Min-Hoi Kim

Concept of rewritable organic ferroelectric random access memory in two lateral transistors-in-one cell architecture

We propose a concept of rewritable ferroelectric random access memory (RAM) with two lateral organic transistors-in-one cell architecture. Lateral integration of a paraelectric organic field-effect transistor (OFET), being a selection transistor, and a ferroelectric OFET as a memory transistor is realized using a paraelectric depolarizing layer (PDL) which is patterned on a ferroelectric insulator by transfer-printing. For the selection transistor, the key roles of the PDL are to reduce the dipolar strength and the surface roughness of the gate insulator, leading to the low memory on–off ratio and the high switching on–off current ratio. A new driving scheme preventing the crosstalk between adjacent memory cells is also demonstrated for the rewritable operation of the ferroelectric RAM.

Dependence of resistive switching on ion-migration process in organic electrochemical metallization memory (Conference Presentation)

Organic-based electrochemical metallization memory (ECM) has been paid much attention for non-volatile memory devices owing to high integration and mechanical flexibility. In such ECM systems, the formation of conductive filaments (CFs) is typically composed of two activation steps: i) the electrochemical redox reactions at an interface between an electrode and an electrolyte and ii) the migration of the cations of metal across the electrolyte. Accordingly, the overall electrical performance of the ECM device is primarily governed by the kinetics of the two steps. However, in the ECM devices using organic electrolytes, a rather compete picture of the resistive switching during the ion-migration process has not been described so far since filamentary paths are barely observable. In this work, we investigated how the resistive switching depends on the ion-migration properties including the drift velocity and the migration path in the organic ECM device. Two types of polymer electrolytes, having different molecular weights, were used for the control of the ion drift velocity. The topography of ion-migration paths was modified by the deposition rate of metal for the top electrode. The formation and the retention of the CFs depend critically on the ion mobility of the polymer electrolyte and the topography of the ion-migration paths as well. These results will provide a useful guideline for constructing high- performance ECM storage systems based on organic materials. This work was supported by the Brain Korea 21 Plus Project in 2018.

Importance of surface modification of a microcontact stamp for pattern fidelity of soluble organic semiconductors

Abstract. We described the effect of the ultraviolet ozone (UVO) treatment of a polydimethylsiloxane (PDMS) stamp on the fidelity of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) patterns produced from a TIPS-PEN/polymer blend by selective contact evaporation (SCE). During the SCE process, the TIPS-PEN in contact with the nanoporous PDMS was absorbed into the PDMS stamp, leaving out the TIPS-PEN patterns, complementary to the PDMS patterns, in the noncontact regions. For the case of the untreated, hydrophobic PDMS surface, the TIPS-PEN patterns developed initially were shrunken and eventually disappeared after 24 h due to the steady absorption of the TIPS-PEN in time. In contrast, for the UVO-treated case, the TIPS-PEN patterns were found to maintain the initial shapes over the period of 24 h since the absorption of the TIPS-PEN was limited by the hydrophilic nature of the UVO-treated PDMS. The modified PDMS surface by the UVO for 30 min yielded the highest fidelity of the TIPS-PEN patterns in both height and width. The patterned TIPS-PEN layer by the SCE was implemented into an organic field-effect transistor to demonstrate the viability of the SCE combined with the UVO treatment for solution-processed organic electronic devices.

Importance of surface modification of a micro-contact stamp for pattern fidelity of soluble organic semiconductors

We described the effect of the ultraviolet ozone (UVO) treatment of a polydimethylsiloxane (PDMS) stamp on the fidelity of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) patterns produced from a TIPS-PEN/polymer blend by selective contact evaporation (SCE). During the SCE process, the TIPS-PEN in contact with the nanoporous PDMS was absorbed into the PDMS stamp, leaving out the TIPS-PEN patterns, complementary to the PDMS patterns, in the non-contact regions. For the case of the untreated, hydrophobic PDMS surface, the TIPS-PEN patterns developed initially were shrunken and eventually disappeared after 24 h due to the steady absorption of the TIPS-PEN in time. In contrast, for the UVO-treated case, the TIPS-PEN patterns were found to maintain the initial shapes over the period of 24 h since the absorption of the TIPS-PEN was limited by the hydrophilic nature of the UVO-treated PDMS. The modified PDMS surface by the UVO for 30 min yielded the highest fidelity of the TIPS-PEN patterns in both height and width. The patterned TIPS-PEN layer by the SCE was implemented into an organic field-effect transistor to demonstrate the viability of the SCE combined with the UVO treatment for solution-processed organic electronic devices.

Low-voltage Organic Thin-film Transistors with Polymeric High-k Gate Insulator on a Flexible Substrates

Abstract We demonstrated low-voltage organic thin-film transistors (OTFTs) with bilayer insulators, high-k polymer and low temperaturecrosslinkable polymer, on a flexible plastic substrate. Poly (vinylidene fluoridetrifluoroethylene) (P(VDF-TrFE)) and poly (2-vinyl-naphthalene) are used for high-k polymer gate insulator and low temperature crosslinkable polymer insulators, respectively. The mobilityof flexible OTFTs is 0.17 cm 2 /Vs at gate voltages -5 V after bending operation. Keywords: Organic thin-film transistor, Leakage current, High capacitance, Flexible substrate 1. 서론 유기물 박막 트랜지스터(Organic thin-film transistor, OTFT)는 차세대 전자소자로서 큰 잠재력을 가지고 있어 다양한 연구들이 활발히 진행되고 있다. OTFT는 뛰어난 유연성과 대면적 저가 공정의 강점을 가지고 있어서 유연 디스플레이나 메모리 소자, RFID등 여러 분야에 응용이 가능하고, 특히 센서소자나 센서의 구동 소자로 응용하는 연구가 활발히 이루어지고 있다[1-6]. 현재 연구된 소자로는 가스나 이온 같은적 화학인 감지나 DNA와 같은 바이오 감지 소자 등으로 응용된 센서들이 있다[7-9]. 하지만 OTFT를 이용한 센서가 상용화되기위해서는 여전히 몇 가지 문제가 남아있다. 첫 번째로, 대부분의 제조공정에서 기판으로 단단한 유리를 사용한다는 것이다. OTFT가 가지는 본연의 장점인 유연함과 낮은 단가의 roll-to-roll 공정을 활용하기 위해서는 유연한 기판에 소자의 제작이 가능해야 한다. 둘째로 OTFT는 비교적 높은 구동 전압을가진다는(≥20 V) 것이다. 트랜지스터의 동작전압을 낮추기 위해 게이트 C커패시턴스 (

Leakage current behavior in MIM capacitors and MISM organic capacitors with a thin AlOx insulator

This is a systematic study of how leakage current behaviors are affected according to the capacitor structures. We fabricated two types of capacitors, a metal-insulator-metal (MIM) capacitor and a metal-insulator-organic semiconductor-metal (MISM) capacitor with an ultra-thin AlOX insulator. In the MIM capacitors, the Fermi-level alignment induced by the difference of the work functions between the top and bottom electrodes leads to the polarity dependence of the leakage current features. In contrast, for the MISM organic capacitors, it is found that the type of organic semiconductor dominantly determines the characteristics of the polarity dependence. These structure-dependent differences of leakage current in a fundamental constituent of the capacitor provide a scientific platform for designing more complex devices for precise operation.

Ambipolar-Type Organic Field-Effect Transistor with Two Stacked Active Layers in Dual-Gate Configuration

We demonstrated an ambipolar-type organic field-effect transistor (OFET) in a dual-gate configuration where two different layers of unipolar organic semiconductors (OSC) are stacked. In our OFET, the hole-channel for the p-type operation depends primarily on the corresponding gate insulator-OSC interface and the electron-channel for the n-type operation on the remaining interface in an independent manner. Using a combination of two independent gate voltages, the charge transport can be efficiently controlled and the on-off current ratio becomes enhanced in a dual-gate configuration. The optimization of the materials, the interfaces, and the device architectures will lead to a wide range of organic electronic applications.

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.

Integrated polymer solar cells in serial architecture with patterned charge-transporting MoOx for miniature high-voltage sources

We develop miniature high-voltage sources from polymer solar cells (PSCs) with charge-transporting molybdenum oxide (MoOx) integrated in a serial architecture through sacrificial layer (SL)-assisted patterning. The MoOx layer, being patterned by the lift-off process of the SL of a hydrophobic fluorinated-polymer, as a hole transporting layer plays a critical role on the reduction of the dark current and the increase of a high open circuit voltage of an integrated PSC array. The underlying mechanism lies primarily on the elimination of the lateral charge pathways in the MoOx layer in the presence of the electrode interconnection. Two miniature voltage sources consisting of 20 PSCs and 50 PSCs are demonstrated in the operation of a liquid crystal display and an organic field-effect transistor, respectively. Our SL-assisted integration approach will be directly applicable for implementing the self-power sources made of the PSCs into a wide range of the electronic and optoelectronic devices.