Gunuk Wang

Evolution of nanomorphology and anisotropic conductivity in solvent-modified PEDOT:PSS films for polymeric anodes of polymer solar cells

A highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film, obtained by addition of a polar solvent, dimethylsulfoxide (DMSO), to an aqueous solution of PEDOT:PSS, was thoroughly investigated to gain a deeper understanding of the fundamental characteristics of the solvent-modified PEDOT:PSS film. Use of the DMSO-modified PEDOT:PSS film as a transparent anode to achieve low-cost and high-efficiency ITO-free organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) was also examined. Changes in the conductivity, morphology, surface composition, work-function, and anisotropic conductivity in both the parallel and perpendicular directions of solvent-treated PEDOT:PSS films that resulted from the addition of various amounts of DMSO were investigated to better understand the nature of the solvent-modified PEDOT:PSS film and the origin of its dramatically enhanced conductivity. Furthermore, the effects of using the modified PEDOT:PSS films as polymer anodes on solar cell performance were investigated by addition of various amounts of DMSO and by the use of PEDOT:PSS films with different thicknesses. The ITO-free OSCs with optimized PEDOT:PSS anodes had a high power conversion efficiency that was comparable to that of conventional ITO-based devices.

Three-Dimensional Integration of Organic Resistive Memory Devices

Since the discovery of conducting polymers [ 1 ] , organic-based electronics such as organic light-emitting diodes, transistors, photovoltaics, and memory devices have been spotlighted as potentially innovative devices given their easy and lowcost fabrication by spin-coating or ink-jet printing, and their fl exibility. [ 2–15 ] Among these, organic memories have been extensively investigated for data-storage application. [ 11 , 14 , 16–21 ]

Three-Dimensional Nanoporous Fe2O3/Fe3C-Graphene Heterogeneous Thin Films for Lithium-Ion Batteries

Three-dimensional self-organized nanoporous thin films integrated into a heterogeneous Fe2O3/Fe3C-graphene structure were fabricated using chemical vapor deposition. Few-layer graphene coated on the nanoporous thin film was used as a conductive passivation layer, and Fe3C was introduced to improve capacity retention and stability of the nanoporous layer. A possible interfacial lithium storage effect was anticipated to provide additional charge storage in the electrode. These nanoporous layers, when used as an anode in lithium-ion batteries, deliver greatly enhanced cyclability and rate capacity compared with pristine Fe2O3: a specific capacity of 356 μAh cm–2 μm–1 (3560 mAh cm–3 or ∼1118 mAh g–1) obtained at a discharge current density of 50 μA cm–2 (∼0.17 C) with 88% retention after 100 cycles and 165 μAh cm–2 μm–1 (1650 mAh cm–3 or ∼518 mAh g–1) obtained at a discharge current density of 1000 μA cm–2 (∼6.6 C) for 1000 cycles were achieved. Meanwhile an energy density of 294 μWh cm–2 μm–1 (2.94 Wh cm–3 or ∼924 Wh kg–1) and power density of 584 μW cm–2 μm–1 (5.84 W cm–3 or ∼1834 W kg–1) were also obtained, which may make these thin film anodes promising as a power supply for micro- or even nanosized portable electronic devices.

A New Approach for Molecular Electronic Junctions with a Multilayer Graphene Electrode

www.MaterialsViews.com C O M M A New Approach for Molecular Electronic Junctions with a Multilayer Graphene Electrode U N IC A Gunuk Wang , Yonghun Kim , Minhyeok Choe , Tae-Wook Kim , and Takhee Lee * IO N Interest in the fi eld of molecular electronics is grounded in the fact that devices based on molecules constitute the ultimate device miniaturization limit that both inorganicand organic-based electronics aspire to reach. [ 1–10 ] The non-linear current–voltage characteristics of molecular junctions have been extensively investigated with a variety of platforms and techniques, such as scanning probe microscope-based techniques, [ 10–13 ] break junctions, [ 5 , 14–17 ] crossed-wire tunnel junctions, [ 18–20 ] and various solid-state device-based methods. [ 4 , 6 , 21–25 ] Within these efforts, the creation of a stable solid-state molecular junction has been a long-standing challenge in terms of understanding molecular charge transport mechanisms and practical device applications. Most fabrication techniques involve evaporating a metal onto the molecules as the top electrode. [ 21–24 , 26 , 27 ] This process causes electrical short circuits and unstable and unexpected current–voltage characteristics due to fi lamentary paths and damage to the molecules. [ 22 , 23 , 26–29 ] These inevitable uncertainties in the fabrication technique lead to relatively large variations in the junction conductance, despite the use of identical molecular components, and this is an obstacle for truly understanding molecular charge transport mechanisms and device applications. New techniques and ideas have been developed to resolve this issue. [ 4 , 6 , 21 , 30 , 31 ] The fabrication of molecular junctions using a conductive polymer (PEDOT:PSS) between the top electrode and the molecules has been one of the most successful techniques in terms of high device yields and stable junctions. [ 21 ] Nevertheless, the use of a conductive polymer has some limitations and presents some uncertainties as a platform for physical-organic studies, because the properties of the interface between the polymer layer and the molecules are not well-understood. [ 21 , 30–33 ] For example, it has been reported that the resistance of the materials fabricated using this technique is signifi cantly different to those of molecular junctions that do not have the polymer interlayer [ 30–33 ] due to poor contact between PEDOT:PSS and

Enhanced Charge Injection in Pentacene Field‐Effect Transistors with Graphene Electrodes

S. Lee , G. Jo , S.-J. Kang , G. Wang , M. Choe , W Park , . Prof. D.-Y. Kim , H. Dr. . Y Kahng , Prof. Lee . TDepartment of Nanobio Materials and Electronics Department of Materials Science and Engineering Gwangju Institute of Science and Technology Gwangju 500–712, Korea E-mail: yhkahng@gist.ac.kr; tlee@gist.ac.kr Dr. Y. H. KahngResearch Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology Gwangju 500–712, Korea

Tuning of a graphene-electrode work function to enhance the efficiency of organic bulk heterojunction photovoltaic cells with an inverted structure

We demonstrate the fabrication of inverted-structure organic solar cells (OSCs) with graphene cathodes. The graphene film used in this work was work-function-engineered with an interfacial dipole layer to reduce the work function of graphene, which resulted in an increase in the built-in potential and enhancement of the charge extraction, thereby enhancing the overall device performance. Our demonstration of inverted-structure OSCs with work-function-engineering of graphene electrodes will foster the fabrication of more advanced structure OSCs with higher efficiency.

Flexible three-dimensional nanoporous metal-based energy devices.

A flexible three-dimensional (3-D) nanoporous NiF2-dominant layer on poly(ethylene terephthalate) has been developed. The nanoporous layer itself can be freestanding without adding any supporting carbon materials or conducting polymers. By assembling the nanoporous layer into two-electrode symmetric devices, the inorganic material delivers battery-like thin-film supercapacitive performance with a maximum capacitance of 66 mF cm(-2) (733 F cm(-3) or 358 F g(-1)), energy density of 384 Wh kg(-1), and power density of 112 kW kg(-1). Flexibility and cyclability tests show that the nanoporous layer maintains its high performance under long-term cycling and different bending conditions. The fabrication of the 3-D nanoporous NiF2 flexible electrode could be easily scaled.

High thermal conductivity of suspended few-layer hexagonal boron nitride sheets

AbstractThe thermal conduction of suspended few-layer hexagonal boron nitride (h-BN) sheets was experimentally investigated using a noncontact micro-Raman spectroscopy method. The first-order temperature coefficients for monolayer (1L), bilayer (2L) and nine-layer (9L) h-BN sheets were measured to be −(3.41 ± 0.12) × 10−2, −(3.15 ± 0.14) × 10−2 and −(3.78 ± 0.16) × 10−2 cm−1·K−1, respectively. The room-temperature thermal conductivity of few-layer h-BN sheets was found to be in the range from 227 to 280 W·m−1·K−1, which is comparable to that of bulk h-BN, indicating their potential use as important components to solve heat dissipation problems in thermal management configurations.

Enhancement of Field Emission Transport by Molecular Tilt Configuration in Metal−Molecule−Metal Junctions

We studied the molecular configuration-dependent charge transport of alkyl metal-molecule-metal junctions using conducting atomic force microscopy (CAFM). The inflection point (or transition voltage V(T)) on the plot of ln(I/V(2)) versus 1/V shifted to a lower voltage with increasing CAFM tip-loading force and decreasing molecular length. Our results indicate that the reduction of gap distance by molecular tilt configuration enhances the transition of the electronic transport mechanism from direct tunneling to field emission transport through molecules. The obtained results are consistent with a barrier height decrease, as affected by the enhancement of the intermolecular chain-to-chain tunneling as molecular tilt, predicted by a multibarrier tunneling model.

Reversible switching characteristics of polyfluorene-derivative single layer film for nonvolatile memory devices

This letter reports on reversible switching behavior of metal-insulator-metal type nonvolatile organic memory devices using polyfluorene-derivative (WPF-oxy-F) single layer film. The current-voltage (I-V) characteristics showed that the WPF-oxy-F single layer film has two distinguished resistance states, low resistance state and high resistance state, with four orders of on/off ratio (Ion∕Ioff∼104). From the analysis of I-V curves, area dependent I-V characteristics, and current images obtained by conducting atomic force microscopy we propose that the space charge limited current with filamentary conduction is a potential mechanism for the reversible switching behavior of WPF-Oxy-F memory devices.