액정기반 발광형 디스플레이를 위한 용액공정 내재형 편광판에 관한 연구

학위논문 (석사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 이신두.Recently, the advancement of the liquid crystal display (LCD) technology has greatly focused on the clear image quality together with the natural color. According to the demand for the image quality, the in-cell polarizers have been attracted much attention owing to the advantages of improving the contrast ratio and reducing the thickness of LCD. In this work, we proposed the QD-based emissive LCD with the in-cell polarizer composed of dichroic dyes. The in-cell polarizer was fabricated through the solution-processing of a dichroic dye solution. The QD layer was constructed on the inner surface of the top substrate, and the in-cell polarizer was subsequently prepared on the QD layer to prevent the depolarization of the emission light and the degradation of the QDs. The intensity of the incident light for exciting QDs was modulated by the phase retardation through the LC layer, depending on the magnitude of the applied voltage. This leads directly to the modulation of the emission spectra of QDs with the color gamut extended to about 80 % of the BT.2020 standard. The architecture based on the in-cell polarizer will provide a simple and viable method of constructing the QD-based emissive LCD with high color purity in a cost-effective manner.최근 액정 디스플레이(LCD) 기술들은 자연스러운 색상과 더불어 선명한 화질을 중심으로 크게 발전했다. 고화질 영상에 대한 수요와 함께, 내재형 편광판 (in-cell polarizer)는 대조율 향상과 LCD 두께 감소라는 다양한 장점들로 인해 더 많은 관심을 끌었다. 본 연구에서는 이색성 염료로 구성된, 내재형 편광판를 적용한 양자점 기반 광 발광 액정 디스플레이를 제안하였다. 양자점 층은 상단 기판의 내부 표면에 구성되었고, 이후 양자점 층에 내재형 편광판를 도입하여 입사광의 편광상태를 유지하고 양자점의 성능 저하를 방지하였다. 내재형 편광판은 용액 공정을 통해 이색성 염료를 정렬하여 제작되었다. 양자점 광 발광을 위한 입사광의 세기는 적용된 전압의 세기에 따라 액정 셀을 통한 위상 지연에 의해 변조되었다. 이는 색 영역이 BT.2020 표준의 약 80%까지 확장된 높은 색 순도를 보여준다. 내재형 편광판에 기반한 액정 디스플레이 구조는 비용-효율적인 방법으로 높은 색 순도를 가진 양자점 기반 광 발광 액정 디스플레이를 구성하는 간단하고 실행가능한 방법을 제공할 것이다.1. Introduction 1 1.1. Overview of liquid crystal-based displays 1 1.1.1. Main LCD Modes 3 1.1.2. Types of backlight unit for LCDs 9 1.2. Outline of thesis 15 2. LCD with QD color filters 16 2.1. Types of polarizers 16 2.2. Architecture of QD-LCD with in-cell polarizer 19 3. Experiments 24 3.1. Fabrication of photoluminescence QD patterns 24 3.2. Dichroic dye-based in-cell polarizer 26 3.3. Solution-processed in-cell polarizer with QD-based LC cell 29 3.4. Measurements of optical and photoluminescence characteristics 30 4. Results and Discussion 31 4.1. Analysis of polarizing characteristics of in-cell polarizer 31 4.2. Photoluminescence characteristics of QD-based LC cell with in-cell polarizer 34 4.3. Microscopic images of QD-based LC cell with in-cell polarizer 35 5. Conclusion 38 Bibliography 39 국문 초록 43Maste

Advanced liquid crystal displays with supreme image qualities

Several metrics are commonly used to evaluate the performance of display devices. In this dissertation, we analyze three key parameters: fast response time, wide color gamut, and high contrast ratio, which affect the final perceived image quality. Firstly, we investigate how response time affects the motion blur, and then discover the 2-ms rule. With advanced low-viscosity materials, new operation modes, and backlight modulation technique, liquid crystal displays (LCDs) with an unnoticeable image blur can be realized. Its performance is comparable to an impulse-type display, like cathode ray tube (CRT). Next, we propose two novel backlight configurations to improve an LCD\u27s color gamut. One is to use a functional reflective polarizer (FRP), acting as a notch filter to block the unwanted light, and the other is to combine FRP with a patterned half-wave plate to suppress the crosstalk between blue and green/red lights. In experiment, we achieved 97.3% Rec. 2020 in CIE 1976 color space, which is approaching the color gamut of a laser projector. Finally, to enhance an LCD\u27s contrast ratio, we proposed a novel device configuration by adding an in-cell polarizer between LC layer and color filter array. The CR for a vertically-aligned LCD is improved from 5000:1 to 20,000:1, and the CR for a fringe field switching LCD is improved from 2000:1 to over 3000:1. To further enlarge CR to fulfill the high dynamic range requirement, a dual-panel LCD system is proposed and the measured contrast ratio exceeds 1,000,000:1. Overall speaking, such an innovated LCD exhibits supreme image qualities with motion picture response time comparable to CRT, vivid color to laser projector, and contrast ratio to OLED. Along with other outstanding features, like high peak brightness, high resolution density, long lifetime, and low cost, LCD would continue to maintain its dominance in consumer electronics in the foreseeable future

High Efficiency and Wide Color Gamut Liquid Crystal Displays

Liquid crystal display (LCD) has become ubiquitous and indispensable in our daily life. Recently, it faces strong competition from organic light emitting diode (OLED). In order to maintain a strong leader position, LCD camp has an urgent need to enrich the color performance and reduce the power consumption. This dissertation focuses on solving these two emerging and important challenges. In the first part of the dissertation we investigate the quantum dot (QD) technology to improve the both the color gamut and the light efficiency of LCD. QD emits saturated color and grants LCD the capability to reproduce color vivid images. Moreover, the QD emission spectrum can be custom designed to match to transmission band of color filters. To fully take advantage of QD\u27s unique features, we propose a systematic modelling of the LCD backlight and optimize the QD spectrum to simultaneously maximize the color gamut and light efficiency. Moreover, QD enhanced LCD demonstrates several advantages: excellent ambient contrast, negligible color shift and controllable white point. Besides three primary LCD, We also present a spatiotemporal four-primary QD enhanced LCD. The LCD\u27s color is generated partially from time domain and partially from spatial domain. As a result, this LCD mode offers 1.5× increment in spatial resolution, 2× brightness enhancement, slightly larger color gamut and mitigated LC response requirement (~4ms). It can be employed in the commercial TV to meet the challenging Energy star 6 regulation. Besides conventional LCD, we also extend the QD applications to liquid displays and smart lighting devices. The second part of this dissertation focuses on improving the LCD light efficiency. Conventional LCD system has fairly low light efficiency (4%~7%) since polarizers and color filters absorb 50% and 67% of the incoming light respectively. We propose two approaches to reduce the light loss within polarizers and color filters. The first method is a polarization preserving backlight system. It can be combined with linearly polarized light source to boost the LCD efficiency. Moreover, this polarization preserving backlight offers high polarization efficiency (~77.8%), 2.4× on-axis luminance enhancement, and no need for extra optics films. The second approach is a LCD backlight system with simultaneous color/polarization recycling. We design a novel polarizing color filter with high transmittance ( \u3e 90%), low absorption loss (~3.3%), high extinction ratio (\u3e10,000:1) and large angular tolerance (up to ±50˚). This polarizing color filter can be used in LCD system to introduce the color/polarization recycling and accordingly boost LCD efficiency by ~3 times. These two approaches open new gateway for ultra-low power LCDs. In the final session of this dissertation, we demonstrate a low power and color vivid reflective liquid crystal on silicon (LCOS) display with low viscosity liquid crystal mixture. Compared with commercial LC material, the new LC mixture offers ~4X faster response at 20oC and ~8X faster response at -20°C. This fast response LC material enables the field-sequential-color (FSC) driving for power saving. It also leads to several attractive advantages: submillisecond response time at room temperature, vivid color even at -20oC, high brightness, excellent ambient contrast ratio, and suppressed color breakup. With this material improvement, LCOS display can be promising for the emerging wearable display market

High-dynamic-range Foveated Near-eye Display System

Wearable near-eye display has found widespread applications in education, gaming, entertainment, engineering, military training, and healthcare, just to name a few. However, the visual experience provided by current near-eye displays still falls short to what we can perceive in the real world. Three major challenges remain to be overcome: 1) limited dynamic range in display brightness and contrast, 2) inadequate angular resolution, and 3) vergence-accommodation conflict (VAC) issue. This dissertation is devoted to addressing these three critical issues from both display panel development and optical system design viewpoints. A high-dynamic-range (HDR) display requires both high peak brightness and excellent dark state. In the second and third chapters, two mainstream display technologies, namely liquid crystal display (LCD) and organic light emitting diode (OLED), are investigated to extend their dynamic range. On one hand, LCD can easily boost its peak brightness to over 1000 nits, but it is challenging to lower the dark state to \u3c 0.01 nits. To achieve HDR, we propose to use a mini-LED local dimming backlight. Based on our simulations and subjective experiments, we establish practical guidelines to correlate the device contrast ratio, viewing distance, and required local dimming zone number. On the other hand, self-emissive OLED display exhibits a true dark state, but boosting its peak brightness would unavoidably cause compromised lifetime. We propose a systematic approach to enhance OLED\u27s optical efficiency while keeping indistinguishable angular color shift. These findings will shed new light to guide future HDR display designs. In Chapter four, in order to improve angular resolution, we demonstrate a multi-resolution foveated display system with two display panels and an optical combiner. The first display panel provides wide field of view for peripheral vision, while the second panel offers ultra-high resolution for the central fovea. By an optical minifying system, both 4x and 5x enhanced resolutions are demonstrated. In addition, a Pancharatnam-Berry phase deflector is applied to actively shift the high-resolution region, in order to enable eye-tracking function. The proposed design effectively reduces the pixelation and screen-door effect in near-eye displays. The VAC issue in stereoscopic displays is believed to be the main cause of visual discomfort and fatigue when wearing VR headsets. In Chapter five, we propose a novel polarization-multiplexing approach to achieve multiplane display. A polarization-sensitive Pancharatnam-Berry phase lens and a spatial polarization modulator are employed to simultaneously create two independent focal planes. This method enables generation of two image planes without the need of temporal multiplexing. Therefore, it can effectively reduce the frame rate by one-half. In Chapter six, we briefly summarize our major accomplishments

High dynamic range display systems

High contrast ratio (CR) enables a display system to faithfully reproduce the real objects. However, achieving high contrast, especially high ambient contrast (ACR), is a challenging task. In this dissertation, two display systems with high CR are discussed: high ACR augmented reality (AR) display and high dynamic range (HDR) display. For an AR display, we improved its ACR by incorporating a tunable transmittance liquid crystal (LC) film. The film has high tunable transmittance range, fast response time, and is fail-safe. To reduce the weight and size of a display system, we proposed a functional reflective polarizer, which can also help people with color vision deficiency. As for the HDR display, we improved all three aspects of the hardware requirements: contrast ratio, color gamut and bit-depth. By stacking two liquid crystal display (LCD) panels together, we have achieved CR over one million to one, 14-bit depth with 5V operation voltage, and pixel-by-pixel local dimming. To widen color gamut, both photoluminescent and electroluminescent quantum dots (QDs) have been investigated. Our analysis shows that with QD approach, it is possible to achieve over 90% of the Rec. 2020 color gamut for a HDR display. Another goal of an HDR display is to achieve the 12-bit perceptual quantizer (PQ) curve covering from 0 to 10,000 nits. Our experimental results indicate that this is difficult with a single LCD panel because of the sluggish response time. To overcome this challenge, we proposed a method to drive the light emitting diode (LED) backlight and the LCD panel simultaneously. Besides relatively fast response time, this approach can also mitigate the imaging noise. Finally yet importantly, we improved the display pipeline by using a HDR gamut mapping approach to display HDR contents adaptively based on display specifications. A psychophysical experiment was conducted to determine the display requirements

High-Dynamic-Range and High-Efficiency Near-Eye Display Systems

Near-eye display systems, which project digital information directly into the human visual system, are expected to revolutionize the interface between digital information and physical world. However, the image quality of most near-eye displays is still far inferior to that of direct-view displays. Both light engine and imaging optics of near-eye display systems play important roles to the degraded image quality. In addition, near-eye displays also suffer from a relatively low optical efficiency, which severely limits the device operation time. Such an efficiency loss originates from both light engines and projection processes. This dissertation is devoted to addressing these two critical issues from the entire system perspective. In Chapter 2, we propose useful design guidelines for the miniature light-emitting diode (mLED) backlit liquid crystal displays (LCDs) to mitigate halo artifacts. After developing a high dynamic range (HDR) light engine in Chapter 3, we establish a systematic image quality evaluation model for virtual reality (VR) devices and analyze the requirements for light engines. Our guidelines for mLED backlit LCDs have been widely practiced in direct-view displays. Similarly, the newly established criteria for light engines will shed new light to guide future VR display development. To improve the optical efficiency of near eye displays, we must optimize each component. For the light engine, we focus on color-converted micro-LED microdisplays. We fabricate a pixelated cholesteric liquid crystal film on top of a pixelated QD array to recycle the leaked blue light, which in turn doubles the optical efficiency and widens the color gamut. In Chapter 5, we tailor the radiation pattern of the light engine to match the etendue of the imaging systems, as a result, the power loss in the projection process is greatly reduced. The system efficiency is enhanced by over one-third for both organic light-emitting diode (OLED) displays and LCDs while maintaining indistinguishable image nonuniformity. In Chapter 6, we briefly summarize our major accomplishments

Flat panel display signal processing

Televisions (TVs) have shown considerable technological progress since their introduction almost a century ago. Starting out as small, dim and monochrome screens in wooden cabinets, TVs have evolved to large, bright and colorful displays in plastic boxes. It took until the turn of the century, however, for the TV to become like a ‘picture on the wall’. This happened when the bulky Cathode Ray Tube (CRT) was replaced with thin and light-weight Flat Panel Displays (FPDs), such as Liquid Crystal Displays (LCDs) or Plasma Display Panels (PDPs). However, the TV system and transmission formats are still strongly coupled to the CRT technology, whereas FPDs use very different principles to convert the electronic video signal to visible images. These differences result in image artifacts that the CRT never had, but at the same time provide opportunities to improve FPD image quality beyond that of the CRT. This thesis presents an analysis of the properties of flat panel displays, their relation to image quality, and video signal processing algorithms to improve the quality of the displayed images. To analyze different types of displays, the display signal chain is described using basic principles common to all displays. The main function of a display is to create visible images (light) from an electronic signal (video), requiring display chain functions like opto-electronic effect, spatial and temporal addressing and reconstruction, and color synthesis. The properties of these functions are used to describe CRT, LCDs, and PDPs, showing that these displays perform the same functions, using different implementations. These differences have a number of consequences, that are further investigated in this thesis. Spatial and temporal aspects, corresponding to ‘static’ and ‘dynamic’ resolution respectively, are covered in detail. Moreover, video signal processing is an essential part of the display signal chain for FPDs, because the display format will in general no longer match the source format. In this thesis, it is investigated how specific FPD properties, especially related to spatial and temporal addressing and reconstruction, affect the video signal processing chain. A model of the display signal chain is presented, and applied to analyze FPD spatial properties in relation to static resolution. In particular, the effect of the color subpixels, that enable color image reproduction in FPDs, is analyzed. The perceived display resolution is strongly influenced by the color subpixel arrangement. When taken into account in the signal chain, this improves the perceived resolution on FPDs, which clearly outperform CRTs in this respect. The cause and effect of this improvement, also for alternative subpixel arrangements, is studied using the display signal model. However, the resolution increase cannot be achieved without video processing. This processing is efficiently combined with image scaling, which is always required in the FPD display signal chain, resulting in an algorithm called ‘subpixel image scaling’. A comparison of the effects of subpixel scaling on several subpixel arrangements shows that the largest increase in perceived resolution is found for two-dimensional subpixel arrangements. FPDs outperform CRTs with respect to static resolution, but not with respect to ‘dynamic resolution’, i.e. the perceived resolution of moving images. Life-like reproduction of moving images is an important requirement for a TV display, but the temporal properties of FPDs cause artifacts in moving images (‘motion artifacts’), that are not found in CRTs. A model of the temporal aspects of the display signal chain is used to analyze dynamic resolution and motion artifacts on several display types, in particular LCD and PDP. Furthermore, video signal processing algorithms are developed that can reduce motion artifacts and increase the dynamic resolution. The occurrence of motion artifacts is explained by the fact that the human visual system tracks moving objects. This converts temporal effects on the display into perceived spatial effects, that can appear in very different ways. The analysis shows how addressing mismatches in the chain cause motion-dependent misalignment of image data, e.g. resulting in the ‘dynamic false contour’ artifact in PDPs. Also, non-ideal temporal reconstruction results in ‘motion blur’, i.e. a loss of sharpness of moving images, which is typical for LCDs. The relation between motion blur, dynamic resolution, and temporal properties of LCDs is analyzed using the display signal model in the temporal (frequency) domain. The concepts of temporal aperture, motion aperture and temporal display bandwidth are introduced, which enable characterization of motion blur in a simple and direct way. This is applied to compare several motion blur reduction methods, based on modified display design and driving. This thesis further describes the development of several video processing algorithms that can reduce motion artifacts. It is shown that the motion of objects in the image plays an essential role in these algorithms, i.e. they require motion estimation and compensation techniques. In LCDs, video processing for motion artifact reduction involves a compensation for the temporal reconstruction characteristics of the display, leading to the ‘motion compensated inverse filtering’ algorithm. The display chain model is used to analyze this algorithm, and several methods to increase its performance are presented. In PDPs, motion artifact reduction can be achieved with ‘motion compensated subfield generation’, for which an advanced algorithm is presented