잉크젯 양자점 발광 다이오드의 성능 향상에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2021. 2. 곽정훈.콜로이드성 양자점은 광소자에 사용하기에 적합한 광학적 전기적 특성을 가지고 있다. 특히나 높은 양자효율, 좁은 발광 파장대, 무기 재료의 내적 열안정성과 광안정성을 가지고 있기에 발광다이오드의 광물질로 사용하기에 적합하다. 그러나 양자점 소재는 콜로이드로 용액 공정 기반이기 때문에 차세대 디스플레이로 사용되기 위해서는 패터닝 기술이 매우 중요하다. 드랍 캐스팅 방법, 미스트 코팅 방법, 트랜스퍼 프린팅 등의 다양한 기술들이 보고되어 왔으나 최근에는 물질 소모 최소화와 고해상도 공정이 가능한 잉크젯 기술이 관심받고 있다. 그러나 잉크젯 기술에는 몇 가지 기술적인 이슈로 인해서 잉크젯 프린팅된 양자점 자발광 소자의 성능은 아직까지 스핀코팅 소자에 비해 낮게 보고되고 있다. 최근에는 잉크젯 프린팅 기반의 양자점 자발광 소자는 용매 혼합물과 뱅크 구조물의 표면 에너지를 바꾸는 방식 등으로 많은 연구가 보고되고 있지만 아직 그에 따른 성능을 분석하는 연구는 부족한 상황이다. 잉크젯 프린팅 기술을 이용한 고성능 양자점 발광 다이오드의 제작을 위해서는 몇 가지 도전과제가 있다. 첫 번째는 커피링 효과와 낮은 러프리스를 낮추는 균일한 박막형성이다. 두 번째는 노즐 클로깅, 머신 흔들림, 오류발생 등에 의한 노즐에서 사출된 방울의 각도변화로 인한 프린팅 실패이다. 이로써 미스-에이밍과 오버플로우가 발생한다. 세 번째는 노즐에서 안정된 드랍을 형성하기 위한 제터빌리티이다. 이는 점도, 표면장력, 내부 장력에 의해서 결정지어진다. 마지막은 용매 제한인데, 아래 층을 녹이지 않으면서 양자점 엉킴을 막으며 사람에게 유해하지 않을 조건을 가지고 있어야 한다. 잉크젯 양자점 발광 다이오드의 성능을 향상시키기 위해 용매 혼합물을 통해서 균일한 양자점 필름을 만들거나 표면 에너지를 바꾸면서 프린팅 실패를 방지하기 위해서 여러가지 연구들이 있었다. 잉크젯 양자점 발광 다이오드의 실용적인 사용을 위해서, 커피링 효과와 프린팅 실패를 잉크젯 소자의 성능과 관련하여 해결할 필요가 있다. 본 연구에서는 소수성의 폴리머를 픽셀 구조물인 뱅크로 사용해서 프린팅 실패를 보상해 픽셀 균일성을 확보하고, 투명한 폴리머인 PMMA가 큐디 잉크에 섞였을 때 필름 모포로지 변화와 그 전기적 상향에 대한 연구를 진행하였다. 소수성 격벽을 사용했을 때, 각도 변화가 있는 프린팅된 양자점 잉크가 뱅크 내부에 잘 위치하였고 발광 영역에 벗어나는 오버플로우가 방지하였다. 잘 형성된 잉크는 소자의 픽셀 균일성을 향상시켰고, 결과적인 소자의 성능은 5300 cd m-2 의 밝기와 0.11 % 의 양자 효율을 낼 수 있었다. 비교적 낮은 성능에도 불구하고, 이는 프린팅 실패의 저항성을 보여주었고, 정교한 최적화를 통하면 소자 성능이 보다 나은 향상을 보일 것이라 생각한다. 또한, 양자점 잉크에 PMMA를 첨가했을 때, 양자점-폴리머 혼합 잉크는 커피링 효과를 줄일 수 있었고 균일한 박막을 형성하였다. 뱅크 격벽의 쌓임 현상 또한 추가적인 폴리머 마랑고니 효과를 통하여 줄일 수 있었다. 게다가, 적절한 폴리머 체인 길이의 PMMA는 표면 거칠기를 줄여서, 소자의 성능도 향상 시킬 수 있었다. 결과적인 잉크젯 양자점 발광다이오드는 73360 cd m-2 의 밝기와 2.8 % 의 양자효율로, 폴리머 첨가물을 넣지 않은 잉크젯 양자점 소자의 비해 눈에 띄게 향상되었다. 본 논문에서 개발된 양자점 발광 다이오드의 소자 격벽 구조와 양자점 폴리머 혼합 잉크에 대한 결과는 고효율 잉크젯 양자점 발광다이오드의 실현에 큰 도움이 될 것으로 생각된다.Colloidal quantum dot light-emitting diodes (QLED) are promising next-generation displays, exhibiting excellence in color purity, low material cost possibility, brightness. Quantum dots, which have many advantages, require patterning technology because of their colloidal status. Various patterning method for colloidal QD have been proposed using drop-casting, mist coating, transfer printing, inkjet printing. Drop casting method can fabricate fast but having weakness in large area process. Mist coating method might be made a monolayer deposition with high accuracy but difficulty in high resolution. Transfer printing method is possible to highest resolution patterning in technologies but having an ink contamination issue. However, ink-jet printing technology is emerging interest for the fabrication of QLEDs because of advantages such as high-resolution pattern possibility, fast processability and tiny material usage by drop-on-demand process. To fabricate high performance QLEDs using inkjet-printing technology, there are some challenges. First is morphology issue which goal is achieving a uniform film deposition against coffee ring effect and bad roughness. Second is a printing failure, which considering accurate positioning of the ink droplet against the angular deviation of the droplet leaving nozzle of the inkjet cartridge. This droplet deflection may be caused by nozzle clogging, machine tremor and error occurrence in inkjet-printing machine, and it leads to two problem such as mis-aiming and overflow. Third is a jeattability for forming a stable drop at nozzle. This is determined by rheological parameters such as viscosity, inertial force, and surface tension. Final is solvent limits which are not dissolving the underlayer, preventing QD aggregation and toxicity to human. There have been studies related to enhancement of inkjet printed QLED by using solvent mixture to form uniform film or using hydrophobic walls to prevent from mis-aiming and overflow, but the reported performance of inkjet-printed QLED is still low. For the practical use of inkjet-printed QLEDs, it is prerequisite to resolve the morphology issue against coffee-ring effect and the printing failure in the relation with the performance of inkjet printed device. In this study, we improved the EQE and CE of inkjet-printed QLED device using hydrophobic walls and QD-polymer composite ink. Hydrophobic walls are used making the droplet positing precise within the bank and it is evaluated a photolithographic property and the resistivity on the printing failure of this material. QD-polymer composite ink can increase viscosity of ink and induce the additional polymer Marangoni effect. When using hydrophobic walls, printed QD ink with the angular deviation is positioned well in the bank and prevent from the overflow out of emission area. Well defined ink induces the pixel uniformity of devices and resulted QLED exhibit the maximum luminance of 5300 cd m-2 and the external quantum efficiency of 0.11 %. Despite of these relatively low performance, it shows the resistivity of printing failure, so I believe it can be further improved through elaborated optimization. Also, when PMMA is added in the QD ink, the QD–polymer composite ink can reduce the coffee-ring effect and form a uniform thin film. Pile-up at the bank wall is also reduced by additional polymer Marangoni effect. In addition, PMMA of suitable polymer chain length can reduce the surface roughness, thereby improving the morphological properties of the thin film. The resulting inkjet-printed QLED emit the highest luminance of 73360 cd m-2 and the external quantum efficiency of 2.8 %, which are conspicuously higher than that of the inkjet-printed QLED without polymer additives. These results in this thesis show the impact of printing accuracy and uniform film formation, and suggest these methods will promise the high performance of inkjet printed QLEDsChapter 1 Introduction 0 1.1 Colloidal Quantum Dots 0 1.2 Fabrication Technology of QLEDs 5 1.3 Key Issues for Inkjet-printed QLED Performance 7 1.4 Outline of Thesis 9 Chapter 2. Experimental Methods 13 2.1 Materials 13 2.1.1 Red-color Emitting CdSe/Zn1-XCdXS Core/shell Heterostructured Quantum Dots 13 2.1.2 Fluorinated photopolymer, PFBI. 15 2.1.4 Organic Material 15 2.1.3 Preparation of ZnO Nanoparticels 17 2.2 Device Fabrication and Characterization Methods 17 2.2.1 Device Fabrication 17 2.2.2 Current-voltage-luminance Measurement 18 2.2.3 Efficiency Calculation Methods 21 2.2.3 Other Characterization Methods 21 2.3 Theory 24 2.2.1 Surface Energy Analysis 24 2.2.2 Coffee Ring Effect and Capillary Flow 25 2.2.3 Marangoni Flow 26 Chapter 3. Printing Accuracy Improvement of Inkjet-printed QLED with Engineered Bank using PFBI as Highly Fluorinated Photopolymer 27 3.1 Introduction 30 3.2 Evalution of Pixelated Structure with PFBI 30 3.3.1 Highly Fluorinated Photopolymer, PFBI for Inkjet-printed QLEDs 32 3.3.2 Characteristics of Pixelated Structure made of PFBI 33 3.3 Evalution of QD Inks on Pixelated Structure with PFBI 38 3.3.1 Morphology Properties of QD Inks on Pixelated PFBI Structure 40 3.3.2 Characteristics of Inkjet-printed QLED using PFBI 43 3.3 Summary 47 Chapter 4. Efficiency Improvement of Inkjet-printed QLEDs Employing Polymer Additives 48 4.1 Introduction of Inkjet-printed QLEDs 50 4.2 Evaluation of QD-PMMA Composite Ink on Planar Substrate 54 4.2.1 QD-PMMA Composite Ink for Reducing Coffee Ring Effect 54 4.2.2 Morphology Uniformity of QD Droplet Film using PMMA Additives 59 4.3 Evaluation of QD-PMMA Composite Ink on Pixelated Structure 64 4.3.1 Morphology Properties of QD Inks on Pixelated Structure 64 4.3.2 Electrical Characteristics of Inkjet-printed QLEDs Employing PMMA Additives 68 4.6 Summary 77 Chapter 5 78 Bibilography 81 한글 초록 86Docto

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Recently, additive manufacturing is one of the most focused research topics due to its explosive development, especially in manufacturing engineering and medical science. In order to build 3D complex scaffolds with multi-biomaterials for clinical application, a new 3D multi-nozzle system with dual-mode drives, i.e. ejection and extrusion was developed. In this paper, much effort was made to gain fine control of droplet and excellent coordination during fabrication. Specifically, the parameters that influence the size and stability of droplet most was intensively studied. Considering that the biomaterials used in the future may have much difference in properties, the combination of parameters was investigated to facilitate the settings for certainsized droplets, which are potentially eligible for bio-printing. The dispensing nozzles can work well both in independent and convergent mode, which can be freely switched. Outstanding to the most currently used 3D bio-printing techniques, this system can fabricate scaffolds with multi-materials of both low viscosity (by pneumatic dispensing) and high viscosity (through motor extrusion). It is highly expected that this system can satisfy clinical application in the near future