Development of a Hybrid Stamp Coated with a Low-Surface-Energy, Diffusion-Blocking Layer for Organic Electronic Devices

다국어 초록 (Multilingual Abstract)

Since the pioneering work by Whitesides et al., termed “soft lithography”, poly(dimethylsiloxane) (PDMS) has been very widely used as a material composing a stamp from which various materials are transferred onto a target substrate. However, even more than 20 years after the first paper reporting this work, applications of PDMS-stamp-based materials transfer have been rather limited to simple cases where materials transfer alone is sufficient for their success and/or the quality of the transfer-bonded interfaces and the cleanliness of the transferred layers do not matter significantly. This is in part due to the following adverse properties of the PDMS stamp: absorption of small molecules by PDMS free volumes and contamination of the transferred layers by uncured oligomers in PDMS.
Here, I develop a hybrid stamp comprised of a PDMS bulk and a perfluoropolyether (PFPE) coating induced by a condensation reaction between not only PDMS and PFPE molecules but also adjacent PFPE molecules. A key role of the PFPE coating layer on the PDMS stamp is effective to prevent organic small molecules from being absorbed into the stamp and the uncured siloxane oligomers of the PDMS from migrating on a layer to be transferred. I prove the effectiveness and versatility of the PFPE-coated PDMS stamp by fabricating an organic light emitting diode whose organic-organic interface is formed by a transfer process and an organic hole-only device with a bottom electrode composed of a graphene bilayer transferred from the stamp. As a result, the mechanically bonded interfaces are sufficiently intimate at the molecular level compared to those of the same interface formed by thermal evaporation. Furthermore, the top surface of the transferred layer that was in contact with the stamp is enough to clean for injecting and extracting charge carriers. The PFPE-coated stamp demonstrated in this work is expected to be widely used in fabricating devices or systems that are especially difficult to realize using high-temperature or wet processes. An exciting example is full-color organic light-emitting device (OLED) displays with a resolution much higher than that of the current displays in smartphones, which is required for virtualreality applications but is difficult to fabricate using the current shadow maskbased patterning.

국문 초록 (Abstract)

폴리디메틸실록산(PDMS)은 다양한 재료를 최종 기판으로 옮길 때 사용하는 소재로써 매우 널리 사용되어 왔다. 그러나 많은 연구자들이 오랫동안 이와 관련된 많은 연구들을 진행했음에도 불구하고, PDMS 도장 기반의 층 또는 패턴 전사 공정은 기계적으로 접합된 계면과 전사된 물질의 청결이 중요하지 않은 응용분야에서만 제한적으로 진행되었다. 이는 다음과 같은 PDMS 도장의 불리한 성질에 기인한다: (i) PDMS의 내부로 유기소분자의 흡수, (ii) PDMS의 경화되지 않은 올리고머에 의한 이동된 층의 오염. 이러한 문제가 해결된다면, 도장을 이용한 패턴된 물질 증착은 기존의 쉐도우 마스크 공정과 진공증착 공정으로 제작되는 유기전자소자 분야에서 다양하고 새로운 구조 및 소자를 제작할 수 있을 것이다.
본 연구에서는 PDMS와 퍼플루오르폴리에테르(PFPE) 분자뿐만 아니라 인접한 PFPE 분자들 사이의 응축 반응에 의해 유도된 PDMS 벌크 및 PFPE 코팅 층으로 구성된 하이브리드 도장을 개발하였다. PDMS 도장 위에 있는 PFPE 코팅 층의 핵심 역할은 PDMS의 경화되지 않은 실록산 올리고머가 전사 될 층 위로 이동하는 것을 막고 전사 될 층의 물질이 도장 내부로 흡수되는 것을 방지하는 것이다. 박막전사 과정에서 가지는 PFPE로 코팅된 PDMS 도장의 효율성과 다 기능성은 전사 공정으로 형성된 유기—유기 계면이 포함된 유기발광 다이오드와 상기 도장으로부터 전사된 그래핀 이중층으로 구성된 하부 전극을 갖는 유기 hole-only 소자를 제작함으로써 증명되었다. 그 결과 기계적으로 결합된 계면의 품질은 열 증발에 의해 형성된 동일한 계면의 품질과 비교하여 비슷한 수준이었다. 또한, 도장과 접촉한 전사 층의 상부 표면은 전하 캐리어를 주입 및 추출하기에 충분하였다. 본 연구에서 개발된 PFPE가 코팅된 도장은 고온 또는 습식 공정을 사용하여 구현하기
가 특히 어려운 전자장치 또는 전자소자 제작에 널리 사용될 수 있을 것으로 예상된다. 가장 흥미로운 예는 가상 현실 어플리케이션에 필요하지만 현재의 쉐도우 마스크 기반 패터닝을 사용하여 제조하기 어려운 스마트 폰의 현재 디스플레이보다 훨씬 높은 해상도를 갖춘 풀 컬러 유기발광소자 (OLED) 디스플레이이다.

목차 (Table of Contents)

• Contents
• Chapter 1 Introduction..........................................................................................1
• 1.1 Overview ........................................................................................................1
• 1.1.1 Limitations of PDMS as a stamp in a contact transfer process……………………1
• 1.1.2 Previous studies to overcome problems with PDMS ............3
• 1.2 Common elements to understand a contact transfer process...........5
• 1.2.1 Conditions for a reliable contact-transfer process.................5
• 1.2.2 Stamp materials for a contact transfer process......................7
• 1.3 Scope of this thesis........................................................................10
• 1.3.1 Development of a process for transferring of graphene patterns and the motivation for the need to coat a PFPE layer on PDMS……………………...……………………………………….12
• 1.3.2 Overcoming limitation of PDMS as a stamp in a contact transfer process .................................................................................12
• 1.3.3 Application of the PFPE-coated PDMS stamp to the fabrication of organic electronic devices..........................................13
• 1.4 References .....................................................................................15
• Chapter 2 Low-Temperature, Dry Transfer-Printing of a Patterned Graphene Monolayer ...................................................................................19
• 2.1 Introduction ...................................................................................19
• 2.2 Results and discussion...................................................................22
• 2.2.1 The low-temperature and dry transfer process for a CVDgrown graphene monolayer...............................................................22
• 2.2.2 Effect of surface on the quality of transfer-printed graphene monolayers........................................................................................27
• 2.2.3 Characterizations of graphene monolayer patterns transferprinted on materials that can be damaged by a wet process .............36
• 2.2.4 Morphological characterizations of transfer-printed graphene
• monolayers........................................................................................39
• 2.3 Conclusions ...................................................................................44
• 2.4 Methods.........................................................................................45
• 2.4.1 Low-temperature, dry transfer-printing process..................45
• 2.4.2 Transfer-printing of patterned graphene layers ...................46
• 2.4.3 Sample preparation for the elemental analysis....................48
• 2.4.4 Preparation of PDMS stamps ..............................................48
• 2.5 References .....................................................................................51
• Chapter 3 Polydimethylsiloxane (PDMS) Stamp Coated with a LowSurface-Energy, Diffusion-Blocking, Covalently Bonded Perfluoropolyether (PFPE) Layer ......................................................................................58
• 3.1 Introduction ...................................................................................58
• 3.2 Results and discussion...................................................................60
• 3.2.1 Dip-coating process for depositing a PFPE layer on a PDMS
• stamp………………………………………………………………. 60
• 3.3 Conclusions ...................................................................................64
• 3.4 Methods.........................................................................................66
• 3.4.1 Preparation of PDMS stamps ..............................................66
• 3.4.2 PFPE dip-coating of PDMS stamps.....................................66
• 3.5 References .....................................................................................69
• Chapter 4 Application of the PFPE-coated PDMS Stamp to the
• Fabrication of Organic Electronic Devices by Layer Transfer ................71
• 4.1 Introduction ...................................................................................71
• 4.2 Results and discussion...................................................................74
• 4.2.1 Transfer-printing of a patterned layer composed of organic
• small molecules from the PFPE-coated stamp to a target substrate .74
• 4.2.2 Fabrication of green fluorescent OLEDs by organic-layer
• transfer .…………………..……………………………..………….85
• 4.2.3 Characteristics of a transferred graphene monolayer using the
• PFPE-coated PDMS stamp ...............................................................89
• 4.2.4 Organic hole-only device with a graphene bottom electrode
• deposited by using the PFPE-coated PDMS stamp ..........................95
• 4.2.5 Characteristics of red phosphorescent OLEDs fabricated by
• patterns transfer...............................................................................100
• 4.2.6 Organic patterns formed by a transfer-printing process ....107
• 4.3 Conclusions .................................................................................107
• 4.4 References ...................................................................................109
• Chapter 5 Conclusions..........................................................................115
• 5.1 Summary......................................................................................115
• 5.2 Further studies .............................................................................117
• 5.2.1 Further applications of a contact-transfer process using PFPEcoated PDMS stamps......................................................................117
• 5.2.2 Further modification of a PFPE-coated PDMS stamp.......118
• 5.3 References ...................................................................................121
• 요 약 (국문초록)........................................................................................123