deposition to Fabricate High-Performing Inverted Organic Photovoltaics

A ripple-structured ZnO film as the electron-collecting layer (ECL) of an inverted organic photovoltaic (OPV) was modified by atomic layer deposition (ALD) to add a ZnO thin layer. Depositing a thin ZnO layer by ALD on wet-chemically prepared ZnO significantly increased the short-circuit current (Jsc) of the OPV. The highest power conversion efficiency (PCE) of 7.96% with Jsc of 17.9 mA/cm2 was observed in the inverted OPV with a 2-nm-thick ALD-ZnO layer, which quenched electron-hole recombination at surface defects of ZnO ripples. Moreover, an ALD-ZnO layer thinner than 2 nm made the distribution of electrical conductivity on the ZnO surface more uniform, enhancing OPV performance. In contrast, a thicker ALD-ZnO layer (5 nm) made the two-dimensional distribution of electrical conductivity on the ZnO surface more heterogeneous, reducing the PCE. In addition, depositing an ALD-ZnO thin layer enhanced OPV stability and initial performance. We suggest that the ALD-ZnO layer thickness should be precisely controlled to fabricate high-performing OPVs.

Multifunctional SWCNT‐ZnO Nanocomposites for Enhancing Performance and Stability of Organic Solar Cells

Photovoltaic systems have been extensively studied and, among them, organic solar cells (OSCs) have attracted particular attention due to their low price and the possibility of using them in fl exible devices. [ 1–4 ] One of the disadvantages of OSCs is their low chemical stability, which is due to the oxidation of their interfaces by oxygen and water and the photodegradation of the active layers. [ 5–9 ] In order to increase their stability, various methods have been employed. Oxygen and water diffusion barriers were employed to protect the interfaces of OSCs from degradation. [ 10 , 11 ] Recently, an OSC with an inverted structure was developed, which was shown to be more stable than conventional solar cells. [ 12 , 13 ] In conventional structures, holes are injected into the transparent conducting electrode (TCE) and, in many cases, materials such as PEDOT: PSS with a high capability of hole injection are deposited on the TCE. In the inverted structure, in contrast, electrons are injected into the TCE. In the past, the improved stability of the inverted structure based on n-type ZnO layers on a TCE with respect to that of conventional OSCs was reported. [ 12 , 13 ] Carbon nanotubes (CNTs) have been widely used in photovoltaic systems. In many cases, bare CNTs and CNTs combined with C 60 , semiconductive and metallic nanoparticles were incorporated in the active layers, and the high capability of CNTs for electron transport has been exploited. [ 14–20 ] On the other hand,

Reduction of Series Resistance in Organic Photovoltaic Using Low Sheet Resistance of ITO Electrode

The series resistance of organic photovoltaic (OPV) devices was decreased by reducing the sheet resistance (R sh ) of the indium tin oxide (ITO) electrode, which leads to increasing device efficiency. The performance of bulk heterojunction OPVs was critically dependent on R sh of the ITO electrode. Upon reducing R sh of the ITO from 39 to 8.5 Ω/□, the fill factor and power conversion efficiency of OPV was improved (from 0.407 to 0.580 and from 1.63 ± 0.2 to 2.5 ± 0.1%, respectively) under an AM1.5 simulated solar intensity of 100 mW/cm 2 . The dependence of the series resistance on R sh of the ITO suggests the dominance of the bulk resistance of the ITO electrode as a limiting factor in practical cell efficiencies.