Quantum dot light emitting diode using graphene oxide as hole injection layer

Song, Daeho Sungkyunkwan university 2018 국내박사

In this dissertation, we discuss the performance improvement of quantum dot light-emitting diode (QLED) using graphene oxide (GO) and reduced graphene oxide (rGO) as the hole injection layer (HIL). Graphene oxide is synthesized by Hummers' method and dispersed in DI water with 0.5 wt%. Since GO has an inherent insulating property, we have confirmed the hole injection capability of GO. First, we compared two types of QLEDs with and without GO HIL. The device without GO-QLED exhibited extremely low luminance and efficiency. However, the device with GO-QLED improved all electro-optical performance of the device. These results imply that the GO can work properly as an HIL layer in QLED, although GO is an insulation layer. Furthermore, we optimized the thickness of the GO to be 2 nm in QLED. Then, we fabricated a QLED device with a GO/PEDOT:PSS bilayer HIL and compared it to counterpart QLED devices with PEDOT:PSS or GO HILs. The QLED with a GO/PEDOT:PSS bilayer HIL exhibited dramatically improved luminance, external quantum efficiency, current efficiency, and turn-on voltage compared to the QLED with GO and PEDOT:PSS HIL. Finally, we used reduced graphene oxide (rGO) instead of GO as the HIL in the QLEDs, where the degree of reduction of rGO was controllable by UV exposure. Interestingly, the electric conductivity of rGO increased with the decrease in oxygen concentration, and the work function of rGO was tunable. As a result of using rGO as the HIL, the QLED device exhibited a high luminance of more than 10,000 cd/m2, a current efficiency of 14.99 cd/A and an external quantum efficiency of 4.23%. These results indicate that GO and rGO can be good alternative materials to replace PEDOT:PSS or transition metal oxide, which are commonly used hole injection layers, and can be used as an HIL of a QLED.

Control of hole distribution in quantum-dot/organic light-emitting diodes and its applications

Song, Sukho Sungkyunkwan university 2021 국내박사

Both quantum-dot light-emitting diodes (QD-LEDs) and organic light-emitting diodes (OLEDs) are self-emissive devices and attracting attention as display light sources because of their advantages such as high color purity, fast response time, and high contrast ratio. However, the QD-LED has a structure wherein hole injection is difficult because of the high ionization potential of the QD, whereas the OLED has a structure wherein electron injection is difficult because of the shallow lowest unoccupied molecular orbital (LUMO) level of the organic electroluminescent materials. The charge imbalance in QD-LEDs and OLEDs results in Auger recombination and triplet-triplet annihilation, respectively, which are non-radiative recombination processes that degrade the device efficiency. Thus, to improve the charge balance, structural approaches were conducted in this thesis. In the case of QD-LEDs, studies on a bi-layered hole injection layer (HIL) structure to enhance the hole injection ability are discussed. The hole current density was significantly improved by applying the bi-layered HIL structure, which was verified to result from the lowered effective hole injection barrier by designing a step-wise energy level. In the case of OLEDs, an OLED structure with a hole modulation layer (HML) inserted in the middle of the emission layer to improve the charge balance was demonstrated. The HML insertion effectively delayed the hole flow and mitigated the charge imbalance, which was confirmed by charge distribution analysis. Improvements in the charge balance in both QD-LED and OLED devices have led to an improvement in the device efficiency, a 65% improvement in the maximum power efficiency (PE) for the QD-LED device, and a 58% improvement in the maximum PE for the OLED device. The device structures introduced in this thesis are expected to effectively reduce the power consumption for actual displays and can be applied in the implementation of ultra-high-resolution displays. In addition, the ‘side-by-side’ geometry method is currently the most widely used technology to reproduce various mixed colors. However, it has limitations in implementing ultra-high-resolution displays because of the inherent low fill factor. In this thesis, we introduce a unique tandem QD-LED structure wherein hole injection from the electrodes is suppressed and only electrons can be injected into the device. Tandem QD-LEDs with configurations where two LED units face each other, with different emission colors stacked in series, are designed. Based on the polarity of the applied voltage, the operating LED unit differs such that the emission color changes within a pixel. Moreover, mixed colors can be reproduced through pulse-width modulation (PWM). Precise and broad color tuning is achieved by simply adjusting the duty cycle of the PWM operation. We expect that the color-tunable tandem QD-LED introduced in this thesis has the potential to be applied to ultra-high-resolution displays in the future. 양자점 발광 다이오드 (quantum-dot light-emitting diode, QD-LED)와 유기 발광 다이오드 (organic light-emitting diode, OLED)는 높은 색 순도, 빠른 응답 속도, 높은 명암비 등의 장점이 있어 디스플레이 광원으로 각광받고 있다. 그러나, QD-LED는 QD의 높은 이온화 에너지로 인하여 정공 주입이 어려운 구조를 지니며, OLED는 유기 발광 재료의 가장 낮은 비점유된 분자궤도함수 (lowest unoccupied molecular orbital, LUMO) 준위가 얕아 전자 주입이 어려운 구조를 지닌다. 이와 같이 불균일하게 전하가 소자에 주입되면 비발광 재결합인 Auger recombination와 triplet-triplet annihilation이 QD-LED와 OLED에서 발생하여 소자의 효율을 저하시킨다. 따라서, 소자의 효율을 향상시키고자 본 논문에서는 구조적인 접근 방법들을 통해 전하 균형을 개선하였다. QD-LED 소자의 경우, bi-layered HIL 구조를 적용하여 hole injection ability를 향상시키는 연구를 진행하였다. Bi-layered HIL 구조를 적용한 경우 정공 전류 밀도가 크게 개선되었는데, 이는 소자의 에너지 레벨을 계단식으로 설계하는 것을 통해 정공 주입 에너지 장벽을 낮출 수 있었기 때문이다. OLED 소자의 경우, hole modulation layer (HML)을 발광층 중간에 삽입하여 전하 균형을 개선하는 연구를 진행하였다. HML 삽입은 정공 흐름을 효과적으로 지연시켜 전하 불균형을 완화하였으며, 이는 전하 분포 분석을 통해 규명하였다. QD-LED 및 OLED 소자의 전하 균형 개선은 소자의 효율 향상으로 이어졌으며, QD-LED의 최대 전력 효율은 65% 개선되었고, OLED의 최대 전력 효율은 58% 개선되었다. 본 논문에서 제시한 소자 구조는 실제 디스플레이의 전력 소모를 효과적으로 감소시키고 초고해상 디스플레이 구현에 적용될 수 있을 것으로 기대한다. 또한, 다양한 혼합색을 구현하는데 있어, 현재 가장 널리 사용되는 방식인 ‘side-by-side’ geometry 기법으로는 한 화소 내에서의 발광 면적을 개선하는데 본질적으로 제한이 있기 때문에 초고해상도 디스플레이를 구현하는데 한계가 있다. 본 논문에서는 전극으로부터의 정공 주입이 억제되어 소자 외부에서는 전자만 주입될 수 있는 tandem QD-LED 구조를 소개한다. Tandem QD-LED의 소자 구조를 서로 다른 색을 발광하는 두 발광 다이오드 유닛이 마주보는 형태로 적층되도록 설계하였다. 이를 통해 인가 전압의 극성에 따라 작동하는 발광 다이오드 유닛이 달라 하나의 화소에서 발광하는 색이 변하는 소자를 구현할 수 있었다. 그리고 펄스 폭 변조 (pulse-width-modulation, PWM) 구동을 통해 두 색상 사이의 다양한 혼합색을 재현할 수 있었다. 단순히 PWM 구동의 duty cycle을 조정하는 것을 통해 정확하고 광범위한 색상 조정이 가능하였다. 본 논문에서 소개하는 색 변조 tandem QD-LED는 향후 초고해상도 디스플레이에 적용 가능하리라 기대한다.

Design and synthesis of polymer-based nanocomposites for bio, sensory and catalytic applications

Song, Sungu Sungkyunkwan university 2021 국내박사

In the modern industry, the increasing demands of higher mechanical, chemical properties and novel characteristics of materials such as bio-compatibility, green synthesis are hard to satisfy with monolithic materials. For this reason, many researchers have focused on the polymer matrix nanocomposites (PNC) due to the synergistic effects of composites inducing enhanced physical and chemical properties and adapting novel characteristics for the applications such as catalyst, bio-medicine, and sensors easily. However, PNC still has limitations in the precise size and morphology control of nanoparticles and polymers, destabilization on disperses, the uneasy orientation of the dispersed phase, and agglomeration problems. Herein, I proposed novel molecules, methods, and mechanisms to develop the novel nanocomposites based on polymers, supramolecules, carbon-nanotubes for bio-, sensory and catalytic applications. I proposed imidazoliums with thiophene groups coordinated with a gold precursor, which can be converted to N-heterocyclic carbene (NHC) polymers with gold nanoparticles (AuNPs) with disproportionation and in-situ polymerization in chapter 2. In chapter 3, a binaphthyl group was introduced for precise control of the size and shape of PNC by solvents types and molecular weights of polymers. I also proposed supramolecular nanocomposite structures using cucurbit[7]uril in chapter 4 to synthesize the various metal nanoparticles (Au, Ag, Pt, Pd, Cu, and Fe) but adopt bio-compatible properties with green synthesis. In chapter 5, I proposed a solid-state synthesis method to synthesize size-controlled dextran/metal nanoparticle PNC to solve the problems of large-scale synthesis, green synthesis, and nanoparticle storage, and test anti-cancer activity. Finally, in chapter 6, I proposed single-walled carbon nanotube-based nanocomposites aligned with a the dielectrophoresis method in portable and selective chemical warfare agent sensors. 고분자 나노복합체는 고분자에 나노사이즈의 분리된 상의 물질이 복합된 형태의 물질로서, 기존 물질의 물리적, 화학적, 전기적 성질을 향상시키는 특성과 더불어, 목표하는 복합체의 물성뿐 아니라, 바이오적합성, 환경친화적인 성질등을 고분자 구조의 설계를 통해서 쉽게 달성할 수 있는 물질로서 큰 관심을 가지고 연구가 되고 있다. 본 연구에서는 고분자 나노복합체는 분류 중 고분자 기반 금속 나노입자의 합성과 응용, 초분자 기반 금속 나노입자의 합성과 응용, 탄소나노튜브 기반 고분자 복합체를 이용한 화학 센서로의 응용에 대하여 연구하였다. 우선, 이미다졸리윰 기반 물질에 금속 전구체가 결합된 형태의 단분자에서, 싸이오펜 그룹을 도입하여 단분자내에서 불균등화반응에 의해 생성된 금나노입자가 이미다졸리윰 물질에 단단히 결합하여 금속나노입자의 위치를 특정할수 있음과 동시에, 전기적 특성을 보이는 싸이오펜 그룹이 금나노입자에 의해 고분자화 반응이 진행되어서, 2TNHC-AuCl 단분자 하나로 고분자 금 나노입자 복합체를 형성하는데 성공하였다. 이에 이에서 바이나프틸 단분자 구조를 도입하여, 금 전구체를 이용해 금속나노입자의 합성과 고분자 합성을 동시에 진행하였으며, 용매에 따른 나노복합체의 크기 조절의 메커니즘 연구를 진행하였다. 이에, 보다 구체적인 복합체의 성질을 도입하고 응용하기 위하여 Cucurbit[7]uril 초분자를 도입하여 NaOH를 통하여 다양한 금속의 나노입자를 합성하고, 생체친화적인 성질을 보이는 CB7의 성질이 도입되었음을 확인하였고 4-나이트로페놀의 촉매 작용 및 항암성질에 응용하였다. 이어서, 초분자가 아닌 덱스트란을 고분자 이용하여 덱스트란의 분자량, 비율등에 의해 고분자 나노복합체의 크기 및 모양이 조절되는 것을 확인하였으며, 고체합성 반응을 도입하여 대량합성 및 나노복합체의 보관에 있어서 높은 안정성을 보이는 물질을 합성하였다. 또한, 보다 다양한 분야에의 응용을 위하여 탄소나노튜브를 도입하여, 유전영동을 통해 고정된 탄소나노튜브에 공유결합과 비공유결합 형태의 탄소나노튜브 기반 나노복합체를 형성하여 정전용량센서에 응용하여 화학작용제를 탐지할 수 있는 센서를 개발하였으며, 유사작용제 및 실작용제를 탐지하는데 성공하였으며, PCA 분석을 통하여 선택성을 보이는 센서를 개발하였다.

(The) impact of psychological distance on user experience on the C2C online shopping platform

Song, Yeosol Sungkyunkwan University 2023 국내석사

Users should feel a sense of belonging and establish a trustworthy environment through social networks on the C2C shopping platform. Reducing the psychological distance among users can help increase trust towards users and reduce uncertainty in the seller on a C2C shopping platform. The study explored the effect of psychological distance on user experience by dividing the psychological distance into physical and social aspects felt among users on an online shopping platform. Study 1 conducted a survey of 506 participants to investigate how physical distance affects trust, attitude and intention for continued use, and Study 2 conducted a 2 (geographical distance: near versus far) × 2 (personal profile: absence versus presence) between-subjects experiment (N = 98) to examine whether they can affect psychological distance and user experience. The results showed that a reduced psychological distance had a positive effect on user experience (Study 1). Moreover, a personal profile affected social-psychological distance and user experience, while geographical proximity did not have a significant impact on user experience (Study 2). Analyzing users’ trust and behavioral intentions in online shopping according to psychological distance has practical implications in that specific service designs were verified through experiments. C2C 쇼핑 플랫폼에서는 사용자들이 소속감을 느끼며, 사회적인 관계망을 통해 신뢰할 만한 환경을 구축하는 것이 중요하다. 거래 상대방에 대한 심리적 거리감을 감소시키는 것은 기존 중고 거래시장의 문제점으로 인식되는 판매자에 대한 신뢰 부족과 불확실성 문제를 해결하는데 도움이 될 수 있으며, 신뢰 이전 이론에 따르면 플랫폼에 대한 신뢰가 형성됨으로써 긍정적인 온라인 거래 환경을 갖출 수 있다. 본 연구는 온라인 쇼핑 플랫폼에서 상대방에 대해 느끼는 심리적 거리감을 물리적, 사회적 측면으로 나누어 사용자 경험에 미치는 영향을 연구했다. 연구 1은 당근마켓 사용자 506명을 대상으로 설문조사를 실시하여 심리적 거리감이 신뢰, 태도, 지속 사용의도에 어떠한 영향을 미치는지를 조사했으며, 연구 2에서는 98명의 참여자들을 대상으로 온라인 거래를 수행하는 실험을 진행하여 지리적 근접성과 개인 프로필의 존재 여부가 심리적 거리감과 사용자 경험에 영향을 줄 수 있는지를 살펴보았다. 결과적으로, 연구 1에서 심리적 거리감의 두 가지 차원 모두 신뢰와 사용자 경험에 긍정적인 영향을 미쳤다. 연구 2에서 개인 프로필의 존재가 심리적 거리감에 영향을 주어 신뢰와 사용자 경험을 증가시키는 것으로 나타났지만, 이와 달리 지리적 근접성에서는 차이가 나타나지 않았다. 온라인 쇼핑에서의 사용자들의 신뢰와 행동의도를 심리적 거리감에 따라 분석함으로써 해석 수준 이론의 범위를 확장하였으며, 실험을 통해 구체적인 서비스 디자인을 검증했다는 실증적 함의를 지닌다.

Childhood and adolescence in the United States and Korea

Song UMI 1990 국내박사

The Impact of Expected and Unexpected Socio-economic Change on Air Pollutants and Carbon

Seyeong Song Ulsan National Institute of Science and Technology 2022 국내석사

This study investigates the impact of unexpected and expected socio-economic changes on air pollutants and carbon. For the impact of socio-economic changes on air pollutants, the analysis used the air quality, thermal power generation, and macroeconomic indices data to identify the effect of thermal power generation and COVID-19 on air quality. The findings are that thermal power generation had worsened the level of CO from 2015 to 2019 and that the impact of COVID-19 improves the air quality in China and South Korea. For the impact of socio-economic changes on carbon, the analysis used firm-level emissions and corporate variables to identify how ETS implementation has influenced carbon productivity (a firm-level revenue created per unit of carbon emission). The findings are that the firms increased carbon productivity after participating in the national-level Emission Trading Scheme (ETS) of South Korea, especially for high-emitting industries, and firms that are profitable, innovative, and managed by CEOs with environment-related educational or working experience successfully increase their carbon productivity. These findings suggest that to improve air pollution and global warming, governments have to establish appropriate air quality control measures on each air pollutant and assist green-minded firms and firms have to increase profitability and invest in eco-friendly technologies by green-minded executives with regulations on high-emitting firms.

The tax system of the Republic of Korea

Song UMI 1990 국내박사

American protestant missionary perceptions of the Korean independence movement of 1919 and its effects upon the churches of Korea

Song UMI 1990 국내박사

Effect of the Tibetan plateau on the Asian summer monsoon circulations : a regional climate model study

송지혜 Graduate School, Yonsei University 2008 국내석사

In order to understand impacts of the Tibetan Plateau (TP) upon the Asian summer monsoon (ASM), a series of numerical experiments using a regional climate model are conducted with the various TP heights ranging from the flat surface (1.2 km) to the 140% of the real height. Compared to what was observed, the control experiment with a realistic TP height reproduces the precipitation and large-scale features properly.From the experiment using flat topography over the TP region, it is found that the presence of the TP leads to the increased (decreased) precipitation over northern (southern) India through both dynamical and thermal effects. The major causes to bring these changes are lower-atmospheric thermal low developed south to the TP and upper-atmospheric Tibetan high north to the TP, which results in consequent rising motion over the Indian summer monsoon (ISM) region. From a series of sensitivity experiments to the various TP height, the dynamical effects are consistently confirmed with somewhat reduced magnitudes; however, the resulting seasonal precipitation is not as robust as revealed in the flat topography run. It indicates that dynamical effects mixed with the weak thermal effects due to the small differences in TP height are not strong enough to bring out sufficient changes in the lower atmospheric circulation that is able to overcome nonlinear diabatic physical processes, so that changes in the ISM precipitation in response to the TP height are not linear.It is found that the presence of the TP intensifies the cyclonic circulation in the lower troposphere over East Asia, which is associated with positive (negative) height anomalies over the TP (east of the TP) in the upper troposphere. These dynamical circulation features due to the TP induce a weakening of the thermal and pressure gradient between land and ocean in the lower atmosphere toward what was observed. A series of sensitivity experiments reveals that both the lower- and higher-TP result in northwestward displacement of the East Asian summer monsoon (EASM) due to the enhanced subtropical northwestern Pacific high. It is concluded that a dominant factor for the EASM due to the TP is the dynamical effect caused by changes in the upper-atmospheric circulation, whereas the role of dynamically driven thermal effect is significant for the Indian summer monsoon.

An efficient data assimilation using an incremental vertical interpolation method

송재익 Graduate School, Yonsei University 2010 국내석사

In order to alleviate the initial imbalance between regional and global model, we adopted the incremental vertical interpolation method. The impact of the incremental vertical interpolation method was investigated over the East Asian region using the Weather Research and Forecasting (WRF) model. For the purpose, four different experiments are conducted: 1) control 2) with the inclusion of the mixing ratio from fist guess field, 3) with an incremental vertical interpolation and 4) with the incremental vertical interpolation and the addition of the mixing ratio in initial condition. Short-range forecasts have been examined for a heavy rain case over Korea on 00 UTC 15 July ~ 00 UTC 17 July 2006 to identify the impact of the incremental interpolation method on the high-impact weather. For the validation of the prediction ability against the analysis data, four different model simulations were compared to the National Centers for Environmental Prediction (NCEP) Final analysis (FNL) data focused on the geopotential height (GPH) and temperature fields, verified with basic statistics analysis. The WRF model is run twice daily during July of 2006, once for 0000 UTC initial conditions and the second time for 1200 UTC initial conditions. Forecasts are integrated for 48hr after initialization time. It is found that experiments adopted the incremental vertical interpolation partially reduce the overestimated rainfall amount compared with the control run, particularly during the first 6-hr of the integration. In the study of the prediction ability, the simulations adopted the incremental vertical interpolation result in more accurate the WRF forecasts than those of the control experiment during the first 6 hour of the integration in the sea level pressure (SLP) and GPH fields. In addition, the statistics scores in the temperature field at middle and upper levels were improved in the experiments using the incremental interpolation method. Thus, methodology designed in this study can be an efficient way to reduce the imbalance between the regional and global models in the short-range forecast.