Juhwan Kim

Bio: Juhwan Kim is an academic researcher from Gwangju Institute of Science and Technology. The author has contributed to research in topics: Polymer solar cell & Organic solar cell. The author has an hindex of 26, co-authored 44 publications receiving 2594 citations. Previous affiliations of Juhwan Kim include LG Chem & University of California, Irvine.

Solution-processable reduced graphene oxide as a novel alternative to PEDOT:PSS hole transport layers for highly efficient and stable polymer solar cells.

Abstract: The potential of a fl exible, roll-to-roll manufacturing process has made bulk-heterojunction (BHJ) organic solar cells (OSCs) very attractive as a promising solution to energy and environmental issues. [ 1–8 ] In polymer solar cells, device characteristics such as fi ll factor (FF), short-circuit current density (J sc ) and open-circuit voltage (V oc ) as well as the cell life-time all are highly dependent on the interface properties between the electrodes and the active layers and on the bulk properties of the materials. [ 9 ] For these reasons, numerous modifi cations of electrodes by introduction of an interfacial layer have been studied intensively for high-performance and stable OSCs, [ 10–15 ] and several key factors such as transparency, conductivity, passivation property, fi lm morphology, stability, and solution-processability have been considered for uses of these promising interfacial layers. [ 9–15 ]

Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation

Abstract: The authors report on a spray deposition method as a cost-efficient technique for the fabrication of organic solar cells (OSCs). Active layers of OSCs were fabricated using conventional handheld airbrushes. Although the spray deposited film showed a relatively rougher surface than spin coated ones, pinhole-free and constant thickness films could be obtained. An optimized OSC showed 2.83% of power conversion efficiency and 52% of incident photon to current conversion efficiency even though the device was fabricated in air. The performance of sprayed OSCs was comparable to that of the spin coated devices fabricated in air.

Efficient Polymer Solar Cells with Surface Relief Gratings Fabricated by Simple Soft Lithography

Abstract: Polymer-based photovoltaic cells, with periodic sub-micrometer structures as an efficient light-trapping scheme, are investigated to improve the performance of organic solar cells based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61. A soft lithographic approach that uses photoresponsive azo polymer films as masters and poly(dimethylsiloxane) as stamps is used to form surface relief gratings (SRGs) on the active layers. The effect of periodic gratings on solar cell performance is precisely investigated according to various grating conditions such as period, depth, and dimension. The solar cells with 1D and 2D SRGs present improved incident-photon-to-current conversion efficiencies and an overall increase in power conversion efficiencies, primarily resulting from the enhancement of short-circuit current density, indicating that periodic structures induce further photon absorption in the active film.

Simple bar-coating process for large-area, high-performance organic field-effect transistors and ambipolar complementary integrated circuits.

Abstract: Solution-processed organic semiconductors are of great potential for large-area, inexpensive, lightweight, and fl exible electronic applications. With respect to these materials, tremendous effort has recently been focused on developing several types of organic electronic and optoelectronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), organic fi eld-effect transistors (OFETs), and organic memory and sensors, using graphic-art printing methods on fl substrates. [ 1‐5 ] OFETs are a fundamental building block of integrated circuits (ICs) and drivers for active-matrix fl at-panel displays. Accordingly, they are a promising candidate to replace the vacuum-processed amorphous inorganic ICs; this would enable the use of drivers in printed and fl exible radio-frequency identifi cation tags, memories, sensors, and display backplanes. [ 6 , 7 ] To realize high-speed organic printed ICs, the complementary IC geometry, which consists of p- and n-channel transistors, has an advantage over those that comprise unipolar transistors because of reduced transition delays, higher noise immunity, and negligible power consumption in the static state. [ 8 ] In solution-processed devices, p- and n-type active channels have been patterned at resolutions as low as a few micrometers using a variety of printing methods such as inkjet, spray, and gravure printing. [ 9 ] However, these printing processes typically result in device-to-device performance deviations because of diffi culties inherent in controlling the morphology (e.g., roughness and crystallinity) of micrometre-sized deposits. For example, organic semiconductor droplets that are deposited via ink jet onto non-absorbing substrates typically show signifi cant coffee

High-Performance Top-Gated Organic Field-Effect Transistor Memory using Electrets for Monolithic Printed Flexible NAND Flash Memory

Abstract: High-performance top-gated organic fi eld-effect transistor (OFET) memory devices using electrets and their applications to flprinted organic NAND fl ash are reported. The OFETs based on an inkjet-printed p-type polymer semiconductor with effi ciently chargeable dielectric poly(2-vinylnaphthalene) (PVN) and high-k blocking gate dielectric poly(vinylidenefl trifl uoroethylene) (P(VDF-TrFE)) shows excellent non-volatile memory characteristics. The superior memory characteristics originate mainly from reversible charge trapping and detrapping in the PVN electret layer effi ciently in low-k/high-k bilayered dielectrics. A strategy is devised for the successful development of monolithically inkjet-printed fl exible organic NAND fl ash memory through the proper selection of the polymer electrets (PVN or PS), where PVN/- and PS/P(VDF-TrFE) devices are used as non-volatile memory cells and ground- and bit-line select transistors, respectively. Electrical simulations reveal that the fl exible printed organic NAND fl ash can be possible to program, read, and erase all memory cells in the memory array repeatedly without affecting the non-selected memory cells.

Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics.

Abstract: Chemically derived graphene oxide (GO) possesses a unique set of properties arising from oxygen functional groups that are introduced during chemical exfoliation of graphite. Large-area thin-film deposition of GO, enabled by its solubility in a variety of solvents, offers a route towards GO-based thin-film electronics and optoelectronics. The electrical and optical properties of GO are strongly dependent on its chemical and atomic structure and are tunable over a wide range via chemical engineering. In this Review, the fundamental structure and properties of GO-based thin films are discussed in relation to their potential applications in electronics and optoelectronics.

Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature

Abstract: Oil spills and industrial organic pollutants have induced severe water pollution and threatened every species in the ecological system. To deal with oily water, special wettability stimulated materials have been developed over the past decade to separate oil-and-water mixtures. Basically, synergy between the surface chemical composition and surface topography are commonly known as the key factors to realize the opposite wettability to oils and water and dominate the selective wetting or absorption of oils/water. In this review, we mainly focus on the development of materials with either super-lyophobicity or super-lyophilicity properties in oil/water separation applications where they can be classified into four kinds as follows (in terms of the surface wettability of water and oils): (i) superhydrophobic and superoleophilic materials, (ii) superhydrophilic and under water superoleophobic materials, (iii) superhydrophilic and superoleophobic materials, and (iv) smart oil/water separation materials with switchable wettability. These materials have already been applied to the separation of oil-and-water mixtures: from simple oil/water layered mixtures to oil/water emulsions (including oil-in-water emulsions and water-in-oil emulsions), and from non-intelligent materials to intelligent materials. Moreover, they also exhibit high absorption capacity or separation efficiency and selectivity, simple and fast separation/absorption ability, excellent recyclability, economical efficiency and outstanding durability under harsh conditions. Then, related theories are proposed to understand the physical mechanisms that occur during the oil/water separation process. Finally, some challenges and promising breakthroughs in this field are also discussed. It is expected that special wettability stimulated oil/water separation materials can achieve industrial scale production and be put into use for oil spills and industrial oily wastewater treatment in the near future.

Few-Layer MoS2: A Promising Layered Semiconductor

Abstract: Due to the recent expanding interest in two-dimensional layered materials, molybdenum disulfide (MoS2) has been receiving much research attention. Having an ultrathin layered structure and an appreciable direct band gap of 1.9 eV in the monolayer regime, few-layer MoS2 has good potential applications in nanoelectronics, optoelectronics, and flexible devices. In addition, the capability of controlling spin and valley degrees of freedom makes it a promising material for spintronic and valleytronic devices. In this review, we attempt to provide an overview of the research relevant to the structural and physical properties, fabrication methods, and electronic devices of few-layer MoS2. Recent developments and advances in studying the material are highlighted.

Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells

Abstract: This article provides an overview on the recent development of solution processed organic, inorganic, and hybrid interfacial materials for bulk-heterojunction polymer solar cells. The introduction of proper interfacial materials to optimize the electronic and electrical properties between the interfaces of the light-harvesting active layer and the charge-collecting electrode has become an important criterion to improve the performance of polymer solar cells. The electronic processes at these interfaces play a critical role in determining the efficiency for photon-to-electricity conversion. An ideal interface requires the formation of Ohmic contact with minimum resistance and high charge selectivity to prevent charge carriers from reaching the opposite electrodes. For long-term stability of polymer solar cells, interfaces with matched surface energy are required to prevent interfacial dewetting and delamination. Several classes of interfacial materials including inorganic metal oxides, crosslinkable charge-transporting materials, conjugated polymer electrolytes, self-assembled functional molecules, and graphene-based materials are highlighted and the integration of these interfacial materials with new low bandgap polymers and fullerene derivatives as active materials in different device architectures is also discussed.