Ohyun Kim

Electrically bistable nonvolatile switching devices fabricated with a high performance polyimide bearing diphenylcarbamyl moieties

In this study, novel nonvolatile memory devices, based on a high performance polyimide, poly(3,3′-bis(diphenylcarbamyloxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-DPC PI), were fabricated with a simple conventional solution coating process. The devices were found to exhibit programmable, rewritable nonvolatile memory characteristics with a high ON/OFF current ratio of up to 109, a long retention time in both ON and OFF states, and low power consumption. Moreover, the active 6F-HAB-DPC PI layer is thermally and dimensionally stable and thus hybridization with a complementary metal-oxide-semiconductor platform is feasible. The advantageous properties and ease of fabrication of the 6F-HAB-DPC PI based devices open up the possibility of the mass production of high performance digital nonvolatile polymer memory devices at low cost.

Bipolar switching characteristics of nonvolatile memory devices based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) thin film

This letter describes the fabrication and electric characteristics of nonvolatile memory devices from a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film sandwiched between Al and indium tin oxide electrodes. These devices have bipolar switching characteristics. The on and off voltages are 0.67 and −1.65V, respectively. The on/off current ratio of the device is up to 103. These characteristics were caused by the formation and destruction of current paths by the reduction and oxidation of PEDOT chains in a PEDOT:PSS thin film. Also, the write-read-erase-read cycle test was operated over 104 times and the retention time was up to 16h.

Enhancement of piezoelectricity via electrostatic effects on a textile platform

We have shown the enhanced piezoelectricity by electrostatic effects on a textile based platform. The electrostatic and piezoelectric effects were hybridized by integrating piezoelectric ZnO nanowires and a charged dielectric film on a wearable textile substrate. The hybrid textile nanogenerator produced an output voltage of 8 V and an output current of 2.5 μA. Using a simple AC–DC converter circuit, we operated the green organic light-emitting diode and a liquid crystal display panel using a 100 dB sonic wave.

Effects of Substrate on Piezoelectricity of Electrospun Poly(vinylidene fluoride)-Nanofiber-Based Energy Generators

We report the effects of various substrates and substrate thicknesses on electrospun poly(vinylidene fluoride) (PVDF)-nanofiber-based energy harvesters. The electrospun PVDF nanofibers showed an average diameter of 84.6 ± 23.5 nm. A high relative β-phase fraction (85.2%) was achieved by applying high voltage during electrospinning. The prepared PVDF nanofibers thus generated considerable piezoelectric potential in accordance with the sound-driven mechanical vibrations of the substrates. Slide glass, poly(ethylene terephthalate), poly(ethylene naphthalate), and paper substrates were used to investigate the effects of the intrinsic and extrinsic substrate properties on the piezoelectricity of the energy harvesters. The thinnest paper substrate (66 μm) with a moderate Youngs modulus showed the highest voltage output (0.4885 V). We used high-performance 76, 66, and 33 μm thick papers to determine the effect of paper thickness on the output voltage. The thinnest paper substrate resulted in the highest voltage output (0.7781 V), and the numerical analyses of the sound-driven mechanical deformation strongly support the hypothesis that substrate thickness has a considerable effect on piezoelectric performance.

Recoverable Residual Image Induced by Hysteresis of Thin Film Transistors in Active Matrix Organic Light Emitting Diode Displays

A recoverable residual image is observed and analyzed in voltage driven active matrix organic light emitting diode (AMOLED) displays of which pixel circuits consist of two thin film transistors (TFTs) and one capacitor. The cause of the residual image is proven to be the hysteresis of the driving TFT in the pixel. The hysteresis of the p-channel TFT can be explained by hole trapping and de-trapping at the interface region of the channel. The recovery time of the residual image also strongly depends on the hysteresis level. We have found that the residual image can be eliminated by reducing the hysteresis level of TFTs.

Electrode-Material-Dependent Switching Characteristics of Organic Nonvolatile Memory Devices Based on Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) Film

The effect of various electrode materials has been studied for organic nonvolatile memory devices using a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film. The bottom electrodes (BEs) were indium tin oxide (ITO) and Al, while the top electrodes (TEs) were Al, Ti, Cr, ITO, Au, Ni, Pd, and Pt. The ITO/PEDOT:PSS/TE devices only had a bipolar switching behavior, while the Al/PEDOT:PSS/TE devices did not have any switching behavior unless a compliance current (CC) was used in the write-operation method. Then, they had a unipolar switching behavior irregardless of the TE material. Therefore, the BE material and the CC have crucial roles in the switching behavior and characteristics.

Bipolar resistive switching in a single layer memory device based on a conjugated copolymer

The feasibility of employing polymer memory devices as future data-storage units was investigated by using thin films of a π-conjugated copolymer, namely, poly(o-anthranilic acid-co-o-nitroaniline), which can be obtained by low-cost solution processing. Investigations on the conjugated-copolymer film showed that the hysteresis in the current-voltage characteristics can be used to perform “write-read-erase-read” memory functions. In addition, the ratio of the on/off current depended on the switching-on compliance current and the maximum voltage applied. This study suggests that the resistive switching of the copolymer device can be attributed to trap charging and discharging, which induce localized filament formation and disappearance.

Effect of the Electrode Material on the Electrical-Switching Characteristics of Nonvolatile Memory Devices Based on Poly(

We investigated the effect of the electrode material on the electrical-switching characteristics (i.e., electrical-switching behavior, switching voltage, and on/off current ratio) of a nonvolatile resistive-memory device based on an active poly(o-anthranilic acid) thin film. The switching characteristics of the active polymer layer were found to depend strongly on the bottom-electrode (BE) material. Depending on which material was used, the devices exhibited two different switching behaviors, namely, a polarity-dependent and a polarity-independent one. The polarity-independent switching behavior was particularly observed in devices fabricated with an aluminum BE, which can be attributed to the formation of a native oxide layer on this substrate.

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In this letter, we investigated the current-dependent switching characteristics of Al/6F-HAB-diphenyl carbamyl (PI-DPC)/Al devices fabricated by spin coating. The current required to switch off the device increased with an increase in the current compliance, limiting the on-state current, whereas the on and off voltages were almost constant. In addition, the turn-off current and the on and off voltages remained almost constant at the constant current compliance. The observed phenomenon indicates that the bistability of the PI-DPC device that is induced by controlling the current is attributed to filament formation and repulsive Coulomb interaction by charge trapping.

-anthranilic acid) Thin Films

We have proposed and fabricated a self-aligned polysilicon thin-film transistor (poly-Si TFT) with a thick dielectric layer at the gate edges near the source and drain. A T-shaped polysilicon gate was successfully formed by the damascene process used in VLSI interconnection technology. During the on state, an inversion layer is induced by the subgate as a drain so that the on current is still high and the poly-Si region under the subgate behaves as an offset, reducing the off-state leakage current during the off-state. As the subgate dielectric becomes 3.5 times thicker than the main gate oxide, the minimum off-state leakage current of the new TFT is decreased from 1.4/spl times/10/sup -10/ to 1.3/spl times/10/sup -11/ without sacrifice of the on current. In addition, the on-off current ratio is significantly improved.