Seonuk Park

A key to room-temperature ferromagnetism in Fe-doped ZnO: Cu

Successful synthesis of room-temperature ferromagnetic semiconductors, Zn1−xFexO, is reported. The essential ingredient in achieving room-temperature ferromagnetism in bulk Zn1−xFexO was found to be additional Cu doping. A transition temperature as high as 550 K was obtained in Zn0.94Fe0.05Cu0.01O; the saturation magnetization at room temperature reached a value of 0.75μB per Fe. A large magnetoresistance was also observed below 100 K.

Al2O3/TiO2 nanolaminate thin film encapsulation for organic thin film transistors via plasma-enhanced atomic layer deposition.

Organic electronic devices require a passivation layer that protects the active layers from moisture and oxygen because most organic materials are very sensitive to such gases. Passivation films for the encapsulation of organic electronic devices need excellent stability and mechanical properties. Although Al2O3 films obtained with plasma enhanced atomic layer deposition (PEALD) have been tested as passivation layers because of their excellent gas barrier properties, amorphous Al2O3 films are significantly corroded by water. In this study, we examined the deformation of PEALD Al2O3 films when immersed in water and attempted to fabricate a corrosion-resistant passivation film by using a PEALD-based Al2O3/TiO2 nanolamination (NL) technique. Our Al2O3/TiO2 NL films were found to exhibit excellent water anticorrosion and low gas permeation and require only low-temperature processing (<100 °C). Organic thin film transistors with excellent air-stability (52 days under high humidity (a relative humidity of 90% and a temperature of 38 °C)) were fabricated.

Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics

We investigated the effects of direct solvent exposure on the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT)/PCBM blend films employed as active layers in, respectively, organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The crystallinity, morphology, and OFET characteristics of the PCBM thin films were significantly influenced by direct exposure to solvent, especially to select alcohols. Control over the nanoscale morphology of the PCBM film, achieved via direct solvent exposure, yielded highly efficient poly(3-hexylthiophene) (P3HT)/PCBM OPVs with a short-circuit current density of 10.2 mA cm−2, an open-circuit voltage of 0.64 V, and a power conversion efficiency of 3.25% under AM 1.5 illumination with a light intensity of 100 mW cm−2. These results indicated that optimal phase separation in the P3HT/PCBM films could be obtained simply by exposing the active layer films for a few seconds to solvent.

Solvent Additive to Achieve Highly Ordered Nanostructural Semicrystalline DPP Copolymers: Toward a High Charge Carrier Mobility

A facile spin-coating method in which a small percentage of the solvent additive, 1-chloronaphthalene (CN), is found to increase the drying time during film deposition, is reported. The field-effect mobility of a PDPPDBTE film cast from a chloroform-CN mixed solution is 0.46 cm(2) V(-1) s(-1). The addition of CN to the chloroform solution facilitates the formation of highly crystalline polymer structures.

Highly stable fluorine-rich polymer treated dielectric surface for the preparation of solution-processed organic field-effect transistors

Highly stable fluorine-rich polymer dielectrics were fabricated using cross-linked poly(3-(hexafluoro-2-hydroxyl) propyl) styrene (PFS), which shows excellent electrical stability, good adhesive surface properties, and good wettability on deposited solution-processed materials. Solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) could be deposited onto the cross-linked PFS dielectrics to yield highly ordered crystalline structured films that did not delaminate. The field-effect mobilities were as high as 0.56 cm2 V−1 s−1, and negligible hysteresis was observed in the organic field-effect transistors (OFETs). The threshold voltage, the ON/OFF ratio, and the subthreshold slope were −0.043 V, ∼107, and −0.3 V per decade, respectively. The OFETs demonstrated excellent device reliability under gate-bias stress conditions due to the presence of highly stable fluorine groups in the cross-linked PFS dielectrics.

A composite of a graphene oxide derivative as a novel sensing layer in an organic field-effect transistor

We report the fabrication of a gas sensor with an oleylamine-modified graphene oxide (OA-GO)/poly(9-9′-dioctyl-fluorene-co-bithiophene) (F8T2) composite as an active layer and demonstrate that it has better sensing ability than a comparable device with an F8T2-only active layer. OA-GO was chosen as the receptor material because of its enhanced interaction with gas analytes and its easy mixing with F8T2. OA-GO was synthesized by a simple condensation reaction between GO and oleylamine (9-octadecylamine), and characterized by Fourier transform infrared spectroscopy. The sensitivities of the gas sensors with respect to acetone and ethanol analytes were investigated by measuring the electrical parameters of the corresponding organic field effect transistor at room temperature. The sensitivity of the OA-GO/F8T2 composite device was up to 34 times that of the F8T2 device for the mobility change of acetone.

Directly Drawn Poly(3-hexylthiophene) Field-Effect Transistors by Electrohydrodynamic Jet Printing: Improving Performance with Surface Modification

In this study, direct micropatterning lines of poly(3-hexylthiophene) (P3HT) without any polymer binder were prepared by electrohydrodynamic jet printing to form organic field-effect transistors (OFETs). We controlled the dielectric surface by introducing self-assembled monolayers and polymer thin films to investigate the effect of surface modifications on the characteristics of printed P3HT lines and electrical performances of the OFETs. The morphology of the printed P3HT lines depended on the surface energy and type of substrate. The resulting OFETs exhibited high performance on octadecyltrichlorosilane-modified substrates, which was comparable to that of other printed P3HT OFETs. In order to realize the commercialization of the OFETs, we also fabricated a large-area transistor array, including 100 OFETs and low-operating-voltage flexible OFETs.

Photo-Patternable ZnO Thin Films Based on Cross-Linked Zinc Acrylate for Organic/Inorganic Hybrid Complementary Inverters

Complementary inverters consisting of p-type organic and n-type metal oxide semiconductors have received considerable attention as key elements for realizing low-cost and large-area future electronics. Solution-processed ZnO thin-film transistors (TFTs) have great potential for use in hybrid complementary inverters as n-type load transistors because of the low cost of their fabrication process and natural abundance of active materials. The integration of a single ZnO TFT into an inverter requires the development of a simple patterning method as an alternative to conventional time-consuming and complicated photolithography techniques. In this study, we used a photocurable polymer precursor, zinc acrylate (or zinc diacrylate, ZDA), to conveniently fabricate photopatternable ZnO thin films for use as the active layers of n-type ZnO TFTs. UV-irradiated ZDA thin films became insoluble in developing solvent as the acrylate moiety photo-cross-linked; therefore, we were able to successfully photopattern solution-processed ZDA thin films using UV light. We studied the effects of addition of a tiny amount of indium dopant on the transistor characteristics of the photopatterned ZnO thin films and demonstrated low-voltage operation of the ZnO TFTs within ±3 V by utilizing Al2O3/TiO2 laminate thin films or ion-gels as gate dielectrics. By combining the ZnO TFTs with p-type pentacene TFTs, we successfully fabricated organic/inorganic hybrid complementary inverters using solution-processed and photopatterned ZnO TFTs.

The effects of organic material-treated SiO2 dielectric surfaces on the electrical characteristics of inorganic amorphous In-Ga-Zn-O thin film transistors

We investigated the influence of organic dielectric surfaces on the electrical characteristics of inorganic amorphous indium-gallium-zinc oxide (a-IGZO)-based thin film transistors (TFTs). To modify the dielectric surface, various self-assembled monolayers and polymer thin films with different functional groups were introduced. Electrical measurements of the a-IGZO TFTs using surface-modified gate dielectrics revealed that the threshold voltages shifted toward positive values as the surface functional groups attract more electrons in the a-IGZO thin films. These results indicate that the channel conductance and carrier density of a-IGZO TFTs could be tuned by simple modification of the dielectric surfaces with organic materials.

Light-responsive spiropyran based polymer thin films for use in organic field-effect transistor memories

Light-responsive spirotype compounds have been receiving attention as attractive smart materials because of their various potential applications in organic optoelectronic devices, based on organic field-effect transistors (OFETs). However, it still remains a challenge to apply the organic flash memory devices using spirotype compounds due to the relatively poor development of new photosensitive electret materials. Here, we report the synthesis of a novel photosensitive polymer electret material, spiropyran, containing poly(3,5-benzoic acid hexafluoroisopropylidene diphthalimide) (6FDA-DBA-SP), and the development of light-responsive flexible memory devices using the 6FDA-DBA-SP electret layer. The charge trapping properties of 6FDA-DBA-SP under light illumination were enhanced by the electron withdrawing properties and lowering energetic barrier of charge trapping between 6FDA-DBA-SP and pentacene analysed by measuring the electronic structures at the pentacene/6FDA-DBA-SP interfaces. The resulting OFETs showed enlarged hysteresis under white-light illumination and exhibited bi-stable current states after the light-assisted programing and erasing processes, and they were utilized in non-volatile flexible memory device applications.