Jin-Hyuk Kwon

Stability Study of Flexible 6,13-Bis(triisopropylsilylethynyl)pentacene Thin-Film Transistors with a Cross-Linked Poly(4-vinylphenol)/Yttrium Oxide Nanocomposite Gate Insulator

We investigated the electrical and mechanical stability of flexible 6,13-bis(triisopropylsilylehtynyl)pentacene (TIPS-pentacene) thin-film transistors (TFTs) that were fabricated on polyimide (PI) substrates using cross-linked poly(4-vinylphenol) (c-PVP) and c-PVP/yttrium oxide (Y2O3) nanocomposite films as gate insulators. Compared with the electrical characteristics of TIPS-pentacene TFTs with c-PVP insulators, the TFTs with c-PVP/Y2O3 nanocomposite insulators exhibited enhancements in the drain current and the threshold voltage due to an increase in the dielectric capacitance. In electrical stability experiments, a gradual decrease in the drain current and a negative shift in the threshold voltage occurred during prolonged bias stress tests, but these characteristic variations were comparable for both types of TFT. On the other hand, the results of mechanical bending tests showed that the characteristic degradation of the TIPS-pentacene TFTs with c-PVP/Y2O3 nanocomposite insulators was more critical than that of the TFTs with c-PVP insulators. In this study, the detrimental effect of the nanocomposite insulator on the mechanical stability of flexible TIPS-pentacene TFTs was found to be caused by physical adhesion of TIPS-pentacene molecules onto the rough surfaces of the c-PVP/Y2O3 nanocomposite insulator. These results indicate that the dielectric and morphological properties of polymeric nanocomposite insulators are significant when considering practical applications of flexible electronics operated at low voltages.

Observation of irreversible current path in polymer dielectric using conductive atomic force microscope

During the measurement of the electrical properties of a metal-polymer-metal capacitor, it was found that the capacitor exhibited write-once-read-many-times (WORM) memory behavior, even though it was made of the dielectric polymer, polystyrene. The initial low conductance state changed to a high conductance state when a threshold voltage was applied, but this final state never reverted to the initial state. This phenomenon only appeared in sub-100-nm-thick films. To understand this phenomenon, conductive atomic force microscopy (CAFM) was used. The current distribution measured with CAFM showed an irreversible current path had formed near particles in the polymer film. For reproducibility, particles were intentionally inserted into the polymer film during the fabrication of metal-polymer-metal capacitors, and the same current mechanism was found. From these results, it is concluded that the purification and cleaning process of organic devices severely affects the device characteristics. In addition, particle-insertion appears to be a promising method for fabrication low-cost and air-stable WORM type memory for various applications.

Improvement of Current Efficiency at High Field Regime Via Description of Roll-off Characteristic in Model Device of OLEDs

We demonstrated the effect of the field dependency of charge carrier mobility on roll-off characteristic of current efficiency. Field dependent factor in Poole-Frenkel model is controlled to describe charge balance of electron and hole concentration at high field. It is well established that roll-off characteristic is dependent upon mobility match which results in charge balance in EML. We exhibited the improvement of the roll-off characteristic of current efficiency and explained how charge balance is induced through this description.

Electrical stability of solution-processed indium oxide thin-film transistors under illumination stress

ABSTRACT We investigated the variation in the electrical characteristics of solution-processed indium oxide (In2O3) thin-film transistors (TFTs) under monochromatic illumination conditions at different wavelengths of 1240, 800, 600, 500, 400, and 350 nm. In our results, there was no discernible difference between the characteristics of the TFT measured in the dark state and those obtained under light illumination at the wavelengths of 1240 nm and 800 nm. On the other hand, the TFT performance was remarkably changed when light having a wavelength of 600 nm or less was illuminated. It is found that the photon energy, which causes the characteristic degradation in the solution-processed In2O3 TFTs, is much lower than the direct band gap energy of In2O3. Consequently, illumination-induced instability in the performance of solution-processed In2O3 TFTs can be explained with the transition phenomena of neutral oxygen vacancies to ionized ones in the In2O3 semiconductor film.

Importance of active layer positioning on gate electrode in organic thin-film transistors

ABSTRACT We describe the importance of active layer positioning for a gate electrode in organic thin-film transistors (TFTs). For this study, we utilizea numerical simulation based on 2-D Atlas, which is a two-dimensional technology computer aided design (so called 2D TCAD) software tool created by Silvaco. Variation in the electrical characteristics of pentacene TFTs is systematically explored by changing the mismatch length (LM) between the active layer and gate electrode in the bottom-gate top-contact configuration. It is found that as the LM increases, the electrical performance of pentacene TFTs is exponentially degraded in terms of drain current on/off characteristics. In particular, we explain this phenomenon by examining variations in charge distribution and gate electric field in the TFT channel region byincreasing the LM.

Dependence of the hybrid solar cell efficiency on the thickness of ZnO nanoparticle optical spacer interlayer

ABSTRACT Optical spacer interlayers are commonly used in solar cells to improve the power conversion efficiency (PCE) of the device. This paper reports a study on the effect of zinc oxide (ZnO) optical spacer interlayer on PCPDTBT:ZnO hybrid solar cell. Optical spacers help to concentrate the spatial distribution of electric field due to light onto the active layer, and thus, improve the light absorption in the active layer. Finite-Difference Time-Domain (FDTD) calculations was carried out on stacked solar cell structure using a simulation software, Lumerical FDTD solutions. Maxwells equations were solved and the light absorbed by the active layer was computed. The design was optimized with regard to active layer thickness and ZnO optical spacer thickness.

Influence of Pentacene Film Thickness on Environmental Stability of Pentacene Thin-Film Transistors

We investigated the influence of pentacene film thickness on the environmental stability of thin-film transistors (TFTs). In order to evaluate the environmental stability, the transfer and output characteristics in pentacene TFTs were measured in air and vacuum environments. The pentacene-thickness-dependent variation of transfer characteristics was shown to originate from the fixed charge at the pentacene/polymeric insulator interface induced by air molecules. In addition, an analysis of parasitic resistance suggests that air molecules degraded the contact property and increased channel conductivity. Particularly, a thick pentacene film was found to passivate the TFT channel region interrupting the penetration of air molecules.

Solvent-Tolerant Patterning of Poly(3-hexylthiophene) Film by Subtractive Photolithography

This study investigates how the fringing field affects the total current flow within a conducting polymer. In order to extract the fringing field component of bar pattern resistors, a solvent-assisted patterning method using subtractive photolithography was successfully established for the conducting polymer poly(3-hexylthiophene). By comparing the current quantities of unpatterned and patterned resistors, a conductance factor for the fringing field was calculated, proving to be almost constant regardless of the resistor length. It is therefore concluded that the length as well as the width of the conducting polymer film need to be suitably patterned for the precise operation of organic electronic devices. In this regard, the patterning method developed will be useful for the fabrication of micro-scale devices.

Effect of a top insulator on turn-on voltage in organic field-effect transistors with a multilayer insulator

We investigated the manner in which a top insulator affects the turn-on voltage in bilayer insulator-structured organic field-effect transistors (BI-OFETs). To systematically analyze the turn-on voltage, we introduced two new concepts: the effective turn-on voltage (VETO) and the normalized effective turn-on voltage (VNETO). By examining the underlying mechanism of turn-on voltage features in BI-OFETs, we found that the values of VNETO of the BI-OFETs are strongly governed by the nature of the top insulator.

Effects of molecular weights of a polymeric insulator on the electrical properties of pentacene thin-film transistors

This paper reports the effects of the molecular weights (MWs) of a polymeric insulator on the electrical properties of a pentacene thin-film transistor. The MWs of polystyrene used as a polymeric insulator were varied from 13,000 to 980,000. The mobility and current on/off ratio in a pentacene thin-film transistor were improved considerably (more than twofold and by a factor of 2, respectively) with increasing MW of polystyrene. In addition, the magnitude of the subthreshold slope was reduced significantly. Both the thickness and dielectric constant remained the same regardless of the MWs of polystyrene. The improved electrical properties could be explained because the higher MW of polystyrene results in the pentacene film with enhanced grain and crystalline intensity. This suggests that a higher MW is important for obtaining better electrical characteristics in organic transistors with polymeric insulators.