Lae Ho Kim

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.

Thermally evaporated SiO thin films as a versatile interlayer for plasma-based OLED passivation.

Silicon monoxide (SiO) thin films were introduced as an efficient interlayer for achieving plasma-based organic light-emitting diode (OLED) surface passivation. The SiO thin films could be consecutively formed via thermal evaporation, without breaking the vacuum, after deposition of the OLED cathode. The plasma resistivity and UV-blocking characteristics of the SiO interlayer protected the OLED devices against electrical and optical degradation during the plasma-enhanced atomic layer deposition (PEALD) and plasma-enhanced chemical vapor deposition (PECVD) passivation processes. In addition, the nonconformal deposition and hydroxyl group-rich surface characteristics of the SiO thin films yielded enhanced surface pinhole coverage and a higher initial film density in the subsequently deposited PEALD-based Al2O3 barrier film. As a result, the OLEDs with a SiO/Al2O3 bilayer passivation layer displayed a remarkably increased device shelf life compared to devices prepared using Al2O3-only passivation. A MOCON test showed that the water vapor transmission rate (WVTR) of the SiO/Al2O3 bilayer film was 0.0033 g/(m(2) day), 2.3 times lower than the rate of a single Al2O3 barrier film. The results of our study demonstrated the multipurpose role of a SiO interlayer in plasma-based OLED passivation. The layer acted as a damage-free protective layer for the underlying OLED devices and an assistant layer to improve the upper barrier film performance.

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.

Fluorinated Polyimide Gate Dielectrics for the Advancing the Electrical Stability of Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) that operated with good electrical stability were prepared by synthesizing fluorinated polyimide (PI) gate dielectrics based on 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI. 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI contain 6 and 18 fluorine atoms per repeat unit, respectively. These fluorinated polymers provided smooth surface topographies and surface energies that decreased as the number of fluorine atoms in the polymer backbone increased. These properties led to a better crystalline morphology in the semiconductor film grown over their surfaces. The number of fluorine atoms in the PI backbone increased, the field-effect mobility improved, and the threshold voltage shifted toward positive values (from -0.38 to +2.21 V) in the OFETs with pentacene and triethylsilylethynyl anthradithiophene. In addition, the highly fluorinated polyimide dielectric showed negligible hysteresis and a notable gate bias stability under both a N2 environment and ambient air.

Vacuum thermally evaporated polymeric zinc acrylate as an organic interlayer of organic/inorganic multilayer passivation for flexible organic thin-film transistors

Polymeric zinc acrylate (pZA) was introduced as an organic interlayer for the inorganic/organic multilayer passivation of flexible organic thin film transistors (OTFTs). The pZA film was deposited by thermally evaporating a zinc diacrylate (ZDA) monomer under high vacuum (<10−5 Torr) and applying UV irradiation. The conversion of ZDA into a polymeric phase was confirmed by FTIR analysis, breakdown voltage measurements and the photopatternability of the film before and after UV irradiation. Vacuum-thermally evaporated pZA film showed good surface smoothness and a high permeation activation energy (53 kJ mol−1) compared to conventional polymeric films. As an interlayer for multilayer passivation, vacuum-thermally evaporated silicon monoxide (SiO) was introduced as the inorganic counterpart of the pZA interlayer. A multilayer film comprising 6.5 pairs of layers showed a water vapor transmission rate (WVTR) of 0.055 g m−2 per day, a 25-fold improvement over the WVTR of a single SiO film (1.207 g m−2 per day). OTFTs encapsulated with 6.5 pairs of polymeric zinc acrylate/silicon monoxide layers showed prolonged stability over 97 days. In addition, the passivation layer did not show crack formation, and sustained barrier characteristics, even after 500 bending tests.

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.