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

Holographic Particle Image Velocimetry of Ink Jet Streams

Ink jet technology is a rapidly growing and diverse field of research. Ink jets are used to deliver very precise and small (picolitre) volumes of fluid to a surface. Recent advances in ink jet technology demand a better understanding of the dynamics of the fluid during jetting. The aim of this project was to design a method capable of measuring the flow velocities inside ink jet streams. This objective has been achieved by the use of digital holographic particle image velocimetry. The difficulty with measuring flows inside tightly curved samples is that the refractive index change over the boundary leads to an optical distortion and therefore particles cannot be viewed or tracked reliably. Optical distortion is compensated for by taking advantage of the ability to replay a holographically recorded wave. The light scattered by particles is propagated numerically back through the sample’s surface, to form a three-dimensional image in which all refractions at the interface have been accounted for. Three dimensional particle fields are then analysed using custom particle detection and correlation code to extract the displacement of individual particles between exposures, which facilitates the construction of full flow profiles. Holograms were recorded with a simple off-axis holographic microscope, comprising two point sources of divergent light, formed from the same objective lens, acting as the source of illumination and reference light, respectively. Experiments were conducted on continuous ink jet streams of water issuing from a nozzle with 100 µm diameter. For a few millimetres after the nozzle exit, the jet is cylindrical, it then starts to form swells and necks; the swells continue to grow at the expense of the necks until the jet breaks up into a stream of droplets. Measurements of the stream wise component of velocity have been successful in the cylindrical parts of the jet, in swells and in necks greater than 20 µm in diameter. To my knowledge measurements of particle velocities on fluid jets at this scale have not been accomplished previously

An Investigation into the relationship between contrast and resolution of a printing system using the RIT contrast resolution test target

A problem arises when different printing systems are used to print images. Different systems have considerably different contrast and resolution capabilities while an individual printing system might have a low resolution capability, the system may have the ability to render low contrast detail. Similarly, if a printing system has a high resolution capability, it does not necessarily mean that such a system has the ability to render low contrast detail well. Such contrast and resolution restrictions may be attributed to the capabilities of the PostScript interpreter, the screening method used by the RIP, the image transfer method of the output device, the substrate used, or a combination of these factors. The RIT Contrast Resolution Test Target has been developed to measure the relationship between contrast and resolution of a printing system. The target measures the contrast-resolution capability of the printing system in both the horizontal and vertical print direction of the printing device. A graph can be plotted to show the Contrast Sensitivity (CS) for the printing system. From this distribution, a contrastresolution- volume (CRV) can be calculated to produce a quantitative contrast-resolution measurement for an individual printing system. The hypothesis of this thesis is that the RIT Contrast Resolution Test Target can provide a method of discriminating the CRV of marking engines and screening methods by using analysis methods intended for use with the target. The target was printed on several printing systems. 12 observers were used to measure the target. The observers were given instruction on proper target reading, and their observations were recorded as CRV measurements. The CRV values for all colors from each system were averaged for each observer. The averaged data was entered into a two-way ANOVA test, where the two dimensions in the test were systems and observers. The results of the ANOVA test showed that there was significant variance in the average CRV values from each system, and the hypothesis of this thesis was accepted. In addition, the ANOVA test indicated that there was significant variance between the observers readings. Although each observer used a different judging criteria, it was concluded that the observers evaluated the different systems relative to one and other in almost the same sequence

Inkjet printing digital image generation and compensation for surface chemistry effects

Additive manufacturing (AM) of electronic materials using digital inkjet printing (DIJP) is of research interests nowadays because of its potential benefits in the semiconductor industry. Current trends in manufacturing electronics feature DIJP as a key technology to enable the production of customised and microscale functional devices. However, the fabrication of microelectronic components at large scale demands fast printing of tight features with high dimensional accuracy on substrates with varied surface topography which push inkjet printing process to its limits. To understand the DIJP droplet deposition on such substrates, generally requires computational fluid dynamics modelling which is limited in its physics approximation of surface interactions. Otherwise, a kind of “trial and error” approach to determining how the ink spreads, coalesce and solidifies over the substrate is used, often a very time-consuming process. Consequently, this thesis aims to develop new modelling techniques to predict fast and accurately the surface morphology of inkjet-printed features, enabling the optimisation of DIJP control parameters and the compensation of images for better dimensional accuracy of printed electronics devices. This investigation explored three categories of modelling techniques to predict the surface morphology of inkjet-printed features: physics-based, data-driven and hybrid physics-based and data-driven. Two physics-based numerical models were developed to reproduce the inkjet printing droplet deposition and solidification processes using a lattice Boltzmann (LB) multiphase flow model and a finite element (FE) chemo-mechanical model, respectively. The LB model was limited to the simulation of single tracks and small square films and the FE model was mainly employed for the distortion prediction of multilayer objects. Alternatively, two data-driven models were implemented to reconstruct the surface morphology of single tracks and free-form films using images from experiments: image analysis (IA) and shape from shading (SFS). IA assumed volume conservation and minimal energy drop shape to reconstruct the surface while SFS resolved the height of the image using a reflection model. Finally, a hybrid physics-based and data-driven approach was generated which incorporates the uncertainty of droplet landing position and footprint, hydrostatic analytical models, empirical correlations derived from experiments, and relationships derived from physics-based models to predict fast and accurately any free-form layer pattern as a function of physical properties, printing parameters and wetting characteristics. Depending on the selection of the modelling technique to predict the deformed geometry, further considerations were required. For the purely physics-based and data-driven models, a surrogate model using response surface equations was employed to create a transfer function between printing parameters, substrate wetting characteristics and the resulting surface morphology. The development of a transfer function significantly decreased the computational time required by purely physics-based models and enabled the parameter optimisation using a multi-objective genetic algorithm approach to attain the best film dimensional accuracy. Additionally, for multilayer printing applications, a layer compensation approach was achieved utilizing a convolutional neural network trained by the predicted (deformed) geometry to reduce the out of plane error to target shape. The optimal combination of printing parameters and input image compensation helped with the generation of fine features that are traditionally difficult for inkjet, improved resolution of edges and corners by reducing the amount of overflow from material, accounted for varied topography and capillary effects thereof on the substrate surface and considered the effect of multiple layers built up on each other. This study revealed for the first time to the best of our knowledge the role of the droplet location and footprint diameter uncertainty in the stability and uniformity of printed features. Using a droplet overlap map which was proposed as a universal technique to assess the effect of printing parameters on pattern geometry, it was shown that reliable limits for break-up and bulging of printed features were obtained. Considering droplet position and diameter size uncertainties, predicted optimal printing parameters improved the quality of printed films on substrates with different wettability. Finally, a stability diagram illustrating the onset of bulging and separation for lines and films as well as the optimal drop spacing, printing frequency and stand-off distance was generated to inform visually the results. This investigation has developed a predictive physics-based model of the surface morphology of DIJP features on heterogeneous substrates and a methodology to find the printing parameters and compensate the layer geometry required for optimum part dimensional accuracy. The simplicity of the proposed technique makes it a promising tool for model driven inkjet printing process optimization, including real time process control and paves the way for better quality devices in the printed electronics industry

디스플레이 및 이미징 시스템으로의 응용을 위한 3D 프린팅 기반 맞춤형 광학 요소의 개발

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2021. 2. 홍용택.일반적으로 제조 공정은 절삭 방식과 적층 방식으로 구분된다. 이 중에서 적층 방식 공정은 저비용 및 단시간으로 복잡한 형태의 구조를 만들 수 있어서 이에 대한 연구와 개발이 꾸준히 진행되어왔다. 특히 3D 프린팅은 적층 방식 공정 중에서 가장 대표적인 방법으로, 기계 부품 및 생체 기관 제조 등의 분야에서는 이미 상용화가 진행되고 있다. 하지만 전자 소자 및 광학 요소 분야에서의 3D 프린팅의 활용은 여전히 연구 개발 또는 시제품 제작 단계에 머무르고 있다. 특히 마이크로 렌즈, 컬러 필터 등이 3D 프린팅으로 응용할 수 있는 가장 가능성 있는 광학 요소로서 디스플레이 및 이미징 시스템에 널리 사용될 것으로 예상되지만 여전히 상용화를 위한 연구가 진행 중이다. 또한 3D 프린팅을 이용한 광학 요소의 제작은 소재, 길이 스케일, 형상 및 응용 방안 등에서도 제한이 많은 상황이다. 따라서 이러한 문제를 극복하기 위해서는 디스플레이 및 이미징 시스템에서의 3D 프린팅 된 광학 요소의 유용성을 확장해야 하며, 다음과 같이 세 가지 측면에서 향상된 성능을 달성해야 한다. 첫째, 다양한 방식의 3D 프린팅 방법을 통해 마이크로미터에서 센티미터까지 광범위의 길이 스케일을 가지는 구조물의 제작이 가능해야 한다. 둘째, 임의의 곡면, 계층적 구조 등 복잡한 형상의 구조물을 쉽게 제작할 수 있어야 한다. 셋째, 단단한 소재 대신 탄성체와 같은 소프트 소재를 이용하여 광학적인 기능을 용이하게 조절할 수 있어야 한다. 이와 같은 동기를 바탕으로 본 학위 논문에서는 디스플레이 및 이미징 시스템으로의 응용을 위한 3D 프린팅 기반 맞춤형 광학 요소의 개발에 대한 내용을 보고한다. 3D 프린팅 기반 광학 요소를 매크로 스케일, 마이크로 스케일 그리고 매크로 및 마이크로 스케일이 혼합된 계층적 구조 등 세 가지 유형으로 분류하고 각각에 대한 응용 분야를 제시한다. 매크로 스케일의 광학 요소로는 가장 기본적인 요소인 렌즈와 거울을 선택한다. 렌즈는 공압식 디스펜싱 방법을 이용하여 실린드리컬 쌍 형태로 제작되었으며, 심리스 모듈러 평판식 디스플레이의 구현을 위해 적용된다. 또한 용융 적층 방식의 3D 프린팅으로 만들어진 몰드를 이용하여 거울을 제작하고, 이를 이용하여 심리스 모듈러 커브드 엣지 디스플레이를 구현한다. 이와 같이 모듈러 디스플레이의 이음새 부분에 3D 프린팅으로 제작된 렌즈 또는 거울을 부착하는 방식으로 화면을 심리스로 확장하는 기술을 제시하고, 다양한 형태의 디스플레이에 적용할 수 있는 가능성을 보여준다. 마이크로 스케일의 광학 요소로는 발광 다이오드에서 색 변환과 광 추출 기능을 동시에 나타내는 색 변환 마이크로 렌즈를 선택한다. 양자 점/광 경화성 고분자 복합체의 전기수력학적 프린팅을 통해 양자 점이 내장된 다양한 형태의 색 변환 마이크로 렌즈를 제작하며, 이를 청색 마이크로 발광 다이오드 어레이의 발광부 상에 적용하여 풀 컬러 마이크로 발광 다이오드 디스플레이로의 응용 가능성을 제시한다. 마지막으로 매크로 및 마이크로 스케일이 혼합된 계층적 구조의 광학 요소로서 디스펜싱 및 건식 러빙 과정의 조합으로 제작된 겹눈 형태를 모사한 렌즈 구조를 제시한다. 반구 형태의 매크로 렌즈를 디스펜싱으로 형성하고, 매크로 렌즈의 곡면 상에 단층의 마이크로 입자의 배열을 얻기 위해 건식 러빙 공정을 진행한다. 이러한 방식으로 형성된 계층적 구조가 소프트한 소재로 복제되어서 신축성을 가지는 겹눈 형태 모사 구조가 완성된다. 마이크로 렌즈 어레이는 매크로 렌즈의 표면을 따라 형성되고 리지드 아일랜드로 역할을 하여, 전체 계층적 구조에 기계적 변형이 가해져 매크로 렌즈의 모양이 변형되어도 마이크로 렌즈는 형상과 해상도, 초점 거리 등의 광학적 특성을 유지할 수 있다. 본 학위 논문은 3D 프린팅을 이용하여 다양한 형태와 스케일의 광학 요소를 제작하고 디스플레이 및 이미징 시스템으로의 여러 응용을 보여줌으로서 앞으로의 새로운 연구 및 개발 방향성을 제시하는 것을 주요 목적으로 한다. 3D 프린팅 설비의 단가가 낮아지고 정밀도 및 해상도가 높아지는 추세에 따라, 광학 요소를 쉽게 만들고 응용할 수 있는 맞춤형 광학 또는 스스로 구현하는 광학 분야가 변형 가능하고 멀티 스케일의 광학계로 점차 확대될 것으로 예상된다. 궁극적으로는 차세대 디스플레이 및 이미징 시스템에 필요한 광학 요소를 위한 기술의 저변을 넓히고, 이를 산업 전반에 응용할 수 있는 기반을 마련하고자 한다.Generally, the manufacturing process is divided into the subtractive (top-down) type and additive type (bottom-up). Among them, the additive manufacturing process has attracted a lot of attention because it can manufacture products with complex shapes in a low-cost and short-time process. In particular, three-dimensional (3D) printing is a representative method, which has already been commercialized in the field of mechanical components and biomedical organ. However, it remains in the research and development step in the field of electronic devices and optical components. Especially, although 3D printed optical components including microlens and color filter are expected to be widely used in display and imaging systems, it is still under investigation for commercialized products, and there are limitations in terms of materials, length scale, shape, and practical applications of components. Therefore, to overcome these issues, it is required for investigating and expanding the potential usefulness for 3D printed optical components in display and imaging systems to achieve better performance, productivity, and usability in three aspects. First, it should be possible to manufacture structures with a wide range of length scales from micrometer to centimeter through various 3D printing methods. Second, complex shapes such as free-from curved surfaces and hierarchical structures should be easily fabricated. Third, it is necessary to add functionality by manufacturing structures in which tunable functions are introduced using soft materials like an elastomer. Based on the above motivations, 3D printing-based customized optical components for display and imaging system applications are introduced in this dissertation. 3D printed optical components are classified into three types and their applications are showed to verify the scalability of 3D printing: macro-scale, microscale, and hierarchical macro/micro-scale. As macro-scale printed optical components, lens and mirror which are the most basic optical components are selected. The lens is fabricated by a pneumatic-type dispensing method with the form of a cylindrical pair and adopted for demonstration of seamless modular flat panel display. Besides, a seamless modular curved-edge display is also demonstrated with a mirror, which is fabricated from fused deposition modeling (FDM)-type 3D printed mold. By simply attaching a printed lens or mirror onto the seam of the modular display, it is possible to secure seamless screen expansion technology with the various form factor of the display panel. In the case of micro-scale printed optical components, the color-convertible microlens is chosen, which act as a color converter and light extractor simultaneously in a light-emitting diode (LED). By electrohydrodynamic (EHD) printing of quantum dot (QD)/photocurable polymer composite, QD-embedded hemispherical lens shape structures with various sizes are fabricated by adjusting printing conditions. Furthermore, it is applied to a blue micro-LED array for full-color micro-LED display applications. Finally, a tunable bio-inspired compound (BIC) eyes structure with a combination of dispensing and a dry-phase rubbing process is suggested as a hierarchical macro/micro-scale printed optical components. A hemispherical macrolens is formed by the dispensing method, followed by a dry-phase rubbing process for arranging micro particles in monolayer onto the curved surface of the macrolens. This hierarchical structure is replicated in soft materials, which have intrinsic stretchability. Microlens array is formed on the surface of the macrolens and acts as a rigid island, thereby maintaining lens shape, resolution, and focal length even though the mechanical strain is applied to overall hierarchical structures and the shape of the macrolens is changed. The primary purposes of this dissertation are to introduce new concepts of the enabling technologies for 3D printed optical components and to shed new light on them. Optical components can be easily made as 3D printing equipment becomes cheaper and more precise, so the field of Consumer optics or Do it yourself (DIY) optics will be gradually expanded on deformable and multi-scale optics. It is expected that this dissertation can contribute to providing a guideline for utilizing and customizing 3D printed optical components in next-generation display and imaging system applications.Chapter 1. Introduction 1 1.1. Manufacturing Process 1 1.2. Additive Manufacturing 4 1.3. Printed Optical Components 8 1.4. Motivation and Organization of Dissertation 11 Chapter 2. Macro-scale Printed Optical Components 15 2.1. Introduction 15 2.2. Seamless Modular Flat Display with Printed Lens 20 2.2.1. Main Concept 20 2.2.2. Experimental Section 23 2.2.3. Results and Discussion 26 2.3. Seamless Modular Curved-edge Display with Printed Mirror 32 2.3.1. Main Concept 32 2.3.2. Experimental Section 33 2.3.3. Results and Discussion 36 2.4. Conclusion 46 Chapter 3. Micro-scale Printed Optical Components 47 3.1. Introduction 47 3.2. Full-color Micro-LED Array with Printed Color-convertible Microlens 52 3.2.1. Main Concept 52 3.2.2. Experimental Section 54 3.2.3. Results and Discussion 57 3.3. Conclusion 65 Chapter 4. Hierarchical Macro/Micro-scale Printed Optical Components 66 4.1. Introduction 66 4.2. Tunable Bio-inspired Compound Eye with Printing and Dry-phase Rubbing Process 69 4.2.1. Main Concept 69 4.2.2. Experimental Section 71 4.2.3. Results and Discussion 73 4.3. Conclusion 79 Chapter 5. Conclusion 80 5.1. Summary 80 5.2. Limitations and Suggestions for Future Researches 83 References 88 Abstract in Korean (국문 초록) 107Docto

Color ink-jet printing: Evaluation of print quality on different paper substrates

Ink-jet printing has gained a dominant position among the technology choices for on demand generation of color hard copies from the computer. Reduction of ink-related problems, low cost, improved resolution, and less-demanding substrate requirements have become important advantages. This study\u27s primary objective is to evaluate, both subjectively and objectively, the effect of various paper properties on the quality of print obtained from a color ink-jet printer. The initial stage of this investigation involved a subjective evaluation of the different prints by a panel of observers. A dual scaling analysis of the subjective responses indicates that for the paper substrates selected, Color Density is the predominant attribute, with Sharpness constituting a lesser second attribute. The product of their respective objective measures, Density Range and Subjective Quality Factor (SQF) appears to be an easily-measurable, overall indicator of print quality. SQF is affected by opacity and brightness of the papers, while density range is affected by the smoothness. It also becomes evident that lower-quality xerographic paper results in better prints than premium xerographic papers under color ink-jet printing. With ink-jet printing, paper substrate affects all the main print quality factors including pattern, sharpness and color reproduction. This study also demonstrates that Penetration and Spreading are two related phenomena, yet are different in terms of their effect on print quality. Spreading can take place either on the surface of paper or inside the paper structure

Inkjet printing digital image generation and compensation for surface chemistry effects

Additive manufacturing (AM) of electronic materials using digital inkjet printing (DIJP) is of research interests nowadays because of its potential benefits in the semiconductor industry. Current trends in manufacturing electronics feature DIJP as a key technology to enable the production of customised and microscale functional devices. However, the fabrication of microelectronic components at large scale demands fast printing of tight features with high dimensional accuracy on substrates with varied surface topography which push inkjet printing process to its limits. To understand the DIJP droplet deposition on such substrates, generally requires computational fluid dynamics modelling which is limited in its physics approximation of surface interactions. Otherwise, a kind of “trial and error” approach to determining how the ink spreads, coalesce and solidifies over the substrate is used, often a very time-consuming process. Consequently, this thesis aims to develop new modelling techniques to predict fast and accurately the surface morphology of inkjet-printed features, enabling the optimisation of DIJP control parameters and the compensation of images for better dimensional accuracy of printed electronics devices. This investigation explored three categories of modelling techniques to predict the surface morphology of inkjet-printed features: physics-based, data-driven and hybrid physics-based and data-driven. Two physics-based numerical models were developed to reproduce the inkjet printing droplet deposition and solidification processes using a lattice Boltzmann (LB) multiphase flow model and a finite element (FE) chemo-mechanical model, respectively. The LB model was limited to the simulation of single tracks and small square films and the FE model was mainly employed for the distortion prediction of multilayer objects. Alternatively, two data-driven models were implemented to reconstruct the surface morphology of single tracks and free-form films using images from experiments: image analysis (IA) and shape from shading (SFS). IA assumed volume conservation and minimal energy drop shape to reconstruct the surface while SFS resolved the height of the image using a reflection model. Finally, a hybrid physics-based and data-driven approach was generated which incorporates the uncertainty of droplet landing position and footprint, hydrostatic analytical models, empirical correlations derived from experiments, and relationships derived from physics-based models to predict fast and accurately any free-form layer pattern as a function of physical properties, printing parameters and wetting characteristics. Depending on the selection of the modelling technique to predict the deformed geometry, further considerations were required. For the purely physics-based and data-driven models, a surrogate model using response surface equations was employed to create a transfer function between printing parameters, substrate wetting characteristics and the resulting surface morphology. The development of a transfer function significantly decreased the computational time required by purely physics-based models and enabled the parameter optimisation using a multi-objective genetic algorithm approach to attain the best film dimensional accuracy. Additionally, for multilayer printing applications, a layer compensation approach was achieved utilizing a convolutional neural network trained by the predicted (deformed) geometry to reduce the out of plane error to target shape. The optimal combination of printing parameters and input image compensation helped with the generation of fine features that are traditionally difficult for inkjet, improved resolution of edges and corners by reducing the amount of overflow from material, accounted for varied topography and capillary effects thereof on the substrate surface and considered the effect of multiple layers built up on each other. This study revealed for the first time to the best of our knowledge the role of the droplet location and footprint diameter uncertainty in the stability and uniformity of printed features. Using a droplet overlap map which was proposed as a universal technique to assess the effect of printing parameters on pattern geometry, it was shown that reliable limits for break-up and bulging of printed features were obtained. Considering droplet position and diameter size uncertainties, predicted optimal printing parameters improved the quality of printed films on substrates with different wettability. Finally, a stability diagram illustrating the onset of bulging and separation for lines and films as well as the optimal drop spacing, printing frequency and stand-off distance was generated to inform visually the results. This investigation has developed a predictive physics-based model of the surface morphology of DIJP features on heterogeneous substrates and a methodology to find the printing parameters and compensate the layer geometry required for optimum part dimensional accuracy. The simplicity of the proposed technique makes it a promising tool for model driven inkjet printing process optimization, including real time process control and paves the way for better quality devices in the printed electronics industry

Stratospheric constituent measurements using UV solar occultation technique

The photochemistry of the stratospheric ozone layer was studied as the result of predictions that trace amounts of pollutants can significantly affect the layer. One of the key species in the determination of the effects of these pollutants is the OH radical. A balloon flight was made to determine whether data on atmospheric OH could be obtained from lower resolution solar spectra obtained from high altitude during sunset

Chemical approaches to ubiquitous computing

Dissertação apresentada para obtenção do Grau de Doutor em Química, perfil de Química Física, pela Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologi