Yongsung Ji

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 ]

Rewritable Switching of One Diode–One Resistor Nonvolatile Organic Memory Devices

[*] Prof. T. Lee, B. Cho, T.-W. Kim, S. Song, Y. Ji, M. Jo, Prof. H. Hwang, Prof. G.-Y. Jung Department of Materials Science and Engineering Gwangju Institute of Science and Technology 1 Oryong-Dong, Buk-Gu Gwangju 500-712 (Korea) E-mail: tlee@gist.ac.kr Prof. T. Lee, Prof. H. Hwang Department of Nanobio Materials and Electronics Gwangju Institute of Science and Technology 1 Oryong-Dong, Buk-Gu Gwangju 500-712 (Korea)

Stable Switching Characteristics of Organic Nonvolatile Memory on a Bent Flexible Substrate

Organic-based electronics have received great attention due to their material variety and advantageous properties such as fl exibility, printability, and light-weightness. [ 1 , 2 ] Their low costs, based on their ease of fabrication and large-area processing capabilities, increase the merits of organic electronics even more. [ 3 , 4 ] Consequently, organic electronics, including organic solar cells, light-emitting diodes, thin-fi lm transistors, and memories, have been extensively investigated for the realization of practical device applications. [ 5–8 ] Among these, organic memories have emerged as an excellent candidate for the nextgeneration information storage media because of their potential application in fl exible memory devices. [ 8–18 ] There are different types of organic memories. They are distinguished as ferroelectric, [ 13 , 14 , 18 ] fl ash, [ 15 , 18 ] and resistive-type organic memories [ 16–18 ]

Flexible and twistable non-volatile memory cell array with all-organic one diode–one resistor architecture

Flexible organic memory devices are one of the integral components for future flexible organic electronics. However, high-density all-organic memory cell arrays on malleable substrates without cross-talk have not been demonstrated because of difficulties in their fabrication and relatively poor performances to date. Here we demonstrate the first flexible all-organic 64-bit memory cell array possessing one diode-one resistor architectures. Our all-organic one diode-one resistor cell exhibits excellent rewritable switching characteristics, even during and after harsh physical stresses. The write-read-erase-read output sequence of the cells perfectly correspond to the external pulse signal regardless of substrate deformation. The one diode-one resistor cell array is clearly addressed at the specified cells and encoded letters based on the standard ASCII character code. Our study on integrated organic memory cell arrays suggests that the all-organic one diode-one resistor cell architecture is suitable for high-density flexible organic memory applications in the future.

Flexible organic solar cells composed of P3HT:PCBM using chemically doped graphene electrodes

Flexible organic solar cells (OSCs) composed of blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated and investigated with chemically doped multilayer graphene films as transparent and conducting electrodes on plastic substrates. The sheet resistance of the chemically doped graphene film was reduced to half of its original value, resulting in a significant performance enhancement of OSCs featuring doped graphene electrodes. Moreover, there was no substantial variation observed in the fill factor and power conversion efficiency values of the flexible OSCs under bending conditions. A power conversion efficiency of ~2.5% for flexible OSCs with doped graphene electrodes was observed under bending conditions, even up to a 5.2 mm bending radius.

High-Performance Pseudocapacitive Microsupercapacitors from Laser-Induced Graphene.

All-solid-state, flexible, symmetric, and asymmetric microsupercapacitors are fabricated by a simple method in a scalable fashion from laser-induced graphene on commercial polyimide films, followed by electrodeposition of pseudocapacitive materials on the interdigitated in-plane architectures. These microsupercapacitors demonstrate comparable energy density to commercial lithium thin-film batteries, yet exhibit more than two orders of magnitude higher power density with good mechanical flexibility.

Enhancement in the photodetection of ZnO nanowires by introducing surface-roughness-induced traps

We investigated the enhanced photoresponse of ZnO nanowire transistors that was introduced with surface-roughness-induced traps by a simple chemical treatment with isopropyl alcohol (IPA). The enhanced photoresponse of IPA-treated ZnO nanowire devices is attributed to an increase in adsorbed oxygen on IPA-induced surface traps. The results of this study revealed that IPA-treated ZnO nanowire devices displayed higher photocurrent gains and faster photoswitching speed than transistors containing unmodified ZnO nanowires. Thus, chemical treatment with IPA can be a useful method for improving the photoresponse of ZnO nanowire devices.

Electrical characterization of organic resistive memory with interfacial oxide layers formed by O2 plasma treatment

We studied organic resistive memory devices with interfacial oxide layers, the thickness of which depended on O2 plasma treatment time. The different interfacial oxide thicknesses sequentially changed the ON and OFF states of the final memory devices. We found that the memory devices that had undergone additional plasma treatment showed higher ON/OFF ratios than devices without the treatment, which was due to the relatively large OFF resistance values. However, a long oxidation process widened the threshold voltage distribution and degraded the switching reproducibility. This indicates that the oxidation process should be carefully optimized to provide practical high-performance organic memory.

Laser‐Induced Graphene in Controlled Atmospheres: From Superhydrophilic to Superhydrophobic Surfaces

The modification of graphene-based materials is an important topic in the field of materials research. This study aims to expand the range of properties for laser-induced graphene (LIG), specifically to tune the hydrophobicity and hydrophilicity of the LIG surfaces. While LIG is normally prepared in the air, here, using selected gas atmospheres, a large change in the water contact angle on the as-prepared LIG surfaces has been observed, from 0° (superhydrophilic) when using O2 or air, to >150° (superhydrophobic) when using Ar or H2 . Characterization of the newly derived surfaces shows that the different wetting properties are due to the surface morphology and chemical composition of the LIG. Applications of the superhydrophobic LIG are shown in oil/water separation as well as anti-icing surfaces, while the versatility of the controlled atmosphere chamber fabrication method is demonstrated through the improved microsupercapacitor performance generated from LIG films prepared in an O2 atmosphere.

Flexible Nanoporous WO3–x Nonvolatile Memory Device

Flexible resistive random access memory (RRAM) devices have attracted great interest for future nonvolatile memories. However, making active layer films at high temperature can be a hindrance to RRAM device fabrication on flexible substrates. Here, we introduced a flexible nanoporous (NP) WO3-x RRAM device using anodic treatment in a room-temperature process. The flexible NP WO3-x RRAM device showed bipolar switching characteristics and a high ION/IOFF ratio of ∼10(5). The device also showed stable retention time over 5 × 10(5) s, outstanding cell-to-cell uniformity, and bending endurance over 10(3) cycles when measured in both the flat and the maximum bending conditions.